The non-invasive acetylene rebreathing method for estimation of cardiac output: influence of breath-by-breath variation.

The inert gas rebreathing method enables non-invasive estimation of pulmonary capillary blood flow, lung tissue volume, transfer factor and functional residual capacity. In the present study, we have examined the influence of breathing pattern during the rebreathing manoeuvre on the precision of the rebreathing method, both theoretically and experimentally. We examined whether the precision of the method could be improved by the guidance and training of the subjects doing the rebreathing manoeuvre. The results of the theoretical study showed that the precision and accuracy of the rebreathing method are practically insensitive to random variation in the breathing pattern. Simulated breath-by-breath variations up to +/-50% of the average 3.01 VT resulted in coefficients of variation of about 3% for QC and about 5% for VTC. Simulated breath-by-breath variations indicate that with mean tidal volume or rebreathing bag volume lower than 1.01 the precision will worsen, and with tidal volume lower than 1.5 1 the accuracy will worsen. The experimental results showed no significant improvement in the precision of the rebreathing method by visual guiding and training of the subjects to optimize the breathing pattern during the rebreathing manoeuvre.