Mechanical Assist Devices

Case Synopsis

A 58-year-old man with past medical history significant for poorly controlled hypertension, diabetes, and an active 30 pack-year smoking history presented to the operating room (OR) in cardiogenic shock following a failed emergency percutaneous coronary intervention (PCI). An intraaortic balloon pump (IABP) was emergently placed via the right femoral artery, and he was intubated before OR transfer for ongoing hemodynamic instability and pulmonary edema. In the OR he became increasingly unstable and was “crashed” on cardiopulmonary bypass (CPB) where he underwent three-vessel coronary artery bypass grafting. During attempted separation from CPB with IABP and high-dose inotropic support, the transesophageal echocardiogram (TEE) showed akinesis of the anterior and lateral walls with an estimated left ventricular ejection fraction of 15% to 20%.

Problem Analysis

Definition

Mechanical circulatory assist devices are used to support the failing heart. A variety of devices are available for both long- and short-term support, as well as univentricular or biventricular support. The simplest and most commonly used of these is the intraaortic balloon pump (IABP), which operates by the principle of counterpulsation and was introduced clinically in 1967. In simple terms, the IABP consists of a catheter-mounted polyurethane balloon that is generally placed percutaneously into the descending thoracic aorta, with the catheter tip just distal to the left subclavian artery. Balloon inflation is timed to the cardiac cycle with the net effect being diastolic augmentation of coronary perfusion and systolic afterload reduction. The overall result is an improvement in the balance between myocardial oxygen supply and demand. Unlike ventricular assist devices (VADs), the IABP does not provide “active” circulatory support, and requires some degree of native cardiac function. General IABP components include the IABP catheter with introducer sheath, a console, a trigger source, a gas source (helium), and a slave cable (for use with intraoperative monitors). The console displays the electrocardiogram (ECG), central aortic pressure waveform, balloon inflation trace, and augmentation. Important controls include trigger selection, start/standby, alarms, and automatic/semiautomatic/manual modes.

More complex VADs used for short- and long-term support include nonpulsatile centrifugal and axial flow pumps. Centrifugal pumps include extracorporeal membrane oxygenation (ECMO), the TandemHeart PLVAD (Cardiac Assist Technologies Inc., Pittsburgh, PA), and the HeartWare HVAD (HeartWare International, Inc., Miami Lakes, FL); axial flow pumps include the Impella (ABIOMED, Inc., Danvers, MA) and the HeartMate II (Thoratec Corporation, Pleasanton, CA).

General indications for VAD support can be divided into four broad categories: (1) bridge to transplant, (2) bridge to recovery, (3) bridge to decision, and (4) destination therapy. A “bridge to candidacy” category has also been described. A general description of these devices is provided in Table 13.1 . Indications for ECMO are unique in that there are two configurations: veno-arterial (V-A) and veno-veno (V-V). V-A ECMO is used for cardiac failure, and V-V ECMO is used to support respiratory failure.

TABLE 13.1

General Overview of Commonly Used Short-Term and Long-Term Ventricular Assist Devices

