The influence of sympathetic nerves on transcutaneous oxygen tension in normal and ischemic lower extremities

The authors evaluated the relationship between sympathetic nerve activity and transcutaneous oxygen tension (TcpO2) in normal and ischemic lower extremities. Dorsal foot TcpO2 was measured by using oxygen-sensing electrodes with surface temperatures of 42 degrees C and 45 degrees C; in theory, changes in sympathetic activity should affect vasomotor tone and TcpO2 in skin beneath an electrode at 42 degrees C (submaximal vasodilation), but not at 45 degrees C (maximal vasodilation). The vasodilation index (TcpO2 at 42 degrees C/TcpO2 at 45 degrees C) was created as an index of vasomotor tone (vasodilation index increases as tone decreases). In normal limbs (n = 24) averages for TcpO2 at 42 degrees C, TcpO2 at 45 degrees C, and vasodilation index were 30.3 mmHg, 62.1 mmHg, and 0.47, respectively. In subjects (n = 5) with quadriplegia and reduced sympathetic tone secondary to cervical cord trauma, TcpO2 at 42 degrees C and vasodilation index were increased (45.0 mmHg and 0.61); TcpO2 at 45 degrees C did not change. When normal subjects (n = 7) were chilled for twenty minutes with a cooling blanket at 5 degrees C (to increase sympathetic tone) average vasodilation index dropped from 0.50 to 0.29. Among ischemic limbs (n = 34) vasodilation index was highly variable (range: 0-0.77); in general, vasodilation index fell as the ischemia worsened. In a subset of patients with ischemic limbs, the vasodilation index increased after the limb was wrapped in a warm dressing (average vasodilation index = 0.25 without dressing, 0.37 with dressing). The authors conclude: TcpO2 can be used to assess the degree of vasomotor tone (and sympathetic activity) in skin; tone generally increases as ischemia worsens; and local warmth can improve cutaneous circulation in ischemic limbs.