Commentary: Differential Hemodynamic Profile of High-Capacity Intraaortic Balloon Pumps – New Promise for an Antiquated Technology?

The intraaortic balloon pump (IABP), now over a half century old, remains the most widely used form of mechanical hemodynamic support for patients with decompensated heart failure and cardiogenic shock.^1,2  Compared to other forms of temporary circulatory support (TCS), IABPs are low cost, easy to insert and maintain, and are associated with an acceptable safety profile across a broad range of patient care scenarios. The goals of IABP counterpulsation include fractional augmentation and indirect unloading of the failing left ventricle and enhancement of coronary perfusion via antegrade/retrograde displacement of native stroke volume within the aorta during diastole, thus favorably influencing the myocardial oxygen supply/demand equation.³ The theoretical precepts of IABP counterpulsation have been clearly articulated from the outset and its potential hemodynamic attributes convincingly demonstrated; however, translation of these characteristics into tangible clinical benefit has proven to be a far harder task, especially in the context of contemporary cardiovascular care. 

Over the last decade, the role for IABP insertion has become increasingly unclear. The 2004 American College of Cardiology/American Heart Association (ACC/AHA) and 2003 European Society of Cardiology (ESC) guidelines for management of patients with ST-elevation myocardial infarction (STEMI) strongly supported the use of IABP in patients with acute myocardial infarction with cardiogenic shock and selected patients without cardiogenic shock with class I recommendations.^4,5 These now outdated guidelines were largely based on the results of prospective and retrospective cohort studies in an era that predated contemporary percutaneous coronary intervention (PCI). A 2009 meta-analysis reviewed seven randomized trials of STEMI without cardiogenic shock and failed to demonstrate survival benefit with IABP.⁶ The same meta-analysis also reviewed the nine cohort studies of patients with STEMI and cardiogenic shock and showed that IABP use was associated with an 18% mortality risk reduction among patients receiving thrombolysis, but only a 6% mortality benefit among patients receiving PCI. Similarly, a 2012 meta-analysis of 15 observational studies involving STEMI and NSTEMI patients showed no mortality benefit with IABP, but did find a 28% reduction in mortality in the subset of patients presenting with cardiogenic shock.⁷ 

Given the paucity of rigorous, prospective trial data, several multicenter, randomized, controlled trials were conducted with the aim of better defining the role of IABP in patients with and without cardiogenic shock. In the CRISP AMI (Counterpulsation to Reduce Infarct Size Pre-PCI Acute Myocardial Infarction) study, a total of 337 patients with acute, anterior STEMI without cardiogenic shock were randomized to IABP prior to PCI or primary PCI alone.⁸  The addition of IABP did not lead to a reduction in infarct size as measured by cardiac magnetic resonance imaging or improvement in short-term survival, but was associated with a significant reduction of the composite secondary endpoint of death, shock, or worsening heart failure at 6 months.⁸ In the IABP-SHOCK-2 (Intra-aortic Balloon Support for Myocardial Infarction with Cardiogenic Shock) study, a total of 600 patients with acute myocardial infarction complicated by cardiogenic shock who subsequently underwent early revascularization with PCI or coronary artery bypass graft (CABG) were randomized to IABP or standard medical care.  Despite early reduction in the simplified acute physiology score (SAPS II), a marker of disease severity in the IABP arm, there was no difference in mortality at 30 days or 12 months.^1,2 It should be noted, however, that IABP-SHOCK-2 enrolled a critically ill patient population with 45% of the aggregate cohort undergoing randomization post-cardiac resuscitation. Additionally, IABP insertion was delayed until after revascularization in 86.6% of patients; thus, any incremental benefit offered by IABP counterpulsation could potentially be underappreciated. 

Uncertainty also now exists in the role of IABP among patients at high risk for hemodynamic instability during elective PCI. Observational studies have consistently supported the use of IABP in high-risk PCI. Brodie et al demonstrated that prophylactic IABP reduced catheterization laboratory events in high-risk patients, particularly among those with cardiogenic shock or low ejection fractions.⁹ Other investigators similarly extended these observations to elective use of IABP in high-risk PCI patients with diminished ejection fraction (<30%) but without shock.^10,11 The BCIS-1 (Balloon Pump-Assisted Coronary Intervention Study) was the first randomized controlled trial investigating elective IABP use in high-risk PCI.¹² Among 300 patients with ejection fractions <30% and severe coronary disease, there was no difference in major cardiac or cerebrovascular events at hospital discharge, but elective IABP use was associated with fewer procedural complications.¹² Interestingly, the trend toward reduced all-cause mortality noted at the primary 6-month endpoint in IABP-treated PCI patients became statistically significant at 51 months of follow-up (hazard ratio, 0.66; 95% confidence interval, 0.44-0.98), albeit without a clear explanation for the late benefit when viewed from the perspectives of jeopardy score, completeness of revascularization, etc.¹³

