ObjectiveTo determine the sympathetic response during cardiopulmonary bypass at mild (34[degree sign]C) and moderate (28[degree sign]C) hypothermia.DesignA randomized study.SettingTertiary university hospital.PatientsAdults undergoing elective coronary artery bypass graft surgery.InterventionsNone.Measurements and Main ResultsPlasma norepinephrine, eplnephrine, and neuropeptide Y concentrations were measured. Moderate, but not mild, hypothermic cardiopulmonary bypass evoked a significant sympathetic response with increases in plasma norepinephrine and neuropeptide Y concentrations. A significantly higher incidence of postoperative atrial fibrillation was also observed in the moderate hypothermic compared with the mild hypothermic group.ConclusionsOur results indicate that the degree of hypothermia significantly influences the sympathetic response during cardiopulmonary bypass. The higher incidence of postoperative atrial fibrillation in the moderate hypothermic group suggests that the enhanced sympathetic response might be one contributing factor in the development of atrial fibrillation. (Crit Care Med 1997; 25:1990-1993)