Regional Circulatory Adjustments to Moderate and Severe Chronic Anemia in Conscious Dogs

The successive changes in regional blood flows and resistances due to moderate and severe anemia were studied serially in nine conscious dogs after they recovered from implantation of ultrasonic Doppler transducers or electromagnetic flow probes on the left circumflex coronary, the mesenteric, the renal, and the iliac arteries and pressure gauges or catheters in the aorta. Anemia was induced by progressive phlebotomy and volume replacement over a period of 2-4 weeks. In moderate anemia (average hematocrit 22%) heart rate increased from 73 beats/min to 105 beats/min, and mean arterial pressure fell from 98 mm Hg to 94 mm Hg. Flow increased to the coronary bed by 75% and to the iliac bed by 45%, but flow to the mesenteric bed was not significantly affected and renal flow fell by 9%. Coronary and iliac resistances fell by 45% and 32%, respectively, but mesenteric and renal resistances were not significantly affected. In severe anemia (hematocrit 14%) heart rate increased to 129 beats/min, and mean arterial pressure was at control levels; coronary flow increased by 227%, iliac flow by 102%, mesenteric flow by 29%, and renal flow by 20%. Resistance decreased in the coronary bed by 69%, in the iliac bed by 52%, in the mesenteric bed by 23%, and in the renal bed by 19%. Exercise during severe anemia increased heart rate to 224 beats/min, mean arterial pressure to 114 mm Hg, coronary flow by 34%, and iliac flow by 215%; it reduced mesenteric flow by 59% and renal flow by 48%. Thus, blood flow increased and resistance decreased to all beds studied, indicating that in the resting conscious dog compensatory reduction of visceral flow is not a feature of the cardiovascular response to severe anemia at rest, although a redistribution of regional blood flow does occur. However, the added stress of exercise during severe anemia results in substantial reductions in mesenteric and renal blood flows.