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The Thoracic Society of Australia and New Zealand (TSANZ) and the Australian and New Zealand Society of Respiratory Science (ANZSRS) commissioned a joint position paper on pulmonary function testing during coronavirus disease 2019 (COVID-19) in July 2021. A working group was formed via an expression of interest to members of both organizations and commenced work in September 2021. A rapid review of the literature was undertaken, with a 'best evidence synthesis' approach taken to answer the research questions formed. This allowed the working group to accept findings of prior relevant reviews or societal document where appropriate. The advice provided is for providers of pulmonary function tests across all settings. The advice is intended to supplement local infection prevention and state, territory or national directives. The working group's key messages reflect a precautionary approach to protect the safety of both healthcare workers (HCWs) and patients in a rapidly changing environment. The decision on strategies employed may vary depending on local transmission and practice environment. The advice is likely to require review as evidence grows and the COVID-19 pandemic evolves. While this position statement was contextualized specifically to the COVID-19 pandemic, the working group strongly advocates that any changes to clinical/laboratory practice, made in the interest of optimizing the safety and well-being of HCWs and patients involved in pulmonary function testing, are carefully considered in light of their potential for ongoing use to reduce transmission of other droplet and/or aerosol borne diseases., (© 2022 The Authors. Respirology published by John Wiley & Sons Australia, Ltd on behalf of Asian Pacific Society of Respirology.)

The recently published Global Lung Function Initiative (GLI) carbon monoxide transfer factor ( T LCO ) reference equations provide an opportunity to adopt a current, all-age, widely applicable reference set. The aim of this study was to document the effect of changing to GLI from commonly utilised reference equations on the interpretation of T LCO results.33 863 T LCO results (48% female, 88% Caucasian, n=930 aged <18 years) from clinical pulmonary function laboratories within three Australian teaching hospitals were analysed. The lower limit of normal (LLN) and proportion of patients with a T LCO below this value were calculated using GLI and other commonly used reference equations.The average T LCO LLN for GLI was similar or lower than the other equations, with the largest difference seen for Crapo equations (median: -1.25, IQR: -1.64, -0.86 mmol·min -1 ·kPa -1 ). These differences resulted in altered rates of reduced T LCO for GLI particularly for adults (+1.9% versus Miller to -27.6% versus Crapo), more so than for children (-0.8% versus Kim to -14.2% versus Cotes). For adults, the highest raw agreement for GLI was with Miller equations (94.7%), while for children it was with Kim equations (98.1%). Results were reclassified from abnormal to normal more frequently for younger adults, and for adult females, particularly when moving from Roca to GLI equations (30% of females versus 16% of males).The adoption of GLI T LCO reference equations in adults will result in altered interpretation depending on the equations previously used and to a greater extent in adult females. The effect on interpretation in children is less significant., Competing Interests: Conflict of interest: D.J. Brazzale has nothing to disclose. Conflict of interest: L.M. Seccombe has nothing to disclose. Conflict of interest: L. Welsh has nothing to disclose. Conflict of interest: C.J. Lanteri has nothing to disclose. Conflict of interest: C.S. Farah has nothing to disclose. Conflict of interest: W.R. Ruehland has nothing to disclose., (Copyright ©ERS 2020.)

