What are the consequences of hemodilution on cardiopulmonary bypass (cardiopulmonary bypass)?

Consequences of Hemodilution During Cardiopulmonary Bypass

Hemodilution during cardiopulmonary bypass (CPB) significantly increases the risk of organ dysfunction, morbidity, and mortality when hematocrit falls below 22%, with optimal outcomes achieved by maintaining hematocrit above 25%. 1

Physiological Effects of Hemodilution

• Hemodilution occurs primarily due to the mixing of the patient's blood with the priming volume of the CPB circuit, resulting in decreased hematocrit and reduced oxygen-carrying capacity 2
• Priming-induced hemodilution leads to reduced plasma protein concentrations, affecting drug pharmacokinetics and pharmacodynamics 2
• Hemodilution combined with hypothermia alters the blood/gas partition coefficient of volatile anesthetics, with hypothermia increasing it and hemodilution decreasing it 2
• During rewarming, temperature increases faster than hematocrit, resulting in a lower blood/gas partition coefficient and increased depth of anesthesia 2

Impact on Oxygen Delivery and Organ Function

• Excessive hemodilution can lead to inadequate oxygen delivery, causing ischemic organ injury and inflammatory responses 1
• Critical oxygen delivery threshold is 272 mL·min⁻¹·m⁻² during CPB; falling below this significantly increases risk of acute renal failure 3
• Hematocrit values below 22% are associated with increased risk of:
  • Stroke and neurological complications 1
  • Myocardial infarction and cardiac arrest 1
  • Renal failure 1, 3
  • Pulmonary complications including prolonged ventilation and pulmonary edema 1
  • Systemic inflammatory response and multiorgan failure 1

Clinical Outcomes and Resource Utilization

• Increased hemodilution severity correlates with:

  • Higher operative mortality rates 1
  • Longer ICU stays (>2 days) and hospital stays (>8 days) 1
  • Increased resource utilization and healthcare costs 1
  • Worse intermediate-term survival (0-6 years) 1

• Even in non-transfused patients, hematocrit values below 25% during CPB are associated with increased major morbidity 4

Strategies to Mitigate Hemodilution Effects

• Minimize pre-CPB intravenous fluid administration to preserve baseline hematocrit (ideally <1600 mL) 5
• Use blood cardioplegia instead of crystalloid cardioplegia in high-risk patients (anemic, low body surface area, chronic kidney disease, or complex procedures) to reduce hematocrit dilution 2
• Implement miniaturized CPB circuits to attenuate inflammatory response and improve hematocrit levels 2
• Apply ultrafiltration techniques:
  • Conventional ultrafiltration during bypass to remove excess volume and raise hematocrit 2
  • Modified ultrafiltration (MUF) following CPB termination to further raise hematocrit and improve pulmonary compliance, especially in pediatric cases 2, 6
• Use retrograde autologous priming to reduce priming volume and minimize hemodilution 2
• Consider cell salvage to process the patient's own blood during surgery 2

Transfusion Considerations

• While severe hemodilution increases transfusion requirements, transfusions themselves carry risks 2
• Current guidelines suggest transfusing packed red blood cells if hemoglobin is <6.0 g/dL or if hematocrit is <21-24% when oxygen delivery falls below 273 mL/min/m² 2
• Transfusion decisions should be guided by monitoring venous saturation (<68%) and oxygen extraction ratio (>39%) 2

Balanced Fluid Management

• Use balanced crystalloid solutions rather than normal saline for priming and maintenance fluids to prevent hyperchloremic metabolic acidosis 2, 7
• Unbalanced solutions with high chloride content (>154 mmol/L) can lead to acidosis, impaired renal function, and tissue injury 7

Monitoring and Management

• Monitor inline systems like CDI 500 to track venous saturation and oxygen delivery during CPB 2
• Adjust pump flow rates to compensate for low hematocrit and maintain adequate oxygen delivery 3
• Consider individualized heparin and protamine management to reduce coagulation abnormalities that may be exacerbated by hemodilution 2

By understanding and addressing the consequences of hemodilution during CPB, clinicians can optimize patient outcomes through appropriate preventive strategies and interventions.