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Impella in Transport: Physiology, Mechanics, Complications, and Transport Considerations

Published:November 14, 2021DOI:https://doi.org/10.1016/j.amj.2021.10.003

      Abstract

      Cardiogenic shock (CS) represents a spectrum of hemodynamic deficits in which the cardiac output is insufficient to provide adequate tissue perfusion. The Impella (Abiomed Inc, Danvers, MA) device, a contemporary percutaneous ventricular support, is most often indicated for classic, deteriorating, and extremis Society for Coronary Angiography and Intervention stages of CS, which describe CS that is not responsive to optimal medical management and conventional treatment measures. Impella devices are an evolving field of mechanical support that is used with increasing frequency. Critical care transport medicine crews are required to transport patient support by the Impella device with increasing frequency. It is important that critical care transport medicine crews are familiar with the Impella device and are able to troubleshoot complications that may arise in the transport environment. This article reviews many aspects of the Impella device critical to the transport of this complex patient population.
      Despite improvements in the recognition and treatment of cardiovascular emergencies, the mortality rate for patients with cardiogenic shock (CS) remains high, exceeding 50% in some studies, especially for patients with a delay to definitive or advanced care.
      • Carnendran L
      • Abboud R
      • Sleeper LA
      • et al.
      Trends in cardiogenic shock: report from the SHOCK Study. The SHould we emergently revascularize Occluded Coronaries for cardiogenic shocK?.
      ,
      • Moghaddam N
      • van Diepen S
      • So D
      • Lawler PR
      • Fordyce CB.
      Cardiogenic shock teams and centres: a contemporary review of multidisciplinary care for cardiogenic shock.
      Mechanical circulatory support (MCS) devices represent an adjunctive strategy for patients with CS who fail maximal medical management or other standard therapies. As technology advances, the indications for and accessibility of these devices continue to expand.
      • Spratt JR
      • Raveendran G
      • Liao K
      • John R.
      Novel percutaneous mechanical circulatory support devices and their expanding applications.
      ,
      • Schwartz B
      • Jain P
      • Salama M
      • Kapur NK.
      The rise of endovascular mechanical circulatory support use for cardiogenic shock and high risk coronary intervention: considerations and challenges.
      As such, the transport provider is likely to encounter an increasing volume of patients requiring some degree of MCS and transport to a tertiary care facility. Prior reports have characterized the feasibility and safety of transporting patients requiring mechanical support.
      • Griffith KE
      • Jenkins E.
      Abiomed Impella(®) 2.5 patient transport: lessons learned.
      • Hori M
      • Nakamura M
      • Nakagaito M
      • Kinugawa K.
      First experience of transfer with Impella 5.0 over the long distance in Japan.
      • Kang TS
      • Ko BS
      • Drakos SG
      • et al.
      Safety of long-distance transfers of patients on acute mechanical circulatory support.
      • Yao H
      • Samoukovic G
      • Farias E
      • Cimone S
      • Churchill-Smith M
      • Jayaraman D.
      Safety and flight considerations for mechanical circulatory support devices during air medical transport and evacuation: a systematic narrative review of the literature.
      This review article specifically focuses on the Impella (Abiomed Inc, Danvers, MA), a percutaneous ventricular assist device, with special attention on its indications, physiology, and complications from the perspective of an emergency transport clinician, as well as a review of the pathophysiology of CS.

      The Pathophysiology of CS

      To better understand the indications for placement and the pathophysiology of patients requiring the Impella for MCS, it is prudent to first review the physiology of CS. CS represents a spectrum of hemodynamic deficits in which the cardiac output (CO) is insufficient to provide adequate tissue perfusion. A commonly used definition, adopted by the SHOCK (Should We Emergently Revascularize Occluded Coronaries for Cardiogenic Shock) and IABP-SHOCK II (Intra-aortic Balloon Pump in Cardiogenic Shock II) trials, requires the presence of 3 objective factors: systolic blood pressure of less than 90 mm Hg for more than 30 minutes or the need for the infusion of catecholamines, clinical signs of pulmonary congestion, and impaired end-organ perfusion.
      • Thiele H
      • Zeymer U
      • Neumann FJ
      • et al.
      Intraaortic balloon support for myocardial infarction with cardiogenic shock.
      ,
      • Hochman JS
      • Sleeper LA
      • Godfrey E
      • et al.
      SHould we emergently revascularize Occluded Coronaries for cardiogenic shocK: an international randomized trial of emergency PTCA/CABG-trial design. The SHOCK Trial Study Group.
      Although a technical definition is necessary for research, this does not fully capture the continuum of disease observed in a clinical setting. To this end, in 2019 the Society for Coronary Angiography and Intervention (SCAI) released a novel classification schema that attempts to clearly define this continuum and bridge the gap from bench to bedside (Table 1).
      • Baran DA
      • Grines CL
      • Bailey S
      • et al.
      SCAI clinical expert consensus statement on the classification of cardiogenic shock: this document was endorsed by the American College of Cardiology (ACC), the American Heart Association (AHA), the Society of Critical Care Medicine (SCCM), and the Society of Thoracic Surgeons (STS) in April 2019.
      The SCAI schema acknowledges that patients present at different clinical stages and may benefit from different treatments along the CS continuum. The 5 classic phenotypes of CS are categorized based on physical examination findings, biochemical makers, and hemodynamic parameters suggestive of a progressive decline in CO and perfusion.
      Table 1Society for Coronary Angiography and Intervention (SCAI) Stages of Cardiogenic Shock

