Definition of expiratory reserve volume
Ask a medical professional for a definition of expiratory reserve volume [ERV] and they’ll offer something along the lines of: “The extra volume of air that can be expired from the lungs with determined effort following a normal tidal volume expiration.”
Let’s make that easier to understand.
Picture yourself sitting normally and breathing as you do when you are not exerting yourself orexercising. The amount of air you breathe in is your tidal volume.
After you breathe out, try to exhale more until you are unable to breathe out any more air. The amount of air you can force out after a normal breath [think about blowing up a balloon] is your expiratory reserve volume.
You can tap into this reserve volume when you exercise and your tidal volume increases.
To sum up: Your expiratory reserve volume is the amount of extra air — above anormal breath — exhaled during a forceful breath out.
The average ERV volume is about 1100 mL in males and 800 mL in females.
Respiratory volumes are the amount of air inhaled, exhaled, and stored in your lungs. Along with expiratory reserve volume, some terms that are often part of a ventilatory pulmonary function test and can be helpful to know include:
- Tidal volume. The amount of air you typically breathe into yourlungs when at rest and not exerting yourself. The average tidal volume is about 500 mL for both men and women.
- Inspiratory reserve volume. The amount of extra air inhaled — above tidal volume — during a forceful breath in. When you exercise, you have a reserve volume to tap into as your tidal volume increases. The average inspiratory reserve volume is about 3000 mL in males and 2100 mL in females.
- Vital capacity. The total usable volume of the lungs that you can control. This is not the entire lung volume as it is impossible to voluntarily breathe all of the air out of your lungs. The average vital capacity volume is about 4600 mL in males and 3400 mL in females.
- Total lung capacity. The total volume of your lungs: your vital capacity plus the amount of air you cannot voluntarily exhale. The average total lung capacity volume is about 5800 mL in males and 4300 mL in females.
The amount of lung capacity varies from person to person based on their physical makeup and their environment.
You are likely to have a larger volume if you:
- are tall
- live at a higher altitude
- are physically fit
You are likely to have a smaller volume if you:
- are short
- live at alower altitude
- are obese
Your expiratory reserve volume is the amount of extra air — above-normal volume — exhaled during a forceful breath out.
Measured with spirometry, your ERV is part of the data gathered in pulmonary function tests used to diagnose restrictive pulmonary diseases and obstructive lung diseases.
European Respiratory Journal 1993 6: 5-40; DOI: 10.1183/09041950.005s1693
1 INTRODUCTION
Lung volumes are subdivided into static and dynamic lung volumes. Static lung volumes are measured by methods which are based on the completeness of respiratory manoeuvres, so that the velocity of the manoeuvres should be adjusted accordingly. The measurements taken during fast breathing movements are described as dynamic lung volumes and as forced inspiratory and expiratory flows.
1.1 Static lung volumes and capacities
The volume of gas in the lung and intrathoracic airways is determined by the properties of lung parenchyma and surrounding organs and tissues, surface tension, the force exerted by respiratory muscles, by lung reflexes and by the properties of airways. The gas volumes of thorax and lung are the same except in the case of a pneumothorax. If two or more subdivisions of the total lung capacity are taken together, the sum of the constituent volumes is described as a lung capacity. Lung volumes and capacities are described in more detail in § 2.
1.1.1 Determinants
Factors which determine the size of the normal lung include stature, age, sex, body mass, posture, habitus, ethnic group, reflex factors and daily activity pattern. The level of maximal inspiration [total lung capacity, TLC] is influenced by the force developed by the inspiratory muscles [disorders include e.g. muscular dystrophy], the elastic recoil of the lung [disorders include e.g. pulmonary fibrosis and emphysema] and the elastic properties of the thorax and adjacent structures [disorders include e.g. ankylosis of joints]. The level of maximal expiration [residual volume, RV] is determined by the force exerted by respiratory muscles [disorders include e.g. muscle paralysis], obstruction, occlusion and compression of small airways [disorders include e.g. emphysema] and by the mechanical properties of lung and thorax [disorders include diffuse fibrosis, kyphoscoliosis].
Assessing the total lung capacity is indispensable in establishing a restrictive ventilatory defect or in diagnosing abnormal lung distensibility, as may occur in patients with emphysema. Measurements of lung volumes are also essential in interpreting data on lung elastic recoil pressure, instantaneous ventilatory flows, airways resistance, and the transfer factor of the lung, since these are all volume dependent.
1.1.2 Restrictive ventilatory defects
Static lung volumes may be diminished by disorders which restrict lung expansion [restrictive ventilatory defects], such as neuromuscular disorders, diseases of the chest wall and abdomen, disorders of the pleural space, increase in lung stiffness, and decrease in the number of available alveolar units [lung resection, atelectasis, scars].
A restrictive ventilatory defect is best described on the basis of a reduced TLC rather than from vital capacity measurements. The vital capacity, i.e. the volume change between RV and TLC, may be diminished by both restrictive and obstructive ventilatory defects; in the latter case it is due to an increase in residual volume due to [premature] airways closure [gas trapping] and airflow limitation at low lung volumes [1, 2], leading to incomplete lung emptying. However, in small airways disease the RV is increased with no change in TLC; accordingly the VC is reduced [with a proportionate decrease in FEV1 [3]]. Hence the vital capacity alone is of little use in discriminating between restrictive, obstructive and mixed ventilatory defects. In some cases of cystic fibrosis, which primarily affects the airways, temporary decreases in TLC have been described, possibly due to partial atelectasis [4].
1.1.3 Hyperinflation
The total lung capacity may be abnormally large in acromegaly and in the case of increased lung distensibility. In the latter situation RV and the functional residual capacity [FRC] are also increased, whereas TLC tends to decrease. The term hyperinflation has been proposed to describe the increase in FRC [5]; however, it is also frequently used to describe increases in TLC or RV. Hyperinflation is usually associated with an obstructive ventilatory defect [see § 1.2.2], but the relationship between the increased FRC and the lowered FEV1 is weak [6].
1.2 Dynamic lung volumes and forced ventilatory flows
1.2.1 Determinants
The ability to move air rapidly in and out of the lungs is essential for normal activity, and any diminution of more than minimal extent will usually cause breathlessness on exertion and hence reduce the capacity for exercise. Ventilatory impairment can arise from changes in the nervous system, the skeleto-muscular system, the skin and subcutaneous tissues, the lungs or the inhaled gas. However the commonest cause is narrowing of the airways. The impairment can be detected by dynamic spirometry, which constitutes the first stage in the assessment of respiratory function. Subsequent stages include consideration of mechanisms, the amount of function which remains, the cause of the condition and the means for its alleviation.
