Mechanical Assist Devices for Heart Failure

The Role of the Intraaortic Balloon Pump

The first step that specifically targets the problem is implementation of intraaortic balloon pump (IABP) counterpulsation. Despite the fact that it was introduced in 1968, the IABP still remains a very commonly used VAD (especially in the United States) because counterpulsation with a properly timed IABP simultaneously increases myocardial oxygen supply and decreases oxygen demand, and is often an effective treatment for left ventricular (LV) failure.

Fig. 22.2 demonstrates a deployed IABP. The device has been inserted percutaneously into the femoral artery and then advanced retrograde up the aorta to its correct position that is just distal to the left subclavian artery. Balloon inflation during diastole occludes the aorta and displaces arterial blood, abruptly increasing the aortic root pressure. This increases coronary perfusion pressure, which increases myocardial oxygen supply (assuming the patient has an adequate level of saturated hemoglobin). Abrupt deflation just before the next systolic ejection decreases the pressure in the aorta in a sudden fashion, facilitating forward ejection from the heart by decreasing impedance to opening of the aortic valve. This results in increased stroke volume and decreased myocardial work and therefore less oxygen demand on the struggling left ventricle. It has been reported that a properly timed, optimally functioning balloon pump can increase cardiac output by 20% or perhaps 30% and decrease afterload by as much as 15%. Of the two, it is generally believed that it is the decrease in oxygen demand that most benefits the failing ventricle supported by this device. In the setting of acute myocardial stunning (eg, as a result of an acute myocardial infarction [AMI]), such a decrease in oxygen demand can help to set the stage for myocardial recovery. In the setting of an acute deterioration of a chronically failing ventricle, the IABP may be used to stabilize hemodynamics as a bridge to intervention. Additional reported benefits of IABP counterpulsation include reduction in systemic acidosis and improvements in cerebral and renal microcirculatory perfusion. However, although a balloon pump is well known to improve cardiac function and overall hemodynamics, as mentioned earlier, it augments forward cardiac output by only 25% to 30% at maximum, and it will not augment anything if there is a complete absence of LV output. As a sole intervention, the IABP cannot be expected to rescue a patient from catastrophic myocardial failure.

Appropriate timing of balloon inflation and deflation is key to realizing the hemodynamic benefits of the device. The usual trigger for balloon inflation is the R wave of the patient's electrocardiogram (ECG); however, an arterial pressure tracing and pacing spikes can also be used. Regardless of the trigger used, as illustrated in Fig. 22.3 , inflation should always coincide with the dicrotic notch of the arterial tracing and should continue throughout diastole. Deflation should always occur just at end-diastole, immediately before the next systolic ejection. Inflation and deflation at any other point in the cardiac cycle must be manually corrected by adjustments in balloon timing. Fig. 22.4 shows and discusses potential timing errors. Helium is used as the inflation gas in the IABP because of its low viscosity and inert nature. Depending on the level of assistance required, the balloon can be triggered with each cardiac cycle (so-called 1 : 1 assistance), every other cycle (1 : 2), every third cycle (1 : 3), and so forth. Ratios of 1 : 2 or 1 : 3 are ideal for optimizing the timing of inflation and deflation.

Contraindications to the use of the IABP include clinically significant aortic insufficiency, aortic aneurysms, and significant friable atherosclerotic plaques in the aorta. However, the widespread availability of echocardiography to assess patients with cardiac problems, as well as the nearly routine use of transesophageal echocardiography (TEE) during IABP placement in the operative setting, can detect significant atherosclerotic disease in the arch and descending aorta and may help identify patients at high risk. While an ascending aortic dissection still contraindicates IABP use, a descending aortic dissection may no longer constitute an absolute contraindication to IABP use, because, in this era of echocardiography, TEE can be used to ensure the device comes to rest in the true lumen of the aorta.

Indications for the IABP have not changed, but routine IABP usage has recently become somewhat controversial, especially in Europe. It is estimated that 5% to 10% of patients will develop cardiogenic shock following an AMI, and early survival rates for these patients have always been reportedly of the order of 5% to 21%. However, 75% of such patients who were unresponsive to pharmacologic interventions were well known to exhibit hemodynamic improvement with IABP therapy alone, and early survival rates in these patients were reported to approach 93% when treated with IABP counterpulsation. Although decades of nonrandomized studies and clinical observational trials reported such benefit of IABP use, until recently, limited data were available from randomized trials regarding the outcomes of patients with AMI cardiogenic shock in whom IABP counterpulsation was employed.

