Two types of pumps are available commercially for extracorporeal circulation: roller pumps and centrifugal pumps. In adults, roller pumps are used rarely, if ever, for temporary circulatory support beyond routine cardiopulmonary bypass applications because of important disadvantages. Although inexpensive, roller pumps are insensitive to line pressure and require unobstructed inflow. Additionally, roller pumps may cause spallation of tubing particles and are subject to tubing failure at unpredictable times. These systems require constant vigilance and are difficult to operate for extended periods. Use of roller pumps beyond 4 to 5 hours is associated with hemolysis and, for this reason, is inappropriate for mechanical assistance that may involve several days to weeks of support.40 Axial flow pumps, in which the pump rotor is parallel to the blood path, have entered the field of temporary support with the introduction of the Impella devices,41 but most of the experience with temporary support devices has been with centrifugal pumps.
Centrifugal pumps are familiar assist systems because of their routine use in cardiopulmonary bypass. In these devices, the blood path is perpendicular to rotor. Although many different pump-head designs are available, they all work on the principle of generating a rotary motion by virtue of moving blades, impellers, or concentric cones. These pumps generally can provide high flow rates with relatively modest increases in pressure. They require priming and deairing prior to use in the circuit, and the amount of flow generated is sensitive to outflow resistance and filling pressures. The differences in design of the various commercially available pump heads are in the numbers of impellers, the shape and angle of the blades, and the priming volume. The only exception is the Medtronic Bio-Pump (Medtronic Bio-Medicus, Inc., Eden Prairie, MN), which is based on two concentric cones generating the rotary motion. The pump heads are disposable, relatively cheap to manufacture, and mounted on a magnetic motorized unit that generates the power. Despite design differences, in vitro and in vivo testing has shown no clear superiority of one pump over the other.40,42 Although earlier designs caused mechanical trauma to the blood elements leading to excessive hemolysis, the newly engineered pumps are less traumatic and can be used for longer periods.
Complications with temporary mechanical assistance are high and are very similar for patients on centrifugal pump support or ECLS. The major complications reported by a voluntary registry for temporary circulatory assistance using primarily left ventricular, right ventricular, and biventricular assist devices (LVADs, RVADs, and BVADs) are bleeding, persistent shock, renal failure, infection, neurologic deficits, thrombosis and emboli, hemolysis, and technical problems. The incidence of these complications in 1279 reported patients differed significantly between continuous perfusion systems and pneumatically driven systems (see the following) with respect to bleeding, renal failure, infection, and hemolysis. Neurologic deficits occurred in approximately 12% of patients, and in Golding's experience, noncerebral emboli occurred equally often.43 Golding also found that 13% of patients also developed hepatic failure. An autopsy study found anatomical evidence of embolization in 63% of patients, even though none had emboli detected clinically.44
Although a meaningful comparison of results of centrifugal support from different institutions is not possible, in general, overall survival has been in the range of 21 to 41%. The voluntary registry reported the experience with 604 LVADs, 168 RVADs, and 507 BVADs; approximately 70% were with continuous-flow pumps and the remainder with pulsatile pumps.45 There were no significant differences in the percentage of patients weaned from circulatory assistance or the percentage discharged from the hospital according to the type of perfusion circuitry. Overall, 45.7% of patients were weaned, and 25.3% were discharged from the hospital.45 The registry also reports that long-term survival of patients weaned from circulatory support is 46% at 5 years.45 Most of the mortality occurs in the hospital before discharge or within 5 months of discharge.
Golding reported an identical hospital survival rate for 91 patients in 1992 using only centrifugal pumps, and Noon reported that 21% of 129 patients were discharged.43,46 Patients who received pulsatile circulatory assistance were supported significantly longer than those supported by centrifugal pumps, but there were no differences in the percentage of patients weaned or discharged. Survivors were supported an average of 3.1 days using continuous-flow pumps. Patients supported for AMI did poorly; only 11.5% survived to be discharged. Joyce reports that 42% of patients supported by Sarns impeller pumps eventually were discharged.47
Extracorporeal Life Support (ECLS/ECMO)
By the 1960s, it was clear that cardiopulmonary bypass was not suitable for patients requiring circulatory support for several days to weeks. The development of ECLS as a temporary assist device (also referred to as extracorporeal membrane oxygenation [ECMO]) is a direct extension of the principles of cardiopulmonary bypass and follows the pioneering efforts of Bartlett and colleagues in demonstrating the efficacy of this technology in neonatal respiratory distress syndrome.48
There are a number of key differences between cardiopulmonary bypass and ECLS. The most obvious difference is the duration of required support. Whereas cardiopulmonary bypass typically is employed for several hours during cardiac surgery, ECLS is designed for longer duration of support. With ECLS, lower doses of heparin are used, and reversal of heparin is not an issue. Because a continuous circuit is used, areas of stasis, such as the cardiotomy suction and venous reservoir, are not present. These differences are thought to reduce the inflammatory response and the more pronounced coagulopathy that can be seen with cardiopulmonary bypass,49 although there is generally a rapid rise of inflammatory cytokines with initiation of ECLS support.50
A typical ECLS circuit is demonstrated in Fig. 18-3. The system consists of the following:
Percutaneous ECMO support is attained via femoral vessel access. Right atrial blood is drained via a catheter inserted into the femoral vein and advanced into the right atrium. Oxygenated blood is perfused retrograde via the femoral artery. Distal femoral artery perfusion is not illustrated.
