BackgroundClinical studies have implicated surgery in promoting infections and compromising immune functions, including natural killer cell activity. Animal studies indicate that surgery‐induced suppression of natural killer cell activity also promotes tumor metastasis. Hypothermia, a common surgical complication, has been suggested to underlie some of the deleterious consequences of surgery. This study evaluated the effect of hypothermia on the activity and number of blood natural killer cells and on host susceptibility to metastasis. The involvement of adrenergic mechanisms was also considered.MethodsFischer‐344 rats remained awake in their cages (control group) or were anesthetized with 70 mg/kg thiopental and maintained for 2.5 h at core body temperatures of 30–32°C (hypothermia group) or 38°C (normothermia group). Thereafter, at several time points, blood was drawn so natural killer cell activity could be assessed, or rats were injected with syngeneic MADB106 tumor cells that metastasize only to the lungs. Lungs were removed 9 h later for assessment of lung tumor retention, or 4 weeks later for counting of metastases.ResultsNormothermic anesthesia reduced natural killer cell activity (lytic units at 30% specific killing, mean ± SEM) to 39 ± 6.2% of control levels and hypothermia further reduced it to 15 ± 6.6%. These changes were not accompanied by alterations in the numbers of circulating natural killer cells. Hypothermia increased tumor retention to 250% of control levels, and the number of metastases increased from 1.1 ± 0.4 to 4.7 ± 1.2. Normothermia had no significant effects on this index. Nadolol (0.4 mg/kg), a &bgr;‐adrenergic antagonist, significantly attenuated the effect of hypothermia on tumor retention.ConclusionsHypothermia under thiopental anesthesia suppresses natural killer cell activity and compromises host resistance to metastatic formation, possiblyviaadrenergic mechanisms. Such suppression may place patients with metastasizing tumors or dormant viral infections at greater risk for complications after intraoperative hypothermia.