SHIVERING THRESHOLD DURING SPINAL-ANESTHESIA REDUCED IN ELDERLY PATIENTS

Background: Both accidental and perioperative hypothermia are common i n the elderly. The elderly are at risk because their responses to hypo thermia may be delayed or less efficient than in those of younger subj ects. For example, the vasoconstriction threshold during isoflurane an esthesia is approximate to 1 degrees C less In elderly than younger pa tients. However, the extent to which other cold defenses are impaired in the elderly remains unclear, especially in those older than 80 yr. Operations suitable for spinal anesthesia provided an opportunity to q uantify shivering thresholds in patients of varying ages. Accordingly, the hypothesis that the shivering threshold is reduced as a function of age during spinal anesthesia was tested. Methods: Twenty-eight ASA Physical Status 1-3 patients undergoing lower extremity orthopedic pro cedures were studied. Spinal anesthesia was induced without preanesthe tic medication, using bupivacaine sufficient to produce a dermatomal l evel near T9. Electrocardiogram signals were recorded at 10-min interv als. Subsequently, an observer masked to patient age and core temperat ure identified the onset of sustained electromyographic artifact consi stent with shivering. The tympanic membrane temperature triggering shi vering identified the threshold. Results: Three patients did not shive r at minimum core temperatures exceeding 36.2 degrees C. Fifteen patie nts aged <80 yr (58 +/- 10 yr) shivered at 36.1 +/- 0.6 degrees C; in contrast, ten patients aged greater than or equal to 80 yr (89 +/- 7 y r) shivered at a significantly lower mean temperature, 35.2 +/- 0.7 de grees C (P = 0.002). The shivering thresholds in seven of the ten pati ents older than 80 yr was less than 35.5 degrees C, whereas the thresh old equaled or exceeded this value In all younger patients (P = 0.0002 ). Conclusions: Age-dependent inhibition of autonomic thermoregulatory control in the elderly might be expected to result in hypothermia. Th at It usually does not suggests that behavioral regulation (e.g., incr easing ambient temperature, dressing warmly) compensates for impaired autonomic control. Elderly patients undergoing spinal anesthesia, howe ver, may be especially at risk of hypothermia because low core tempera tures may not trigger protective autonomic responses. Furthermore, hyp othermia in the elderly given regional anesthesia may not be perceived by the patient (who typically feels less cold after induction of the block), or by the anesthesiologist (who does not observe shivering). C onsequently, temperature monitoring and management usually is indicate d in these patients.