The peripheral vascular response to severe exercise in untethered dogs before and after complete heart block

The peripheral vascular response to severe exercise was studied in 11 healthy conscious dogs instrumented with Doppler ultrasonic flow probes on the mesenteric, renal, and iliac arteries, and miniature pressure gauges in the aorta. The response to severe exercise was restudied in six of these dogs after recovery from a second operation producing complete heart block by the injection of formalin into the atrioventricular (AV) node. Three of these dogs also exercised while their ventricles were paced at rates of 100/min and 200/min. The untethered normal dogs ran at speeds of 15-25 miles/hr behind a mobile recording unit for a distance averaging 1.5 miles, while continuous measurements of arterial blood pressure and blood flow were telemetered and recorded on magnetic tape. Severe exercise in normal dogs increased heart rate from 84 to 259/min, arterial pressure from 89 to 140 mm Hg, flow resistance in the mesenteric and renal beds by 59 and 52% respectively, and iliac blood flow 479% above control, while mesenteric and renal blood flows remained constant and iliac resistance decreased by 73%. In dogs with complete AV block, severe exercise at speeds of 10-18 miles/hr increased heart rate from 47 to 78/min, mean arterial pressure from 81 to 89 mm Hg, iliac flow 224%, resistance in the renal bed by 273%, and mesenteric bed by 222% while it decreased blood flow in mesenteric and renal beds by 61 and 65% respectively, and iliac resistance by 62%. A similar response occurred during exercise with pacing at 100/min, but when paced at 200/min a more normal exercise response reappeared. Thus, in normal dogs the peripheral vascular response to severe exercise involved increases in heart rate, arterial pressure and visceral resistance but visceral blood flow did not decrease. In dogs with heart block, where the ability to increase heart rate is severely compromised, compensatory reduction of mesenteric and renal blood flows occurred.