Variability, reproducibility and observer difference of body plethysmographic measurements.

The variability (coefficient of variation of five consecutive measurements), reproducibility (difference of results at 1 h and 24 h), and interobserver difference (independent reading of the tracings by two observers) of airways resistance (Raw) and static lung volumes (residual volume, functional residual capacity, total lung capacity) using a body plethysmograph were assessed in 14 healthy subjects and in 25 patients with various respiratory disorders. The variability was low for TLC (4-5%), moderate for FRC (7-8%) and high for Raw (28%). No significant changes of Raw or lung volumes were found for the groups at 1 h and 24 h. Between observers, a slight difference existed for FRC and Raw in normal subjects; the difference was higher (4.5% for FRC and 11% for Raw) and became significant in patients. The overestimation of Raw by observer 2 as compared to observer 1 was more important at larger values. The present findings call for caution when pooling results obtained by several observers in large-scale studies, or when comparing figures obtained by different technicians in the pulmonary function laboratory.

The unreliability of indirect lung compliance in healthy subjects and patients with chronic lung disorders.

The clinical usefulness of lung compliance calculated indirectly was reassessed in a large number of healthy subjects and patients with chronic airflow obstruction or restrictive lung disorders. Indirect compliance was calculated from maximal expiratory flow-volume (MEFV) curve and airway resistance measured plethysmographically according to two approaches. In the first (approach A) all calculations were done at the functional residual capacity, whereas in the second (approach B) they were obtained over the 50-75% volume range of the forced vital capacity; values were compared to those of direct compliance measured concurrently. For the group as a whole, the correlations between indirect and direct values were poor regardless of the approach. Examined separately, the best correlations were found for the healthy group using approach A (r = 0.501) and for the obstructive group using approach B (r = 0.312). Failure to derive a valuable indirect compliance is due to the fact that there is a very poor correlation between upstream resistance and airway resistance measured by body plethysmography.U

Comparison of four methods for calculating the total lung capacity measured by body plethysmography.

Static lung volumes were measured plethysmographically one hour apart in healthy subjects (n = 14) and in patients with chronic pulmonary disorders of various etiologies (n = 25). The total lung capacity (TLC) obtained from paired measurements of functional residual capacity (FRC) and inspiratory capacity (IC) was calculated according to the four following methods: 1) average FRC plus the largest IC, 2) average FRC plus the average IC, 3) largest sum of FRC and corresponding IC, and 4) average of individual FRC and IC sums. The data, analysed for average values and for reproducibility in the group as a whole and in the healthy subjects and patients separately indicate that: a) For the group as a whole the largest average TLC values were found with method 3 followed by methods 4 and 1. The differences were statistically significant for all comparisons but one (method 1 vs method 4). A similar pattern was found for the healthy subjects and patients separately. b) For the group as a whole, the one hour reproducibility tended to be worse from method 1 through method 4 but the intermethod differences were not statistically significant. For the healthy subjects, the reproducibility tended to be better for methods 3 and 4 and for the patients this was the case for methods 1 and 2. The average reproducibiltiy of methods 1 and 2 was similar for both the healthy and patient groups and these methods seemed more suitable for TLC calculations. Because it is more widely employed, method 1 is recommended.