Dynamic lung volumes and flows are assessed during forced inspiration or expiration, or during forced breathing when maximal effort is applied throughout the respiratory manoeuvre. In this it differs from the measurement of static lung volumes where the maximal effort is generated only at the beginning and end of the manoeuvre. The results of dynamic spirometry are usually expressed in terms of the relationships of inspired or expired volume to time as described by the volume-time curve [spirogram: fig. 1]. They are also expressed as the relationship of maximal flow to lung volume described by the flow-volume curve [fig. 2].
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Figure 2–
Maximal expiratory flow-volume curves of healthy subjects and in patients with an obstructive and restrictive ventilatory defect. In the top panel the curves are superimposed at TLC, the lower panel shows maximal expiratory flow as a function of absolute lung volume.
1.2.2 Obstructive ventilatory defect
The obstructive ventilatory defect is defined as a decrease in FEV1 [the volume exhaled in 1 second during a forced expiration started at the level of TLC] out of proportion of any decrease in VC, i.e. a decrease in the FEV1/VC ratio. The flows during a forced expiration are reduced concomitantly.
The determinants of expiratory flow during a forced expiration are complex. During the forced expiration both the pleural and alveolar pressures are greatly increased above pressure at the mouth. However, after a brief effort dependent phase, which includes the peak expiratory flow, the pressure drop from alveoli to the mouth causes pressure in intrathoracic airways to become less than the surrounding pleural pressure; hence these airways are dynamically compressed and act as flow limiting segments, causing forced expiratory flows to be effort independent. At this stage the forced expiratory flow is determined by a complex interplay between lung elastic recoil pressure, the resistance to airflow of airways upstream of the flow limiting segment, and the elastic properties of the compressed airway «tube law» [7]. In healthy, young subjects near residual volume the high intrathoracic pressures and hence airway compression cannot be maintained, so that expiratory flows may become effort dependent once more. The elastic properties of the extrathoracic airways vary with the stretch to which they are subjected, such as with flexion and extension of the neck [8]. Within an individual the lung elastic recoil pressure varies with volume and hence with the level of lung inflation. During a forced expiration, the effect of reduction in lung volume due to gas compression is more pronounced in a patient with obstructive lung disease than in a healthy subject; this is because the absolute volume may be increased and hence the volume compressed greater, and with a limited VC this represents a greater proportion of VC. Thus the patient's lung is at a lower point on its pressure-volume curve and hence at a lower elastic recoil pressure. This pressure would be higher if less expiratory force were to be applied. Since the recoil pressure determines the expiratory flow the latter can sometimes be increased by the patient exerting less expiratory effort. Under these circumstances the volume expired in 1 second [FEV1] is larger than during a maximal effort [9–12].
1.2.3 Site of expiratory flow limitation
The [forced] vital capacity and instantaneous flows can be obtained from both volume-time and flow-volume curves. Time averaged flows or forced expiratory times are derived from volume-time curves. In healthy subjects, maximal flows at large lung volumes reflect mainly the flow characteristics of the trachea and central bronchi, whilst those at small lung volumes are usually held to reflect more the characteristics of the smaller intrathoracic airways [13, 14]. In the latter the flow is laminar, whilst in the large airways it is at least partly turbulent. The turbulent component of the airway resistance but not the laminar component is reduced by replacing the respired air with gas of low density, for example, helium with 20% oxygen. However, the reduction in gas density also has other effects which make interpretation difficult. On this account the procedure is not recommended for routine assessment [15, 16].
In lung diseases including asthma, which cause acute changes in ventilatory capacity, the larger «central» intrathoracic airways are supposed to be the principal site of reversible airflow limitation. The smaller «peripheral» airways contribute to the limitation when there is bronchiolitis as can occur with infection, asthma or chronic exposure to fumes, fibrosis of the respiratory bronchioles as in asbestosis, or reduced lung elasticity; the latter is a feature of emphysema. In patients with emphysema the site of flow limitation moves peripherally [13, 14]. Intravenous administration of histamine in dogs similarly moves the flow limiting segment towards the periphery, but this is overruled by adding a central obstruction [17]. In heart-lung transplant patients who develop bronchiolitis obliterans the first physiological abnormality is a decrease of flows at low lung volumes with a progressive increase in convexity towards the volume axis of the terminal part of the flow-volume curve [18].
A progressive deviation from normal ventilatory capacity can be detected by longitudinal measurements [§ 4.2]; the principal constraints are then the appropriateness and reproducibility of the tests. For measurements at one point in time an additional constraint is what constitutes normal function for the individual in question. The estimate of normal lung function is called the reference value [§ 5].
1.2.4 Extrathoracic airway obstruction
During a forced expiration the extrathoracic airways are subjected to a positive transmural pressure, whereas during forced inspiratory manoeuvres their calibre decreases due to the negative transmural pressure. Accordingly extrathoracic airway obstruction is best detected during a forced inspiration [see § 2.3].
1.3 Application of tests
Assessments of lung volumes and forced ventilatory flows described in this report are usually the first to be applied in clinical and non-clinical work. They are used in:
1 The diagnosis of subjects with known or suspected lung disease, e.g. to identify intra- or extrathoracic airflow limitation or a restrictive ventilatory defect.
2 The treatment of patients with lung disease, to monitor the effect of preventive measures [e.g. allergen avoidance, therapeutic interventions [effects of drugs], or diagnostic procedures [e.g. the use of the FEV1 in tests of bronchial responsiveness].
3 Establishing a prognosis, based on e.g. the severity and extent of respiratory impairment, on the effectiveness of therapeutic interventions, or the rate of deterioration over a period of time.
4 Making a pre-operative assessment with a view to estimating the risk from respiratory complications and optimising the patient's condition pre-operatively.
5 Evaluating pulmonary disablement.
6 Monitoring the respiratory health of populations in epidemiological studies and by monitoring at the workplace.
7 Aiding in the interpretation of other lung function tests which are volume dependent, e.g. the transfer factor etc.
Positive test results reveal functional patterns rather than a particular disease. For example, the VC and the FEV1 may be reduced by a restrictive and by an obstructive ventilatory defect, and these conditions may occur concurrently [see § 2.1.10]. The tests contribute information which can complement that obtained in other ways.
2 INDICES AND DEFINITIONS
A schematic representation of static lung volumes is given in figure 3. The definitions of lung volumes and capacities given here are in agreement with those proposed by the European Society for Clinical Respiratory Physiology and the World Health Organization, Europe Branch [5]. Unless otherwise specified, volumes are expressed in l BTPS [see § 3.3] and flows in l BTPS s−1. The units adopted are those of the Système international d'unités [SI-units], and based on recommendations for use of SI units in respiratory physiology [19–32]. The units relevant for this report, including, non-SI units retained and allowed within the European Community, are given in table 1, while table 2 gives the conversion factors to go from conventional to SI units. In keeping with international recommendations in respiratory physiology the units for pressure and volume are kPa and litre respectively. A list of abbreviations, symbols and units, with a translation of terms into the languages of the European Community, is to be found elsewhere in this volume.
