The 24 hour cycle and nocturnal depression of human cardiac output

The NASA-developed version of the impedance plethysmographic method for the determination of left ventricular stroke volume was applied to the investigation of daily variation of cardiovascular function. Hourly determinations of cardiac output and arterial pressure (auscultation) were obtained from two male subjects over the course of a 48-h period in the laboratory. Cardiac output displayed a strong 24-h cycle of nearly 1 1/min, peak-to-peak, cresting in the late evening hours. Nocturnal sleep was associated with a reduction of cardiac output characterized by a minimum in the early morning hours. It was concluded that a nocturnal, 24-h cyclic depression of waking cardiac output is exacerbated during sleep periods, and mechanisms for this exacerbation were suggested.