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Ann Thorac Surg 2003;75:S79-S85
© 2003 The Society of Thoracic Surgeons

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Supplement

Continuous flow pumps and total artificial hearts: management issues

^a Cardiovascular Surgical Research Laboratories, Texas Heart Institute at St. Luke’s Episcopal Hospital, Houston, Texas, USA

^* Address reprint requests to Mr Myers, Texas Heart Institute, 6770 Bertner Ave, MC 1-268, Houston, TX 77030, USA
e-mail: tmyers{at}heart.thi.tmc.edu

Presented at the Heart Failure & Circulatory Support Summit, Cleveland, OH, Aug 22–25, 2002.

Abstract

Feasibility studies are underway for new axial flow ventricular assist systems and with a total artificial heart (TAH). The axial flow pumps provide continuous flow from the left ventricle (LV) to the aorta; the TAH provides pulsatile flow to the pulmonary and systemic circulation. Understanding the differences between these systems is necessary for appropriate patient selection and management. We compared the Jarvik 2000 axial-flow pump and the AbioCor TAH. The Jarvik 2000 pump is placed in the LV with its outflow graft anastomosed to the aorta. This system is used for bridge-to-transplantation and destination therapy. The AbioCor TAH provides complete circulatory support. The AbioCor is used for destination therapy in patients expected to die in less than 30 days. Worldwide, 45 patients have received the Jarvik 2000 as a bridge to transplantation (n = 34) or destination therapy (n = 11) for an average duration of support of 132.8 days (5 to 853 days). In 30 bridge-to-transplantation cases, 14 patients (47%) have undergone heart transplantation, 5 (17%) continue to be supported with the Jarvik 2000 device, and 11 (37%) have died. Five of 7 patients supported by the AbioCor TAH survived beyond the perioperative period; 4 were ambulatory, 2 were discharged from the hospital, and 1 is at home 13 months after implantation. Anticoagulation therapy and infection management are necessary for both systems. Therapy with inotropic agents, vasoactive drugs, a pacemaker, and electrolyte normalization is necessary for Jarvik patients. AbioCor-supported patients do not require medications to support heart function. Vasoactive agents may be useful for controlling blood pressure.

Mechanical circulatory support (MCS) systems are being used with increasing frequency to support patients with severe heart failure. Implantable ventricular assist systems (VAS) are now available commercially for use as a bridge to transplantation [1, 2] and as a bridge to recovery of myocardial function [3, 4]. Recently, government approval was granted for the use of the HeartMate LVAS (Thoratec Inc, Pleasanton, CA) for destination therapy [5]. The first generation of VAS produces intermittent, or pulsatile, blood flow at physiologic rates, thus mimicking the normal circulation. Although the implantable VAS devices have been shown to be safe and effective, serious and frequent complications associated with their use persist [6]. Therefore, new MCS devices are being developed in an attempt to provide more cost-effective systems that produce fewer complications. Presently, no single MCS system can be applied in all heart failure patients. Ideally, clinicians who treat the increasing number of heart failure patients would be able to choose from a variety of extracorporeal and implantable circulatory support systems. The systems currently under development vary greatly in design and intended use. New MCS systems range from small intraventricular blood pumps that provide partial cardiac assistance to total cardiac replacement systems.

Some of the new VAS designs, which are based on axial flow technology, are small, relatively easy to implant and operate, and provide continuous blood flow. The Jarvik 2000 axial flow pump (Jarvik Heart Inc, New York, NY) is placed within the ventricle and provides 3 to 6 L/min of circulatory support. This system is designed for partial assist of the failing heart [7].

In contrast to the new VAS designs, an implantable total artificial heart (TAH) system is being developed for eventual use in patients who require total circulatory support for the remainder of their lives [8]. The TAH provides pulsatile, intermittent blood flow at physiologic rates. Another significant difference is that the axial flow VAS provides submaximal blood flow from the left heart only, whereas the TAH provides total circulation to the pulmonary and systemic arterial systems. The TAH and all of its necessary components are located within the body, making this type of MCS system more complex than any of the VAS designs.

The type and amount of blood flow provided by the axial flow pumps and the TAH differ, as do the physiologic response of heart failure patients to each system. Consequently, understanding the differences between these two systems is important to ensure the appropriate selection of recipients and proper patient care and management.

In this report we present the mechanical and physiologic differences between a representative axial flow pump (Jarvik 2000) and a TAH (AbioCor; AbioMed Inc, Danvers, MA), discuss the capabilities and limitations of each, and identify important management issues related to their use.

