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

Background: Cardiac output (Q)over dot monitoring can support the management of high-risk surgical patients, but the pulmonary artery catheterisation required by the current 'gold standard'-bolus thermodilution ((Q)over dot(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)over dot(IsT)). Methods: 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)over dot(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)over dot(IST) were compared with (Q)over dot(T) during pharmacologically induced haemodynamic changes, before and after repeated lung lavages, in eight anaesthetised pigs. Results: Before lung lavage, changes in (Q)over dot(IST) and (Q)over dot(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)over dot(IST) and (Q)over dot(T) was 92.5% as assessed by four-quadrant analysis, and polar plot analysis revealed a mean angular bias of 5.98 degrees (95% CI, -24.4 degrees-36.3 degrees). After lung lavage, concordance was slightly reduced (89.4%), and the mean angular bias widened to 21.8 degrees (-4.2 degrees, 47.6 degrees). 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.