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By Michael H. Crawford, MD
Professor of Medicine, Lucie Stern Chair in Cardiology, Director, Cardiology Fellowship Program, Chief of Clinical Cardiology, University of California, San Francisco
Dr. Crawford reports no financial relationships relevant to this field of study. This article originally appeared in the March 2014 issue of Clinical Cardiology Alert.
Synopsis: The authors concluded that in an apparently healthy, middle-aged population, a prolonged PR interval can normalize over time in almost one-third of subjects, and even if persistent, is not associated with death or cardiovascular morbidity and mortality.
Source: Aro AL, at al. Prognostic significance of prolonged PR interval in the general population. Eur Heart J 2014;35:123-129.
Although long believed to be benign, first-degree av block has recently been shown to be associated with atrial fibrillation development, pacemaker need, and all-cause mortality. This group of investigators from Finland had access to a 30-year follow-up study of almost 11,000 apparently healthy, middle-aged Finns where the prognostic value of the ECG PR interval could be assessed. In 10,957 men and women aged 30-59 years at entry between 1966-1972, a complete history, targeted physical examination, routine laboratory studies, and an ECG were done. They were followed for a mean of 30 ± 11 years until 2007. Less than 2% were lost to follow up. The endpoints of death, cardiovascular death, sudden death, and hospitalizations were obtained from Finland government data and medical record reviews. Also, a second ECG was performed after a mean of 6 years into the study.
A PR interval > 200 msec was observed in 2.1% of the subjects and was more common in obese older men and related to a slower heart rate and suspected cardiac disease. At the 6-year ECG, 71% of the subjects with a prolonged PR at entry still had it. After adjustment for age and sex, prolonged PR interval was not associated with all-cause cardiovascular or sudden death, and these results were not changed by multivariate adjustment, including use of a PR interval of > 220 msec. Also, the risk of hospitalization for atrial fibrillation, CAD, heart failure, or stroke did not differ from the rest of the population. The authors concluded that in an apparently healthy, middle-aged population, a prolonged PR interval can normalize over time in almost one-third of subjects, and even if persistent, is not associated with death or cardiovascular morbidity and mortality.
Occasionally, we see isolated PR interval prolongation on ECG in an otherwise healthy individual without overt cardiac disease and wonder if we should be concerned. This study in almost 11,000 middle-aged (30-59 years) subjects followed for a mean of 30 years sheds considerable light on the issue. When combined with prior studies, several conclusions can be reached. Overall, in an otherwise healthy, middle-aged population, first-degree AV block is not a harbinger of death, sudden death, or cardiovascular (CV) morbidity when adjusted for age, sex, and risk factors for CV disease. So why is the PR interval prolonged in some healthy people? It is related to heart rate and really should be adjusted for heart rate the way the QT interval is, but since this is only relevant at heart rates < 60 beats per minute we don’t bother. It is influenced by autonomic nervous system tone. Increased PR can be seen in athletes and others with high vagal tone. It has a circadian variation, so the time of day the ECG is done can be important. These observations probably explain the disappearance of first-degree block in this study at the 6-year follow-up in about one-third of the population studied. There is also a genetic component and it can be associated with atrial arrhythmias and the development of AV block. This is not surprising because we know there is a large genetic influence on the risk of developing atrial fibrillation.
Clearly a prolonged PR interval can be a harbinger of future conduction system disease in some people, especially older subjects where cardiac degenerative diseases are more common. Also, if a patient develops diastolic dysfunction, a prolonged PR interval can adversely affect diastolic filling and lead to diastolic mitral valve regurgitation, which under the right conditions could contribute to the development of heart failure. So how should we handle patients with first-degree heart block? If they are young and overtly healthy, I would repeat the ECG in 1-3 years because it may just disappear with age or deconditioning. In older patients, it would prompt me to repeat the ECG at the time of their routine health maintenance follow-up exams, which is usually every 1-2 years depending on age. However, patients should be reassured that most people with this finding have normal longevity and are free from CV disease.