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Minimum hematocrit for normothermic cardiopulmonary bypass in dogs
D J Cook1, T A Orszulak, R C Daly
1Department of Anesthesiology, Mayo Foundation and Mayo Clinic, Rochester, Minn 55905, USA. cook.david@Mayo.edu
Insights
The critical hematocrit for canine brain oxygenation during normothermic cardiopulmonary bypass is between 0.09 and 0.14. Below this level, cerebral oxygen delivery and metabolism decline, despite increased cerebral blood flow.
Area of Science:
- Cardiovascular Research
- Neuroscience
- Physiology
Background:
- Cerebral oxygenation is vital during cardiopulmonary bypass (CPB).
- Understanding the minimum hematocrit for adequate brain oxygen supply during normothermic CPB is crucial.
Purpose of the Study:
- To determine the minimum hematocrit supporting cerebral oxygenation during normothermic CPB in dogs.
- To investigate the effects of hemodilution on cerebral blood flow (CBF), cerebral metabolic rate (CMRO2), and cerebral oxygen delivery (CDO2).
Main Methods:
- Progressive normovolemic hemodilution was performed in eight anesthetized dogs during 37.5°C CPB.
- CBF was measured using the sagittal sinus outflow technique.
- CMRO2 and CDO2 were calculated, and intracranial pressure was monitored.
Main Results:
- Cerebral blood flow increased reciprocally with hemodilution, reaching 240% at a hematocrit of 0.09 compared to 0.39.
- Cerebral oxygen delivery was maintained until a mean hematocrit of 0.14 ± 0.02.
- Both cerebral oxygen delivery and metabolism declined at a hematocrit of 0.09 ± 0.02, indicating the critical threshold.
Conclusions:
- The critical hematocrit for brain oxygen balance during normothermic CPB in dogs is between 0.09 and 0.14.
- This study provides a physiological basis for understanding hemodilution tolerance in certain patient populations during CPB.
- The relationship between hematocrit and cerebral oxygen balance shows a plateau before a critical decline.
Background:
The purpose of this study was to determine the minimum hematocrit supporting cerebral oxygenation during normothermic cardiopulmonary bypass (CPB) in dogs. The effect of hemodilution on cerebral blood flow (CBF), cerebral metabolic rate (CMRO2), and cerebral oxygen delivery (CDO2) was determined over a range of hematocrits.
Methods And Results:
Measurements were obtained during 37.5 degrees C CPB with progressive normovolemic hemodilution (hematocrit 0.39 to 0.9) in eight anesthetized animals. Dextran 70 (6%) was used as a diluent. CBF was measured using the sagittal sinus outflow technique. CMRO2 and CDO2 were calculated using standard formula. Hemodilution was associated with a reciprocal rise in CBF. CBF at a hematocrit of 0.09 was 240% of the CBF when the hematocrit was 0.39. Increases in CBF compensated for decreased arterial oxygen content and CDO2 was maintained to a mean hematocrit of 0.14+/-0.02. At a hematocrit of 0.09+/-0.02, CDO2 and CMRO2 declined. Intracranial pressure remained stable throughout. Thus, the critical hematocrit for brain (the hematocrit at which metabolism becomes delivery-dependent) was between 0.09 and 0.14 during normothermic CPB in dogs.
Conclusions:
This is the first systematic attempt to determine the critical hematocrit supporting cerebral oxygenation during warm CPB. The curve describing the relationship between hematocrit and cerebral oxygen balance has a broad plateau and a genu near a hematocrit value of 0.15. While we do not advocate hemodilution to a hematocrit of 15% during "warm" CPB, these data provide a physiological foundation for our hemodilution practice and offer an explanation why low hematocrits are tolerated in certain patient populations.