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(Circulation. 2003;108:951.)
© 2003 American Heart Association, Inc.

Clinical Investigation and Reports

Relation Between Hospital Intra-Aortic Balloon Counterpulsation Volume and Mortality in Acute Myocardial Infarction Complicated by Cardiogenic Shock

Edmond W. Chen, MD; John G. Canto, MD; Lori S. Parsons, BS; Eric D. Peterson, MD; Katherine A. Littrell, PhD, RN; Nathan R. Every, MD; C. Michael Gibson, MS, MD; Judith S. Hochman, MD; E. Magnus Ohman, MD; Morris Cheeks, MD; Hal V. Barron, MD, for the Investigators in the National Registry of Myocardial Infarction (NRMI) 2

From the Division of Cardiology (E.W.C.), Kaiser Permanente Medical Group, Inc, Richmond, Calif; Division of Cardiology (E.W.C., H.V.B.), University of California, San Francisco, Calif; Division of Cardiovascular Diseases (J.G.C.), Chest Pain Center and Center for Outcomes and Effectiveness Research and Education, University of Alabama at Birmingham, Birmingham, Ala; Ovation Research Group (L.S.P.), Seattle, Wash; Department of Medicine (E.D.P.), Duke University Medical Center, Durham, NC; Department of Medical Affairs (K.A.L., M.C., H.V.B.), Genentech, Inc, South San Francisco, Calif; Northwest HSR&D Field Program (N.R.E., C.M.G.), VA Puget Sound Healthcare System and the University of Washington, Seattle, Wash; Division of Cardiology (J.S.H.), New York University School of Medicine, New York, NY; and Department of Medicine (E.M.O.), Division of Cardiology, The University of North Carolina at Chapel Hill, Chapel Hill, NC.

Correspondence to Edmond W. Chen, MD, Division of Cardiology, Department of Medicine, The Permanente Medical Group, Inc, 901 Nevin Ave, Richmond, CA 94801-3195.

Received July 15, 2002; de novo received February 7, 2003; revision received May 16, 2003; accepted May 21, 2003.

	Abstract

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Background— Increasing evidence suggests an inverse relationship between outcome and the total number of invasive cardiac procedures performed at a given hospital. The purpose of the present study was to determine if a similar relationship exists between the number of intra-aortic balloon counterpulsation (IABP) procedures performed at a given hospital per year and the in-hospital mortality rate of patients with acute myocardial infarction complicated by cardiogenic shock.

Methods and Results— We analyzed data of 12 730 patients at 750 hospitals enrolled in the National Registry of Myocardial Infarction 2 from 1994 to 1998. The hospitals were divided into tertiles (low–, intermediate–, and high–IABP volume hospitals) according to the number of IABPs performed at the given hospital per year. The median number of IABPs performed per hospital per year was 3.4, 12.7, and 37.4 IABPs at low-, intermediate-, and high-volume hospitals, respectively. Of those patients who underwent IABP, there were only minor differences in baseline patient characteristics between the 3 groups. Crude mortality rate decreased with increasing IABP volume: 65.4%, lowest volume tertile; 54.1%, intermediate volume tertile; and 50.6%, highest volume tertile (P for trend <0.001). This mortality difference represented 150 fewer deaths per 1000 patients treated at the high IABP hospitals. In the multivariate analysis, high hospital IABP volume for patients with acute myocardial infarction was associated with lower mortality (OR=0.71, 95% CI=0.56 to 0.90), independent of baseline patient characteristics, hospital factors, treatment, and procedures such as PTCA.

Conclusions— Among the myocardial infarction patients with cardiogenic shock who underwent IABP placement, mortality rate was significantly lower at high–IABP volume hospitals compared with low–IABP volume hospitals.

