Use of Cardiac Procedures and Outcomes in Elderly Patients with Myocardial Infarction in the United States and Canada
Jack V. Tu, M.D., Ph.D., Chris L. Pashos, Ph.D., C. David Naylor, M.D., D.Phil., Erluo Chen, M.B., M.P.H., Sharon-Lise Normand, Ph.D., Joseph P. Newhouse, Ph.D., and Barbara J. McNeil, M.D., Ph.D.
Background Acute myocardial infarction is a leading cause ofmorbidity and mortality in the United States and Canada. Weperformed a population-based study to compare the use of cardiacprocedures and outcomes after acute myocardial infarction inelderly patients in the two countries.
Methods We compared the use of invasive cardiac procedures andthe mortality rates among 224,258 elderly Medicare beneficiariesin the United States and 9444 elderly patients in Ontario, Canada,each of whom had a new acute myocardial infarction in 1991.
Results The U.S. patients were significantly more likely thanthe Canadian patients to undergo coronary angiography (34.9percent vs. 6.7 percent, P< 0.001), percutaneous transluminalcoronary angioplasty (11.7 percent vs. 1.5 percent, P<0.001),and coronary-artery bypass surgery (10.6 percent vs. 1.4 percent,P<0.001) during the first 30 days after the index infarction.These differences in the use of cardiac procedures narrowedbut persisted through 180 days of follow-up. The 30-day mortalityrates were slightly but significantly lower for the U.S. patientsthan for the Canadian patients (21.4 percent vs. 22.3 percent,P = 0.03). However, the one-year mortality rates were virtuallyidentical (34.3 percent in the United States vs. 34.4 percentin Ontario, P = 0.94).
Conclusions Short-term mortality after an acute myocardial infarctionwas slightly lower in the United States than in Ontario, butthese differences did not persist through one year of follow-up.The strikingly higher rates of use of cardiac procedures inthe United States, as compared with Canada, do not appear toresult in better long-term survival rates for elderly U.S. patientswith acute myocardial infarction.
The debate over health care reform in the United States hasstimulated interest in comparing medical care in Canada andthe United States, two countries with very different methodsof financing health care. In particular, recent studies of acutemyocardial infarction in the two countries, notably the Survivaland Ventricular Enlargement (SAVE) and Global Utilization ofStreptokinase and Tissue Plasminogen Activator for OccludedCoronary Arteries (GUSTO) trials, suggested that the higherrates of revascularization in the United States than in Canadain the immediate postinfarction period slightly improve thequality of life, although convincing benefits in terms of survivalhave not been demonstrated.1,2 These studies, however, had limitedstatistical power to detect clinically significant differencesin mortality, because they involved relatively small numbersof patients. Furthermore, the patients were enrolled in randomizedclinical trials and are therefore not representative of thefull spectrum of patients with myocardial infarction in thetwo countries. For example, patients in Ontario, Canada, whowere enrolled in the GUSTO trial were younger, had fewer coexistingillnesses, and were much more likely to undergo coronary revascularizationprocedures than patients in Ontario with myocardial infarctionwho were not enrolled in the GUSTO trial.3 Elderly patientsconstitute the majority of patients with myocardial infarctionsin both countries but are often excluded from clinical trialsof new therapeutic strategies.4,5 For these reasons, we undertooka study to compare the use of cardiac procedures and mortalityin two population-based cohorts of elderly patients who hadnew acute myocardial infarctions in 1991 — one cohortin the United States and one in Ontario.
Methods
Sources of Data
In this study we used data on demographic characteristics, coexistingillnesses, use of procedures, and survival obtained from linkedadministrative and claims data bases in the two countries. TheU.S. data were for calendar year 1991, whereas the Canadiandata were for fiscal 1991 (April 1, 1991, through March 31,1992).
