Review the published study, “The effect of ultrafiltration on end-cardiopulmonary bypass hematocrit during cardiac surgery,” published in the July 2018 edition of Perfusion.
Cardiac surgery and cardiopulmonary bypass, which provides circulatory support during the operation, lowers the concentration of red blood cells (“hematocrit”) in patients under the best of circumstances. Many patients also develop fluid overload while on the heart-lung machine that further lowers their hematocrit and heightens their risk for a blood transfusion—in addition to increasing their potential for heart tissue damage once they’re disconnected from the bypass pump.
Many techniques exist to prevent this fluid gain, such as minimally invasive approaches requiring less on-pump time and procedures that eliminate the need to be on the heart-lung machine at all. Perfusionists also have multiple ways of removing the excess fluid with an “ultrafiltration” (UF) device that uses the principles of hydrostatic pressure. The practice is most often utilized for patients undergoing valve surgery, where fluid buildup can be particularly problematic.
Overall, UF is less popular than retrograde autologous priming (RAP), when patients’ blood displaces the saline solution used to prime the heart-lung machine. RAP tends to lessen the need for UF, especially for patients undergoing coronary artery bypass surgery.
A research team at SpecialtyCare in Nashville, Tennessee, recently conducted a retrospective study looking at the effect of UF on the hematocrit of adult cardiac surgery patients at 197 institutions across the U.S.
Researchers reviewed 73,506 cardiac procedures performed between April 2012 and October 2016 from the SpecialtyCare Operative Procedure Registry (SCOPE). Cases included in the analysis involved neither a blood transfusion during the surgery nor a “zero-balance UF”—the practice favored by some surgeons whereby fluid is continually added and removed in equal measure to avoid the potentially harmful effects of excessive electrolytes in the blood.
The primary endpoint was the last hematocrit reading before patients were taken off the bypass pump. The secondary endpoint was urine output during cardiopulmonary bypass. The study controlled for numerous confounding factors, including patents’ initial hematocrit while on bypass, total volume of clear fluid infused into their body, their estimated circulating blood volume, and procedure type.
Patients were divided into one of five subgroups based on the UF volume removed during their surgery, as well as by procedure type. The statistical model for the primary outcome was applied only to the 65,617 cases with no missing registry data. The secondary outcome included 25,041 cases where UF was used.
The majority of patients (61.9%) had no UF volume removed. Of the remainder, the removal amount variably ranged from 1 mL to 1.0 L (9.3%), 1.0 L to 1.5 L (8%), 1.5 L to 2.4 L (10.9%), and 2.4 L or more (9.9%).
The relationship between UF volume removed and the last hematocrit on bypass had a nonlinear pattern. For most procedures, UF raised hematocrit readings no matter how much fluid was removed—but most effectively at amounts between 1 mL and 2.5 L. An association was also found between UF volume removed and urine output on bypass, although the effect markedly weakened as UF volumes increased.
Study findings, “The effect of ultrafiltration on end-cardiopulmonary bypass hematocrit during cardiac surgery,” were published in the July 2018 edition of Perfusion.
Many studies report the benefit of UF, notably in high-risk adult patients undergoing valvular and combined atrial and mitral valve procedures.
A few studies point to the potential harm of UF, with some researchers suggesting it raises the risk of acute injury if excessive amounts of fluid are removed. Others have found an increased inflammatory response or oxygen deficiency in the blood with the use of UF in adult cardiac surgery patients. The process of UF can also decrease the volume of circulating blood that the body may replace with some of the fluid in the spaces around cells, though the effect of this phenomenon is not well understood.
CLINICAL EXCELLENCE IN PRACTICE
The study under discussion supports the benefits of UF, notably when fluid volumes below 2.5 L are removed. But concerns about the potential overuse of UF remain, especially given the alternative of RAP that raises a patient’s hematocrit going into surgery.
Further research is needed to define an optimum hematocrit reading, as well as the benefits and harms to UF patients from having a greater concentration of proteins and electrolytes in their blood. Also unknown is the impact of having UF take over the work of the kidneys to flush out excess water—and all that comes with it—from the body.
THE RESEARCH TEAM
The study team for this research consisted of Linda B. Mongero, CCP; Alfred H. Stammers, MSA, CCP; Eric A. Tesdahl, Ph.D.; Andrew J. Stasko, MBA, CCP; and Samuel Weinstein, M.D. They are all members of the SpecialtyCare Medical Department established to maintain a quality-first focus on all clinical matters at the organization, and routinely publish in peer-reviewed journals.