Mechanical Circulatory Support in the New Era: An Overview

Kiran Shekar; Shaun D. Gregory; John F. Fraser

Disclosures

Crit Care. 2016;20(66) 

In This Article

Mechanical Circulatory Support Strategies

Intra-aortic Balloon Pumps

Despite the controversies around their efficacy in the setting of cardiogenic shock,[8] IABPs can be seen as a bridge between conventional medical therapy and MCS. IABPs are more widely available than other MCS systems, lower risk, less invasive, easy to institute and may be a useful first-line MCS option while we await definitive evidence for their use in various clinical settings that may lead to cardiogenic shock. More detailed reviews of IABPs and a summary of evidence can be found elsewhere.[9] However, the IABP remains a useful adjunct and it should be noted that while it may improve native cardiac performance by reducing afterload and myocardial oxygen demand, it cannot partially or completely replace cardiopulmonary function. More advanced MCS options need to be considered and an early referral to a MCS center should be considered in a young patient with presumed reversible acute cardiomyopathy or in whom there are no overt contraindications for heart transplantation in the absence of cardiac recovery. Current data suggest that IABPs may assist aortic valve opening in patients requiring peripheral VA-ECMO and they should not necessarily be removed prior to VA-ECMO support.

Venoarterial ECMO

There has been a significant uptake of ECMO technology in adults since the 2009 H1N1 Influenza pandemic. This pandemic not only led to many new ECMO centers but also created greater awareness of the process of ECMO. Success achieved with venovenous (VV) ECMO during the pandemic with contemporary technology has certainly encouraged clinicians to apply ECMO technology to provide cardiorespiratory support in a variety of clinical settings. Providing tailored temporary MCS to patients with acute refractory cardiac failure using ECMO technology is a rapidly evolving area where intervention may be time-critical and mortality is higher than for isolated respiratory failure.[10,11]

The indications listed in the Extracorporeal Life Support Organization (ELSO) guidelines for ECMO for cardiac failure in adults are shown in Table 1.[12] The use of ECMO in the setting of CPR is discussed elsewhere.[13,14] International Society for Heart and Lung Transplantation guidelines[15] for MCS provide evidence-based recommendations for long-term MCS options for patients with cardiac failure. These guidelines strongly recommend consideration of the use of temporary MCS in patients with multiorgan failure, sepsis, or on mechanical ventilation, to allow successful optimization of clinical status and neurologic assessment prior to placement of a long-term MCS device. The severity of non-cardiac organ system failures may be used to identify suitable patients and a sequential organ failure assessment (SOFA) score >/15 has been considered a contraindication to VV-ECMO;[16] similar criteria may be applicable for VA-ECMO or for the use of an ECMO circuit as a temporary VAD.

The underlying cause of cardiac dysfunction and projected time course of recovery, severity of pulmonary dysfunction and projected time course of recovery, functional reserve of each ventricle, the presence and severity of valvular pathology, risk of arterial access and size of vessels, severity of coagulopathy and risk of sternotomy, planned future surgery such as long term VAD or transplant may all have to be considered prior to finalizing an individualized MCS strategy.[3]

For patients with predominant cardiac failure and preserved pulmonary function, several MCS strategies may be considered. Central VA-ECMO has traditionally been applied as a bridge to recovery in patients who fail to wean from cardiopulmonary bypass (CPB) after cardiac surgery (Fig. 1a). Central VA-ECMO outside this setting in adults is uncommon. Femoral VA-ECMO (Fig. 1b) is the more commonly used MCS modality in adults requiring urgent cardiac support as it can be initiated rapidly and a sternotomy with its concomitant bleeding is avoided. One of the major limitations of peripheral femoro-femoral VA-ECMO is LV afterload mismatch and inadequate LV decompression. Patients with very low native cardiac output states and severe mitral valve regurgitation are at a greater risk of developing hydrostatic pulmonary edema and further reduction of myocardial oxygenation by the distended LV cavity compressing the intracoronary circulation. Although, some centers use an IABP in conjunction with peripheral VA-ECMO to reduce LV afterload and pulmonary congestion, no definitive data exist to support its routine use. Femoral VA-ECMO is also limited by femoral arterial size and thus cannula size and the requirement for distal limb perfusion. Although use of smaller arterial return cannulae may minimize the need for routine back-flow cannula insertion for distal limb perfusion, early insertion of these cannulae should be considered in all these cases until more supportive evidence becomes available.