The complete equation of motion for a single compartment model (SCM) includes an inertance term to describe pressure changes in phase with acceleration, as well as terms for resistance and elastance. Inertance has traditionally been excluded from the model when measuring respiratory mechanics at conventional ventilatory frequencies in mature respiratory systems. However, this omission has been questioned recently for measurements of respiratory mechanics in intubated infants where higher ventilation frequencies and smaller tracheal tubes are the norm. We investigated 1) the significance of inertance in an immature respiratory system during mechanical ventilation, and 2) the effect of omitting it from the model on estimates of respiratory mechanics. Six anesthetised, paralysed and mechanically ventilated puppies (2.6-3.9 kg) were studied. A SCM, including an inertance term was fitted to measurements of flow and airway opening (P(AO)) or transpulmonary (P(TP)) pressure using multiple linear regression to estimate respiratory system and lung resistance (R(RS), R(L)), elastance (E(RS), E(L)) and inertance (I(RS), I(L)) respectively, at various ventilation frequencies (0.2-2 Hz). Data obtained at each ventilation frequency were also fitted with a similar model without the inertance term. Inertance contributed significantly to the model at frequencies greater than approximately 0.3-0.5 Hz (20-30 breaths per minute), with I(RS) dominated by the lung. The importance of including the inertance term in the model increased as ventilation frequency increased. Exclusion of inertance from the model led to underestimation of E(RS) and E(L), but no errors in estimates of R(RS) or R(L). The errors increased with ventilation frequency to approximately 10-20% for E(RS) and approximately 10-40% for E(L) at 2 Hz. While inertance contributed significantly to the SCM at ventilation frequencies typically required to maintain normal gas exchange in puppies, the errors from excluding this term were small: <3% for E(RS) and <9% for E(L)., (Copyright 1999 Wiley-Liss, Inc.)

Air leak around a tracheal tube (TT) during mechanical ventilation is likely to occur during the inspiratory phase because airway pressure is high for a prolonged period. The presence of a leak may introduce errors in measurements of respiratory mechanics made at the airway opening. If so, respiratory mechanics can be measured more accurately when data are collected during the expiratory phase of ventilation. We examined this phenomenon in a lung model. When a leak was introduced into the model, simulating a leak around the TT, the leak occurred predominantly during the inspiratory phase of respiration. As the magnitude of the leak increased, the overestimation of resistance progressively increased, when calculated from pressure and flow measured at the airway opening. A large leak (38%) resulted in an overestimation of respiratory system resistance by 51% and an underestimation of elastance (Ers) by 23% when calculated from the entire ventilatory cycle. However, there was no under- or overestimation in Rrs when calculated from the expiratory phase only, and ERS was overestimated by only 6.1%. Varying peak inspiratory pressure, end-expiratory pressure, and expiratory time did influence the effect of leak, however, respiratory mechanics could still be calculated accurately from the expiratory phase under these conditions. We conclude that measurements of lung mechanics from the expiratory phase is a promising approach to dealing with the problem of measuring respiratory mechanics in mechanically ventilated infants with leaks around the tracheal tube.

A single combined intramuscular dose of betamethasone and l-thyroxine (T4) or placebo was injected into the shoulder of fetal lambs 48 hours prior to delivery at days 121 (n = 14), 128 (n = 25) or 135 (n = 20) of gestation. Respiratory mechanics were calculated using multiple linear regression analysis. Both respiratory system resistance (RRS) and elastance (ERS) decreased approximately 4 fold between gestational days 121 (D121) and 135 (D135). Both variables were also reduced by hormone treatment. Reduction in ERS was due to a reduction in both lung (EL) and chest wall (EW) components. In absolute terms EW decreased with gestational age; however, EW as a proportion of total elastance (% EW) increased. Inclusion of a volume-dependent elastance term in the multiple linear regression model enabled us to separate total elastance into volume-independent (E1) and volume-dependent (E2V) components. E1 decreased almost 8-fold compared with only a 2.5-fold fall in E2V between D121 and D135. %E2, the proportion of ERS which is volume-dependent and which provides an index of overventilation, doubled over this time period. Hormone treatment affected E1 and E2V components equally hence %E2 was not altered. Both excised lung volume and end expiratory alveolar volume increased with gestational age and with treatment. The response to treatment was qualitatively similar at each of the gestational ages examined, however, for all mechanics variables, except resistance and E1, the magnitude of response to treatment was significantly smaller in D135 animals compared with other age groups.

