Comparison of methods of measuring cardiac output in newborn lambs

To define agreement between methods, we measured cardiac output (CO) in chronically instrumented lambs by four different methods. In 23 lambs we measured CO simultaneously by the Fick and microsphere methods and with an electromagnetic flowmeter over a wide range of CO (80-454 ml/kg/min) in different experimental conditions on 97 occasions. When the electromagnetic flowmeter was corrected for coronary flow (about 8% of CO) calculated from microspheres, the mean cardiac outputs were almost identical for these three methods (237, 235, and 236 ml/kg x min, respectively). Comparisons between any two of the methods showed a correlation coefficient greater than 0.89. In four lambs, 52 measurements of CO by Fick and thermodilution had a correlation coefficient of 0.94. We conclude that any of the four methods, if appropriately applied, adequately measures CO in a variety of circumstances and may be used with confidence for physiologic or other studies.