Noninvasive cardiac output monitoring in a porcine model using the inspired sinewave technique: a proof-of-concept study.

Abstract

Cardiac output (Q˙) monitoring can support the management of high-risk surgical patients, but the pulmonary artery catheterisation required by the current 'gold standard'-bolus thermodilution (Q˙_T)-has the potential to cause life-threatening complications. We present a novel noninvasive and fully automated method that uses the inspired sinewave technique to continuously monitor cardiac output (Q˙_IST). Over successive breaths the inspired nitrous oxide (N₂O) concentration was forced to oscillate sinusoidally with a fixed mean (4%), amplitude (3%), and period (60 s). Q˙_IST was determined in a single-compartment tidal ventilation lung model that used the resulting amplitude/phase of the expired N₂O sinewave. The agreement and trending ability of Q˙_IST were compared with Q˙_T during pharmacologically induced haemodynamic changes, before and after repeated lung lavages, in eight anaesthetised pigs. Before lung lavage, changes in Q˙_IST and Q˙_T from baseline had a mean bias of -0.52 L min^-1 (95% confidence interval [CI], -0.41 to -0.63). The concordance between Q˙_IST and Q˙_T was 92.5% as assessed by four-quadrant analysis, and polar plot analysis revealed a mean angular bias of 5.98° (95% CI, -24.4°-36.3°). After lung lavage, concordance was slightly reduced (89.4%), and the mean angular bias widened to 21.8° (-4.2°, 47.6°). Impaired trending ability correlated with shunt fraction (r=0.79, P<0.05). The inspired sinewave technique provides continuous and noninvasive monitoring of cardiac output, with a 'marginal-good' trending ability compared with cardiac output based on thermodilution. However, the trending ability can be reduced with increasing shunt fraction, such as in acute lung injury.