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Previous Volume 331:1625-1629 December 15, 1994 Number 24 Next

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ABSTRACT

Background Previous studies have found that hospitals at which more procedures, such as coronary-artery bypass grafting (CABG) and other vascular surgery, are performed have lower rates of mortality related to these procedures than hospitals where fewer such procedures are performed.

Methods We examined the relation between the number of percutaneous transluminal coronary angioplasty (PTCA) procedures performed at hospitals (volume) and short-term mortality in a population of 217,836 Medicare beneficiaries 65 years of age or older who underwent angioplasty in the United States from 1987 through 1990.

Results The unadjusted in-hospital mortality among patients who underwent PTCA increased from 2.5 percent among the 10 percent of patients treated in hospitals with the highest volume of such procedures to 3.9 percent among the 10 percent of patients treated in hospitals with the lowest volume. The rate of bypass surgery after PTCA also increased, from 2.8 percent among patients in the highest-volume hospitals to 5.3 percent among those in the lowest-volume hospitals. Higher rates of mortality and CABG persisted in all the groups of patients treated in hospitals that performed fewer than 100 angioplasty procedures per year in Medicare beneficiaries; this volume in Medicare beneficiaries can be extrapolated to an overall annual volume of 200 to 400 angioplasty procedures. In a logistic-regression model, the volume of PTCA procedures at a hospital was found to be a highly significant predictor of in-hospital mortality (P<0.001). These results suggest that if the hospitals with the lowest volume had achieved the experience and technical results of the highest-volume hospitals, 381 fewer patients would have undergone CABG and there would have been 300 fewer in-hospital deaths in the population we studied.

Conclusions Hospitals that perform more PTCA procedures have lower short-term mortality rates after the procedure. These data provide evidence in support of the regionalization of angioplasty services.

On the basis of the assumption that ongoing experience with percutaneous transluminal coronary angioplasty (PTCA) is required to maintain competence, the guidelines of the American College of Cardiology and the American Heart Association for coronary angioplasty recommend that hospitals maintain institutional volumes of "at least 200" PTCA procedures annually and that "a minimum of 75 PTCA procedures performed per year as the primary operator" be required for physicians⁴. Despite the fact that over 300,000 angioplasty procedures are performed annually in the United States, there are few empirical data to support these guidelines⁵. One study of nearly 25,000 angioplasty procedures in California found that significantly more patients were referred for CABG after angioplasty at low-volume than at high-volume hospitals but that there were no differences in short-term mortality⁶. In this study, we examined the relation between the volume of angioplasty procedures and short-term mortality among patients 65 years of age or older who underwent angioplasty in the United States during the period from 1987 through 1990.

Methods

Sources of Data

We obtained data from the Medicare Provider Analysis and Review (MEDPAR) file maintained by the Health Care Financing Administration (HCFA) for hospitalized Medicare enrollees. This file contains the Health Insurance Claim (HIC) number; demographic information, including age, sex, and race; and limited clinical information, specifically discharge status (including death) and up to five discharge diagnoses and three procedures identified by codes from the International Classification of Diseases, 9th edition, Clinical Modification (ICD-9-CM)⁷. By matching the patients' HIC numbers, we obtained data on Medicare eligibility and 30-day mortality from MEDPAR finder files maintained by the HCFA.

Patient Population

We obtained data on 256,632 patients in the MEDPAR file from 1987 through 1990 who had procedure codes for coronary angioplasty at discharge (ICD-9-CM codes 36.01, 36.02, and 36.05). To avoid double counting of patients in the analysis of mortality and CABG after angioplasty, we examined only the first hospitalization of those who had more than one admission for angioplasty during this time.

We excluded 38,796 patients (15.1 percent) from the cohort for one or more of the following reasons: Medicare eligibility for reasons other than age (disability, end-stage renal disease, or Railroad Retirement Board entitlements; 15,905 patients excluded); treatment in federal hospitals, since the HCFA receives incomplete data on Medicare beneficiaries treated at these facilities (333 excluded); or residence outside the United States (320 excluded). In addition, patients were excluded if the primary illness (as indicated by the discharge codes) was suspected to be something other than ischemic heart disease (23,213 excluded).