Table 1– Selected SI base and derived units, and non-SI units retained
Table 2– Factors for converting conventional units to SI units
2.1 Static lung volumes
2.1.1 Vital capacity
The vital capacity [VC] is the volume change at the mouth between the positions of full inspiration and complete expiration. The measurement may be made in one of the following ways:
1 Inspiratory vital capacity [IVC]: the measurement is performed in a relaxed manner, without undue haste or deliberately holding back, from a position of full expiration to full inspiration;
2 Expiratory vital capacity [EVC]: the measurement is similarly performed from a position of full inspiration to full expiration;
3 Two-stage vital capacity: the vital capacity is determined in two steps as the sum of the inspiratory capacity [IC] and expiratory reserve volume [ERV];
4 Forced vital capacity [FVC]: this denotes the volume of gas which is exhaled during a forced expiration starting from a position of full inspiration and ending at complete expiration.
Subdivisions of the vital capacity include the tidal volume [TV, VT], inspiratory reserve volume [IRV], expiratory reserve volume [ERV]; the inspiratory capacity [IC] is the sum of IRV and VT [fig. 3].
The average within-subject standard deviation of repeated measurements of the vital capacity is between 90 and 200 mL, a weighted average being 148 mL [see overview in [33]]. The variability is also expressed as the coefficient of variation, which varies between 0.3 and 11.4 per cent within the same individual; this index assumes that the variability is proportional to the mean, which is most often not true for ventilatory indices. In healthy subjects differences between the FVC and IVC are minimal. The relaxed expiratory vital capacity, and particularly the forced vital capacity [34], may be considerably less than the IVC in patients with airflow limitation; therefore when the total lung capacity is computed as the sum of RV and VC it will be underestimated unless the IVC is used: TLC = RV+IVC. Similarly the Tiffeneau index [FEV1%VC] will be spuriously high in patients with airflow limitation unless the IVC is used in the denominator. Hence, where a measurement of VC is used, it shall normally be the IVC; if this is not feasible then the relaxed VC is a good alternative. The two-stage vital capacity is not recommended for routine use; its measurement may occasionally be useful in very dyspnoeic patients.
2.1.2 Expiratory reserve volume
The expiratory reserve volume [ERV] is the volume that can be maximally expired from the level of the functional residual capacity. It is less in the supine than in the sitting posture [33] and decreases in obesity [35–40]; ERV is rarely used as an independent index.
2.1.3 Inspiratory reserve volume
The inspiratory reserve volume [IRV] is the maximal volume that can be inspired from the mean end-inspiratory level. It is of theoretical interest only.
2.1.4 Inspiratory capacity
The inspiratory capacity [IC] is the maximal volume that can be inspired from functional residual capacity; it is equal to the sum of tidal volume and inspiratory reserve volume. It is not different in the supine compared to the sitting position [42].
2.1.5 Tidal volume
The tidal volume [VT, TV] is the volume of gas which is inspired or expired during a respiratory cycle; although listed under static lung volumes, it is a dynamic lung volume, which varies with the level of physical activity. It is commonly measured at the mouth and varies with measuring conditions [rest, exercise, posture]. The average value of at least six breaths should be used.
2.1.6 Functional residual capacity
The functional residual capacity [FRC] is the volume of gas present in the lung and airways at the average end-expiratory level. It is the sum of expiratory reserve and residual volume. The latter volume can only be measured indirectly; the method of measurement as well as the measuring conditions should be specified.
The functional residual capacity can be assessed by a «gas dilution» method, by body plethysmography or by radiography. In healthy subjects the three methods yield very similar results [43–50]. The coefficient of variation of repeated measurements on the same subject is usually less than 10% [51]. In patients with severe airflow limitation or emphysema the true lung volume is underestimated by the dilution method, unless mixing time is prolonged to at least 20 min [44, 45, 52, 53]. The gas dilution method is widely used because it is simple to perform and the equipment is relatively inexpensive. The plethysmograph method is mandatory for studies of instantaneous lung volume such as are required for interpretation of airways resistance and forced expiratory flow. The plethysmographic FRC includes non-ventilated as well as ventilated lung compartments. On this account in patients with gas trapping and lung cysts the method gives higher results than the gas dilution technique; the difference between plethysmographic and gas dilution measurements provides information about non-ventilated air spaces within the thorax. The plethysmographically assessed lung volume may be further increased by gas present in the abdomen [54–56]. In the case of severe air-flow limitation the lung volume may be systematically overestimated [47, 57–69] by the plethysmographic method when the measurement is performed at respiratory rates in excess of 1 s−1.
The FRC varies considerably with the level of physical activity and with posture, being smaller when lying down than when sitting or standing; it is also greatly influenced by the quantity of body fat. This is because gross obesity decreases total and chest wall compliance [41, 70] and diminishes the ERV and FRC [35–40].
2.1.7 Residual volume
The residual volume [RV] is the volume of gas remaining in the lung at the end of a full expiration. It is calculated by subtracting the expiratory reserve volume from the functional residual capacity: RV = FRC−ERV, or RV = TLC−IVC. The coefficient of variation of repeated measurements on the same subject is about 8% [71].
2.1.8 Total lung capacity
The total lung capacity [TLC] is the volume of gas in the lung at the end of a full inspiration. It is either calculated from: TLC = RV+IVC, or from: TLC = FRC+IC; the latter is the preferred method in body plethysmography. It can also be measured directly by the radiologic technique. The method of measurement [gas dilution, body plethysmography, radiology] should be specified.
2.1.9 Thoracic gas volume
The thoracic gas volume [TGV] is the volume of gas in the thorax at any point in time and any level of thoracic compression. It is usually measured by the whole body plethysmograph method [55], which is the method of choice in patients with airflow limitation, in whom gas is often trapped behind occluded airways; however, when there is severe airways obstruction thoracic gas volume may be overestimated [see § 2.1.6] [47, 57–60, 64–68].
The thoracic gas volume may be determined at any level of lung inflation; the level should be specified [e.g. FRC]. Alternatively the TLC or RV can be obtained by adding to the TGV the volume which can be inhaled to total lung capacity, or by subtracting the volume that can be exhaled to residual volume. In the latter case, the inspiratory or expiratory manoeuvres should be performed immediately after the measurement of TGV.
2.1.10 Clinical usefulness
Measurements of VC have a well-established basis in assessing lung volumes in health and disease. However, the information provided by VC may be ambiguous, and clinically relevant information may be obtained only from considering additional indices. For instance, coexistence of an obstructive and restrictive ventilatory defect cannot be deduced from simple spirometric measurements; the diagnosis of a mixed ventilatory defect should be limited to the combination of a reduced TLC and FEV1/VC ratio. However, in patients with airflow limitation and emphysema, TLC is not very sensitive to processes usually associated with a restrictive pattern such as lobectomy [72] or cryptogenic fibrosing alveolitis [73].
