The Association between On-Site Cardiac Catheterization Facilities and the Use of Coronary Angiography after Acute Myocardial Infarction
Nathan R. Every, Eric B. Larson, Paul E. Litwin, Charles Maynard, Stephan D. Fihn, Mickey S. Eisenberg, Alfred P. Hallstrom, Jenny S. Martin, W. Douglas Weaver, for The Myocardial Infarction Triage and Intervention Project Investigators
Background During the past decade the use of coronary angiographyafter acute myocardial infarction has substantially increased.Among the possible contributing factors, the increasing availabilityof cardiac catheterization facilities was the focus of our investigation.
Methods We investigated whether the availability of cardiaccatheterization facilities at an admitting hospital was associatedwith the likelihood that a patient would undergo coronary angiography.After adjusting for age, sex, cardiac history, and cardiac complicationsduring hospitalization, we evaluated this association in 5867consecutive patients with acute myocardial infarction admittedto 19 Seattle-area hospitals. We also assessed the associationbetween the presence of on-site cardiac catheterization facilitiesand in-hospital mortality.
Results Patients admitted to hospitals with on-site cardiaccatheterization facilities were far more likely to undergo coronaryangiography (odds ratio, 3.21; 95 percent confidence interval,2.81 to 3.67) than patients admitted to hospitals where transferto another institution would be required to perform cardiaccatheterization. Admission to a hospital with on-site facilitieswas more strongly associated with the use of coronary angiographythan any characteristic of the patient. Although our study hadlimited power to detect differences in mortality, the availabilityof coronary angiography had no discernible association within-hospital mortality rates (odds ratio for mortality amongpatients admitted to hospitals with on-site facilities vs. patientsadmitted to hospitals without such facilities, 0.88; 95 percentconfidence interval, 0.71 to 1.09).
Conclusions In this community-wide study, the availability ofon-site cardiac catheterization facilities was associated witha higher likelihood that a patient would undergo coronary angiography.However, admission to hospitals with these facilities did notappear to be associated with lower short-term mortality.
Performing coronary angiography after acute myocardial infarctioncan be useful in obtaining prognostic information1,2 as wellas details of the coronary anatomy required for subsequent coronaryrevascularization. Some authorities recommend that coronaryangiography be performed routinely before discharge in mostpatients who have had an acute myocardial infarction3. However,the Joint Task Force of the American College of Cardiology andthe American Heart Association has recommended angiography onlyfor selected patients in whom either spontaneous or exercise-inducedischemia develops or who have evidence of impaired ventricularfunction4. Nonetheless, the proportion of patients undergoingdiagnostic coronary angiography after acute myocardial infarctionhas increased dramatically since 1984, and in one study almosthalf the patients at low risk who underwent angiography didnot have a recommended indication5.
The increasing use of coronary angiography after acute myocardialinfarction may be due in part to the perceived benefit of revascularizationafter infarction. However, growth in the number of cardiac catheterizationfacilities, in and of itself, may contribute to increased usein much the same way that an increase in the number of physicians6and hospital beds7 leads to greater use of medical services.Since 1980, the proportion of hospitals with on-site cardiaccatheterization facilities in the metropolitan Seattle areahas increased from 31 percent to 69 percent, resulting in nearlyidentical services in neighboring hospitals. This increasedavailability of cardiac services may promote the use of proceduresirrespective of clinical indications.
To determine whether the availability of on-site cardiac catheterizationfacilities was associated with increased use of invasive cardiacprocedures, we examined the rates at which coronary angiographywas performed at hospitals with and without on-site cardiaccatheterization facilities in a large cohort of consecutivelyadmitted patients with acute myocardial infarction. Since in-hospitalmortality may be affected by the rapid availability of coronaryrevascularization at hospitals with on-site facilities, we comparedshort-term mortality rates among these patients at the two typesof hospitals. We hypothesized that in an urban area where neighboringhospitals offered similar cardiac services, the availabilityof cardiac catheterization facilities might substantially increaseuse without measurably affecting short-term mortality.
Methods
Patients
We studied 5867 patients with a diagnosis of acute myocardialinfarction who were admitted to the 19 Seattle-area hospitalsparticipating in the Myocardial Infarction Triage and InterventionProject. The characteristics of the registry, data collection,and the reliability of the data have been described previously8.In brief, this project is a collaborative effort to evaluatenew treatment strategies for patients with acute myocardialinfarction, and it supports a registry of all patients admittedfor suspected myocardial infarction as confirmed by the logsof coronary care units and reviews of discharge diagnoses frommedical records. The registry contains detailed data about allpatients who had an acute myocardial infarction at the timeof discharge or death. This study was approved by the Universityof Washington Human Subjects Review Committee.