Hollow-fiber membrane oxygenator with an integrated heat-exchange system. The microporous membrane provides the necessary gas-transfer capability via the micropores where there is direct blood-gas interface with minimal resistance to diffusion. By virtue of the membranes being close to each other, the diffusion distance has been reduced without a significant pressure drop across the system.51 Control of oxygenation and ventilation is relatively easy. Increasing the total gas flow rate increases CO2 removal (increasing the "sweep") by reducing the gas-phase CO2 partial pressure and promoting diffusion. Blood oxygenation is controlled simply by changing the fraction of O2 in the gas supplied to the oxygenator.
Centrifugal pump. These pumps are totally nonocclusive and afterload-dependent. An increase in downstream resistance, such as significant hypertension, will decrease forward flow to the body. Therefore, flow is not determined by rotational flow alone, and a flowmeter needs to be incorporated in the arterial outflow to quantitate the actual pump output. If the pump outflow should become occluded, the pump will not generate excessive pressure and will not rupture the arterial line. Similarly, the pump will not generate significant negative pressure if the inflow becomes occluded. This protects against cavitation and microembolus formation. The newer generation magnetically levitated centrifugal pumps have been also used recently in the ECLS circuit and may have less traumatic effect on the blood elements.52
Heat exchanger. This allows for control of blood temperature as it passes through the extracorporeal circuit. Generally, the transfer of energy occurs by circulating nonsterile water in a countercurrent fashion against the circulating blood. Use of water as the heat-exchange medium provides an even temperature across the surface of the heat exchanger without localized hot spots. The use of a heat exchanger allows for maintenance of normothermia given the potential heal loss that can occur through the long circuit.
Circuitry interfaced between the patient and the system. The need for systemic anticoagulation on ECLS and the complications associated with massive coagulopathy and persistent bleeding during the postcardiotomy period led to the development of biocompatible heparin-bonded bypass circuits. In 1991, the Carmeda Corporation in Stockholm, Sweden, released a heparin-coating process that could be used to produce an antithrombotic surface.51 This process was applied to extracorporeal tubing and the hollow-fiber microporous oxygenator surface.53 Initial experience suggested that the need for systemic anticoagulation had been eliminated. In addition, heparin coating has been associated with a decrease in the inflammatory response with reduced granulocyte54 and complement activation.55 Bindslev and colleagues56 and Mottaghy and colleagues57 reported excellent hemodynamic support with minimal postoperative blood loss in experimental animals for up to 5 days. Magovern and Aranki reported similar excellent results with clinical application.58,59
Although these heparin-bonded circuits were initially thought to completely eliminate the need for heparinization, thrombus formation without anticoagulation remains a persistent problem. In a study of 30 adult patients with cardiogenic shock who underwent ECLS using the heparin-bonded circuits and no systemic anticoagulation, 20% of patients developed left ventricular thrombus by transesophageal echocardiography, and an additional 6% had a visible clot in the pump head.60 Protamine administration after starting ECLS can precipitate intracardiac clot. If the left ventricle does not eject and blood remains static within the ventricle, clot formation is more likely. Intracavitary clot is more likely in patients with MI owing to expression of tissue factor by the injured cells. Protamine may bind to the heparinized coating of the new circuit and negate an anticoagulant effect.61
The main difference between the centrifugal pump and ECLS is the presence of an in-line oxygenator. As a result, ECLS can be used for biventricular support by using central or peripheral cannulation. Intraoperatively, the most common application of ECLS has been for patients who cannot be weaned from cardiopulmonary bypass after heart surgery. In these cases, the existing right atrial and aortic cannulas can be used. An alternative strategy is to convert the system to peripheral cannulas, which potentially permits later decannulation without opening the chest.62
Cannulation is accomplished by surgical cut down or percutaneous insertion. The entire vessel does not need to be mobilized, and exposure of the anterior surface of the vessels typically suffices. A pursestring suture is placed over the anterior surface of the vessel. The largest cannula that the vessel can accommodate is selected. Typically, arterial cannulae of 16 to 20 French and venous cannulae of 18 to 28 French are used. The cannulation is performed under direct vision using Seldinger's technique. A stab incision is made in the skin with a no. 11 blade knife, a needle is inserted through the stab incision into the vessel, and a guidewire is advanced gently. Dilators then are passed sequentially to gently dilate the tract and the insertion point in the vessel. The cannulae then are inserted, the guidewire is removed, and a clamp is applied. For venous drainage, a long venous cannula is directed into the femoral vein to the level of the right atrium under transesophageal echocardiographic guidance.
To minimize limb complications from ischemia, one strategy is to place an 8- to 10-French perfusion cannula in the superficial femoral artery distal to the primary arterial inflow cannula to perfuse the leg (Fig. 18-4). This cannula is connected to a tubing circuit that is spliced into the arterial circuit with a Y-connector. The distal cannula directs continuous flow into the leg and significantly reduces the incidence of leg ischemia. It should be noted, however, that limb ischemia associated with long-term peripheral cannulation relates not just to arterial perfusion, but also to the relative venous obstruction that can occur with large venous lines. In such circumstances distal venous drainage can be established by splicing another small venous cannula into the circuit.
Surgical exposure of the femoral vessels facilitates cannulation for ECMO. A small 10-French cannula is used to perfuse the distal femoral artery.
