These results indicate that lithium dilution is a suitable method for measuring cardiac output in horses. As well as being accurate, it avoids the need for pulmonary artery catheterization and is quick and safe to use. Monitoring cardiac output during anesthesia in horses may help reduce the high anesthetic mortality in this species.

We have developed a new pulse contour cardiac output (PulseCO) algorithm based on frequency analysis studies of the arterial system. PulseCO was compared with thermodilution cardiac output (TDCO) in 10 patients undergoing cardiac surgery. Results from one patient were unsuitable for analysis. In the remaining nine patients, 142 TDCO determinations were compared with PulseCO after logarithmic transformation and after being normalized by the initial cardiac output in each patient. Each determination was usually the average of three measurements. Least squares regression gave y = 0.77 x (r2 = 0.81) and the limits of agreement were from -26% to +21%. The accuracy of PulseCO in determining short-term changes in cardiac output was assessed by comparing the ratios of consecutive PulseCO determinations with the ratios of the corresponding, consecutive TDCO determinations. Least squares regression gave y = 0.71 x (r2 = 0.70) and the limits of agreement were from -21% to +25%. After phenylephrine had been given to five patients, PulseCO showed an increase in systemic vascular resistance consistent with the known pharmacological actions of the drug. The PulseCO algorithm was incorporated into a computer program that acquires arterial pressure data from an analogue-to-digital converter and displays beat-to-beat trend values.

We describe a new indicator dilution method of measuring cardiac output in man. A bolus injection of lithium chloride 0.6 mmol was given via a central venous catheter and arterial plasma [L/+] recorded using a specially developed sensor incorporating an LP-selective electrode. Cardiac output was derived from the lithium dilution curve, with a correction for packed cell volume. Lithium dilution cardiac output (LiDCO) was compared with thermodilution cardiac output (TD) using 22 lithium sensors in nine patients. For each sensor, one LiDCO was measured immediately before and one immediately after three TD estimations and mean values of LiDCO and TD derived. The correlation coefficient, r, was 0.89; slope of the regression 0.84; y intercept 0.72; bias 0.3 (0.5) litre min' 1 (mean (TD-LiDCO) (1 SD). LiDCO appeared to be a safe, simple and accurate technique which does not require insertion of a pulmonary artery catheter.

The overall agreement between the two methods was good. The variability of the thermodilution measurements was greater than that of the lithium dilution measurements. The lithium dilution method is at least as accurate as bolus thermodilution and, since pulmonary artery catheterization is not needed, it has the advantages of being safe and quick to perform.

These results suggest that the LiDCO method can be used to provide safe and accurate measurement of cardiac output in paediatric patients. The method is simple and quick to perform, requiring only arterial and venous catheters, which will already have been inserted for other reasons in these patients.