	Pump Type	Type of Support	Flows/Pump Speed	Cannulation	Indications	General Description
Short Term
ECMO	Centrifugal	R,L	Flows generated will depend on cannula size; 0–4,000 rpm; up to 7 L/min	V-A ECMO inflow: femoral artery, axillary artery, aorta (central); outflow: femoral vein/IVC, SVC, right atrium. V-V ECMO: femoral vein–femoral vein; femoral vein–SVC, or V-V ECMO: Avalon catheter (dual lumen; IVC, RA)	Cardiac support (V-A ECMO); respiratory support (V-V ECMO)	Extracorporeal centrifugal pump consists of impellers or rotating cones set in a clear plastic housing that is electromagnetically coupled with a motor. Rotary motion draws blood from pump head to a return cannula. Continuous veno-veno hemofiltration possible.
Impella	Microaxial	R,L	Impella LP 2.5, up to 2.5 L/min (maximum rotational speed 50,000 rpm); Impella LP 5.0, up to 5 L/min (maximum rotational speed 33,000 rpm); Impella RD 5.0, up to 5 L/min	9-Fr catheter in femoral artery (LVAD); Impella LP 5.0 requires cutdown; Impella RD 5.0 requires sternotomy for insertion into RA	LV support in cardiogenic shock; can serve as a bridge to recovery or as a bridge to a longer-term device; RV support in cases of postcardiotomy RV failure	Miniaturized axial flow pump. Impella LP 2.5 most useful for high-risk PCI; LP 5.0 better when increased cardiac support required, as in cardiogenic shock.
TandemHeart	Centrifugal	L	Up to 5 L/min; 3000–7500 rpm	21-Fr femoral venous transseptal cannula; 12–19-Fr femoral arterial cannula; can be placed percutaneously or at time of cardiac surgery	LV support in cardiogenic shock; can serve as a bridge to recovery or as a bridge to a longer-term device	Transseptal LA catheter has 14 side-holes to optimize drainage. Provides 80%–90% unloading of LV and physiologic pressures (90 mm Hg). Six-blade impeller is magnetically driven and is cooled and lubricated by a fluid infusion system.
Long Term
HeartMate II	Axial	L	Up to 10 L/min flow possible; speed range 6000–15,000 rpm (clinical range 8600–9800 rpm)	Ventricular apex (inflow) and ascending aorta (outflow) made of woven Dacron and requires preclotting	Bridge to transplant; bridge to recovery; bridge to candidacy; destination therapy	Rotor sits in titanium pump housing; rotor is only moving part; motor in pump housing creates spinning magnetic field that spins rotor.
HeartWare HVAD	Centrifugal	L	Up to 10 L/min flow possible; 1800–4000 rpm (clinical operating range 2400–3200 rpm; lower ranges used for weaning from CPB)	Ventricular apex (inflow cannula); ascending aorta (outflow)	Bridge to transplant; bridge to recovery; awaiting approval for destination therapy	Miniaturized centrifugal pump sits in pericardial space. Inflow cannula integral to pump; impeller suspended by magnetic and hydrodynamic forces, and draws blood through pump; frictionless rotation at 1800–2400 rpm.

IVC, inferior vena cava; L, left; PCI, percutaneous coronary intervention; R, right; RA, right atrium; SVC, superior vena cava; V-A, veno-arterial; VAD, ventricular assist device; V-V, veno-veno.

In simple terms, a VAD diverts blood from the left side of the heart to the systemic circulation (LVAD), or from the right side of the heart to the pulmonary circuit (RVAD), thereby bypassing the failing ventricular chamber. With LVAD support, blood is diverted from the left side of the heart (left atrium [LA] or left ventricle [LV]) to a pump that propels or ejects blood into the systemic circulation via an outflow cannula or graft (typically anastomosed to the ascending aorta for long-term support)—if the majority of support is derived from the pump, the aortic valve will remain closed; an RVAD will divert blood from the right atrium (RA) or right ventricle (RV) to the pulmonary artery (PA).

It is important to understand the nomenclature used when describing VADs. “Inflow” refers to the flow of blood directly into the pump, whereas “outflow” refers to propulsion of blood out of the pump and systemically. This is different from cardiopulmonary bypass, where systemic flow from the pump is into the aorta, femoral artery, or axillary artery (inflow), and flow to the pump is from the venous cannula (outflow) into a reservoir.

Recognition

Intraaortic Balloon Pump

The IABP is used both in the critical care setting and in the operating room. Placement is generally percutaneous via the femoral artery; however, alternate sites, including the axillary artery and aorta, have been used when femoral access was of poor quality. Proper positioning can be verified using chest radiography, fluoroscopy, or TEE. Indications and contraindications for IABP use are outlined in Box 13.1 .

BOX 13.1

Indications and Contraindications for IABP Support

Indications

Postinfarct cardiogenic shock
Postinfarct mechanical complications (VSD, ruptured papillary muscle)
Ischemic arrhythmias
Intractable angina
High-risk cardiac catheterization/PCI
Major emergency (noncardiac) surgery in presence of severe CAD/ischemia
RV failure with pulmonary hypertension
Hypodynamic septic shock
Augmentation of thrombolysis with high risk for vessel reocclusion
Myocardial contusion
Failure to wean from cardiopulmonary bypass

Contraindications

Aortic insufficiency
Severe atherosclerotic disease (femoral, aortoiliac)
Aortic dissection
Aortic aneurysm
Infection at insertion site
Irreversible cardiac disease in a nontransplant candidate
“Do not resuscitate” status
Aortic, iliofemoral stents/grafts

CAD, Coronary artery disease; PCI, percutaneous coronary intervention; VSD, ventricular septal defect.