      The Pressure-Volume Loop

      The fundamental principles of CS can be understood through perturbations of the classic graphic representation of cardiovascular physiology—the pressure-volume (PV) loop (Fig. 1). The PV loop depicts the cardiac cycle in 4 distinct phases, creating a closed loop.
      Figure 1
      Figure 1The PV loop. The 4 distinct phases of the PV loop are ventricular filling, isovolumetric contraction, ejection, and isovolumetric relaxation. The transition between each phase is displayed as a point on the PV loop. Point A represents the pressure and volume in the ventricle at the end of diastolic filling (ie, end-diastolic pressure and end-diastolic volume [EDV]). After isovolumetric contraction (phase 2), point B corresponds to aortic valve opening when the intraventricular pressure overcomes aortic diastolic pressure. Point C occurs after ejection when the aortic valve closes, representing the end systolic pressure and end systolic volume. Phase 4 represents isovolumetric relaxation of the left ventricle (LV) between point C (aortic valve closure) and point D (mitral valve opening). After mitral valve opening occurs at point D, ventricular filling (phase 1) commences, restarting the cycle. The area within the PV loop represents the SW, the work done by the myocardium in 1 cardiac cycle. ESV, end systolic volume; Vo, volume must fill the ventricle before it can generate any pressure.
      The PV loop is bound by the end-systolic pressure-volume relationship (ESPVR) and the end-diastolic pressure-volume relationship (EDPVR). The ESPVR represents the maximum pressure that can be generated for a given ventricular volume and corresponds with contractility, such that increases in ventricular contractility will result in an upward and leftward shift of the ESPVR. Similarly, decreases in ventricular contractility result in a downward and rightward shift of the ESPVR. The slope (EES) is a load-independent variable of ventricular contractility and changes proportionally with contractility.
      • Burkhoff D
      • Mirsky I
      • Suga H.
      Assessment of systolic and diastolic ventricular properties via pressure-volume analysis: a guide for clinical, translational, and basic researchers.
      ,
      • Sagawa K.
      The end-systolic pressure-volume relation of the ventricle: definition, modifications and clinical use.
      The EDPVR is a nonlinear PV relationship that characterizes the passive ventricular properties observed in the relaxation of the ventricle. The work done by the ventricle to eject each stroke volume is the stroke work (SW), which is roughly equal to the product of the stroke volume and the mean arterial pressure (MAP). The SW is represented by the area within the PV loop. The overall contractile function of the heart is represented by the cardiac power output (CPO), which equals the product of the SW and heart rate, which is proportional to the product of the CO and MAP.

      The Death Spiral of CS

      CS generally occurs because of impaired ventricular contractility, leading to a spiral of reduced CO, hypotension, and further coronary insufficiency and culminating in impaired end-organ perfusion, shock, and death. The PV loop and the classic shock paradigm as illustrated by Hollenberg can aid in characterizing the hemodynamic effects of acutely decreased ventricular contractility.
      • Hochman JS.
      Cardiogenic shock complicating acute myocardial infarction: expanding the paradigm.
      In the initial stages of decompensated CS as ventricular contractility is reduced, the ESPVR shifts rightward and downward, whereas a small elevation is observed in the EDPVR. There is a concomitant reduction in the SW and CPO. Clinically, this results in a decreased CO and coronary perfusion pressure producing hypotension, increased left ventricular end diastolic pressure/volume (LVEDP/V, the pressure and volume in the ventricle at the end of diastolic filling), pulmonary capillary wedge pressure (PCWP), and central venous pressure (CVP). These changes lead to a spiral in both the systolic and diastolic phases, ultimately contributing to further ischemia and progression of the myocardial dysfunction.
      Compensatory responses in CS can also be characterized by the PV loop (Fig. 2C). Neurohormonal activation (ie, the release of catecholamines from sympathetic nerve endings and the adrenal glands) is the first response to an acute decrease in ventricular contractility. The release of catecholamines results in an increase in total peripheral resistance and heart rate. In addition to arterial and arteriolar vasoconstriction, venoconstriction also occurs, shifting intravascular volume from high-capacity reservoirs (eg, splanchnic circulation) to low-capacity reservoirs and increasing central venous circulation.
      • Funk DJ
      • Jacobsohn E
      • Kumar A.
      The role of venous return in critical illness and shock-part I: physiology.
      Collectively, these neurohormonal compensatory effects increase blood pressure but cause a rightward shift of the PV loop through an increase in the LVEDP/V. These compensatory mechanisms, although necessary for immediate hemodynamic stabilization, place additional stress on an already ischemic myocardium, increase myocardial oxygen demand (MVO2), and exacerbate left ventricular (LV) dysfunction.
      Figure 2
      Figure 2The PV loop of CS. LVD, left ventricular distention; NH, neurohormonal. (Data from Furer A, Wessler J, Burkhoff D. Hemodynamics of Cardiogenic Shock. Interv Cardiol Clin. 2017;6:359-371 and Hochman JS. Cardiogenic shock complicating acute myocardial infarction: expanding the paradigm. Circulation. 2003;107:2998-3002.)
      The ability to increase systemic vascular resistance through vasoconstriction is an important compensatory response consistent with the classic shock paradigm and associated with a decrease in mortality.
      • Menon V
      • Slater JN
      • White HD
      • Sleeper LA
      • Cocke T
      • Hochman JS.
      Acute myocardial infarction complicated by systemic hypoperfusion without hypotension: report of the SHOCK trial registry.
      CS is often complicated by a systemic inflammatory response syndrome atypical of the classic shock paradigm as shown in Figure 2D. The systemic inflammatory response is mediated by the release of inflammatory cytokines from the heart, most commonly after acute myocardial infarction (AMI), leading to decreased systemic perfusion, coronary perfusion, and contractility and worsening the state of CS.
      • Hochman JS.
      Cardiogenic shock complicating acute myocardial infarction: expanding the paradigm.
      This process persists until it is interrupted by pharmacologic or mechanical intervention.

      MCS and the Impella

      Although the treatment of CS is quite broad, including both pharmacologic and mechanical interventions, for many patients traditional management efforts are not sufficient to meet the metabolic perturbations discussed earlier. As such, many patients will require some degree of temporary MCS. For most patients, initially, this strategy will involve a percutaneous ventricular support device. This is most often indicated for classic, deteriorating, and extremis SCAI stages of CS, which describe CS that is not responsive to optimal medical management and conventional treatment measures. Percutaneous ventricular support devices are an evolving field of mechanical support used in high-risk elective cases and the emergent setting of AMI complicated by CS, among other causes. This review article focuses on the Impella family of MCS devices for left heart support because they have unique indications, complications, and management strategies.

