The Controversy of Normothermic vs. Hypothermic CPB: What Does the Evidence Say?

Introduction

Cardiopulmonary bypass (CPB) has transformed cardiac surgery, allowing for safe and controlled procedures on the arrested heart. However, a long-standing debate in perfusion science is whether CPB should be conducted under normothermic (≥35°C) or hypothermic (<32°C) conditions. Hypothermia has traditionally been used to reduce metabolic demand, whereas normothermia is increasingly advocated for improved tissue perfusion and faster recovery. This article critically examines the evidence, advantages, and risks associated with both strategies.

1. Physiological Basis of Temperature Regulation in CPB

Temperature management during CPB affects:

• Metabolic Rate: Every 1°C drop in temperature decreases oxygen consumption (VO₂) by 7–10% (Murkin et al., 2010).
• Oxygen Delivery (DO₂): Hypothermia reduces oxygen demand but also affects oxygen unloading from hemoglobin (Gold et al., 2012).
• Coagulation and Inflammatory Response: Hypothermia impairs coagulation, while normothermia may exacerbate inflammation (Engelman et al., 2005).
• Neurological Outcomes: The effect on neuroprotection remains controversial (Reed et al., 2018).

2. Hypothermic CPB: Rationale and Evidence

Concept: Cooling the patient to 28–32°C reduces metabolic demand, providing a theoretical protective effect, especially during prolonged surgeries.

Potential Benefits:

✔ Myocardial Protection: Decreased oxygen demand may reduce ischemic injury (Murkin et al., 2010). ✔ Neuroprotection: Historically thought to protect against ischemic brain injury (Engelman et al., 2005). ✔ Reduced Oxygen Consumption (VO₂): Helps in situations with limited oxygen delivery (Gold et al., 2012).

Potential Risks:

✖ Coagulopathy: Hypothermia impairs platelet function and clotting cascade, increasing bleeding risks (Reed et al., 2018). ✖ Delayed Metabolism of Drugs: Can prolong the effects of anesthetic agents and muscle relaxants (Biccard et al., 2019). ✖ Vasoconstriction and Maldistribution of Flow: May lead to inadequate tissue perfusion (Engelman et al., 2005).

Clinical Evidence Supporting Hypothermia

• Engelman et al. (2005) suggested mild hypothermia (32-34°C) might reduce post-operative cognitive dysfunction.
• However, Gold et al. (2012) found that deep hypothermia (<28°C) increases bleeding and the need for transfusion.

3. Normothermic CPB: Rationale and Evidence

Concept: Maintaining a body temperature ≥35°C to preserve physiological homeostasis and improve microcirculatory perfusion.

Potential Benefits:

✔ Improved Oxygen Delivery: Maintains normal hemoglobin-oxygen affinity, ensuring optimal tissue oxygenation (Reed et al., 2018). ✔ Reduced Coagulopathy: Less platelet dysfunction and decreased transfusion needs (Biccard et al., 2019). ✔ Faster Postoperative Recovery: Patients often regain consciousness and organ function more quickly (Murkin et al., 2010).

Potential Risks:

✖ Increased Inflammatory Response: Higher temperature may activate inflammatory mediators, increasing post-CPB complications (Reed et al., 2018). ✖ Risk of Ischemic Injury: If perfusion is inadequate, normothermia can increase oxygen demand beyond supply (Gold et al., 2012).

Clinical Evidence Supporting Normothermia

• A meta-analysis by Reed et al. (2018) showed normothermic CPB was associated with reduced transfusion rates and shorter ICU stays.
• A large RCT (Murkin et al., 2010) found no significant difference in neuroprotection between normothermia and mild hypothermia.
• Biccard et al. (2019) reported normothermic CPB had lower rates of acute kidney injury (AKI) compared to hypothermia.

4. Key Controversies and Current Perspectives

• Neurological Protection: Some studies favor hypothermia, while others show no advantage.
• Bleeding vs. Inflammation: Hypothermia increases bleeding risk, whereas normothermia may worsen systemic inflammation.
• Cardiac Outcomes: Recent trends favor normothermia for shorter pump runs but hypothermia for prolonged surgeries (e.g., complex aortic cases).

Consensus from Major Guidelines

• EACTS/EACTA (2022): Normothermia is preferred for routine CPB, but mild hypothermia may be beneficial for prolonged cases.
• STS Guidelines (2023): Recommend individualized temperature strategies based on patient risk factors.

5. Conclusion: Striking the Right Balance

Both normothermic and hypothermic CPBhave their advantages and risks. The choice should be tailored based on:

. Surgical Procedure Complexity (e.g., long procedures may benefit from mild hypothermia). . . . Patient-Specific Factors (e.g., pre-existing coagulopathy favors normothermia).

.Institutional Protocols and Perfusion Techniques (e.g., use of modified ultrafiltration in hypothermic cases).

Take-Home Message

• Normothermia is emerging as the preferred strategy for routine CPB due to better oxygenation and faster recovery.
• Mild hypothermia (32-34°C) may still have benefits in selected cases, particularly for neuroprotection and prolonged surgeries.
• Perfusionists should adopt an individualized approach, optimizing temperature control based on patient condition and surgical needs.

References

1. Engelman, R., et al. (2005). Temperature Management and Neurologic Outcomes in Cardiac Surgery. Annals of Thoracic Surgery, 79(2), 666–672.
2. Gold, J. P., et al. (2012). The Impact of Temperature on Coagulation During CPB. Journal of Cardiothoracic Surgery, 7(1), 34.
3. Murkin, J. M., et al. (2010). Normothermic vs. Hypothermic CPB: A Randomized Controlled Trial. Circulation, 121(7), 1033–1041.
4. Reed, J. C., et al. (2018). Meta-Analysis of CPB Temperature Strategies: Effects on Outcomes. Journal of Thoracic and Cardiovascular Surgery, 155(5), 1876–1883.
5. Biccard, B. M., et al. (2019). Temperature Management and Renal Function in CPB. Anesthesia & Analgesia, 129(4), 1092–1098.
6. EACTS/EACTA Guidelines (2022). Temperature Management in Cardiopulmonary Bypass. European Journal of Cardiothoracic Surgery, 62(3), 415–432.
7. Society of Thoracic Surgeons (STS) Guidelines (2023). Best Practices in CPB Temperature Regulation. Annals of Thoracic Surgery, 116(4), 920–935.