Figure 1.

Cannulation sites for venoarterial extracorporeal membrane oxygenation (VA-ECMO). VA-ECMO can be instituted (a) centrally by cannulating the right atrium/inferior vena cava and the aorta, or peripherally using (b) femoral vein and femoral artery (dark blue arrow arterial return cannula, light blue arrow back flow cannula for distal limb perfusion), or (c) axillary/subclavian artery. The choice is often guided by the clinical setting, expected duration of support and pulmonary function. From[3] with permission

LV and aortic root stasis from lack of cardiac ejection and failure of aortic valve opening may result in catastrophic intra-cardiac and aortic root thrombosis. Increased intensity of anticoagulation to minimize this risk may precipitate bleeding. Less invasive strategies, such as percutaneous trans-septal left atrial decompression[17] and subxiphoid surgical approaches to drain the left ventricle,[18] have been described to reduce LV distension. The residual atrial defect may require surgical correction once the patient has been weaned from the MCS. Use of a pVAD to decompress the distended left ventricle has also been reported in this setting[19] alleviating the need for a high risk septostomy or surgical venting.

Given its less invasive nature (compared to central MCS strategies) peripheral VA-ECMO, with attention to optimal LV afterload, minimizing LV distension with optimal fluid and inotrope therapy, anticoagulation and pulmonary management remains a viable first-line option for patients with isolated acute cardiac failure refractory to conventional management.

Other Temporary Mechanical Circulatory Support Configurations

Configurations Based on the ECMO Circuit. The limitations of peripheral VA-ECMO have prompted the use of ECMO devices[20] to facilitate ventricular unloading by changing to a temporary left or right VAD or a BiVAD configuration. Any perfusion strategy that creates a right-to-left shunt requires an oxygenator in the circuit. Oxygenators may additionally facilitate temperature management. This strategy effectively provides biventricular support and gas exchange through a double (Fig. 2a) or single (Fig. 2b) pump configuration with the ability to cease RV support when not required and thereafter to discontinue the oxygenator. However, this configuration requires a sternotomy and cannulation of the left ventricle (or left atrium), aorta and/or pulmonary arteries. A reoperation (sternotomy or thoracotomy) is then required for explantation of the cannula from the left ventricle or left atrium upon cardiac recovery or for implantation of a long-term mechanical assist device.

Figure 2.

Temporary biventricular support strategies. a Biventricular assist and oxygenation support using two centrifugal pumps. Cannulation details: transfemoral right atrium (RA) drainage → allograft to pulmonary artery (PA) for returning oxygenated blood; and left ventricle (LV) apex drainage to aorta return. An oxygenator was included in the right ventricular assist device circuit. b Biventricular and oxygenation support provided using a single centrifugal pump. Dual drainage cannulas positioned in the LV apically and right atrium transfemorally. Oxygenated blood was returned to the ascending aorta through central cannulation. Insert demonstrates how the two drainage tubes were merged using a Y-connector to enable usage of a single pump

RV support for up to several months can be provided with a CentriMag ECMO system through percutaneous femoral venous access to the right atrium and return to the pulmonary artery via a cannulated exteriorized Dacron graft. Alternatively, venous drainage can also be achieved through a centrally placed right atrial cannula. This strategy is described for temporary support of the right ventricle with insertion of a long term LVAD but is applicable to other causes of severe isolated RV dysfunction, such as post-massive pulmonary embolism. Inclusion of an oxygenator into the circuit at this stage ensures adequate oxygenation, CO2 removal and temperature regulation whilst facilitating protective ventilation. Upon RV recovery, the pulmonary artery graft can be ligated and buried upon decannulation without re-sternotomy.