An alternative strategy is to completely mobilize the common femoral artery and sew a 8- or 10-mm short Dacron graft to its anterior surface as a "chimney." The graft serves as the conduit for the arterial cannula, and no obstruction to distal flow exists. This strategy also allows for a more secure connection and avoids problems with inadvertent dislodgement of the cannulae because of loosening of the pursestrings. In general, complete percutaneous placement of arterial cannulae is avoided to prevent iatrogenic injury during insertion and ensure proper positioning of the cannula. However, when venovenous bypass is the only mode of support needed, percutaneous cannulation is performed. Surgical exposure is not necessary, and bleeding is less with this technique. Although traditionally the perfusion circuit involves atrial drainage and femoral reinfusion (aortofemoral flow), a recent prospective study has shown the reverse circuit (femoroatrial flow) to provide higher maximal extracorporeal flow and higher pulmonary arterial mixed venous oxygenation.63
Central cannulation sometimes is indicated because of either severe peripheral vascular disease or the desire to deliver the highly oxygenated blood directly to the coronary arteries and cerebral circulation. In patients with an open chest, aortic and right atrial cannulae are used. Reinforcing pursestring sutures are placed and tied over rubber chokers and buttons for later tying at decannulation. The catheters are brought through the chest wall through separate stab wounds, and after bleeding is secured, the chest is covered, but not closed, over mediastinal drainage tubes.64
An alternative central cannulation site is the axillary artery. Direct cannulation of this artery has been associated with progressive edema of the arm.65 Therefore, the best strategy to maintain arm perfusion is to expose the axillary artery and sew a 8- or 10-mm graft to the vessel as a "chimney." The cannula then is placed in the graft and tied securely with several circumferential umbilical tapes.
Once instituted, the system is simple enough to be monitored by trained ICU nurses and maintained by a perfusionist on a daily basis. Evidence of clots in the pump head requires a change. Leakage of plasma across the membrane from the blood phase to the gas phase may be a problem, gradually decreasing the efficiency of the oxygenator and increasing resistance to flow and necessitating oxygenator exchanges. Using this system, ECLS flows of 4 to 6 L/min are possible at pump speeds of 3000 to 3200 rpm. Higher pump speeds are avoided to minimize mechanical trauma to blood cells. Other means of improving flow include transfusion of blood, crystalloid, or other colloid solutions to increase the overall circulating volume.
Physiologically, ECLS will unload the right ventricle but will not unload the compromised left ventricle, even though left ventricular preload is reduced.59 In normal hearts, the marked reduction in preload and small increase in afterload produced by the arterial inflow from the ECLS system reduces wall stress and produces smaller end-diastolic left ventricular volumes because the heart is able to eject the blood it receives. However, if the heart is dilated and poorly contracting, the marked increase in afterload provided by the ECLS system offsets any change in end-diastolic left ventricular volume produced by bypassing the heart. The heart remains dilated because the left ventricle cannot eject sufficient volume against the increased afterload to reduce either end-diastolic or -systolic volume. ECLS, therefore, theoretically may increase left ventricular wall stress and myocardial oxygen consumption unless an IABP or other means is used to unload the left ventricle mechanically and reduce left ventricular wall stress.59
As mentioned, the versatility of ECLS is that it allows rapid restoration of circulation by peripheral cannulation during active resuscitation in the setting of acute cardiac arrest, acute pulmonary embolism, or patients in cardiogenic shock who cannot be moved safely to the operating room.
An isolated RVAD is rarely indicated in the postcardiotomy setting because, in general, these patients have global biventricular dysfunction. ECLS as an RVAD (with outflow to the pulmonary artery) may be used only in patients with good function of the left ventricle who manifest right ventricular failure and hypoxia.
The experience in adults with ECLS for postoperative cardiogenic shock is associated with high rates of bleeding. Surgical bleeding from the chest is exacerbated by anticoagulation and the consumptive coagulopathy caused by the ECLS circuit.66 Pennington reported massive bleeding in six of six adults supported by ECLS after cardiac surgery. Even without the chest wound, bleeding was the major complication in a large study of long-term ECLS for acute respiratory insufficiency.67 Muehrcke reported experience with ECLS using heparin-coated circuitry with no or minimal heparin.68 The incidence of reexploration was 52% in the Cleveland Clinic experience; transfusions averaged 43 units of packed cells, 59 units of platelets, 51 units of cryoprecipitate, and 10 units of fresh-frozen plasma. Magovern reported somewhat less use of blood products, but treated persistent bleeding by replacement therapy and did not observe evidence of intravascular clots; two patients developed stroke after perfusion stopped. Other important complications associated with ECLS using heparin-coated circuits included renal failure requiring dialysis (47%), bacteremia or mediastinitis (23%), stroke (10%), leg ischemia (70%), oxygenator failure requiring change (43%), and pump change (13%).69 Nine of 21 patients with leg ischemia required thrombectomy and one amputation. Half the patients developed marked left ventricular dilatation, and six patients developed intracardiac clot detected by TEE. Intracardiac thrombus may form within a poorly contracting, nonejecting left ventricle or atrium because little blood reaches the left atrium with good right atrial drainage. We have observed intracardiac thrombus in heparinized patients and those perfused with pulsatile devices and a left atrial drainage cannula. Therefore, the problem is not unique to ECLS or the location of the left-sided drainage catheter, but is related to left ventricular dysfunction and stagnant flow. In patients on temporary VADs in whom LV clot forms, the clot is removed at the time of permanent VAD implantation.