      Indications for Impella Placement

      Before delving into the physiologic changes produced by Impella support, it is essential to review the data commonly used to inform the decision to place a patient on MCS. This comprehensive decision process is incredibly complex, based on predicted clinical trajectory, underlying pathophysiology, and ultimately the expert opinion of the care team, and beyond the scope of this review. Dynamic data obtained from a pulmonary artery catheter (PAC) are often central to the process of MCS implementation. Although a comprehensive review of PAC interpretation can be found elsewhere,
      • Magder S.
      Invasive hemodynamic monitoring.
      a brief overview will be helpful in understanding the decision to use the Impella for cardiac support.
      The PAC is commonly used in intensive care settings across the world to directly measure intracardiac pressures and collect data that can be used to determine CO. A percutaneous catheter is placed into a central vein, traversing the right-sided heart chambers into the pulmonary artery (PA). Pressures are obtained in the superior vena cava (CVP), right atrium, right ventricle, and the PA. A small balloon at the distal tip of the PAC is inflated, and the catheter is advanced into the PA, capturing the PCWP, which approximates the LVEDP. CO is calculated using either thermodilution or the Fick method using an estimation of whole-body oxygen consumption.
      • Kresoja KP
      • Faragli A
      • Abawi D
      • et al.
      Thermodilution vs estimated Fick cardiac output measurement in an elderly cohort of patients: a single-centre experience.
      The cardiac index (CI) is CO that has been normalized to the body surface area of the patient and is often used to help define CS. Classic CS is characterized by a drop in CI accompanied by increasing PCWP, right-sided pressures, and CVP. Society guidelines include PAC data in the definition of CS, most commonly PCWP > 18 mm Hg paired with CI < 1.9 or < 2.2 while on inotropic support (Table 1).
      • Spratt JR
      • Raveendran G
      • Liao K
      • John R.
      Novel percutaneous mechanical circulatory support devices and their expanding applications.
      Indications for Impella placement vary based on several factors, including the underlying pathophysiology and the center caring for the patient. As alluded to earlier, the Impella serves as a bridge to recovery in most cases and was originally approved by the Food and Drug Administration for use in the context of CS after AMI or postpericardiotomy syndrome in patients refractory to traditional therapies for less than 4 to 6 days depending on the model.

      Abiomed Inc. U.S. Food and Drug Administration premarket approval (PMA). Available at: https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfpma/pma.cfm?id=P140003S005. Accessed July 1, 2020.

      There are numerous off-label uses in the literature, including CS due to mitral regurgitation,
      • Jalil B
      • El-Kersh K
      • Frizzell J
      • Ahmed S.
      Impella percutaneous left ventricular assist device for severe acute ischaemic mitral regurgitation as a bridge to surgery.
      ,
      • Harmon L
      • Boccalandro F.
      Cardiogenic shock secondary to severe acute ischemic mitral regurgitation managed with an Impella 2.5 percutaneous left ventricular assist device.
      support during high-risk PCI and aortic valve valvuloplasty,
      • Badawi RA
      • Grise MA
      • Thornton SN.
      Impella 2.5 assisted balloon aortic valvuloplasty and percutaneous coronary intervention as a bridge to heart transplantation.
      and LV unloading during extracorporeal support.
      • Vallabhajosyula S
      • O'Horo JC
      • Antharam P
      • et al.
      Venoarterial extracorporeal membrane oxygenation with concomitant Impella versus venoarterial extracorporeal membrane oxygenation for cardiogenic shock.
      The Impella device comes in several versions, with escalating levels of support based on the available liters of flow (eg, Impella CP, 5.0, and 5.5 [Abiomed Inc]). The Impella RP (Abiomed Inc) is specifically designed for right-sided support, whereas the remainder of devices are indicated for isolated LV failure. For biventricular failure, the “Bipella” technique has been used with concomitant Impella RP and CP insertion. Unless otherwise mentioned, this review focuses on the LV support devices.

      Impella's Hemodynamic Support

      The Impella consists of an electric microaxial flow pump that continuously extracts blood from the left ventricle through an inlet cage, bypassing the aortic valve before ejecting it into the ascending aorta.
      AbiomedInc
      • Werdan K
      • Gielen S
      • Ebelt H
      • Hochman JS.
      Mechanical circulatory support in cardiogenic shock.
      • Burzotta F
      • Trani C
      • Doshi SN
      • et al.
      Impella ventricular support in clinical practice: collaborative viewpoint from a European expert user group.
      The hemodynamic support offered by the Impella is the result of supplementing forward flow across the aortic valve and pressure augmentation (an increase in aortic and coronary pressure), leading to increased CPO, CO, and MAP. The Impella provides 2.5 to 6.0 L/min support depending on the specific model (Impella 2.5, CP, 5.0, LD, or 5.5). Support level settings, termed “P” level, and the aortic-ventricular pressure gradient are directly and inversely related to forward flow, respectively. Valgimigli et al
      • Valgimigli M
      • Steendijk P
      • Sianos G
      • Onderwater E
      • Serruys PW.
      Left ventricular unloading and concomitant total cardiac output increase by the use of percutaneous Impella Recover LP 2.5 assist device during high-risk coronary intervention.
      reported a total net CO increase of 23% associated with Impella 2.5 support, whereas multiple others have reported increased active forward flow.
      • Burzotta F
      • Trani C
      • Doshi SN
      • et al.
      Impella ventricular support in clinical practice: collaborative viewpoint from a European expert user group.
      ,
      • Dixon SR
      • Henriques JP
      • Mauri L
      • et al.
      A prospective feasibility trial investigating the use of the Impella 2.5 system in patients undergoing high-risk percutaneous coronary intervention (The PROTECT I Trial): initial U.S. experience.
      It should be noted that the increased CO is a net increase, accounting for both the supplemental flow from the device and native CO (Fig. 3). Remmelink et al
      • Remmelink M
      • Sjauw KD
      • Henriques JP
      • et al.
      Effects of left ventricular unloading by Impella recover LP2.5 on coronary hemodynamics.
      reported augmentation of the aortic blood pressure in addition to the forward flow reported by the device. This hemodynamic support provides increased coronary and systemic perfusion, disrupts the cycle of CS,
      • Hochman JS
      • Sleeper LA
      • Godfrey E
      • et al.
      SHould we emergently revascularize Occluded Coronaries for cardiogenic shocK: an international randomized trial of emergency PTCA/CABG-trial design. The SHOCK Trial Study Group.
      ,
      • Burzotta F
      • Trani C
      • Doshi SN
      • et al.
      Impella ventricular support in clinical practice: collaborative viewpoint from a European expert user group.
      ,
      • Kapur NK
      • Esposito M.
      Hemodynamic support with percutaneous devices in patients with heart failure.
      unloads the left ventricle, reduces MVO2 with minimization of the infarct size,
      • Burzotta F
      • Trani C
      • Doshi SN
      • et al.
      Impella ventricular support in clinical practice: collaborative viewpoint from a European expert user group.
      ,
      • Rihal CS
      • Naidu SS
      • Givertz MM
      • et al.
      2015 SCAI/ACC/HFSA/STS clinical expert consensus statement on the use of percutaneous mechanical circulatory support devices in cardiovascular care: endorsed by the American Heart Assocation, the Cardiological Society of India, and Sociedad Latino Americana de Cardiologia Intervencion; Affirmation of Value by the Canadian Association of Interventional Cardiology-Association Canadienne de Cardiologie d'intervention.
      ,
      • Seyfarth M
      • Sibbing D
      • Bauer I
      • et al.
      A randomized clinical trial to evaluate the safety and efficacy of a percutaneous left ventricular assist device versus intra-aortic balloon pumping for treatment of cardiogenic shock caused by myocardial infarction.
      and limits the duration of CS.
      • Samuels LE
      • Kaufman MS
      • Thomas MP
      • Holmes EC
      • Brockman SK
      • Wechsler AS.
      Pharmacological criteria for ventricular assist device insertion following postcardiotomy shock: experience with the Abiomed BVS system.
      Additionally, although the device itself does not provide oxygenation, the Impella protects ischemic myocardium from further insult by indirectly improving oxygen supply and reducing oxygen demand, as further described later.

