ADVANTAGES OF CONTINUOUS MEASUREMENT OF CARDIAC-OUTPUT 24 H A DAY

To test the hypothesis that continuous measurement of cardiac output 2 4 h a day would provide a better day-by-day reproducibility of the dai ly average cardiac output than acute measurements, we developed a comp uter-assisted method to monitor cardiac output continuously using an e lectromagnetic flow transducer. Because the diastolic aortic flow, whi ch is used as a zero-flow reference, can drift significantly with elec tromagnetic flow probes, automatic tracking of the diastolic flow base line was considered essential for long-term measurements. To accomplis h this, the analog pulsatile flow signal was digitally converted and p rocessed by an IBM PC to correct for signal drift on a beat-per-beat b asis. Using this computerized system in 19 chronically instrumented do gs, we compared the values of cardiac output during 5 consecutive cont rol days, measured either for 20 h each day (allowing 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 ind ividual dog was three times smaller when cardiac output was measured 2 0 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, s troke volume, and total peripheral resistance. Because of this greater day-by-day reproducibility, continuous monitoring of cardiac output i s likely to be more sensitive to small changes in cardiac output induc ed by experimental protocols.