Congenital heart malformations are often associated with altered pulmonary hemodynamics. Lesions associated with increased pulmonary blood flow (PBF) or increased mean pulmonary artery pressure (MPAP) may in turn alter respiratory mechanics. Surgical correction of these cardiac defects frequently involves the use of cardiopulmonary bypass (CPB), during which the lung may be partially or completely atelectatic for lengthy periods, further compromising lung mechanics. The aims of this study were to document the effect of PBF on respiratory mechanics in children and to determine whether the detrimental effects of CPB were outweighed by the potentially positive effects of the corrective surgery. Twenty-three children (2-120 mo) undergoing surgery were studied while anesthetized, paralyzed, and mechanically ventilated. Pulmonary to systemic blood flow ratio was used as an index of PBF. Seventeen children had lesions associated with increased PBF (group 1), while six had decreased or normal PBF (group 2). Respiratory mechanics were measured just before the commencement of CPB and within approximately 2 h after the cessation of CPB, with the chest closed. Dynamic elastance (Ers,dyn) and resistance (RRS) were calculated from flow, volume (V), and pressure (Pao) measurements, using multiple linear regression with a volume-dependent single compartment model. Static elastance (ERS,st) was calculated from Pao and V measurements obtained when deflating the lung in steps from a maximal Pao of 30 cm H2O. ERS,dyn, ERS,st, and RRS increased significantly with increasing PBF to 220-330% predicted. There was no correlation between MPAP and respiratory mechanics. After CPB, ERS, dyn and RRS fell to normal levels in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

To determine the role of M1 muscarinic receptors in the response of the pulmonary parenchyma to inhaled methacholine (MCh), 20 mongrel, out-bred puppies, 8-10 weeks of age were challenged following pretreatment with either saline (control), UH-AH37 (a combined M1 & M3 receptor blocker), or pirenzepine (a relatively selective M1 receptor blocker). In addition, eight fox hound-beagle puppies, born and raised in a clean animal house, were studied. Relatively selective doses of pirenzepine produced a dose-dependent shift to the right of the parenchymal dose-response curves (P = 0.031), with no effect on the airway dose-response curve (P = 0.102). The fox hound-beagle puppies showed less parenchymal response (P <0.0005), but equivalent airway response (P = 0.468), to MCh compared with the mongrel puppies. High doses of pirenzepine (10 000 mu g/kg) and UH-AH37 (3 mg/kg) markedly inhibited both the parenchymal and airway responses to MCh. Data from the present study demonstrate that: (1) while both the airway and pulmonary parenchyma respond to inhaled MCh, the mechanisms by which they respond differ; (2) stimulation of M1 subtype muscarinic receptors are responsible, at least partly, for the parenchymal response; and (3) experimental conditions, such as the breed and housing conditions of animals, may have major influences on the parenchymal response to inhalational challenge tests.

To investigate the influence of nonlinearities on estimates of respiratory mechanics, differing patterns of mechanical ventilation patterns were analyzed from 8 puppies and 14 children. Respiratory mechanics were calculated using multiple linear regression to fit a linear single-compartment model, a volume-dependent single-compartment model (VDSCM), and a flow-dependent single-compartment model. The ratio of the compliance of the last 20% of the dynamic volume-pressure (V-P) curve to the total compliance (C20/C) and the contribution of a volume-dependent elastance to total elastance [%E2 = E2 (VT)/[(E1 + E2)VT], where E1 + E2 is total elastance, E2 is the volume-dependent component, and VT is tidal volume] were used as the indexes of over-distension. By positioning the dynamic loops on the static V-P curves, ventilation patterns were classified as overdistended or nonoverdistended. In the overdistended group, the C20/C was significantly lower (0.71 +/- 0.10 vs. 0.92 +/- 0.16; P < 0.0001) and %E2 was significantly higher (43.4 +/- 15.0 vs. 0.51 +/- 18.02%, P < 0.0001) than in the nonoverdistended group. The mode of ventilation (pressure controlled vs. volume controlled) and the resistive pressures that resulted in widening of the dynamic V-P loop were found to alter C20/C but not %E2. When the respiratory system was overdistended, i.e., ventilated up to the flattened portion of the V-P curve, the VDSCM gave more accurate estimates of respiratory mechanisms. Furthermore, %E2 calculated from VDSCM is a useful parameter for estimating respiratory system overdistension that is not affected by resistive pressures.