      Improved Oxygen Supply

      Coronary blood flow is dependent on the pressure gradient within the coronary vessels and the resistance of the vasculature. If the distal (ie, venous) pressure within the coronary vessels is assumed to be fixed, the coronary blood flow is proportional to the ratio of the aortic pressure and the microvascular resistance. Furthermore, coronary flow is also dependent on coronary perfusion pressure, which factors in the mean aortic pressure and the downstream pressure, in this case LVEDP, as the coronaries fill in diastole. In addition to increasing the aortic pressure by propelling blood into the aorta, the Impella unloads the left ventricle by removing blood directly from the cavity, reducing the LVEDP/V, and thus decreasing ventricular wall stress. This translates to a reduction in the resistance of the coronary circulation leading to the desired increase in coronary blood flow. Augmented flow has been demonstrated in many different models including technetium-99m sestamibi myocardial perfusion imaging
      • Remmelink M
      • Sjauw KD
      • Henriques JP
      • et al.
      Effects of left ventricular unloading by Impella recover LP2.5 on coronary hemodynamics.
      ,
      • Sauren LD
      • Accord RE
      • Hamzeh K
      • et al.
      Combined Impella and intra-aortic balloon pump support to improve both ventricular unloading and coronary blood flow for myocardial recovery: an experimental study.
      ,
      • Aqel RA
      • Hage FG
      • Iskandrian AE.
      Improvement of myocardial perfusion with a percutaneously inserted left ventricular assist device.
      and appears to directly correlate with increasing levels of support.
      • Vallabhajosyula S
      • O'Horo JC
      • Antharam P
      • et al.
      Venoarterial extracorporeal membrane oxygenation with concomitant Impella versus venoarterial extracorporeal membrane oxygenation for cardiogenic shock.
      A more recent publication demonstrated continuous support via a percutaneous ventricular assist device improved coronary perfusion in patients with critical coronary stenosis.
      • Alqarqaz M
      • Basir M
      • Alaswad K
      • O'Neill W.
      Effects of Impella on coronary perfusion in patients with critical coronary artery stenosis.
      Thus, the Impella provides myocardial protection by decreasing LVEDP/V and improving coronary flow during diastole.

      Reduced Oxygen Demand

      Peak systolic wall stress is a key determinant of MVO2. By reducing LVEDV, the wall stress is significantly reduced at increasing levels of Impella support. Additionally, the active forward flow provided by the Impella reduces the mechanical work by the native heart, dropping MVO2 and protecting at-risk myocardium from the progression of ischemia.
      The ventricular unloading accomplished by the Impella can be both quantified clinically and characterized by changes in the PV loop (Fig. 4). In the setting of isolated LV failure, clinically the LVEDP/V, PCWP, and CVP decrease under Impella support. The effect of continuous unloading shifts the PV loop leftward and downward, producing a triangular shape. As the PV loops narrows, it signifies the decrease in SW (the area within the PV loop), whereas the leftward shift points toward the reduction in potential energy (the area between the systolic and diastolic pressure–volume relations and bounded by isovolumic relaxation) and SW. Thus, the Impella affords a decrease in both determinants of pressure volume area (SW and potential energy) and myocardial oxygen consumption.
      Figure 4
      Figure 4The Impella's impact on the PV loop. (Data from Bastos MB, Burkhoff D, Maly J, et al. Invasive left ventricle pressure-volume analysis: overview and practical clinical implications. Eur Heart J. 2020;41:1286-1297.)

      The Evidence and Safety Profile of the Impella

      Despite the physiologic data suggesting that the Impella should ameliorate the downward spiral of CS, there are no randomized controlled trials to demonstrate a mortality benefit with the use of this device to date. Seven randomized controlled trials have been attempted to evaluate the effect of the Impella on mortality in CS, of which 5 were stopped early because of poor enrollment.

      Massetti M, Sabatier R. Comparison of standard treatment versus standard treatment plus extracorporeal life support (ECLS) in myocardial infarction complicated with cardiogenic shock. 2006. Available at: https://clinicaltrials.gov/ct2/show/NCT00314847. Accessed July 1, 2021.

      • Ouweneel DM
      • Engstrom AE
      • Sjauw KD
      • et al.
      Experience from a randomized controlled trial with Impella 2.5 versus IABP in STEMI patients with cardiogenic pre-shock. Lessons learned from the IMPRESS in STEMI trial.
      • Henriques JP
      • Remmelink M
      • Baan J
      • et al.
      Safety and feasibility of elective high-risk percutaneous coronary intervention procedures with left ventricular support of the Impella Recover LP 2.5.
      • Griffith BP
      • Anderson MB
      • Samuels LE
      • Pae WE
      • Naka Y
      • Frazier OH.
      The RECOVER I: a multicenter prospective study of Impella 5.0/LD for postcardiotomy circulatory support.

      O'Neill W. Trial Using Impella LP 2.5 System in Patients With Acute Myocardial Infarction Induced Hemodynamic Instability (RECOVER II). 2008. Available at: https://www.clinicaltrials.gov/ct2/history/NCT00972270?V_2=View. Accessed July 1, 2021.

