Circadian (approximately 24 hr) rhythms in heart rate (HR) and rhythm, stroke volume (SV), and cardiac output (CO) have been found. However, few studies describing temporal patterns in these variables have included critically ill adults. It was unknown whether these variables continue to be rhythmic in critically ill postoperative, cardiac surgical patients and if so, whether the rhythms could be detected.

To describe simultaneously measured temporal pattern in HR and rhythm, SV, and CO in critically ill adults in a cardiac surgical intensive care unit (ICU), six patients aged 33-68 years, mean 48.3 $pm$ 15.3 years, were observed continuously for 36-48 hours. HR and rhythm were measured using a Holter tape recorder. Thermodilution CO was measured every 1-4 hours; SV was derived. Treatments and environmental events were noted on the graphed variables.

Cosinor analyses of HR using whole data set and divided data set revealed 24-hour rhythms in all analyses except for one patient in the whole data analysis (p $<$ 0.056). Mesors, amplitudes, and acrophases differed between the two study days. Ultradian (12 hr, 6 hr, 4 hr) rhythms occurred in some subjects and differed between study days.

Cosinor analysis of arrhythmias (n = 4) revealed 24-hour rhythms in PACs (n = 1; p = 0.004), PVCs (n = 2; p $<$ 0.062), and ventricular couplets (n = 2; p = 0.001). 4-hour rhythms were found in PACs (n = 1; p = 0.004), atrial tachycardia (n = 1; p = 0.034), and PVCs (n = 4; p $<$ 0.089). Acrophases for arrhythmias varied among individuals. Mesors, amplitudes, and acrophases differed between study days. On study day two, the 4-hour-rhythm acrophases in HR and arrhythmias were related to the timing of respiratory therapy.

SV and CO had significant 24-hour rhythms in two subjects (SV, p = 0.000, 0.096; CO, p = 0.006, 0.056). Mesors and amplitudes in these two subjects were similar; acrophases did not overlap.

This is the first study to describe the temporal patterns of HR and rhythm, SV, and CO simultaneously measured in critically ill adults in a cardiac surgical ICU. Knowledge that rhythms are occurring and that their characteristics are changing from day to day adds some explanatory value in understanding why changes in these variables may be seen clinically and in research settings.