A lowered RV is occasionally the sole physiological abnormality [74] in patients with chest wall problems [skeletal deformity, fibrothorax] or parenchymal disease [congestive heart failure, sarcoidosis, infections]. RV measurements are also useful in evaluating the interaction between smoking and interstitial lung disease. In smokers and ex-smokers with parenchymal sarcoidosis RV and FRC are lower than in nonsmokers [75], whereas in idiopathic pulmonary fibrosis RV is higher in smokers [76]; in these studies groups did not differ with respect to VC and FEV1. Increases in RV without changes in FEV1 and FEV1/VC are seen in patients at risk of developing chronic obstructive pulmonary disease, such as middle-aged females with heterozygous α1-antichymotrypsin deficiency [77]. A slight increase in RV is the most frequent functional abnormality in young patients after an episode of idiopathic spontaneous pneumothorax [78], in whom CT examination suggests centrilobular emphysema in upper lung zones [79]. Longitudinal studies over a mean interval of 3.5 years in middle-aged cigarette smokers without established lung disease demonstrated more consistent increases in RV and TLC than in decreases in VC or FEV1 [80].
Measurements of FRC unaccompanied by information about RV and TLC are of interest in patients with chest wall disorders. FRC and ERV are markedly reduced in persons with morbid obesity when VC and FEV1 are still within the normal range [40]. In subjects with the sleep apnoea syndrome with normal VC, RV and TLC the level of nocturnal hypoxaemia can be predicted from the decrease in FRC and ERV when changing from the sitting to the supine posture [81]. In other conditions FRC and RV are normal when VC and TLC are severely reduced, e.g. after surgical correction for funnel chest [82]. These examples illustrate the complex interactions between lung and chest wall [83], and underline the need for measuring several indices in addition to spirometric measurements.
Useful information can also be derived from assessing TLC by different methods in the same patient. In healthy subjects TLC assessed by the single breath helium dilution method [see § 3.7.2] is somewhat smaller [down to 83%] than when assessed with the multi-breath technique [71, 84, 85]. The difference can be accentuated in patients with asthma even during a period when no airflow limitation is detected, indicating abnormal unevenness of ventilation distribution. Also the trapped gas volume [§ 3.8.7] is of interest in patients with bullous emphysema in whom surgical correction is considered [86, 87].
2.2 Forced expiration
2.2.1 Forced vital capacity
The forced vital capacity [FVC] is the volume of gas delivered during an expiration made as forcefully and completely as possible starting from full inspiration [fig. 1, 2 and 4]. The FVC is to be distinguished from the relaxed expiratory vital capacity and from the inspiratory vital capacity, where the emphasis is put only on the completeness of the manoeuvre, not on the speed. The FVC may be underestimated if not enough time is allowed for lung emptying at low lung volumes, where the emptying rate is determined by airflow limitation [1, 2].
2.2.2 Time averaged maximal expiratory flow
The timed forced expiratory volume [FEV1] is the volume of gas exhaled in a specified time from the start of the forced vital capacity manoeuvre; conventionally, the time used is 1 s, symbolized FEV1. It is an extensively used index with good reproducibility; the standard deviation of repeated measurements within healthy subjects in various studies varies from 60 to 270 mL, 183 mL being the weighted average [computed from overview in [33]]. The FEV1 can be standardized for the vital capacity, when it is called FEV1% [FEV1%VC, the VC needs to be specified]. The use of the inspiratory, the relaxed expiratory or the two-stage vital capacity in the denominator yields a more sensitive index of airflow limitation; the first of these is the Tiffeneau index [88].
The maximal mid-expiratory flow [MMEF, FEF25–75%], also called forced mid-expiratory flow, is the mean forced expiratory flow during the middle half of the FVC [89]. It is extensively used and is reported to have a good sensitivity for diagnosing minimal airflow limitation, but interpretation is difficult if the vital capacity is abnormal. The index should not be used for assessing changes in airflow limitation, for example following inhalation of a bronchodilator drug [§ 4.1].
The forced late expiratory flow [FEF75–85%] is the mean flow during exhalation from 75 to 85% of the FVC. The index has a poor reproducibility and is little used. Other indices have been suggested; they are poorly reproducible and have not been shown to provide information which is not provided by other indices.
2.2.3 Instantaneous maximal expiratory flows
The peak expiratory flow [PEF] is the maximal flow during a forced expiratory vital capacity manoeuvre starting from a position of full inspiration. In healthy subjects the index reflects the calibre of «central» airways and the force exerted by the expiratory muscles. The PEF is widely used in the management of patients with variable airflow limitation, in whom it is significantly influenced by the calibre of peripheral airways. The index is effort dependent. The results are influenced by the definition of peak flow which is adopted, for example with respect to its duration; in addition results using different equipment are not always comparable. Thus further work is needed [90]. A working party of the European Respiratory Society will shortly issue recommendations on peak expiratory flow, hence this item will not be addressed in great detail in this report.
The maximal expiratory flow at a specified lung volume [V′max, x%V, MEFx%V, FEFx%V] is the expiratory flow achieved at the designated lung volume during a forced expiratory manoeuvre starting from TLC [91]. Variations in lung volume are measured either at the mouth or from the chest using a whole body plethysmograph. The two methods can yield substantially different results, on account of the former making no allowance for the reduction in lung volume which occurs by compression of the alveolar gas during the forced expiratory manoeuvre. The reduction in turn reduces the lung elastic recoil and hence the calibre of lung airways [§ 1.2.2]. In practice, the flow is determined at a volume defined in one of the following ways:
a. That obtained when a given percentage of the FVC remains to be expired [e.g. MEF25% FVC, V′max,25%FVC]. The flow was also expressed in terms of the proportion of FVC which has been exhaled [e.g. FEF75% FVC]. These indices are complementary, which easily leads to confusion; hence it is recommended to use only MEFx% FVC.
b. That obtained when a given percentage of the actual or the predicted total lung capacity remains in the lung [for example MEF60% TLC, MEF60%pred TLC].
The result is expressed either as a flow [l·s−1] to be compared to the reference value, or divided by an observed or a predicted lung volume [FVC or TLC]. The former procedure is recommended.
The forced expiratory flows when 50% or 25% of the vital capacity remains in the lung are widely measured. However, the measurements are of only moderate reproducibility [92] and are often subject to instrumental error, which contributes to differences in absolute values between laboratories. In addition interpretation is difficult if the vital capacity is abnormal, whilst incomplete expiratory effort can cause a large overestimation of MEF25% FVC. For groups of healthy subjects, the results are poorly described by multiple linear regression equations on height and age. Thus these indices have not yet fulfilled early expectations as to their usefulness.
2.2.4 Forced expiratory times
The forced expiratory time [FETb] is the time required to exhale a specified portion b of the FVC; for example, the FET95% FVC is the time required to deliver the first 95% of the FVC. The test is seldom used.
The time constant of the upstream segment of intrathoracic airways is the reciprocal of the slope of the flow-volume curve over a specified range [93]. The index is reported to reflect the compliance of the airways at the choke point [94]. It is probably of limited usefulness.
