Measurement of lung volumes in health subjects and in subjects with airflow limitation: emphasis on patients with airways obstruction resulting in gas mixing disorders

The validity of the forced rebreathing method was assessed by comparing ventilatory function results determined by the tidal breath helium dilution, whale body plethysmography and the forced rebreathing nitrogen washout methods in healthy subjects and patients with COPD. In 13 healthy subjects the mean RVN was insignificantly less than the mean RVHE (P>0.5). In a group of 21 healthy subjects the mean RVHE was only 105 ml. Less than the mean RVbox. From these observations it was concluded that in healthy subjects the methods give similar results. An interpolation procedure has been described in this study. It takes into account the effects of gas solubility in body fluids and tissues, and changes in the respiratory exchange ratio. Using this approach, in 28 patients with COPD (mean FEV1%FVC 48.28) the mean RVN was insignificantly less than RVHe (P>0.1). In another group of 12 patients in which concurrent plethysmographic measurements were made, RV was computed from changes in alveolar pressure measured at the mouth (RV no) or esophagus (RVoes)' RVN and RVHe were 2 significantly less than RVmo and RVoes (P<0.001). The mean difference was larger between gas dilution RV and RVmo than between gas dilution RV and RVoes mainly due to an overestimation of RVmo by the plethysmograph. Analysis of RV per breath was used to determine the rate of apparent change in lung volume during forced rebreathing manoeuvres in healthy subjects and patients with COPD. Patients have large apparent increase in RV (18-69 mI/breath) when compared with 7-11 ml/breath and 13-15 ml/breath in healthy non-smokers and smokers respectively, despite the patients' reduced alveolar ventilation and large lung volume. The large apparent increases in lung volume are considered to be a result of increased airways resistance, which leads to an increase in the work of breathing and oxygen uptake from the system. Having demonstrated that the forced rebreathing technique gives results similar to those obtained by the helium dilution method in healthy subjects and in patients with COPD, the technique was employed to measure RV and TLC, including spirometric measurement of other lung volumes in healthy indigenous adolescents in Tanzania; the results were compared with those of Caucasian counterparts. The FEVI1%FVC was similar in the two races until about 18 years of age in boys and 20 years in girls when a decline at a rate of about 0.81%/year was observed in Tanzanian subjects. It is concluded that the forced rebreathing method is useful in both healthy subjects and in patients with COPD. It has advantages over the helium dilution method in that considerably less time is required to complete a single measurement and that it does not require use of a foreign gas. It has an advantage over plethysmography in that it does not require bulky and expensive equipment.