Physical Exertion as a Trigger of Acute Myocardial Infarction
Stefan N. Willich, Michael Lewis, Hannelore Lowel, Hans-Richard Arntz, Frauke Schubert, Rolf Schroder, for The Triggers and Mechanisms of Myocardial Infarction Study Group
Background It is controversial whether the onset of myocardialinfarction occurs randomly or is precipitated by identifiablestimuli. Previous studies have suggested a higher risk of cardiacevents in association with exertion.
Methods Consecutive patients with acute myocardial infarctionwere identified by recording all admissions to our hospitalin Berlin and by monitoring a general population of 330,000residents in Augsburg, Germany. Information on the circumstancesof each infarction was obtained by means of standardized interviews.The data analysis included a comparison of patients with matchedcontrols and a case-crossover comparison (one in which eachpatient serves as his or her own control) of the patient's usualfrequency of exertion with the last episode of exertion beforethe onset of myocardial infarction.
Results From January 1989 through December 1991, 1194 patients(74 percent of whom were men; mean age [±SD], 61 ±9years) completed the interview 13 ±6 days after infarction.We found that 7.1 percent of the case patients had engaged inphysical exertion ( 6 metabolic equivalents) at the onset ofinfarction, as compared with 3.9 percent of the controls atthe onset of the control event. For the patients as comparedwith the matched controls, the adjusted relative risk of havingengaged in strenuous physical activity at the onset of infarctionor the control event was 2.1 (95 percent confidence interval,1.1 to 3.6). The case-crossover comparison yielded a similarrelative risk of 2.1 (95 percent confidence interval, 1.6 to3.1) for having engaged in strenuous physical activity withinone hour before myocardial infarction. Patients whose frequencyof regular exercise was less than four and four or more timesper week had relative risks of 6.9 and 1.3, respectively (P<0.01).
Conclusions A period of strenuous physical activity is associatedwith a temporary increase in the risk of having a myocardialinfarction, particularly among patients who exercise infrequently.These findings should aid in the identification of the triggeringmechanisms for myocardial infarction and improve preventionof this common and serious disorder.
Throughout this century, the possible association of externalstimuli with the onset of acute myocardial infarction has beendebated1,2,3,4,5,6,7,8,9,10. Among the factors that have beensuggested as precipitating myocardial infarction are strenuousphysical activity and emotionally upsetting life events. Somestudies have suggested that cardiac events occur with increasedfrequency during or within a few hours after physical exertion1,2,3,8,9,10.Others have concluded that acute myocardial infarction is arandom event among patients with coronary artery disease4,7.Many of these studies, however, were based on anecdotal reports,studied selected patient populations, or were limited by thelack of adequate control data.
The documentation of circadian variation in the incidence ofacute myocardial infarction indicated that the onset of infarctionis not a random event; such studies thus provided a basis forthe study of the triggering mechanisms11,12,13. The Triggersand Mechanisms of Myocardial Infarction Study was designed todetermine the frequency and importance of physical activityand other external stimuli associated with the transition fromchronic coronary artery disease to acute nonfatal myocardialinfarction.
Methods
Study Population
In Berlin, Germany, data were recorded for all patients admittedto the coronary care unit at Klinikum Steglitz with a primarydiagnosis of acute myocardial infarction. In Augsburg (a midsizecity in southern Germany), a predefined general population of330,000 residents (48 percent men and 52 percent women, 25 through74 years of age) was monitored with use of the registrationprocedures developed for the Monitoring Trends and Determinantsin Cardiovascular Disease project of the World Health Organization14.The study personnel in Augsburg identified patients with myocardialinfarctions by routine monitoring of hospitalizations in theCentral Hospital (on a daily basis), in the other 12 hospitalswithin the study area (once a week), and in 13 hospitals adjacentto the study area (once a week)15.
The diagnostic criteria for acute myocardial infarction includedchest pain lasting 20 minutes or more that was not relievedby nitrates, electrocardiographic changes suggestive of evolvingmyocardial infarction according to the Minnesota coding system,16and a subsequent increase in the level of at least one of threecardiac enzymes (creatine kinase, aspartate aminotransferase,and lactate dehydrogenase) to more than twice the upper limitof normal. Electrocardiographic data (available for 98 percentof the patients) were coded independently by two specially trainedcoders, and a final decision about the diagnosis was subsequentlymade by a supervisor. Exclusion criteria for the study werepoor health (such as a critical illness or moribund condition),inability to communicate with the investigators, and death beforethe scheduled interview. Informed consent for participationwas requested and received from all patients.