We studied the effect of a single-dose, corticosteroid treatment on preterm lambs (gestational age: 128 d). A low, medium, or high betamethasone dose (0.1, 0.5, and 2.0 mg/kg) or saline control was administered directly to the fetus by ultrasound-guided intramuscular injection 48 h before delivery. A second group received either the high dose of betamethasome or saline 24 h before delivery. The lambs were delivered at 128 d gestation, anesthetized with ketamine, and ventilated for 50 min. Respiratory system elastance and resistance were measured at 10-min intervals using multiple linear regression analysis of pressure, flow, and volume. Similarly, estimates of lung mechanics were calculated from transpulmonary pressure. The viscoelastic time constant (tau) was calculated by fitting an exponential to the pressure changes occurring after occluding the airway during expiration. Excised lung volume at 40 cm H2O and lung weight were used to calculate specific elastance and resistance correcting for lung size using volume or weight, respectively. Of the 13 lambs in the 48-h high-dose betamethasone group, five developed pulmonary interstitial emphysema (PIE) as did 3 of 11 animals in the high-dose group treated 24 h before delivery. These animals were analyzed separately. The lambs receiving medium- or high-dose (24 and 48 h predelivery) betamethasone had significantly lower elastance and a trend toward lower resistance when compared with the control groups. Ten minutes after delivery, the animals that developed PIE all had elastance values comparable to that of the control animals despite corticosteroid treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Forced expiratory flow-volume (FEFV) curves can be generated from end-tidal inspiration in infants with use of an inflatable jacket. We have developed a technique to raise lung volume in the infant before generation of FEFV curves. Measurements of pressure transmission to the airway opening by use of static maneuvers have shown no change with increasing lung volume above end-tidal inspiration. The aim of this study was to determine, under dynamic conditions (i.e., during rapid thoracic compression), whether the efficiency of pressure transmission across the chest wall is altered by raising lung volume above the tidal range. Dynamic pressure transmission (Ptx,dyn) was measured in five infants (age 6-17 mo). Jacket pressure (Pj), esophageal pressure, and volume were measured throughout passive and FEFV curves at lung volumes set by 10, 15, and 20 cmH2O preset pressure. The group mean Ptx,dyn was 37 +/- 6% (SE) of Pj at end-tidal inspiration, and no change was seen with further increases in lung volume. However, a mean decrease in Ptx,dyn of 42% was evident throughout the tidal volume range (i.e., from end-tidal inspiration to end expiration). Isovolume static pressure transmission (Ptx,st) was measured in three of the five infants by inflation of the jacket in a stepwise manner with the airway closed. Measurements were made at end-tidal inspiration and lung volumes at 10, 15, and 20 cmH2O preset pressure. Resulting changes in Pj, esophageal pressure, and airway opening pressure were compared using linear regressions to determine Ptx,st.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurements of respiratory mechanics are frequently made in ventilated infants and children. Esophageal pressure measurements (Pes) using a balloon on a catheter have been used to partition the respiratory mechanics into lung and chest wall components. Appropriate positioning of this balloon is crucial to obtain accurate estimates of pleural pressure. Traditionally, in spontaneously breathing subjects the balloon position is assessed with an occlusion test. In ventilated subjects, it is not always possible to perform an occlusion test prior to paralysis, and even if such a test is performed it may be relevant under conditions of positive pressure ventilation. By occluding the airway opening and applying gentle pressure to the abdomen or rib cage, positive swings in pressure can be measured by both Pes and airway opening pressure (Pao). We compared traditional occlusion tests measured in 16 spontaneously breathing puppies to the positive pressure occlusion test performed after paralysis. In 2 pups we were unable to obtain a reasonable traditional occlusion test (> 15% difference between Pes and Pao) but we obtained 10 traditional occlusion tests in each of the remaining 14 pups (2.1-14 kg). In 11 of these animals delta Pes was within 10% of delta Pao. This compared well to positive pressure occlusion test using abdominal pressure performed after analysis, where delta Pes was within 10% of delta Pao in 10 animals. In 9 of these pups occlusion tests were also performed by applying pressure on the rib cage, where delta Pes was within 10% of delta Pao in 6 animals.(ABSTRACT TRUNCATED AT 250 WORDS)

