Your question is an interesting one. I think it does an excellent job of articulating the complex world within which perfusionists have to work and all the influences that have to be considered when trying to implement a logical and incremental approach to improving patient outcomes in this area.
Let’s discuss the first part of your question regarding the effects of time on bypass. The length of time on bypass can provide unique challenges for the perfusionist when it comes to preserving the formed components of the blood and controlling the need for allogeneic transfusion. The mechanism of damage to the clotting factors and the red cells is well understood and documented. This damage is directly related to the mechanical interaction of the blood with the foreign surfaces they encounter. Obviously, the extent of this damage will determine the amount of transfusion required to offset the loss of oxygen carrying and clotting capabilities. The types of foreign surface effects can be broken down into two broad categories: surface dependent blood activation and surface independent blood activation.
The effects of both forms of activation are amplified by the length of time on bypass, the flow rate over time, the amount of blood/air interface in the forms of venous air and cardiotomy suction and vent return and the size of the surface area of your CPB circuit design. All of these factors, other than the length of time on bypass, can be modified or controlled to varying degrees through circuit redesign, and the utilization of autologous blood therapies.
Autologous blood therapies can impact banked blood use through two mechanisms. The first method, RAP, seeks to limit the initial hemodilution the patient experiences with the initiation of CPB. The goal is to not pull the “transfusion trigger,” to eliminate the need for banked blood to start the procedure. This technique can aid in maintaining overall homeostasis.
The second type of autologous blood therapy involves the use of intra-operative hemodilution techniques. The benefit of these techniques is to sequester whole blood and protect it from the activating forces associated with bypass. Return of these intra-operatively stored products is accomplished after protamine reversal and before the patient leaves the operating room. Given your particular circumstance of having to work blood management strategies around long bypass times, perhaps this area of autologous blood therapy is one to which that you should give strong consideration.
You probably already incorporate the use of hemoconcentrators. This is another effective technique in helping to ward off the need for intra-operative blood products.
So, combining the use of circuit size reduction options, including the use of sizing the oxygenator to the patient, the development of a small adult circuit to reduce foreign surface area exposure, along with autologous blood therapy techniques could help you achieve your blood management goals.
There is no single “silver bullet” approach or therapy to optimize the cardiopulmonary bypass system. The evidence is very compelling in support of reducing the size of your circuit to meet the needs of the specific patient you are working with. Circuit optimization will reduce both the foreign surface area to blood exposure that takes place over time and also hemodilution which aids in the preservation of clotting factors and hematocrit.
Considering the evidence outlined in the STS/SCA blood conservation guidelines, it is safe to say that anything you can do to reduce the need for and the attendent morbidity associated with banked blood transfusion by incorporating perfusion techniques or technologies should benefit the patient’s outcome. The time on bypass is an important consideration and anything you and your team can do to modify this parameter, even in an incremental way, will add benefit. The cardiac teams who we have seen to be very successful with blood management have one thing in common: a common vision and a structure to communicate and evaluate performance.
References:
Perioperative Blood Transfusion and Blood Conservation in Cardiac Surgery: Society of Thoracic Surgeons and The Society of Cardiovascular Anesthesiologists Clinical Practice Guideline. Ann Thorac Surg, 2007;83:27-86.
Ranucci, M, et al. A Systematic Review of Biocompatible Cardiopulmonary Bypass Circuits and Clinical Outcome
Ann Thorac Surg, 2009;87:1311-1319.
Waters, JH. Blood Management: Options for Better Patient Care. AABB Press, 2008, 441 pages.
