Thermal Management of the Surgical Patient

Sigg DC, Houlton AJ, Iaizzo PA: The potential for increased risk of infection due to the reuse of convective-air warming/cooling coverlets. Acta Anaesthesia Scandinavica. 43: 173-176, 1999.

SUMMARY

Background: The use of convective air warming and/or cooling for the prevention of hypothermia or to induce hypothermia is growing rapidly. To date, there is no information available as to the potential risks for infection associated with either the postsurgical reuse or the repositioning of coverlets closer to the wound. We hypothesized that use of coverlets either intra- or postoperatively leads to increased contamination.

Methods: The bacterial contamination of commercially available coverlets before (control group, n=10) and after patient application (n=18) was investigated. From three predetermined sites, 1cm x 2cm pieces of coverlet were removed and analyzed for bacterial contamination.

Results: Even prior to use, coverlet samplings provided identifiable contamination (3 out of 30 sites, 10%), but this could be within our study's sampling error. Nevertheless, following clinical use the frequency for contamination was considerably increased; 17 out of 57 sampled sites (31.5%) elicited contamination (p<0.05, Fisher's exact test).

Conclusion: This study demonstrates, that the use of the coverlets, intra- or postoperatively, can lead to significant bacterial contamination. It is concluded that it is not advisable to reuse coverlets for multiple clinical applications.

Iaizzo PA, Jeon Y, Sigg D: Facial warming increases the threshold for shivering. Journal of Neurosurgical Anesthesiology 11:231-239, 1999.

SUMMARY

Background: As little as 1°C core hypothermia provides significant protection against cerebral ischemia. However, shivering usually prevents induction of hypothermia in unanesthetized subjects. We, therefore, tested the hypothesis that facial and airway heating reduces the shivering threshold and enables core cooling in unanesthetized subjects. Such a procedure could be applied to unanesthetized or lightly sedated neuropatients (head injury, acute stroke) in the ICU or even initiated while these patients are being transported to a medical center.

Methods: Nine experimental trials were performed on a group of seven healthy male volunteers. Each subject was positioned supine on a circulating-water mattress (8-15 °C) with a convective air coverlet (15-18°C) extending from the neck to the feet. A dynamic study protocol governed by individualized physiological responses was utilized. Focal facial (and airway) warming was employed to suppress such involuntary motor activity and, thereby, to enable non-invasive cooling to lower the core temperature. The following parameters were monitored throughout: 1) heart rate; 2) blood pressure; 3) core temperature (tympanic, axilla and rectal); 4) cutaneous temperatures; and 5) a subjective shiver index (scale 1-10). In three of the experimental trials, electromyograms (EMG) and infra-red thermographs were also obtained.

Results: Upon cooling without facial and airway warming, involuntary motor activity (muscle tensing and shiver) increased until it was widespread. This vigorous motor activity prevented any significant lowering of core temperature or caused it to slightly increase. Subsequently, in all subjects, within seconds after the application of facial focal warming, motor activity was suppressed almost completely, and within minutes core temperatures significantly decreased. Also, concomitant with facial and airway warming was an increased subjective tolerance for the active cooling.

Conclusion: Preliminary studies described here indicate that focal facial warming applied during active whole body cooling to induce hypothermia minimizes the need to pharmacologically suppress involuntary motor activity. Such a procedure could be used to initiate, as soon as possible (e.g., during emergency transport), cerebral mild hypothermia in order to maximize protection and thus improve outcome in appropriate patients.