Volume-Outcome Relationships for Intra-Aortic Balloon Pump in Acute Myocardial Infarction

Abstract

Background: Acute myocardial infarction (AMI) is a major scenario for the use of an intra-aortic balloon pump (IABP), particularly when complicated by cardiogenic shock, although the utilization of mechanical circulatory support devices varies widely per hospital. We evaluated the relationship, at the hospital level, between the volume of IABP use and mortality in AMI.

Methods and Results: Using a Japanese nationwide administrative database, 26,490 patients with AMI undergoing primary percutaneous coronary intervention (PCI) from 154 hospitals were included in this study. The primary endpoint was the observed-to-predicted in-hospital mortality ratio. Predicted mortality per patient was calculated using baseline variables and averaged for each hospital. The associations among PCI volume for AMI, observed and predicted in-hospital mortality, and observed and predicted IABP use were assessed per hospital. Of 26,490 patients, 2,959 (11.2%) were treated with IABP and 1,283 (4.8%) died during hospitalization. The annualized number of uses of IABP per hospital in AMI was 4.5. In lower-volume primary PCI centers, IABP was more likely to be underused than expected, and the observed-to-predicted in-hospital mortality ratio was higher than in higher-volume centers.

Conclusions: A lower annual number of IABP use was associated with an increased mortality risk at the hospital level, suggesting that IABP use can be an institutional quality indicator in the setting of AMI.

Acute myocardial infarction (AMI) is one of the major scenarios for the use of mechanical circulatory support (MCS) devices, particularly when complicated by cardiogenic shock (CS). Despite recent advances in early reperfusion therapy with primary percutaneous coronary intervention (PCI), clinical outcomes of AMI with CS remain poor, with high in-hospital mortality of up to 50%.^1,2 For patients with CS refractory to pharmacological treatment, temporary MCS devices such as an intra-aortic balloon pump (IABP), extracorporeal membrane oxygenation (ECMO), and intravascular microaxial left ventricular assist device (Impella; Abiomed Inc., Danvers, MA, USA) may be indicated.³ Despite the lack of clinical evidence, MCS devices have been persistently or even increasingly used in the setting of AMI in daily practice;^1,4 of these devices, IABP is the most readily available form of support, and thus one of the most common MCS devices in many countries, even in the current era.^5–9 For instance, a recent Japanese administrative database study demonstrated that although IABP use has declined from 2010 to 2020, IABP was still the most frequently used MCS device in a setting of CS in Japan, accounting for approximately two-thirds of patients with CS who were treated with MCS devices in 2020.¹⁰ Even though IABP is a less complex and resource-intensive device than ECMO and the Impella, the utilization of IABP varies widely among institutions and regions.^7–9 Although a large-scale database study in the US demonstrated that risk-adjusted mortality rates did not differ significantly between hospitals with higher and lower MCS (predominantly IABP) volume,⁸ another nationwide study in Japan showed that a higher number of IABP use per year per hospital was associated with better 30-day mortality in patients with AMI complicated by CS.¹¹ Thus, there may be a volume-outcome relationship that higher IABP volume centers can provide better care, but the association remains uncertain. The aim of the present study was to evaluate the relationship between the volume of IABP use and mortality at the hospital level in patients with AMI undergoing primary PCI.

Methods

Data Source

This study used the nationwide Diagnosis Procedure Combination (DPC) database, which comprises administrative claim information during hospitalization to calculate reimbursements from insurers to acute care hospitals in Japan.^12–15 The DPC database contains inpatient data such as demographics, disease diagnosis, comorbidities, the presence or absence of CS (Killip class) on admission, the use of MCS devices (e.g., IABP and ECMO), and medications. Data on the use of medical resources, diagnostic tests, and interventional procedures are also included in the DPC database. The International Classification of Disease 10th revision (ICD-10) codes were used to identify disease diagnosis. In a previous study, the accuracy of the disease diagnosis of the DPC database using ICD-10 codes was validated.¹⁶

The present study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee at Chiba University Graduate School of Medicine (No. 3309). Because of complete data anonymization and the observational study design, the requirement for written informed consent was waived.