Percutaneous VADs. Percutaneously inserted LVADs, such as TandemHeart and Impella,[9] are potential options for short-term MCS in the acute setting (Fig. 3). However, there is a paucity of supportive evidence[21] for their use and the complications with arterial access, such as bleeding and limb ischemia, cannot be understated. They may also be viable options to vent the distending left ventricle during peripheral VA-ECMO support. These devices in many ways are likely to form a significant part of our armamentarium whilst providing individualized MCS to a patient with acute cardiac failure.

Figure 3.

Schematic representation of two commercially available percutaneous ventricular assist devices. a the TandemHeart pVAD (Cardiac Assist Inc., Pittsburgh, PA, USA), b the Impella pVAD (AbioMed Europe, Aachen, Germany). From[53] with permission

The TandemHeart uses a centrifugal pump to drain the left atrial blood from a catheter placed transeptally via the femoral vein and returns it to the femoral artery. The Impella uses an axial pump that is inserted retrogradely across the aortic valve via the femoral artery. These devices provide some LV support but lack the ability to provide extracorporeal respiratory support if required. However, there are case reports pertaining to their successful use as RV assist and or biventricular assist devices.[22,23] Similarly, minimally invasive percutaneous right VADs have been developed (Impella RP system, Abiomed and TandemHeart, CardiacAssist) and may be significant additions to the spectrum of available MCS therapies in the future.

There has been a radical shift in VAD technology and new generation implantable rotary blood pumps are now a viable bridge to destination or heart transplant.[24,25] The shortage of appropriate donor organs and the expanding pool of patients waiting for heart transplantation have led to growing interest in alternative strategies, particularly in longer term MCS.

Long-term Implantable VADs

Indications for Support. Eligible patients with progressive, non-reversible, chronic heart failure may be placed on these devices as bridge to destination or heart transplant. Meticulous patient selection and timely insertion of the device/s is the key to positive outcomes.[6,24] The temporary MCS bridging strategies described above in many ways may eliminate the need for placement of these very expensive devices in critically ill patients. This is important, as urgency of VAD placement has also been shown to play a factor in survival. Patients receiving emergent LVADs have a lower rate of survival than patients who are less unwell when the LVAD is implanted.[26]

There are several risk models to predict the survival of heart failure patients.[27,28] These may be used to identify high-risk patients for potential LVAD therapy. The identified preoperative risk factors for mortality based on the results of the Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) indicate that older age, ascites, increased bilirubin, and cardiogenic shock (INTERMACS level 1) are highly associated with post-implant mortality.[29] While it is increasingly obvious that implanting a VAD in these patients is associated with poor survival, refinements in devices and surgical techniques raise an important question: when is it too soon to implant a VAD in a patient with progressive, non-reversible, chronic heart failure? The following sections will briefly discuss the available VAD options and common early complications that intensivists may encounter following VAD implantation.

Devices. Improved results and the increased applicability and durability of LVADs have enhanced this treatment option for end-stage heart failure patients. Results using non-pulsatile continuous flow pumps as a bridge to transplant or destination subsequent to the landmark Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) Trial[30] are very promising and significantly better when compared with pulsatile LVADs.[31] In 2006, 78 of the 98 implanted devices recorded on the INTERMACS registry were pulsatile, intracorporeal devices,[29] whereas in 2013, 2420 of the 2506 implanted devices recorded on the INTERMACS registry were continuous flow intracorporeal devices.[29] Therefore, this section focuses solely on the continuous flow VADs which are commonly used in the clinical setting and does not report on devices no longer clinically available or those under development. A summary of the technical aspects of the devices in this review is provided in Table 2. We briefly discuss the two commonly used rotary blood pump-based VADs in this article.

