Lung volumes measured by helium dilution and by body plethysmography with mouth and oesophageal pressures: a comparative study in patients with lung disease.

A comparison was made between the helium dilution technique and body plethysmography--using both mouth pressure and oesophageal pressure against box pressure--for measuring functional residual capacity (FRC). In patients with restrictive lung disease (n = 9) no major differences in FRC were noted between the techniques. In patients with obstructive lung disease (n = 17), helium dilution underestimated FRC by a mean of 0.9 l and conventional body plethysmography (mouth-box pressure) overestimated it by 0.4 l in comparison with FRC obtained by oesophageal pressure against box pressure. The difference between the techniques increased with increasing lung volumes. It is concluded that conventional body plethysmography measures FRC more accurately than the helium dilution technique in patients with obstructive lung disease.