Predictive Hemodilution

It's obvious that Perfusionists must have a clear understanding of the many ways we influence intravascular hemodilution, other than just looking at the priming volume. Despite well-designed formal and didactic perfusion programs, clinical choices for conducting cardiopulmonary bypass procedures are often driven by personal bias, clinical impressions, experience from previous employers, industry or peer pressure and current evidence based literature.

For any one profession to have complete control over patient hemodilution is a dream and is simply not possible because the whole perioperative process requires the buy in and cooperation of several disciplines (administration, surgeon, anesthesiologist, perfusionist and nurse) … not just during the intraoperative period but also during the patient’s entire perioperative stay. Current practice does not have room for individual clinicians to dismiss their own responsibilities and contributions to excessive hemodilution and allogeneic transfusions by passing off transfusion outcomes solely to the actions, or relative inactions, of other perioperative team members. 

In a survey by Likosky et al on the assessment of current practices of perfusionists, surgeons and anesthesiologists regarding guidelines for perioperative blood transfusion in cardiac surgery, the authors concluded that at the institutional level, those individuals with leadership roles have an obligation to provide other hospital leaders, practitioners, people with support roles, and even practitioners beyond the hospital environs, with the information, tools and mandate to practice patient-centered and cost-effective care of the surgical patient. In other words, unless the centers operative team and their post-op care team buy into the practice of reducing hemodilution, the goal to reduce allogeneic blood transfusions will be nothing but an academic practice at best.

Intraoperative hemodilution is not just a matter of how much or how little fluid we put into the bypass circuit, or how much we take off. It reflects many factors and concerns, all of which should be recognized and addressed by the various practitioners involved in cardiac surgery. A concept I like to call Predictive Hemodilution© (PH) is an attempt to bring global attention to all the conflicting and sometimes misdirected aspects involved in the process of hemodilution during cardiac surgery, and to help remove some of the myths and uncertainties around the fluids, devices, techniques and opinions that are used in every day cardiopulmonary bypass.

PH is simply a process of information that gives any practitioner the ability to look at their programs present perioperative approach to volume management in a more comprehensive and open minded way, which then allows them to use available evidence based information, current clinical practices and simple mathematical calculations to help reduce excessive hemodilution … thereby reducing allogeneic blood use and the morbid effects associated with these transfusions.

As previously stated, reducing transfusion of allogeneic blood in adult cardiac surgical patients is not just a matter of reducing extracorporeal priming volumes. If it were that simple, transfusion rates in all cardiac surgical centers across the globe would be at an all-time low. In fact, determining our clinical practice by focusing solely on prime volume reduction may lead the clinician to make choices in other areas that may inadvertently place not only themselves at risk, but the patient and their health care institution as well.

One of the first understandings we need in PH is the implementation of a process to detect the presence of anemic patients before they get to the day of surgery. Since it was shown that preoperative anemia is a strong independent risk factor for allogeneic transfusions, intuition should guide us to find ways to discover this at risk group (before surgery) and then provide the necessary means and protocols to take them out of this at risk category. Does this mean that if we do not identify anemic patients prior to their day of surgery they will probably be transfused… according to the literature and statistics the answer is yes. The implementation of hospital Blood Management Programs are instrumental to addressing this issue.

Another clear understanding is that the practice of anesthesia volume loading patients during the induction has been around for decades. However, more and more anesthetists have been moving in the direction to reduce this practice to avoid excessive hemodilution even before the perfusionist places the patient on bypass. Considering the acute effect these large fluid volumes have on excessive hemodilution and damage to the endothelial Glycocalyx lining, volume loading is just as disruptive and possibly unnecessary to the patient as going on bypass with 2-3 liters of prime volume. It should therefore be intuitive that in preparation for cardiac surgery, volume management and goal directed therapy is better for the patient than volume loading.

The discovery of ways we excessively hemodilute patients also reinforces the knowledge we already have about the value of using autotransfusion devices and hemoconcentrators in a variety of ways and cases during cardiac surgery. It is difficult for me to understand why we are still waiting for that one definitive multi center, prospective, peer reviewed paper to tell us that hemoconcentration during adult bypass is extremely useful and therefore should be part of every cardiopulmonary bypass case. The problems we envision are usually not even related to the cautious and well planned use of a cell saver or hemoconcentrator. However, problems can occur with any unplanned or hurried use of the latter blood conservation devices, especially when people use them off label or have the impression that the more volume we remove, the better the patient is … instead of having a specific therapeutic goal with the considerations of their postoperative implications in mind.

