DO2 Calculation on Cardiopulmonary Bypass
Oxygen delivery (DO2) during cardiopulmonary bypass is calculated as: DO2 (mL/min/m²) = Pump Flow (L/min) × Arterial Oxygen Content (mL O2/L) × 10 / BSA (m²), where arterial oxygen content = (Hemoglobin × 1.36 × SaO2) + (PaO2 × 0.0031). 1
The Complete Formula Breakdown
DO2 = Pump Flow × CaO2 × 10 / BSA
Where the components are:
- Pump Flow: Measured in L/min from the cardiopulmonary bypass circuit 1
- CaO2 (Arterial Oxygen Content): Calculated as (Hb g/dL × 1.36 mL O2/g × SaO2) + (PaO2 mmHg × 0.0031 mL O2/mmHg/dL) 1
- BSA: Body surface area in m² 1
- The factor of 10: Converts units to mL/min/m² 1
Critical Target Values
Maintain DO2 ≥280 mL/min/m² during normothermic CPB to reduce acute kidney injury (Class I, Level A recommendation). 1
- The 2024 EACTS/EACTAIC/EBCP guidelines specifically recommend maintaining DO2 between 280-300 mL/min/m² during normothermic CPB for optimal clinical outcomes (Class IIa, Level B) 1
- Even a single DO2 measurement below 280 mL/min/m² is associated with increased prolonged ventilation and longer postoperative length of stay 2
- Multiple readings below threshold (≥2 measurements <280 mL/min/m²) correlate with 7.0 days postoperative length of stay versus 5.2 days when all readings remain ≥280 mL/min/m² 2
Practical Application During CPB
Calculate DO2 at CPB initiation and every 30 minutes thereafter, adjusting pump flow immediately when DO2 falls below 280 mL/min/m². 2
The adequacy of pump flow should be verified using multiple parameters:
- Mixed venous oxygen saturation (SvO2): Target >75% 1
- Oxygen extraction ratio (O2ER): Monitor for excessive extraction 1
- Regional cerebral oxygen saturation (rcSO2): Assess cerebral perfusion 1
- Arterial lactate levels: Detect inadequate tissue oxygenation 1
Adjusting DO2 Components
When DO2 falls below threshold, you have three primary interventions:
- Increase pump flow: The most direct method, though must be balanced against BSA and temperature 1
- Increase hemoglobin: Transfuse packed red blood cells if hematocrit <18% (Hb 6.0 g/dL) during CPB (Class I recommendation) 1
- Optimize oxygenation: Ensure adequate SaO2, though this typically has minimal impact as SaO2 is usually near 100% on CPB 1
Special Considerations for Obese Patients
In patients with obesity, calculate pump flow rates based on lean body mass rather than total BSA to avoid overestimating metabolic needs (Class IIb, Level B). 1
- BSA may not adequately reflect metabolic demands in obese patients 1
- Lean body mass provides a more accurate estimate of systemic oxygen consumption 1
Temperature Adjustments
The DO2 threshold of 280 mL/min/m² applies specifically to normothermic CPB 1. Under moderate hypothermia, oxygen consumption decreases, and lower DO2 values may be acceptable, though specific thresholds are not well-validated 1. The pump flow rate must be determined before CPB initiation based on both BSA and planned temperature (Class I recommendation) 1.
Critical Pitfall to Avoid
Do not rely solely on pump flow calculations without measuring actual DO2, as hemodilution during CPB significantly reduces arterial oxygen content despite adequate flow rates. 1
- A patient on miniaturized CPB may have lower pump flows but equivalent DO2 due to less hemodilution 1
- The relationship between oxygen consumption and delivery shows that VO2 becomes supply-dependent when DO2 falls below 300 mL/min/m² 3
- At DO2 values below the critical threshold, oxygen consumption plateaus and lactate accumulates, indicating anaerobic metabolism 3
Goal-Directed Perfusion Strategy
Implement goal-directed perfusion by continuously monitoring DO2 and adjusting pump flow and hemoglobin to maintain DO2 ≥280 mL/min/m² throughout CPB (Class I, Level A). 1