Key Words: mortality • balloon • myocardial infarction • shock • epidemiology

	Introduction

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Acute myocardial infarction (AMI) complicated by cardiogenic shock remains a leading cause of death in AMI patients, with a mortality rate as high as 60% to 70%.^1–3 Before the modern reperfusion era, intra-aortic balloon counterpulsation (IABP) was shown to have favorable hemodynamic effects in patients with cardiogenic shock.^4,5 Although it appears that conventional fibrinolytic therapy alone is often unsuccessful,⁶ in the Global Utilization of Streptokinase and TPA for Occluded Coronary Arteries (GUSTO-I) trial, the use of IABP in patients undergoing reperfusion therapy led to a trend toward lower 30-day and 1-year mortality.⁷ In the Should We Emergently Revascularize Occluded Coronaries for Cardiogenic Shock (SHOCK) Trial Registry, patients who underwent IABP had a significantly lower mortality rate, especially in those who had received concomitant thrombolytic treatment.⁸ Recently, the SHOCK trial showed that early revascularization with a high utilization of IABP led to a significant improvement of 6-month and 1-year mortality rates.^9,10 In the 23 180 patients in the National Registry of Myocardial Infarction 2 (NRMI-2) with cardiogenic shock, the use of IABP in conjunction with thrombolytic therapy decreased the odds of death by 18%.³

The outcome of selected medical procedures is increasingly associated with hospital and operator volume, particularly in the setting of coronary artery bypass graft (CABG) surgery.^11,12 Recent studies have now confirmed that the mortality rate of patients undergoing percutaneous coronary interventions is similarly affected by hospital procedural volumes.^13–17 It remains unknown, however, whether the number of IABP procedures performed at a given hospital is associated with mortality in patients with cardiogenic shock.

The purpose of the present study, therefore, was to determine the relationship between hospital-specific IABP volume and in-hospital mortality for patients with AMI complicated by cardiogenic shock. To address this issue, we performed a retrospective cohort study of patients who received IABP for cardiogenic shock in NRMI-2, a large registry of AMI patients.

	Methods

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Data Sources
NRMI-2, a voluntary, observational database, sponsored by Genentech, Inc (South San Francisco, Calif), which began in June 1994 and ended in April 1998, contains chart-abstracted data of AMI patients admitted to registry hospitals. The characteristics and reliability of the registry have been previously described.¹⁸

Patient and Data Collection
Patients were eligible for entry into NRMI-2 if they were diagnosed with AMI at a participating hospital. The method of diagnosis of AMI was as previously described (both ST-elevation myocardial infarction and non–ST-elevation myocardial infarction patients are included).³ Patients were included in this analysis if they had cardiogenic shock or Killip class IV on initial presentation, or if cardiogenic shock developed during hospitalization with AMI (n=39 425). Cardiogenic shock was defined as "low blood pressure (<90 mm Hg) and signs of hypoperfusion (cool, clammy skin, oliguria, or altered sensorium), nonresponsive to fluid resuscitation or pressors" (Barron et al,³ p 934). Further analysis was then performed on the 750 hospitals in the registry that performed IABP and the 12 730 patients who underwent IABP treatment during their hospitalizations.

Data Analysis
The primary outcome and the outcome variable for all risk models were in-hospital mortality. Patients were stratified on the basis of their associated hospitals’ annual volume of IABP used in AMI. Hospitals were divided into tertiles (low–, intermediate–, and high–IABP volume hospitals). Patient characteristics and outcomes were compared across volume strata in univariate analyses. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated for all covariates in the multivariate models. All probability values were derived from 2-tailed analyses of trends.

To evaluate the association between IABP volume and hospital mortality rates independently of differences in patient and hospital characteristics, we performed a series of logistic regression analyses with hospital death as the outcome variable. Factors such as baseline patient characteristics, hospital factors, and procedures are listed in Tables 1 through 3. Covariates were chosen for inclusion in the multivariate analyses on the basis of significance in the univariate analyses (P0.05) and clinical importance. Covariates were allowed to enter the model in a stepwise fashion in the following blocks: demographics, medical history, presentation characteristics, procedures, hospital characteristics, and other additional characteristics. Additional analyses were performed by forcing variables such as number of AMI cases per hospital per year, number of cardiogenic shock cases per year, number of staffed beds, and number of shock patients receiving IABP.

