Heat loss during surgical skin preparation.

BACKGROUND: Hypothermia develops rapidly during the 1st h of anesthesia and results in part from evaporative heat loss during surgical skin preparation. The authors tested the hypothesis that evaporation of skin preparation solution contributes significantly to hypothermia.
METHODS: Five healthy, unanesthetized volunteers were studied in a 22 +/- 0.4 degrees C environment. One thigh of each volunteer was washed for 10 min, using each of the following representative solutions: (1) water; (2) 50% ethanol in water (EtOH/H2O; similar to tincture of iodine); and (3) povidone-iodine gel. Water and EtOH/H2O each were tested at ambient temperature (cold), warmed to 40 degrees C before application (warm), and with radiant heating of the skin, and gel only at ambient temperatures, resulting in seven study states. Heat loss and skin temperatures on the washed thighs were measured using thermal flux transducers, and values compared with the data obtained from the contralateral unwashed thighs. Change in mean body temperature (per 70 kg) due to washing was calculated by integrating measured heat loss over time and multiplying by the specific heat of human tissue. A mathematical model was developed to predict cutaneous heat loss using only skin temperature, independent of the type and temperature of skin-preparation solution or the use of radiant heating during preparation.
RESULTS: Heat loss from the unwashed thigh was approximately 14 kcal/m2 during radiant warming and approximately 39 kcal/m2 without warming. Net heat loss (increment produced by washing) was approximately 30 kcal/m2 with water and gel without radiant warming, but loss was larger with EtOH/H2O than with water under all study conditions. Radiant warming reduced total heat loss (increment produced by washing and environment) during both the EtOH/H2O and water trials, compared with warm or cold EtOH/H2O and water alone. The calculated decreases in mean body temperature per 70 kg ranged from -0.2 to -0.7 degree C/m2. The smallest decrease occurred during radiant warming and washing with water, and the largest decreases during warm or cold EtOH/H2O.
CONCLUSIONS: Heat loss was significantly less with water-based than with alcohol-based solutions. Though heating the solutions and radiant warming decreased heat loss, such loss under each tested condition, even per square meter of washed surface, was small compared to other causes of perioperative hypothermia. Consequently, the authors recommend that efforts to maintain intraoperative normothermia be directed elsewhere.