      Moller J, Eiskjaer H, Junker A, Hassager C, Shaefer A, Werner N. Danish Cardiogenic Shock Trial (DanShock). 2012. Available at: https://clinicaltrials.gov/ct2/show/NCT01633502. Accessed July 1, 2021.

      The Impella has not been shown to improve outcomes in patients with CS compared with an intra-aortic balloon pump (IABP), although both published randomized controlled trials were underpowered. As such, at the current time, national and international guidelines for the management of CS do not recommend one MCS device over another.
      The IMPRESS (Impella Versus IABP Reduces Mortality in STEMI Patients Treated With Primary PCI in Severe Cardiogenic Shock) trial randomized 48 mechanically ventilated patients with severe CS complicating AMI to percutaneous mechanical support (n = 24) or IABP (n = 24).43 This was a high-risk cohort with nearly 92% of patients (Impella = 100%, IABP = 83%) experiencing cardiac arrest before randomization. Mortality was similar at 30 days and 6 months, although the baseline mortality in the sample was much lower than anticipated, invalidating the power calculation. As such, the authors elected to publish their data as an exploratory safety study. Given this high-risk cohort, these findings may not be generalizable to patients in earlier stages of shock.
      The ISAR-SHOCK (Efficacy Study of LV Assist Device to Treat Patients With Cardiogenic Shock) trial was a prospective, 2-center, randomized, open-label study that evaluated whether the Impella 2.5 provided superior hemodynamic improvement or reduced mortality compared with IABP for patients suffering CS from an AMI. The Impella 2.5 improved CI compared with the IABP at 30 minutes (Impella: ΔCI = 0.49 ± 0.46 L/min/m2; IABP: ΔCI = 0.11 ± 0.31 L/min/m2; P = .02) but had no effect on 30-day mortality.
      • Seyfarth M
      • Sibbing D
      • Bauer I
      • et al.
      A randomized clinical trial to evaluate the safety and efficacy of a percutaneous left ventricular assist device versus intra-aortic balloon pumping for treatment of cardiogenic shock caused by myocardial infarction.
      Recent bench studies demonstrating a reduced infarct size in at-risk myocardia have sparked discussion regarding the utility of LV unloading before reperfusion in AMI.
      AbiomedInc
      ,
      • Werdan K
      • Gielen S
      • Ebelt H
      • Hochman JS.
      Mechanical circulatory support in cardiogenic shock.
      Based on these preliminary data, the STEMI-DTU trial, an appropriately powered prospective investigation, is now enrolling.
      • Briceno N
      • Annamalai SK
      • Reyelt L
      • et al.
      Left ventricular unloading increases the coronary collateral flow index before reperfusion and reduces infarct size in a swine model of acute myocardial infarction.
      • Swain L
      • Reyelt L
      • Bhave S
      • et al.
      Transvalvular ventricular unloading before reperfusion in acute myocardial infarction.
      • Kapur N
      • O'Neill W.
      Primary Unloading and Delayed Reperfusion in ST-Elevation Myocardial Infarction: The STEMI-DTU Trial.
      The Danish Cardiogenic Shock (DanShock) trial, a randomized multicenter study allocating patients to conventional circulatory support or the Impella device and inotropic support before revascularization, is currently enrolling.

      Moller J, Eiskjaer H, Junker A, Hassager C, Shaefer A, Werner N. Danish Cardiogenic Shock Trial (DanShock). 2012. Available at: https://clinicaltrials.gov/ct2/show/NCT01633502. Accessed July 1, 2021.

      A total of 360 patients are planned to be enrolled with the primary end point of death. If successful, this will be the first randomized trial adequately powered to report mortality outcomes.
      As with all medical interventions, MCS comes with a number of potential complications, and the Impella is no exception. The IMPRESS trial demonstrated that both bleeding (33.3%) and hemolysis (8.3%) are frequent complications.
      • Ouweneel DM
      • Eriksen E
      • Sjauw KD
      • et al.
      Percutaneous mechanical circulatory support versus intra-aortic balloon pump in cardiogenic shock after acute myocardial infarction.
      A recent systemic review by Hill et al

      Hill J, Banning A, Burzzotta F, et al. A systematic literature review and meta-analysis of impella devices used in cardiogenic shock and high risk percutaneous coronary interventions. Interv Cardiol, 11, 2019, 161-171.

      demonstrated rates of hemolysis of 7.8% and a rate of limb ischemia of 5.9%. A pooled analysis of retrospective studies found that complications were relatively infrequent, except for bleeding (21.4%), and included device malfunction (2.5%), in-hospital stroke (3.7%), limb ischemia (3.6%), hematoma (4.9%), and hemolysis (8.1%). Although Impella use is variable in its application and outcomes, support was associated with higher rates of adverse events and costs compared with non-Impella MCS in a recent single-arm study by Amin et al.
      • Amin AP
      • Spertus JA
      • Curtis JP
      • et al.
      The evolving landscape of Impella use in the United States among patients undergoing percutaneous coronary intervention with mechanical circulatory support.

      Practical Considerations for the Impella

      Patients requiring MCS, including devices from the Impella family are, by definition, critically ill. Close attention must be given to patient selection, meticulous placement, and careful management of the device once it is deployed. The following section reviews the technical and practical aspects of caring for a patient with an Impella, with a special focus on troubleshooting for the transport provider.

