OSCILLATIONS IN MEAN ARTERIAL BLOOD-PRESSURE IN CONSCIOUS DOGS

Oscillations in mean arterial blood pressure (MABP) with periods near 1.5 hours were observed in conscious male dogs with pressure transducers implanted in their aortas. Cardiac output (CO) was measured with an electromagnetic flow probe implanted about the ascending aorta, and total peripheral resistance (TPR) was calculated as the ratio of MABP to CO. Heart rate (HR) was measured with a cardiotachometer. Coherence functions were calculated among the four variables (MABP, TPR, CO, and HR) to determine significant oscillations at the frequency of the MABP oscillation. Cross-spectral calculations produced phase relationships among the variables. TPR and CO were both oscillating at the same frequency as MABP. However, TPR lagged MABP by 63 degrees, whereas the phase angle for CO and MABP was not significantly different from zero. The authors concluded that CO must be producing the MABP oscillations, and that the TPR oscillations arose as a reaction to the MABP or CO changes. HR was oscillating in phase with CO, indicating that CO fluctuations were induced in part by HR changes and that the sympathetic nervous system was probably driving the heart. In further experiments, chronic dietary sodium changes did not alter the power spectra for MABP. Thus, the renin-angiotensin system (RAS) was not responsible for the oscillations; conversely, the oscillations were not acting as a signal to the RAS during sodium-deficient states, a conditions known to stimulate the RAS.