Abstracts of Posters Presented at the International Anesthesia Research Society; 72nd Clinical and Scientific Congress; Orlando, FL; March 7-11, 1998: Cardiovascular Anesthesia
Introduction: Remifentanil is an ultrashort acting opioid. It allows for precise control of hemodynamic parameters, and therefore is a valuable drug in cardiac surgery. Our initial experience with remifentanil in patients undergoing cardiopulmonary bypass (CPB) with a hemoconcentrator (HC) in the CPB circuit demonstrated increased remifentanil requirements. We hypothesized that the HC (Minntech, Minneapolis) removes temifentanil from the CPB circuit. This study was designed to evaluate this observation.
Methods: Thirty patients undergoing cardiac surgery and placed on CPB were included in this study. A continuous infusion of remifentanil was started at the induction of anesthesia. This infusion was maintained throughout the procedure and during CPB to maintain the mean arterial pressure (MAP) between 60 and 80 mmHg. CPB blood flow rates were adjusted to maintain a cardiac index (C.I.) of 2.4 to 2.6 L/min/m2. Our hypothesis was then tested by adjusting the circuitry to administer the remifentanil on either the arterial or venous side of the CPB circuit (i.e. distal or proximal to the HC). Remifentanil was administered proximal (n=10) or distal (n=10) to the HC, as well as without the use of a HC (n=10). Remifentanil infusion rates were recorded during CPB before (baseline), during and after use of the HC.
Results: There were no significant differences between groups in age, height, weight and preoperative C.I. Without a HC in the CPB circuit, remifentanil infusion rate did not require adjustment. There was an 82% increase in remifentanil infusion rate when administered on the venous side of the concentrator, compared to baseline (p<0.05). When given on the arterial side of the HC, the infusion rate did not need to be adjusted (p>0.05 when compared to baseline) (Table 1)
Discussion: These results suggest that remifentanil is removed in the hemoconcentrating process. The HC filters molecules smaller than 65000 daltons. Remifentanil has a molecular weight of 412.9 daltons. 
Conclusion: We therefore recommend the administration of remifentanil on the arterial side of the hemoconcentrator to obtain maximal efficacy of the drug during cardiopulmonary bypass.
1. Anesth Analg 1996;83:646-51