Epidural anesthesia reduces the gain and maximum intensity of shivering

Shivering can be characterized by its threshold (triggering core temperature), gain (incremental intensity increase), and maximum intensity. The gain of shivering might be preserved during epidural or spinal anesthesia if control mechanisms compensate for lower-body paralysis by augmenting the activity of upper-body muscles. Conversely, gain will be reduced approximately by half if the thermoregulatory system fails to compensate. Similarly, appropriate regulatory feedback might maintain maximum shivering intensity during regional anesthesia. Accordingly, the gain and maximum intensity of shivering during epidural anesthesia were determined.

Seven volunteers participated on two randomly ordered study days. On one day (control), no anesthesia was administered; on the other, epidural anesthesia was maintained at a T8 sensory level. Shivering, at a mean skin temperature near 33 degrees C, was provoked by central-venous infusion of cold fluid; core cooling continued until shivering intensity no longer increased. Shivering was evaluated by systemic oxygen consumption and electromyography of two upper-body and two lower-body muscles. The core temperature triggering an increase in oxygen consumption identified the shivering threshold. The slopes of the oxygen consumption versus core temperature and electromyographic intensity versus core temperature regressions identified systemic and regional shivering gains, respectively.

The shivering threshold was reduced by epidural anesthesia by approximately 0.4 degrees C, from 36.7 +/- 0.6 to 36.3 +/- 0.5 degrees C (means +/- SD; P < 0.05). Systemic gain, as determined by oxygen consumption, was reduced from -581 +/- 186 to -215 +/- 154 ml x min(-1) x degrees C(-1) (P < 0.01). Lower-body gain, as determined electromyographically, was essentially obliterated by paralysis during epidural anesthesia, decreasing from -0.73 +/- 0.85 to -0.04 +/- 0.06 intensity units/degrees C (P < 0.01). However, upper-body gain had no compensatory increase: -1.3 +/- 1.1 units/degrees C control versus 2.0 +/- 2.1 units/degrees C epidural. Maximum oxygen consumption was decreased by one third during epidural anesthesia: 607 +/- 82 versus 412 +/- 50 ml/min (P < 0.05).

These results confirm that regional anesthesia reduces the shivering threshold. Epidural anesthesia reduced the gain of shivering by 63% because upper-body muscles failed to compensate for lower-body paralysis. The thermoregulatory system thus fails to recognize that regional anesthesia reduces metabolic heat production, instead responding as if lower-body muscular activity remained intact.