TOTAL ARTIFICIAL HEART REPLACEMENT VS. HEART TRANSPLANTATION - WHAT YOU NEED TO KNOW

Medically Reviewed by Dr. Sony Sherpa (MBBS) - September 20, 2024

Since the early 80s, total artificial hearts have been used to help bridge the gap to transplants for those with heart failure. With refinements to the technology, it may be possible to replace the failing heart permanently.

The discussion below sheds light on what artificial hearts are, how they are designed to prolong life, and the latest advancements with regard to artificial heart replacement technology. Additionally, the pros and cons are compared between the world’s best artificial hearts and ventricular assist devices as bridges to transplant options.

Heart Failure and Heart Replacement

When the ventricles of the heart are deformed, it can result in heart failure, a potentially fatal condition. It is the number one cause of cardiovascular mortality and dramatically increases the risk of arrhythmias, thromboembolism, and heart attack. The prevalence in adults is estimated to be roughly 1-2%, affecting approximately 26 million individuals worldwide.[1]

Causes. In a healthy heart, the ventricles pump blood in and out of the heart and to the rest of the body. Each ventricle pumps blood simultaneously due to the action of the myocardium, which is the thick muscle that surrounds the heart and keeps it beating.[2] In heart failure, the myocardium does not work as well, resulting in a substantially reduced heart rate and suboptimal circulation. The most common causes of heart failure are myocardium and/or ventricle deterioration. Other causes promote excessive thickness of heart tissues and/or severe circulatory problems (e.g., hypertension, hypovolemia, embolism, etc.) that detract from the ability of the heart to pump blood.

Types. Heart failure is classified into three main types in accordance with the left ventricle ejection fraction (EF), which refers to the functionality of the left ventricle and the amount of oxygenated blood that leaves the heart. Patients with a reduced EF (EF <40%) have a worse prognosis than those with preserved EF (EF >50%) and mid-range or borderline EF (EF between 41-49%). Preserved EF is more common than other types and is indicative of excessive ventricular stiffness in comparison to reduced EF, resulting in more pressure on the left of the heart as opposed to the right. While more often recommended for those with reduced EF, those with preserved or borderline EF may also require heart replacements if symptoms do not improve in response to other treatments.

Treatment. All heart failure patients receive treatment for common risk factors and comorbidities, including hypertension, diabetes, and dyslipidemia. This includes pharmacological management (after receiving diagnostic confirmation), as well as instituting dietary and lifestyle modifications. In early detection, the patient may be eligible for regenerative treatment such as stem cell therapy. If the patient does not respond to these treatment options, surgical interventions are advisable and include coronary revascularization (for suitable patients), internal pacemakers, ventricular assist devices, artificial heart replacement, and heart transplantation.

Heart Transplantation in Advanced Heart Failure. Heart transplantation is advised when heart failure reaches a critical stage. Advanced heart failure could be diagnosed in a patient with persistent symptoms that do not respond to pharmacologic medical treatment. It is often accompanied by reduced EF, edema, and/or severely reduced peripheral blood flow, as well as extreme systolic and/or diastolic cardiac dysfunction. Advanced heart failure patients who require heart transplantation are often placed on a waiting list. Despite the urgency, there are not often donor hearts available for transplant, which has led to the development of ventricular assist devices and artificial heart replacement.

Artificial Heart Replacement for Advanced Heart Failure

Artificial hearts have been in development for nearly 40 years and have managed to prolong the lives of hundreds of people with heart failure.

What is an Artificial Heart?

A total artificial heart is a device designed to replace a failing heart, usually for patients waiting to receive a heart transplant. It consists of two man-made ventricles that are surgically implanted and connected to the upper chambers of the heart (atria) and the major arteries (aorta and the pulmonary vein).

The artificial heart is also connected to two tubes, known as drive lines, that leave the body through two incisions on the side of the patient’s abdomen. The tubes connect to an external device, referred to as the driver, which serves as a heart pump machine. This comes with a battery and can be recharged.

How Does an Artificial Heart Work?

