The study, “The effectiveness of acute normolvolemic hemodilution and autologous prime on intraoperative blood management during cardiac surgery,” published in the September 2017 issue of Perfusion.
There is near-universal agreement among clinicians that multiple strategies are needed to limit blood loss during cardiac surgery and prevent the need for a blood transfusion, which negatively affects patient outcomes. But results of multiple studies using two common blood conservation methods—acute normolvolemic hemodilution (ANH) and autologous priming (AP)—have been ambiguous.
ANH is similar to making a blood bank donation, except the giver and receiver are always the same person. The anesthesiologist draws blood in the hours leading up to surgery, and the blood gets reinfused back into patients near the end of the operation. In the interim, large volumes of clear fluids are used to replace the removed blood that inevitably lowers their concentration of oxygen-carrying red blood cells (“hematocrit”) and circulating clotting factors. A small amount of a mild anticoagulant drug is also added to the bagged blood to prevent it from clotting.
With AP, the saline solution used to prime the heart-lung machine gets displaced by patients’ own blood before cardiopulmonary bypass begins. This lessens the dilutional impact on the hematocrit—and for zero added cost. The technique has a minimal impact on total procedure time.
A research team with SpecialtyCare in Nashville, Tennessee, recently examined the impact of ANH and AP—used separately, in combination, or not all—on hematocrit and transfusion rates among 18,024 adult patients who had undergone cardiac surgery with cardiopulmonary bypass at 171 hospitals across the U.S.
Case records on patients operated on between January and October 2016 were drawn from the SpecialtyCare Operative Procedure Registry (SCOPE) for analysis. Patients were excluded if their record was missing data or any of the 100-plus quality indicators collected by the registry.
Researchers divided patients into subgroups based on the volume of blood taken and reinfused into patients during ANH, and the volume of saline solution displaced by patients’ own blood during AP. Regression models then measured the impact on initial hematocrit reading while on bypass (primary endpoint) and subsequent readings and need for transfusion (secondary endpoints). Patients receiving a transfusion were excluded from the hematocrit analysis since the dataset did not indicate when the transfusion occurred.
The primary endpoint controlled for 15 known confounding variables. Secondary endpoints used two additional controls, one for total transfusion time and another for cross-clamp time—the period when the heart is intentionally stopped so that surgery can be safely performed.
AP only was the most common hospital practice (96.5%), followed by a combination of ANH and AP (50.3%), ANH only (19.9%), and neither procedure (2.9%). Significantly more males were in the ANH group.
Those in the ANH and ANH + AP groups had higher starting blood volumes, the highest hematocrit upon entering the operating room and after being given an anticoagulant (heparin) and experienced the greatest hematocrit changes across all measured time periods. When ANH alone was used, patients’ circulating blood volume was higher than when ANH was combined with AP. Importantly, higher hematocrits and lower transfusions were seen with the combined approach.
The ANH group, compared to all other cohorts, had the lowest first hematocrit reading during cardiopulmonary bypass and the lowest transfusion rate. The latter finding reflects the response of clinicians to having patients’ reserved blood available to give in lieu of a transfusion. Among patients where neither ANH nor AP was used, prime volume was highest, and surgeries were lengthier.
The ANH + AP group had the highest first hematocrit while on bypass. This group also saw the second highest change in hematocrit across time periods, but these differences disappeared when AP volumes were relatively high and AHN volumes were low. In other words, men with higher circulating blood volumes had a reduced transfusion risk regardless of ANH and AP, while women with lower circulating blood volumes were at heightened risk.
For men, overall transfusion risk was extremely low. Regardless of gender, patients who were smaller or anemic preferentially benefitted from AP when it came to maintaining higher on-pump hematocrits with fewer transfusions.
Previous studies have tied the beneficial effects of ANH to the volume of blood removed from patients, the type of surgery being performed, the ability of the retained blood to retain its coagulation properties, and whether or not additional blood conservation measures were performed. Infusing patients with large volumes of fluids to replace their withdrawn blood has also been found to potentially reduce circulating coagulation proteins.
Prior evidence confirms that the use of ANH and AP in tandem has a positive effect on red blood cell count and the volume of whole blood that can be removed from cardiac surgery patients. AP has become a popular blood-sparing technique, and is not difficult to do, and one study found significantly lower clear fluid volumes given during bypass when it was used rather than ANH.
The SpecialtyCare team has formerly shown that women are at increased transfusion risk related to their lower circulating blood volume and pre-bypass hematocrit. Being female is also an independent risk factor for excessive bleeding following cardiac surgery.
Fluid restriction is recognized as effective in reducing the likelihood of a blood transfusion. A long list of patient-related factors also influences transfusion rates, most importantly pre-existing anemia and clotting disorders. The “tipping point” for an increased risk of adverse outcomes appears around the time patients’ hematocrit hits 24%.
CLINICAL EXCELLENCE IN PRACTICE
Utilization of both ANH and AP are effective in reducing the likelihood of receiving a transfusion of red blood cells and should be considered staples of effective blood management for patients undergoing cardiac surgery. AP, in particular, deserves consideration given that it is a quick, cost-free procedure that poses very little risk to patients.
But for women, the volume of blood reserved during ANH and the prime solution displaced during AP matter a great deal when it comes to how soon they reach the clinical trigger for a transfusion. Careful consideration of the preoperative estimated circulating blood volume should be made so that the use of combining ANH and AP does not result in over hemodilution and induced anemia.
THE RESEARCH TEAM
The study team for this research consisted of Alfred H. Stammers, MSA, CCP; Linda B. Mongero, CCP; Eric A. Tesdahl, PhD; 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.