Comparison and reliability of two non-invasive acetylene uptake techniques for the measurement of cardiac output

Comparison and reliability of two non-invasive acetylene uptake techniques for the measurement of cardiac output. Thirteen trained male cyclists performed CO₂ rebreathing (CO₂RB) at intensities from rest to 200 W, and open-circuit acetylene uptake (OpCirc) and single-breath acetylene uptake (SB) at intensities from rest to 300 W, with all procedures using 50 W increments. Oxygen consumption \((\dot V{\text{O}}_{2}),\) cardiac output \((\dot Q),\) and heart rate (HR), were measured at each stage, and the values for each variable were compared within each intensity to determine reliability of the measuring device. Both the OpCirc and SBs were shown to be reliable measures of cardiac output (r = 0.95 and 0.92, respectively) with decreasing coefficients of variation (CV) as intensity increased, and were similar to published data. The \(\dot Q{\text{--}}\dot V{\text{O}}_{2}\) relationship using the SB diverged from the regression line for OpCirc and CO₂RB. Linear regression of the \(\dot Q{\text{--}}\dot V{\text{O}}_{2}\) relationship for CO₂RB was \(y = 6.18 \times \dot V{\text{O}}_{2} + 2.59,\) for OpCirc was \(y = 6.12 \times \dot V{\text{O}}_{2} + 2.98,\) and for SB was \(y = 5.05 \times \dot V{\text{O}}_{2} + 3.76. \) The OpCirc and SBs were both shown to be reliable techniques for measuring cardiac output, comparable to previously reported cardiac output measurements, and suitable for use in exercise testing. However, the SB, requiring a constant, slow exhalation rate, made the procedure difficult to perform at higher exercise intensities.