Prognostic Value of a Prolonged PR Interval
Prognostic Value of a Prolonged PR Interval
Abstract & Commentary
By John P. DiMarco, MD, PhD
Source: Cheng S, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA. 2009;301:2571-2577.
This report from the Framingham heart study looks at the prognostic significance of PR interval prolongation in ambulatory, relatively healthy individuals. The baseline for this report was the 11th biennial examination of the original Framingham cohort performed in the years 1968 to 1971 and the first offspring cohort examination, which was performed between 1971 and 1974. Patients with a history of atrial fibrillation or use of antiarrhythmic drugs or a pacemaker at baseline were excluded. This left 7,575 participants who had an ECG with an analyzable PR interval. A single lead (lead II) was used for analysis, and measurements were made by technicians using digital calipers and a magnifying tablet. Subsequent examinations of the original cohort were performed every two years and in the offspring cohort every four years. Patients in the Framingham Study are routinely followed for death and cardiovascular events, including myocardial infarction, coronary insufficiency, stroke, and heart failure. In addition, data on the occurrence of atrial fibrillation and pacemaker implantations are also recorded. The prognostic value of the PR interval was analyzed first using the PR as a continuous variable and then as a categorical variable based on the standard definition of first-degree AV block (PR interval > 200 m/sec). Multivariate models were created that adjusted for age, heart rate, hypertension, body mass index, lipid status, smoking, and diabetes. For the AF analysis, valvular heart disease, left ventricular hypertrophy, and atrial premature beats were included as additional covariates.
At enrollment, the mean age of the group was 47 years; 54% were women. One-third of the individuals had hypertension, but only 2% had a history of myocardial infarction or heart failure. The median PR interval was 149 m/sec; 124 participants had a PR interval of > 200 m/sec. During the follow-up period, 481 participants developed atrial fibrillation, 124 patients received a permanent pacemaker, and 1,739 participants died. The PR interval was related to the development of atrial fibrillation, the need for pacemaker implantation, and all-cause mortality. For patients having first-degree AV block (PR > 200 m/sec), the absolute risk increase for atrial fibrillation was 1.04% per person-year, for pacemaker implantation 0.53%, and all-cause mortality 2.05%. Cox proportional hazard model showed that each 20-second increment in the PR interval was associated with an adjusted hazard ratio of 1.11 for atrial fibrillation, 1.22 for pacemaker implantation, and 1.08 for all-cause mortality. For patients with or without first-degree AV block, the hazard ratios were 2.06 for atrial fibrillation, 2.89 for pacemaker implantation, and 1.44 for-all cause mortality. During long-term follow-up, 161 (3%) individuals developed new first-degree AV block in the 12 years after their follow-up examination. New first-degree AV block was associated with increased risks for atrial fibrillation and pacemaker implantation, but was not associated with all-cause mortality.
Cheng et al conclude that PR prolongation is related to risk for atrial fibrillation, pacemaker implantation, and all-cause mortality.
Commentary
The PR interval is the sum conduction through the right atrium, the AV node, and the His-Purkinje system. Intra-atrial conduction and His-Purkinje conduction are relatively independent of heart rate and autonomic tone, but the AH interval, the measure of AV nodal conduction, is both rate-dependent and highly sensitive to changes in autonomic tone. Cheng et al show that PR interval prolongation predicts several important cardiovascular events, though this paper won't be terribly helpful to clinicians. Although an increased relative risk was shown with PR intervals above the median of 149 m/sec, the absolute risk of atrial fibrillation, pacemaker implantation, or mortality is not great, even in those with PR intervals greater than 200 m/sec. Therefore, no specific action should be taken if first-degree AV block were detected. The value of these data on the PR interval is likely to be for calculating risk scores for atrial fibrillation. As shown in an earlier paper from the same group (Lancet. 2009;373:739-745), mild or moderate PR interval prolongation may be included with other factors to yield a risk score for atrial fibrillation. By itself, the PR interval, in any individual, will require interpretation, and will justify therapy only in rare cases.
This report from the Framingham heart study looks at the prognostic significance of PR interval prolongation in ambulatory, relatively healthy individuals.Subscribe Now for Access
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