Control subjects were recruited from the general populationof Augsburg and matched to the Augsburg patients in terms ofage, sex, and precinct. Case patients were frequency-matchedto controls in a projected ratio of 2:1. All residents in Germanyare legally required to register with the local administration.After approval of the study by the Department of Health, stateofficials in Augsburg and the surrounding communities provideda stratified random sample of residents matched to the casepatients. Within seven days after mailing them a request forcooperation in a "study on heart disease," we contacted themembers of the sample by telephone and asked them to participatein the study. Seventy percent of the initial random sample ofcontrols were successfully contacted and consented to a telephoneinterview. The stratification scheme was preserved by the useof backup stratified samples of matched residents.
Collection of Data
All the study patients underwent a standardized interview aftertheir transfer from the coronary care unit to a general ward.The interview covered demographic variables, medical history,cardiac risk factors, the timing and characteristics of symptoms,physical activity, unusual life events, location and circumstancesof the myocardial infarction, sleeping and waking habits, thetimes of awakening and arising, medications used, typical mealsand activities, and any factors suspected by the patients oftriggering myocardial infarction.
During an initial pilot phase,17 the questionnaire was testedand revised to improve its clarity, minimize redundancy, andfacilitate statistical analysis. The interviews with the patientswere conducted by three study nurses with extensive clinicalexperience with cardiovascular disease. Adherence to standardizedinterview and coding procedures was ensured by the initial trainingof the interviewers, subsequent routine supervision, and a policyof immediately contacting the study investigators in the caseof unforeseen problems. If necessary, supporting informationwas retrieved from the patients' medical records or obtainedfrom their physicians (for example, to obtain complete informationon medications). If data conflicted, medical reports were consideredmore reliable than information furnished by the patients.
The telephone interview with the controls was conducted by studypersonnel who used a standardized questionnaire that was almostidentical to the patient-interview form but that excluded questionsabout acute symptoms and suspected triggers of myocardial infarction.The controls were instructed by the interviewer to assume thatthey had experienced acute chest pain (the "control events")at specific times within the previous 24 hours, distributedin a circadian pattern identical to the pattern of onset ofmyocardial infarction among the patients.
Physical activity was measured in terms of metabolic equivalents(MET), defined as the energy expended per minute by a restingsubject. Activity was categorized as follows: sleep (1 MET),sedentary activity (2 MET), mild-to-moderate activity (3 to5 MET), or strenuous activity ( 6 MET) on the basis of the subjects'assessment; this method was similar to that used by Paffenbargeret al18. The time of occurrence of the myocardial infarctionwas defined as time of onset of the most severe symptoms.
Statistical Analysis
After the questionnaires were checked for completeness and accuracy,the data were coded and double-entered into a D-BASE IV database on a PS/2-70 computer, and the plausibility of the responseswas checked. The data were evaluated with SAS and Epi Info 5statistical-software packages.
The primary statistical approach (Figure 1) was a case-controlanalysis of the frequency of exposure (in this case, the frequencyof strenuous physical activity) in the patients as comparedwith the matched controls. We also used a self-matched case-crossovermethod that was recently developed to identify transient effectson the risk of acute events19. Briefly, the patients' usualfrequency of physical exertion over the past year (for purposesof the analysis, the daily frequency of strenuous activity assuming16 potential one-hour episodes of activity per day and eighthours of sleep) was compared with the length of time betweenthe last episode of physical activity and the onset of myocardialinfarction. The risk period was prospectively defined as thehour before the onset of myocardial infarction. For each patient,we calculated the observed odds of occurrence of physical exertion(1:0 or 0:1) within the hour before the onset of myocardialinfarction and the expected odds (x:y) that exertion would haveoccurred within one hour before the onset of symptoms, giventhe usual frequency of physical exertion. Algebraically, thesum of y in patients who reported exertion within one hour beforeonset represents the numerator and the sum of x in patientswho reported no exertion represents the denominator of the riskratio.
The time point 0 indicates the onset of acute myocardial infarction in the patients and coincides with the control event for the matched controls (a specified time during the 24 hours before the interview). The case-control analysis compared the patients' exposure to a suspected trigger (strenuous physical activity) before the onset of myocardial infarction (risk period) with the controls' exposure to the same trigger before the control event ("risk" period). An additional case-crossover analysis compared the patients' exposure to a suspected trigger (strenuous physical activity) during the risk period with their usual frequency of exposure.
In our analysis of the length of time from exertion to the onsetof myocardial infarction, adjusted for the time of awakening,the probability of onset in patients in whom symptoms were presentwhen they awakened was evenly distributed during the six hoursbefore the time of awakening, as described previously17.
