TITLE: COMPARISON OF COMPENSATORY MECHANISMS FOR ACUTE NORMOVOLEMIC HEMODILUTION (ANHD) IN ANESTHETIZED AND AWAKE SHEEP

AUTHORS: J S. Funston, MD, Luiz A. Vane, MD, Mali Mathru, MD, Donald J. Deyo, DVM, George C. Kramer, PhD
AFFILIATION: The Univ. of Texas Medical Branch, Galveston, TX.

INTRODUCTION: Acute normovolemic hemodilution (ANHD) is one therapeutic strategy used to decrease transfusion requirements. The safety of ANHD is based on the premise that in the presence of decreased arterial oxygen content (CaO₂), tissues maintain constant O₂ consumption (VO₂) by compensa- tory mechanisms that include increasing cardiac output, regional blood flows and oxygen extraction. Controversy exists in the literature about these mechanism's ability to respond in the anesthetized state. In our clinical experience cardiac output response to ANHD was blunted during both opiate and inhala- tional anesthesia. In the present study we used sheep to model the response intraoperatively and postoperatively to ANHD in surgical patients undergoing abdominal surgery.
METHODS: Twelve adult sheep were anesthetized with 1.5-2% isoflurane. Vascular catheters (arterial, venous, and a Swan Ganz) were placed and a splenectomy was performed. After baseline, sheep were subjected to hemorrhage and ANHD with lactated ringers (LR) to reduce hemoglobin to 5g/dl, while LR infusions were used to keep filling pressures at baseline values. After ANHD, sheep were randomized to be maintained with LR only or infused with 2 units of packed RBC's, monitored while anesthetized for 2 more hours then awakend. All sheep were recovered and monitored for 2 post-operative days.
RESULTS: Prehemorrhage hemoglobin levels were similar in both groups (8.5±0.5g/dl before and 5.1±0.03 after ANHD) but increased to 7.9±0.5g/dl after packed RBC's. The figure shows mean cardiac output±SEM and time in hours(hr) after ANHD. Filling pressures were identical in both groups and unchanged from baseline. Cardiac output remained at baseline in both groups while anesthetized, but during post-operative recovery the CO increased in both groups, with the highest levels in the LR group.
CONCLUSION: Cardiac output responsiveness to hemo- dilution may be blunted during anesthesia and surgical stress leaving increased oxygen extraction as the mechanism respons- ible for maintaining oxygen consumption