Jarvik 2000

Device components
The Jarvik 2000 (Fig 1) is an implantable VAS that produces axial flow by means of a single, rotating, vaned impeller [9]. The system (Fig 2) consists of a blood pump, 16-mm outflow graft, percutaneous power cable, pump-speed controller, and direct-current power supply. The blood pump weighs 90 g, is 2.5 cm in diameter, and is about the size of a common C-cell battery. A brushless direct-current motor contained within the housing creates the electromagnetic force necessary to rotate the impeller. All the blood-contacting surfaces within the pump are made of smooth titanium. Blood flow is directed through the outflow graft by stator blades located near the pump outlet. The power cable is constructed of pacemaker-type wires insulated with polycarbonate polyurethane and partially covered with Dacron (C.R. Bard, Haverhill, PA). The impeller, which is composed of a neodymium-iron-boron magnet and hydrodynamic titanium blades, is held in position by two ceramic bearings. The hydrodynamic blades are located on the outer surface of the impeller, and outflow stator blades are located downstream from the impeller. The electromagnetic force created by the motor spins the impeller at 8,000 to 12,000 rpm, generating an average flow rate of 3 to 6 L/min at 4 to 6 W of power.

View larger version (113K): [in this window] [in a new window]   Fig 1. The Jarvik 2000 axial-flow blood pump with a portion of the outer housing cut away.

View larger version (55K): [in this window] [in a new window]   Fig 2. The Jarvik 2000 system and its components.

Jarvik 2000 clinical study summary
Clinical study of the Jarvik 2000 is now being conducted at eight centers in the United States and five centers in Europe. In the United States, the study has involved only temporary use of the Jarvik 2000 in heart transplant candidates. In Europe, however, the system is being used both as a bridge to transplantation and for destination therapy. All study candidates must have New York Heart Association (NYHA) class IV heart failure, must have a cardiac index less than 2.0 L · min^-1 · m^-2, must be receiving maximal medical support with an intraaortic balloon pump, and must not have significant comorbidities.

Between March 2000 and October 2002, the Jarvik 2000 was implanted in 45 patients worldwide. The inclusion and exclusion criteria for the US bridge-to-transplantation study are listed in Table 1. A more generic list of inclusion and exclusion criteria from the European destination-therapy study is found in Table 2. The specific criteria for the destination-therapy study vary somewhat among the centers.

Serious complications during support occurred in 18 of 45 patients. Patient-related deaths include 5 due to sepsis and multiple organ failure, 4 due to myocardial ischemia, 1 due to coagulopathy, and 3 due to right heart failure. Two of the destination-therapy patients died of subdural hematoma; the hematomas results from a complication associated with the skull-mounted pedestal in 1 case and a fall unrelated to the device in the other. Three device-related deaths were attributed to device thrombosis, stroke, and an improper power cable disconnection by the patient at home, respectively. Nonlethal complications include left ventricular thrombus, coronary thrombosis, and gastrointestinal bleeding. Three patients have been treated successfully with tissue plasminogen activator (tPA) for lysis of ventricular thrombi. Hemolysis has been minimal except in the 2 cases in which thrombolytics were given. No significant device-related infections were noted, although 1 patient developed a superficial infection at the power-cable exit site.

AbioCor total artificial heart

Patient selection
The AbioCor TAH is intended for use in end-stage heart failure patients with irreversible left and right ventricular failure for whom conventional surgery or medical therapy is inadequate [16]. The inclusion and exclusion criteria are listed in Table 4. All study patients have a higher than 70% probability of dying within 30 days. Although the AbioCor is used in patients who are not eligible for heart transplantation, supported patients may become heart transplant candidates when their condition improves sufficiently and they meet the eligibility criteria. Study candidates may be ineligible for the AbioCor system if there is potential for myocardial recovery or if they receive chronic hemodialysis, have irreversible hepatic failure, or have had a recent cerebrovascular accident. Also, patients with significant coagulation disorders may not undergo the implantation surgery until their coagulation status is normalized.

View larger version (34K): [in this window] [in a new window]   Fig 3. The AbioCor total artificial heart system.

The external components include a computer console, the external TET coil, and external battery packs. The computer receives information about pump performance through the RF Comm system. Also, pump control parameters may be sent to the implanted controller from the computer through the RF Comm. The external TET coil provides power to the pump from the console or from the external battery packs. The external battery packs can power the AbioCor TAH for 2 to 4 hours, depending on the number of battery packs carried.

