Measurement of cardiac output by open-circuit acetylene uptake: a computer model to quantify error caused by ventilation–perfusion inequality

Abstract

The ability to assess cardiac output noninvasively has been the focus of interest for many researchers. While the open-circuit acetylene (C2H2) method seems promising, it is prone to error due to ventilation–perfusion inequality. Measurements during exercise, at high altitude or in patients with chronic obstructive pulmonary disease (COPD) could be unreliable and further validation studies under these circumstances may be needed. We used a computer model based on formulae derived from the multiple inert gas elimination technique to quantify error in measurements resulting from inequality at rest, during exercise or at high altitude. Moreover, potential errors encountered in patients with COPD were quantified. In healthy subjects, inequality related measurement error seems negligible, under both normoxic and hypoxic conditions and especially during exercise. In COPD, errors up to 20% at rest and up to 15% during exercise are expected. It is therefore concluded from our model that the open-circuit C2H2 uptake method is expected to be accurate in normal subjects. Its validity in COPD needs further study.