BLOOD FLOW, PERIPHERAL RESISTANCE AND VASCULAR TONUS, WITH OBSERVATIONS ON THE RELATIONSHIP BETWEEN BLOOD FLOW AND CUTANEOUS TEMPERATURE

Abstract

The relationship between vasomotor activity, peripheral resistance, blood flow and subcut. temp. was studied by recording either the arterial inflow or the venous outflow at a series of perfusion pressures in various vascular beds in the hind limbs of anesthetized dogs. Avg. blood flows at mean arterial pressure were 6.9 ml./l00 gs. of muscle. In the combined skin and muscle of the lower half of the hind extremity, the avg. flow was 3.2/100 gs. of extremity and in the skin of the latter, the avg. flow was 1.6 ml./lOO gs. of extremity. During a constant state of vasomotor activity the increment of blood flow/increment of perfusion pressure in skin and often in muscle increased regularly from zero upward when flow was measured immediately after establishment of the perfusion pressure. The relationship between perfusion pressure and flow in these expts. could be represented by [image], where n has values between 1 and 5, usually about 2. This relationship appears to be explainable on a purely physical basis. When the rate of flow was measured after 1-2 mins. after establishment of the perfusion pressure in muscle and in, the whole distal part of the extremity the flows at pressures below mean arterial pressure appeared to be relatively greater than they would be on the basis of a purely physical relationship between pressure and flow. This was probably due to a vasodilation induced locally by the accumulation of metabolic products as a result of the slowed flow at low perfusion pressures, but might also be due to a lowering of the extravascular pressure. The result was a sigmoid rather than a parabolic shaped curve relating perfusion pressure and flow. In all expts., increase of vasomotor activity decreased the rate of flow at each perfusion pressure. In those expts. in which [image], increase of vasomotor activity increased the magnitude of the constant C and of the exponent n. The term PRU is proposed as a unit of peripheral resistance, where [image]. Peripheral 1 ml/min resistance was found to be altered when the perfusion pressure was raised or lowered, even when vasomotor activity remained constant. Conversely, change of vasomotor activity was not in all cases correctly indicated by an appropriate alteration of peripheral resistance. Various methods of expressing change of vasomotor activity were discussed. The only completely satisfactory method is to compare the plot relating pressure to flow over the complete range of pressures from zero to mean aortic pressure in a control period, with a similar plot obtained in the exptl. period. However, this method is laborious and it is difficult to depict the progressive changes in these plots with time. As a compromise the next most satisfactory method appears to be to determine during a control period the relationship between pressure and flow over the entire range of flows anticipated during the subsequent exptl. period and thereafter to make only single detns. of pressure and flow, usually at a perfusion pressure equal to the existing mean aortic pressure. The results may then be expressed in terms of the ratio of the peripheral resistance (or perfusion pressure) in the exptl. period to the peripheral resistance (or perfusion pressure) observed during the control period at the same rate of flow.