Hemolysis, Patient Management
Best Practices for Patients Supported with Impella® (Part 3): Hemolysis
What is hemolysis?
Dr. Tehrani explains that hemolysis can occur when red blood cells are subjected to a variety of mechanical and shear forces that disrupt the integrity of the red blood cell membrane. For this reason, it is important to examine the forces with which blood cells are pulled into and ejected from the Impella heart pump.
How is Impella designed to avoid hemolysis?
The speed of the Impella pump is designed to avoid hemolysis. Dr. Tehrani explains that the rotational speed of the impeller is optimized to create the maximum flow within safe limits of shear force. This speed ranges from about 51,000 rpm for the Impella 2.5® to 33,000 for Impella 5.0®. In addition, the size of flow channels around the impeller are maintained above minimum requirements to avoid damaging red blood cells or other blood components, such as platelets or white blood cells.
Dr. Tehrani then shows a schematic depicting the range of Impella shear forces—the forces Impella is designed to impose on red blood cells—and the range or shear forces known to cause red blood cell rupture. Impella shear forces are below 400 Pa (pascals) while the shear forces known to cause red blood cell rupture range between 425 and 700 Pa. “Inherently it should not cause hemolysis,” Dr. Tehrani explains. “If you’re having hemolysis, always do an echo to ensure position is correct in these patients.”
What marker best measures hemolysis?
Referring to a retrospective review published in The Journal of Heart and Lung Transplantation in 2016, Dr. Tehrani explains, “the most specific marker to look at hemolysis is actually not LDH, but rather plasma free hemoglobin (pfHgb) at about 40 mg/dL, 2 times over a period of 24 hours. And this is a much better marker of assessing hemolysis. More specific than LDH.”
What causes hemolysis?
Dr. Tehrani explains types of obstructions to Impella flow that can increase the potential for hemolysis.
- Inflow obstructions, such as ventricular structures obstructing the Impella inflow windows, may cause blood to travel faster and more turbulently, increasing shear forces and potentially resulting in hemolysis.
- Clots, fibers, or kinks can cause cannula obstructions, narrowing the passageway for blood and exposing blood to higher shear forces.
- Outflow obstructions from the aortic valve or wall of the aorta may cause blood to exit unobstructed windows at higher speeds, causing shear stress on red blood cells.
“So, positioning, positioning, positioning!” Dr. Tehrani emphasizes. “At the end of the day it really comes down to having a solid understanding and a foundation of knowledge for positioning these devices.”