To investigate the effect of ventilation rate on respiratory mechanics, 21 neonates ventilated in the neonatal period for various reasons were studied while being ventilated at 30 and 80 breaths/min. Dynamic respiratory system elasticity (ERS), dynamic respiratory system resistance (RRS), and alveolar pressure at end expiration (EEP) were calculated by using multilinear regression to fit the equation of motion of a linear single-compartment model. Technically satisfactory data were obtained from 13 neonates. With the fast ventilation rate, tidal volume and RRS decreased by a mean of 41.3% (p < 0.01) and 17.5% (p < 0.01), respectively, ERS and EEP increased by a mean of 8.3% (p < 0.05) and 22.2% (p < 0.01), respectively. Fast ventilation produced a shorter effective time constant during expiration, limiting the changes in EEP and, hence, in end-expiratory lung volume. The same changes in respiratory mechanics were also observed in neonates who did not show an increase in EEP even at high frequency. These neonates had a high elastance and time constant short enough to ensure adequate lung emptying. These results suggest that the respiratory mechanics of ventilated neonates are frequency dependent and that neonates with higher ERS, such as those with hyaline membrane disease, can cope with fast rate ventilation without developing dynamic hyperinflation.

The simple model best able to describe the viscoelastic behavior of lung tissues contains a Kelvin body. Some facets of the viscoelastic behavior of the model can be characterized by a time constant. We have performed a series of experiments to demonstrate this viscoelastic time constant in open-chest mechanically ventilated puppies by recording the stress recovery after midexpiratory flow interruption. In 19 8- to 10-wk-old puppies, the mean time constant was 0.64 +/- 0.01 (SE) s. This time constant remained unchanged after inhalation challenge with histamine, methacholine, or hypertonic saline, despite marked changes in tissue mechanics (resistance and elastance) and in hysteresivity. The constancy of the viscoelastic time constant demonstrates a tight coupling of the parameters of the Kelvin body model, effectively reducing the viscoelastic parameters to a single degree of freedom, and may explain the coupling between the dynamic elastic and resistive properties of the lung tissues previously demonstrated.

The relationship between respiratory mechanics, changes in pulmonary blood flow (PBF), pulmonary arterial pressure, and left atrial pressure is unclear. Conventional methods for the measurement of respiratory mechanics model the respiratory system as a single compartment, which may not adequately represent the respiratory system in a diseased state. The interrupter technique models the respiratory system as two compartments, with the "flow resistance" of the conducting airways and chest wall (Raw) considered separately from Pdif, a measure of the viscoelastic properties of the lung and chest wall, together with any pendelluft present. The respiratory mechanics of 15 infants in the first year of life were studied during cardiac catheterization with the use of conventional methods and the interrupter technique. The infants had a PBF-to-systemic blood flow ratio ranging from 0.6 to 4.0:1. The specific dynamic compliance of the respiratory system was not related to the PBF; however, there was a significant relationship between PBF and the total resistance of the respiratory system (Rrs) [analysis of variance (ANOVA) F = 5.69, P < 0.05], Raw (ANOVA, F = 12.30, P < 0.01), and Pdif (ANOVA, F = 3.79, P < 0.05). Rrs increased significantly with an increase in mean left atrial pressure (ANOVA, F = 6.92, P < 0.05); however, dynamic compliance, Raw, and Pdif did not. These results suggest that the relationship between Rrs and PBF is due an increase in the resistive properties of the conducting airways and tissue components.

