Objective:To determine whether a correlation exists between concentrations of intracellular and extracellular potassium and to determine the frequency of ventricular ectopy in patients after cardiac operations.Design:Prospective, observational clinical evaluation.Setting:Surgical-respiratory intensive care unit of a university-affiliated tertiary care center.Patients:Continuous 24-hr electrocardiographic monitoring was performed, and serum (extracellular) and erythrocyte (intra-cellular) potassium concentrations ([K+]eand [K+]i) were determined, before cardiopulmonary bypass, immediately postoperatively, and at 2, 4, 12, and 20 hrs after elective coronary bypass grafting in 31 patients.Interventions:None. Potassium replacement was left to the discretion of the attending physicians.Measurements and Main Results:Although the mean [K+]evaried significantly during the postoperative 24-hr period (p< .0001), the [K+]idid not (p= .953). No significant correlations were found between premature ventricular beats and [K+]i, [K+]e, or [K+]i/[K+]e(allp> .05). However, among the few patients who had one or more episodes of ventricular tachycardia (VT) within 30 mins of a study K+sample, the mean [K+]ewas significantly lower during the episode(s) of VT compared with the mean [K+]ein the absence of VT (p< .01).Conclusions:Although it is clear that over the clinically acceptable range of [K+]eand [K+]iconcentrations seen in this population, there is no correlation between potassium concentrations and the occurrence of premature ventricular beats, the infrequent association of more serious ventricular ectopy, VT, with lower [K+]econcentrations supports the practice of using serum potassium to guide potassium replacement in patients after cardiac operations.