The present analysis included all registry patients with acutemyocardial infarction admitted between January 1988 and April1991; it excluded patients admitted after they had been resuscitatedafter cardiac arrest outside the hospital, as well as thosein whom acute infarction had complicated the treatment of anothercondition (e.g., orthopedic surgery). In the 6.4 percent ofpatients admitted more than once for myocardial infarction duringthe study period, only the first admission was included in theanalysis.
To investigate whether the presence of an on-site facility forcardiac catheterization was associated with the rate at whichpatients underwent this procedure, the patients were dividedinto two groups according to whether such a facility was availableat the admitting hospital. For purposes of this analysis, allprocedures performed in a patient were attributed to the originaladmitting hospital, even if actually performed after the patienthad been transferred to another facility. The admitting hospitalwas chosen by the patient or the paramedic system (or both together)and was not determined by randomization or assigned.
Clinical Data
Trained abstracters collected detailed data from the patients'records within three months after their discharge or death.Demographic variables in this analysis included age, sex, andrace (white vs. nonwhite). Prehospitalization variables includedthe type of transportation to the hospital (emergency transportof patients ["911 call"] vs. patients' transporting themselves)and the duration of symptoms before evaluation in the emergencydepartment. Previous cardiac events were recorded (myocardialinfarction, heart failure, angina, percutaneous coronary angioplasty,or bypass surgery). Data related to the hospital course includednew evidence of congestive heart failure, shock, extension ofthe infarct, or recurrent chest pain; the left ventricular ejectionfraction; the use of thrombolytic therapy, cardiac catheterization,coronary angioplasty, or bypass surgery; and death during hospitalization.
Statistical Analysis
The chi-square test and Student's t-test were used to identifydifferences in base-line characteristics9 between patients admittedto hospitals with on-site cardiac catheterization facilitiesand those admitted to hospitals without such facilities. Totest the hypothesis that patients were more likely to undergocardiac catheterization when admitted to hospitals with on-sitefacilities, a series of logistic-regression models10 was constructedthat included patients in whom the values for all variableswere known (5255 patients, or 90 percent of the cohort). Variablesfound to be significantly associated with cardiac catheterizationin univariate comparisons (P<0.10), as well as variablesconsidered clinically relevant, were entered into a multivariatemodel in a stepwise fashion, and the variable for the type ofhospital (with vs. without on-site cardiac catheterization)was forced into the model in the final step.
A similar set of logistic models was constructed that includedpatients evaluated by cardiologists who had privileges at botha hospital with and a hospital without a cardiac catheterizationfacility, to determine whether differences in the rate of usewere due to the availability of facilities or the characteristicsof the physicians practicing at these hospitals.
To evaluate the influence of the availability of on-site cardiaccatheterization facilities on in-hospital mortality, a logisticmodel was constructed that included variables shown to be significantlyassociated (P<0.10) with in-hospital mortality by univariatecomparisons. These variables were entered into the multivariatemodel in a stepwise fashion, and the variable for the availabilityof coronary angiography was forced into the model in the finalstep.
Results
Base-Line Characteristics
A total of 5867 patients with acute myocardial infarction wereincluded in the analysis. Their mean age was 66 years; 66 percentwere men, and 92 percent were white. Of the 19 hospitals studied,10 did not have cardiac catheterization facilities, including2 hospitals serving staff-model health maintenance organizations;these 10 hospitals had an average of 208 beds. The nine hospitalswith on-site cardiac catheterization facilities had an averageof 348 beds; none of them served staff-model health maintenanceorganizations.
Patients admitted to hospitals with cardiac catheterizationfacilities were younger, more likely to be men, and more likelyto have had angina pectoris, bypass surgery, or coronary angioplastythan patients admitted to hospitals without such facilities(Table 1). However, the distribution of other factors that mighthave influenced the use of procedures, such as the durationof symptoms, previous heart failure, or previous myocardialinfarction, was similar in both groups.
Table 1. Characteristics of the Hospitals and Patients, According to Availability of Cardiac Catheterization Facilities.
The hospital courses of the patients in the two groups weresimilar (Table 2). There were no differences in the proportionof patients with shock or heart failure at the time of admission,the rate of use of thrombolytic therapy, or the occurrence ofinfarct extension. However, the patients admitted to hospitalswith cardiac catheterization facilities had more recurrent chestpain and less evidence of new heart failure during hospitalization.Values for the left ventricular ejection fraction were availablefor 35 percent of all the patients; the ejection fraction wasan average of 4 percent higher in patients admitted to hospitalswith cardiac catheterization facilities.