Hospital Volume

We calculated the volume of PTCA procedures for each hospital as the number of discharges with an ICD-9-CM code for coronary angioplasty (36.01, 36.02, or 36.05) per calendar year for each year from 1987 to 1990. In calculating hospital volume, we counted all angioplasty procedures recorded in the MEDPAR file for each enrollee; some had undergone multiple procedures.

Statistical Analysis

We first examined the relation between volume and in-hospital mortality by plotting in-hospital mortality against hospital volume. Each patient was assigned to 1 of 10 groups, each containing approximately 10 percent of the sample, according to the annual volume of angioplasty procedures in Medicare beneficiaries at the hospital where he or she was treated; each of these 10 hospital-volume groups contained approximately 21,800 patients. The numbers of procedures performed per year by the hospitals that treated patients in the 10 groups were as follows: 1 to 46, 47 to 70, 71 to 93, 94 to 114, 115 to 137, 138 to 166, 167 to 206, 207 to 261, 262 to 371, and 372 to 987. Plots were produced for 30-day mortality and CABG before discharge. Tables comparing outcomes were generated for the patients treated in hospitals with low volume (<50 procedures per year), intermediate volume (50 to 100 procedures per year), and high volume (>100 procedures per year) after stratification according to sex, race, age, presence or absence of acute myocardial infarction, number of arteries treated by angioplasty, and characteristics of the hospitals, including medical school affiliation and availability of bypass surgery. The cutoff points for the three broad hospital-volume categories were selected to produce distinct groups of hospitals as displayed in the plots.

A logistic-regression model was developed to examine the relation between hospital volume and mortality after adjustment for the patient-specific variables age, sex, race, and year of procedure. We did not adjust the analysis for clinical indicators of the severity of illness, since the diagnoses in the MEDPAR file were not coded with sufficient accuracy to allow us to do so and the file lacked important indicators of severity, such as the left ventricular ejection fraction and the extent of coronary disease⁸. We also did not correct for either correlation between patients in the same hospital or correlation among the results in different years at the same hospital. These correlations are likely to be small, would not bias the estimates of regression coefficients, and would have little effect on estimates of standard errors. We used various mathematical transformations to explore the relation between volume and short-term mortality. These analyses indicated that the logarithmic transformation of volume was appropriate for the logistic model; log (volume) appeared to be linearly associated with the logit of short-term mortality.

Patients were designated as having an acute myocardial infarction if they had either a principal diagnosis of acute myocardial infarction (ICD-9-CM code 410) or complications of an acute myocardial infarction with a secondary diagnosis of acute myocardial infarction⁹. According to coding guidelines, any myocardial infarction occurring within eight weeks of admission is coded as an acute myocardial infarction. Patients with ICD-9-CM codes 36.10 through 36.19 were classified as having undergone bypass surgery. Bypass surgery performed on the day of angioplasty or later in the admission was classified as CABG after unsuccessful angioplasty. By linking all hospital records, we identified patients who transferred to other hospitals for CABG during the same admission.

The medical school affiliations of hospitals were identified by Medicare provider-of-services files as "major," "limited," "graduate," or "no affiliation." Hospitals that did not submit any claims to Medicare for CABG in a given year were considered not to have CABG available during that year.

Results

We studied 217,836 patients 65 years of age or older who underwent angioplasty between 1987 and 1990. The mean age was 71.5 years (range, 65 to 100). Forty-two percent were women; their racial distribution was 94.7 percent white, 2.9 percent black, 1.6 percent other, and 0.7 percent unknown. According to their discharge diagnoses, 71,340 patients (32.7 percent) had an acute myocardial infarction at the time of the initial angioplasty procedure. According to ICD-9-CM procedure codes, 86.0 percent had single-vessel angioplasty without mention of a thrombolytic agent, 4.4 percent had single-vessel angioplasty with a thrombolytic agent, and 10.1 percent had multiple-vessel angioplasty with or without a thrombolytic agent (more than one code related to PTCA was present for 0.5 percent of the patients).

