Cardiac output ðQ_ Þ monitoring can support the management of high-risk surgical patients, but the pulmonary artery catheterisation required by the current ‘gold standard’dbolus thermodilution ðQ_ TÞdhas 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 ðQ_ ISTÞ.

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.