View this table: [in this window] [in a new window]   TABLE 1. Baseline Demographics and Clinical Characteristics of Patients

View this table: [in this window] [in a new window]   TABLE 2. Baseline Hospital Characteristics

View this table: [in this window] [in a new window]   TABLE 3. Clinical Treatments and Outcomes of Patients

	Results

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During the study period from 1994 to 1998, 39 425 patients were diagnosed with cardiogenic shock, of whom 12 771 patients (35%) underwent IABP placement during their hospitalization. Of the 1674 participating hospitals, 769 hospitals (46%) performed IABP. The majority of the patients (75%) developed cardiogenic shock after initial presentation to the hospital. Overall, 750 hospitals met the criteria of the study, with a total of 12 730 patients. Hospital IABP volumes were stratified across tertiles. The hospitals were split into 3 even groups: low–, intermediate–, and high–hospital IABP volume groups. The median number of IABPs performed per year was 3.4, 12.7, and 37.4 in the low-, intermediate-, and high-volume groups, respectively.

The baseline demographics and clinical characteristics of this population are shown in Table 1. The mean age of patients in the study was 67 years; 60% were men, and 85.5% were of white ethnicity. Thirty percent of the patients were diabetic; 11% had a history of congestive heart failure; 9.7% had a history of prior PTCA; 12.4% had prior CABG; and 7.7% had prior stroke. The low–IABP volume hospitals compared with the intermediate– and the high–IABP volume hospitals had fewer patients who were transferred in from other hospitals and slightly more patients with a history of prior myocardial infarction. However, the low-volume hospitals had fewer patients with current smoking history, hypertension, and angina. Patients at the low-, intermediate-, and high-volume hospitals had similar initial systolic blood pressure, Killip class, and ECG findings.

The baseline characteristics of the hospitals that performed IABP are shown in Table 2. The mean number of IABPs performed at the low, intermediate, and high–IABP volume hospitals were 3.2±1.4, 12.1±3.6, and 50.2±37.9 IABPs per year, respectively. In addition to variation in IABP numbers, the 3 groups differed in the number of AMI cases admitted each year and in the total number of staffed beds. The low–IABP volume hospitals were smaller hospitals with fewer staffed beds and fewer patients with AMI admitted per year, compared with the intermediate- and high-volume hospitals. The low–IABP volume hospitals also had fewer cases of cardiogenic shock per year. In addition, the proportion of cardiogenic shock patients receiving IABP was lower at the low–IABP volume hospitals (30.6±18.9% [mean]), as compared with the intermediate- and high-volume hospitals (43.6±15.3% and 52.0±11.7% [P<0.001], respectively). All 3 hospital groups had similar cardiac catheterization capability; however, fewer low-volume hospitals had PTCA and/or CABG availabilities.

The proportion of patients with IABP undergoing reperfusion therapy was similar among the 3 hospital groups (Table 3), with more patients undergoing thrombolytic therapy and fewer patients undergoing primary PTCA at the low-volume hospitals. Similar numbers of patients underwent rescue angioplasty, but nonprimary PTCA utilization was lower at the low-volume hospitals. Fewer patients underwent CABG during their hospital stay at the low-volume hospitals. Overall adjunctive medical therapy was similar among the 3 groups, except the use of aspirin was lower at the low-volume hospitals.

The in-hospital mortality rate was lower at the intermediate- and high-volume hospitals, with the lowest mortality rate at the high-volume centers. The mortality rate of all patients who received IABP was significantly lower at the high– versus low–IABP volume centers (50.6% versus 65.4%; P<0.001) (Table 3). In contrast, secondary outcomes such as major bleeding, stroke, and nonfatal stroke were all higher at the intermediate- and higher-volume hospitals (Table 3). Across the volume strata of hospitals split by deciles, the higher the median hospital IABP volume, the lower the corresponding in-hospital mortality rate (Figure 1). The volume–mortality relationship was consistent across treatment combinations in patients who received IABP alone or in combination with thrombolytic therapy, primary or elective PTCA, any reperfusion therapy, or CABG (Figure 2).