Abiocor clinical study summary
Since the clinical trials of the AbioCor were initiated in June 2001, 7 patients have been supported by this system. Four of the 7 patients lived beyond the 60-day study end point, which is twice their predicted life expectancy. Two patients died within 24 hours of the implant surgery. Four of the patients who survived beyond the perioperative period were supported for 151, 142, 56, and 294 days, respectively. One patient continues to be supported at more than 385 days and is living at home. Two patients have lived outside of the hospital and 5 patients were ambulatory while supported by the TAH. As with other artificial heart systems, and despite strict anticoagulation protocols, thromboembolism has been a problem in patients supported with the AbioCor. Thrombi have been observed on support struts within the atrial cuffs, which struts have since been removed from the device. Serious device-related infections have not occurred.

To date, the AbioCor system has demonstrated a high degree of reliability. No pump failures have been experienced and only minor technical problems have been encountered. The system has functioned as intended, and hemodynamic stability is easily achieved. The TET system has functioned reliably with no cases of power interruption. Quality of life was significantly improved in 4 of 7 patients. The feasibility trial will continue until 15 patients have been enrolled. Detailed results of the feasibility study will be published once the 15 studies are completed.

Care and management issues

The type and the amount of blood flow provided by the axial flow pumps and the TAH differ substantially. Supportive medical management also differs greatly. The Jarvik 2000 axial flow pump provides submaximal unloading of the left ventricle, whereas the TAH provides total circulation to pulmonary and systemic arteries. In TAH-supported patients with normal intravascular volume and vascular wall tone, blood flow and pressure will be within a normal range. However, this is not always true in patients supported by an axial flow pump. Because axial flow pumps cause continuous blood flow throughout the cardiac cycle, diastolic blood pressure increases, systolic blood pressure remains unchanged, and the mean blood pressure increases (Fig 4). The pulse pressure is normally reduced during support with axial flow pumps, but some pulsatility in the arterial pressure should always be present.

View larger version (63K): [in this window] [in a new window]   Fig 4. A representative arterial pressure waveform from a Jarvik 2000-supported heart failure patient. The pulse pressure progressively decreases as the pump speed increases.

View larger version (99K): [in this window] [in a new window]   Fig 5. A representative echocardiogram from a Jarvik 2000-supported heart failure patient showing that blood flow through the Jarvik 2000 outflow graft is continuous throughout the cardiac cycle.

Serial echocardiogram studies with simultaneous blood pressure measurements are useful to determine the appropriate Jarvik 2000 pump speed setting. Some ejection of blood is desirable from the left ventricle out the aortic valve to prevent stasis of blood in the aortic root. The pump speed is adjusted from 8,000 to 12,000 rpm in 1000-rpm increments while the aortic valve is observed. A general guideline is that the optimal pump speed setting is 1,000 rpm less than the speed at which the aortic valve remains closed. Another guide for determining the appropriate pump speed is when the pulse pressure (systolic-diastolic) is more than 20 mm Hg; the aortic valve will normally be open during systole.

One of the complications that can occur with axial flow pumps is thrombosis within the left ventricle, adjacent to the device’s inflow conduit, which may lead to inflow obstruction and recurrent heart failure. Partial or complete obstruction of the inflow can be determined by an acute increase in the power requirements of the pump motor. The left ventricular thrombus can be diagnosed or confirmed with echocardiogram studies. This complication has occurred in 2 of the US bridge-to-transplantation patients supported by the Jarvik 2000. In the cardiac catheterization laboratory, recombinant tPA is continuously infused into the left ventricle at 1 mg/min. The speed of the LVAD is increased to 11,000 rpm to allow maximal tPA perfusion of the device. The power readout is observed for a decrease in the power necessary to rotate the impeller, indicating successful thrombolysis. Thrombolytic therapy is often contraindicated because of recent surgery or gastrointestinal bleeding. In both cases mentioned above, we were able to treat the thrombus by infusing a thrombolytic agent directly into the left ventricle. By monitoring the result and administering only as much as of the thrombolytic agent as necessary to achieve thrombolysis, we were able to achieve a successful result with a minimal dose.

Because the Jarvik 2000 is a partial assist system, usual heart failure support with inotropic and vasoactive agents is often necessary. Pacemaker use and electrolyte normalization may be necessary to maintain an adequate cardiac rhythm. Maintenance of adequate left heart volume with a low or normal systemic vascular resistance is essential for patients supported by an axial flow pump. Pulmonary vasodilators, such as prostaglandin or nitric oxide, are often needed to keep the pulmonary vascular resistance within a normal range. The right ventricle should be supported with inotropic agents when the pulmonary vascular resistance is elevated or if the right ventricular function is poor. Optimal pump flow and cardiac output are usually achieved when the mean arterial blood pressure is in the range of 65 to 75 mm Hg. Vasodilator therapy is often necessary for proper blood pressure maintenance.