According to the MEDPAR file, 1194 hospitals performed angioplasty during the study period: 736 during all four years, 77 in three of the four years, 99 in two of the four years, and 282 in only one of the four years. Figure 1 shows the distribution of hospitals according to the annual volume of angioplasty procedures in Medicare beneficiaries during the first and last years of the study. The number of procedures performed annually in each hospital ranged from 1 to 987. Fifty percent of the patients were treated in hospitals performing 54 or fewer angioplasty procedures in Medicare beneficiaries per year. Of the hospitals performing angioplasty during at least two years, 258 performed fewer than 10 procedures in Medicare beneficiaries during a given year. There were 24 hospitals performing angioplasty in all four years that performed fewer than 10 procedures in each year.

View larger version (51K): [in this window] [in a new window]   Figure 1. Distribution of Hospitals According to Volume of Coronary Angioplasty Procedures in Medicare Beneficiaries, 1987 and 1990.

 
Among the hospitals where CABG was not available, 1938 patients (0.9 percent) underwent angioplasty at 338 hospitals (28.3 percent). Of the 1581 of these patients who were treated at 335 low-volume hospitals, 54.1 percent had an acute myocardial infarction at admission.

Overall in-hospital and 30-day mortality rates were 2.9 percent and 3.2 percent, respectively. Unadjusted mortality rates according to hospital-volume category are presented in Figure 2. In-hospital mortality fell from 3.9 percent among patients treated in the lowest-volume hospitals (1 to 46 procedures per year) to 2.5 percent among those treated in the highest-volume hospitals (372 or more procedures per year). Thirty-day mortality also fell, from 4.2 percent among the patients in the lowest-volume category to 2.7 percent among those in the highest. The greatest decline in mortality was from the lowest category of volume (1 to 46 procedures per year) to the second-lowest category (47 to 70 per year).

View larger version (13K): [in this window] [in a new window]   Figure 2. Mortality among Medicare Beneficiaries Who Underwent PTCA from 1987 through 1990, According to the Volume of PTCA Procedures at the Hospitals Where They Were Treated. The points shown on the curves correspond to the mortality rates among patients treated at hospitals with volumes of 1 to 46, 47 to 70, 71 to 93, 94 to 114, 115 to 137, 138 to 166, 167 to 206, 207 to 261, 262 to 371, and 372 to 987 procedures per year.

 
Of the patients who underwent angioplasty, 3.8 percent also underwent bypass surgery before discharge. Figure 3 shows the relation between hospital volume and CABG before discharge. Of the patients in the lowest-volume category, 5.3 percent underwent CABG, as compared with 2.8 percent in the highest-volume category. Patients who underwent CABG before discharge had an in-hospital mortality rate of 7.8 percent, as compared with 2.7 percent for those who did not undergo bypass surgery (P<0.001).

View larger version (9K): [in this window] [in a new window]   Figure 3. Rate of CABG after Angioplasty among Medicare Beneficiaries Who Underwent PTCA from 1987 through 1990, According to the Volume of PTCA Procedures at the Hospitals Where They Were Treated. The points shown on the curve correspond to the mortality rates among patients treated at hospitals with volumes of 1 to 46, 47 to 70, 71 to 93, 94 to 114, 115 to 137, 138 to 166, 167 to 206, 207 to 261, 262 to 371, and 372 to 987 procedures per year.

 
Table 1 shows in-hospital mortality and rates of CABG according to age, sex, presence or absence of acute myocardial infarction, type of angioplasty (single vessel or multivessel), availability of CABG, and medical school affiliation. For all the subgroups we examined, the rates of in-hospital mortality and CABG declined with increasing hospital volume of PTCA procedures. In a logistic-regression model with adjustment for age, sex, race, and year of procedure, hospital volume remained significantly associated with mortality (P<0.001).

View this table: [in this window] [in a new window]   Table 1. In-Hospital Mortality and Rates of CABG According to Hospital Volume of Angioplasty Procedures and Characteristics of Patients and Hospitals.