Conventional statistical techniques were used to determine thedifferences between the study groups in terms of categoricalor continuous variables. The relative risk of myocardial infarction(odds ratio) associated with a suspected trigger and a correspondingestimate of the variance of its logarithm were calculated withuse of the Mantel-Haenszel procedure19,20,21,22. Multivariatelogistic-regression analysis was performed to adjust for factorsknown to influence the risk of myocardial infarction or to differbetween cases and controls. All P values were two-tailed; valuesbelow 0.05 were considered to indicate statistical significance.
Results
Study Population
From January 1989 through December 1991, a total of 1194 patients(74 percent men and 26 percent women; mean age [±SD],61 ±9 years) were enrolled in the study and completedthe interview 13 ±6 days after myocardial infarction.Of all patients hospitalized during this period with documentednonfatal myocardial infarctions, 75 percent were eligible andconsented to participate in the study. The differences betweenthe 224 patients from Berlin and the 970 patients from Augsburg(Table 1) were probably related mainly to the study design.A total of 532 controls were matched to the Augsburg patients(Table 1). The controls did not differ from the patients inage, sex, marital status, proportion of blue-collar workers,educational level, or usual frequency of physical exertion,but a higher proportion of the control group was employed. Aswould be expected, there were substantial differences betweenthe patients and the controls in medical history and use ofcardiac medications (Table 1).
Table 1. Demographic Characteristics and Medical History of the Study Population.
Physical-Activity Status
As compared with the matched controls, the Augsburg patientshad a crude relative risk of 1.9 (95 percent confidence interval,1.1 to 3.5) of having engaged in strenuous physical activity( 6 MET) at the onset of myocardial infarction (69 patients[7.1 percent] vs. 21 controls [3.9 percent]). The activitiesreported by the patients included housework (9 patients), physicalactivity at work (6), yardwork (4), shopping and carrying bags(8), lifting heavy objects (6), walking up flights of stairs(8), jogging (5), bicycling (2), aerobics (1), and other activities(19); in the case of 1 additional patient, myocardial infarctionbegan during an exercise treadmill test. Patients were lesslikely to have been sleeping at the onset of myocardial infarctionthan were the controls at the times of the control events (194patients [20 percent] vs. 157 controls [30 percent]). A totalof 509 patients (52 percent) and 233 controls (44 percent) hadbeen sedentary, and 198 patients (20 percent) and 121 controls(23 percent) had been engaged in mild-to-moderate activity;these percentages did not differ significantly between patientsand controls.
Multivariate logistic-regression analysis with adjustment forfactors known to influence the risk of myocardial infarctionor to differ between case patients and controls confirmed anindependent relative risk of 2.1 (95 percent confidence interval,1.1 to 3.6) that the patients had engaged in physical exertionat the onset of myocardial infarction (Table 2).
Table 2. Adjusted Odds Ratios of Physical-Activity States among Augsburg Patients at the Onset of Myocardial Infarction as Compared with Controls.
Patients with a previous diagnosis of coronary heart disease(myocardial infarction or angina pectoris) appeared not to havean increased risk in association with physical exertion (Table 3),but this finding may be an artifact caused by the smallnumber of controls who reported physical activity. Subgroupsof patients defined according to sex and age had similar relativerisks of myocardial infarction in association with physicalexertion: men, 2.0 (95 percent confidence interval, 0.9 to 3.7),and women, 2.0 (95 percent confidence interval, 0.5 to 9.0);patients 60 years of age or older, 2.4 (95 percent confidenceinterval, 1.0 to 5.9), and patients less than 60 years of age,1.6 (95 percent confidence interval, 0.8 to 3.5). Among patientswho reported exertion before the event, the onset of myocardialinfarction was fairly evenly distributed throughout the day.
Table 3. Effect of Physical Exertion in Augsburg Patients at the Onset of Myocardial Infarction as Compared with Controls, According to Presence or Absence of Previous Coronary Heart Disease.
The case-crossover comparison of the combined patient population,in which each patient served as his or her own control (Table 4),yielded a relative risk of 2.1 (95 percent confidence interval,1.6 to 3.1) of having engaged in strenuous physical activitywithin the hour before the onset of myocardial infarction; forpatients in Berlin, the relative risk of having engaged in physicalexertion was 3.2 (95 percent confidence interval, 1.5 to 6.9),whereas for patients in Augsburg the relative risk was 1.9 (95percent confidence interval, 1.3 to 2.9). The subgroups of patientswho usually engaged in exertional physical activity less thanfour times a week (n = 125) had a relative risk of having engagedin physical exertion in the hour before the onset of myocardialinfarction of 6.9 (95 percent confidence interval, 4.1 to 12.2),as compared with 1.3 (95 percent confidence interval, 0.8 to2.2) for patients who reported a usual frequency of physicalactivity of four or more times a week (P<0.01). Patientshad a similar relative risk whether or not they had a previousdiagnosis of coronary disease (Table 4).