The mean transit time is the mean time taken by gas molecules to leave the lung during the performance of the FVC manoeuvre. It is obtained by applying moment analysis to the volume-time curve, which is considered as a cumulative distribution of transit times; the analysis also yields the standard deviation of transit times and an index of skewness of their distribution [derived from the second and third moments respectively] [95]. Advantages claimed for this approach include a high signal-to-noise ratio and independence of lung volume; however, the methodology is not yet fully standardized [96, 97] and more information is needed on its usefulness.
2.3 Forced inspiration
The manoeuvre of forced inspiration is used to detect obstruction to flow in extrathoracic airways [98], such as in laryngeal or tracheal obstruction. The procedure is considered unpleasant by many subjects so is seldom used in other circumstances. However, it can be used to differentiate expiratory airflow limitation due to airways obstruction from that attributable solely to low elastic lung recoil from pulmonary emphysema; in the latter case inspiratory flows would be little affected [99]. Extra care should be given to hygienic measures with inspiratory measurements as compared with tests which entail expiratory manoeuvres only.
Inspiratory flows are also useful in distinguishing between extrathoracic and intrathoracic airways obstruction; thus a ratio of MEF50%FVC/MIF50%FVC>1.0, but similarly a ratio of FEV1 [mL] to PEF [l·min−1] >10.0 and a ratio of FEV1 to FEV0.5 of 1.5 or more are all compatible with upper airway obstruction [100–103]; such ratios should be accompanied with reproducible inspiratory flow plateaus.
2.3.1 Forced inspiratory vital capacity
The forced inspiratory vital capacity [FIVC] is the maximal volume of air which can be inspired during forced inspiration from a position of full expiration [104].
2.3.2 Timed forced inspiratory volume
The timed forced inspiratory volume [FIV1] is the volume of air inhaled in a specified time during the performance of the forced inspiratory vital capacity, e.g. FIV1 for the volume of air inhaled in the first second as defined above. Advantages claimed for FIV1 are that it is little affected by low lung recoil, so that a low FEV1 and a normal FIV1 can be taken as evidence for low lung elastic recoil.
2.3.3 Maximal inspiratory flow
The maximal inspiratory flow [MIFx% FIVC] is the maximal flow observed when a specified percentage x of the FIVC has been inhaled.
2.3.4 Peak inspiratory flow
The peak inspiratory flow [PIF] is the maximal instantaneous flow achieved during a FIVC manoeuvre.
2.4 Maximal voluntary ventilation
The maximal breathing capacity [MBC] is the volume of air expired per minute during maximal breathing; the breathing can be by voluntary effort or driven by exercise or carbon dioxide. Maximal voluntary ventilation [MVVf] is assessed during forced breathing. The breathing time is usually 15 s except for the sustained maximal voluntary ventilation when it can be up to 4 min. In the latter case the inspired gas should contain carbon dioxide in order to prevent hypocapnia. The respiratory frequency [f] should be specified; for example, MVV30 is MVV performed at 30 breaths per minute. The procedure can cause respiratory muscle fatigue which is some-times amenable to physical training. MVV is now superseded by FEV1, with which it is highly correlated except during resistance breathing. It remains an important functional dimension of the lung on account of its relationship to the maximal ventilation during exercise. Usually maximal exercise ventilation is less than MVV but can exceed it in the presence of severe airflow limitation.
3 METHODS
3.1 Introduction
Volume changes of the lung are usually measured at the mouth, preferably by means of a spirometer or else a pneumotachometer and integrator, but other methods of measurement [e.g. rotating vane anemometer, hot wire anemometer] are gaining acceptance. Alternatively volume changes can be measured from the body surface by means of a volume displacement whole body plethysmograph, which also takes account of volume changes due to expansion or compression of gas [105]; it is mainly used for research purposes. These methods are suited to measure the vital capacity and its subdivisions. When lung volumes which include the residual volume are measured, this is done by gas dilution methods, whole body plethysmography or radiographic methods.
For sufficient accuracy as well as for comparability of measurements between laboratories and in longitudinal studies, it is imperative that the measurements and procedures are standardized; this includes frequent calibration of all equipment. Ventilatory manoeuvres should preferably be recorded and/or displayed in order to facilitate quality control.
3.2 Measurement variability
Instruments used to measure indices of ventilatory function should meet the requirements for accuracy delineated in § 6.4. Laboratory personnel using the equipment need to be trained in its use and be familiar with its operation, so that problems can be easily detected and remedied promptly.
3.2.1 Accuracy and precision
Measurements are subject to errors of accuracy and
precision. The accuracy error is the systematic difference between the true and the measured value. For example, if exactly 3 litre is delivered to a spirometer, and the readings are 2.90, 2.834, 2.801, 2.874, 2.890 [mean 2.860], the spirometer is inaccurate because the readings are systematically low by almost 5%. The precision error, usually denoted reproducibility, is the numerical difference between successive measurements. In the case of many measurements this quantity is described by the
standard deviation [SD]. It can be computed as follows. Let Xl …. Xn be the value of n measurements of the same quantity, then the mean [
Each individual observation differs from the mean by an amount which is called the deviation. The standard deviation is the square root of the sum of all the deviations squared divided by n which is the number of observations [or in the case of small numbers n -
1]:
In the above example the mean is 2.860 litre, SD 0.041; hence the instrument deviates from the true value by on average 140 mL, and the standard deviation of 5 repeated measurements is 41 millilitre. The accuracy may be improved by calibrating the instrument, i.e. the act of checking the instrument
against a known standard. In the above example, provided the inaccuracy is a proportional one, multiplying all readings by 3/2.860 = 1.049 [calibration factor] would greatly improve the accuracy. The use of the factor would not improve the precision, which in this case could be expressed as a coefficient of variation
[100·SD/
3.2.2 Sources of variability
In addition to the above instrumental errors, the measurements are subject to biological variability and to errors attributable to the observer. Biological variability is independent of errors due to the instrument or operator. In healthy persons such variability may be related to the time of the day, when it is often called diurnal variation, exposure to tobacco smoke or other chemical or physical stimuli; also the respiratory system may be affected by the measurement procedure; for example deep inspiration can cause bronchodilation and a change in the elastic properties of the lung. Within subject variability in lung volumes and ventilatory flows may be due to variability in the activity of a disease process [infection, exposure to occupational inhalant and allergen], challenge testing, exercise or exposure to fog, cold air, tobacco smoke, or to environmental pollutants in subjects with hyperresponsive airways; obviously ventilatory function will also be influenced by drugs that affect airway calibre. Observer errors can be technical, for example from differences in the technique of reading charts, computational procedures, the handling or transferring of data, but also from differences in the way the subjects are approached and instructed.
3.2.3 Reducing variability
It is the objective of quality control to achieve maximal accuracy and precision. Biological variability is minimised by careful attention to the time and circumstances of the test [e.g. with respect to environmental conditions].