The U.S. data base included all elderly Medicare patients witha principal diagnosis of acute myocardial infarction (codes410.0 through 410.9 in the International Classification of Diseases,9th Revision, Clinical Modification [ICD-9-CM]).6 The data camefrom the Health Care Financing Administration's Health InsuranceSkeletonized Write-off data base and were linked to longitudinaldata on the use of cardiac procedures in the Medicare ProviderAnalysis and Review and Standard Analytical Outpatient files,as described elsewhere.4
The Canadian data included all elderly patients in Ontario witha primary diagnosis of acute myocardial infarction (ICD-9-CMcodes 410.0 through 410.9). These data came from the CanadianInstitute for Health Information (CIHI) data base, an administrativedata base containing hospital-discharge abstracts, and werelinked to longitudinal data on the use of cardiac proceduresin the CIHI data base and provincial vital-statistics data (fromthe Ontario Cancer Treatment and Research Foundation) by meansof unique patient identifiers.5 Coding for cardiac procedureswas based on the Canadian Classification of Diagnostic, Therapeutic,and Surgical Procedures.7
A primary diagnosis in Ontario is the discharge diagnosis mostresponsible for the length of a patient's hospital stay, whereasa principal diagnosis in the United States is the dischargediagnosis responsible for the hospital admission.4,5 An auditof data on Canadian patients with a primary diagnosis of acutemyocardial infarction at one hospital in Ontario has shown thatthis is the equivalent of a principal diagnosis of acute myocardialinfarction in the United States 98 percent of the time (datanot shown).
Study Cohorts
We identified comparable cohorts of elderly patients with anew myocardial infarction in the two countries using a varietyof exclusion criteria.4 Patients excluded from both cohortswere those who were not representative of the elderly U.S. population(i.e., patients under the age of 65 years and patients withend-stage renal disease), patients who were likely to have beenmisclassified as having an acute myocardial infarction (i.e.,those discharged from the hospital after a stay of less than5 days and those transferred to another hospital within 2 daysafter admission when the admitting diagnosis at the receivinghospital was not a myocardial infarction), and patients witha myocardial infarction in the preceding 365 days. In the U.S.cohort, patients enrolled in health maintenance organizationswere also excluded, because they were unlikely to have completelongitudinal data available. After these exclusions, a totalof 224,258 patients remained in the U.S. cohort.
Two other exclusion criteria were applied to the Canadian cohort,in addition to those just described, to ensure comparabilitywith the U.S. sample. Patients who were initially admitted fornoncardiac surgery were excluded on the assumption that thesepatients probably had postoperative myocardial infarctions,and patients who died of a myocardial infarction outside a coronarycare unit or intensive care unit on the day of admission wereexcluded on the assumption that they had died in the emergencyroom. Patients in the first category would not have been includedin the U.S. cohort because their principal diagnosis would havebeen the type of surgery being performed, whereas patients inthe second category would not have been included in the U.S.cohort because deaths in the emergency room are not consideredhospital admissions for purposes of Medicare reimbursement.Patients without a valid Ontario health-card number were alsoexcluded because we could not obtain complete follow-up dataon them. After these additional exclusions, 9444 patients remainedin the Canadian cohort.
