Physiologic changes during extracorporeal circulation, flow rate and pressure management, and hemodilution concepts.
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- What happens to mean arterial pressure at the onset of full CPB compared to pre-bypass pressure?
- It typically drops due to acute hemodilution and loss of pulsatile cardiac output, producing a lower systemic vascular resistance state initially
- What is the typical target full-flow index for adult CPB in liters per minute per square meter of body surface area?
- About 2.2 to 2.5 L/min/m2
- Why is flow rate on CPB often indexed to body surface area rather than given as an absolute number?
- Metabolic oxygen demand scales with body size, so cardiac index better matches perfusion to patient size
- What happens to required flow rate when a patient is cooled to moderate hypothermia during bypass?
- Flow requirements decrease because hypothermia lowers whole-body oxygen consumption and metabolic rate
- What is hemodilution in the context of CPB?
- The drop in hematocrit and plasma protein concentration caused by mixing the patient's blood with the circuit priming volume
- Name two clinical benefits of controlled hemodilution during bypass.
- Reduced blood viscosity (improving microcirculatory flow, especially during hypothermia) and reduced need for allogeneic blood transfusion
- What is a common lower hematocrit limit perfusionists try to avoid during routine adult bypass?
- Roughly 20 to 25 percent, to preserve adequate oxygen-carrying capacity
- How does hypothermia affect blood viscosity during CPB?
- It increases blood viscosity, which is one reason hemodilution is used to offset the effect and preserve microvascular flow
- What happens to the oxyhemoglobin dissociation curve with hypothermia?
- It shifts to the left, increasing hemoglobin's affinity for oxygen and reducing oxygen unloading to tissues
- What is alpha-stat blood gas management?
- A pH management strategy that does not correct blood gas values for actual patient temperature, keeping intracellular pH-enzyme relationships stable across temperatures
- What is pH-stat blood gas management?
- A strategy that corrects blood gas values to the patient's actual temperature, typically requiring added CO2 to maintain a temperature-corrected normal pH during hypothermia
- Which blood gas management strategy is generally preferred for adult CPB without deep hypothermic circulatory arrest?
- Alpha-stat management
- Which blood gas management strategy is often favored during deep hypothermic circulatory arrest, particularly in pediatric cases?
- pH-stat management, because it increases cerebral blood flow and may improve brain cooling uniformity
- What is the physiologic rationale for using nonpulsatile flow during standard CPB?
- Roller and centrifugal pumps most commonly generate continuous (nonpulsatile) flow, which is technically simpler even though it differs from native pulsatile physiology
- What happens to circulating catecholamine and stress hormone levels during CPB?
- They rise, reflecting the systemic stress and inflammatory response triggered by extracorporeal circulation and surgery
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