Variability in the measurement is minimised by frequent checks of instrument performance, instrument maintenance, proper instrument use, adequate instruction of the person being tested, and well-trained personnel who can administer the test professionally and according to a standard protocol.
3.3 Correcting to standard conditions
All measurements of gas volumes should relate to conditions in the lung where the gas is at body temperature, pressure, saturated with water vapour [designated BTPS]. They should not relate to conditions in the measuring equipment [ATP = ambient temperature, pressure; when saturated with water vapour designated ATPS]. The correction from one set of conditions to the other will be dealt with for spirometers. Corrections for pneumotachometers are more complex: it is difficult to know the condition of the gas, which depends on how the instrument is heated, how close it is to the mouth, whether the gas is inspired or expired; a further complication is that the gain of the instrument varies with gas temperature. Corrections for pneumotachometers are dealt with in appendix A.
Spirometric
recordings made at temperatures and water vapour pressures that differ from those in the lung should be corrected to BTPS conditions as follows:
where t = ambient temperature [°C], PB = barometric pressure [kPa] and PH2O = water vapour pressure [kPa] of the ambient gas. Note that «ambient» relates to the temperature and saturation of inspired gas or that attained by the gas when it is exhaled into an instrument; this condition may be that of room gas, but in all other circumstances it is the temperature and saturation of gas inhaled from or delivered into another system, such as a spirometer.
The relationship between temperature and water vapour pressure [PH2O] of fully saturated gas is shown in
table 3. Between 16 and 37 °C it can be approximated as follows:
Table 3– Relationship between water vapour pressure of fully saturated gas and Celsius temperature, and factor for correcting to BTPS at sea level
At sea level barometric pressure can be assumed to be 101.3 kPa, but with severe storms there may be significant deviations from this pressure. Under stable conditions at sea level the factor for converting volumes recorded at ATPS conditions to BTPS is as in table 3. All spirometers should be equipped with a thermometer [see § 3.5.1.6]. Note that if «ambient» gas is not fully saturated with water, the actual PH2O rather than the one listed in table 3 must be substituted in the above equation. Occasionally it may not be feasible to assess ambient water vapour pressure. In such circumstances it is usually reasonable to assume a 50% saturation; the associated factor for conversion to BTPS is given in table 3.
In practice gas delivered to volume displacement spirometers and their tubing does not attain a stable temperature immediately [106–108]; when operating at 3 °C errors occur in FEV1 of 7.7 to 14% in spite of BTPS correction. It is recommended that gas temperatures in the spirometer should not be less than 17°C and not more than 40 °C [109].
3.4 Measurement procedures
3.4.1 General
The subject should have been at rest 15 minutes prior to the test. The procedure should be carefully described to the subject with emphasis on the need of avoiding leaks round the mouthpiece, and where appropriate of making a maximal inspiratory and expiratory effort; the latter should be sustained until the expiration is complete. With inexperienced subjects the trained operator, whose performance should preferably have been validated against a practiced operator, should demonstrate the procedure using a detached mouthpiece, then allow two practice attempts which should be recorded.
When employing a spirometer without gas conditioning hypoxaemia and hypercapnia are prevented by flushing the spirometer with air after the subject has performed two vital capacity manoeuvres. During flushing the subject should be disconnected from the mouthpiece.
If the spirometer has an absorber for removing CO2 but no oxygen is added, the vital capacity delivered to the spirometer will be underestimated to a small amount [approximately 2–3%].
A noseclip is mandatory for measurements made during normal breathing and maximal voluntary ventilation. Whilst it is difficult to exhale [partly] through the nose during a forced vital capacity manoeuvre the use of a noseclip is nevertheless recommended during such manoeuvres; it should be used if the forced expiratory time is greatly prolonged. It should also be used in children and in persons with blocked nasal passages. Dentures, unless fitting very badly so that they come loose and obstruct air flow, should not be removed, since the lips and cheeks then lose support, which promotes air leaks from the mouth. The mouthpiece should be inserted between the teeth and held by the lips. The use of disposable mouthpieces obviates the need for laborious disinfection.
3.4.2 Body posture
The measurements are to be made with the subject seated in an upright posture. This is because subdivisions of lung volumes are highly influenced by body position, being lower when supine than when seated or standing [110–112]. The vital capacity is on average 70 mL less in the sitting than the standing position in middle-aged persons [113] but not in younger persons [114]; it can drop markedly in the case of diaphragmatic paralysis when changing from the sitting to the recumbent position [115].
During the breathing manoeuvres the thorax should be free to move freely; hence tight clothing should be loosened. The practice of leaning forward as the expiration proceeds towards residual volume is undesirable, since it will compress the trachea and leads to saliva dripping into the mouthpiece assembly. The best position is achieved by using an adjustable stool and a rigid mouthpiece assembly or a flexible tube carrying the mouthpiece; this should be adjusted to suit the subject, so that the head is not tilted during measurements.
3.4.3 Volume history
In tests which entail measuring forced expiratory flows and FEV1 it is important that the volume history is standardised, i.e. that there is a smooth transition from inspiration to TLC, preferably about a 2 s pause at or near TLC and subsequent forced exhalation with minimal pause. This is because the effects of the inspiratory manoeuvre on airway and lung hysteresis are different; in addition stress relaxation of visco-elastic lung elements is time dependent, so that a forced expiration immediately after stretching the lung leads to higher expiratory flows than after some pause [116, 117], the latter being the most feasible in the majority of subjects. These phenomena lead to bronchodilator effects in healthy subects [118–123] except after administration of a bronchodilator drug. In asthmatics bronchoconstrictor [121, 124–131] as well as bronchodilator effects [127–132] have been reported; after induced bronchoconstriction there is usually a bronchodilator response to deep inspiration in asthmatics [125, 129, 131–138]. No such effects are observed in chronic obstructive pulmonary disease [119]. In order to circumvent bronchodilator or bronchoconstrictor effects partial expiratory flow-volume [PEFV] curves can be used, where the forced expiration is started after a normal inspiration [121, 139].
3.4.4 Effort dependence of maximal expiratory flows
The procedure of forced expiration causes compression of alveolar gas; on that account the lung volume, and hence the lung elastic recoil pressure [a determinant of maximal expiratory flow] diminishes. Thus the compression can reduce the rate of emptying of the lungs; this is particularly marked in subjects with airflow limitation [§ 1.2.2]. Conversely submaximal effort, because it causes less alveolar gas compression, can be associated with unrepresentatively high values for most indices of forced expiratory flow. The principal exceptions are the peak expiratory flow and MEF75%FVC' which are effort-dependent. Expirations performed with submaximal effort are seldom reproduced exactly. For this reason, and to reduce breath-to-breath variations, the result is usually based on three blows which were performed correctly and with maximal effort [table 4].