 
Discussion

We found an inverse relation between the number of coronary angioplasty procedures performed by an institution and short-term mortality after the procedure. The patients in our study represented the majority of persons over 65 years of age who underwent angioplasty in the United States during the study period. Patients who underwent angioplasty at the hospitals with the lowest volume of procedures had the highest mortality rates. This relation was observed in all subgroups and remained significant after adjustment for age, sex, race, and year of procedure. In addition to the 1.4 percent higher mortality at the lowest-volume hospitals, 1.8 percent more patients underwent CABG before discharge at these hospitals than at the highest-volume hospitals, a finding previously observed by Ritchie et al⁶. These results suggest that if the hospitals with the lowest volume had achieved the experience and technical results of the highest-volume hospitals, there would have been 381 fewer bypass operations and 300 fewer in-hospital deaths.

Relation of the Volume of Angioplasty Procedures in Medicare Beneficiaries to Overall Volume

In this study we used the number of patients insured by Medicare who underwent angioplasty as a measure of the volume of angioplasty procedures. Although the volume of any procedure in Medicare beneficiaries is likely to be highly correlated with overall volume, the varying proportions of patients over 65 years of age who undergo angioplasty at given hospitals confound the conversion of our estimates to hospital volume for all age groups. In the National Heart, Lung, and Blood Institute's registry for the years 1985 and 1986, patients over 65 years of age represented 27 percent of the entire patient group¹⁰. More recent reports indicate that patients over 65 years of age represented up to 54 percent of the total at some institutions^11,12. Therefore, the hospital volumes for Medicare beneficiaries reported in this study probably represent between 25 and 50 percent of the hospital volumes for patients of all ages. The largest difference in mortality was between patients treated at hospitals with the lowest volume (1 to 46 procedures per year) and those treated at hospitals with the second-lowest volume (47 to 70 procedures per year). In converting Medicare volume to overall volume, 47 angioplasty procedures in Medicare beneficiaries at an institution would probably translate to 100 to 200 procedures for all age groups. An annual volume below 200 was also the level associated with the highest rate of subsequent referral for bypass surgery in California hospitals⁶.

The implications of the volume of angioplasty procedures in Medicare beneficiaries for the volume of procedures performed by individual physicians are also problematic. In the registry of the Society for Cardiac Angiography and Interventions, each hospital had an average of 6.7 physician operators, suggesting that the annual number of procedures per physician that is associated with better short-term mortality may be substantially lower than the number per hospital¹³. Although the relation of mortality and volume per physician cannot be examined with data from the MEDPAR file because it does not include physician identifiers, our findings are consistent with the recommendations of the American College of Cardiology and the American Heart Association for minimal annual volumes per hospital and per operator⁴. They also support the referral of patients to higher-volume hospitals in the same region.

Limitations of Counting Procedures

A limitation of the MEDPAR data for determining hospital volume is that the number of procedures performed is based on a discharge abstract that does not recognize repeated procedures performed during the same hospitalization. Moreover, only three procedures of any type can be recorded on hospital claim forms. Therefore, our method of determining volume may underestimate the actual number of procedures in Medicare patients in all volume categories.

Possible Explanations for the Volume-Mortality Relation

Three hypotheses first proposed by Luft and colleagues may explain the inverse relation between volume and mortality¹. First, greater experience at high-volume hospitals may lead to better short-term outcomes -- the "practice makes perfect" theory. High-volume hospitals are more likely to have more experienced physicians and ancillary personnel and greater availability of and experience in the use of devices for the treatment of complications of angioplasty, such as atherectomy and percutaneous cardiopulmonary bypass.

The second explanation is that lower-volume hospitals perform angioplasty on sicker patients, with resulting higher mortality. When outcomes were stratified according to indicators of the severity of illness, the inverse relations between in-hospital mortality or CABG after angioplasty and hospital volume persisted for all subgroups. Previous studies suggest that insurance-claims data bases such as the one we used have deficiencies in describing the severity of illness and may fail to identify more than half the patients with important coexisting conditions^8,14,15. However, more detailed data on severity from the PTCA Registry of the Society for Cardiac Angiography and Interventions do not support this hypothesis¹³.

