Comparison of Compensatory Reserve Index to Changes in Stroke Volume and Intravascular Volume

Sequential Blood Loss: Blood loss will be simulated by withdrawal of 20% of estimated total blood volume. Estimated total blood volume for men = 75 mL/kg and for women = 65 mL/kg. The calculated target withdrawal volume is also consistent with an allowable estimated blood loss equation assuming a starting hematocrit (hct) of approximately 45% in males and 40% in females, where the expected drop in hct for this volume withdrawn is 7 percentage points in males and 6 percentage points in females. An actual drop in hct may not be observed in this study due to the relatively expedient removal of blood and no simultaneous administration of fluids (i.e., no "hemodilution" with crystalloid). Blood will be removed in aliquots up to ~333 mL. For safety, men will have no more than 4 units of blood removed (1,333 mL) and women will have no more than 3 units of blood removed (1,000 mL).

The blood will be withdrawn via the venous catheter and we anticipate that it will take 15-20 min for each stage of blood withdrawal to occur. A total of 3 aliquots will be withdrawn over approximately sixty minutes in women and 4 aliquots withdrawn over 80 minutes in men. The blood will be stored using preservatives and anticoagulation procedures similar to those used for perioperative hemodilution and/or intraoperative cell saver. The blood will remain in the study room until it is re-infused. We will use standard "auto-transfusion" blood collection equipment identical to that used in the ORs for perioperative hemodilution techniques. The blood will be drained by gravity from an indwelling vascular catheter into anti-coagulant preservative solution using tubing and bags systems provided by the autologous transfusion team that we routinely work with in the ORs. For the first several studies we will schedule them at a time when auto-transfusion OR personnel can be present and after that the anesthesiologists will be directly involved in drawing the blood, subsequent handling and reinfusion. The blood will never leave the autologous transfusion bag and a closed system will be used. The blood will be held at room temp with gentle agitation for ~60-80 minutes and then re-infused over 60-80 minutes via a large bore IV; monitoring will continue during reinfusion. Following completion of the reinfusion subjects will de-instrumented and observed in the physiology lab for two hours.