Given the emerging uncertainty of IABP efficacy, current ACC/AHA and ESC guidelines do not address the role of IABP in high-risk STEMI without shock and have downgraded IABP use in cardiogenic shock to IIa and IIb recommendations, respectively.^14,15 Furthermore, current ESC guidelines recommend elective IABP before PCI only in patients with hemodynamic instability, particularly among patients with cardiogenic shock or mechanical complications.¹⁶ Current ACC/AHA guidelines assign a IIb recommendation on the use of elective IABP before PCI in selected high-risk patients.¹⁷

In this issue of the Journal of Invasive Cardiology, Kapur and colleagues offer insight into the differential hemodynamic effects of newer large-capacity (50 cc) IABPs versus the standard-capacity (40 cc) IABPs predominantly used in the aforementioned trials.¹⁸ In this single-center, retrospective study, fifty-two consecutive patients received either the 40 cc IABP (n = 26) or 50 cc IABP (n = 26) via the traditional femoral approach. The study population was all inclusive and included patients with acute decompensated heart failure (33%; n = 17), NSTEMI (6%; n = 3), STEMI (27%; n = 14), cardiogenic shock (23%; n = 12), and high-risk PCI (12%; n = 6). IABP hemodynamic tracings were acquired and analyzed within 24 hours of device implantation and correlated with in-hospital clinical outcomes and to data from pulmonary artery (PA) catheters in a subset of 20 patients. Notably, use of the 50 cc IABP led to enhanced diastolic augmentation (71 ± 26 for 50 cc vs 56 ± 18 for 40 cc; P=.02) and systolic unloading (13 ± 7 for 50 cc vs 9 ± 4 for 40 cc; P=.01) than use of the 40 cc IABP.  Similarly, the 50 cc IABP and not the 40cc IABP was associated with a significant reduction in PA diastolic pressure (21 ± 10 mm Hg vs 30 ± 7 mm Hg; P<.05) and pulmonary artery occlusion pressure (21 ± 8 mm Hg vs 31 ± 7; P<.05) as well as a significant increase in cardiac output (4.8 ± 0.7 L/min vs 3.5 ± 1 L/min; P<.05), cardiac index (2.4 ± 0.4 L/min/m² vs 1.7 ± 0.3 L/min/m²; P<.05), and PA oxygen saturation (60 ± 9% vs 49 ± 8%; P<.05). Interestingly, although the study was not powered to detect a difference in the clinical endpoints, a small, non-significant, but potentially important signal of reduced in-hospital mortality was observed in the 50 cc IABP group (27% vs 19%; P=NS). 

As with any small retrospective analysis, the study by Kapur and colleagues has several limitations. IABP size selection was operator dependent, and thus operator bias might have influenced patient selection into each cohort, although the authors note that IABP selection was largely institutionally driven. Furthermore, there were no differential size inclusion criteria for either balloon size. Rather, patients were included if they met a minimum height requirement of 162 cm for either IABP type. It is conceivable that the optimal IABP size might depend on a patient’s height or body surface area and that a “one size fits all” approach may not offer the greatest potential advantage. Furthermore, balloon size relative to body size is only one of many variables that might predict “responsiveness” to IABP therapy. Additional demographic, hemodynamic, and laboratory variables such as the indication for IABP insertion, aortic compliance, ejection fraction, and degree of coronary burden may also predict an optimal response to IABP therapy. On the issue of aortic compliance as a potential determinant of both native and augmented aortic waveform contour (the so-called “Windkessel effect”), it should be noted that the overwhelming majority of study subjects were older male patients with atherosclerotic heart disease. These patients may be presumed to have greater rigidity of the large arteries with lower elastic reservoir than younger patients receiving IABP for non-ischemic cardiomyopathies, myocarditis, etc. Thus, neither the absolute pressure changes observed within groups with versus without counterpulsation, nor the relative differences observed between groups, may be reliably extrapolated to the aforementioned population who still presumably possess elastic aortic reservoir. Larger studies are necessary to further define this and other variables that may predict IABP responsiveness. Lastly, short-term hemodynamic improvement, while important, is still a surrogate endpoint. Both persistence of initial treatment effect as well as translation of mechanistic benefit to durable clinical benefit must be confirmed in appropriately powered, prospective investigations. 

Overall, the study by Kapur et al is an important first and necessary step in redefining the role for IABP in the modern era of acute cardiovascular care. The authors should be commended for their meticulous assessment of all hemodynamic parameters associated with counterpulsation. This study reminds us that despite the ongoing controversy, IABPs likely still have a role in the management of high-risk patients with and without cardiogenic shock. The 50 cc balloon appears to offer both an enhanced as well as more consistent hemodynamic response when compared with the 40 cc balloon. The “million dollar question” of whether this will translate into meaningful morbidity and mortality benefits thus remains and should whet our appetite for future prospective studies.

References

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From the Section of Cardiology, Department of Medicine, University of Chicago, Chicago, Illinois.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Nathan reports personal fees from Abiomed and Maquet. Dr Grinstein reports no conflicts of interest regarding the content herein.

Address for correspondence: Sandeep Nathan, MD, University of Chicago Medical Center, 5841 South Maryland Ave, MC 5076, Chicago, IL 60637. Email: snathan@medicine.bsd.uchicago.edu