Study Population

From February 2014 to March 2018, a total of 96,396 patients aged ≥18 years from 170 hospitals in Japan underwent PCI for AMI or non-AMI, including stable coronary artery disease and unstable angina. AMI included both ST-segment elevation myocardial infarction (MI) and non-ST-segment elevation MI, which was defined by ICD-10 codes for AMI (I21.0, I21.1, I21.2, I21.3, I21.4, and I21.9) with the implementation of primary PCI within 24 h from hospital admission.¹⁴ Major exclusion criteria were as follows: patients with no antithrombotic medications used; patients from a hospital where no in-hospital death was recorded and no primary PCI for AMI was performed; PCI procedures for elective cases; and PCI for AMI with ECMO (Figure 1). Thus, a total of 26,490 patients with AMI undergoing PCI from 154 hospitals were included in the present analysis.

Figure 1.

Study flow. AMI, acute myocardial infarction; ECMO, extracorporeal membrane oxygenation; IABP, intra-aortic balloon pump; PCI, percutaneous coronary intervention.

Outcomes and Statistical Analysis

The primary outcome of the present study was in-hospital mortality and the observed-to-predicted mortality ratio at the hospital level. The main interest of this study was the relationship between the volume of IABP use and the observed-to-predicted in-hospital mortality ratio per hospital.

All data are expressed as the mean±SD, median with interquartile range (IQR), or frequency (%), as appropriate. Using a multivariate mixed-effects logistic regression model, the 10 variables shown in Table 1 (age, sex, body mass index, hypertension, diabetes, dyslipidemia, anterior MI, Killip class, cardiac arrest, and mechanical complications) were included in calculating predicted in-hospital mortality and the probability of IABP use for each patient. The model’s accuracy was evaluated using the C-statistic. The model was applied to all patients to predict in-hospital mortality and the probability of IABP use, and then the patient-level predicted rates were averaged at the hospital level. Observed (crude) in-hospital mortality and probability of IABP use were divided by predicted mortality and IABP use at the hospital level to calculate observed-to-predicted in-hospital mortality and IABP use ratios.^14,17 The associations among observed in-hospital mortality and IABP use, annual PCI volume for AMI, and the observed-to-predicted in-hospital mortality and IABP use ratios at the hospital level were assessed using the locally weighted scatterplot smoother plot with 95% confidence intervals (CIs). All volume metrics were annualized, because the time frame (≥3 months) for enrollment varied among participating hospitals. P<0.05 was considered statistically significant. Statistical analyses were performed by biostatisticians using SAS software version 9.4 (SAS Institute, Cary, NC, USA).

Table 1.

Baseline Characteristics

	All (n=26,490)	IABP(−) (n=23,531)	IABP(+) (n=2,959)	P value
Age (years)	69.3±12.7	69.2±12.8	70.1±12.3	<0.001
Male sex	20,064 (75.7)	17,778 (75.6)	2,286 (77.3)	0.04
BMI (kg/m2)	24.0±3.8	24.0±3.9	23.6±3.8	<0.001
Hypertension	18,254 (68.9)	16,696 (71.0)	1,558 (52.7)	<0.001
Diabetes	8,373 (31.6)	7,394 (31.4)	979 (33.1)	0.07
Dyslipidemia	19,164 (72.3)	17,530 (74.5)	1,634 (55.2)	<0.001
Anterior MI	11,188 (42.2)	9,617 (40.9)	1,571 (53.1)	<0.001
Killip class at presentation				<0.001
Class I	12,509 (47.2)	12,007 (51.0)	502 (17.0)
Class II	6,553 (24.7)	5,967 (25.4)	586 (19.8)
Class III	1,858 (7.0)	1,550 (6.6)	308 (10.4)
Class IV	3,012 (11.4)	1,748 (7.4)	1,264 (42.7)
Undetermined	2,553 (9.6)	2,259 (9.6)	294 (10.1)
Cardiac arrest	769 (2.9)	447 (1.9)	322 (10.9)	<0.001
Mechanical complicationsA	189 (0.7)	132 (0.6)	57 (1.9)	<0.001

Unless indicated otherwise, data are given as the mean±SD or n (%). ^AMechanical complications include papillary muscle rupture, ventricular septal perforation, and free wall rupture. BMI, body mass index; IABP, intra-aortic balloon pump; MI, myocardial infarction.

