The standard plethysmographic method of measuring total lung capacity (TLC) has been reported to result in spuriously high estimates in patients with severe airway obstruction. The helium-dilution method is known to underestimate TLC in the same patients. To determine the magnitude of these possible errors we measured TLC by four methods in 20 patients with varying degrees of chronic obstructive lung disease and in 11 normal subjects. TLC was measured by (1) helium dilution (TLCHe); (2) a volume-displacement body plethysmograph, box volume being plotted against mouth pressure (TLCm); (3) the same body plethysmograph with volume plotted against pressure measured with an oesophageal balloon (TLCes); and (4) a radiological technique (TLCxr). In normal subjects there was no difference between TCLm (6.57 +/- 1.20) and TLCes (6.51 +/- 1.24). In the patients with chronic obstructive lung disease TLCm gave results significantly higher than those of any other method. If TLCes is taken as the closest estimate of true TLC, TLCm consistently overestimates and TLCHe underestimates TLC. There was no relationship between the degree of airway obstruction and (TLCm - TLCes) but there was between (TLCes - TLCHe) and severity of airway obstruction. We conclude that using mouth pressure in the plethysmographic measurement of TLC in patients with chronic obstructive lung disease results in consistent but slight overestimation of TLC.
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