Prolonged QTc intervals on admission electrocardiograms: prevalence and correspondence with admission electrolyte abnormalities.

Background: Prolonged QTc (corrected QT) interval and torsades de pointes (TDP) are associated with hypocalcemia, hypomagnesemia, hypokalemia, possibly alkalosis and may result in syncope and sudden cardiac death.

Hypothesis: The purpose of this study was to determine the prevalence of prolonged QTc intervals on admission electrocardiograms (ECGs) and its correspondence with simultaneous admission electrolyte values, adjusted for admission diagnoses, comorbidities, other ECG abnormalities, and medications potentially associated with QTc prolongation.

Methods: Consecutive patients (N=258) admitted to a general medical service with the admission electrolytes and ECGs. For each ECG, QTc was classified in 10 msec. intervals from QTc > or = 450 to QTc > or = 500 msec. as determined by the electronically measured QTc (Marquette ECG system). Medical records were reviewed for prespecified admission diagnoses, comorbidities and medications associated with QTc prolongation. The correspondence of admission electrolyte abnormalities and QTc prolongation was evaluated using univariate and multivariate statistical methods.

Results: The prevalence of abnormal QTc intervals varied by the criterion applied, ranging from 25.2%, using the most lenient criterion of abnormality (QTc > or =450 msec.), to 3.5%, when the most restrictive criterion was applied (QTc > or = 500 msec.). In univariate analyses, there were no significant associations between QTc intervals and admission values for any of the electrolytes. In multivariate analyses, after adjusting for age, gender, ECG abnormalities and other covariates, none of the admission electrolyte values were significantly associated with prolonged QTc.

Conclusions: Using conventional criteria, electronically measured prolonged QTc intervals were quite common at the time of admission among general medicine service patients, hospitalized for non-cardiac complaints. Admission electrolyte values were not associated with QTc intervals. We conclude that the association of metabolic derangements and QTc abnormalities may not be as strong as is widely believed.