Total lung capacity measured by body plethysmography and by the helium dilution method. A comparative study in different patient groups.

The helium dilution method is known to underestimate the total lung capacity (TLC) in patients with poorly or non-ventilated areas in the lungs. The standard plethysmographic method has been reported to overestimate TLC in patients with severe airway obstruction. To determine the magnitude of the difference between the two methods, a comparison was made in different patient groups. In a group of patients with normal lung function tests (n = 20) there was a small but significant average difference in TLC between plethysmography and the helium dilution method, the larger values being obtained with the latter. In patient groups with moderately obstructed airways (n = 23), severely obstructed airways (n = 20), or emphysema (n = 19), there were no significant average differences, although in two patients in the emphysema group the plethysmographic values were considerably larger than those obtained by helium dilution. We conclude that the gas dilution methods and plethysmography with a pressure-compensated volume displacement plethysmograph gave estimates of TLC which agreed even in patients with airway obstruction or emphysema, except in patients with very severe lung disease.