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The concept of circulatory assistance is not new.1 The need for such temporary support for hours to days has been recognized for more than a decade; and still exists. The most common form of acute, iatrogenic, potentially reversible circulatory failure in man is that following cardiopul-monary bypass. If means were available to support temporarily the circulation of such patients, the current low mortality associated with open-heart surgery would be further decreased and, perhaps, the indications for high-risk patients broadened. For example, following otherwise uncomplicated procedures it is common for patients to require reinstitution of cardiopulmonary bypass for brief periods to allow the biologic left ventricle to resume its pumping function to support the peripheral and central circulations. There is a correlation between the duration of the necessary period of re-support with the degree of acute left ventricular decompensation encountered. The limitations of prolonged bypass are hemolysis, protein denaturation and the appearance of hemorrhagic diatheses. Cardiopulmonary bypass is a controlled form of shock and applicable for only short periods of time.
The techniques proposed for circulatory assistance have included aortic bypass pumping (vide supra), diastolic intraaortic balloon pumping, counterpulsation with an ectopic ventricle, external regional counterpulsation, synchronous airway pulsation, veno-venous oxygenation, veno-arterial oxygenation and mechanical synchronous sternal compression. All have been utilized in man with varying degrees of success.
The purpose of this communication is to provide an anticipatory refrence responsive to questions regarding the proposed short-term use of an abdominal left ventricular assist device,2,3 and to submit these concepts and principles to peer review. This is appropriate in the current climate of an increasing involvement of medicine, technology, industry and government with society. Clearly, the possible use of this abdominal left ventricular assist device could involve not only the aforementioned sectors, but also impinge on moral, legal and ethical interests. It should be emphasized that, over the last two years, these implications have been given careful consideration. The new National Heart, Blood Vessel, Lung and Blood Act of 1972 reaffirms the responsibility for research and development in this area; there is to be both research into, and establishment of, programs dealing with devices and instrumentation. The details can be found in Sections 413 (A), items 3 and 4. Legislation is not specified.
The abdominal left ventricular assist device (ALVAD) is an implantable blood pump actuated by an external pneumatic power source.4 This pump is a modification of the Model VII left ventricular assist device developed under NHLI sponsorship over the past 6 years. Blood is received from the apex of the left ventricle during systole and ejected into the infrarenal portion of the abdominal aorta during diastole. The pump is of cylindrical design and operates axio-symmetrically. The pump ventricle consists of a polyurethane bladder which collapses when pneumatic pressure is applied to the space between the bladder and the stainless steel casing. Unidirectional flow is imparted by disc valves at the inflow and outflow orifices. In vitro tests of this pump have shown that its output is essentially a linear function of pumping rate up to maximum flows of approximately 13 liters per minute. Two pump sizes have been used for animal experiments, with 65 and 100 ml stroke volumes for dogs and calves, respectively.
All blood contacting surfaces, including the inflow tube, the valve struts and the pump bladder, excluding the valve discs and the outflow tube graft, are “flocked” with a coating of polyester fibrils. These filaments are 10 mils in length and 1 mil in diameter and are applied with a density of 100–150 fibrils per square millimeter. This flocked surface promotes a rapid deposition of fibrin which serves as base for the development of a stable, blood compatible surface. Microscopically, the neointima formed on flocked polyurethane appears to consist of a fibrin lattice containing pyknotic nuclei of fibrocytes. A thin neointima of endothelial-like cells is gradually deposited over this fibrin layer. The observation of lymphocyte aggregates at the blood-biomaterial interface suggests the possibility that the neoendothelial cells originate from circulating leukocytes by a process of redifferentiation.
The pump drive console provides both EKG triggered synchronous pumping and fixed rate asynchronous pumping over a range of 40 to 140 strokes per minute. Variable ejection delay and pulse duration are provided.5 The console has redundant pneumatic and electronic systems. The primary system has a vacuum assisted fill cycle and is capable of operating in either synchronous or asynchronous modes over the complete range of rates. The redundant system provides drive pressures at adjustable fixed rates and pulse durations. A four channel oscilloscope displays EKG, arterial pressure, pump drive pressure and left ventricular pressure. Left ventricular pressure is obtained from an inflow pressure transducer incorporated into the ALVAD. Fail-safe systems are included for: 1) EKG malfunction or arrhythmias; 2) primary system mechanical or electronic failure; 3) loss of pneumatic power; and 4) loss of AC line power.6
Implantation of the ALVAD is accomplished by utilizing a modification of the method developed by Bernhard and LaFarge.7 For acute experiments in dogs, a median sternotomy with midline abdominal extension is used. For chronic experiments in calves, a left thoracotomy and a left transverse laparotomy are preferred. The woven outflow tube graft is sutured end-to-side to the abdominal aorta below the renal arteries. The pump and inflow tube are primed with saline. A Teflon felt sewing ring is sutured to the left ventricular apex. A small incision is made in the apex and a Foley catheter is inserted into the left ventricular cavity utilizing a central stylet. The catheter balloon is inflated and with gentle withdrawal pressure is applied against the apical wall. A circular knife is passed down the catheter and a full thickness segment of apical myocardium is excised. The catheter balloon provides hemostasis. Under a carbon dioxide atmosphere, the balloon is deflated and quickly withdrawn and the inflow tube of the pump is rapidly inserted into the left ventricle. A purse string suture is then drawn tight around the collar of the sewing ring creating a seal around the inflow tube. The pumping chamber resides in the abdomen with its inflow tube traversing the diaphragm. The pneumatic drive line is covered with a sleeve of polyester velour. The drive line and the pressure transducer instrumentation cable are brought out of the abdomen through a separate small incision.
Two advantages result from the abdominal pump placement: 1) pulmonary function is minimally impaired; and 2) removal may be accomplished through the original abdominal incision without re-entering the thorax.
Removal of the ALVAD is performed by re-opening the previously made celiotomy incision. The pump inflow tube is attached to the pump by a rotary quick connect-disconnect fitting. The pump is removed and the inflow tube is left in situ in the ventricular apex, occluded with a special obturator. The outflow tube graft is divided and oversewn.
When viewed objectively, a series of questions can be raised regarding the use in man of such a device. Those which are immediately apparent were first considered two years ago and were included in proposals* to the Division of Technological Applications of the National Heart and Lung Institute.
An implantable left ventricular assist pump has been developed to the point where initial clinical trials can be seriously considered. These latter considerations involve a patient population faced with imminent demise. The pump functions well in vivo, according to design specifications, and can be easily removed. If used clinically in the projected circumstances, legal, moral and ethical questions arise. The potential of life sustaining benefits would seem to outweigh the inherent risks.
* RFP NHLI-MDAP-72-21, April 1972 and Proposal No. TE 2436-200 Continuation of Contract No NO1-HL-73-2946, May 1973
* Mr. Dayton W. Selby, Administrative Office, Washington Greater Metropolitan Blue Shield, 550 12th St., S.W., Washington D.C., Source: Dr. J. K. Roche, NHLI.
This article has been reprinted from Cardiovascular Diseases: Bulletin of the Texas Heart Institute 1974;1:251–64.