Magovern reported improved results in 14 patients supported by a heparin-coated ECLS circuit after operations for myocardial revascularization.59 Eleven of 14 patients (79%) with revascularization survived, but none of three patients with mitral valve surgery and none of four patients who underwent elective circulatory arrest survived. Overall, 52% of the whole group survived, but two patients developed postperfusion strokes that probably were the result of thrombi produced during perfusion. Although the Cleveland Clinic experience with heparin-coated ECLS circuits produced a survival rate of 30%, the patient population was more diversified and represented only 0.38% of cardiac operations done during the same time period.60
The Cleveland Clinic reported their results looking at 202 adults with cardiac failure.34 With an extended follow-up up of 7.5 years (mean 3.8 years), survival was reported to be 76% at 3 days, 38% at 30 days, and 24% at 5 years. Patients surviving 30 days had a 63% chance of being alive at 5 years, demonstrating that the high early mortality remains the Achilles heal of this technology. Interestingly, patients who were weaned or bridged to transplantation had a higher overall survival (40 and 45%, respectively). Failure to wean or bridge was secondary to end-organ dysfunction and included renal and hepatic failure and occurrence of a neurologic event while on support.34 Another report from the Cleveland Clinic looking at 19,985 patients undergoing cardiac operations found that 107 (0.5%) required ECLS for postcardiotomy failure.4 Younger age, number of reoperations, emergency operations, higher creatinine concentration, greater left ventricular dysfunction (LVD), and history of MI were significant predictors of the need for mechanical support. Although overall survival was 35%, in the subgroup bridged to a chronic implantable device, survival was 72%.
The TandemHeart PTVA (percutaneous ventricular assist) System (CardiacAssist, Inc., Pittsburgh, PA) has 510k FDA approval for short-term (<6 hours) mechanical support. It was envisioned for the short-term support of high-risk percutaneous interventions in the catheterization lab.70 The device is powered by a small hydrodynamic centrifugal pump that resides in a paracorporeal location. The rotor of the pump is suspended and lubricated by a fluid interface of heparinized saline. Cannulas are introduced from the femoral vessels by either percutaneous or direct insertion techniques. Pump inflow is achieved by a novel, proprietary 21-French cannula delivered across the atrial septum. Outflow typically is directed into the common femoral artery (Fig. 18-5). Position is facilitated and confirmed by fluoroscopy and intracardiac ultrasound (ICE).71
Transeptal inflow cannula of the TandemHeart device. (Reproduced with permission from CardiacAssist, Inc., Pittsburgh, PA.)
As opposed to an ECLS circuit, excellent left atrial decompression is achieved as long as the inflow cannula is appropriately positioned. The device can be introduced either in the catheterization laboratory using fluoroscopy or directly in the operating room using TEE guidance. Most of the experience with the device has been in the catheterization laboratory, where it has been used extensively to facilitate high-risk percutaneous interventions.72 Flows of up to 4 L are typical. With surgically implanted larger cannulae flows up to 8 L are possible.
The TandemHeart can be configured in many ways to achieve effective mechanical support. The inlet and outlet connectors to the pump are 3/8 – 3/8 connectors, and as such can be connected to any of a number of commercially available percutaneous or surgically implanted cannulae.
Patients with this device are typically kept in bed, given that the device is inserted through the femoral vessels. An ACT of 200s is targeted while the patients are on support. The TandemHeart is a very versatile system that can be deployed and discontinued rapidly. One beneficial aspect for postcardiotomy support is that the entire device can be removed in the ICU without reopening the patient's chest. RVAD configurations with right atrial inflow and outflow to the main pulmonary artery are possible in both open73 and percutaneous configurations.74 With a percutaneous approach the 21-French transeptal cannula is directed into the main pulmonary artery under flouroscopy.74
Complete percutaneous biventricular support is also possible, but is somewhat cumbersome given the vascular access requirements. When faced with a patient that needs biventricular support, most groups are splicing an oxygenator into the circuit and using right atrial or biatrial drainage. In essence such a configuration converts the assist system to ECLS circuit.75 With transeptal drainage the issues of left-sided congestion, which sometimes plague ECLS patients, are nonexistent. Gregoric and the group from Texas Heart Institute reported on the outcomes of nine patients with refractory shock, supported by the TandemHeart. Eight patients had an IABP and three were undergoing active chest compressions. Once neurologic and end-organ recovery ensued, the patients were transitioned to permanent continuous-flow VADs. Three patients were ultimately transplanted and 1-year survival for the entire group was 100%.76 The Texas Heart Group has also had extensive experience with using to the TandemHeart to support conventional surgery and recently reported the outcomes of eight patients in shock secondary to critical aortic stenosis supported with preoperative TandemHeart. Five were receiving chest compressions at the time of TandemHeart Insertion. All underwent conventional aortic valve replacement after a mean duration of support of 6 days. One patient died of postoperative sepsis. The other seven patients were discharged from the hospital and were all alive at the time of the report.76
Brinkman and the group at Medical City Dallas reported on the outcomes of 22 patients supported with the TandemHeart device. Mean duration of support was 6.8 days with no pump failures or pump related neurologic events. Three patients developed bleeding and two patients had lower extremity ischemic complications. In 11 patients who were neurologically intact at the time of TandemHeart Insertion, five went on to receive transplants while on TandemHeart support, three went on to permanent LVAD placement, and two recovered. Of 11 patients with indeterminate neurologic status or multiorgan failure, seven died, two went on to receive permanent LVADs, one was transplanted, and one recovered.77 It should be noted that use of a TandemHeart beyond 6 hours and use with an oxygenator are off-label uses of the device. It is important to remember that when placing a long-term VAD in a patient on Tandem Support, it is necessary to repair the atrial septum. Failure to do so can lead to hypoxia caused by entrainment of unoxygenated blood across the atrial septal defect created by the TandemHeart inlet cannula.
