Validity of total lung capacity determination by the single breath nitrogen technique

Total lung capacity (TLC) derived from the single breath oxygen test (dilution of alveolar nitrogen) was compared with the plethysmographic TLC in 40 healthy males aged 25 to 55 years and in 16 patients with chronic obstructive respiratory disease. In healthy subjects TLC02 was 8% less (P less than 0.001) than TLC pleth. The two results were highly correlated (r = 0.89; P less than 0.001); their difference tended to increase with age and correlated significantly with the slope of phase III (P less than 0.01). In patients TLC02 underestimated the lung volume by 5 to 50% (average 28%) (P less than 0.001), and the relation between the two measurements were weaker (r = 0.59; P less than 0.02). The TLC02/TLC pleth. ratio was significantly related to FEV1.0% predicted (r = 0.68) and to specific conductance (r = 0.60), and highly significantly (P less than 0.001) related to obstructive ventilatory impairment (r = 0.77), to distribution unevenness (r = -0.85) and overinflation (r = 0.94). Although an overestimation of lung volume by plethysmography in the presence of severe obstruction cannot be excluded, most of the discrepancy reported is to be ascribed to underestimation by single breath nitrogen dilution due to impaired distribution of ventilation.