Study findings, “The Unintended Consequences of Over-Reducing Cardiopulmonary Bypass Circuit Prime Volume,” were published in the June 2017 edition of The Annals of Thoracic Surgery.
Clinical practice guidelines of multiple professional societies recommend minimizing the “prime volume” (PV) of fluid used to ready the cardiopulmonary bypass pump (aka heart-lung machine) to take over the work of the heart during cardiac surgery. The fluid interface is a necessary step that prevents patients’ blood from sticking to the device’s dry tubing, not unlike rinsing a drinking glass with water before pouring in soda to reduce some of the fizz.
Many studies have tied a reduction in prime volume to less dilution in the concentration of red blood cells in the body, thereby lowering patients’ risk for a blood transfusion and a bad surgical outcome. But it is unknown what level of PV reduction will lead to this protective effect.
A research team led by Benjamin Sun, M.D.—a thoracic surgeon with the Minneapolis Heart Institute in Minneapolis—recently took a closer look at the effectiveness of restricting cardiopulmonary bypass PV on the incidence of transfusion among 47,273 adult patients who had undergone coronary artery bypass graft procedures at 189 hospitals across the U.S. The research was enabled by surgical procedural data collected by SpecialtyCare in Nashville, Tennessee.
Case records for patients operated on between April 2012 and May 2015 were drawn from the SpecialtyCare Operative Procedure Registry (SCOPE) for the controlled observational study. Among the data elements collected were demographic information, blood conservation practices, fluids infused by anesthesiologists and perfusionists during surgery, and transfusions performed. Other data collected included patients’ red blood cell concentration (“hematocrit”) and how long they were on the bypass machine and their heart was isolated from the rest of their blood circulation (“cross-clamp” duration).
Researchers also calculated the ratio of prime volume to estimated blood volume (“PV ratio”) for each patient to assess the effect of PV by body size.
The primary outcome was transfusion of at least one unit of red blood cells. The secondary outcome was the transfusion of four or more units of red blood cells among those being transfused at all. Cases with missing data were excluded from the regression analysis.
Close to one-quarter (22.2 percent) of all patients received a blood transfusion, with the majority (9.9 percent) getting two units of red blood cells and the minority (3.0 percent) getting three units. In between were those receiving one unit (5.9 percent) or four or more units (3.3 percent). Relative to patients receiving no transfusion, the transfusion groups had a higher proportion of women, elderly (70 or older), and non-elective procedures as well as more patients with a body mass index below 31 (no more than mildly obese) and lower estimated blood volume.
Transfused patients had longer cross-clamp and bypass times, lower average hematocrit readings, and more often required mechanical assistance to support their heart function and blood flow. Based on PV ratios, their average prime volume was also 25 percent larger than nontransfused patients. Additionally, transfused patients were given larger volumes of anesthesia, clear fluid while on the bypass machine, and ultrafiltration to reduce excess fluid.
PV ratios did not correlate evenly with the primary or secondary outcome but appeared to have a stronger effect on transfusion risk among low-hematocrit patients. A ratio of 15 percent PV to estimated blood volume was associated with the lowest predicted probability of a transfusion, but more pronounced negative effects were seen with ratios above and below this threshold for patients who were more anemic.
Many of the studies supporting the reduction of PVs are based on randomized trials comparing miniaturized cardiopulmonary bypass circuits to the traditional circuits commonly used in the U.S., where surgical teams employ other means to minimize PV—including displacing clear fluid in the heart-lung machine with patients’ own donated blood (“autologous priming”). The studies are uniformly small, making their findings more anecdotal than scientifically conclusive.
CLINICAL EXCELLENCE IN PRACTICE
Common wisdom says using less PV lessens the impact on patients’ hematocrit, and this turns out to be true—up to a point. After that, it could unintentionally increase their risk of a transfusion.
Based on findings of this well-powered study, surgical teams might consider lowering PV to no more than 15 percent to protect patients from the need for a transfusion. Efforts to minimize it further may be counterproductive and, in some cases, heighten rather than lower transfusion risk.
The researchers stopped short of declaring a PV ratio of 15 percent as the absolute minimum due to variability in bypass circuits used by institutions and, perhaps more important, the hematocrit reading that generally triggers a transfusion.
THE RESEARCH TEAM
Assisting Dr. Sun in this research were Donald S. Likosky, PhD, with the Center for Health Outcomes and Policy at the University of Michigan at Ann Arbor, and members of the SpecialtyCare Medical Department established to maintain a quality-first focus on all clinical matters at the organization. These included Timothy A. Dickinson, CCP; Eric A. Tesdahl, PhD; Christopher Wells, M.D. MBA; and CEO Samuel Weinstein, M.D.