Cardiology

Other Mechanical Circulatory Support Systems

General description of procedure, equipment, technique

The SynCardia Total Artificial Heart is a pneumatic orthotopic replacement for use as a bridge to cardiac transplantation in patients with severe, irreversible, biventricular failure and severe cardiac failure with the onset of multiorgan failure. A destination therapy trial has been launched in the US in the first quarter of 2016. The device now comes in 2 sizes. The first and commercially available size consists of two 70 ml ventricles that result in a total volume displacement in the chest of 400 ml, and a combined weight of 150 g. The smaller 50ml ventricle size is approximately one third less in volume displacement and weight and available in the US as part of an FDA trial. Computerized virtual fitting is available for both sizes. Two thirds of the 50ml ventricle implants have been in women. The implantation and function of the smaller heart are similar to the 70ml version. In this discussion focus will be upon the 70 ml device.

It delivers pulsatile flows in vivo of 7 to 9 L/min at a central venous pressure ranging from 6 to 12 mm Hg. The chambers are lined with smooth polyurethane and have four layered polyurethane diaphragms that are displaced by air.

Thus, with air on one side and blood on the other, a pulse is generated by nonsucking partial fill (50 to 60 ml of the maximal 70 ml possible) of the ventricles and full ejection with each beat. The valves are mechanical and on the inflow side are 27 mm in diameter and on the outflow side 25 mm.

The beat rates are usually 120 to 130/min and the console computer displays instantaneous pump output, filling curves, and pressure curves for both ventricles, as well as trending the outputs for both sides. Imbalance between the two ventricles does not occur and the device has a "built in" Starling response, increasing output with increased filling. Thus, if the rate is 130 beats per minute and the fill volume is 50 ml, there is a 20 ml reserve volume with each beat and the device can increase output in response to increasing preload up to 2.6 liters/min (20 ml X 130 beats/min). For example, if a single right sided beat delivers 10 additional ml. to the left, it is fully ejected with the next beat. This explains the automaticity of pneumatic technology.

Hospital discharge is possible with the Freedom driver. It is fully portable weighing 13.5 lb and having the size of a small briefcase.

Implantation of this device is very similar to cardiac transplantation. The two 70 ml ventricles will fit into the chest of any patient with a left ventricular end diastolic dimension that is at least 70 mm.

Indications and patient selection

Indications for use

There are some situations that oblige the surgeon to use a complete replacement of the heart. These include: failing cardiac grafts, transplant coronary vasculopathy or rejection, massive myocardial infarction, acquired ventricular septal defect, diffuse left ventricular thrombus, ventricular rupture, unrepairable aortic root, stone heart, diastolic heart failure (severe hypertrophic disease, amyloid and other infiltrative diseases, fibrotic disease, restrictive cardiomyopathy), cardiac malignancy, and some congenital heart problems (failed Fontan, and failing uni and biventricular hearts).

This device is ideal when total control of the circulation is desirable and a high pulsatile perfusion pressure and low central venous pressure are believed to be necessary for recovery from multiple organ failure. These include: Intermacs 1 patients ("crash and burn") that are judged to have hours to days of survival, patients with biventricular failure, patients with right ventricular failure (i.e., mild elevation of the pulmonary artery pressure but high (>18 mm Hg central venous pressure) central venous pressure, patients that have high risk scores for left ventricular assist device (LVAD) implantation (Lietz Miller score of >19), and patients with multiple organ failure.

Current FDA approval is for use as a bridge to transplantation, or destination therapy approval is pending.

Selection criteria

  1. Patient is potentially a heart transplant candidate.

  2. Fit criteria: In order for the device to "fit" and not compress the inferior vena cava or the left pulmonary veins, the following criteria should be considered: Heart size should be large (by chest x-ray: cardiothoracic ratio >0.5; by echo: left ventricular end diastolic dimension of at least 70 mm). The chest size should be large (the distance from posterior sternum to anterior vertebral body should be at least 10 cm). In other words, the patient should have a very large heart and/or a large chest cavity.

