The shift from deep hypothermia (33°C) to milder temperature control (36°C) for out-of-hospital cardiac arrest (OHCA) patients has sparked debate in the medical community. Initially driven by the findings of major studies such as TTM1 and TTM2, hospitals began prioritizing fever prevention over aggressive cooling. However, real-world evidence from retrospective studies suggests this change may have unintended consequences, including reduced compliance, increased rates of fever, and worse clinical outcomes. This article examines the practical effects of fever prevention compared to hypothermia, highlighting whether maintaining a temperature of 36°C truly benefits patients or if deeper cooling remains a superior strategy for improving neurologic outcomes and survival rates.
Therapeutic hypothermia, once hailed as a breakthrough in the management of comatose survivors of OHCA, involves cooling the body to protect against brain damage. For years, 33°C was the standard target temperature. However, following studies like TTM1 and TTM2, a shift toward a target of 36°C — effectively normothermia with aggressive fever control — gained traction. Despite initial optimism, emerging evidence suggests that this change may not always benefit patients.
This article will explore the real-world effects of transitioning from hypothermia to fever prevention, based on studies by Bray et al. (2017) and Johnson et al. (2020).
Why Shift from Hypothermia to Fever Prevention?
The rationale behind the shift was twofold:
- Safety Concerns: Cooling to 33°C carries risks, such as arrhythmias, infections, and metabolic disturbances.
- Study Findings: The TTM1 and TTM2 trials found no significant differences in survival or neurologic outcomes between patients cooled to 33°C and those maintained at 36°C.
However, interpreting these results as evidence that hypothermia offers no benefit may oversimplify the issue.
Real-World Impact of Changing Protocols
1. Study by Bray et al. (2017): Increased Fever and Reduced Compliance
Bray and colleagues analyzed the outcomes of patients treated at 33°C versus 36°C in a Melbourne hospital. The change to 36°C led to:
- Higher fever rates: Fever occurred in 19% of patients after the shift to 36°C, compared to none in the 33°C group.
- Reduced time at target temperature: Patients in the 36°C group spent less time within the target range, indicating poor compliance.
- Worse outcomes: Although not statistically significant, there was a trend toward lower rates of discharge and favorable neurologic outcomes in the 36°C group.
Key Takeaways:
Maintaining normothermia (36°C) requires active intervention, including sedation and muscle relaxation. Without proper management, patients risk developing fever, which can exacerbate brain injury.
2. Study by Johnson et al. (2020): Better Neurologic Outcomes with Hypothermia
Johnson and colleagues conducted a retrospective analysis comparing outcomes at 33°C and 36°C in an urban trauma center. Their findings included:
- Improved neurologic outcomes at 33°C: 40% of patients in the 33°C group had favorable neurologic outcomes, compared to 30% in the 36°C group.
- Faster initiation of cooling: Patients treated at 33°C reached target temperature more quickly, which may have contributed to better outcomes.
- No significant difference in mortality: Hospital mortality rates were similar between the two groups.
Key Takeaways:
Deeper cooling (33°C) was associated with higher rates of neurologically intact survival. This suggests that hypothermia may offer neuroprotective benefits beyond simple fever prevention.
Challenges in Preventing Fever
1. Difficulty in Achieving 36°C
While 36°C may seem easier to maintain, achieving and sustaining this temperature is challenging. Sedation and muscle relaxation are often required, adding complexity to patient management. In Bray’s study, compliance dropped significantly after the protocol change, highlighting the practical difficulties of maintaining normothermia.
2. Risks of Fever
Fever is a well-known risk factor for worse outcomes after cardiac arrest. Hyperthermia exacerbates brain injury, increasing metabolic demand and inflammation. Preventing fever may be crucial, but the risk of underestimating its impact remains high without aggressive temperature control.
Hypothermia vs. Fever Prevention: Which is Better?
Arguments for Hypothermia (33°C):
- Neuroprotection: Cooling reduces cerebral metabolism and limits secondary brain injury.
- Faster Cooling: As shown in Johnson’s study, hypothermia protocols may lead to faster initiation and more consistent temperature management.
- Favorable Outcomes: Evidence suggests improved neurologic outcomes at 33°C, even if overall survival rates are similar.
Arguments for Fever Prevention (36°C):
- Reduced Complications: Avoiding deep hypothermia may reduce risks such as arrhythmias and infections.
- Simpler Protocols: Maintaining normothermia might simplify care, although this depends on adequate sedation and fever prevention strategies.
Practical Recommendations
- Focus on Individualized Care: Not all patients may benefit equally from deep hypothermia. Tailoring temperature management based on patient characteristics and risk factors is crucial.
- Improve Compliance: Whether targeting 33°C or 36°C, strict adherence to protocols is essential. This requires adequate resources, including sedation and cooling technologies.
- Monitor for Fever: Regardless of the target temperature, aggressive fever prevention is critical. Early detection and treatment of fever can mitigate its harmful effects.
Conclusion
The shift from deep hypothermia (33°C) to fever prevention (36°C) was intended to improve patient safety, but real-world evidence suggests it may come at the cost of worse neurologic outcomes. While preventing fever is essential, deeper cooling may still offer unique neuroprotective benefits. Moving forward, a balanced approach that prioritizes both patient safety and effective temperature management is needed. By refining protocols and ensuring compliance, clinicians can optimize outcomes for OHCA patients.
Sources:
Resuscitation 2017 Apr, Janet E Bray, Dion Stub, Jason E Bloom
Crit Care Med 2020 Mar, Nicholas J Johnson , Kyle R Danielson, Catherine R Counts
Targeted Temperature Management at 33 Versus 36 Degrees: A Retrospective Cohort Study