What are the guidelines for cooling and rewarming in cardiopulmonary bypass (CPB)?

Cooling and Rewarming Guidelines in Cardiopulmonary Bypass

Temperature Management Strategy

Mild hypothermia (32-34°C) should be used during CPB rather than moderate hypothermia (28°C), as it significantly reduces postoperative atrial fibrillation without compromising myocardial or cerebral protection. 1

Cooling Phase

• Target core temperature of 32-34°C during routine cardiac surgery procedures, measured via nasopharyngeal monitoring 1
• Cool arterial blood to 25.0°C to achieve nasopharyngeal temperature of 28.0-28.5°C when deeper hypothermia is required for complex procedures 2
• Modern CPB techniques utilize lower prime volumes and milder degrees of systemic hypothermia compared to historical practices that used 2L+ prime volumes and cooling to 30°C 1
• Deep hypothermia (<20°C) with circulatory arrest should be reserved only for procedures requiring complete circulatory arrest 1

Critical Monitoring During Cooling

• Monitor nasopharyngeal temperature as the primary indicator of brain temperature during both cooling and rewarming phases 2, 3
• Arterial blood temperature should be monitored simultaneously as a marker of potential cerebral hyperthermia 2
• Bladder temperature alone is inadequate as it lags behind brain temperature and may remain <37°C even when brain temperature exceeds 38°C 3

Rewarming Phase: The Critical Period

During rewarming, limit arterial blood temperature to a maximum of 38°C and maintain nasopharyngeal temperature ≤37°C to prevent inadvertent cerebral hyperthermia. 2, 3

Rewarming Protocol

• Raise arterial blood temperature to maximum 38.0°C during rewarming in hypothermic cases 2
• For normothermic CPB (37°C), maintain arterial blood temperature at maximum 37.0°C to keep nasopharyngeal temperature at 36.5-37.0°C 2
• Brain temperature should not exceed 34°C at the time of separation from CPB to optimize neuroprotection 4
• Temperature increases faster than hematocrit during rewarming, resulting in altered pharmacokinetics of volatile anesthetics requiring dose adjustments 1

Common Pitfalls During Rewarming

• Peak brain temperatures near 39°C are commonly achieved inadvertently during conventional rewarming protocols 4, 3
• Nasopharyngeal and tympanic temperatures frequently exceed 38°C during rewarming even when bladder temperature remains <37°C 3
• Rigid adherence to 37.0°C during normothermic CPB can cause cerebral overheating 2
• The blood/gas partition coefficient decreases during rewarming, leading to increased depth of anesthesia and requiring increased volatile anesthetic concentrations 1

Anesthetic Management During Temperature Changes

Cooling Phase Adjustments

• Reduce volatile anesthetic dose by approximately 30% for each 5°C drop in temperature after wash-in is complete 1
• Anesthetic requirements are temperature-dependent, with lower requirements during systemic hypothermia 1
• BIS monitoring has limitations below 32°C, with values reduced by approximately 1.2 units per 1°C reduction 1

Rewarming Phase Adjustments

• Increase volatile anesthetic concentrations during rewarming due to reduced blood/gas partition coefficient and faster wash-in 1
• Monitor oxygenator exhaust concentrations of volatile agents, which should be at least equal to pre-CPB levels, and increased during rewarming 1, 5
• The oxygenator exhaust concentration correlates well with arterial concentration 1

Cardioplegia Temperature Considerations

• Cold blood cardioplegia administered intermittently every 20-30 minutes maintains cardiac arrest and hypothermia 1
• Warm blood cardioplegia (normothermic) may reduce postoperative myocardial infarction compared to cold cardioplegia but requires continuous delivery to prevent warm ischemic injury 1
• Patient-centered myocardial protective strategies should be based on clinical condition and procedural complexity rather than fixed institutional protocols 1

Temperature-Related Complications to Avoid

• Incomplete rewarming can lead to cardiac arrest 2
• Cerebral hyperthermia during rewarming may cause neuropsychological deficits, as mild temperature elevation (even 2-5°C above normal) is markedly deleterious to brain tissue 4, 3
• Moderate hypothermia (28°C) increases postoperative atrial fibrillation to 48.5% compared to 21.9% with mild hypothermia (34°C) 1