Table 2. Hospital Events and Procedures, According to Availability of Cardiac Catheterization Facilities.
Use of Cardiac Procedures
Without adjustment for potentially confounding factors, therate of cardiac catheterization among patients treated in hospitalswith on-site angiography was nearly twice that among patientstreated in hospitals without this capability (65.7 percent vs.36.3 percent; P<0.001), and the rate of coronary angioplastywas more than three times higher (28.2 percent vs. 9.1 percent;P<0.001) (Table 2).
After adjustment for potentially confounding clinical and demographiccharacteristics by multivariate comparisons, the odds of undergoingcoronary angiography were more than three times greater amongthe patients admitted to hospitals with on-site cardiac catheterizationfacilities than among those admitted to hospitals without thesefacilities (odds ratio, 3.21; 95 percent confidence interval,2.81 to 3.67) (Figure 1). The presence of an on-site cardiaccatheterization facility was more strongly associated with alikelihood of undergoing coronary angiography than any clinicalfactor, including the use of thrombolytic therapy (odds ratio,2.06; 95 percent confidence interval, 1.74 to 2.43), recurrentchest pain during hospitalization (odds ratio, 2.15; 95 percentconfidence interval, 1.86 to 2.50) infarct extension duringhospitalization (odds ratio, 1.75; 95 percent confidence interval,1.26 to 2.44), or previous coronary angioplasty (odds ratio,1.72; 95 percent confidence interval, 1.23 to 2.39). Coronaryangiography was much less likely to be performed in patientswith a history of heart failure (odds ratio, 0.45; 95 percentconfidence interval, 0.37 to 0.56), nonwhite patients (oddsratio, 0.68; 95 percent confidence interval, 0.54 to 0.86),and women (odds ratio, 0.81; 95 percent confidence interval,0.70 to 0.93); it was also less likely to be performed in olderpatients (odds ratio per 10-year increment, 0.55; 95 percentconfidence interval, 0.52 to 0.59). Neither a history of anginaor myocardial infarction nor previous bypass surgery was significantlyassociated with the use of cardiac catheterization.
Figure 1. Odds Ratios with 95 Percent Confidence Intervals for Factors Influencing the Use of Coronary Angiography before Discharge in 5255 Patients Hospitalized after Acute Myocardial Infarction.
Bars representing factors associated with greater use of angiography are shown to the right of the (dotted) line of identity. After adjustment for all clinical factors by multivariate analysis, the odds of undergoing angiography were 3.2 times higher (95 percent confidence interval, 2.8 to 3.7) among patients admitted to hospitals with on-site catheterization facilities than among patients admitted to hospitals without such facilities.
To ensure that a large number of patients who were dischargedafter their index myocardial infarction without having undergonecardiac catheterization did not simply undergo this procedureas outpatients in the weeks after discharge, we determined therates of cardiac catheterization within 14 and 60 days afterdischarge in a 15 percent random sample of each of the two groupsof patients. The proportions of patients admitted to both typesof hospitals who underwent coronary angiography after dischargewere similar (hospitals with catheterization facilities vs.those without them, 5.0 percent vs. 4.7 percent [P = 0.93] at14 days and 10.0 percent vs. 8.1 percent [P = 0.63] at 60 days).
Since patients with known coronary disease may select a hospitalon the basis of existing relationships with physicians or theprevious performance of procedures there, we carried out anadditional analysis in a subgroup of 2432 patients who had notpreviously had cardiac symptoms, disease, or procedures. Theuse of coronary angiography was strongly associated with theavailability of on-site cardiac catheterization in this subgroup(odds ratio, 4.19; 95 percent confidence interval, 3.45 to 5.10).
Physicians' Decisions and Use of Angiography
To determine the effect of the availability of coronary angiographyon physicians' decisions whether to use this procedure, we performedan analysis of 614 patients evaluated by cardiologists who hadprivileges at both hospitals with and hospitals without on-sitecardiac catheterization facilities. This enabled us to determinehow often the same physicians decided to perform coronary angiographyin patients admitted to each type of hospital. These physiciansperformed the procedure in 73 percent of patients admitted tohospitals with on-site cardiac catheterization facilities, butin only 47 percent of patients admitted to hospitals withoutsuch facilities (P<0.001). After adjustment for differencesbetween the groups of patients at base line, the odds of undergoingcoronary angiography among the patients treated by physiciansat hospitals with on-site cardiac catheterization facilitieswere still more than twice the odds among the patients treatedby the same physicians at hospitals without on-site facilities(odds ratio, 2.54; 95 percent confidence interval, 1.76 to 3.70).