The artificial heart pump ensures that blood is pumped pneumatically or hydraulically through the body, sending either compressed air or water through the driver lines. The heart rate can be controlled by the driver alongside other parameters.[3]

The shape and compact size of the artificial heart allow more blood to flow through the device during a shorter space of time compared to a failing biological heart. This increases the output of oxygenated blood, reduces venous pressure, and improves overall tissue perfusion. The artificial ventricles do not completely fill with each breath, allowing for blood pressure changes that would ordinarily occur in response to exercise or other activities that can increase both respiration and heart rate[4]. Despite these functions, patients with an artificial heart are still unable to exert themselves as much as a person with a healthy heart can.[5]

Types of Artificial Heart. There are currently two well-known artificial hearts available on the global market, the SynCardia and the Carmat prosthetic heart. Their different features and how they work are described below:

• The SynCardia Total Artificial Heart (SynCardia TAH) was developed from the first prototypes available in the late 1990s and early 2000s. Formerly known as the CardioWest, it is comprised entirely of synthetic ventricles made from Polyurethane that does not resemble a human heart. The SynCardia is electro-pneumatically powered and is set at a fixed rate of 100-130 beats per minute, which limits the degree to which it can compensate for changes in activity, respiration, and blood pressure. The driver is often very large and inconvenient for the patient to carry around despite being portable enough to fit in a large carrier bag.
• The Carmat Bioprosthetic Heart is a new type of artificial heart that was first released in Europe in 2017. It is a completely different technology from SynCardia and is comprised of biocompatible materials (including cow skin) and pressure sensors that allow for subtle changes that more closely resemble the functionality of a biological heart.[6] Driven by electro-hydraulics, the Carmat driver is much more portable, less noisy, and housed in a small sling bag. Unlike mechanical hearts, it has the ability to automatically adjust its flow rate in response to pressure, allowing the patient to engage in more activity without worrying about blood pressure-related problems.[7]

Future technologies are expected to incorporate softer artificial material that more closely resembles the shape and function of the human heart, improved grafting techniques, and robotics for better heart pump control.

Purpose of a Total Artificial Heart Replacement

A total artificial heart is most commonly used to support patients with critically advanced heart failure, in which both ventricles of the heart are not functioning properly. It may also be recommended to the advanced heart failure patient who is non-responsive towards treatment or for whom a ventricular assist device is contraindicated.

Bridge to Transplant. Total artificial heart replacements are most commonly employed temporarily as a bridge to transplant, allowing the patient to live with a better quality of life and a higher chance of survival until they can undergo heart transplantation.

Destination Therapy. Despite being an option, patients do not often opt for an artificial heart as destination therapy. Some patients with total artificial hearts are unable to get a transplant and may need to rely on an artificial heart or ventricular assist device for a prolonged period of time.

Artificial Heart Replacement Risks and Complications

Common risks and complications of artificial heart replacement include:

• Hypertension
• Bleeding
• Thrombosis, Cardioembolism or Stroke
• Infection
• Anemia
• Liver or Kidney Failure

Pharmaceuticals are still prescribed to heart failure patients with artificial hearts to minimize risks and complications.

Artificial Heart Alternatives and How They Compare

Before heart transplantation and total artificial heart replacement are considered for heart failure, other alternatives are available that help to maintain core heart functions. These include surgical techniques such as coronary bypass surgery, as well as pacemakers and ventricular assist devices.

Those with advanced heart failure can often decide between total artificial heart replacement and ventricular assist devices. The below section discusses VADs while comparing them to total artificial heart (TAH) replacements.

Total Artificial Heart vs. LVAD and BiVAD

Ventricular assist devices (VADs) are more frequently used than total artificial hearts. They are devices that are surgically inserted into the patient, which replace functionality in either the left or both ventricles. These are referred to as Left VADs or Biventricular VADs, respectively, and are approved for both destination therapy and bridge to transplant for heart failure patients eligible for transplantation. Like artificial hearts, they connect to an external electrical device.

Most heart transplant patients on a waiting list opt for a left VAD (LVAD). Patients with biventricular failure or severe structural abnormalities tend to have the worst prognosis and are eligible for total artificial heart replacement or biventricular VAD. While risks are similar, LVADs are generally seen as superior to biventricular VADs, which pose a higher risk of developing strokes and heart complications such as arrhythmia and valve dysfunction[8].