Results

Among the 26,490 patients with AMI undergoing PCI from 154 hospitals, 2,959 (11.2%) were treated with IABP, and 1,283 (4.8%) died during hospitalization over a median length of hospital stay of 13 days (IQR 10–18 days; Figure 1). Table 1 presents baseline characteristics, all of which were significantly associated with in-hospital mortality and used to calculate the predicted mortality. The median period for enrollment per hospital was 30 months (IQR 24–42 months), and the median institutional PCI volume for AMI per year was 61.4 (IQR 37.5–94.4). The median annualized number of IABP use per hospital in AMI was 4.5 (IQR 2.4–10.7; Figure 2). IABP was used at least once during the study period in 145 of 154 (94.2%) hospitals. IABP was used in 42.0% of patients with CS (i.e., Killip Class IV; Table 1). The length of hospital stay was significantly longer for patients treated with IABP than their counterparts (median 19 [IQR 13–30] vs. 13 [IQR 9–17] days, respectively; P<0.001). In-hospital mortality was higher among patients treated with than without IABP (18.5% vs. 3.1%, respectively; P<0.001). Predicted in-hospital mortality and the probability of IABP use were calculated using the variables in Table 1 (Table 2), resulting in a C-statistic of 0.93 (95% CI 0.92–0.93) and 0.83 (95% CI 0.83–0.84), respectively.

Figure 2.

Annual number of intra-aortic balloon pump (IABP) use in acute myocardial infarction (AMI) across hospitals. IQR, interquartile range.

Table 2.

ORs for In-Hospital Mortality and IABP Use

	OR for in-hospital mortality (95% CI)	P value	OR for IABP use (95% CI)	P value
Age	1.06 (1.05–1.07)	<0.001	1.00 (0.99–1.00)	0.17
Male sex	0.99 (0.84–1.16)	0.87	1.24 (1.10–1.39)	<0.001
BMI	1.01 (0.99–1.03)	0.28	1.00 (0.98–1.01)	0.51
Hypertension	0.43 (0.37–0.51)	<0.001	0.67 (0.60–0.74)	<0.001
Diabetes	0.72 (0.61–0.85)	<0.001	1.12 (1.01–1.23)	0.03
Dyslipidemia	0.23 (0.19–0.27)	<0.001	0.67 (0.61–0.75)	<0.001
Anterior MI	1.13 (0.98–1.32)	0.10	1.62 (1.48–1.78)	<0.001
Killip class at presentation
Class I	Ref.		Ref.
Class II	2.14 (1.64–2.80)	<0.001	2.09 (1.83–2.40)	<0.001
Class III	5.26 (3.95–7.00)	<0.001	4.42 (3.73–5.23)	<0.001
Class IV	8.19 (6.41–10.47)	<0.001	16.66 (14.54–19.09)	<0.001
Undetermined	4.06 (2.97–5.55)	<0.001	3.15 (2.58–3.85)	<0.001
Cardiac arrest	2.94 (2.31–3.74)	<0.001	2.03 (1.66–2.48)	<0.001
Mechanical complications	11.89 (8.14–17.35)	<0.001	2.02 (1.37–2.99)	<0.001

CI, confidence interval; OR, odds ratio. Other abbreviations as in Table 1.

The relationships of annual primary PCI volume for AMI to observed IABP use and the observed-to-predicted IABP use ratio are shown in Figure 3. A higher primary PCI volume was associated with an increase in the number of observed IABP use. In hospitals with an annual primary PCI volume of approximately ≥50 cases per year, IABP use was as predicted in number (i.e., the observed-to-predicted IABP use ratio was around 1), whereas in the lower-volume centers IABP was underused compared with expected (Figure 3). Overall, the observed-to-predicted in-hospital mortality ratio was high in hospitals with a low number of annual IABP use, especially in those hospitals with annual primary PCI volume <50, whereas the association was attenuated in high-volume centers (Figure 4). The observed-to-predicted mortality ratio was lowest in hospitals with an observed-to-predicted IABP use ratio of around 1 (Figure 5).

