Walking up the stairs to your room one evening, you stop to catch your breath and think “I haven't always felt this tired.” Week by week, you start to notice even going out to get the mail is exhausting. Eventually, you can't keep up with your kids without being completely worn out. A few months go by. One night, you wake up gasping for air, feeling like you are drowning despite being asleep just moments before. This is the normal progression of heart failure, a disease that results in over 1 million hospitalizations in the United States each year.1
Heart failure is a progressive and debilitating disease in which the heart is no longer able to effectively pump blood through the body. As a result, the pressure in the heart backs up into the lungs and causes fluid to leak into the airways. This leakage results in congestion, shortness of breath, and the sensation of drowning. As the fluid builds up throughout the body, it can also result in painful swelling in the extremities, further restricting mobility.
Surprisingly, for many of these patients, the root cause of heart failure cannot be attributed to the heart alone. If the circulatory system were a smoothly operating assembly line, heart failure would represent the slowdown of one crucial step. As product keeps coming down the line, it encounters the slowdown, causing congestion up the line. Down the line, there isn't enough product being shipped out the door, perpetuating the congestion.
In the body, the kidneys eliminate excess fluid through an elaborate and balanced filtration mechanism. When blood flow to the kidneys is choked due to heart failure, the body is unable to sufficiently eliminate fluid, which adds to the congestion in the heart, lungs, and throughout the body. This feedback cycle, known as cardiorenal syndrome, is self-perpetuating and eventually spirals out of control.
Cardiorenal syndrome occurs in up to 64 percent of acute decompensated heart failure hospitalizations.2 Kidney dysfunction cuts expected survival in half and increases lengths of hospital stays and rates of rehospitalization. As a result, kidney dysfunction is now recognized as a leading prognostic indicator of heart failure outcomes. Despite the significance of disease, there are no proven therapies or guidelines for patients who develop kidney dysfunction in the setting of heart failure.
Physicians have attempted to remedy this problem using pharmacologic agents, such as loop diuretics and artificial ultrafiltration devices, which supplement the fluid elimination function of the kidneys. Loop diuretics, while effective early in the disease progression, eventually cause the kidneys more harm than good, reducing filtration rate (GFR) and causing long-term damage. Although ultrafiltration devices supplement fluid elimination with an external filtration system, optimal patient selection and filtration rates are not well understood. This has resulted in limited adoption due to potential complications and increased cost.
Due to the shortcomings of existing solutions, heart failure cardiologists have broad interest in a treatment that directly addresses the problem of reduced kidney perfusion while supporting the heart. This article discusses the innovation behind the design and development of a heart pump to address this disease.
A Pump for More Than the Heart
In Houston, TX, a team of life science entrepreneurs is developing the first heart pump designed to remedy the root cause of cardiorenal syndrome. The small but powerful AortixTM micro pump is designed to disrupt the cardiorenal syndrome cycle and assist the natural function of the heart. Uniquely positioned between the heart and the kidneys, the pump provides a solution for patients with limited treatment options.
Aortix is a catheter-delivered axial flow pump, implanted through an artery in the leg to the aorta, the primary artery supplying the kidneys, vital organs, and lower extremities. The novel placement in the aorta, which is downstream of the heart and upstream of the kidneys, gives Aortix an unrivaled ability to have a positive impact on cardiorenal syndrome.
In contrast to many other circulatory assist devices, which must be surgically implanted, Aortix can be introduced by a cardiologist in a percutaneous procedure. The pump's small size and unique design are intended to help lower procedural risk. A cardiologist can quickly deliver the device (measuring approximately 6 mm in diameter and 5 cm long) through the femoral artery to the aorta between the heart and the kidneys. Once the delivery tools are removed, self-expanding nickeltitanium anchors deploy to affix the pump to the aortic wall. The location of Aortix downstream of the heart and arteries supplying the brain has the additional benefit of reducing the risk of thrombotic stroke or damage to the heart and valves.
Once the pump is implanted, its thin, flexible electrical lead is connected to an external controller. Once activated, the pump operates at a range of speeds around 25,000 RPM, depending on the level of support required. Blood is drawn into the inlet, accelerated, then ejected in high-velocity jets through the outlet ports. The jet pump entrains native aortic blood flow, transferring energy to the surrounding blood and creating a pressure gradient across the pump. Downstream of the pump, the kidneys see increased pressure, which leads to improved kidney function. Upstream of the pump, Aortix reduces the pressure the heart has to pump against (“unloading the heart”). This pressure relief reduces the overall workload of the heart and improves cardiac output, further disrupting the cardiorenal syndrome and improving downstream perfusion.
The newest generation of the Aortix pump allows for the same level of pumping support as previous generations of the device, but in a shorter overall form factor. The shortened length of the device allows for easier implantation and navigation of tortuous vascular anatomy. The new pump design has been optimized using computational fluid dynamics and whole blood benchtop models to reduce the likelihood of hemolysis, or the rupture of blood cells. Hemolysis is a common adverse event in pumps with this form factor.
After studying the regions of the device that impart the highest shear forces on the surrounding blood, the pump was optimized to reduce turbulent flow and shear stress on blood cells. As a result, the standardized benchtop flow test outperformed existing percutaneous pumps, and no hemolysis was observed for approximately 70 minutes in the initial patients implanted with the Aortix pump.
Lower Speeds, Lower Positioning Provides Greater Benefit
In a recent work with an ovine model of chronic heart failure, the newest generation of the Aortix device provided some intriguing results.3 In contrast to the hypothesis that higher speeds and closer proximity to the heart would provide superior benefits, independent investigators at Tufts University found that moderate speeds and more distal positioning provided larger pressure changes at the kidneys, superior cardiac output, reduced afterload, and lower effective systemic vascular resistance, all of which are indicators of reduced cardiac workload or increased cardiac performance.
These findings have implications on future development, indicating that positioning and speed may play a role in efficacy of the Aortix pump. Lower positioning may be more effective because a larger volume of blood (upstream of the pump) can be moved away from the heart. The resulting sufficiency of moderate speeds could mean Aortix could be reduced in size without sacrificing effectiveness.
Additionally, this study examined the safety of reducing aortic root pressure. Extreme drops in pressure could negatively impact blood flow to the heart muscle or brain. Investigators used Doppler flow probes to study coronary and carotid blood flow and found no significant changes due to the Aortix pump. This data demonstrates that within its normal operating range, Aortix poses a low risk of causing patients to suffer negative effects from aortic root pressure drop.
Clinical Study and Future Work
In the first clinical study with the Aortix device in an acute setting, the device was implanted and retrieved in six patients with no complications or adverse events.4 Kidney function as measured by urine output was dramatically increased, pointing to the potential for benefit in heart failure.
Cardiorenal syndrome continues to be one of most common presentations of the heart failure epidemic. Without sufficient treatment options, patients are left fighting for breath, with extremely poor quality of life. In order to remedy this condition, a solution must address the source of the problems with both the heart and the kidneys. At Procyrion, current research and development is focused on the application of the Aortix technology to patients with cardiorenal syndrome.
This article was written by Will Clifton, MD, Clinical Consultant Procyrion, Inc. Note: The Aortix device is in the research and development phase and is not approved for sale in any country. Procyrion was the 2015 grand prize winner in the Create the Future Design Contest for its Aortix heart pump. For more information, visit here.