Pros

• BiVAD Post-Transplant Survival Rates May Be Higher Than SynCardia. Survival rates are higher by an average of 5-6% after transplantation in patients that made use of biventricular VAD as compared to the SynCardia TAH after 1 month, 1 year, and 3 years. This suggests a similar efficacy and that VADs are perhaps a mildly safer alternative for those waiting for heart transplantation. [9] Other studies confirm these results, revealing that there are higher transplantation successes and survival rates on average for those with VADs, averaging 77% after 5 years[10] (compared to 64% with post-SynCardia patients). A minimum of 14% average mortality is seen with both techniques across all studies. [11] [12]
• Lower Thrombosis Risk. Thrombosis rates are roughly 10-15% lower in the latest VAD models than seen in patients with SynCardia TAH (2.2 - 7% vs. 19%). [13] The risk is nearly equitable in old VAD models.
• Reduced Morbidity Rates. When compared to the HeartMate 3 LVAD, SynCardia TAH revealed a higher degree of morbidity and an increased risk of kidney failure.[14]
• More Portable. The external device connected to the internal VAD is handheld, making it a lot more portable than current artificial heart drivers.
• Can Be Monitored More Easily. Newer VADs are able to take direct readings of arterial pressure levels and pulmonary resistance. VADs can also be easily read by external devices, such as CardioMEMs. Artificial hearts can be regulated by the driver, yet it may be more difficult to assess their real-time efficacy in the patient.
• Chance of Total Heart Recovery. A small percentage of patients with LVAD exhibit symptoms of myocardial regeneration and opt to have the device removed. Left ventricle unloading appears to improve cardiac regeneration with positive implications for growth, collagen metabolism, the viability of myocytes, and beta-receptor sensitivity. This may be more applicable for healthier, younger patients and short-term use, as prolonged use is more commonly associated with ventricular degeneration in heart failure patients. Other evidence suggests that for patients who received simultaneous coronary artery bypass and LVAD implants, the chances of full or partial heart recovery are higher.[15]

Cons

• Requires a Surgical Pocket. The internal components of VADs come in variable shapes and sizes, all of which take up extra space over and above the patient’s heart. These require surgical pockets and may be uncomfortable for the patient. The latest models are very compact to reduce the inconvenience.
• Risk of Valve Atrophy. Prolonged use of VADs can promote cardiac atrophy, weakness, and abnormal calcium cycling, which can all lessen the heart's capacity to pump blood. [16] This may incur additional costs for valve replacement.
• Higher risk of bleeding and stroke. Roughly 10 - 30% of patients with VADs were shown to suffer strokes[17] (this could be as high as 60% in those with BiVADs). Bleeding risk was also shown to be much higher than seen with total artificial heart replacement, with an average prevalence of anywhere between 23 - 43% across studies.
• Infection Risk. Studies report an exceedingly high variance with respect to infection rate (13-80%)[18], which may indicate that better precautions are taken with regard to total heart replacement.

SynCardia Total Artificial Heart. Approximately 2000 artificial heart transplants have been carried out to date globally with the SynCardia total artificial heart. Current evidence regarding its efficacy by comparison to VADs is summarized below.

Pros

• Takes Up Less Internal Space. As it completely replaces the heart, valves, and all, SynCardia does not require a surgical pocket or any additional internal space in contrast to a VAD.
• Lower Hospitalization Time. TAH may prove to be more cost-effective than VADs due to requiring a diminished length of stay in the hospital.[19] Patients may be sent home after 1 week of hospitalization with a total artificial heart.
• Fewer Complications. There may be a marginally lower risk for some complications with respect to TAH, including bleeding (~14%) and stroke (~8%).[20]
• Possibly Better Infection Rates. Infection rates are still roughly equitable to LVADs in most studies. However, there may be a slight advantage with SynCardia TAH due to a more narrow range of prevalence across clinical trials (38-53%).

