How should pump flows and temperature be managed during cardiopulmonary bypass (CPB) to prevent complications?

Managing Pump Flows and Temperature During Cardiopulmonary Bypass

Pump flow rates during CPB must be determined initially by BSA and planned temperature, then continuously adjusted based on oxygen delivery (DO₂) targets of ≥280 mL/min/m² during normothermic conditions, with adequacy verified through metabolic markers including SvO₂ >75%, rather than relying on flow rate alone. 1, 2

Initial Pump Flow Determination

Pre-CPB flow calculation should be based on:

• Body surface area (BSA) with target flows of 2.2-2.8 L/min/m² under moderate hypothermia to normothermic conditions 1
• In obese patients, calculate pump flow based on lean body mass rather than total BSA to avoid overestimating metabolic needs and causing relative over-perfusion (Class IIb, Level B) 1, 2, 3
• Planned temperature target, as metabolic demands vary significantly with temperature 1, 4

Goal-Directed Perfusion Strategy: The Superior Approach

The most critical paradigm shift is moving from BSA-based flow rates to oxygen delivery-guided perfusion:

• Adjust pump flow according to arterial oxygen content to maintain DO₂ ≥280-300 mL/min/m² during normothermic CPB (Class IIa, Level B) 1, 2
• Calculate DO₂ as: DO₂ (mL/min/m²) = Pump Flow (L/min) × Arterial Oxygen Content (mL O₂/L) × 10 / BSA (m²) 2
• Where arterial oxygen content = (Hemoglobin × 1.36 × SaO₂) + (PaO₂ × 0.0031) 2
• Goal-directed perfusion reduces stage 1 AKI by 55% (RR 0.45,95% CI 0.25-0.83) compared to conventional BSA-based perfusion 1, 2

Real-Time Monitoring to Verify Adequate Perfusion

You cannot rely on pump flow rate alone—continuous monitoring of metabolic markers is essential:

Primary Monitoring Parameters (Class IIa, Level B):

• Mixed venous oxygen saturation (SvO₂) target >75% from the venous reservoir 1, 2
• Oxygen extraction ratio (O₂ER) to detect excessive extraction 1
• Near-infrared spectroscopy (NIRS) for regional cerebral oxygen saturation 1, 4
• Arterial lactate levels to detect inadequate tissue oxygenation 1
• Carbon dioxide production (VCO₂) as a metabolic indicator 1

Critical Pitfall to Avoid:

Low values of SvO₂, DO₂, NIRS and high values of O₂ER and lactates are markers of inadequate perfusion associated with adverse outcomes including AKI, neurologic injury, and mortality 1

Temperature Management and Flow Adjustments

Temperature profoundly affects safe minimum flow rates:

• At 15°C with hematocrit 20-30%, flows as low as 10 mL/kg/min are safe for up to 2 hours 4
• At 34°C (near-normothermic), the same flow rate of 10 mL/kg/min is very likely to cause neurologic injury 4
• During hypothermia, pump flows at 2.4 L/min/m² result in relative over-perfusion, suggesting temperature-adjusted flow reduction is appropriate 3
• During rewarming, metabolic demands increase dramatically—failure to increase flows accordingly predicts worse neurologic outcomes 4

Temperature-Related Algorithm:

1. Deep hypothermia (15-18°C): Lower flows acceptable if metabolic markers remain adequate 4
2. Moderate hypothermia (25-28°C): Standard BSA-based flows (2.2-2.8 L/min/m²) 1
3. Normothermic (34-37°C): Higher flows required, guided by DO₂ ≥280 mL/min/m² 2, 4
4. During rewarming: Progressively increase flows and monitor for functional capillary density recovery 4

When Temperature "Drifts" Occur: Immediate Actions

If unplanned temperature changes occur during CPB:

For Unintended Cooling:

• Reduce pump flow proportionally to decreased metabolic demand 5, 3
• Verify SvO₂ remains >75% to confirm adequate perfusion at lower flow 1
• Monitor NIRS to ensure cerebral perfusion remains adequate 4

For Unintended Warming:

• Immediately increase pump flow rate to match rising metabolic demands 4
• Increase DO₂ by either increasing flow or transfusing PRBCs if hematocrit <18% 2, 6
• Monitor lactate levels closely—rising lactate indicates inadequate perfusion 1
• Aggressive rewarming practices contribute to neurologic injury—control warming rate 7

Adjusting DO₂ Components When Perfusion is Inadequate

When metabolic markers indicate inadequate perfusion, adjust DO₂ through:

1. Increase pump flow rate (most direct method) while balancing against BSA and temperature 2, 6
2. Transfuse packed red blood cells if hematocrit <18% (Hb <6.0 g/dL) during CPB (Class I recommendation) 2
3. Verify adequate depth of anesthesia—light anesthesia increases metabolic demands 1, 6
4. Ensure adequate oxygenation (SaO₂) from the oxygenator 2

Critical Evidence:

The risk of developing stage 1 AKI is significantly and inversely associated with DO₂—lower DO₂ directly increases AKI risk regardless of pump flow rate alone 1

Blood Pressure Management During Flow Adjustments

After adjusting pump flow, manage MAP appropriately:

• Target MAP 50-80 mmHg during CPB 1
• Use vasodilators if MAP >80 mmHg after checking anesthesia depth and confirming adequate pump flow (Class I, Level A) 1
• Use vasoconstrictors if MAP <50 mmHg after verifying pump flow is adequate (Class I, Level A) 1
• Do NOT use vasopressors to force MAP >80 mmHg (Class III, Level B)—this is harmful 1

For Vasoplegic Syndrome During CPB:

• First-line: α1-adrenergic agonist vasopressors (Class I, Level C) 1, 8
• Refractory cases: vasopressin, terlipressin, or methylene blue (Class IIa, Level B) 1, 8
• Consider hydroxocobalamin 5g IV for refractory vasoplegia (Class IIb) 8

Common Pitfalls and How to Avoid Them

Critical errors that lead to organ injury:

1. Relying solely on BSA-based flow rates without monitoring DO₂—this ignores hemodilution effects and individual metabolic variation 1, 2
2. Failing to adjust flows during temperature changes—metabolic demands change exponentially with temperature 5, 4
3. Using total BSA in obese patients—this overestimates metabolic needs and causes relative over-perfusion 1, 2, 3
4. Ignoring rising lactate or falling SvO₂—these are early markers of inadequate perfusion before organ injury becomes irreversible 1
5. Aggressive rewarming without increasing flows—this causes supply-demand mismatch and neurologic injury 7, 4