The HeartMate II (Thoratec Corporation, Pleasanton, CA) is the most widely implanted rotary blood pump (Fig. 4a), with a second-generation design that relies on a pivot bearing; however minimal wear is reported.[32] To date, over 20,000 HeartMate II devices have been implanted with support duration exceeding 8 years.[33] The HeartMate II received Food and Drug Administration (FDA) approval for bridge to transplant in 2008 and for destination therapy in 2010.[34] In 2014, Thoratec started clinical trials for the HeartMate 3, a third-generation centrifugal flow design with a magnetically levitated impeller to increase blood flow gaps and reduce blood trauma. The HeartMate 3 includes a small artificial pulse to enhance pump washout and textured blood-contacting surfaces to encourage tissue integration. The HeartWare HVAD (Fig. 4b) is a centrifugal, third generation device with passive magnetic and hydrodynamic forces levitating the impeller and two axial flux motors for redundancy in case one fails. The HVAD has also been used for RV support,[35] although CE or FDA approval for this purpose has not been obtained. In 2015, HeartWare started clinical trials of the miniaturized MVAD, an axial flow pump approximately one-third the size of the HVAD with similar impeller levitation principles and capable of less-invasive implantation due to its smaller size.[36]

Figure 4.

Two commonly used rotary ventricular assist devices (VADs). a Thoratec HeartMate II, b HeartWare HVAD

Apart from early postoperative hemostatic complications, a major issue in the early postoperative course is that of RV failure. While controversy remains around pre-emptive mechanical RV support using a temporary RVAD based on ECMO circuitry or an implantable LVAD on the right (there is no customized, long-term rotary RVAD at this stage), it should be noted that re-operation to institute mechanical RV support once RV failure sets in adds to mortality and morbidity in these patients.[37] A high index of suspicion preoperatively and vigilance and prompt escalation of pharmacological and mechanical RV support intra- and postoperatively is the key. We refer the readers elsewhere for a more detailed summary of outcomes and complications.[38]

Total Artificial Hearts

Compared to the dramatic increase in continuous intracorporeal pump implants over the last decade, clinical use of total artificial hearts has been much slower. In 2007, the INTERMACS database reported 22 pulsatile intracorporeal total artificial heart implants, which had increased only to 66 by 2013.[29] The lack of a long-term, low-wear device with small wearable components, as seen in the latest generation of VADs, may have contributed to the slow uptake of total artificial hearts. Meanwhile, the 'safety net' provided with a VAD, where remnant ventricular contractility may sustain life until emergency intervention, could also explain why total artificial hearts are only used when absolutely necessary.

Although several total artificial hearts, such as the Liotta-Cooley, Akutsu III and the AbioCor devices, have been used to support patients,[7] these devices are no longer used clinically. The Carmat (Vélizy Villacoublay, France) total artificial heart is currently in clinical use, however very few patients have been supported since the first implant in December 2013. Meanwhile, the use of a dual LVAD configuration for total artificial heart support has been reported using dual HeartMate II[39,40] or HeartWare HVAD[41] devices; however clinical experience with this technique is limited. The only total artificial heart currently available to fully support the circulation for which there is substantial clinical experience is the SynCardia total artificial heart (SynCardia, Tucson, AZ).

Initially developed as the Jarvik 7 and renamed as the Symbion, Cardiowest and now SynCardia total artificial heart, this pulsatile first generation device consists of two pneumatically operated chambers which provide total systemic and pulmonary flow. A pneumatic driver, for which a 6.1/kg portable version now exists, supplies pulses of compressed air through percutaneous leads to the left and right chambers to deliver almost 70/ml/beat. The beat rate can be changed to deliver flow rates up to 9.5/l/min from the device, which weighs 160/g. Unidirectional flow is achieved with four tilting valves, which have reportedly never failed, while the pumping diaphragms have a failure rate of less than 1 %.[42] Although the SynCardia total artificial heart has been in clinical use for several decades with CE and FDA approval for bridge-to-transplant (1999 and 2004 respectively) and FDA investigational device exemption for destination therapy (2015), widespread clinical use has been slow with over 1440 implants to date.[42] The longest duration of support with the SynCardia total artificial heart currently stands at 1374 days;[42] however typical support duration is closer to 15–90 days at different centers.[7] Meanwhile, SynCardia have recently had FDA investigational device exemption approval for a smaller total artificial heart version with 50/ml pneumatic chambers.

The quest for a durable, safe, practical and affordable total artificial heart continues and rotary blood pump technology has the potential to deliver the same. In the meantime, the temporary and long term MCS options discussed thus far will have to be used in an individualized, tailored fashion so that positive patient outcomes may be achieved whist making the most of available technology.

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