Should they be used in all cases ... in my opinion the answer is yes. Considering the improvements in hematocrit and the removal of the many metabolites associated with plasma water, you must conclude that a hemoconcentrator should be just as common in the extracorporeal circuit as an oxygenator. Autotransfusion devices are also very useful and a beneficial addition to all cardiac surgical cases and they should be considered for all CPB cases to reduce the hemodynamic consequences of pericardial enzymes and proinflammatory mediators that are found in surgical blood loss. Quite often, any postoperative problems associated with either of these devices is not due to the technology itself, but the aggressive overuse of the technology.

Many clinicians are reluctant to use simple techniques like Retro-Autologous Priming (RAP) and/or Venous Antegrade Priming (VAP) to remove 600-800 mls from the priming volume. But unfortunately, they are missing out on a simple, cost effective and safe technique that can help reduce hemodilution in all adult patients undergoing cardiac surgery. Like all techniques used by perfusionists, the use of RAP/VAP must include a knowledge of the patient’s preoperative volume status and their present state of hydration (dehydrated, hypovolemic, hypervolemic) to remain effective during bypass. To me, it seems useless to remove 600 mls of blood from a dehydrated patient going on bypass, only to have to infuse 600 - 1000 mls of crystalloid after being on bypass to maintain a safe operating level. If there was a downside to using RAP/VAP, it would have to be the small costs associated with a blood recovery bag.

Even the simple act of collecting blood for analysis during the entire perioperative period can have a negative influence on the patient’s hemoglobin status. This is one variable in blood management that is easy to address and modify. This routine and often frequent practice of taking large blood samples (5.0 to 10.0 mls) for chemistry and hematology analysis during the perioperative period is a potential phlebotomizing action that can lead to unnecessary postoperative allogeneic transfusions. To address this problem is to simply reduce the sample volumes and the frequency of protocol analysis, then evaluate/justify the need for routine or increased sampling with the medical staff on an individual basis during the entire intraoperative and postoperative periods.

The administration of Mannitol during cardiac surgery is a common and widely practiced part of cardiopulmonary bypass. We have always known that during Mannitol infusion, systemic pressures will take a transient drop when the drug is in the prime or is rapidly added to the perfusion circuit. To my knowledge, there is no urgency to rapidly infuse Mannitol at any stage during bypass. With the Ravussin paper we now have an explanation of why those transient drops in pressure occur, which is the rapid osmotic hemodilution and the sudden decreased blood viscosity. This would explain the medications dramatic effect on decreasing intravascular hemoglobin and hematocrit. This alone provides a reason why we should not react to the first adverse set of hemoglobin/hematocrit values we collect, which is generally done within ten minutes after going on bypass.  Subsequent blood analysis during bypass allows for the effects of fluid shifting related to Mannitol, a drug that was originally intended to reduce plasma free hemoglobin during CPB (5 decades ago) by increasing glomerular filtration rate.

One of the most clearly defined, yet least understood tools the perfusionist and postop care nurses use to measure transfusion triggers and levels of hemodilution… is the Point of Care (POC) blood gas analyzer. There are numerous scientific papers and references relating to the inaccuracy and unreliability of using a POC technology called conductivity for measuring hematocrits during and after cardiac surgery, yet conductivity may be the most frequently used methods for measuring this parameter in the cardiac operating room and intensive care units.

To my knowledge, there is no paper or scientific report relating to the hemoglobin/hematocrit inconsistency of another POC technology called co-oximetry, which is also used for the measurement of hemoglobin during any surgical or postoperative period. Considering that one of the most critical transfusion parameters we measure during CPB is the hemoglobin/hematocrit, the use of conductivity to measure these parameters does not make clinical or diagnostic sense ... or even good clinical judgement. A simple difference in hemoglobin results of only one gram could lead to a patient being unnecessarily transfused a unit of allogeneic blood. It is unfortunate that an individual may receive a unit of allogeneic blood due to a technology difference.