Mortality
The in-hospital mortality in the study cohort was 10.1 percent.The unadjusted mortality rate was 9.6 percent among patientstreated at hospitals with on-site cardiac catheterization facilitiesand 11.0 percent among those treated at hospitals without them.However, after adjustment for base-line differences, the availabilityof cardiac catheterization at the admitting hospital had nodiscernible effect on in-hospital mortality (odds ratio, 0.88;95 percent confidence interval, 0.71 to 1.09) (Figure 2). Variablesassociated with increased in-hospital mortality were the presenceof signs and symptoms of shock on admission (odds ratio, 7.47;95 percent confidence interval, 4.87 to 11.46), infarct extension(odds ratio, 4.57; 95 percent confidence interval, 3.27 to 6.40),evidence of heart failure during hospitalization (odds ratio,2.68; 95 percent confidence interval, 2.19 to 3.28), recurrentchest pain during hospitalization (odds ratio, 1.31; 95 percentconfidence interval, 1.04 to 1.65), and increasing age (oddsratio per 10-year increment, 1.55; 95 percent confidence interval,1.41 to 1.70). The use of coronary angiography during hospitalizationwas associated with lower mortality (odds ratio, 0.42; 95 percentconfidence interval, 0.31 to 0.57). However, neither bypasssurgery (odds ratio, 1.15; 95 percent confidence interval, 0.79to 1.67) nor coronary angioplasty (odds ratio, 1.30; 95 percentconfidence interval, 0.92 to 1.85) was associated with in-hospitalmortality. Race, sex, and a history of heart failure, myocardialinfarction, or angina pectoris were not significantly associatedwith mortality in the multivariate model.
Figure 2. Odds Ratios with 95 Percent Confidence Intervals for Factors Influencing In-Hospital Mortality among 5255 Patients Hospitalized after Acute Myocardial Infarction.
Bars representing factors associated with higher mortality are shown to the right of the (dotted) line of identity. After adjustment for all clinical factors by multivariate analysis, there was no association between the availability of on-site catheterization facilities and in-hospital mortality (odds ratio, 0.88; 95 percent confidence interval, 0.71 to 1.09).
Discussion
Although several well-designed clinical trials have helped todetermine the role of coronary angiography and coronary angioplastyafter the administration of thrombolytic therapy,11,12 the valueof performing angiography after acute myocardial infarction,particularly in patients not eligible for treatment with thrombolyticagents, remains uncertain. As a result, variation in the useof this procedure among practitioners is considerable. Despitethe uncertainty about the value of angiography after acute myocardialinfarction, the proportion of patients studied in this fashionhas substantially increased5.
In the present investigation, we studied one likely contributingfactor to the increased use of coronary angiography after acutemyocardial infarction: whether a cardiac catheterization laboratorywas conveniently available at the hospital where a patient wasadmitted, or whether transport to another institution was required.After adjusting for important clinical differences, we foundthat patients admitted to hospitals with on-site cardiac catheterizationfacilities were far more likely to undergo coronary angiographythan patients admitted to hospitals without this capability.The availability of a cardiac catheterization facility at theadmitting hospital had a stronger effect on the use of catheterizationthan did any clinical factor, including evidence of left ventriculardysfunction, recurrent chest pain, or infarct extension. Onthe other hand, the availability of such a facility at the admittinghospital had no discernible effect on in-hospital mortality,although our study had limited power to detect differences inmortality. As noted in previous studies of this registry population,13,14both women and nonwhite patients were less likely to undergoangiography, according to multivariate analysis.
Previous studies of factors influencing the use of procedureshave implicated inappropriate use of services,15,16 the numberof hospital beds, and the number of surgeons per capita17 ascauses of geographic variation in the rates of procedures. Hillmanet al. noted significantly increased use of radiologic proceduresby self-referring physicians as compared with physicians whoreferred patients to radiologists18. Those authors speculatedthat both financial incentives and patients' convenience wereresponsible for the increased use of services by self-referringphysicians.
The present study has several strengths as compared with otherstudies analyzing the use of procedures. The extensive clinicaland demographic data available from the project registry allowadjustment for known differences between patients treated ateach type of hospital that might affect the rate of use. Also,this registry is a comprehensive data base that permits patternsof care in the community to be assessed without selection biasbecause it includes all patients with acute myocardial infarctionadmitted to many hospitals. Finally, our study included outcomedata, thereby allowing analyses to assess whether differencesin the availability of cardiac catheterization facilities hada measurable effect on in-hospital mortality.
