Twenty-four-hour pattern of circulation by radiotelemetry in the unrestrained dog

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Abstract
Eight dogs with indwelling transducers for continuous radiotelemetry of the hemodynamic function were acclimatized in an unisolated room under 12:12 LD photoperiods, in phase with the solar day cycle, and constant temperature (20 +/- 1 degrees C). Light was turned on at 0600 and off at 1800. Mean aortic blood flow (ABF), mean aortic blood pressure (ABP), heart rate (HR), respiratory rate (RR), and cardiac work (CW) were maximal at about 1800 and minimal at time 0600. Total peripheral resistance (TPR) was highest and stroke volume (SV) lowest at about 1200. The 24-h mean was for HR 77 beats/min, SV 28 ml/beat, RR 11 breaths/min, ABF 177 ml/min per kg body wt, ABP 102 Torr, TPR 3,960 dyn . s . cm-5, and CW 218 Torr . 1/min. The range of amplitude fluctuations for daytime means was about 6% for ABP and SV and 11-17% for all others. It is suggested that the homeostatic balance and timing of circulation is circadian time dependent.