Priming Techniques in Cardiopulmonary Bypass: An Overview.

Introduction Cardiopulmonary bypass (CPB) is a cornerstone of modern cardiac surgery, facilitating complex intracardiac procedures by temporarily assuming the function of the heart and lungs. One of the most critical steps in initiating CPB is the priming of the extracorporeal circuit. Priming ensures the circuit is free from air and ready for immediate use, while also playing a significant role in the patient’s hemodynamic stability, oxygen-carrying capacity, and inflammatory response.

Priming solutions vary widely in their composition and effects. The selection of an appropriate priming technique must be guided by patient-specific factors such as age, weight, hematocrit, renal function, and surgical complexity. This article explores the types of priming solutions—crystalloid, colloid, and blood—along with their clinical applications and implications. In addition, it provides practical guidance on priming volume calculations and final hematocrit estimation.

Types of Priming Solutions

1. Crystalloid Prime Crystalloid solutions are aqueous solutions of mineral salts or other water-soluble molecules. The most commonly used crystalloid solutions in CPB include Ringer’s Lactate, Normal Saline, and Plasmalyte-A.

• Advantages: Inexpensive and readily available. Low risk of allergic reactions. Easy to handle and store.
• Disadvantages: Contributes to significant hemodilution, which can lower hematocrit and reduce oxygen delivery. May lead to tissue edema due to low oncotic pressure. Electrolyte imbalances, especially if large volumes are used.
• Applications: Suitable for adult patients with normal baseline hematocrit.Ideal for short and uncomplicated procedures.

2. Colloid Prime Colloid solutions contain larger molecules that stay in the vascular compartment longer and help maintain oncotic pressure. Common colloids include Hydroxyethyl Starch (HES), gelatin-based solutions, and human albumin.

• Advantages: Better maintenance of oncotic pressure. Reduced risk of tissue edema. Potential renal protective effect (depending on the type used)
• Disadvantages: Higher cost. Some colloids, especially HES, are associated with coagulopathy and renal dysfunction. Risk of allergic reactions.
• Clinical Applications: Beneficial in patients with low serum protein levels. Used when minimizing fluid overload and tissue edema is a priority.

3. Blood Prime Blood priming involves adding packed red blood cells (PRBCs), and sometimes fresh frozen plasma (FFP), to the prime solution. This technique is vital in pediatric and neonatal patients due to their small circulating volume and high metabolic demands.

• Advantages: Maintains adequate hematocrit and oxygen-carrying capacity. Minimizes risk of severe hemodilution.
• Disadvantages: Risk of transfusion reactions and infections. Cost and logistical issues (blood availability, cross-matching)
• Clinical Applications: Mandatory in neonates and infants.Adults with anemia or those at risk of low hematocrit during CPB.

4. Hybrid or Mixed Prime In many clinical scenarios, a mixed approach is used, combining crystalloid with colloid and/or blood components. This provides the benefits of each solution while mitigating their disadvantages.

• Examples: Crystalloid + HES, Crystalloid + PRBCs, Crystalloid + Albumin + PRBCs.

Priming Volume Calculations Accurate calculation of the total priming volume and its effect on patient hematocrit is essential to guide clinical decision-making and optimize perfusion.

• Total Prime Volume (TPV):

• Patient Blood Volume (PBV):

• Final Hematocrit (Hct):

Example Calculation: A 70 kg adult with a hematocrit of 40% and a prime volume of 1200 mL:

• PBV = 70 × 70 = 4900 mL
• Final Hct = (4900 × 0.40) / (4900 + 1200) = 32.1%

This hematocrit is generally acceptable in adults but could be insufficient in pediatric populations.

Clinical Considerations

• Pediatric patients have lower total blood volume, requiring blood or hybrid prime to prevent severe hemodilution.
• Renal dysfunction necessitates caution with HES; albumin is preferred.
• Jehovah’s Witness patients require bloodless strategies including hemoconcentration and use of autologous blood recovery systems.

Summary Table.

Conclusion :

Priming is not just a mechanical step in the preparation for cardiopulmonary bypass—it is a strategic choice that affects patient outcomes. The type and volume of the priming solution determine the degree of hemodilution, the stability of oncotic pressure, and the efficiency of oxygen delivery. Tailoring the priming strategy to each patient’s physiological status and surgical requirements allows the perfusionist to optimize safety, efficacy, and clinical outcomes.

References

1. Gravlee GP et al. Cardiopulmonary Bypass: Principles and Practice.
2. Khonsari S, Sintek C. Cardiac Surgery: Safeguards and Pitfalls.
3. Relevant journal articles and institutional protocols