The third explanation for the relation of higher mortality and lower volume is that patients are more often referred to hospitals that have better outcomes. Practitioners know which institutions have better outcomes and selectively refer more patients to such hospitals. The influence of selective referral is difficult to separate from the influence of improved outcomes with practice; presumably, both factors contribute to better outcomes. If selective referral leads to better outcomes for more patients, providing additional information about hospital outcomes would increase such referrals. More detailed data may provide important insights into ways in which all hospitals can improve outcomes.

CABG as an Indicator of Outcome

The inverse relation between angioplasty volume and the frequency of CABG during the index hospitalization is also consistent with poorer outcomes at low-volume hospitals. Bypass surgery after angioplasty denotes an unsuccessful initial procedure, since only patients in whom there are problems after angioplasty are referred for CABG. In addition to a 5.1 percent higher mortality, patients who undergo CABG during the same hospitalization have greater morbidity, including that associated with median sternotomy incisions, and longer hospitalization and recovery periods¹⁶. The same hypotheses that explain the higher mortality after angioplasty at hospitals with low volume may also explain the higher rates of CABG: less experience, greater severity of disease, or selective referral of patients to hospitals with higher success rates.

Low-Volume Hospitals without Surgical Backup

The 1581 patients undergoing angioplasty at low-volume hospitals with no capability to perform CABG had the highest in-hospital mortality rate (9.9 percent, as compared with 2.9 percent overall; P<0.001). This difference may be attributed in part to the higher incidence of acute myocardial infarction in this group (54.1 percent, as compared with 32.7 percent overall; P<0.001). A selection bias involving the need for emergency angioplasty may have also increased the mortality rate. More of these patients may have presented with complications of acute infarction that required immediate angioplasty (such as hemodynamic collapse) that precluded transfer to a high-volume hospital with surgical backup. Future recommendations for minimal volumes of angioplasty procedures need to take into account exceptional situations in which the risk of transfer outweighs the risk of immediate intervention.

Supported by research grants (HS-06503 and HS-05635) from the Agency for Health Care Policy and Research, a grant (HL-17670) from the National Heart, Lung, and Blood Institute, and a grant (LM04613) from the National Library of Medicine.

Source Information

From the Division of Cardiology, Department of Medicine (J.G.J., E.D.P., D.B.M., S.R.C., D.B.P.), and the Division of Biometry, Department of Community and Family Medicine (E.R.D., L.H.M.), Duke University Medical Center, Durham, N.C. Presented in part at the 66th Scientific Session of the American Heart Association, Atlanta, November 10, 1993.

Address reprint requests to Dr. Jollis at Box 3254, Duke University Medical Center, Durham, NC 27708-3254.

References

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9. Annual report of the California Hospital Outcomes Project. Sacramento, Calif.: Office of Statewide Planning and Development, 1993.
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11. Thompson RC, Holmes DR Jr, Gersch BJ, Mock MB, Bailey KR. Percutaneous transluminal coronary angioplasty in the elderly: early and long-term results. J Am Coll Cardiol 1991;17:1245-1250. [Abstract]
12. Jollis JG, Jackman JD, Dean LS. Percutaneous transluminal coronary angioplasty in the elderly. In: Roubin GS, Califf RM, O'Neill WW, Phillips HR, Stack RS, eds. Interventional cardiovascular medicine: principles and practice. New York: Churchill Livingstone, 1994:343-53.
13. Krone RJ, Vetrovec GW, Noto TJ, Johnson LW, Registry Committee. PTCA operator variability: procedure complexity and outcome from the Registry of the Society for Cardiac Angiography and Interventions. Circulation 1993;88:Suppl I:I-300.abstract 
14. Jencks SF, Williams DK, Kay TL. Assessing hospital-associated deaths from discharge data: the role of length of stay and comorbidities. JAMA 1988;260:2240-2246. [Abstract]
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