Elective transfer from cardiopulmonary bypass to centrifugal blood pump support in very highrisk cardiac surgery

The decision to preemptively deploy an LVAD must balance safety with efficacy. To improve outcome in borderline survival situations following high-risk cardiac surgery the Oxford Group decided to wean directly from CPB to a new temporary centrifugal blood pump designed to reduce bleeding and thromboembolic complications [98]. The rationale for elective LVAD

deployment is straightforward. The failing heart beats more than 120,000 times per day pumping in excess of 7000 L of blood against an increased afterload. A dilated heart has greater wall tension and increased myocardial energy demand but suffers reduced subendocardial blood flow. After surgery ischemia-reperfusion injury may trigger the downward spiral into cardiogenic shock and multiorgan failure. Inotropic agents temporarily elevate systemic blood pressure but they exert deleterious effects by increasing heart rate, after-load, and subendocardial ischemia. In contrast an LVAD unloads the ventricle, boosts coronary and systemic blood flows, and promotes myocar-dial recovery. Some inotropic support may be required to sustain right ventricular contractility in the face of increased venous return.

The Levitronix Centrimag short-term VAD is an extracorporeal system composed of a single-use centrifugal blood pump, a motor, a console, a flow probe, and a tubing circuit (Fig. 8). The device is composed of a bearingless motor that combines the drive, the magnetic bearing, and the rotor function in a single unit. The motor generates the magnetic bearing force that levitates the rotor into the pump housing while also generating the torque necessary to produce the unidirectional flow. The importance of magnetic levitation is the absence of bearings and seals resulting in minimal friction or heat generation in the blood path. The rotor surface is uniformly washed which minimizes the areas of blood stagnation and turbulence in the pump. To reduce hemolysis the mechanical gaps in the pump are greater than 0.6 mm to allow sheer forces to be low. This device can produce flows up to 10 L/min under normal physiologic conditions with a priming volume of 31 mL. The device is CE mark approved for short-term circulatory support and is commercially available through Europe. Initial European clinical trials in postcardiotomy car-diogenic shock have been encouraging over mean support periods of 2 weeks and the longest support period of 64 days. Overall 30-day mortality was 50%, which compares favorably with that reported for other devices. The system is reliable and versatile so that it can be quickly implemented in situations of rapid deterioration. Device mechanical reliability and relatively low complication rates make the Levitronix pump safe to use for patients who need time for evaluation for cardiac transplantation or a longer-term device.

For elective transfer from CPB to centrifugal blood pump support the patients at highest risk for postcardiotomy cardiogenic shock are

Cardiac Surgical Care Unit
Fig. 8. The Levitronix system supporting two patients in the cardiac intensive care unit.

selected before surgery. Candidates may have chronic left ventricular dysfunction with LVEF less than 20%, recent acute myocardial infarction, impaired renal function, or aortoiliac disease precluding IABP use. LVAD implantation is undertaking during 30 minutes of reperfusion time before discontinuation of CPB. Conduits for the inflow and outflow cannulas

Fig. 9. Method to improve the safety of decannulation after blood support. Conduits are sewn to the left superior pulmonary vein and ascending aorta through which the inflow and outflow cannulas are attached to the heart. (Adapted from Westaby S, Balacumaraswami L, Evans BJ, et al. Elective transfer from cardiopulmonary by pass to centrifugal blood pump support in very high-risk cardiac surgery. J Thorac Cardiovasc Surg 2007;133(2):577; with permission. Copyright © 2007, The American Association for Thoracic Surgery.)

Fig. 9. Method to improve the safety of decannulation after blood support. Conduits are sewn to the left superior pulmonary vein and ascending aorta through which the inflow and outflow cannulas are attached to the heart. (Adapted from Westaby S, Balacumaraswami L, Evans BJ, et al. Elective transfer from cardiopulmonary by pass to centrifugal blood pump support in very high-risk cardiac surgery. J Thorac Cardiovasc Surg 2007;133(2):577; with permission. Copyright © 2007, The American Association for Thoracic Surgery.)

are used to improve the safety of decannulation (Fig. 9). A tube of descending aortic homograft (8 cm x 10 mm diameter) is sewn to an incision at the junction of the superior pulmonary vein with the left atrium. Through this conduit is introduced the 32F right-angled wire-reinforced venous cannula into the center of the left atrium. Ligatures are placed around the homograft tube to retain the inflow cannula in position. The distal end of the venous cannula is brought through the skin below the sternotomy wound and then filled by raising left atrial pressure. A Dacron polyester fabric graft (8 mm) is then sewn to the ascending aorta with a side clamp. The straight 22F arterial inflow cannula is inserted through this graft, secured into place by ligatures, and brought out through the skin adjacent to the venous cannula. The system is filled during reperfusion and de-airing of the native heart. The patient is then weaned directly from CPB onto LVAD flow to provide between 3 and 4 L/min. Antegrade cardiac ejection continues to provide systemic pulsatility. Combined output from the device and the native left ventricle is around 3.0 L/min/m2. Transesophageal echocardiography is used to confirm the position of the inflow cannula and the efficacy of de-airing.

After protamine administration the sternot-omy wound is closed to allow extubation during support. To minimize bleeding no anticoagulation is given for 12 hours. Once the chest tube drainage is less than 50 mL/h heparin infusion is given to provide an activated partial thromboplastin time ratio of 1.5 to 2.5. For recovery after ischemic arrest the support duration is usually less than 7 days. In this time frame the Levitronix pump is reliable, safe, and effective. It is readily managed by nursing staff and easily portable. Reoperation for bleeding and decannulation problems are avoided by the use of the conduits.

With a view to explant, myocardial function is assessed daily with the pump flow turned down to 2.0 L/min/m2. After sustainable improvement in myocardial function has been achieved, the patient is returned to the operating room, the pump is switched off, and the cannulas are withdrawn. The grafts are ligated close to their insertion to prevent thrombus formation. As part of the step-down process an IABP is used for a further 24 to 48 hours.

The elective circulatory support strategy has been used successfully in a small series of patients who had coronary and aortic valve disease, all with LVEF less than 15% [98]. All patients who had chronic heart failure survived. The only mortality occurred in relation to extensive recent myo-cardial infarction with ventricular septal defect in which a salvage procedure proved unsuccessful and support was withdrawn because of advanced hepatorenal failure.

To date the Levitronix Centrimag pump has been used in more than 300 patients, of whom 45% of cases have been salvage postcardiotomy support with a mean of 9 days and 53% survival. It is likely that the 47% mortality could have been reduced substantially by anticipating postoperative deterioration and using the blood pump electively to prevent cardiogenic shock during the duration of reversible postischemic stunning.

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