      Impella Placement

      Arterial access is the first step in Impella placement. Most commonly, arterial access is obtained percutaneously via 1 of the common femoral arteries. The axillary artery is a reasonable alternative for percutaneous Impella insertion in patients with suboptimal femoral access. A surgical cutdown may be required for models with a larger bore (ie, Impella 5.5). Regardless of the access site or method, given the large arterial sheath required for Impella placement, access should be obtained meticulously to minimize complications.
      Once arterial access is obtained, serial dilation is completed over a guidewire for the appropriately sized peel-away introducer sheath (13F-23F depending on the device, Fig. 5A). An activated clotting time (ACT) greater than 250 seconds is typically confirmed after heparin administration. Next, the Impella is loaded over the guidewire via the peel-away sheath across the aortic valve under fluoroscopic guidance. On echocardiography, the inlet area is positioned approximately 3.5 cm below the aortic valve annulus and in the middle of the ventricular chamber. The peel-away introducer sheath allows for smooth catheter insertion and manipulation followed by removal of the sheath with no movement of the catheter or inserted device. Potential contraindications to Impella placement are listed in Table 2.
      Figure 5
      Figure 5A, Peel-away sheath. B, Repositioning sheath.
      Table 2Contraindications to the Impella Device
      Mural thrombus in the left ventricle
      Mechanical aortic valve or heart constrictive device
      Aortic valve stenosis/calcification (equivalent to a valve area ≤ 0.6 cm2)
      Moderate to severe aortic insufficiency (echocardiographic assessment of aortic  insufficiency graded as ≥ +2)
      Severe peripheral artery disease
      Refractory respiratory failure needing an oxygenator
      Known large ventricular septal defect
      Left ventricular rupture
      Cardiac tamponade
      Data from Abiomed Inc.
      AbiomedInc
      Although a peel-away introducer sheath is often used for Impella placement, a repositioning sheath (Fig. 5B) often replaces the peel-away introducer sheath when extended-duration support is required. The repositioning sheath allows for continuous arterial access, improved hemostasis, and improved distal perfusion due to a small caliber relative to the peel-away introducer sheath. After the repositioning sheath is placed, the sterile sleeve cover should be attached. The proceduralist ensures there is no slack in the aorta via fluoroscopy and the Tuohy-Borst valve (a leak-proof, locking valve that allows the introduction of procedural devices) is tightened to prevent migration. Critical care transport medicine providers should be familiar with the peel-away introducer and repositioning sheath because they may both be encountered in the transport environment.

      The Purge Cassette and Anticoagulation

      The success of the Impella depends on appropriate anticoagulation and ensuring blood does not enter the microaxial motor (Fig. 6). Blood is excluded from the microaxial motor by a heparin-infused dextrose-based countercurrent purge solution, which maintains pressure within and lubricates the motor. The purge solution typically contains 25,000 units of unfractionated heparin in 500 mL 5% dextrose solution (50 U/mL), with the device console determining the flow rate (2-30 mL/h) to maintain a purge pressure between 300 and 1,100 mm Hg. It is imperative that all transports have a purge solution running through the impeller at all times and that providers do not use normal saline in the purge system.
      AbiomedInc
      If the patient has bleeding complications, the purge solution can temporarily be replaced with a 5% dextrose solution without heparin.
      • Seyfarth M
      • Sibbing D
      • Bauer I
      • et al.
      A randomized clinical trial to evaluate the safety and efficacy of a percutaneous left ventricular assist device versus intra-aortic balloon pumping for treatment of cardiogenic shock caused by myocardial infarction.
      Anticoagulation parameters should be monitored frequently with subtherapeutic and supratherapeutic values promptly corrected through systemic anticoagulation or changes in the purge solution concentrations.
      • Allender JE
      • Reed BN
      • Foster JL
      • et al.
      Pharmacologic considerations in the management of patients receiving left ventricular percutaneous mechanical circulatory support.

      Automated Impella Controller

      The Automated Impella Controller (Abiomed Inc) is the primary interface for providers to monitor the Impella function, address alarms, and assess the catheter's position (Fig. 7). The Automated Impella Controller is powered by alternating current (AC) power or an internal battery, which can provide power for 60 minutes when fully charged. We recommend placing the Automated Impella Controller on AC power when secured within the transport vehicle.
      Figure 7
      Figure 7The Abiomed Automated Impella Controller.
      The alarm window is displayed at the top of the home screen and displays up to 3 alarms simultaneously in order of priority, with instructions displayed to the right of the alarm window. The color of the alarm corresponds with priority (red, critical; yellow, serious; white, advisory; and gray, previously resolved alarms).
      At the bottom of the display, from left to right, is the Impella catheter flow, purge system, and system power. The Impella catheter flow displays minimum flow, maximum flow, current flow rate in L/min, and the LVEDP (with SmartAssist, Abiomed Inc, Danvers, MA). The flow numbers are white if the catheter is in the correct position and yellow if the position is incorrect or unknown. The purge system marquee scrolls from left to right, with slow scrolling indicating a normal purge flow rate and fast scrolling representing a bolus or priming flow rate. If the Impella is equipped with SmartAssist, you may be able to directly measure LVEDP in real time.

      How is Impella CP with Smart Assist different than Impella CP? Available at: https://www.heartrecovery.com/education/education-library/faq-how-is-impella-cp-with-smartassist-different-from-impella-cp. Accessed July 12, 2021.

      A full green battery represents a fully charged battery, with a partial green battery representing > 50% charge, yellow representing 16% to 50% charge, and a partial red bar representing < 15% charge. A moving gray bar represents the battery being charged. Beside the battery icon is a plug icon. A green plug indicates that the controller is running on AC power, whereas a gray plug with a red X indicates no AC power is detected.

      Placement Screen

      The placement screen displays the placement signal and the motor current waveforms and maximum, minimum, and average values (Fig. 8). The display soft button is used to navigate this screen. The placement signal waveform is useful in determining the location of the open pressure area of the catheter with respect to the aortic valve by evaluating the current pressure waveform as an aortic or ventricular waveform.
      Figure 8
      Figure 8A, Correct placement of the Impella. B, Placement of the Impella in the aorta (migrated out). C, Placement of the Impella in the ventricle (migrated in).
      The motor current is a measure of energy intake, which varies with different motor speeds and aortic-ventricular pressure differences. When the Impella is positioned correctly (inlet area in the left ventricle and the outlet in the ascending aorta), the motor current is pulsatile given that the pressure difference between the inlet and outlet areas changes with the cardiac cycle. If, for example, the inlet and outlet areas are both distal to the aortic valve, the motor current would be flat.

      Assessment, Complications, and Transport Considerations for the Transport Provider

