Ventricular Assist Device Therapy in Advanced Heart Failure

Technology of Left Ventricular Assist Devices

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  The initial LVADs were volume displacement pumps known as pulsatile-flow devices.

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  However, pumps had multiple moving parts, including bearings, valves, and pusher plates that were subject to failure.

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  A paradigm shift in the field of assisted circulation occurred with the introduction of durable, implantable continuous-flow devices.

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  The rationale for continuous flow was the observation that the initial pulsatile flow in the aorta is progressively dampened, transforming into continuous nonpulsatile flow at the level of the capillary ( Fig. 25.1 ).

  

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  Continuous-flow LVADs have only a single moving part and propel blood forward in a steady, continuous fashion with an axial or centrifugal rotor or an impeller.

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  With this simplified design, the risk of mechanical failure has been greatly reduced.

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  Currently, the HeartMate II (St. Jude Medical) and the HeartWare HVAD (HeartWare) are the only US Food and Drug Administration (FDA)–approved continuous-flow LVADs ( Fig. 25.2 ).

  

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  The HeartMate II is capable of providing up to 10 L/min of support and is surgically inserted into a preperitoneal pocket (See Fig. 25.2A ).

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  Blood is pulled out of the LV into the LV inflow cannula, accelerated by a rotor, and then ejected into the outflow graft, which is anastomosed to the ascending aorta.

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  A percutaneous driveline exits in the upper abdomen and connects the device with a portable controller and two batteries for mobile operation, or to a power base unit and a wall outlet when a patient is stationary for several hours (e.g., while sleeping).

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  The device provides a constant flow of blood with one backup speed used in case of a sudden drop of preload.

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  The typical operating speed range is 8600 to 9600 rpm.

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  Despite antithrombotic coating with titanium microbeads, anticoagulation with a target international normalized ratio (INR) of 2.0 to 3.0 and aspirin are recommended.

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  In distinction to the axial-flow HeartMate II, the HVAD is a miniaturized centrifugal pump (see Fig. 25.2B ).

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  The smaller size of this device allows implantation into the pericardial space and, often, a shorter operation.

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  The housing contains an impeller suspended by magnets and the device is capable of providing 10 L/min of flow.

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  The usual operating speed range is 2400 to 2800 rpm.

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  The presence of a continuous-flow device does not necessarily eliminate the presence of a palpable pulse on physical examination.

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  This has an important consequence when measuring a patient's blood pressure.

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  In the absence of a palpable pulse, mean arterial pressure (MAP) is measured via Doppler.

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    The blood pressure cuff is inflated above where the Doppler tones are heard, and then the return of sounds auscultated via Doppler is taken as the MAP.

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    MAP measured by Doppler overestimates pressure in the presence of palpable pulse and should not be reported.

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    Instead, auscultation of the Korotkoff sounds should be performed.