Noninvasive transcutaneous cardiac pacing (NTP) is a rapid, safe, and easily utilized form of emergency cardiac pacing, with hemodynamics similar to right ventricular endocardial pacing. Although the technique has proven effective for hemodynamically significant bradycardias and early use during cardiopulmonary resuscitation, NTP under anesthetic conditions has been poorly characterized. In particular, it is unknown to what degree the multiple physiologic perturbations of cardiac surgery and cardiopulmonary bypass (CPB) affect myocardial thresholds and the efficacy of the unit itself. Patients undergoing procedures utilizing CPB (n = 23) were studied in an effort to address these issues. All patients were able to be paced at all points throughout the 24-h study interval, although four patients developed hemodynamic instability during this period causing their exclusion from additional investigation. Only one patient requested discontinuation from the study due to discomfort. A statistically significant increase in mean current requirements for capture was demonstrated over time (P less than 0.0001), with baseline thresholds being significantly less than other study points (P less than or equal to 0.05). Thresholds following chest wall closure were significantly greater than all other study points (P less than or equal to 0.05), possibly due to accumulation of pericardial and mediastinal air. Multiple measured variables changed significantly during the study, but only increases in cardiac output and core temperature were related to statistically significant increases in current thresholds (P less than or equal to 0.05). Increasing age and pump time were of borderline importance. NTP represents an effective pacing alternative in cardiac surgical patients.