Figure 3.

Relationships of annual percutaneous coronary intervention (PCI) volume for acute myocardial infarction (AMI) to observed intra-aortic balloon pump (IABP) use and the observed-to-predicted IABP use ratio per hospital.

Figure 4.

Relationships of the annual number of intra-aortic balloon pump (IABP) use to the observed-to-predicted in-hospital mortality ratio per hospital. AMI, acute myocardial infarction; PCI, percutaneous coronary intervention.

Figure 5.

Relationship between the observed-to-predicted intra-aortic balloon pump (IABP) use and in-hospital mortality ratios.

Discussion

The present nationwide database study showed that in patients with AMI undergoing primary PCI, IABP was used in >10% of the entire study population and in >40% of patients with CS. The median institutional IABP use per year in a setting of AMI was 4.5. In centers with a low primary PCI volume (e.g., annual primary PCI volume <50), IABP was more likely to be underused, and the observed in-hospital mortality ratio was higher than predicted in centers with a lower IABP volume. The observed-to-predicted mortality ratio seemed lowest in hospitals where the observed-to-predicted IABP use ratio was around 1.

IABP in AMI

MCS devices have been frequently used in patients with AMI when complicated by CS. A multicenter observational study in the US reported that AMI was the leading indication for IABP use between 2017 and 2018, accounting for 33% of shock and >50% of CS.⁵ The prognosis remains poor in this clinical scenario of patients with AMI and CS, with in-hospital mortality of up to 50%.² Despite the lack of robust evidence that MCS devices can improve clinical outcomes in patients with AMI and/or CS, IABP, ECMO, Impella, and others have been persistently used in daily clinical practice.¹ For instance, the randomized IABP-SHOCK II trial demonstrated that routine IABP use had no effect on all-cause mortality at 30 days and long-term follow-up up to 6 years.^18,19 In the recent Japanese guidelines for acute coronary syndrome, routine IABP is not recommended in patients with CS (Class III), whereas selective IABP use is recommended in patients with treatment-resistant CS (Class IIa).²⁰ Although Impella use has been increasing recently and the use of IABP has declined, IABP is still one of the most common MCS devices in many countries,^6,7 and the use of Impella rather than IABP was reportedly associated with increased medical cost and risks of short-term and 1-year mortality, bleeding events, and kidney replacement therapy in patients with AMI and CS in observational studies.^21–23 In a nationwide cross-sectional study in the US, the mean number of IABP use per hospital per year was 3.2 in patients with AMI complicated by CS.⁷ In the present study, the mean (not median) annual IABP use per hospital was 7.3, and CS accounted for approximately 40% of the indications for IABP use. Thus, the annual mean IABP use per hospital in patients with AMI and CS in the present study was around 4, which is in line with the previous report from the US.⁷ Even though IABP is a less complex device than ECMO and Impella, it requires skills and expertise as an MCS device, especially in the setting of AMI. Given the wide variation in IABP utilization among institutions and regions,^7–9 there may be a volume-outcome relationship for IABP, and IABP use could be an institutional quality indicator in AMI.

IABP Volume and Outcomes in AMI

Recently, a large-scale database study in Japan showed that a higher annual number of IABP use per hospital was associated with a better survival rate at 30 days in patients with AMI complicated by CS.¹¹ Interestingly, however, the Japanese study also demonstrated higher 30-day mortality among patients with AMI and no CS in centers with a high IABP volume.¹¹ In addition, a large-scale observational study in the US indicated that an institutional MCS volume including IABP had a neutral effect on adjusted mortality in patients with AMI complicated by CS.⁸ In this context, the present study revealed the volume-outcome relationship for IABP in patients with AMI, particularly in hospitals where volumes of primary PCI for AMI and IABP use were low, although the direct association of IABP volume with mortality outcomes was uncertain.

