Influence of the panting technique on the plethysmographic measurement of thoracic gas volume

In 7 normal subjects we studied the effect of different panting techniques on the measurement of thoracic gas volume (VTG). When inspiratory efforts against the occluded airway were performed primarily with intercostal and accessory muscles, the value of VTG was significantly larger than during efforts performed primarily with the diaphragm. The difference could be as large as 900 ml during the same occlusion and was due to compression and decompression of abdominal gas. The divergence of the measured VTG from the true VTG depended on the volume of abdominal gas (Vab) and the ratio of gastric to mouth pressure changes (ΔPg/ΔPm). In our normal subjects, Vab was 358 ± 65 ml (mean ± SE) and ΔPg/ΔPm ranged from 0.7 to -2.5. In 10 randomly selected patients with a variety of pulmonary disorders, the mean value of ΔPg/ΔPm was 0.32. In one subject with asthma who increased his total lung capacity by one liter after excercising, ΔPg/ΔPm did not change significantly from the control value. Our results indicated that the pattern of panting is an important determinant of the accuracy of plethysmographic measurement of VTG. However, preliminary results from studies of patients suggest that the error is small and does not account for the large changes in lung volume measured in patients with acute asthma.