Table 4– Common faults in performance of a forced ventilatory manoeuvre
3.4.5 Required number of forced vital capacity manoeuvres
Each subject performs a minimum of three blows. In the event of the procedure being faulty, the defective blow should be repeated; if eight forced vital capacity manoeuvres have not led to a set of satisfactory blows, the test is best terminated since the results will be of little value [140]. Stress incontinence in elderly subjects may be an underrated problem leading to submaximal performance in tests which entail increasing the intra-abdominal pressure, such as FEV1 and FVC.
3.4.6 Acceptable forced vital capacity manoeuvres
To achieve acceptable tracings the subject should follow all instructions, inspiratory efforts should be to total lung capacity and expiratory efforts to residual volume. Forced inspiratory and expiratory efforts should be performed with maximal effort and without hesitation, leading to smooth curves. Irregularities in the resulting curves may be due to the tongue obstructing the mouthpiece, coughs, leaks, pauses, and loose false teeth [table 4].
3.4.7 Serial measurements
Due to diurnal variations in lung function, the time of the day at which measurements are made should ideally be fixed and repeat measurements preferably be made at the same time of the day. Ideally the subject should not have smoked 1 h prior to the measurements, and these should not be made shortly after meals. A record should be made of the date, the time of day and at altitude, the barometric pressure; at sea level the pressure is unlikely to deviate much from normal except in relation to severe storms. It is also helpful to record the type and time of any recent medication, the extent to which the subject complied with the operator's instructions and any untoward reactions, for example coughing.
For serial measurements, the circumstances of the tests should preferably be similar on all occasions with respect to time of day, season of year, apparatus, and the test be administered by an experienced operator.
3.4.8 Timing of forced vital capacity manoeuvre
When measuring timed volumes, such as FEV1 or FIV1, the starting point of the forced ventilatory manoeuvre should be obtained by backward extrapolation to zero volume change of the steepest part of the volume-time curve [dV/dt] [141–143]. This is illustrated in figure 4. In acceptable tracings the extrapolated volume should not exceed 100 mL or 5% of the FVC, whichever is greater [109]. Alternatively, or when assessing the forced expiratory or inspiratory time, the starting point can be defined as that when the inspiratory or expiratory flow exceeds 0.5 l·s−1, and the end of the breath when the volume change in 0.5 s does not exceed 25 mL.
3.4.9 Summary statistics of FVC manoeuvre
The largest of the first three technically satisfactory vital capacities [be it the IVC, EVC or FVC] and of the first three technically acceptable FEV1s should be reported; the chosen value should not exceed the next highest one by more than 5% or 0.1 l, whichever is greater. However, in some patients the manoeuvre may induce broncho-constriction, so that consecutive measurements become less; this trend should be noted and the largest VC reported [124, 144]. In addition variability in ventilatory indices is greater in subjects with obstructive airways disease [145] than in healthy subjects, so that patients are more likely to be unable to meet these reproducibility criteria. These criteria should not be applied to reject a patient's data but may lead to collecting more than the minimum of three technically acceptable manoeuvres. If even then the reproducibility criteria cannot be met, a note to that effect should accompany the best test results in the report form.
For indices taken from flow-volume curves the chosen curves should be of similar shape and have a peak which is representative and not flattened. To this end the curves should be available for inspection by the operator at the time of measurement. When curves are selected by the computer a useful criterion is that the PEF should be within 10% of the maximal value. Flow-volume indices should be obtained from three technically satisfactory FVC manoeuvres in either of two ways. The first one [envelope method, fig. 5] entails superimposing the curves from total lung capacity to form a composite maximal curve [146]; the largest FVC is used to delineate the highest instantaneous flows at specified lung volumes. The second method, which leads to equally reproducible results [146], is to take the highest instantaneous flow from three technically satisfactory FVC manoeuvres; the FVC from the chosen flow-volume curves should not differ from the largest FVC by more than 5%. The two methods lead to equally reproducible results [146, 147]; with the latter method using the mean of 2 or 3 values improves the reproducibility. The practic of taking five definitive blows instead of three improves the reproducibility to a small extent, but the improvement is not usually cost-effective [147]. The Working Party has considered the recommendation to derive maximal expiratory flows from the «best curve», i.e. from the flow-volume curve with the highest sum of FEV1 and FVC [109, 141]. Whilst on average the flows do not differ much from those obtained with the above procedures, the reproducibility of indices derived from the «best curve» compares unfavourably [146, 147], so that the method is not recommended.
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Figure 5–
Using the «envelope method», a composite curve is obtained from a set of maximal expiratory flow-volume curves by superimposition at the level of total lung capacity and by reporting the largest flow values at given percentages of the largest FVC. Note that the variability in maximal flow shown here may arise from different flexion of the neck at each FVC manoeuvre.
3.5 Spirometry
Spirometers are the instruments of choice for measuring the vital capacity and its subdivisions. They can be divided into two broad categories which reflect their construction and measuring features [148], namely a] spirometers with facilities for gas conditioning: the conditioning relates primarily to facilities for controlling the concentrations of oxygen and carbon dioxide and to measures taken to ensure unidirectional gas flow; b] spirometers designed to have good dynamic properties. They are either of the wet type [such as the classical bell spirometer with a water seal] or dry type [bellows, piston, wedge or rolling seal spirometer].
3.5.1 Spirometers with facilities for gas conditioning
Spirometers with facilities for gas conditioning are suitable for investigations lasting from several minutes to many hours, depending on the conditioning of the gas in the spirometer. In a closed system, oxygen lack is the first and greatest danger, especially if the carbon dioxide concentration is kept very low.
3.5.1.1 Capacity
The spirometer should be capable of recording the full vital capacity [at least 8 l volume
displacement] as a function of time. The smaller the volume of the spirometer circuit, the more attention needs to be paid to the conditioning of the gas. Gas circulation is produced preferably by a pump with an output at least ten times the volume of the spirometer per minute [minimal flow 180 l·min−1] or alternatively by low-resistance one-way valves. The rationale for the recommended flow is as follows. In a valveless spirometer a flow of 3 l·s−1
suffices to prevent rebreathing of expired gas except during tests which entail maximal voluntary ventilation. In addition this flow ensures rapid gas mixing within the spirometer. The concentration c of a substance at time t which is made to mix in a gas container can be approximated by
where E is the substance added in l·min−1, V is the volume to which the substance is being added in litres, and n is the number of times the volume V is being circulated per minute. If the spirometer volume is on average 6 litres, n comes to 180/6 = 30 times per minute = 0.5 times per second. The time constant for mixing is 1/n = 1/0.5 = 2 s. This ensures both rapid gas mixing and a time constant which approximates or is better than the response time of most commercially available helium meters.