A cross-sectional survey involving 51 children, ranging in age from 3 wk to 15 yr, was performed to examine the changes in respiratory mechanics with age in mechanically ventilated children, using both a single-compartment model of the respiratory system and a more sophisticated two-compartment model. Children were studied while under anesthesia for urological surgery and were considered to have normal lungs. They were paralyzed and mechanically ventilated throughout measurements. Respiratory mechanics were measured during ventilation by applying a single-compartment model and by using multilinear regression to calculate dynamic compliance and respiratory system resistance (Rrs). We then used the interrupter technique, which allowed us to partition Rrs into airway resistance and a tissue viscoelastic component known as Pdif. A static volume-pressure curve was constructed from multiple occlusions made at different lung volumes throughout expiration, and static compliance was determined. Rrs and airway resistance decreased as height increased. There was a progressive increase in respiratory system compliance with height. Pdif fell in the first 2 yr of life and then subsequently increased after the age of approximately 5 yr.

A double-blind, randomized, placebo-controlled, parallel-group study was undertaken in 120 clinically stable asthmatic children (aged 6-19 years) to determine the ability of 8 weeks of treatment with nedocromil sodium (4 mg, three times a day) to reduce the level of histamine responsiveness. Despite the subjects being clinically stable and reporting few asthma symptoms, approximately one-third had abnormal pulmonary function on enrollment into the study. Statistically significant increases in pulmonary function were seen in the group treated with nedocromil sodium but not in the control group (p = 0.01). Furthermore, 52% of the individuals with abnormal pulmonary function returned to normal following treatment with nedocromil sodium compared to 11% of those with abnormal pulmonary function who received placebo. However, there were no differences in the level of histamine responsiveness between the two treatment groups at baseline or after 4 or 8 weeks of treatment with nedocromil sodium or placebo. These data suggest that the level of histamine responsiveness is not intimately related to the level of asthma control in children.

The site of action of inhaled hypertonic saline was determined in 8- to 10-wk-old puppies by combining measurements of respiratory mechanics, made during mechanical ventilation and after midexpiratory flow interruptions, with direct measurements of alveolar pressure. Under both control conditions and after inhalation of 10% saline, we were able to partition lung mechanics into components representing the airways and tissue viscoelastic properties. Hypertonic saline challenge altered lung mechanics by increasing airway resistance and did not have any effect on elastic or viscoelastic properties of the lung.

Twelve open-chest mongrel puppies, 8-10 wk old, were studied to localize the site of action of inhaled methacholine within the lungs. Six puppies were challenged with methacholine aerosols and six were challenged with an equal number of nebulizations of normal saline (control group). Pulmonary mechanics were measured during mechanical ventilation and after midexpiratory flow interruptions. Alveolar pressure was measured to allow the partitioning of pulmonary mechanics into airway and tissue components. Good matching between airway opening and alveolar pressures was seen throughout the study. After methacholine challenge, lung resistance increased fivefold. Increases in airway resistance and in the parameters reflecting tissue viscoelastic properties contributed to this increase in lung resistance. Dynamic lung elastance also increased threefold. The response of the methacholine group was statistically different from that of the control group. These data indicate that both the airways and pulmonary parenchyma contribute to the response to inhaled methacholine in 8- to 10-wk-old puppies.

