When Bad Hearts Get Worse
An air medical transport team is called to the post-anesthesia care unit (PACU) of a regional cardiac referral center for a 58-year-old male who underwent a three-vessel coronary artery bypass graft (CABG) earlier that morning. The patient was unable to wean from the cardiopulmonary bypass circuit and required emergent invasive assistance in the operating room. An Abiomed BVS system was placed to assist the left ventricle.
Upon arrival to the bedside, the crew finds the patient on a ventilator with settings that are providing good chest wall compliance and adequate oxygenation and ventilation per clinical assessment parameters and blood gas readings (although his last lactate level was 6.4 mmol/liter). He has two mediastinal chest tubes draining dark red blood at approximately 200 ml per hour each. He has received four units of type-specific packed red blood cells (PRBCs) since arriving from the operating room. The abiomed console reads flows of 2.1 liters per minute and a rate of 52 beats per minute. His vital signs include an ECG heart rate of 116 beats per minute and sinus, a blood pressure by invasive line in the right radial artery of 85/68 with a rate of 52 beats per minute and an oxygen saturation of 91 percent (FiO2 of 1.0).
Are these parameters acceptable? What interventions can be done at the bedside to maximize hemodynamics during transport? Possibly a more compelling question might be: do I remember what an abiomed is and how it works? This month's discussion will focus on the last question and give a brief review and overview of this particular ventricular assist device.
Providing Desperately Needed Assistance
An Abiomed is a ventricular assist device or Circulatory Support System (CSS). It is designed to aid those suffering from reversible ventricular dysfunction. For example, those patients who have undergone successful cardiac surgery and subsequently developed low cardiac output (CO), patients with acute cardiac disorders and hemodynamic instability, and patients who simply need a reduction in ventricular work and time to allow the heart to recover adequate function will benefit from this type of support. It can assist one or both failing ventricles. It is designed for short-term use only.
Indications and Contraindications
Primary indications for this device include those patients who:
• fail to wean from cardiopulmonary bypass (CPB) following heart surgery.
• require ventricular assist following heart transplantation.
• require right ventricular assist support while on implantable left ventricular assist device support.
• are in need for resting the native heart.
• major cardiac or extra cardiac catastrophes occurring intraoperatively or postoperatively that preclude survival.
• central nervous system damage resulting in fixed and dilated pupils.
The right heart is supported by a cannula placed into the right atrium and is connected to an assist device. The assist device is either a dual chambered blood pump or an artificial ventricle. Both devices are pneumatically driven and are volume dependant. The blood passes through the assist device and is returned to the patient through a cannula attached to the main pulmonary artery. The left heart is assisted by a cannula placed into the left atrium and returned by a cannula anastomosed to the ascending aorta. This effectively bypasses the ventricles to provide rest and myocardial recovery.
A console designed to support either a blood pump or artificial ventricle ensures proper system function. The system automatically responds to changes in preload and afterload. The left and right sides of the pump work independently of each other and automatically adjust heart rates and flows accordingly. Back-up systems and a hand pump are available if the unit loses electrical power. The system is designed to deliver a stroke volume of approximately 80 ml and a cardiac output up to six liters per minute.
Patient Management Considerations
Maintenance of hemostasis is of utmost importance with respect to patients receiving this type of support. Blood products and clotting factors (e.g. platelets, fresh frozen plasma (FFP), and cryoprecipitate) are employed to control non-surgical bleeding. Once bleeding is controlled, anticoagulation is utilized in order to decrease the risk of thrombus formation.
Hemodynamic management is vital. These devices are volume dependent so crystalloids, colloids, and blood products are employed first. Pharmacologic therapy is utilized to assist with systemic vascular resistance (SVR). Controlling the patient’s pulmonary status helps prevent hypoxia, which can lead to pulmonary vasoconstriction that will impede pump ejection and flows.
While function of the native cardiac conduction system is not required, arrhythmia management aids in preventing thrombus formation. Chest compressions and precordial thump is contraindicated as cannulae are surgically implanted. CPR is provided if the system malfunctions with the attached hand pump.
If defibrillation or cardioversion is indicated, there is no need to turn off or electrically isolate the system. Temporary pacemakers can be utilized to support electrical conduction in bradycardia and asystole. Pharmacological supports per ACLS algortihms are administered in usual fashion.
The safe transport of patients requiring advanced cardiovascular support is becoming commonplace for many critical care transport teams. Understanding system components, care and normal operation are important. Next month's discussion will focus on troubleshooting our patient introduced in this month’s scenario.
1. Abiomed Clinical Reference Manual: Circulatory Support Systems. Abiomed Inc. Danvers, MA. 2006
2. University of Michigan. CVCICU guidelines for managing Abiomeds. UMHS Health System. Last revised 2007.
3. University of Michigan: Survival Flight Medical Protocols. Abiomed Transports. UMHS Health System. Last revised September 2008.