Overall, the number of IABP use was significantly correlated with annual PCI volume for AMI at the hospital level in the present study (Figure 2). Because the hospital volume is reportedly associated with outcomes in AMI, as shown in our previous study,¹⁴ the relationship between IABP volume and in-hospital mortality may represent, at least in part, the volume-outcome relationship. Using the DPC database, we previously reported that the observed in-hospital mortality after AMI and observed-to-predicted mortality ratio were higher in hospitals with lower primary, elective, and total (primary plus elective) PCI volumes.¹⁴ Among hospitals with ≥50 primary PCIs for AMI per year, the observed-to-predicted IABP use ratio was around 1, indicating that IABP was used as predicted. Conversely, among hospitals with <50 primary PCI per year, the curve of observed-to-predicted IABP use ratio declines steeply (Figure 3), indicating that IABP was less likely to be used than predicted in centers with a lower primary PCI volume. Although the cut-off value of 50 was arbitrarily determined, this finding suggests that the lower likelihood of IABP use in lower primary PCI volume centers was potentially attributable to not only patient risk profiles, but also institutional factors, such as medical resources, including intensive care unit, medical staff expertise, and the number of IABP machines, which could not be directly assessed in the present dataset. In centers with a low primary PCI volume, a lower IABP volume was associated with an increased risk of higher observed-to-predicted mortality ratio, although this trend was not clear in centers with a high primary PCI volume, illustrating that the prognostic impact of IABP volume at the hospital level may be more evident in centers with a low PCI volume. Although this prognostic effect found in centers with a lower primary PCI volume did not necessarily indicate that the infrequent IABP use was directly associated with higher mortality, the low rate of IABP use may be related to limited resources and worse outcomes in such hospitals. One of the important findings in this study was the relationship between the observed-to-predicted IABP use ratio and the observed-to-predicted in-hospital mortality ratio (Figure 5). When the observed-to-predicted IABP use ratio equals 1, indicating that the propensity of IABP use in a hospital follows overall standards (at least in number), the observed-to-predicted mortality ratio was apparently low. This finding suggests that among hospitals where IABP was used as predicted in number, clinical outcomes may be better. In contrast, a higher observed-to-predicted mortality ratio was observed in hospitals with potential IABP underuse (i.e., the observed-to-predicted IABP use ratio was <1). Nonetheless, given the small sample size of hospitals with a low observed-to-predicted IABP use ratio (Figure 5), it is challenging to establish the relationship. Further studies are warranted to confirm our results and to investigate whether the standardized approaches in medical care improve clinical outcomes, particularly in the setting of AMI.

Study Limitations

The present study has some limitations. This study was performed in a retrospective manner using the DPC administrative database, in which detailed clinical data, including transfer information, door-to-balloon time, and laboratory, electrocardiography, echocardiography, and angiography findings, were unavailable. In addition, the timing of IABP use (e.g., before or after PCI) and device-related complications were not assessable. The sample size of the present study is large enough to reflect routine practice for AMI in Japan,^24–27 but the IABP volume per hospital was relatively small. Therefore, the prognostic impact of IABP volume in AMI patients treated with IABP at the hospital level could not be addressed. Nevertheless, this study showed IABP volume as a potential surrogate of the quality of institutional care in patients with AMI who did and did not undergo IABP treatment, even though the prognostic impact of IABP has not been established in the setting of AMI with CS. The present study excluded patients treated with ECMO, and the effects of Impella were not analyzed because it was launched in Japan in 2019.²⁸

Conclusions

The present Japanese nationwide database study demonstrated that IABP was used in >10% of patients with AMI undergoing PCI. The utilization of IABP at each hospital varied widely, with a median of 4.5 cases per year in the setting of AMI. In centers with a low primary PCI volume, IABP may be underused compared with what is expected, and the observed-to-predicted mortality ratio seemed higher in centers with a low rather than high IABP volume. The observed-to-predicted mortality ratio was low when the observed-to-predicted IABP use was around 1. Together, these findings suggest that the IABP use may be an institutional quality indicator in AMI.

Sources of Funding

None.

Disclosures

Y. Kobayashi is a member of Circulation Journal’s Editorial Team. The remaining authors have no conflicts of interest to declare.

IRB Information

This study was approved by the Ethics Committee at Chiba University Graduate School of Medicine (Approval no. 3309).

Data Availability

The data will not be shared.

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