A comparison of cardiac output derived from the arterial pressure wave against thermodilution in cardiac surgery patients

In three clinical centres, we compared a new method for measuringcardiac output with conventional thermodilution. The new methodcomputes beat-to-beat cardiac output from radial artery pressureby simulating a three-element model of aortic input impedance,and includes non-linear aortic mechanical properties and a self-adaptingsystemic vascular resistance. We compared cardiac output bycontinuous model simulation (MF) with thermodilution cardiacoutput (TD) in 54 patients (18 female, 36 male) undergoing coronaryartery bypass surgery. We made three or four conventional thermodilutionestimates spread equally over the ventilatory cycle. In 490series of measurements, thermodilution cardiac output rangedfrom 2.1 to 9.3, mean 5.0 litre min^–1. MF differed +0.32(1.0) litre min^–1 on average with limits of agreementof –1.68 and +2.32 litre min^–1. Differences decreasedwhen the first series of measurements in a patient was usedto calibrate the model. In 436 remaining series, the mean differencebecame –0.13 (0.47) litre min^–1 with limits of agreementof –1.05 and +0.79 litre min^–1. When consecutivemeasurements were made, the change was greater than 0.5 litremin^–1, on 204 occasions. The direction of change was thesame with both methods in 199. The difference between the methodsremained near zero during surgery suggesting that a single calibrationper patient was adequate. Aortic model simulation with radialartery pressure as input reliably monitors changes in cardiacoutput in cardiac surgery patients. Before calibration, themodel cannot replace thermodilution, but after calibration themodel method can quantitatively replace further thermodilutionestimates. Br J Anaesth 2001; 87: 212–22