Although there is evidence that some patients may benefit fromrevascularization after acute myocardial infarction,19,20 theavailability of coronary angiography at the admitting hospitalwas not associated with a reduction of in-hospital mortality.This finding is consistent with the results of Rouleau et al.,who noted that mortality rates among selected patients admittedwith acute myocardial infarction to Canadian hospitals weresimilar to those among comparable patients admitted to Americanhospitals, despite significantly greater use of procedures inthe United States21. One possible explanation for our findingis that the patients most likely to benefit from revascularizationwere identified and transferred from hospitals without catheterizationfacilities so that they could receive more aggressive therapybefore any catastrophic event occurred, whereas patients withequivocal indications were treated more conservatively.
Although provocative, these findings must be viewed with caution.First, the present study lacks adequate statistical power todetect a moderate (<30 percent) survival advantage amongpatients admitted to hospitals with on-site cardiac catheterizationfacilities (for example, 10 percent vs. 7 percent mortalityin the two types of hospitals). Second, previous studies evaluatingthe effect of revascularization on mortality among patientswith coronary artery disease have required longer follow-up(more than three years) to demonstrate a survival benefit. Thus,although we found no association between the availability ofcoronary angiography and in-hospital survival, our findingsneed to be verified in a larger group of patients with longerfollow-up.
Although the availability of on-site cardiac catheterizationfacilities was not associated with in-hospital mortality thereappeared to be a survival advantage among patients at eithertype of hospital who underwent coronary angiography. Since coronaryangiography is a diagnostic test, this survival benefit mustbe either a result of revascularization after angiography ora result of selection bias among patients chosen to undergoangiography. It is unlikely that this observed survival benefitis a result of revascularization, since neither bypass surgerynor coronary angioplasty during hospitalization was associatedwith survival in the multivariate analysis. The most likelyexplanation is that "healthier" patients were selected to undergoangiography, and that this effect cannot be fully explainedby measured factors such as age, sex, and cardiac history.
We have attempted to account for what we perceive to be themajor potential sources of bias in this study. First, cardiologistswho practice at hospitals with cardiac catheterization facilitiesmight be expected to favor performing invasive cardiac proceduresafter myocardial infarction. However, our analysis of patientstreated by cardiologists who practiced at both types of hospitalsshowed the same increase in the rate of use at hospitals withon-site facilities that we observed in all patients. Second,patients with a history of cardiac disease or previous cardiacprocedures might be more likely to be admitted to hospitalsthat offer tertiary care. However, when we excluded such patientsfrom our analysis, the multivariate model demonstrated an evenstronger association between the availability of on-site cardiaccatheterization and the rate of use of coronary angiography.Finally, we were concerned that there might be a high rate ofpost-discharge coronary angiography among patients admittedto hospitals without cardiac catheterization facilities. However,there was no difference between the two groups of patients inthe rate of coronary angiography 14 and 60 days after discharge.
Other potential limitations in this study could not be fullyaddressed. Although the cohort was large, it represented patientsfrom only one region. In particular, the proximity of hospitalswith and without cardiac catheterization facilities in thisregion may have helped make patient transfer easy and safe.Also, although we adjusted for known differences in the base-linecharacteristics of the patients, bias may have been introducedby unmeasured differences between the two groups of patients.
The availability of technological resources has important implicationsfor health policy. In the traditional fee-for-service environment,cardiac procedures are among those most profitable for hospitals22.Therefore, there are economic incentives for hospitals to buildmore cardiac catheterization facilities that attract both physiciansand patients. Competition among hospitals in urban areas hasresulted in the proliferation and duplication of cardiac services23.In metropolitan Seattle, as in many urban areas in the UnitedStates, the proportion of hospitals with on-site cardiac catheterizationfacilities has increased dramatically over the past decade24.From the present study, however, it appears that such growthin the availability of cardiac services may increase use withoutmeasurably affecting short-term mortality. Although regulatoryapproaches such as the requirement of a certificate of needbefore approval of capital expenditures appear to have had littlesuccess in controlling the growth and use of new techniques,25other regulatory mechanisms, such as regional global budgets,may decrease incentives to expand the availability of such innovations.
Supported by a grant (R01-HL-38454) from the National Heart,Lung, and Blood Institute and an Educational Grant from Genentech,Inc.
Source Information
From the Northwest Health Services Research and Development Field Program, Seattle Veterans Affairs Medical Center (N.R.E., S.D.F.), and the Departments of Medicine/Cardiology (E.B.L., P.E.L., C.M., M.S.E., J.S.M., W.D.W.) and Biostatistics (A.P.H.), University of Washington -- both in Seattle.
Address reprint requests to Dr. Every at the MITI Coordinating Center, 1910 Fairview Ave. E., Suite 205, Seattle, WA 98102.
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