The simplified version of Boyle's Law leads to errors in the measurement of thoracic gas volume.

When using Boyle's Law for thoracic gas volume (Vtg) measurement, it is generally assumed that the alveolar pressure (Palv) does not differ from barometric pressure (Pbar) at the start of rarefaction and compression and that the product of the change in volume and pressure (delta P x delta V) is negligibly small. In a gentle panting maneuver in which the difference between Palv and Pbar is small, errors introduced by these assumptions are likely to be small; however, this is not the case when Vtg is measured using a single vigorous inspiratory effort. Discrepancies in the Vtg between the "complex" version of Boyle's Law, which does not ignore delta P x delta V and accounts for large swings in Palv, and the "simplified" version, during both a panting maneuver and a single inspiratory effort were calculated for normal control subjects and patients with cystic fibrosis or asthma. Defining the Vtg from the complete version as "correct," the errors introduced by the simplified version ranged from -3 to +3% for the panting maneuver whereas they ranged from 2 to 9% for the inspiratory maneuver. Using the simplified equation, the Vtg for the inspiratory maneuver was 0.135 +/- 0.237 L greater (p < 0.02) than for the panting maneuver. This discrepancy disappeared when the complete equation was used. While the errors introduced by the use of the simplified version of Boyle's Law are small, they are systematic and unnecessary.

Redefining end of test (EOT) criteria for pulmonary function testing in children.

In order for an individual FVC maneuver to be considered acceptable according to the 1994 American Thoracic Society (ATS) standards it must meet end of test (EOT) criteria of a minimum expiration time of 6 s with minimal volume change (0.03 L) over 1 s. We have found that while these criteria are often not met in children, most of the tests do meet the ATS criteria for reproducibility with repeated efforts. We, therefore, sought to develop new EOT criteria that would be more appropriate for children and in keeping with the findings for reproducibility. Using an exponential curve fitting of the volume time tracing, we determined a theoretical curve that closely approximated the actual curve (mean difference between actual and derived FEV1 0.01 +/- 0.04 L). The theoretical FVC (FVCFULL) at the point where the curve reached its asymptote was then determined using the fitted curve. Since this point could be difficult to reach for some patients, 95% of FVCFULL (FVC95) was proposed as the new EOT in children. Data from 382 patients were reviewed. Their ages ranged from 5 to 18 yr and their FEV1s from 21 to 120% of predicted. Only 19% of the patients met current ATS EOT requirements despite the fact that 91% met the reproducibility criteria for both FEV1 and FVC. However, 90% of them reached their FVC95. When this was broken down by age, 37% of those < or = 7 yr failed to reach FVC95 whereas only 4% of the older children failed to do so. It is proposed that patients be coached to obtain maximal effort and that the curves be individually fitted to an exponential equation to determine whether the patient has achieved EOT as defined by FVC95.

An alternate method for the determination of functional residual capacity in a plethysmograph.

The validity of measuring thoracic gas volume using a single inspiratory effort against an occlusion (Vtginsp) was determined in children and young adults (8 normal control subjects and 17 patients with cystic fibrosis [CF] or asthma). In addition, the validity of the measurement of Vtg at FRC in children at a low panting frequency (Vtgpant) was also determined. During both the panting (encouraged to be about 1 Hz) and inspiratory maneuvers, mouth pressure (Pm) and esophageal pressure (Pes) were measured simultaneously. Hence, Vtgpant and Vtginsp were determined using both delta Pm and delta Pes. Vtginsp using delta Pm was found to be similar to Vtginsp using delta Pes. Values for Vtgpant using either delta Pm or delta Pes were also found to be similar. The percent difference between Vtginsp (using delta Pm) and the average of Vtgpant and Vtginsp (using delta Pes for both) was calculated as an indication of the error of the inspiratory method. The error ranged from -13 to +13% and did not correlate with indices of air-flow limitation or hyperinflation. We have found that Vtginsp can be used to determine FRC in normal children and young adults as well as in those with CF or asthma. In addition, we have also validated the use of Vtgpant at a low panting frequency in these subjects.

Effect of salbutamol on gas compression in cystic fibrosis and asthma.

In cystic fibrosis (CF), it has been suggested that increases in FEV1 postbronchodilator (BD) can be accompanied by paradoxical decreases in isovolume maximal flow at 25% of vital capacity (V25iso) measured from maximum expiratory flow-volume curves (MEFVC), raising concerns about determining the benefits of BD in CF. MEFVC measured using expired volume has been shown to be subject to errors due to gas compression. In the present study, BD response was assessed in 91 patients with asthma and 78 with CF using the percentage of change in FEV1 and V25iso determined using MEFVC from both mouth (m) and plethysmograph (p) volumes. From the two curves, volume of compression (Vcomp) was measured. Baseline measurements were similar for both groups, except that the residual volume to total lung capacity ratio (RV/TLC) was higher and Vmax25 and FVC were lower in CF. Both groups showed significant (p < 0.05) increases in FVC, FEV1, and V25iso after BD. The percentage of change in FEV1 correlated with the percentage of change in V25iso (r = 0.53 for CF and 0.66 for asthma, p < 0.001). Baseline Vcomp25 was higher in asthma than in CF. The percentage of change in V25iso was not related to the change in Vcomp25iso for either group. Only four patients with CF showed a paradoxical decrease in V25iso, and the differences in flow were small. Two had an increase in Vcomp and two had a decrease. We conclude that paradoxical decreases in V25iso are rare, are associated with small changes in flow, and are not related to changes in Vcomp.(ABSTRACT TRUNCATED AT 250 WORDS)