Validity of the forced rebreathing method in the measurement of residual volume in patients with airflow limitation.

The validity of the forced rebreathing method (FRM) in the measurement of residual volume (RVn) was assessed in two groups (28 and 12) of patients with significant airflow limitation. The mean FEV1% FVC% were 48.28%, SD = 8.91% and 54.3%, SD = 0.23% respectively. Patients performed at least 30 forced rebreathings into a bag and bottle system at about 28 breaths per minute. RVn per breath was regressed on breath and gas dilution was considered complete at the breath number where the regression line deviated from the nitrogen washout curve. Residual volume computed four breaths after the breath number where the regression line deviated from the curve were compared with results derived from helium dilution (RVHe), mouth (RVmo) and oesophageal (RVOeS) pressure changes in the body plethysmograph. The mean RVn was similar to RVHe (P > 0.98) and correlated well with RVHe (r = 0.908, P < 0.001). RVn and RVHe were significantly smaller than RVmo and RVoes (P < 0.001). The difference between RVn and RVmo was smaller than the difference with RVoes. RVmo was larger than RVoes (P < 0.001) and correlated well with RVoes (r = 0.939, P < 0.001). It is concluded that the FRM can be used with reasonable accuracy to measure residual volume in patients with airflow limitation, and has advantages over the plethysmographic and conventional helium dilution methods.