How the Heart Pumps with an LVAD
With an LVAD, the heart operates in a parallel circulation where the device continuously draws blood from the left ventricle through an inflow cannula and pumps it directly into the ascending aorta via an outflow cannula, creating continuous flow that fundamentally alters normal pulsatile cardiac physiology.
Basic Mechanical Function
The LVAD creates a parallel circuit alongside the native left ventricle 1. Blood flows through two potential pathways:
- LVAD pathway: Continuous flow from LV → inflow cannula → pump → outflow cannula → aorta
- Native pathway: LV → aortic valve → aorta (when valve opens)
Modern continuous-flow LVADs (axial or centrifugal pumps) operate at fixed speeds (typically 8,000-12,000 rpm), generating continuous rather than pulsatile flow 2. This means the arterial pulse is often reduced or completely absent in LVAD patients 3.
Left Ventricular Mechanics
The LVAD fundamentally unloads the left ventricle by:
- Reducing LV end-diastolic pressures and volumes 2
- Decreasing LV wall tension and myocardial oxygen demand
- Allowing the LV chamber to decompress throughout the cardiac cycle
Critical point: At optimal pump speeds, the aortic valve can still open intermittently, allowing the native LV to contribute to cardiac output even at maximum pump speeds 2. However, continuous LVAD flow increases the aortic transvalvular gradient, which promotes prolonged aortic valve closure and predisposes to valve stasis [@1,2,3,4@].
Right Ventricular Effects
LV unloading creates paradoxical effects on the right ventricle [@2,3@]:
Beneficial effects:
- Reduces RV afterload (by decompressing the pulmonary circulation)
Detrimental effects:
- Increases RV preload (more blood returns from now-decompressed LV)
- Alters septal geometry (leftward shift from LV decompression)
- Reduces availability of cardiac myocytes for RV contraction (ventricular interdependence disruption)
This explains why right heart failure occurs in 28-34% of patients within 2 years despite successful LV support [@1,2,3,4@].
Hemodynamic Patterns and Pulsatility
Unlike the native heart's pulsatile ejection, continuous-flow LVADs produce near-constant flow [@5,6@]. The degree of residual pulsatility depends on:
- Pump speed settings
- Native LV contractility
- Whether the aortic valve opens during systole
Pump speed optimization is critical: Too high speeds prevent aortic valve opening entirely (promoting aortic insufficiency); too low speeds provide inadequate circulatory support 4.
Aortic Valve Dynamics
The continuous flow pattern creates a persistent pressure gradient across the closed aortic valve [@1,2,3,4@]. This leads to:
- Prolonged valve closure and stasis
- Altered shear forces in the ascending aorta
- Aortic leaflet injury, commissural fusion, and leaflet thickening
- Progressive aortic insufficiency (5.6-11.5% at 2 years, increasing to 19-35% long-term) [@1,2,3,4@]
Coronary and End-Organ Perfusion
Despite reduced pulsatility, myocardial perfusion remains adequate 2:
- The ratio of myocardial oxygen consumption to coronary blood flow stays constant
- Subendocardial-to-subepicardial blood flow ratio is preserved
- Brain and kidney microcirculation remain uncompromised at all pump speeds 2
Clinical Implications
The parallel circulation creates unique management challenges:
- Displayed pump parameters (speed, flow, power) can mislead clinicians unfamiliar with LVAD physiology 1
- The heart-pump interaction is dynamic and changes with loading conditions 4
- Optimal speed selection requires balancing adequate LV unloading against maintaining some aortic valve opening 5
Common pitfall: Increasing pump speed to address aortic insufficiency paradoxically increases preload to the RV, which may precipitate right heart failure if RV ventriculoarterial coupling is already compromised [@2,3@].
The fundamental physiology is that the LVAD doesn't replace the heart—it creates a hybrid circulation where mechanical and biological pumps work in parallel, with complex interdependencies that require careful hemodynamic optimization to minimize complications while maintaining adequate perfusion [@1,2,3,4@].