Clinical death and the measurement of stressed vascular volume

Objectives: To measure stressed vascular volume in humans and to review the concepts of stressed and unstressed vascular volume. Design: Observational study during surgical procedure. Setting: Operating room at a university hospital, Patients: Five patients undergoing hypothermic circulatory arrest for surgery on major vessels, Intervention: We measured the volume that drained from the patient to the reservoir of the pump when the pump was turned off. Measurements and Main Results: Stressed volume was 20.2 +/- 1.0 mL/kg: which is 30% of the predicted blood volume of these patients. Conclusion: The amount of blood volume that determines vascular filling pressure is only about a quarter of the total predicted volume, which means that there is a large reserve of unstressed volume that can be recruited to maintain vascular filling pressure.