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When It Comes to Exercise, Maybe More is Better
Abstract & Commentary
By Barbara A. Phillips, MD, MSPH, Professor of Medicine, University of Kentucky; Director, Sleep Disorders Center, Samaritan Hospital, Lexington. Dr. Phillips serves on the speaker's bureaus for Cephalon, Resmed, and Respironics.
Synopsis: In this meta-analysis, individuals who exercised or had sex episodically had an increased risk of acute cardiac events during those activities compared with those who had high levels of habitual physical activity.
Source: Dahabreh IJ, Paulus JK. Association of episodic physical and sexual activity with triggering of acute cardiac events: Systematic review and meta-analysis. JAMA 2011;305: 1225-1233.
This study resulted from a meta-analysis that was done to investigate the association of episodic physical or sexual activity with acute coronary syndromes, including myocardial infarction (MI) and sudden cardiac death. The investigators used combinations of search terms related to exposure (exertion, exercise, physical activity, sexual activity) and to outcomes (MI, acute coronary syndrome, sudden [cardiac] death). They also searched the reference lists of eligible studies. To be included in this analysis, studies had to have a case-crossover design and had to examine the effect of episodic physical or sexual activity (exposures) on the risk of acute coronary syndromes (outcomes). Each study was independently reviewed by two reviewers who extracted data about activity and risk, and standardized activity to make it comparable between studies. Studies were also assessed for validity and heterogeneity. The authors used data from the Framingham Heart Study1 and the U.S. vital statistics mortality data2 to establish the absolute (baseline) event rate for sudden cardiac death and MI.
Although the investigators identified more than 5000 articles that were potentially of use for this systemic review, they were left with only 13 eligible papers to include in the analysis after elimination of duplicate articles, reviews, editorials, letters, those without case-crossover design, those without data about the relevant outcomes or exposures, those with experimental design, and those not originally published in English.
Ten studies provided data on episodic physical activity, three on sexual activity, and one had data for both of these activities. Of the studies that assessed episodic physical activity, seven studies enrolled patients with MI, three enrolled patients with sudden cardiac death, and one enrolled patients with mixed diagnoses of acute coronary syndrome. All of the studies assessing sexual activity as the exposure of interest included only patients with MI. The individuals in the analyzed reports had a mean or median age of older than 60 years, and most were male. All 10 studies of episodic physical activity quantified the intensity of the exposure based on multiples of metabolic equivalents (METs). Moderate activity in all studies was fairly uniformly defined as exertion of at least 5 to 6 METs.
Overall, the studies suggested a strong association between episodic physical activity and MI (relative risk [RR] = 3.45, P < 0.001). Three studies assessed the potential of episodic physical activity to trigger sudden cardiac death. Overall there was evidence of an increase in the risk of sudden cardiac death triggered by episodic physical exertion (RR = 4.98, P = 0.01). Four studies (2960 patients) investigated the association between sexual activity and triggering of MI. Overall, sexual activity was associated with an acutely increased risk of infarction (RR = 2.70, P = 0.001)
Overall, subgroups of patients with higher habitual activity levels tended to be less susceptible to the triggering effect of causing a coronary event by episodic physical activity. In groups with the lowest habitual activity, the RR for the triggering effect of episodic physical activity ranged from 4.47 to 107 for MI, indicating a very substantial increase in risk during or immediately following exertion. The corresponding range in the highest habitual activity groups was 0.86 to 3.3, indicating much smaller increases in risk. Similar patterns were observed for the associations of episodic physical activity with sudden cardiac death and of sexual activity with MI, although the differences were less pronounced and fewer studies contributed data.
Based on estimates in these studies, the authors estimated that the RR of MI triggered by episodic physical activity was decreased by approximately 45% for each additional (unquantified) time per week a person was habitually exposed to physical activity. The relative risk of sudden cardiac death triggered by episodic physical activity was decreased by approximately 30% for each additional time per week a person was habitually exposed to physical activity. Unfortunately, the studies of sexual activity did not provide enough data to estimate the risk reduction afforded by the number of events of sexual activity per week. There was a dose-response relationship between episodic physical and both MI and sudden cardiac death regardless of the boundaries used to define physical activity levels.
This study got a lot of attention in the lay press, and your patients may be asking about it. What to say? First of all, there is a well-established beneficial effect of regular physical activity on the risk of acute coronary events,3 and regular physical exercise is clearly part of a healthy lifestyle. The current study shows that during the period of acute exposure to physical or sexual activity, an individual's risk of an event is increased compared with unexposed periods of time, but regular physical activity may reduce this risk by more than 30%. In this study, individuals with the lowest habitual levels of physical activity had the highest risk for all coronary events during exercise or sex. The authors of this study estimate that for each additional time an individual is exposed to physical activity per week, the relative risk of MI or sudden cardiac death associated with this exercise is reduced by approximately 45% and 30%, respectively.
There were a few things about this study that were not clear to me. First, what is "episodic" exercise? I believe that this is exercise less frequent than once a week, since most of the reviewed studies classified habitual exertion as weekly frequency based on the definitions used in the analyzed studies. And how much "exercise" or physical activity counts? In the reviewed studies, moderate physical activity was typically defined as exertion of at least 5 to 6 METs. Some activities that consume 5 to 6 METs are presented in the Table.
What this means to us clinically is that our advice about exercise needs to be tailored to each individual. Those who are sedentary should be counseled to increase the frequency and intensity of physical activity gradually. We can point out that exercising does not necessarily have to involve having a gym membership. House and garden work and walking are great forms of exercise that most people can work into their daily routines. And everybody needs to be counseled that regular exercise will reduce the risk of having an MI or of dying while exercising, or worse having sex.
1. Incidence and Prevalence: 2006 Chart Book on Cardiovascular and Lung Diseases. Bethesda, MD: National Heart, Lung, and Blood Institute; 2006.
2. Zheng ZJ, et al. Sudden cardiac death in the United States, 1989 to 1998. Circulation 2001;104:2158-2163.
3. Shiroma EJ, Lee IM. Physical activity and cardiovascular health: Lessons learned from epidemiological studies across age, gender, and race/ethnicity. Circulation 2010;122:743-752.
4. Goldman L, et al. Comparative reproducibility and validity of systems for assessing cardiovascular functional class: Advantages of a new specific activity scale. Circulation 1981;64:1227-1234.