EVALUATION OF HEMODYNAMICS IN PATIENTS WITH CONTINUOUS FLOW LEFT VENTRICULAR ASSIST DEVICES
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.
Drawings and cross-sections of selected continuous-flow left ventricular assist devices (LVADs). A. HM II (subdiaphragmatic pump). Arterial blood passes from the left ventricle into the pump through the inflow (IF) conduit; blood flow direction is straightened by the inflow stator (IS); the rotor (R) controlled by the motor (M) spins to generate the needed force for blood to pass through the outflow stator (OS), then through the outflow (OF) conduit. Typical operating speeds are 8800 to 9400 rpm. (Adopted and modified from Stainback et al. J Am Soc Echocardiogr. 2015;28:853-909. Heatley et al. J Heart Lung Transplant. 2016;35:528-536.) B. HM 3 (intrapericardial pump). The rotor is magnetically levitated via electromagnetic coils and rotated via motor drive coils. The levitated rotor produces wide recirculation passages as shown in the magnified schematic on the right side with view of the gaps around the rotor and magnetic fields. (Adopted and modified from Netuka et al. JACC. 2015;66(23).) C. HVAD (intrapericardial pump). The continuous flow of blood through the centrifugal pump is shown. Blood is conveyed through the pump via an impeller that is suspended by a combination of magnetic and hydrodynamic forces, allowing frictionless rotation at operating speeds of 1800 to 2400 rpm. (Reprinted with permission from Aaronson et al. Circulation. 2012;125:3191-3200.)
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.