QTc-measurements: a case-control study on serum electrolytes

This is a report on a prospective case-control study, in which a group of patients with an electrocardiographic diagnosis of QTc-prolongation (cases) was compared with a group of patients with a normal QTc (controls) for the presence of potential causative factors. Twelve cases and twelve carefully-matched controls were entered during a 1-year recruitment period. Ventricular arrhythmias were more frequent amongst the cases than amongst the controls (7/12 the cases, only 2/12 controls). Blood was sampled around the time of the ECG-recording in all patients. Forearm exercise (pumping) to increase venous blood pooling was prohibited before this blood sampling. The serum cation levels (potassium, calcium, magnesium) were determined in one central laboratory. Serum potassium levels were inversely correlated with the QTc-interval. Such a correlation was not found for the other two cations, although calcium levels also tended to be lower in patients with a long QTc. The findings, in addition, suggest that serum potassium values in the lower normal range (i.e. < 4 meq/l) might predispose to the development of ventricular arrhythmias. In view of these results, the diagnostic work-up of a patient with a long QTc is incomplete without a serum--K+, and possibly serum--Ca2+, assessment.