In the era of thrombolysis as a primary management of AMI, the IABP enjoyed a class I recommendation in international guidelines. However, in current international guidelines ( Box 22.1 ), in the era of percutaneous coronary interventions (PCIs), the recommendation for routine IABP use in the setting of AMI cardiogenic shock has now been downgraded from class I to class IIa in the 2013 American Heart Association (AHA) guidelines and to a class IIb recommendation in European guidelines, on the basis of registry data and a small number of retrospective meta-analyses and randomized trials that failed to demonstrate a mortality benefit from use of the device. However, a number of serious concerns and criticisms (eg, regarding patient selection and timing of intervention) have been raised about the methodologies and protocols used in these trials (and thus in trials analyzed in the meta-analyses), and their negative conclusions have been questioned at the international level because several modern-era trials and analyses have demonstrated outcome benefits from IABP use in the AMI cardiogenic shock population.

Box 22.1

From ACCF/AHA guideline for the management of ST-elevation myocardial infarction: executive summary: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines: developed in collaboration with the American College of Emergency Physicians and Society for Cardiovascular Angiography and Interventions. Catheter Cardiovasc Interv . 2013;82(1):E1–E27.

2013 ACCF/AHA STEMI Guidelines for the Use of IABP and VADs

Class I

• 1.

  Emergency revascularization with either PCI or CABG is recommended in suitable patients with cardiogenic shock due to pump failure after STEMI irrespective of the time delay from MI onset. (Level of evidence: B)

• 2.

  In the absence of contraindications, fibrinolytic therapy should be administered to patients with STEMI and cardiogenic shock who are unsuitable candidates for either PCI or CABG. (Level of evidence: B)

Class IIa

• 1.

  The use of IABP counterpulsation can be useful for patients with cardiogenic shock after STEMI who do not quickly stabilize with pharmacologic therapy. (Level of evidence: B)

Class IIb

• 1.

  Alternative LVADs for circulatory support may be considered in patients with refractory cardiogenic shock. (Level of evidence: C)

ACCF , American College of Cardiology Foundation; AHA , American Heart Association; CABG, coronary artery bypass graft; IABP, intraaortic balloon pump; LVADs, left-ventricular assist devices; MI, myocardial infarction; PCI, percutaneous coronary intervention; STEMI, ST-segment elevation myocardial infarction; VAD, ventricular assist device.

A summary of available published data at the time of this writing regarding the routine use of the IABP to treat patients with AMI cardiogenic shock is as follows:

• 1.

  There are no strong data to support the routine use of the IABP in the management of AMI with or without cardiogenic shock, certainly when the device is deployed after PCI.

• 2.

  Conversely, no strong data support the avoidance of the use of an IABP in a timely manner in appropriately selected patients who might benefit from the hemodynamic optimization it can provide. Overall, minimal harm has been demonstrated from its use, specifically regarding the incidence of stroke, bleeding, peripheral ischemic complications, and sepsis.

• 3.

  There are data suggesting that the routine inclusion of an IABP before high-risk PCI decreases the number of procedural complications and the need for rescue.

• 4.

  There are data suggesting that long-term mortality is improved by timely inclusion of an IABP when placed before PCI to ameliorate myocardial ischemia, decompress the ischemic left ventricle, and assist forward flow.

Clearly, the IABP remains useful for stabilizing and improving the hemodynamics of selected patients with low cardiac output. It cannot, however, substantially augment forward cardiac output in patients with severe LV failure. This is where more formal MCS comes into play. Despite the absence of strong data and only a class IIb recommendation in current American College of Cardiology (ACC)/AHA guidelines regarding MCS for acute situations (see Box 22.1 ), the immediate survival of acute cardiogenic shock of any origin will be minimal if nothing is done and is disappointingly low (<20%) if medical management alone is instituted.

Implementation of Mechanical Circulatory Support

When the patient with a failing ventricle has failed to improve substantially following all the usual attempts to optimize and maximize, including an IABP, signs that the patient likely needs formal MCS include the following:

• Hypotension (mean arterial pressure [MAP] <60 mm Hg or SBP <90 mm Hg)

• Cardiac index <2 liters per minute [LPM]/m ²

• PCWP or right atrial pressure (RAP) >20 mm Hg

• Systemic vascular resistance (SVR) >2000 dyne-sec/cm

• Oliguria, low mixed venous oxygen saturation, and rising lactate.

It cannot be overemphasized that fixing numbers will not inevitably improve outcome. Even if somewhat acceptable central hemodynamics can be created pharmacologically, it is very important to consider evidence of poor organ and peripheral perfusion, such as oliguria, low mixed venous oxygen saturation, and rising serum lactate as indications of the need to support the circulation in a more formal way.

Moreover, it is critically important that the failure of the usual maneuvers to adequately stabilize the patient be promptly recognized, because experience of the past few decades has shown that the timing of implementation of MCS is the most important factor in patient outcome.

