The Influence of Sympathetic Nerves on Transcutaneous Oxygen Tension in Normal and Ischemic Lower Extremities

Abstract

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 ° C and 45°C; in theory, changes in sympathetic activity should affect vasomotor tone and TcpO₂ in skin beneath an electrode at 42 °C (submaximal vasodilation), but not at 45°C (maximal vasodilation). The vasodilation index (TcpO₂ at 42°C/TcpO ₂ at 45°C) was created as an index of vasomotor tone (vasodilation index increases as tone decreases). In normal limbs (n=24) averages for TcpO ₂ at 42°C, TcpO₂ at 45°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, TcpO ₂ at 42 ° C and vasodilation index were increased (45.0 mmHg and 0.61); TcpO₂ at 45 ° C did not change. When normal subjects (n=7) were chilled for twenty minutes with a cooling blanket at 5 ° 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=0.25without dressing, 0.37 with dressing). The authors conclude: (1) TcpO₂ can be used to assess the degree of vasomotor tone (and sympathetic activity) in skin; (2) tone generally increases as ischemia worsens; and (3) local warmth can improve cutaneous circulation in ischemic limbs.