Cons

• Reduced Survival Rates. In a long-term review, SynCardia proved to have a lower survival rate post-transplantation compared to those with VADs and other devices.[21]
• More Invasive Surgical Procedures. Despite not requiring a surgical pocket, the surgery for a total artificial heart replacement is a lot more invasive. VAD surgery may only require a couple of small incisions to achieve.
• Less Portable External Devices. SynCardia makes use of a very cumbersome driver that is not nearly as portable as the external device attached to a VAD. With great effort, the patient may move it around in a large carrier bag. It is also relatively noisy.
• Thrombosis Risk may be higher for SynCardia patients than those who opted for next-generation VADs.
• A Higher Kidney Failure Risk is seen in TAH patients (up to 38%) than in BiVAD patients (up to 25%[22]).
• Anemia is worse in TAH patients than VAD patients on average.

Carmat Bioprosthetic Heart. Recent studies show that the European Carmat bioprosthetic total artificial heart performs a lot better than previous artificial heart models. However, studies are extremely limited, and very little is known about survival and complication rates. Positives are reviewed below:

• Significantly Lower Thrombosis and Stroke Risk. A small trial on 10 patients revealed that the Carmat bioprosthetic heart radically reduced the risk of blood clots due to the absence of hemolysis as a result of the acquired von Willebrand factor.[23] Von Willebrand factor is a protein released by the cells that comprise blood vessel walls in response to injury. They are large glycoprotein molecules that quickly form blood clots for wound repair. However,  they are also known to greatly increase the risk of thrombosis, heart attack, or stroke due to being more insoluble than other clotting factors[24]. Those with heart failure are especially prone to elevations of the von Willebrand Factor, particularly after the insertion of cardiac assist devices, including mechanical hearts and VADs. By comparison, the study participants with a bioprosthetic heart exhibited von Willebrand Factor within a healthy range, a greatly reduced need for anticoagulant medications[25], and a lower risk for developing anemia, strokes, or thrombosis[26]. If these results are replicable, bioprosthetic hearts may well become safer alternatives to VADs in the future.
• May Require Servicing. As it is a relatively new technology, adjustments may be required while making use of the Carmat prosthetic heart. In recent studies, most adjustments occurred within the first 30 days after an operation[27]. It is not clear yet whether this device can be used for destination therapy or if biocompatible components inside the prosthetic will need replacing over time.

Can A Total Artificial Heart Replace Transplantation?

The truthful answer is no. Current technology for total artificial heart replacement does not exceed heart transplantation for heart failure patients.

Patients who make it to successful heart transplantation have been shown to live for an average of more than 12 years, highlighting its success in treating heart failure and lowering the mortality risk.[28] There is little to no evidence that suggests patients can live with artificial hearts long-term, and if they were able to, their quality of life would be extremely dismal by comparison. Some of the longest reported cases range between 1000 - 2000 days, and these are deemed to be extraordinary exceptions.

Heart transplantation presents itself with different challenges to artificial heart replacement. Patients are at risk for graft rejection and cancer, yet the risk for all other morbidities is greatly reduced, including stroke and bleeding[29]. Excessive blood clotting and thrombosis, as well as infection risk, remain constant with respect to mechanical hearts or heart transplantation.

Artificial heart technology has the potential to replace transplantation in the future, although there is still a lot that requires improvement. It is more likely that regenerative techniques will surpass the need for either transplantation or mechanical assistance before artificial hearts can be relied on for life.

Conclusion

While many improvements have been made with regard to heart replacement, artificial hearts are still unable to compare to the superior design of the healthy human heart. Heart failure patients benefit from both total artificial heart replacement and ventricular assist devices when on a waiting list for heart transplantation. By comparison, those with the SynCardia total artificial heart may be more vulnerable during the waiting period due to being at a higher risk for comorbidity and thrombosis. The Carmat bioprosthetic heart represents next-generation technology and has been shown to negate the risk of thrombosis almost entirely in preliminary trials. Ventricular assist devices come with a higher risk for bleeding and stroke. However, they also offer the chance for complete recovery of the patient’s heart and are sometimes removed as a result. They may also be preferred due to being less noisy, a lot more portable, and providing real-time access to the patient’s vitals.

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Sources:

• [1] https://www.ncbi.nlm.nih.gov/books/NBK553115/
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