Coexisting Illnesses, Use of Procedures, and Mortality
Coexisting conditions were considered to be those identifiedby the first seven secondary-diagnosis codes for each patientin the two data bases.8 Rates of coronary angiography, percutaneoustransluminal coronary angioplasty (PTCA), and coronary-arterybypass grafting (CABG) were determined 30 days and 180 daysafter the index infarction. Data on outpatient coronary-angiographyprocedures were not uniformly available in either country. However,previous studies have shown that they constitute only a smallpercentage of all angiography procedures in elderly patientsin the early postinfarction period9; including them in the analysiswould therefore not have affected the overall conclusions substantially.The unadjusted 30-day and 1-year mortality rates among patientsin the two countries were compared. Mortality rates adjustedfor age and sex were also calculated for the Canadian cohort,with standardization to the age and sex distribution of theU.S. cohort.10
Characteristics of the Hospitals
The availability of facilities for catheterization and revascularization(PTCA, CABG, or both) at hospitals in both countries was determined.Hospitals were considered to provide catheterization only ifthey performed 5 or more cardiac catheterizations and fewerthan 10 revascularization procedures annually in elderly patientswith myocardial infarction, whereas hospitals were categorizedas providing revascularization if they performed 10 or morerevascularization procedures annually in elderly patients withmyocardial infarction.11
Statistical Analysis
All continuous variables were compared with use of unpairedt-tests; categorical variables were compared with use of thechi-square statistic.12 Relative rates of use of cardiac procedures30 days and 180 days after the index infarction in the UnitedStates as compared with Ontario were calculated, with 95 percentconfidence intervals determined with use of a Taylor seriesexpansion.10 The cumulative procedure-rate curves and cumulativemortality curves in the two cohorts were compared with use ofa log-rank statistic.12 All P values are two-sided. The SASstatistical package was used for statistical analysis.13
Results
Characteristics of the Cohorts
Table 1 shows the demographic characteristics and coexistingillnesses of the two cohorts of elderly patients with acutemyocardial infarction in 1991. Patients in the U.S. cohort wereslightly older, and this cohort contained a greater proportionof women (P<0.001 for both comparisons). The prevalence ofcoexisting diseases in the two cohorts was similar; the largestdifference was in the prevalence of chronic pulmonary disease,which was higher in the U.S. cohort (P<0.001).
Table 1. Characteristics of Elderly Patients with Acute Myocardial Infarction in the United States and Ontario, 1991.
Rates of Cardiac Procedures
Rates of use of cardiac procedures are shown in Table 2 andFigure 1 and Figure 2. Overall, the relative rate of coronaryangiography in the U.S. patients as compared with the Canadianpatients was 5.2 (95 percent confidence interval, 4.8 to 5.7),and the relative rate of revascularization was 7.9 (95 percentconfidence interval, 7.0 to 8.9) during the first 30 days afterthe myocardial infarction. The rates of use of PTCA were higherthan the rates of use of CABG in the early postinfarction period,whereas in the late postinfarction period the rates of use ofCABG were higher than those of PTCA (Figure 2). Eighty-one percentof the U.S. patients, as compared with 48 percent of the Canadianpatients, who underwent revascularization within the first 180days after a myocardial infarction underwent the procedure within30 days after the index infarction. Differences in rates ofuse of cardiac procedures persisted through 180 days of follow-up;6 months after the index infarction, the rate of revascularizationprocedures in the U.S. cohort was quadruple that in the Canadiancohort. The cardiac-procedure-rate curves in the 26 weeks afterthe acute myocardial infarction were all significantly higher(P<0.001) for the U.S. cohort than for the Canadian cohort(Figure 1 and Figure 2).
Figure 2. Cumulative Rates of Percutaneous Transluminal Coronary Angioplasty (PTCA) and Coronary-Artery Bypass Grafting (CABG) after Acute Myocardial Infarction among Elderly Patients in the United States and Ontario, 1991.
Characteristics of the Hospitals
A higher proportion of U.S. than Canadian patients were initiallyadmitted to hospitals that were able to perform both catheterizationand revascularization procedures (34.5 percent vs. 14.0 percent,P<0.001) as well as to hospitals that performed catheterizationonly (22.8 percent vs. 4.1 percent, P<0.001). Overall, approximatelyfour times as many U.S. hospitals had the capability to performrevascularization procedures, and about eight times as manywere catheterization-only hospitals (Table 3).
Table 3. Characteristics of Hospitals in the United States and Ontario, 1991.