View larger version (26K): [in this window] [in a new window]   Figure 1. A, Annual median hospital IABP volume by deciles. B, In-hospital unadjusted mortality of patients who received IABP by deciles.

View larger version (48K): [in this window] [in a new window]   Figure 2. In-hospital mortality of patients who received IABP. CABG indicates coronary artery bypass graft surgery; TTx, thrombolytic therapy; PPTCA, primary PTCA; and PTCA, any PTCA.

In the multivariate analysis, factors that were associated with IABP use entered into the model in a stepwise fashion. Intermediate and high IABP volumes were evaluated using low IABP volume as the reference. Hospital IABP volume was inversely associated with in-hospital mortality, independent of baseline patient presentation, hospital characteristics, hospital procedures, and whether patients were transferred in from other hospitals (Table 4). Increasing age, female gender, diabetes, prior CABG, and a history of prior congestive heart failure were associated with increased mortality. Conversely, high–IABP volume hospitals, primary PTCA, nonimmediate revascularization with either subsequent PTCA or CABG surgery, and the ability of the hospital to perform PTCA were associated with improved survival.

View this table: [in this window] [in a new window]   TABLE 4. Predictors of In-Hospital Mortality: Multivariate Analysis

To examine the potential for bias due to transfer, we re-ran the analysis using only hospitals with full interventional capabilities (where transfer rate was <3%), and our results remained unchanged (data not presented in Table 4).

	Discussion

Top Abstract Introduction Methods Results Discussion Conclusion References

 
In this study, we examined the use of IABP in patients with AMI complicated by cardiogenic shock. When in-hospital mortality was assessed at hospitals that performed IABP, an inverse volume–mortality relationship was observed. Specifically, a lower in-hospital mortality rate was observed at the hospitals that performed more IABPs per year for AMI. Moreover, the association of IABP hospital volume and mortality was independent of patient and hospital characteristics such as size of the hospital, the number of AMI patients per year, the number of cardiogenic shock patients per year, and the number of cardiogenic shock patients receiving IABP per year. The volume–mortality association was also independent of in-hospital procedures such as primary PTCA, nonprimary PTCA, or CABG. Although information about complications directly related to IABP use was not available for current analysis, complications from IABP likely contributed very little to overall mortality. Review of contemporary literature and practice suggests that the complication rate of appropriate IABP use is consistently low.¹⁹ In contrast, bleeding and stroke were higher at the higher-volume hospitals. Although these rates were probably higher because of a greater number of invasive procedures performed, the intermediate- and the high-volume hospitals still had lower mortality than the low-volume hospitals. The present finding is consistent with prior studies documenting an inverse association between hospital procedural volume and outcomes after procedures such as PTCA and CABG surgery.^11–17

Multiple studies have suggested mortality benefits of IABP in patients with cardiogenic shock complicating AMI.^3,7,8,20,21 The American College of Cardiology (ACC) and the American Heart Association (AHA) recommend the use of IABP in patients with shock refractory to pharmacological therapy and as a stabilizing measure for urgent revascularization.²² Nevertheless, the use of IABP in the treatment of cardiogenic shock remains remarkably low. In GUSTO-I and GUSTO-III, IABP was used in 22% of the patients.^7,23 In the Worcester community and in NRMI-2, the use of IABP was observed in 25% and 31% of the patients, respectively.^1,3 However, when IABP was aggressively integrated into standard medical therapy for cardiogenic shock, IABP was used in 51% of the patients in the SHOCK Trial Registry. In the SHOCK Trial Registry, the use of IABP was associated with lower mortality, with or without thrombolytic therapy. In particular, those who had received IABP placement and subsequently underwent revascularization with PTCA or CABG surgery had dramatically lower in-hospital mortality (39% versus 78%; P<0.001).⁸ In the SHOCK Trial, 86% of all patients underwent IABP placement as part of standard medical therapy. Although the invasive strategy did not alter 30-day mortality rate, the 6-month and 1-year mortality rates were significantly lower.^9,10 This underscores the importance of IABP in the care of patients with cardiogenic shock.