  3. Patient is sick enough:

    • On support with multiple inotropic agents and at least one pressor agent and failing hemodynamically.

    • Judged to have hours to days to survive (by Intermacs criteria: Intermacs 1 crash and burn, or Intermacs 2 failing in hospital maximal therapy)

    • Central venous pressure of >16 mm Hg.

    • Systolic blood pressure of <100 mm Hg.

    • Cardiac index of <2.0 L/min per square meter

    • Evidence of incipient multiple organ failure characterized by: serum creatinine of >2.0 mg/dl, total bilirubin of >1.5 mg/dl, prothrombin time of >16 seconds, platelet count of <148,000.

  4. Patient is not a candidate for an LVAD: nonfunction or severe failure of the right ventricle, injury to the right ventricle, risk stratification that indicates extreme high risk (i.e., Lietz Miller score of >19 indicates 70%, 1-year mortality; >22 indicates 90%, 1-year mortality with an LVAD, right ventricular stroke work index of <8.8 g/m).

Contraindications

  1. Inadequate size (see above)

  2. Severe cachexia

  3. Advanced physiologic age (>75 years)

  4. Chronic multiple organ failure so severe as to be incompatible with organ recovery

  5. Cirrhosis

  6. On ECMO (extracorporeal membrane oxygenation) for CHF for a support period of >3 days.

Details of how the procedure is performed

Evaluation

If time permits, patient preparation should include the same steps used for cardiac transplant evaluation. Briefly, this includes a complete laboratory evaluation; toxicology screen of urine; blood typing; HLA typing; anti-HLA antibody screening; serology for cytomegalovirus (CMV), Epstein-Barr virus (EBV), HIV, human T-lymphotrophic virus (HTLV), and toxoplasmosis; chest x-ray; cardiac echo; cardiac catheterization of both left and right heart with careful transpulmonary gradient (TPG) and pulmonary vascular resistance (PVR) measurements; head, chest, and abdominal CT scans; pulmonary function tests; dental panorex; and psychosocial screening.

Patient preparation

Make the patient as free of problems and as stable as possible, realizing that a decision to postpone implantation may decrease his or her chance for survival. If possible, have the patient off antiplatelet agents and coumadin. Antibiotic prophylaxis: 24 hours of vancomycin starting intraoperatively. Nasal mupirocin.

The procedure

On full cardiopulmonary bypass, the heart is arrested, aorta cross-clamped, and the ventricles are removed at the level of the atrioventricular (AV) groove. A 2 mm edge of each of the AV valves is left in place for strength.

Atrial connectors are sewn to the atrial cuffs and outflow conduits with proximal connectors are anastomosed to the great vessels. The ventricles are then snapped into place, the heart is deaired, and cardiopulmonary bypass is discontinued as the total artificial heart (TAH) beat rate is increased gradually, taking over the circulation.

Generally, the beat rate is set at 120 to 135 beats/min, the left-sided pump pressure at 180 mm Hg, right-sided pump pressure at 90 mm Hg, the percent systole at 50, and the vacuum at minus 10.

The device is run so as to provide partial prosthetic ventricular filling of 55 to 60 ml on both sides and to full ejection of all blood on every beat. This leads to a cardiac output of 7 to 8 L/min and a central venous pressure (CVP) of 8 to 12 mm Hg.

Interpretation of results

Fill volumes are an index of total blood volume. If they exceed 65 ml/beat consistently, increase the beat rate.

If the ventricles are allowed to run fill volumes of 70 ml, any increase of intravascular volume will increase filling pressure. If there is "space" for an additional 10-20 ml per stroke, the device will automatically receive and eject additional intravascular volume.

Example: A patient running fill volumes of 65 to 70 ml, with a beat rate of 120 beats/min, cannot tolerate much more volume before filling pressure central venous pressure (CVP) and airway pressure (PAW) go up. To drop the filling volume to safer levels, the heart rate is increased to 125 or 130 beats /min.

This drops the fill volume and allows the device to pump more blood. The filling curves are an even more sensitive reflection of ventricular filling. They represent the rate of filling.

