Advantages of continuous measurement of cardiac output 24 h a day.

To test the hypothesis that continuous measurement of cardiac output 24 h a day would provide a better day-by-day reproducibility of the daily average cardiac output than acute measurements, we developed a computer-assisted method to monitor cardiac output continuously using an electromagnetic flow transducer. Because the diastolic aortic flow, which is used as a zero-flow reference, can drift significantly with electromagnetic flow probes, automatic tracking of the diastolic flow baseline was considered essential for long-term measurements. To accomplish this, the analog pulsatile flow signal was digitally converted and processed by an IBM PC to correct for signal drift on a beat-per-beat basis. Using this computerized system in 19 chronically instrumented dogs, we compared the values of cardiac output during 5 consecutive control days, measured either for 20 h each day (allowed 4 h for special care) or for 30 min in the morning when the trained dogs were required to lie quietly in their cages. The results show that the coefficient of variation of the five daily averages in cardiac output for each individual dog was three times smaller when cardiac output was measured 20 h each day (2.9 +/- 0.3 vs. 9.7 +/- 1.0%). Whole-day coefficients of variation were also smaller for mean arterial pressure, heart rate, stroke volume, and total peripheral resistance. Because of this greater day-by-day reproducibility, continuous monitoring of cardiac output is likely to be more sensitive to small changes in cardiac output induced by experimental protocols.