Increasing evidence suggests that higher operator volume is associated with lower mortality for various common interventional and surgical procedures such as PTCA^13–17 and CABG.^11,12 This is particularly true in percutaneous interventions, for which high-volume centers have lower short-term and long-term mortality and fewer complications.^14–16 Even at low-volume centers with fewer than 200 angioplasties per year, in-hospital mortality can improve over time.²⁴ Unlike percutaneous interventions or even diagnostic coronary angiography, the insertion of IABP can be safely done at the bedside by trained medical professionals.⁵ However, after its placement, the proper use and management of IABP may require competent medical and nursing staff.

The ACC and AHA have specific guidelines governing interventional procedures such as primary or elective coronary angioplasty.²⁵ Even a bedside procedure with questionable clinical benefits, such as pulmonary artery catheterization, has warranted recent calls for task forces and consensus conferences.²⁶ However, an underutilized and clinically beneficial procedure such as IABP has received little or no attention. Similar to the use of pulmonary artery catheters, formal training requirements and educational programs may be necessary to improve care and increase the number of physicians and nurses formally and competently trained to operate IABP.

Urgent revascularization is imperative in patients with cardiogenic shock complicating AMI. In GUSTO I, the use of early invasive strategy with catheterization and revascularization was associated with a significant short-term in-hospital and long-term mortality rate reduction.^27,28 In patients who underwent revascularization within 30 days of AMI, mortality rate was reduced by 40%.²⁷ More recently, in the SHOCK Trial, aggressive medical therapy with early stabilization and urgent revascularization led to substantial improvement of long-term outcome.¹⁰ Similarly, in patients with acute coronary syndrome without ST-segment elevation, early invasive strategy has been shown to have favorable outcome.^29,30 Findings of the present study support urgent revascularization, especially by mechanical means. However, most hospitals in the United States do not have available cardiac catheterization capabilities, thus making urgent revascularization difficult and requiring transfers to other institutions. In hemodynamically compromised patients with cardiogenic shock, any delay invariably results in worse outcome. This underscores the role of IABP in helping to facilitate reperfusion therapy and to allow transfer to other facilities for complete revascularization.

Limitations
Although the information was collected prospectively, the present analysis is retrospective and has the many limitations associated with this type of clinical research. The nonrandom allocation of treatment of IABP and the noncollected variables are potential confounders. Information about complications associated specifically with IABP, timing of cardiogenic shock, and timing of IABP were not prespecified variables in the registry and were unavailable for analysis. The present analysis is restricted to IABP use for AMI patients only and does not address the use of IABP for other indications. Despite the comprehensive multivariate analysis, there remains the possibility that some unmeasured confounding treatment or procedure may be responsible for the current observation of improved outcome in patients treated at high–IABP volume hospitals.

	Conclusion

Top Abstract Introduction Methods Results Discussion Conclusion References

 
IABP remains underutilized in AMI complicated by cardiogenic shock. Among patients with cardiogenic shock who underwent IABP placement, mortality was significantly lower at high–IABP volume hospitals as compared with low–IABP volume hospitals. Greater efforts should be directed to increasing IABP rate and training and improvement of care of patients with cardiogenic shock.

	Acknowledgments

 
This work was supported by a Grant from Datascope Corp, Fairfield, NJ, and Genentech, Inc, South San Francisco, Calif.

	Footnotes

 
Drs Barron, Littrell, and Cheeks are employees of Genentech, Inc, South San Francisco, Calif, which is a sponsor of the NRMI.

	References

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