One cannot wait to initiate support until there is profound cardiogenic shock with deterioration in major organ function. Some recovery may be possible with restoration of adequate perfusion, but it is very difficult to predict, and study after study has shown that patient status at the time of implantation is the primary determinant of outcome. The longer one waits, the worse the outcome.

To provide formal MCS, the heart and great vessels must be cannulated and connected to a pump. Fig. 22.5 shows the classic cannulation strategies in the heart and great vessels that, until recently, were the only options, regardless of which manufacturer's device was selected to provide the support.

While diversion of blood to the pump provides the stroke volume that is ejected, it also facilitates decompression of the failing ventricle, which is critical, because decreased ventricular wall tension dramatically reduces myocardial oxygen demand, interrupting the cycle of worsening ventricular failure. It should also be noted that currently available VADs do not provide any oxygenation or removal of waste from the blood but simply act as pumps that can promote perfusion of the arterial circulation downstream from the failing ventricle. It is possible with some devices, however, to introduce an in-line membrane oxygenator and extracorporeal carbon dioxide (CO ₂ ) removal system for patients with concomitant respiratory failure.

Once the decision to provide MCS has been made, an appropriate device is selected. A number of different devices may be available, and the device selected for a given patient depends primarily on the following factors:

• 1.

  The anticipated duration of support required (different devices have different intended durations of use rooted in their engineering, and there are also considerations of FDA approval of the various devices).

• 2.

  Whether univentricular or biventricular support is required (some devices are intended to support just the left ventricle, although some can be configured to support either, or both simultaneously).

• 3.

  The degree of pulmonary dysfunction such that ECMO is required.

• 4.

  The urgency of the situation (some devices can be deployed rapidly, perhaps even at the bedside, whereas others require transfer to the operating room for sternotomy and cardiopulmonary bypass [CPB]).

• 5.

  The availability of the device.

As patient management experience in the field has grown and the outcomes have improved with more advanced devices, there has been an interesting shift away from the question, who needs a VAD? to the more pertinent question, who probably should not receive one? The only absolute contraindications to even temporary VAD use are prognostic factors that would preclude survival even with the restoration of perfusion to the vital organs and peripheral tissues. Thus, the potential for recovery is the paramount consideration. On many occasions, however, the myocardial insult is at least initially the primary problem, and more relative contraindications to VAD support will need to be considered.

Box 22.2 lists a number of commonly encountered considerations and relative contraindications to VAD support that include a variety of anatomic issues and other patient factors that create management issues, make VAD placement or use difficult, make complications more likely, or make meaningful recovery unlikely. Although the advent of modern devices and management strategies has rendered some of these relative contraindications essentially moot, all must be considered and addressed.

Box 22.2

Conditions or Comorbidities That Make Ventricular Assist Device Placement or Use Difficult, Make the Patient More Likely to Have Major Complications, or Make Meaningful Recovery Unlikely

Absolute Contraindication

• The patient will not survive regardless of the restoration of adequate systemic perfusion

Relative Contraindications or Issues That Need to Be Addressed

• Patient is not a transplant candidate (unless destination therapy or bridging to improved candidacy is the intention and a durable LVAD is being implanted)

• In situ prosthetic valves

• Clinically significant aortic insufficiency

• Clinically significant tricuspid insufficiency

• Mitral or tricuspid stenosis

• Congenital heart disease

• Intracardiac shunts

• Previous cardiac surgery

• Poor nutritional status

• Extremes of body surface area

• Advanced systemic disease (severe COPD, malignancy, ESLD, ESRD, sepsis, progressive neurologic disorder, etc.)

COPD, Chronic obstructive pulmonary disease; ESLD, end-stage liver disease; ESRD, end-stage renal disease; LVAD, left ventricular assist device.

Available Devices for Short-Term Support

Before 1992, when the Abiomed BVS 5000 became clinically available, standard centrifugal pumps were used to provide either univentricular or biventricular short-term mechanical circulatory assistance. Currently, this type of very basic device would be used for pediatric applications (with small-caliber cannulas to limit flow) or for ECMO; clinicians are beginning to incorporate ECMO more frequently into resuscitative efforts as a bridge to next decision. Such a strategy has been termed extracorporeal life support , in which a patient with refractory cardiogenic shock with uncertain outcome is placed on ECMO for a few days. In this fashion, a less expensive centrifugal device is used to determine if there is reasonable likelihood of survival, before committing the patient to a more formal (and much more expensive) VAD. As experience grows, ECMO utilization is likely to increase in these circumstances, concurrent with the availability of more advanced devices that are replacing standard centrifugal pump head technology.