The Levitronix CentriMag pump is a centrifugal pump with a fully magnetically levitated impeller (Fig. 18-6).78 Very little friction is generated and it requires only a very small priming volume. It can be configured for both right- and left-sided heart support, typically with central cannulation via median sternotomy. With good cannula placement, more than 9 L of support can be achieved. It has FDA 510K approval for 6 hours of use as an LVAD and has FDA approval for use as an RVAD for 30 days.
Levitronix CentriMag. (Reproduced with permission from Levitronix, Waltham, MA.)
The group at the University of Minnesota reported the outcomes of 12 patients supported with CentriMag BiVADs.3 Of 12 patients who presented in cardiogenic shock, eight went on to receive long-term implantable VADs, two recovered, and two died. Thirty-day survival was 75% and 1-year survival was 63%.3
The group at the University of Pittsburgh recently reported the results of using CentriMag as a temporary RVAD. The indication for RV support was postcardiotomy RV failure in seven (24%), RV failure postcardiac transplant in 10 (35%), and RV failure post-LVAD placement in 12 (41%) patients. The RVAD was able to be weaned in 43% of postcardiotomy patients, 70% of transplant patients, and 58% of LVAD patients after a mean duration of support of 8 days. The authors concluded that the CentriMag was easy to implant, provided effective support, and was easy to wean,79 with low overall morbidity.
As with similar pumps, the CentriMag can be configured with an oxygenator to create an ECLS circuit.80
The Impella pump has been acquired by ABIOMED, Inc. and is being marketed as the Impella Recover system. The device is a microaxial pump, with both peripheral and central cannulation configurations available. In either case, the pump is directed across the aortic valve into the left ventricle (Fig. 18-7). The cannula portion of the device, which sits across the aortic valve, is contiguous with the integrated motor that comprises the largest-diameter section of the catheter (see Fig. 18-7). The small diameter of the cannula is designed to allow easy coaptation of the aortic valve leaflets around it, resulting in minimal aortic valve insufficiency. Its hemodynamic support results from the design feature that provides active forward flow that increases net cardiac output, and its ability to address the needs for myocardial protection stems from simultaneously unloading work from the ventricle (decreasing myocardial oxygen demand) and augmenting coronary flow (increasing oxygen supply).81–83
Impella device. (Reproduced with permission from ABIOMED, Danvers, MA.)
The device comes in two sizes depending on indication for use and desired flow. The smaller Impella 2.5 is capable of generating flows up to 2.5 L/min. Experience with Impella 2.5 is largely with high-risk percutaneous interventions in which the device maintains hemodynamic stability during balloon inflation and stent deployment. In addition, in cases of myocardial infarction it may help in reducing infarct size. In a recent prospective randomized trial 20 patients underwent high-risk percutaneous coronary intervention while being supported with the Impella 2.5. All patients had poor left ventricular function and had interventions on the left main or the last remaining patent conduit. Patients with recent ST-segment elevation myocardial infarction or cardiogenic shock were excluded. The primary safety end point was the incidence of major adverse cardiac events at 30 days. The primary efficacy end point was freedom from hemodynamic compromise during intervention. The Impella 2.5 device was implanted successfully in all patients. The mean duration of circulatory support was 1.7 ± 0.6 hours (range, 0.4 to 2.5 hours). Mean pump flow during PCI was 2.2 ± 0.3 L/min. At 30 days, the incidence of major adverse cardiac events was 20%. (Two patients had a periprocedural myocardial infarction, and two patients died at days 12 and 14.) There was no evidence of aortic valve injury, cardiac perforation, or limb ischemia. Two patients (10%) developed mild, transient hemolysis without clinical sequelae. None of the patients developed hemodynamic compromise during PCI.84
The larger-size Impella 5.0 is designed to generate flows of up to 5 L/min and is more appropriate for hemodynamic support for cardiogenic shock. The device can be inserted percutaneously or directly through the aorta in the operating room. As with all left ventricular assist devices, the actual flow depends on adequacy of blood return to the left side on the heart, which in return depends on right ventricular function, pulmonary vascular resistance, and adequate blood volume. Although the device provides superior hemodynamic support compared with IABP, a recent meta-analysis suggested that early survival is not necessarily improved.85
The results of this limited study likely have to do more with the acuteness and duration of hemodynamic collapse in the patients than the efficacy of restoring circulatory flow. Several isolated reports suggest the novel ways that patients can be supported using this technology, including one as a bridge to a more long-term device, and another for support of primary graft failure status post-heart transplant.86
A right ventricular support device also has been developed and reported.87 The development and implementation of this kind of device can be particularly useful in patients in need of chronic LVAD therapy. Theoretically, a percutaneously implanted device can support the right heart during the critical days after LVAD implantation to allow for recovery of the right ventricle and the gradual reduction in pulmonary vascular resistance. This kind of device will significantly facilitate patient management and reduce the need for high doses of inotropic support.88
The ABIOMED BVS 5000 blood pump is an extracorporeal device designed to provide pulsatile univentricular or biventricular support. In 1992, it became the first such device to receive FDA approval. It has been used in thousands of patients in Europe and the United States for the purpose of postcardiotomy pump failure. The system is widely utilized for postcardiotomy support and is available in over 450 centers in the United States. The pump is configured as a dual-chamber device containing both an atrial chamber and a ventricular chamber that pumps the blood pneumatically to the outflow cannula (Fig. 18-8). The two chambers and the outflow tract are divided by trileaflet polyurethane valves that allow for unidirectional blood flow.