Table 4. Case-Crossover Comparison of the Patients' Usual Frequency of Physical Exertion with the Last Episode of Physical Exertion before Myocardial Infarction.
Time of Myocardial Infarction Adjusted for Time of Awakening
There was significant circadian variation (P<0.01) in theonset of myocardial infarction, with a peak during the morningfrom 8 to 11 a.m. (Figure 2). The relative risk of myocardialinfarction during this three-hour period was 1.8 (95 percentconfidence interval, 1.5 to 2.1). After adjustment for the individualawakening times of patients, the morning peak of myocardialinfarction was markedly sharper. The relative risk of myocardialinfarction during the initial three-hour period after awakeningwas 2.7 (95 percent confidence interval, 2.3 to 3.1). The lengthof time between awakening and arising was less than 15 minutesfor 94 percent of the patients.
Figure 2. Distribution of Times of Onset of Myocardial Infarction.
The time of onset of myocardial infarction showed a significant circadian variation (P<0.01), with a peak incidence from 8 to 11 a.m. (upper panel). The relative risk of myocardial infarction during this three-hour period, as compared with other times of day, was 1.8 (95 percent confidence interval, 1.5 to 2.1). After adjustment for the individual patients' times of awakening (lower panel), the morning peak was more pronounced. The relative risk of myocardial infarction during the initial three-hour period after awakening was 2.7 (95 percent confidence interval, 2.3 to 3.1).
Unusual Life Events
The cumulative frequency of unusual life events in the 24 hoursor the 24 hours to four weeks before either a myocardial infarctionor a control event was similar in the patient and control populations(Table 5). However, as compared with the control subjects, patientsmore often reported unusual emotionally upsetting events (P<0.05)in the 24 hours before the onset of myocardial infarction orfrom 24 hours to four weeks before it, and there was a nonsignificanttrend toward more frequent reporting of stress at work from24 hours to four weeks before the event (Table 5). Within thesame time period, the death of a friend and disease in the respondentwere reported more frequently by the controls than by the patients(P<0.05). The other types of life events that we evaluatedoccurred with similar frequency among patients and controls.
Table 5. Unusual Life Events within 24 Hours and between 24 Hours and Four Weeks before the Onset of Myocardial Infarction or before the Control Event in Augsburg Patients and Controls.
Discussion
We have documented an increased risk of acute myocardial infarctionduring strenuous physical activity or within the one-hour periodafter it. In a subgroup analysis a particularly high risk wasassociated with physical exertion by patients who usually exercisedonly infrequently. Moreover, an analysis of the time of onsetof myocardial infarction adjusted for the time of awakeningdemonstrated an approximately threefold increase in risk duringthe three-hour period after awakening, as compared with othertimes of day.
On the basis of anecdotal reports and clinical experience, precipitatingfactors of acute myocardial infarction have previously beensuspected1,2,3,4,5,6,7,8,9,10,11,12,13. Since the potentialtriggers of myocardial infarction are likely to be nonspecific,however, adequate control data are needed to determine theirimportance. To overcome the limitations of previous studies,we used a case-control design in which patients from a large,predefined, and carefully monitored population were comparedwith matched controls. To minimize possible recall bias, thecontrols were asked about their activities at a specific timeduring the 24 hours before the interview. Although the rateof participation among the initial group of eligible controlswas high (70 percent), the possibility of selection bias cannotbe entirely excluded. Similarly, the difference in techniquebetween the interviews conducted with the case patients andthose conducted with the controls permits the possibility ofsome bias with regard to the information obtained.
In order to eliminate these sources of potential bias, we undertookan additional case-crossover comparison, in which each patientserved as his or her own control19. This analysis confirmedthe results of the conventional case-control analysis. Althoughonly patients who exerted themselves reasonably often and forwhom complete information on physical activity could be obtainedwere included in this analysis (23 percent of total), the statisticalpower was sufficient to demonstrate a significant association.The slightly higher relative risk associated with physical exertionin this analysis may be due to the longer exposure period --activity one hour before the onset of myocardial infarction,as compared with activity at the onset of the event in the case-controlanalysis. The case-crossover method appears to be a powerfultool, although it may not eliminate the influence of transientconfounding factors or preclude information bias due to differentialrecall of the time of events (for example, patients may overestimatepossible triggers in the recent past and underestimate the frequencyof their usual exposure to those factors).
