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Effects of sino-atrial ischaemia on the heart rate response to phasic burst stimulation of the right cervical vagus in the dog.

The effects of occluding sinus node arteries on the pacemaker responses to graded phasic burst stimulation of the right cervical vagus were studied in anaesthetised dogs. In six dogs (Group I), phase response curves (vagally affected atrial cycle plotted against the phase of the stimulus in the cycle) to 1, 3 and 5 pulse burst stimulations were determined at control and hourly for 3 h after the occlusion. In four dogs, the occlusion produced a slowing of the heart rate, a shift of pacemaker outside sinus node region and a displacement of the phase response curves upward and to the right. Identical stimulation bursts resulted in significantly longer atrial cycles than at control, and bursts introduced with a longer phase remained effective and resulted in significantly longer atrial cycles than the maximum one reached at control. However, when the vagal responses were corrected for occlusion-induced changes in spontaneous PP intervals, the occlusion was found not to significantly affect the vagal responses except to increase their variability. Time-related differences in the effects of the occlusion were not statistically significant. In six control dogs (Group II) submitted to the same protocol, occlusion excepted, the vagal response curves changed very little over the 3 h experiments, thus confirming the stability of the experimental preparation used and of the vagal responses obtained in the absence of ischaemia.(ABSTRACT TRUNCATED AT 250 WORDS)