Effects of Intra-arterial and Intravenous Epinephrine on Disappearance of Nal from Calf Muscle and on Calf Blood Flow

• It has often been considered that pie thy smographic foot or hand, and calf or forearm, measurements are representative of skin or of muscle blood flow, respectively, because of the preponderant amounts of each tissue in these areas. However, Cooper et al. have demonstrated that 60% of the total forearm flow may be in the skin under certain conditions. It is obvious, therefore, that increases or decreases of blood flow in skeletal muscle, as measured plethysmographieally in the calf or forearm, may be hidden or exaggerated by changes in skin flow. In 1949, Kety introduced the technic of tissue clearance of radioisotopes as an index of blood flow through capillaries near the site of deposition of the isotope. An important discrepancy between the two technics is said to occur during the intravenous administration of epinephrine; large changes occur in plethysmographic calf or forearm blood flow measurements while the rate of clearance of radioisotope from skeletal muscle is unaffected. The explanation given for this finding is that epinephrine probably opens arteriovenous anastomotic channels whose flow would be measured by the plethysmograph but not by a technic indicating capillary blood flow. The further implication is that this is evidence for the existence of functional arteriovenous shunts in skeletal muscle.