Outcomes after Acute Myocardial Infarction
The outcomes of the patients with acute myocardial infarctionin the two cohorts are shown in Figure 3. The cumulative mortalityrates diverged relatively early and slightly favored the U.S.cohort throughout the early postinfarction period. The divergencein the mortality curves coincided with the period in which manyU.S. patients were undergoing revascularization procedures (Figure 2).The unadjusted 30-day mortality rates were slightly butsignificantly lower in the U.S. cohort than in the Canadiancohort (21.4 percent vs. 22.3 percent, P = 0.03). However, thissmall difference in early mortality gradually narrowed overtime, and the unadjusted one-year mortality rates were virtuallyidentical in the U.S. and Canadian cohorts (34.3 percent and34.4 percent, P = 0.94). The age- and sex-adjusted one-yearmortality in the Canadian cohort (35.1 percent) was still notsignificantly different from that in the U.S. cohort after standardizationto the age and sex distribution of the U.S. cohort (P = 0.14).Overall, the cumulative mortality curves for the two cohorts,shown in Figure 3, did not differ significantly (P = 0.36).
Figure 3. Cumulative Mortality after Acute Myocardial Infarction among Elderly Patients in the United States and Ontario, 1991.
Discussion
In this study we compared the use of invasive cardiac proceduresand the mortality rates for two population-based cohorts ofelderly patients with acute myocardial infarction — onein the United States and one in Ontario, Canada. Ontario isCanada's largest province, home to 37 percent of its populationand 21 percent of its hospitals. We found striking differencesin the rates of use of invasive cardiac procedures, with theelderly U.S. patients undergoing substantially more coronaryangiography and revascularization procedures, particularly duringthe 30 days immediately after the index infarction. We alsofound significant differences in the short-term outcomes ofpatients in the two countries, with lower 30-day mortality amongthe U.S. cohort, although these differences in mortality werenot sustained at 1 year of follow-up. The results of our studysuggest that the greater use of revascularization proceduresin the United States does not improve the long-term survivalrates of elderly U.S. patients with acute myocardial infarction.
The differences we found in the use of cardiac procedures arelarger than those found in previous studies and could explainthe small short-term difference in mortality favoring the UnitedStates. The SAVE and GUSTO comparison studies noted two- to-threefolddifferences in the use of revascularization procedures and similarmortality rates among nonelderly patients in the two countriesenrolled in those clinical trials,1,2 whereas we found thaton a population-wide basis, there were larger differences inthe rates of use of cardiac procedures for elderly patientswith myocardial infarction between the two countries (relativerate of procedure use in the United States as compared withOntario, 3 to 7). Revascularization procedures were used soonafter the index infarction in approximately 3 percent of theelderly Canadian patients in our study, as compared with 12percent and 14 percent of the Canadian patients enrolled inthe SAVE and GUSTO trials, respectively.1,2 Elderly patientsare at significantly higher risk of dying shortly after a myocardialinfarction and may be significantly more likely to benefit fromthe use of revascularization procedures during this period.The results of our study are consistent with the findings ofthe Thrombolysis in Myocardial Infarction (TIMI) IIIB clinicaltrial, which showed that the aggressive use of revascularizationprocedures (coronary angiography in all patients within 48 hoursof admission, followed by revascularization if appropriate)significantly improved the short-term survival rates among patients65 years of age or older who presented with unstable anginaor a non–Q-wave infarction.14
The absence of a sustained survival benefit over the one-yearfollow-up period probably reflects factors other than the differencesin the use of revascularization procedures. Many medical therapies(e.g., beta-blockers and aspirin) are known to improve long-termsurvival after acute myocardial infarction,15 and it is possiblethat they were used more frequently in the Canadian cohort.The GUSTO study suggested that Canadian physicians were morelikely to follow published guidelines for the medical care ofnonelderly patients with myocardial infarction than their colleaguesin the United States,2 although we did not have the data totest this hypothesis. The phenomenon of similar or better short-termsurvival rates in the United States followed by equivalent orbetter long-term survival rates in Canada has also been observedin other studies comparing the outcomes of elderly postsurgicalpatients and patients with cancer in the two countries.16,17,18It has been suggested that the better short-term outcomes inthe United States may be a reflection of the intensity and timelinessof U.S. hospital care, whereas the better long-term outcomesin Canada may reflect greater access to primary care, prescriptiondrugs, and long-term care, which are universally provided tothe elderly with minimal copayments, or none, under the Canadianhealth care system.16,17,19,20
