Introduction to Ventricular Assist Devices
The impetus for development of ventricular assist devices is the rapidly increasing number of patients with heart failure (5.3 million in the United States alone) combined with the unavailability of organs for transplantation (~ 2,200 per year in the U.S.) and the utter inadequacy of medical therapy. In fact, medical therapy (which may include beta blockers, ACE-inhibitors, diuretics, antiarrhythmics, and even ionotropic agents) does not affect mortality [Thunberg CA et al. J Cardiothorac Vasc Anesth 24: 656, 2010]. The recent approval of the HeartMate II by the FDA for destination therapy, based on favorable long-term data from the “Thoratec HeartMate II Left Ventricular Assist System [LVAS] for Bridge to Cardiac Transplantation” trial [Pagani FD et al. J Am Coll Cardiol 54: 312, 2009], has significantly expanded the potential recipient population for these devices
Classification of VADs
Most first generation devices (ex. the HeartMate I, of the Thoratec PVAD) generated pulsatile flow. However, continuous flow devices appear to be superior to pulsatile flow devices [Slaughter MS et al. N Engl J Med 361: 2241, 2009], hence the development of second-generation devices (below)
Second-generation devices (ex. HeartMate II, TandemHeart [percutaneous VAD], Jarvik 2000) provide pulsatile flow.
HeartMate II LVAD
Recent approved by the FDA for destination therapy, based on favorable long-term data from the “Thoratec HeartMate II Left Ventricular Assist System [LVAS] for Bridge to Cardiac Transplantation” trial [Pagani FD et al. J Am Coll Cardiol 54: 312, 2009]. Despite attempts to reduce embolic events (ex. coating the inflow and outflow conduits in an attempt to promote endothelial cell coverage), the HeartMate II does require anticoagulation. Can generate up to 10 L/min of output. Complications associated with long-term usage include aortic valve closure (due to fusion, presumably from inactivity), coronary sinus thrombosis, AVM formation and GI bleeding. In an attempt to allow the clinician to assess the relative contribution of the HeartMate II to total blood flow, the HMII displays a “pulsatility index” (1-10, dimensionless), which quantifies the pulsatility of flow. When the HMII is the predominant source of flow, the PI is low. As the native LV provides an increasing contribution to total blood flow, pulsatility (and hence the PI) will increase. A PI of 3-4 is considered normal [Thunberg CA et al. J Cardiothorac Vasc Anesth 24: 656, 2010]
Significant for its small size, of particular interest because it can be inserted into pediatric patients. Also, because of its small size can be inserted via a left thoracotomy incision without the need for Cardiopulmonary Bypass
Similar to the Javik 2000 in size, the only FDA approved LVAD for the pediatric population
Percutaneous VAD, approved for up to 14 days of use. Delivers up to 5 L/min of flow rate when inserted percutaneously (up to 8 L/min if inserted surgically)
Anesthetic Concerns in VAD Placement
Patients who require VAD implantation have a myriad of medical problems arising from their end-stage heart failure, including but not limited to pulmonary hypertension, kidney failure, and liver dysfunction. Importantly, these patients have a decreased Vd, which increases the plasma concentration of intravenously administered anesthetic agents.
Note that for patients in end-stage heart failure, CO is severely limited and relatively dependent on heart rate. Many of these patients live on the right-most portion of the Starling curve, making them unresponsive to additional preload. These patients have high sympathetic tone at baseline and may respond strongly to agents that reduce sympathetic outflow
Particular attention to sterile technique is mandatory, as sepsis the most common cause of death in these patients (peak risk at 3 weeks, most common agents are staphylococcus or Candida). Antibiotic prophylaxis is extensive, and likely should include vancomycin, levofloxacin, reifampin, fluconazole, and mupriocin [Thunberg CA et al. J Cardiothorac Vasc Anesth 24: 656, 2010]