      Assessment

      In addition to the thorough standard physical examination required for all critically ill patients, there are specific considerations that should be considered for patients being treated with the Impella. The assessment of the patient should also include an assessment of the Impella insertion site, the Impella device, and a thorough neurovascular examination. Impella-specific assessment and documentation checklists are present in Tables 3 and 4, respectively.
      Table 3Assessment Checklist of the Impella Patient
      • Make sure the Tuohy-Borst valve with sterile sleeve is locked.
      • Assess pulses; check pulses distal from the insertion site upon acceptance of the patient and upon arrival at the destination. Doppler may be needed for pulse checks.
      • The Impella device should produce a placement and motor current waveform.
      • Assess insertion site (specifically the presence of active bleeding, a hematoma, or ecchymosis).
      • Mark the Impella at the skin to allow for easy assessment of accidental displacement of the device.
      Table 4Documentation Checklist for the Impella Device
      • Type of device (2.5, 5.0, CP, RP, etc)
      • P level
      • Impella flow (max/min and mean)
      • Placement signal (max/min and mean)
      • Motor current (max/min and mean)
      • Insertion date/time/depth
      • Last ACT if known
      ACT = activated clotting time.
      Table 5.
      Table 5Access Site Bleeding Checklist
      • Sheath type: repositioning sheath preferred
      • Proper positioning
      • Dressing
      • Knee immobilizer/patient positioning
      • Minimize movement
      • Check anticoagulation status
      When assessing the hemodynamic status of the patient, the arterial pressure (labeled placement signal) on the Impella console is a reflected pressure of the aorta used for catheter positioning only and is not a true arterial pressure. Clinical decision making for interventions must be based on blood pressures recorded by a noninvasive blood pressure cuff or a separate arterial line. If pulsatility is minimal and the noninvasive blood pressure is inaccurate or unobtainable, using the MAP from the placement signal is acceptable.

      Complications of the Impella Device

      Device Malposition

      When positioned properly, the Impella device sits across the aortic valve with the blood inlet area sitting within the left ventricle and the blood outlet area within the aortic root. When determining the position of the Impella through imaging, fluoroscopic imaging is the best method. However, a transthoracic echocardiogram or a standard chest radiograph can be used and is often more readily available. The following should be kept in mind:
      • 1.
        If the “device position wrong” or “device position unknown” alarms while at the referring facility, request confirmation imaging and physician repositioning. The pump offers no support and can cause harm if malpositioned.
      • 2.
        At the bedside, the transport provider can perform transthoracic echocardiography to rapidly assess the appropriate position of the Impella. In the parasternal long view, the visible inflow port should be no more than 4.0 cm and no less than 3.0 cm into the left ventricle from the aortic valve annulus. The outlet port should be between 1.5 and 2 cm distal to the sinuses of Valsalva. In addition, the inflow port should be clear of mitral valve structures and the left ventricle wall.
        • Stainback RF
        • Estep JD
        • Agler DA
        • et al.
        Echocardiography in the management of patients with left ventricular assist devices: recommendations from the American Society of Echocardiography.
      • 3.
        Incorrect positioning can also be identified by checking the controller display.
        • a.
          Correct Impella position (Fig. 8A): when positioned correctly, the red aortic signal waveform will demonstrate a typical aortic pressure tracing like an arterial line. The white LV placement signal will show a typical ventricular waveform with tall pressure waves. The green motor current will be pulsatile, and the home screen will show “Impella Position OK.”
        • b.
          Impella is too deep in the left ventricle (Fig. 8C): the alarm will read “Impella position in Ventricle.” Both the red waveform and white waveform, if equipped with SmartAssist, will demonstrate a typical ventricular waveform, and the green motor current line will be flat.
        • c.
          Impella is fully in the aorta (Fig. 8B): the alarm reads “Impella position in aorta”; both the red waveform and the white waveform show an aortic pressure waveform. The green motor current line is flat.
        • d.
          Unknown Impella location: home screen will read “Impella Position Unknown.” This could be due to low native heart pulsatility and is relatively common.
      • 4.
        If the alarm occurs during transport, assess the waveform to ascertain whether the pump is too far in or too far out. Do not attempt repositioning during transport; any changes need to be made by a trained clinician, likely with support from real-time imaging. Closely monitor the patient and anticipate the need for inotropic and vasopressor agents to support cardiac function. Keep the device and purge system running to prevent thrombus formation. The receiving facility should be contacted for medical direction and to request an echocardiography technician and physician be available upon arrival for repositioning. Under medical direction, transport providers may consider decreasing the “P” level if the device is malpositioned.
      • 5.
        If the Impella is malpositioned, repositioning by a cardiologist or other credentialed provider should be attempted under direct imaging guidance. If the Impella is completely out of the ventricle, any reattempts should be done under fluoroscopic guidance strictly under physician discretion.

      Suction

      A suction event may occur if there is inadequate blood volume in the left ventricle for the Impella to pump, limiting the amount of support that can be provided by the device. Suction events can be seen in patients with right ventricular failure, hypovolemia, or other pathophysiologic states with diminished preload or right-to-left ventricular transit and can result in hemolysis. The Automated Impella Controller, when running in “Auto Mode,” will reduce the motor speed on the Impella if a suction event is detected. If the suction event resolves, the Impella will return to the original motor speed. If the suction event is still detected at the lower speed, the “suction” alarm will occur. If the “suction” alarm occurs, contact medical control and consider the following steps under their direction:
      • 1.
        Decrease the “P” level by 1 or 2 to reduce the effects of suction. Do not drop below “P2” because retrograde flow can occur at lower levels of support.
      • 2.
        Assess for hypovolemia and volume resuscitate as indicated.
      • 3.
        Evaluate for right ventricular dysfunction with CVP or echocardiography when able. Consider ionotropic support for right ventricular dysfunction if suction alarms are persistent and echocardiographic findings or CVP are suggestive of right ventricular failure. If a PAC is in place, the assessment of CVP, PA pressures, and calculation of the pulmonary artery pulsatility index can help quantify right ventricular dysfunction and filling pressures.
      • 4.
        Gradually return “P” level to previous setting.

      Access Site Bleeding

      If a patient has significant bleeding from the access sheath, multiple factors should be considered (Table 5). A repositioning sheath has improved hemostasis and decreased complications relative to a peel-away introducer sheath. The peel-away introducer sheath is designed to “peel away” with scored edges that are easily disrupted with an untampered body near the proximal portion of the sheath. Assessing for active bleeding or sheath complications before transport is key.
      If a repositioning sheath is present, improving the positioning and bolstering the dressing of the sheath should be considered. Because the repositioning sheath is a tapered sheath, it should be as far into the vessel as possible with the first suture ring buried under the skin. Appropriate dressing of the sheath can decrease unnecessary oozing. The Impella catheter should be “tented” (Fig. 9) to eliminate a shallow angle and maintain the angle of entry into the artery, ultimately decreasing drag on the arteriotomy site. Close attention should be paid to whether the sheath or device has moved in or out of the patient. If possible, transport providers may consider leaving the access site visible during transit to closely monitor for signs of bleeding or movement.
      Figure 9
      Figure 9Correct positioning of the Impella catheter.
      Movement of the patient should be minimized, with the head of the bed kept at less than 30 degrees. A knee immobilizer can be used to assist in preventing unintended movement of the lower extremities. Lastly, the patient's anticoagulation status must be considered, assessing the ACT if possible. Systemic anticoagulation should not be started until the ACT is less than 160 seconds. If the therapeutic goal is maintained with the purge solution, systemic anticoagulation is typically deferred because starting systemic anticoagulation with an ACT > 180 seconds increases bleeding risk.
      Other purge system complications include the following:
      • 1.
        Air in the purge system: follow the instructions on the screen for deairing the purge system. Always keep the purge bag and syringe above the level of the purge cassette to minimize the chance of air entering the cassette.
      • 2.
        Purge pressure low: check tubing for leaks and ensure all connections are tight.
      • 3.
        Purge system blocked: check tubing for kinks and obstruction.
      • 4.
        Placement signal lumen blocked: the placement of the signal lumen may be clotted off due to a closed or partially closed roller clamp on the saline bag or the pressure bag not being inflated appropriately. If the line will not aspirate, do not flush because this may dislodge thrombi if present.

