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

BACKGROUND: Cardiac output [Formula: see text] monitoring can support the management of high-risk surgical patients, but the pulmonary artery catheterisation required by the current ‘gold standard’—bolus thermodilution [Formula: see text] —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 [Formula: see text]. METHODS: Over successive breaths the inspired nitrous oxide (N(2)O) concentration was forced to oscillate sinusoidally with a fixed mean (4%), amplitude (3%), and period (60 s). [Formula: see text] was determined in a single-compartment tidal ventilation lung model that used the resulting amplitude/phase of the expired N(2)O sinewave. The agreement and trending ability of [Formula: see text] were compared with [Formula: see text] during pharmacologically induced haemodynamic changes, before and after repeated lung lavages, in eight anaesthetised pigs. RESULTS: Before lung lavage, changes in [Formula: see text] and [Formula: see text] from baseline had a mean bias of –0.52 L min(−1) (95% confidence interval [CI], –0.41 to –0.63). The concordance between [Formula: see text] and [Formula: see text] 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). CONCLUSIONS: 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.