Our results should also be interpreted in the context of otherstudies that have shown marked regional, national, and internationalvariation in the use of cardiac procedures in the period aftermyocardial infarction.1,2,21,22 Most of these studies have focusedon nonelderly patients enrolled in randomized clinical trialsand have not found significant differences in mortality betweenareas with high rates of use and those with low rates. The numberof patients in our study is several times as high and the differencesin procedure rates are several times as large as in most otherstudies, yet we also found no substantial differences in outcome.The results of our study are likely to stimulate debate aboutthe costs and effectiveness of the more aggressive U.S. approachto revascularization in the elderly. Although some patients'lives may have been saved by the greater use of coronary revascularizationprocedures in the United States and many more may have receivedsymptomatic benefit, there may also have been other U.S. patientswhose lives ended prematurely because of the high risks associatedwith cardiac procedures in the elderly.23
Our study has several important limitations. First, we did nothave data on the quality of life of the patients in the twocohorts, and it is possible that there were important differencesin functional status between the elderly U.S. and Canadian patients;such differences have been observed in other studies of nonelderlyU.S. and Canadian patients with myocardial infarction.1,2 Second,one can never be sure that confounding has been completely controlledfor in observational comparisons of data from administrativedata bases, and it is possible that unmeasured differences inpatients' characteristics or medical therapies other than thosewe measured may explain the short-term and long-term outcomeswe found. Third, we had data on procedure rates and outcomesafter a myocardial infarction only for the province of Ontario,and our results may not be generalizable to other parts of Canada.
In conclusion, we found that one-year mortality rates for elderlypatients with myocardial infarction were similar in the UnitedStates and Ontario in 1991, in spite of a small short-term survivaldifference favoring the United States. Higher rates of use ofcardiac procedures did not translate into better long-term survivalrates for elderly patients in the United States. Given the manyother factors known to influence short-term and long-term survivalafter a myocardial infarction,24,25 additional population-basedstudies with more detailed data on medical treatments and patients'characteristics are required before firm conclusions can bedrawn about the reasons for the outcomes in our study. Althoughthe overall results of our study appear to favor the more conservativeCanadian approach to revascularization, the difference in short-termmortality favoring the United States and the question of thequality of life of elderly patients with myocardial infarctionin the two countries warrant further investigation.
Supported by a grant (HS08071) from the Agency for Health CarePolicy and Research. Dr. Tu is the recipient of a Health ResearchPersonnel Development Program Fellowship (04544) and Dr. Nayloris the recipient of a Career Scientist Award (02377), both fromthe Ontario Ministry of Health.
We are indebted to Edward Guadagnoli, Ph.D., for helpful commentson an earlier version of the manuscript; to Bud Davies, PeterGaccione, Fung-Yea Huang, Keyi Wu, and Claus Wall for assistancewith statistical programming; and to the Ontario Cancer Treatmentand Research Foundation for supplying vital-statistics dataon the Canadian cohort.
Source Information
From the Institute for Clinical Evaluative Sciences in Ontario, North York, Ont., Canada (J.V.T., C.D.N., E.C.); the Department of Medicine, Sunnybrook Health Science Centre, University of Toronto, Toronto (J.V.T., C.D.N.); Abt Associates, Cambridge, Mass. (C.L.P.); the Departments of Biostatistics (S.-L.N.) and Health Policy and Management (J.P.N.), Harvard School of Public Health, Boston; the Kennedy School of Government, Cambridge, Mass. (J.P.N.); and the Department of Health Care Policy, Harvard Medical School, Boston (J.V.T., C.L.P., S.-L.N., J.P.N., B.J.M.). The opinions and conclusions in this study are those of the authors, and no official endorsement by the Ontario Ministry of Health is intended or should be inferred.
Address reprint requests to Dr. Tu at the Institute for Clinical Evaluative Sciences, G-106, 2075 Bayview Ave., North York, ON M4N 3M5, Canada.
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A correction has been published: N Engl J Med 1997;337(2):139.
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