VO2 Calculation on Cardiopulmonary Bypass
Oxygen consumption (VO2) during cardiopulmonary bypass is calculated using the Fick equation: VO2 (mL/min) = Pump Flow (L/min) × [Arterial O2 Content - Mixed Venous O2 Content] × 10, where oxygen content = (Hemoglobin × 1.36 × Saturation) + (PO2 × 0.003). 1, 2
Core Calculation Components
The calculation requires three essential measurements during CPB:
- Pump flow rate measured by ultrasonic flow monitoring on the arterial line (Class I recommendation) 3
- Arterial oxygen content (CaO2) = (Hemoglobin g/dL × 1.36 mL O2/g × SaO2) + (PaO2 mmHg × 0.003 mL O2/mmHg) 1, 2
- Mixed venous oxygen content (CvO2) = (Hemoglobin g/dL × 1.36 mL O2/g × SvO2) + (PvO2 mmHg × 0.003 mL O2/mmHg) 2
The dissolved oxygen component (PO2 × 0.003) contributes minimally but should be included for precision 2.
Practical Measurement During CPB
Continuous monitoring of mixed venous oxygen saturation (SvO2) from the venous reservoir is recommended during CPB (Class I, Level B). 3 This provides real-time assessment of the adequacy of oxygen delivery relative to consumption 3.
The adequacy of VO2 relative to metabolic demand should be verified using multiple parameters 3:
- SvO2 target >75% indicates adequate oxygen delivery relative to consumption 3, 1
- Oxygen extraction ratio (O2ER) = (CaO2 - CvO2)/CaO2, with elevated values suggesting inadequate delivery 3
- Arterial lactate levels to detect tissue hypoxia 3, 1
- Regional cerebral oxygen saturation (rcSO2) for cerebral perfusion adequacy 3, 1
Relationship to Oxygen Delivery
The critical threshold for oxygen delivery (DO2) is ≥280 mL/min/m² during normothermic CPB to prevent acute kidney injury (Class I, Level A). 3, 1 Below this threshold, VO2 becomes supply-dependent, meaning tissue oxygen consumption decreases proportionally with delivery 4.
DO2 is calculated as: DO2 (mL/min/m²) = Pump Flow (L/min) × CaO2 (mL O2/L) × 10 / BSA (m²) 1. When DO2 exceeds 300 mL/min/m², VO2 typically plateaus at approximately 105 ± 13 mL/min/m² in patients without lactic acidosis 4.
Important Methodological Considerations
The Fick-derived VO2 calculation during CPB systematically overestimates oxygen consumption by approximately 30% compared to respiratory gas analysis methods. 5 This occurs because the Fick method captures total body VO2 including intrapulmonary oxygen consumption, which accounts for approximately 32% of whole body consumption 5.
However, during CPB when the lungs are not ventilated or are minimally ventilated, this discrepancy becomes less relevant 5. The Fick method remains the standard approach during CPB because respiratory gas analysis is not feasible when the lungs are bypassed 6, 5.
Common Pitfalls to Avoid
- Do not estimate VO2 using standard equations (e.g., 125 mL/min/m² × BSA) during CPB, as these are inaccurate with mean errors exceeding 50 mL/min, particularly in obese patients and men 7
- Ensure blood samples are truly mixed venous from the venous reservoir during CPB, not from peripheral veins 2
- Account for hemodilution effects on hemoglobin concentration, which directly impacts both oxygen content calculations 3
- In obese patients, calculate pump flow based on lean body mass rather than total BSA to avoid overestimating metabolic needs (Class IIb, Level B) 3, 1
Goal-Directed Perfusion Strategy
Implement continuous monitoring of both DO2 and VO2, adjusting pump flow and hemoglobin to maintain DO2 ≥280 mL/min/m² throughout CPB (Class I, Level A). 1 This goal-directed approach reduces stage 1 acute kidney injury with a relative risk of 0.45 (95% CI 0.25-0.83) compared to conventional perfusion 1.
When VO2 calculations suggest inadequate oxygen delivery (rising O2ER, falling SvO2 <75%, rising lactate), increase DO2 by 3, 1:
- Increasing pump flow rate as the primary intervention
- Transfusing packed red blood cells if hematocrit <18% (Hb <6.0 g/dL)
- Ensuring adequate depth of anesthesia to minimize metabolic demand