HEMATOCRIT PREDICTION AND PRESERVATION FOR CARDIOPULMONARY BYPASS

Low hematocrit (Hct < 20%) during cardiopulmonary bypass (CPB), is associated with increased risk of in-hospital mortality, intra- or postoperative placement of an intra-aortic balloon, and return to CPB after attempted separation, for patients undergoing coronary artery bypass grafting . These findings caused our center to develop a systematic approach to perfusion management of patients, aimed at avoiding intra-operative anemia , . Since Hct is largely determined by hemodilution, the patient's body size and pre-operative Hct play a large role in the extent of intra-operative anemia. We use a smaller, lower prime (LP) CPB circuit for patients with smaller body size. We also further reduce hemodilution by displacement of the solution in the perfusion circuit with the patient's own blood just prior to initiating CPB, a technique that we refer to as "autopriming" previously described by Debois and colleagues . An important part of our Hct management is the prediction of the CPB Hct for each patient prior to surgery. Once this is done, the appropriate strategies to prevent a low Hct may then be selected.

Circuit Descriptions

The LP circuit has a smaller oxygenator (Capiox SX-10, Terumo Medical Corp., Somerset, NJ, U.S.A.) in place of our standard adult oxygenator (M301, Sorin Biomedical Corp., Irvine, CA, U.S.A.) and employs a 9.5 mm. ID (internal diameter) venous line and arterial pump boot as opposed to 12.7 mm. ID in the SP circuit, which reduces the prime 400 ml. We determined that this venous line could drain at the rate of approximately four liters per minute, by gravity, and that the oxygenator can safely perform at this flow rate.

Prediction Formula

With patients with a BSA of <1.7 M2 we use the LP circuit which has a total prime volume (PV) of 1,120 ml. Patients with a BSA>1.7 M2 receive the standard prime (SP) circuit with 1,520 ml. Typically 600-800 ml of additional priming volume may be removed using the autopriming technique. The Hct prediction formula without volume intervention is: (Hct x BV) / (BV + PV + IV). IV (intravenous fluid administered pre-CPB) varies, but we found the formulae were more statistically significant if an approximation of this volume was included. We further calculate what the CPB Hct would be if we were to sequester 600 ml. of whole blood from the patient and/or autoprime either 600 ml. (for the LP group) or 750 ml. (SP), and if one or two units of packed red blood cells were added to the pump pre-CPB. Using these predicting formulae, we are able to foresee the impact of various techniques and choose the technique(s) that result in an Hct of greater than 20% during CPB. (See appendix)

The Autologous Priming Technique (autopriming)

The autologous priming (AP) technique consists of attaching an empty one-liter transfer pack to a stopcock placed in a 3.2 mm. internal diameter tube interposed between a Luer-Lok connector placed directly distal to the arterial roller pump and a stopcock on another Luer-Lok connector in the venous line, near its attachment to the venous reservoir. After the aortic cannula is placed and secured, the stopcock at the transfer pack is turned to allow the patient's blood to enter the arterial line slowly, displacing pump prime into the transfer pack. The anesthesiologist may elect to administer phenylephrine and/or utilize Trendelenberg Position in order to maintain the patient's systolic blood pressure at or above 80 mm Hg. At any time during the autologous priming, should the patient not tolerate this transfer of blood into the CPB circuit, autopriming is discontinued. When the patient's blood has filled the arterial filter, heat exchanger, membrane oxygenator, and reaches the stopcock, the stopcock is turned off. Following venous cannulation, with the venous return line clamped between the venous reservoir and the previously mentioned venous Luer connector, the stopcocks are positioned so as to allow that line to fill, siphoning additional prime into the transfer pack. During autopriming of the venous line, the arterial pump may be used to return up to 250 ml. of blood to the patient, lowering the prime level in the venous reservoir, but restoring some of the patient's blood volume and reducing any hypotension that may have occurred.

Use of this approach has lead to a reduction in the incidence of Low Hct during CPB. The incidence of Hct < 21% decreased from 26.9% to 7.5% after these interventions were adopted without increasing the use of autologous blood (see figure below).

Summary