We recently demonstrated that most of the change in lung function following inhaled histamine in 8-10 week old puppies was due to changes in the tissue visco-elastic properties and argued that vagally-mediated reflexes may be responsible for this phenomenon (Sly and Lanteri, J. Appl. Physiol. 1990; 68: 1562-1567). To test this hypothesis we compared 5 puppies treated with inhaled lignocaine or bilateral cervical vagotomy did not alter baseline pulmonary mechanics and there were no differences in the response to histamine challenge between the three groups. These results show that vagal reflexes, mediated via the central nervous system, are not involved in the response of the pulmonary tissues to inhaled histamine, but do not exclude the possible involvement of local axonal reflexes.

Respiratory mechanics were studied in five anaesthetised children, aged 3 to 33 months undergoing urological surgery, in both the supine position and with extreme truncal flexion. Extreme truncal flexion was associated with a reduced respiratory system compliance. Dynamic compliance decreased significantly, by 30% (range 12-55%) and static compliance decreased significantly, by 40% (range 18-65%). There were no changes in respiratory system resistance. Tidal volume was also significantly reduced (mean 20%) despite a significant increase (mean 22%) in peak ventilator pressure. These changes in mechanics must be recognised to avoid alveolar hypoventilation, with a consequent decrease in gas exchange during surgery.

Wheezy infants, less than 6 months of age, were given either inhaled salbutamol or saline in a double-blind study. A significant change in maximal flow at functional residual capacity (VmaxFRC) was defined as being greater than twice the coefficient of variation of the baseline measurements. There was no difference in the infants' response to saline or salbutamol. Wheezy infants, less than 6 months of age, do not have an increase in VmaxFRC following a single dose of inhaled salbutamol.

Airway and pulmonary tissue responses to aerosolized histamine were studied in five mongrel puppies (8-10 wk old). Alveolar pressure was measured by use of alveolar capsules and respiratory mechanics calculated during tidal ventilation and flow interruptions. Aerosolized histamine caused an increase in the tissue viscoelastic properties, which was measured as an increase in pulmonary resistance during tidal ventilation. An increase in stress recovery of the pulmonary tissues was measured with the interrupter technique after aerosolized histamine. These data demonstrate that aerosolized histamine caused an increase in the tissue viscoelastic properties. The most reasonable explanation for the mechanism of this increase would seem to be via reflex pathways stimulated by centrally located receptors.

It has been suggested that thoracic gas volume (TGV) measured in infants in a plethysmograph most accurately represents true lung volume when calculated from end-inspiratory airway occlusions. The rationale proposed is that pressure measured at the mouth underestimates alveolar pressure more at end-expiration than at end-inspiration, presumably due to small airway closure, and this results in greater overestimation of TGV. To investigate this possibility we calculated TGV in 40 wheezy infants from occlusions at both end-inspiration (TGVei) and end-expiration (TGVee) using a 60 L whole body plethysmograph. TGV was corrected for equipment dead space and tidal volume. When a significant change in TGV was defined as lying outside the 95% confidence interval of the TGVee measurements, 8 of the 40 infants tested had significantly higher TGV values measured from occlusions made at end-expiration, while two infants had significantly lower TGV values measured from occlusions made at end-expiration. This trend was not more common in infants with "concave" flow-volume curves. Although it is technically easier to make occlusions at end-expiration, occluding at end-inspiration may minimize errors of TGV measures in a few individuals due to small airway closure at low lung volumes.

Partial expiratory flow-volume curves are commonly used in infant pulmonary function testing. The flow measurements are volume dependent and thoracic gas volume (TGV) is often measured in conjunction with forced expiratory maneuvers. Since it is not possible to make continuous, simultaneous measurements of TGV during forced expiration, it is assumed that lung volume returns to its original value after forced expiration. To test this assumption we measured TGV using a whole body plethysmograph in 14 wheezy infants before and after a series of forced expirations produced with an inflatable jacket. Forced expiration did not cause a significant change in group mean TGV measurements. Examination of individual data did not show any systematic difference between TGV measured before and after forced expiration. These results suggest that repeated forced expirations do not alter TGV within the time scale of usual pulmonary function testing protocols.