The IABP catheter has two lumens: one to the balloon, where helium gas is rapidly shuttled back and forth as the balloon inflates and deflates; and a second lumen that allows insertion of the catheter over a long guidewire and subsequent monitoring of central aortic pressure. Proper positioning of the catheter tip 1 to 2 cm distal to the left subclavian artery is important to prevent occlusion of major vessels (i.e., cerebral, renal, mesenteric).

Coronary blood flow depends on a number of factors, including perfusion pressure, myocardial extravascular intramural coronary compression, myocardial metabolic rate, and neurohumoral factors. Keeping in mind that coronary blood flow is equal to diastolic blood pressure minus left ventricular end diastolic pressure (CBF = DBP − LVEDP), and that the majority of left coronary blood flow occurs in diastole (right coronary flow occurs in both systole and diastole), it becomes easy to understand how balloon counterpulsation enhances blood supply to the heart. Rapid balloon deflation before systole displaces blood from the aorta, leading to reduced aortic pressure, improved forward ejection, decreased myocardial oxygen consumption (MVO ₂ ), decreased wall stress, and improved myocardial performance. Balloon inflation at the time of aortic valve closure increases coronary perfusion, leading to improved contractility, decreased filling pressures, and overall improved diastolic performance.

Ventricular Assist Device

Short-Term Devices

The Impella consists of a miniaturized microaxial flow pump mounted at the tip of a catheter that is placed either percutaneously (Impella 2.5) or by cutdown (Impella 5.0) via the femoral artery and advanced retrograde up the descending thoracic aorta and into the left ventricular outflow tract (LVOT). With activation, blood is continuously propelled from the LVOT into the aortic root/ascending aorta. The Impella RD is a miniaturized RVAD that is inserted via median sternotomy (generally at the time of heart surgery to support postcardiotomy RV failure). The inflow cannula is implanted directly into the right atrium, and the outflow resides in the main PA. Flows up to 5 L/min can be generated.

The TandemHeart diverts blood from the left atrium (via a transseptal puncture and femoral venous access) to a small centrifugal pump, which in turn propels blood systemically to the aorta via a femoral arterial cannula. It can be inserted percutaneously or surgically while on CPB.

Long-Term Devices

After the landmark publication of the REMATCH study (Randomized Evaluation of Mechanical Assistance in the Treatment of Chronic Heart Failure) in 2001, long-term support of the failing heart with a mechanical circulatory assist device became a reality. The evolution of long-term VADs from the original first-generation volume displacement pulsatile devices (HeartMate I, Novacor LVAS, formerly World Heart Corporation, now a division of HeartWare; Thoratec paracorporeal PVAD) to second-generation (HeartMate II; DeBakey Micromed—MicroMed Technology, Houston TX; Jarvik 2000—Jarvik Heart Inc., New York, NY) and third-generation (HeartWare, HeartMate III) nonpulsatile devices has resulted in pumps with smaller components, thinner drivelines, fewer moving parts, less friction and heat generation, and more quiet operation. These latter pumps could also be implanted in smaller patients. First-generation devices were plagued with mechanical failures, thromboembolism, and infectious complications associated with larger drivelines, valves, and other moving parts. Despite these early problems, the REMATCH study was able to demonstrate superior outcomes and quality-of-life measures in transplant-ineligible patients supported with the HeartMate I compared with maximal medical therapy. When data from the REMATCH study were extrapolated and compared with outcomes from the HeartMate II Destination Therapy Trial, the newer-generation pump showed even better outcomes and fewer complications, most notably a reduction in mechanical failures and driveline-associated infections. Currently, roughly 25% of patients awaiting cardiac transplantation are supported with a VAD. Indications and contraindications are highlighted in Box 13.2 .

BOX 13.2

Indications and Contraindications for VAD Support

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here