INTRODUCTION

End-stage heart failure (HF) is characterized by specific hemodynamic abnormalities. These abnormalities are a result of left ventricular (LV) systolic and diastolic dysfunction. This in turn leads to right ventricular (RV) dysfunction due to secondary pulmonary hypertension (PH). Systolic dysfunction in end-stage HF is manifested by low cardiac output and elevated left ventricular end-diastolic pressure (LVEDP) as measured by pulmonary capillary wedge pressure (PCWP). The elevation of PCWP increases pulmonary arterial pressure and hence increases afterload of the RV. In both ischemic and nonischemic cardiomyopathies, RV often has intrinsic contractile dysfunction and its performance is worsened by increased loading due to secondary PH. Left ventricular assist devices (LVADs) have been shown to decrease left ventricular end-diastolic volume and pressure and, in turn, decrease left atrial pressure (LAP) and pulmonary arterial pressure (PAP), while increasing effective cardiac output (CO).

Currently, the 2 most widely used durable LVADs are HeartMate II (St Jude Medical, Pleasanton, CA) and HeartWare (HVAD, HeartWare Inc., Framingham, MA), both of which are continuous-flow devices (CF-LVAD) (Figure 38-1). Although the 2 devices have a different mechanism of pumping blood—HeartMate II is an axial flow pump and HVAD is an intrapericardial, centrifugal pump—the hemodynamic effects are similar. A newer generation HM III device is currently under investigation.

For simplicity, this chapter will focus on the hemodynamic effects of continuous-flow LVADs. Furthermore, the role of echocardiography in evaluation of device function will be discussed.

GOALS OF LEFT VENTRICULAR ASSIST DEVICE SUPPORT AND EFFECT ON PRESSURE-VOLUME RELATIONSHIP

Pressure volume loops (PVLs) have played an important role in understanding the hemodynamic effects of LVADs.¹ Figure 38-2A shows the changes in PVL with HF and that of a patient with CF-LVAD. As the name denotes, CF-LVADs continuously unload the heart, irrespective of the cardiac cycle. This results in loss of the isovolumetric phase of the cardiac cycle. The normal PVL is trapezoidal and with loss of the isovolumetric phase, it transforms into a triangular shape. With increasing pump flow rates, the peak LV pressure generation decreases and the LV becomes increasingly unloaded. This causes a shift in the PVL toward the left. This decreases the pressure volume area (PVA) and reflects a decrease in MVO2. As the LV becomes more unloaded, the LAP and the PCWP decrease.

EFFECT OF CONTINUOUS-FLOW LEFT VENTRICULAR ASSIST DEVICE ON BLOOD PRESSURE AND AORTIC VALVE OPENING

With incremental increase in speed of the CF-LVAD, the peak LV pressure decreases and with overall rise in flow rates, the aortic pressure increases. This leads to dissociation between peak LV and aortic pressure (Ao) (Figure 38-2B to E). The aortic valve (AV) opening is dependent on the LV-Ao pressure gradient. As the pump speed increases, the LV-Ao dissociation occurs to a point where the AV no longer opens. This in turn leads to lack of pulsatility. The LV-Ao dissociation is only part of the physiological process that explains opening or closure of the AV. The LV preload and contractility can also play an important role in determining this process.

As the CF-LVAD speed increases (increased revolutions per minute [RPMs]), the LV preload decreases to a point where there is not enough LV volume to cause ejection. At the bedside, this is evident as the RPMs increase, the pulsatility decreases, to a point where there is no longer a palpable pulse. The speed at which the pulse drop occurs varies from patient to patient. Generally speaking 60% of the patients with CF-LVADs do not have a pulse.

Native contractility of the LV affects AV opening with more pressure generation in the LV cavity. This is particularly true for patients with myocardial recovery, where the AV valve may open at every beat.

BLOOD PRESSURE IN CONTINUOUS-FLOW LEFT VENTRICULAR ASSIST DEVICES

Depending on whether CF-LVAD patients have a pulse, the automatic or manual cuff blood pressure (BP) measurements may be inaccurate. For patients without a palpable pulse, Doppler devices are needed to measure BP accurately. Doppler signals are first heard as the perfusion pressure approaches the mean arterial BP. The mean BP goal in LVAD patients is between 60 and 85 mm Hg. Consistent high BP (defined as mean BP over 90 mm Hg) is associated with higher risks of strokes and bleeding.

EFFECT OF AFTERLOAD ON PUMP FUNCTION

CF-LVADs are sensitive to afterload as pump output depends on the pressure gradient between the pump outflow graft and aorta. Centrifugal flow LVADs are more sensitive to afterload than axial flow pumps. Higher afterload therefore results in decreased flow and in turn decreased LV unloading. Maintaining an effective mean BP is important for appropriate device function of the pump and optimal LV unloading.

EFFECT OF PRELOAD AND PUMP FUNCTION

Optimal preload is important for proper pump function. The preload is determined by 2 factors in patients who are supported by an LVAD: (1) overall volume body status, and (2) native RV function. In conditions where the effective body volume is reduced (such as dehydration and gastrointestinal [GI] bleeding), excess unloading can lead to suction events. The ventricular septum and the lateral free wall can be drawn into the inflow cannula causing both intermittent inflow obstruction and ventricular tachycardia.

EFFECT OF NATIVE VENTRICULAR CONTRACTILITY ON PUMP FUNCTION

Most patients with CF-LVADs do not have much native contractility. Although rare, but with the continuous LV unloading and optimal HF medications, the LV can completely reverse remodel. Increased LV contractility can lead to suction events, which is associated with higher risk of both pump thrombosis and ventricular arrhythmia.

Echocardiography plays a central role in surveillance of and detection of complications related to LVADs. The recent ASE document provides a comprehensive overview of recommendations regarding acquisition and protocol for surveillance, optimization of device function, troubleshooting complications, and evaluation of myocardial recovery.²

SURVEILLANCE ECHOCARDIOGRAPHY

Surveillance echocardiography in patients with CF-LVADs is used to detect the following:

1. 
   

   Complications related to chronic LVAD support such as thrombosis and right HF, and

   
   
2. 
   

   Pump speed optimization to ensure effective LV unloading.

Particular attention needs to be paid to LV size (and decrement compared to baseline), AV opening, location of the interatrial and interventricular septums, severity of aortic and mitral regurgitation (MR), and RV function. Decreased LV size, impaired AV opening, reduction in MR severity, and reduction of pulmonary artery systolic pressures (PASPs) are all indices of optimum unloading.

Optimal LVAD hemodynamics, as explained earlier, is a complex interplay of patient characteristics, native heart function, and device settings. Physicians must be cautious when selecting optimum LVAD speed in the perioperative period because preload and afterload conditions are altered by surgery. RV dysfunction often worsens in the postoperative period due to aortopulmonary bypass. However, rare cases may improve postoperatively, which may allow complete LV unloading and an increase in LVAD speed. There is variability among clinicians regarding appropriate speed selection; some physicians advocate partial LV unloading in the immediate postoperative period to avoid acute RV failure due to volume overload, whereas others are more liberal with LVAD speed. A comprehensive table for evaluation of CF-LVAD post implant is shown in Table 38-1.