Frequency and characteristics of exercise-induced second-degree atrioventricular block in patients undergoing stress testing
Introduction
Exercise-induced second-degree atrioventricular block (Ex2AVB) is uncommon in the general population [1]. The frequency is higher in older, long-term endurance athletes (up to 8%) [2,3]. Depending on the level of the block, Ex2AVB can be an important limiting factor in the ability of a patient or an athlete to perform and may require placement of a permanent pacemaker (PPM) [4,5]. The largest study to date has included 14 patients in a case series [1].
Exercise, atropine, and other vagolytic maneuvers physiologically increase the sinus rate and decrease the refractory period of the atrioventricular (AV) node, thereby allowing 1:1 AV conduction without any blockade [1,6]. The pathophysiologic basis of a second-degree AV block (AVB) can be disease of the conduction system generally below the level of the AV node, which then affects the refractory period of that particular part of the conduction system [7,8]. The disease is usually located in the bundle of His or at the level of the bundle branches. Without a concomitant decrease in the refractory period at the distal part of the conduction system, second-degree AVB is likely to occur [9]. One theory for explaining the cause of disease at the end of the conduction system in endurance athletes is myocardial remodeling and myocardial fibrosis in the left atrium, the right atrium, and the right ventricle [10,11]. Benign resting second-degree AVB is generally due to increased vagal tone (e.g., from endurance training), and it usually disappears with an increased heart rate (HR) (e.g., with exercise) [6].
We sought to provide a clearer picture of the frequency and characteristics of Ex2AVB by describing the patient population presenting with Ex2AVB documented in a large exercise-testing database. We hypothesized that the number of patients would be small, but, when induced by exercise, Ex2AVB would generally require invasive treatment such as PPM implantation.
Section snippets
Methods
We retrospectively reviewed the Mayo Clinic Integrated Stress Center database for nonimaging tests performed from January 2006 through December 2010. All exercise test results coded as second-degree AVB were captured and reviewed through the electronic health record. The exercise testing protocol was the Bruce protocol or the Mayo protocol [12,13]. The Mayo exercise stress test protocol includes 2-minute stages beginning at 2 mph and 0% incline and increasing by 2 metabolic equivalent tasks at
Results
During the study period, 40,715 patients underwent nonimaging exercise stress tests. Baseline characteristics of the 40,715 patients included the following: mean ± SD age, 53.7 ± 14.7 years (range, 5–96 years); 14,268 (35%) were female; mean ± SD body mass index, 28.5 ± 5.6; 3787 (9.3%) had diabetes mellitus type 1 or 2; and 3106 (7.6%) were current smokers. Baseline cardiovascular status of the whole cohort (40,715 patients) is summarized in Fig. 1. Of the entire cohort, 6540 patients (16.1%)
Summary of findings
After reviewing a large database from the Integrated Stress Center at Mayo Clinic with 40,715 patients, we identified 19 patients (0.05%) who presented with Ex2AVB. Our findings confirmed that, despite Ex2AVB being uncommon, a small group of patients do have Ex2AVB with associated clinical symptoms such as dyspnea and syncope. Additionally, 7 of those 19 (37%) in our cohort who had Ex2AVB also received a PPM after the exercise test. Ex2AVB can be persistent or intermittent, and the patients in
Conclusion
Although Ex2AVB is uncommon, patients with Ex2AVB have limited exercise capacity. Our study results showed that Ex2AVB is more likely to be present in patients with congenital heart disease, AHD, or underlying CAD. The block generally occurs below the AV node (proximal or distal to the bundle of His or infra-Hisian). Exercise-induced AV block may require an EP study to determine the level of the block and the need for a PPM. Exercise testing is a noninvasive and effective method for evaluating
Conflict of interest
None.
Role of the funding source
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sections.
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Dr. Barout is now with UMass Memorial Medical Center, Worcester, Massachusetts.