The ABIOMED BVS 5000. (Left) The atrial chamber empties through a one-way valve into the ventricular chamber (diastole). (Right) The pneumatically driven pump compresses the ventricular chamber, and blood flows through a one-way valve into the patient (systole). During pump systole, the atrial chamber fills by gravity.
The pump chamber itself consists of a collapsible polyurethane bladder with a capacity of 100 mL. With the BVS 5000 and BVS 5000i consoles, filling of the atrial chamber depends on gravity (the height of the chamber relative to the patient's atrium), the central venous pressure (preload), and the central venous capacitance. The atrial bladder operates in a fill-to-empty mode and therefore can be affected by changes in the height of the pump relative to the patient or the volume status of the patient. The pump usually is set approximately 25 cm below the bed. The adequacy of filling can be assessed visually because the pump is transparent. The passive filling (absence of negative-pressure generation) is designed to prevent atrial collapse with each pump cycle as well as to prevent entrainment of air into the circuitry.
The ventricular chamber requires active pulsatile pumping by a pneumatic driveline. Compressed air is delivered to the chamber, causing bladder collapse and forcing blood out of the pump to the patient. During diastole, air is vented to the atmosphere, allowing refilling of the chamber during the next cycle. The rate of pumping and the duration of pump systole and diastole are adjusted by the pump microprocessor that operates asynchronous to the native heart rate. The pump makes adjustments automatically to account for preload and afterload changes and delivers a constant stroke volume of approximately 80 mL. The maximum output is approximately 5 to 6 L/min with the BVS 5000i console. This design requires minimal input by personnel except during periods of weaning. Medical management should include optimizing the patient's hydration status and outflow resistance because the pump's performance depends on these parameters.
The device has not demonstrated any significant hemolysis, and provides excellent ventricular decompression if a proper cannulation strategy is employed. As opposed to the centrifugal pump and ECLS, patients can be extubated and have limited mobility, such as transfer from bed to chair or dangling of the legs from the bed. The BVS 5000 blood pump can be driven by both the BVS 5000 console and the newer AB5000 console.
The AB5000 circulatory support system, introduced in 2004, consists of the pneumatically driven AB5000 ventricle and the updated AB5000 console, which utilizes vacuum- assisted drainage. The ventricle is designed for short to intermediate (<3-month) support and incorporates many of the features of the ABIOCOR total artificial heart. It is driven pneumatically with valves that are constructed of Angioflex, Abiombed's proprietary polyether-based polyurethane plastic. The AB5000 system uses the same cannulae as the BVS 5000 system. Inflow cannulae can be configured for atrial or ventricular placement. Perhaps the most attractive aspect of the upgraded system is that bedside conversion from the BVS 5000 pump to the ventricle is made possible by quick-connect attachments. As such, patients implanted with a BVS 5000 system at an outlying hospital can be converted to the more long-term ventricle at the receiving institution without reopening the patient's chest or going on bypass. However, the combination of the high vacuum setting and smaller cannula size can lead to hemolysis.89 Optimal cannula placement without chamber collapse or a high-velocity jet at the inflow cannula tip must be confirmed by transesophageal echocardiography before leaving the operating room. Anticoagulation to an activated clotting time (ACT) of 200 seconds is recommended.
ABIOMED cannulae are constructed from polyvinyl chloride and have a velour body sleeve that is tunneled subcutaneously. Three sizes of wire-reinforced inflow cannulas are available commercially. These include malleable 32-, 36-, and 42-French cannulas. Arterial cannulas have a precoated Dacron graft attached and are available in two sizes: a 10-mm graft for anastomosis to the smaller and lower-resistance pulmonary artery and a 12-mm graft for anastomosis to the ascending aorta.
Careful cannula insertion is important for optimal performance. Venous inflow must be unimpeded, and outflow grafts must not be kinked. In addition, careful consideration must be given to cannula position when bypass grafts cross the epicardial surface of the heart. Depending on the location of these grafts, these cannulae must be placed such that graft compression cannot occur. The three-dimensional layout of this geometry must be visualized and thought out in advance, particularly if chest closure is planned. Any graft compression will make recovery unlikely.
It is technically much easier to use cardiopulmonary bypass for placement of these cannulas, although off-pump insertion is possible and may be preferable in certain clinical situations, particularly for isolated right-sided support. A side-biting clamp typically is used on the aorta to perform the outflow anastomosis. If the patient is on cardiopulmonary bypass, the pulmonary artery anastomosis can be done without the need of a partial cross-clamp. The length of the graft is measured from the anticipated skin exit site to the site of anastomosis, and the preclotted Dacron graft is cut to an appropriate length such that there is no excessive tension or any kinking. The cutaneous exit site is planned so that approximately 2 cm of the velour cuff extends from the skin and the remainder is in the subcutaneous tunnel. The cannula is not tunneled subcutaneously until after completion of the anastomosis. For the aortic anastomosis, incorporation of a Teflon or pericardial strip helps to control suture-line bleeding. If an off pump insertion is planned, cannulae must be tunneled before anastomosis.
