Ultra- and circadian fluctuations in arterial pressure and electromyogram in conscious dogs.

Mean arterial blood pressure was measured for 2 days in conscious dogs to estimate the frequency of ultradian fluctuations and to compare the magnitudes of the ultra- and circadian fluctuations. The ultradian fluctuation had an average frequency of 0.38 +/- 0.04 cycle/h and 60% more power than the circadian rhythm. The ultradian fluctuation was consistently more powerful during daylight than nighttime hours, but there was no consistent change in the frequency with time of day. Electromyogram (EMG) activity was also measured to test a possible causal relationship between the two variables. There were both circ- and ultradian fluctuations in EMG activity, and the peak cross-spectral power was also in these frequency bands. The coherence function was also high, but the phase angle between the two variables was so variable, both in sign and in magnitude, that it was judged unlikely that there was tight coupling between them.