In contrast, the AbioCor TAH does not normally require strict control of pulmonary and systemic vascular resistance. The mechanical pumps can generate adequate blood flow against a high resistance. However, maintaining proper intravascular volume is critical. The AbioCor system can pump as much as 8 L/min, and it will do so as long as the right and left atria have sufficient blood volume. Too much intravascular volume may cause severe pulmonary edema or hepatic congestion; therefore, the left atrial pressure and central venous pressures should be maintained within normal range. Although hemolysis has not been observed in patients supported by the AbioCor, frequent hematologic assessments should be made to monitor for anemia.

All critical care involves the monitoring and maintenance of cardiac rhythm. In the case of AbioCor recipients, however, there is no electrocardiogram to assess and no need for medications to support heart function because the natural heart is removed and a mechanical system is functioning. Inotropic medications are not useful, and the usual cardiac rhythm problems associated with severe electrolyte imbalance are of no concern. Pulmonary vasodilator therapy may be used, although this therapy may not always be necessary because a mechanical right heart pump is present. Hypertension and anticoagulation therapy places these patients at risk for hemorrhagic stroke; therefore, systemic vasoactive agents should be used as necessary to control the arterial blood pressure.

Anticoagulation therapy is necessary to prevent thrombosis in both the Jarvik 2000 and the AbioCor. Thrombosis in either of these devices may result in device failure or may cause infarction in other organs. Typically, a combination of low-molecular-weight dextran, heparin, warfarin, dipyridamole, and aspirin is used to prolong clotting times and to suppress platelet function. For all cardiac support systems, with the exception of the HeartMate LVAS, the initiation of anticoagulation therapy depends on the amount of postoperative bleeding and the coagulation status of the patient. The partial thromboplastin time is used to assess heparin dosage, and the prothrombin time and international normalized ratio are used to determine the dosage of warfarin. The thromboelastogram is used intermittently to determine the global efficiency of anticoagulation therapy. The goal is to begin heparin administration at a dose of 10 U · kg^-1 · h^-1 within the first 3 days and simultaneously start warfarin administration at a dose of 5 mg/d. The heparin dose is reduced in increments of 200 U/h when the partial thromboplastin time is longer than 60 seconds, and the warfarin dose is adjusted to achieve an international normalized ratio in the range of 3.0 to 4.5 by postoperative day 10. Antiplatelet therapy is also begun at the same time as the anticoagulation therapy. Dextran 40 at a dose of 25 mL/h is continued until the patient can take aspirin orally. Dipyridamole (100 mg) is given every 6 hours, and the dose is increased 100 mg each day until the thromboelastogram indicates that the patient is not hypercoagulable. Dipyridamole and aspirin are continued daily.

Nutritional support and exercise are important for all patients supported with a cardiac support system. Early aggressive physical therapy is necessary for progress toward ambulation. A key part of the rehabilitation process is proper education of the patient and family on the operation of the system, since one of the main goals after implantation is to prepare the patient to live outside the hospital.

Maintenance and care of the Jarvik 2000 and AbioCor systems are different. The external hardware for the Jarvik 2000 system is small and easily operated. Usually, Jarvik 2000-supported patients need only to be able to change their batteries and recognize alerts on the controller. The AbioCor system naturally requires more training, care, and maintenance because of the increased complexity. Patients need to remove and replace the TET coil, change batteries, identify multiple alarm conditions, and maintain numerous pieces of accessory equipment. Although the AbioCor system is complex, it is designed to be maintained by patients.

As with all implantable blood pumps, aggressive infection management is necessary to prevent bacterial colonization of the implanted foreign material. Chronic infection of these devices often leads to serious complications and death. Both the AbioCor and the Jarvik 2000 systems are designed to minimize infectious complications. Antimicrobial therapy for AbioCor and Jarvik 2000 recipients is not unique and is the same as the therapy given to any patient undergoing cardiac surgery. The TET system of the AbioCor helps to reduce device infections by eliminating the need to penetrate the skin. The relatively small amount of foreign material in the Jarvik 2000 may also result in fewer infection-related problems. To date, there have been no reports of colonization of either device. The clinical results so far indicate that the infectious complications associated with the Jarvik 2000 and the AbioCor are minimal.

Comment

Although the Jarvik 2000 and the AbioCor systems are significantly different, they both have an important role to play in supporting heart failure patients. Continuing studies are necessary to establish appropriate patient selection and to develop management protocols.

References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Myers, T. J. Articles by Frazier, O. H. Search for Related Content PubMed PubMed Citation Articles by Myers, T. J. Articles by Frazier, O. H. Related Collections Mechanical Circulatory Assistance