If the rate of filling drops to zero during diastole, the ventricle may be full (as in volume overload) or there may be some obstruction to ventricular filling (as in atrial tamponade or inflow valve obstruction).

Pressure curves show instantaneous pressure delivery to the ventricles. Once the ventricle has fully ejected, there is an overshoot of the curve. This is used as a marker to ensure full ejection. If the ventricle does not fully eject, pressure and volume back up may occur and the instantaneous pump output (cardiac output) will be an overestimate.

Outcomes (applies only to therapeutic procedures)

Used primarily in very sick patients, the survival rate to transplantation has been 70% to 80% and the posttransplant survival rate has been 77% to 86% at 1 year.

Alternative and/or additional procedures to consider

Biventricular assist device (BiVAD) implantation with extra corporeal ventricles is an option for patients who are too small for a TAH, but otherwise would be reasonable candidates.

Extracorporeal membrane oxygenation (ECMO) has been used in very sick patients and is the method of choice if a reversible process (viral or peripartum cardiomyopathy) is suspected. This is sometimes inadequate either because of inadequate flow from the ECMO or from ECMO failure that is a time-related problem. Usually ECMO is recommended for around 1 to 2 weeks because of this. Unfortunately for those patients the lungs receive very little flow and slow weaning is indicated. In patients on prolonged A-V ECMO, sudden return of normal pulsatile perfusion, overflow pulmonary edema is likely to occur. Then treatment with VV ECMO is obligatory. To avoid this scenario, I recommend limited AV ECMO prior to TAH implantation (no more than 3 days).

The only other alternative for these patients may be cardiac transplantation. Because of the shortage of donor hearts, the long waiting time for transplantation, and the rapid decline seen in most of these patients, receiving a donor heart is unlikely.

Complications and their management

Complications and percent occurrence: takeback for hemorrhage 25%, stroke 8%, serious infection 5%. Takeback for hemorrhage is a therapeutic procedure that accomplishes control of hemorrhage and tamponade.

The ventricles, being rigid, are not directly subject to cardiac tamponade. But the atria are highly susceptible to external compression and some degree of "atrial" tamponade is nearly always seen when there has been brisk postoperative bleeding.

Discrete right or left atrial tamponade has been seen. Since the TAH is preload dependent, tamponade of one or both atria can have a profound effect on pump output.

Strokes are seen most commonly in the 48 hours postimplantation and have in most cases been attributed to preoperative and intraoperative events. Following the usual diagnostic and therapeutic anticoagulation protocol for this device has led to a low incidence of stroke (0.08 events per patient year) after the first 48 hours.

Infections are diagnosed and treated as usual with the device. Often they are related to the preoperative condition of the patient. Infections of the mediastinum are rare (<5%) and in most cases are subclinical being diagnosed at the time of explantation by routine mediastinal cultures.

We have seen three out of 101 patients with clinical mediastinal infections post-TAH implantation. Two had strong risk factors (failed BiVAD and Crohn's disease) and both died. One survived debridement irrigation and subsequent transplantation. Prevention of infection consists of the usual prophylactic antibiotic therapy for valve surgery, 1 to 2 doses of vancomycin.

What’s the evidence?

Copeland, JG, Smith, RG, Arabia, FA. "Cardiac replacement with a total artificial heart as a bridge to transplantation". N Engl J Med. vol. 351. 2004. pp. 859-67.

(This is a report of the multi-institutional FDA study of the TAH. It is the key article documenting the usefulness of the TAH.)

Copeland, JG, Arabia, FA, Tsau, PH. "Total artificial hearts: Bridge to transplantation". Cardiol Clin. vol. 21. 2003. pp. 101-13.

Copeland, JG, Copeland, H, Gustafson, H. "Experience with more than 100 total artificial heart implants". J Thorac Cardiovasc Surg. vol. 143. 2012. pp. 727-734.

(This article documents 101 consecutive TAH implants at one institution with no exclusions.)(An in-depth article documenting institutional experience with the TAH, including a review of bleeding and anticoagulation.)
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