3.5.1.2 Construction
A long-cylinder spirometer bell is the simplest in construction and mechanically the least vulnerable. For the measurement of static lung volumes, a bell cross-sectional area of 300 to 400 cm2 and a moving mass of maximally 600 g is acceptable. A wide-cylinder bell [cross-sectional area 2000 to 3000 cm2] has considerably better dynamic properties for the same volume, weight and material. Such a bell requires a specially constructed suspension and electrical amplification of the displacement signal. A larger surface without the problems of suspension can be provided by a wedge-shaped spirometer.
3.5.1.3 Connections
The gas connections of the spirometer serve the following purposes:
1. Oxygen supply to compensate for O2-consumption and stabilization of the oxygen concentration in the spirometer;
2. Supply of indicator gas [usually helium] for determining the functional residual capacity;
3. The connections for the subject are usually provided by a two-way tap;
4. A connection to and from the spirometer should be available, so that the tracer gas concentration in the spirometer [for example helium] can be measured.
Carbon dioxide should be adsorbed by soda lime contained in a canister. The fractional CO2 concentration in the spirometer should be kept below 0.005. The hose connecting the patient to the spirometer should be sufficiently stiff to prevent spurious volume deflections, such as may occur with concertina-hoses when handled during respiratory manoeuvres.
3.5.1.4 Kymograph
The paper speed should be 3 cm·min−1 for recording semistatic manoeuvres, and at least 120 cm·min−1 for recording dynamic lung volumes and ventilatory flows.
3.5.1.5 Pressure, leaks
The maximal pressure at the mouth during a forced expiratory ventilatory manoeuvre should not exceed 0.6 kPa. The driving pressure required to achieve a volume deflection should not exceed 0.03 kPa. The circuit should be free from leaks. These are looked for by placing a weight on the spirometer bell to raise the pressure by at least 0.2 kPa; the recording should remain level over at least a 1 min period. Tests for leaks should be performed each week.
3.5.1.6 Temperature
The spirometer should be equipped with a thermometer which should be carefully located. For correcting inspired gas to BTPS conditions the temperature may be measured at the inspiratory line near the mouthpiece. For expired gas the situation is more complex since temperature may rise considerably at the level of the soda lime when CO2 is adsorbed. In a water-seal spirometer the water temperature can be used for correcting inspiratory and expiratory gas volumes. In spirometers equipped with a gas circulation pump the gas temperature at the outlet of the spirometer, or under the spirometer bell, is an acceptable compromise; in spirometers with a common gas inlet and outlet the inspiratory temperature should be measured at that point; the site for expiratory temperature corrections should be carefully chosen within the spirometer [106, 149].
3.5.1.7 Calibration
The spirometer and recording equipment should be calibrated at least every three months by means of an airtight 3 litre calibrated syringe; the latter should be accurate within 25 mL. The displacement should be linear over the entire volume range and capable of being recorded with an accuracy of ±3% of the reading or ±50 mL, whichever is greater; accounting for the potential error in the volume displacement from the calibrated syringe this implies that an error of up to 3.5% or 70 mL, whichever is greater, is acceptable. A change in volume of 25 mL should be detectable. Similarly the recorder speed should be checked at least quarterly with a stopwatch, and be accurate within 1%. In spirometers where the time recording is initiated when the expired gas exceeds a certain volume, the acceleration of the electric motor is critical. This is difficult to check, but can be done with a calibrator based on explosive decompression [150, 151] or equipment which delivers precisely known flow patterns [152].
3.5.2 Spirometers for recording forced ventilatory manoeuvres
Spirometers with good dynamic properties are required to record rapid volume changes, and, by electronic or digital differentiation of the volume, flow during forced ventilatory manoeuvres. The characteristics of such spirometers should be the same as those for pneumotachometers which are described below. However, due to the differentiation procedure the signal to noise ratio of spirometers tends to be less than that of pneumotachometers.
3.6 Pneumotachometry
3.6.1 Devices
Numerous devices are available for measuring gas flow, of which the most widely used are Lilly and Fleisch type pneumotachometers [screen and parallel capillary tubes respectively]. They should be used in conjunction with an appropriate differential pressure transducer, amplifier and a DC-coupled analogue or a digital integrator. The present recommendations are limited to these two types only. Emphasis in the context of measuring lung volumes is put on the following features: linearity, stability and calibration.
3.6.2 Linearity
The gain of the system should not change with flow. This implies that the volume reading should be the same when a fixed volume of gas is administered from a calibrated syringe at varying flows. An accuracy of 3% of the reading or 50 mL, whichever is greater, [accounting for errors in the volume displacement of the syringe relaxes this to 3.5% or 70 mL, whichever is greater] is acceptable. Alinearity is a feature of some pneumotachometers; it should be corrected electronically or digitally prior to integration.
3.6.3 Stability
The volume signal often exhibits an unstable baseline «drift» for various reasons. The most important one is electric off-set of the flow signal, which is very often not constant over a prolonged period of time; it is usually minimized by allowing an appropriate warming-up period for the electronic equipment and by thermal isolation of the pressure transducer. In addition, inspired and expired gases are different because the respiratory exchange ratio is not unity, and because inspired and expired gas usually have a different temperature, water vapour content and gas composition, all of which affect the flow measurements [see appendix A]. Finally the measuring device may give different signals for the same flow in opposite directions. For these reasons baseline «drift» of the volume signal is unavoidable. If it is minimal [0.3 should form the lower limit, as the errors in lung volume increase rapidly when the ratio is less than this [fig. 9].
B.3 Solubility of helium
If gas mixing is continued long enough, then helium will not only mix between spirometer and lung, but will also equilibrate with fue blood and subsequently with. body water and fat. The helium uptake has been estimated at 0.3 mL·min−1 per per cent helium in alveolar gas
[155], i.e. 0.5 mL·s−1 per unit fractional helium concentration. This does not take into account that body fluids and fat become saturated with helium, so that its uptake ultimately diminishes. Therefore the error will be approximated as follows. Let us assume that VL = 0.043·W, where VL is in litre and W = body mass in kg; in addition we assume that
total body water is 0.6 W and body fat 0.1 W; for a person of 70 kg VL comes to 3 litre. The blood/gas partition coefficient for helium is 0.0088, that for oil/water 1.7 [cf 289]. Hence body fluids and body fat are equivalent to a gas compartment of
In this example, if gas mixing is
continued long enough to achieve equilibration between gas, body fluids and body tissues, the helium equilibration will lead to an overestimate of VL by
B.4 Imperfect oxygen supply
Gas concentrations are not only influenced by the gas mixing process and helium uptake, but also by the continuous oxygen consumption not being perfectly matched by oxygen supply. From VL, Vsp, Fsp,He,1 and Fsp,He,2 as defined
above, we obtain:
Disregarding any influence of changes in oxygen and nitrogen concentration on the measuring device the error can be approximated as follows. Let the volumetric error Verr in oxygen supply at any time t be
Note that when the volumetric error in oxygen supply is small relative to the spirometer volume, this will safeguard against a dangerous drop in inspired oxygen concentration in the case of prolonged measurements.
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