Prolonged QT interval: accuracy of the '' rule of thumb'' method for measuring the QT interval in the elderly attending geriatric clinical practices
Background Long QT syndrome (LQTS) is characterized by a prolonged QT interval on the electrocardiogram (ECG), a risk for sudden cardiac death. A simple 'rule of thumb' method states that if a patient's heart rate is between 60-100 bpm, the QT interval should not be more than half the R-R interval. The clinical accuracy of this method has not been tested in the elderly. Objectives To determine if the 'rule of thumb' to calculate QT interval prolongation, is accurate, compared to the corrected QT interval calculated using Bazett's formula. Secondary objectives include the prevalence of long QT and risk factors for QT prolongation. Methods The QT interval was calculated using Bazett's formula, and the 'rule of thumb' method, from ECG's collected from patients over 60 years old, on their first visit to a geriatric clinical service. Only data from patients with heart rates in the range 60-100 were analyzed. Medications and electrolyte levels were recorded. Results A total of 1000 ECGs were collected. 776 ECGs were included in the study. Prevalence of prolonged QT interval was 37.8% using Bazett's formula. Compared to Bazett's formula, the 'rule of thumb' method had a sensitivity of 65.2% and a specificity of 96.9%. ECG computer analysis calculated QTc was available for 42.5% of the cases and had a sensitivity of 58.1% and specificity of 95.3% compared to Bazett's formula. Of the 23.3% of patients taking medications known to prolong the QT interval only 38.6% had a prolonged QT interval. There was a significant association between QT interval prolongation and hypokalaemia. Conclusion The 'rule of thumb' method to determine QT interval prolongation, has high specificity but low sensitivity. This bedside measure is similar in accuracy to QT determination using an ECG computer analysis calculation in this population of older persons.
Rule of Thumb