      Resuscitation

      Patients receiving MCS are universally critically ill. As such, they are at high risk for cardiac arrest from coronary ischemia, spontaneous arrythmia, critical hypotension, or progression of the original insult. Standard advanced cardiac life support (ACLS) principles apply, and high-quality chest compressions are essential and generally safe in patients with an Impella.

      Ventricular Fibrillation/Ventricular Tachycardia Without a Pulse

      Ventricular tachycardia or fibrillation usually renders the Impella ineffective because of concomitant right ventricular failure. Reduce the flow to approximately 1.5 L/min (P2), begin cardiopulmonary resuscitation (CPR), and defibrillate as indicated per ACLS. During defibrillation, do not touch the Impella catheter, cables, or Automated Impella Controller because this could injure the provider.

      Asystole/Pulseless Electrical Activity

      Reduce the flow to approximately 1.5 L/min (“P2”) and initiate CPR and standard ACLS therapy. With return of spontaneous circulation, return the “P” level to the previous setting and ensure the receiving facility is aware that the patient underwent CPR during transport. The patient will require echocardiography to check that the Impella is positioned correctly across the aortic valve after chest compressions. Monitor the device very closely for the alarms described previously.

      Transport Considerations

      When a critical care transport request is received for a patient currently located in a cardiac catheterization laboratory, we recommend that the transport service routinely ask if the patient is supported by an Impella or other MCS device. Some transport teams and some transport vehicles may be unable to transfer patients supported by these therapies, either because of inadequate training or logistical issues, including weight limits, spatial limitations, or a lack of AC power on the aircraft. Furthermore, those teams who are able to do so will benefit from earlier knowledge that their potential patient is supported by an Impella in terms of readiness and preparation prior to arrival at the bedside. The following steps should be performed:
      • 1.
        The automated Impella Controller should be fully charged before transport.
        • a.
          The device will run for a maximum of 60 minutes on battery power alone. If taking the Impella from the sending facility, ensure that it is plugged in while preparing for transport. If the device has not been fully charged, the 60 minutes of battery power will not be attainable.
        • b.
          A built-in direct current to AC power inverter is needed if the transport is greater than 60 minutes and is highly recommended for any interfacility transport in the event that transport delays occur.
        • c.
          Keep the controller connected to AC power (or an AC inverter) whenever possible.
      • 2.
        Positioning and securing the Automated Impella Controller
        • a.
          Always ensure that the cooling vents are not blocked.
        • b.
          A bed mount is present on the back of the housing that may be used to hang the controller on the stretcher while the patient is transported inside the hospital or between the hospital and the transport vehicle.
        • c.
          When within the transport vehicle, the Automated Impella Controller should be strapped to a flat, secure surface with the bed mount used as a loop to secure the strap.
        • d.
          The controller should be positioned to allow team members’ easy access to the display screen and soft buttons to view alarms and make any necessary changes.
        • e.
          If possible, place the controller and the red Impella plug at the level of the patient's heart during transport.
        • f.
          Do not stress the connector cable from the controller to the Impella catheter. Tension could move the catheter out of the correct position and compromise patient circulatory support.
      • 3.
        Air should be removed from the purge fluid bag and always positioned right side up to prevent the need to remove air from the system during transport. Carefully monitor purge pressures during changes in altitude.
      • 4.
        During transport, the Impella Controller may be exposed to strong electromagnetic disturbances causing it to display soft button menu selections that were not previously selected by the user. No intervention is required because the operating parameters should not be affected. However, patient hemodynamics should be closely monitored to confirm normal Impella function.
      When transferring a patient with the Impella device in place, do not raise the head of the patient's bed higher than 30 degrees because this may induce kinking of the catheter or increase the bleeding risk.

      Summary and Conclusion

      CS generally occurs due to impaired ventricular contractility leading to a spiral of reduced CO, hypotension, and further coronary insufficiency, culminating in impaired end-organ perfusion and shock. Clinically, providers will observe hypotension, volume overload, and poor end-organ perfusion. Despite neurohormonal compensatory responses and pharmacologic interventions, cardiac dysfunction often remains, in part secondary to the increased MVO2, which exacerbates ventricular dysfunction.
      Although the treatment of CS suffers from a lack of robust randomized clinical trial data, MCS devices are increasingly used for this diagnosis, with the Impella device being a frequent choice of interventionalists. The hemodynamic support offered by the Impella is the result of supplementing forward flow across the aortic valve and pressure augmentation. This hemodynamic support provides increased coronary and systemic perfusion, disrupts the cycle of CS, unloads the left ventricle, reduces MVO2 with minimization of the infarct size, and limits the duration of CS.
      Once deployed, meticulous monitoring and management of the Impella device are required by a provider familiar with the device and the automated Impella controller. Before transport, a thorough physical examination should be completed in accordance with Table 3, and providers should vigilantly monitor for complications throughout transport. Patients supported by an Impella are at high risk for cardiac arrest. Standard ACLS principles apply, and high-quality chest compressions are essential and generally safe in these patients.
      With the proliferation of its use, in conjunction with the centralized care models used by many health systems in the United States, critical care transport providers are likely to encounter these patients with regularity. Transporting patients receiving MCS, while logistically challenging, can be performed safely in well-trained systems. It is imperative that transport providers caring for patients supported by an Impella be well versed in the underlying pathophysiology of CS, the Impella, and any complications they may encounter.

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