The results of this study are compatible with the findings ofanother study in which the case-crossover analysis was the primarymethod used23. Several additional studies have found that theonset of myocardial infarction occurred during physical exertionin 5 to 13 percent of cases, which is in the same range as ourfindings2,3,6,7.
During the morning and the hours just after awakening, the riskof acute coronary events is markedly increased as compared withthe risk at other times of the day -- a pattern confirmed byour results11,12,13,24,25,26,27,28. In multivariate analyseswe found independent significant associations between the onsetof myocardial infarction and both physical exertion and thetime of awakening (Table 2). This suggests that both physicalexertion at any time during the day and routine activities afterawakening and arising may be triggers of myocardial infarctionin patients with coronary artery disease. Suspected underlyingphysiologic changes responsible for circadian variation in theincidence of myocardial infarction include the sharp morningincreases in blood pressure, heart rate, coronary tone, andplatelet aggregability and the simultaneous decrease in fibrinolyticactivity29,30,31,32,33,34,35. Platelet activation has been reportedto occur during exercise-induced myocardial ischemia in patientswith coronary artery disease, directly after physical exercisein normal subjects, and during other types of stress36,37,38,39.Furthermore, in persons who do not exercise regularly, endogenousfibrinolytic activity is lower both at base line and after bicycleexercise testing than in trained controls; this reduction furtherenhances the tendency to thrombosis40. Consistent with thesepathophysiologic observations is our finding of a particularlyhigh relative risk of myocardial infarction among "unfit" patientsduring or shortly after exertion.
The benefit of regular physical activity in providing long-termprotection against the manifestations of coronary heart diseasehas been documented18,41,42. The risk of an acute cardiac event,however, may actually be increased during vigorous exercise,9,10,43at least in the case of patients with coronary disease who donot exercise regularly -- a concept that is supported by thepresent results. In the light of the large number of peoplewho have myocardial infarctions in Western industrialized societies,44even a small excess risk associated with physical exertion maytranslate into a hazard for many patients with coronary disease.Our results also suggest that it is not only patients with known,previously diagnosed coronary heart disease who may be at increasedrisk during exertion, but also those without a previous event-- that is, those with "silent" coronary disease. The importantclinical implications of this increase in risk emphasize theneed to investigate further the relations among the type andduration of physical activity, training status, physiologicchanges, and the risk of acute coronary disease, including suddendeath45.
The increased incidence of acute myocardial infarction duringthe first hours after awakening and arising suggests that, inaddition to strenuous exertion, routine morning activities areassociated with an increased risk in some patients. Therefore,scheduling cardiac medication some time after arising may notbe the most protective method of administration. Controlledstudies are needed to investigate the preventive potential ofeither long-acting medication taken at night or medication takenbefore arising. Although physical activity is an essential aspectof life and may reduce the incidence of coronary disease,18,41,42pharmacologic or behavioral intervention may be required toattenuate the risk of cardiac events associated with physicalexertion in patients with coronary artery disease, especiallyif they are usually sedentary.
Supported in part by grants from the Schwarz Pharma Companyand the Deutsche Forschungsgemeinschaft (Wi 957/2-1).
We are indebted to Petra Pitschi, Christine Winter, and GabrieleZimmermann for conducting the interviews with the patients,to Bernhard Schwertner for help in organizing the case-controlstudy, to Ingeborg Schwarzwalder and Dieter Janku for statisticalprogramming, to Karl Wegscheider, Ph.D., for statistical consultation,to Dorothea Lukitsch for data entry, and to the patients andcontrol subjects for their cooperation.
Source Information
From the Cardiopulmonary Division, Department of Medicine, Klinikum Steglitz, Free University of Berlin, Berlin (S.N.W., H.-R.A., F.S., R.S.), and Zentralklinikum Augsburg, Augsburg (M.L., H.L.) -- both in Germany.
Address reprint requests to Dr. Willich at the Department of Medicine, Klinikum Steglitz, Free University of Berlin, Hindenburgdamm 30, D-12200 Berlin, Germany.
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6 metabolic equivalents) at the onset of infarction, as compared with 3.9 percent of the controls at the onset of the control event. For the patients as compared with the matched controls, the adjusted relative risk of having engaged in strenuous physical activity at the onset of infarction or the control event was 2.1 (95 percent confidence interval, 1.1 to 3.6). The case-crossover comparison yielded a similar relative risk of 2.1 (95 percent confidence interval, 1.6 to 3.1) for having engaged in strenuous physical activity within one hour before myocardial infarction. Patients whose frequency of regular exercise was less than four and four or more times per week had relative risks of 6.9 and 1.3, respectively (P<0.01). 
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