For atrial cannulation, a double-pledgeted pursestring suture using 3-0 polypropylene is placed concentrically. Tourniquets must be secured firmly to prevent inadvertent loosening of the pursestring suture and bleeding from the insertion sites. In addition, the heart generally is volume loaded to prevent air embolism during insertion.
For pump inflow, the 36-French malleable cannula is typically used because it provides versatility to accommodate variations in anatomy and clinical conditions. Left atrial cannulation can be achieved via the interatrial groove, the dome of the left atrium, or the left atrial appendage. The right atrial appendage provides the most hemostatic way to cannulate the right atrium because securing ligatures can be placed about the appendage and cannula to afford hemostasis. Alternatively, the body of the right ventricle or left ventricular apex may be cannulated. In the absence of left ventricular clot, the cannula can be introduced through a cruciate-shaped ventriculotomy. Ventricular cannulation offers the advantages of excellent ventricular decompression, which may enhance ventricular recovery. Bleeding in the setting of a recent MI is a consideration, but usually is not a problem if careful reinforced sutures are placed. A purse string of 00 polypropylene suture passed through a collar of bovine pericardium is a helpful hemostatic adjunct in particularly friable ventricles. Additionally, a hand-made or preordered chimney made from a preclotted graft sewn to a felt collar facilitates ventricular cannulation. A "top-hat" type conduit is constructed and with the "brim" sewn to the left ventricle using mattress sutures. A ventriculotomy is made and a cannula is introduced through the conduit. If recovery occurs, the graft can be stapled to achieve hemostasis. It is simply removed and the site closed or converted to a more formal inlet cannulation if a long-term device must be used.
The consoles for the ABIOMED device are relatively simple to operate. The control system automatically adjusts the duration of pump diastole and systole primarily in response to changes in preload. Pump rate and flow are visible on the display monitor. With the AB5000, console vacuum is adjusted to the lowest possible setting that provides adequate flow.
As with all patients who require postcardiotomy mechanical support, complications are frequent. Guyton reported 75% bleeding complications, 54% respiratory failure, 52% renal failure, and 26% permanent neurologic deficit.90 Infection occurred in 13 patients (28%) while on the device, but only three cases were considered device-related. Other complications included embolism in 13% and hemolysis in 17% and mechanical problems related to the atrial cannula site in 13% of patients. No major changes in platelet count or blood chemistries occur during the period of circulatory support.
Jett reported on 55 patients supported on the ABIOMED for a variety of indications, including postcardiotomy failure (28), failed transplant allograft (8), AMI (2), and myocarditis (1).37 They reported a 40% incidence of bleeding, 50% respiratory complications, and 25% neurologic complications. Marelli and colleagues also reported a similar incidence of complications in 19 status I patients, with three developing renal failure, nine reexplored for bleeding, and three dying of sepsis and multisystem organ failure.91 As with all acute mechanical support systems, these relatively high complication rates are a reflection of the significant preexisting hemodynamic insult necessitating implementation of mechanical support. Early device insertion should be considered and may improve overall outcome.36
The ABIOMED system is available in more than 500 U.S centers, with more than 6000 patients supported to date. Results from several reports have been summarized in Table 18-2 In a multicenter study, Guyton and colleagues reported that 55% of postcardiotomy patients were weaned from support and 29% were discharged from the hospital.90 However, 47% of patients who had not experienced cardiac arrest before being placed on circulatory support were discharged. Of 14 patients who had experienced cardiac arrest, only one (7%) was discharged. In another report of 500 patients treated with the BVS 5000 system that included 265 (53%) who could not be weaned from cardiopulmonary bypass, 27% of patients were discharged from the hospital.92 Recent data using this device in a wide range of clinical situations, including postcardiotomy failure, have indicated successful wean in 83% and discharge to home in 45% of patients. These excellent results are also reported by Marelli and colleagues in 14 of 19 patients who were weaned or transplanted with a 1-year survival of 79%.91 Korfer also reported 50% hospital discharge in 50 postcardiotomy patients supported with the ABIOMED and 7 of 14 patients transplanted with a 1-year survival of 86%.93 The ABIOMED worldwide registry experience suggests that better results can be expected from experienced centers with heart transplant programs.37
Table 18-2 Clinical Experience with Abiomed Support for Postcardiotomy Cardiogenic Shock ||Download (.pdf)
Table 18-2 Clinical Experience with Abiomed Support for Postcardiotomy Cardiogenic Shock
|Reference||Patients (no.)||Biventricular Support (%)||Mean Duration of Support (Days)||Weaned, no. (%)||Discharged, no. (%)|
|Guyton||31||52||4.7||17 (55)||9 (29)|
|Minami||26||31||NR||16 (62)||3 (50)|
|Körfer||55||NR||5.7 ± 6.9||33 (60)||27 (49)|
|ABIOMED postmarket surveillance study registry*||876||50||5||NR||271 (31)|
A more recent report by Anderson and colleagues looked at the outcomes of patients transferred on the BVS 5000 at "spoke" centers and converted to the AB 5000 at "hubs."94 Fifty patients were studied over a 2-year period with a survival to either recovery, transplant, or destination VAD of 42%.94
Thoratec Ventricular Assist Device
The Thoratec paracorporeal (PVAD) (Thoratec Laboratories Corp., Pleasanton, CA) was introduced clinically in 1976 under an investigational device exemption (IDE) and was approved for bridge to heart transplantation in 1995 and postcardiotomy support in 1998, respectively.
