Author: Suhani Patel
Editors: Shamsia Ahmed and Liane Xu
Artists: Denise Suarez
The heart is arguably the most vital organ in the human body. Without this muscular organ, life is unimaginable because it is responsible for pumping blood through blood vessels to different parts of our body. The human heart beats 100,000 times a day, pumping 2,000 gallons of blood across a 60,000-mile network of arteries, capillaries, and veins. Unfortunately, heart disease has become the leading cause of death for humans in the U.S., killing more than 600,000 annually and accounting for about one in every four fatalities. Doctors and scientists have been trying for a long time to create something similar or equivalent to the heart. The purpose is to develop a temporary machine or pump for a person who has a disease of the heart and their survival without a transplant is impossible. A series of medical breakthroughs in the latter half of the 20th century birthed devices that could assist the heart or even replace one side of it. More recently, a total artificial heart that replaces the function of a patient’s native organ has emerged as a viable option for people suffering from a variety of ailments: congenital heart defects, coronary heart disease, or circumstances where more than one side of a heart has failed, requiring bi-ventricular support. These devices mark a crucial development in the face of this epidemic and provide the last line of defense when a patient runs out of options.
SynCardia's artificial heart compared to a human heart. (4 June 2010, SynCardia Systems, Inc.) The human heart has four chambers — two atria that collect blood and two ventricles that pump it to the lungs or circulatory system — as well as four valves that control the direction of blood flow through the organ. A prosthetic heart works much the same way: Blood is pumped in and out of two ventricle chambers, each equipped with two valves. The ventricles are connected to the atria, aorta, and pulmonary artery by inflow and outflow connectors. Made of segmented polyurethane and about the size of two fists, the device is attached to a pair of tubes called drivelines, which exit the body through the abdominal wall and connect to an external driver that pumps the artificial heart by sending pulses of air through the drivelines into the ventricles. Originally, these drivers weighed upwards of 400 pounds and were the size of a washing machine, which unfortunately kept patients confined to the hospital. But advances in technology have downsized these drivers significantly in recent years, giving patients the freedom to move around with a smaller unit outfitted with wheels. Today, patients have the option of an even smaller driver: a portable pump powered by batteries that fits in a backpack.
This allows stable patients to leave the hospital when their condition improves and return home while they wait for a donor heart for transplant. Currently, the only commercially approved total artificial heart (TAH) available in the U.S. is manufactured by Tucson, Arizona-based SynCardia Systems. The most widely used artificial heart in the world, the SynCardia artificial heart received FDA approval in 2004 after a decadelong clinical study. Since then, more than 1,800 patients have received a SynCardia heart. The company’s original model, the 70cc TAH, is essentially an updated, smaller version of the Jarvik-7, the first artificial heart to be successfully implanted in a human in 1982. The 70cc TAH is primarily used for adult men as well as some women and adolescents. A newer, smaller model, the 50cc TAH — which is approved in Canada and Europe and undergoing a clinical trial in the U.S. — is used mostly for women and children.
Artificial hearts have improved vastly over time, but the technology still presents a variety of limitations. While mechanical failure is rare, bleeding and the risk of infection are common, and anticoagulants like warfarin are required to stop blood clots from forming. Of course, owners also need to lug around a 13-pound external air compressor at all times, and a single battery charge lasts only about four hours, so a backup unit needs to be on hand as well. Some physical activities, like swimming, are out.
The TAH has supported patients as young as 9 years old and as old as 80. The TAH is manufactured in two different sizes — 70cc and 50cc — to fit more patients. The 70cc TAH fits a majority of men and some women, and it can generate blood flow of up to 9.5 liters per minute, depending on the patient’s needs. The smaller 50cc TAH is designed to fit a majority of women and some adolescents, and it can generate blood flow of up to 7.5 liters per minute, depending on the patient’s needs. In comparison, the average human heart at rest pumps an average of 5 liters of blood per minute. The longest a person has been supported by an artificial heart was Italian patient Pietro Zorzetto, who had a SynCardia Total Artificial Heart for nearly four years—1,374 days—prior to his successful heart transplant on September 11, 2011. Another patient was supported for 1,172 days—more than three years—on the SynCardia Heart until he was transplanted on June 5, 2014. As of January 20, 2015, two other patients have now been supported for three-and-a-half years on the SynCardia Heart as they continue to await donor heart transplants. One-third of current SynCardia Total Artificial Heart patients have been supported for more than a year (47% outside of the United States, 21% in the U.S.), including some who have been supported by the device for two years or more. When all of the time that patients have been supported by total artificial hearts are added up, the SynCardia Total Artificial Heart accounts for 98.4%, or 443 patient-years, of support. The other designs account for seven years.
The greatest setback with artificial hearts right now is that they remain a temporary solution; a bridge keeping patients alive until a transplant donor can be found. This is unfortunate, considering that the demand for donor hearts far exceeds supply. About 4,000 people are waiting for a heart transplant in the U.S. at any given time, but only 2,000 hearts become available every year. In the pediatric transplant world, the donor pool is even smaller, typically about 500 hearts every year. Then there are those who don’t qualify for a transplant for age or health reasons. There’s an appreciable irony in that, in order to receive a heart transplant, patients need to be sick enough to be put on a transplant list — severely ill, in other words — but healthy enough to survive the arduous transplant process, which usually takes at least six months. For this reason, some researchers have turned to artificial hearts as a destination therapy or final, end-goal solution as opposed to a transitional stage before transplantation. A permanent artificial heart could one day make donor hearts unnecessary and render transplant rejection a problem of the past. Today, the technology has actually gotten good enough and the risk profile has become low enough that this could conceivably become reality. Oregon Health and Science University’s (OHSU) School of Medicine is one institution currently developing what it hopes will become the first permanent total human heart replacement. OHSU’s work is based on concepts developed by now-retired surgical resident Richard Wampler, M.D., who previously invented a patented rotary-like left ventricle assist system, the HVAD, to treat severe congestive heart failure. OHSU’s proposed device completely reimagines how the human heart works, replacing its ventricles and valves with a simple pump design concept, where a hollow titanium tube housing hydrodynamic bearings suspend a rod that moves back and forth, shuttling blood first to the lungs and then to the circulatory system. The single-moving-part design is not only efficient but also reduces the risk of blood damage and blood clots. It fits most adults as well as patients as young as 10 years old. Like SynCardia’s unit, OHSU’s artificial heart is powered by an external controller and rechargeable battery unit but is so small that patients can fit it in their pockets. A prototype of OHSU’s total artificial heart has been successfully tested in cows, and researchers now plan to implant a smaller model in sheep for a series of short-term studies. They hope that clinical trials on human patients could begin within the next decade.
Developments are being made in other areas as well, particularly related to current limitations in the external driver. Some researchers have turned their focus to the idea of a completely self-contained artificial heart, where smaller batteries could be implanted inside the patient and recharged with a transcutaneous energy transfer across the skin. Of course, these feats of cardiovascular engineering present their own challenges; one benefit of an external drive is that users can simply switch to a backup in the event of a power issue. If something goes wrong in a self-contained unit, the patient may require surgery. While none of these developments suggest a panacea, they bode well for a future that can provide better patient outcomes and options while maintaining quality of life.
Citations:
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