Impella Position is Key
Marvin Slepian, MD, who specializes in blood rheology, discusses the nature and causes of hemolysis as well as how to prevent and detect hemolysis. Dr. Slepian is a professor of medicine & biomedical engineering and a Regents professor at the University of Arizona.
Dr. Slepian explains that all mechanical circulatory support devices cause some degree of hemolysis as they drive blood through the device and through the vasculature. As blood moves through these narrow spaces, velocity increases, imparting a shear force. Fortunately, Dr. Slepian explains, red blood cells are designed to bend and flex and withstand a certain amount of shear force. But excessive or repeated shear force can distort or “pull apart” the cell membrane, causing the cell to hemolyze, or rupture and release its contents.
With regard to Impella®, Dr. Slepian tells interviewer Dr. Shon Chakrabarti, “The device is exquisitely well designed, so there isn’t that much hemolysis by the device itself.” Dr. Slepian also states, “The beauty of the design is it’s small enough, yet it’s capable enough to move enough blood, yet it only imparts shear just in a small little area that’s potentially hemolyzing. The bulk of it will not cause hemolysis.”
When asked what causes hemolysis in patients, Dr. Slepian responds, “The answer is simple. Physicians have to pay attention to the position of this device.” He emphasizes the importance of positioning Impella so that the outlet area is free in the aorta so that blood can flow smoothly through the Impella and into the aorta. Hemolysis occurs when the outlet abuts the aortic valve and blood flow exiting Impella becomes turbulent. This unexpected flow, he explains, that has nothing to do with the device, is what actually causes hemolysis.
“The red cell and the platelet are the only cells in the body that can’t repair themselves.” Dr. Slepian explains that these cells accumulate shear as they circulate and encounter bends and angles. “It’s also a surface area thing,” he adds, comparing the “very tight, concise” Impella device to ECMO, noting that ECMO is “a football field of surface and material and shear and bend.”
How should clinicians diagnose and assess hemolysis? “Best assay: plasma-free hemoglobin,” Dr. Slepian responds without hesitation. Patients supported with Impella often have comorbidities and common hemolysis indicators, such as urine in the blood and increases in LDH, which often indicate something other than true hemolysis. Therefore, Dr. Slepian recommends plasma-free hemoglobin (PFH), which provides a direct measurement of the liberated hemoglobin in the plasma.
The take home message regarding Impella and hemolysis: “Position it correctly… The cells will be happy and the patient will be happy.”
To learn more about the Impella® platform of heart pumps, including important risk and safety information associated with the use of the devices, please visit: abiomed.com/important-safety-information