The device is a pneumatically driven pulsatile pump that contains two seamless polyurethane bladders within a rigid housing.95 The inlet and outlet ports contain mono-strut tilting-disk valves to provide unidirectional flow. The effective stroke volume of each prosthetic ventricle is 65 mL. The pneumatic drive console applies alternating negative and positive pressures to fill and empty each prosthetic bladder. Multiple settings can be adjusted to potentially optimize pump filling to provide univentricular (LVAD or RVAD) or biventricular support (BiVAD).
Thoratec pumps reside on the upper abdominal wall and are connected to the heart with large wire reinforced cannula. The Dual Drive Console (DDC) is a large, wheeled pneumatic controller that is used early in the patient's course to optimize VAD parameters. The TLC-II driver is smaller and is approved for out-of-hospital use.
Device implantation typically is performed on cardiopulmonary bypass. It is important to select the cannula position and cutaneous exit sites carefully. The pump should be planned to rest on the anterior abdominal wall. Lateral placement may lead to excessive tension at the skin exit sites and prevent formation of a seal. Approximately 1.5 to 2 cm of the felt covering of the cannulas must extend beyond the skin exit site, with the remainder in the subcutaneous tunnel to promote ingrowth of tissue and create a seal. The distance between the inlet and outlet portion of the pump is 4 cm, and the distance between the inlet and outlet cannulas of the pump should be planned accordingly. The cannulae must be long enough to allow for pump connection but should be trimmed to prevent the pump from kinking when the patient sits.
The arterial cannulas are available with a 14-mm graft (for the pulmonary artery) or an 18-mm graft (for the aorta) and must be cut to length after the appropriate exit site has been selected. They come in two lengths, 15 and 18 cm, which again are selected based on the patient's anatomy and the planned exit site. The graft is generally sewn on the aorta or pulmonary artery after applying a partial-occluding clamp and sewn with 4-0 polypropylene suture with or without a strip of pericardium or Teflon felt for reinforcement. Inflow can be accomplished by cannulation of the atria or the ventricles.96 All cannulations generally are reinforced with a double layer of pledgeted concentric pursestring sutures. For atrial cannulation, a 51-French right-angled cannula is available in two lengths, 25 and 30 cm. For the left atrium, the cannula is inserted through the atrial appendage, the interatrial groove, or the superior dome of the left atrium. For right atrial cannulation, the cannula is inserted ideally into the right atrial appendage and directed toward the inferior vena cava.
Inflow cannulation of the left ventricle is preferred over left atrial cannulation,96 because it provides better drainage, higher flows, and perhaps improves the chance of myocardial recovery. Left ventricular cannulation also decreases the amount of stasis in the LV in the poorly contractile heart, which decreases thrombus formation. Atrially cannulated patients who develop thrombus are at high risk for thromboembolic complication because the ventricle usually continues to eject. Left ventricular cannulation is achieved by placing a concentric layer of pledgeted horizontal mattress sutures at the apex of the left ventricle or the acute margin of the right ventricle (superior to the posterior descending artery). The previously placed sutures are passed sequentially through the cuff, the apex of the heart is elevated, the left ventricle cored, and the cannula seated. The cannula then is inserted and secured by tying the sutures. The free end can then be directed out through the previously planned cutaneous exit site, and a tubing clamp placed to maintain hemostasis until the pump is connected. Connecting the cannulae to the pump is difficult and must be done with care. The connections to the pump have a sharp, beveled edge that should be directed carefully under gentle pressure to fit the cannulas without damaging the inner surface of the tube. In addition, if this tip bends, it may provide a nidus for thrombus formation. Gentle hand pumping can be performed to ensure complete air evacuation using an aortic vent. Deairing of PVADs is best achieved by keeping the heart full and keeping the pumps full at all time.
The complications reported for patients bridged to transplantation are similar to those reported for postcardiotomy patients. In a multicenter trial, the most common complications were bleeding in 42%, renal failure in 36%, infection in 36%, neurologic events in 22%, and multisystem organ failure in 16% of patients.95 Similar complications have been reported from other centers.97,98
Most contemporary usage of the Thoratec PVADs has been for smaller individuals needing mechanical support or for those needing biventricular support as a bridge to transplant. Especially in cases of postpartum cardiomyopathy or myocarditis, recovering of adequate native heart function has been reported.99 PVADs can also be used for postcardiotomy and post–MI shock patients, as well as for graft failure posttransplant. After cardiotomy, results are similar to those obtained with ECLS and the ABIOMED BVS 5000. In a review of 145 patients with nonbridge use of the Thoratec device, 37% were weaned and 21% were discharged. More experienced centers have achieved hospital survival rates of greater than 40%.97,98
The Thoratec premarket approval experience for the treatment of 53 patients with postcardiotomy heart failure had an in-hospital survival of 28%. The majority of these patients were supported with a BiVAD. The Bad Oeynhausen group, however, has reported a 60% survival for postcardiotomy patients supported with the Thoratec device.97 Clearly the greatest advantage that the Thoratec device offers is that it can be applied for longer duration of support than any other temporary device mentioned previously. This feature may be uniquely advantageous, particularly because the duration of support necessary is usually unclear in advance. All other devices mentioned have an increasing complication rate with longer duration of support. Furthermore, the Thoratec device allows for physical rehabilitation, ambulation, and home discharge. The durability advantage of the PVAD must be weighed against the ease with which the newer generation of acute support devices can be inserted and then easily be transitioned to newer-generation chronic devices with improved complication profiles.