Relationship between intracranial pressure, mild hypothermia and temperature-corrected PaCO₂ in patients with traumatic brain injury

Abstract

Objective: To study the effects of mild hypothermia and associated changes in temperature-corrected PaCO₂ (cPaCO₂) on intracranial pressure (ICP), mean velocity of the middle cerebral artery (Vm), and venous jugular saturation in O₂ (SjvO₂) in patients with severe traumatic brain injury (TBI).¶Design: Prospective, observational study.¶Setting: Intensive care unit.¶Patients: Severe TBI patients mechanically ventilated, sedated and paralyzed.¶Interventions: Twenty patients were subjected to four consecutive periods: (a) normocapnia-normothermia; (b) hypocapnia-normothermia, where hypocapnia was induced by an increase in minute volume; (c) hypocapnia-hypothermia, where hypocapnia was induced by hypothermia maintaining the ventilatory settings constant; (d) normocapnia-hypothermia, where normocapnia was achieved by a decrease in minute volume.¶Measurements and results: cPaCO₂ was 41 ± 8 mmHg in periods 1 and 4, and 31 ± 7 mmHg in periods 2 and 3. Core temperature was 37.1 ± 0.8 °C in periods 1 and 2, and 34.1 ± 1.1 °C in periods 3 and 4. End-tidal CO₂ and cPaCO₂ values showed no difference between periods 1 and 4 and periods 2 and 3. ICP and Vm were dependent on cPaCO₂ but independent of core temperature values. SjvO₂ was related to cPaCO₂ and was significantly higher during period 3 than during period 2 (P < 0.05).¶Conclusion: The decrease in ICP was similar when hypocapnia was induced by hyperventilation or as a result of hypothermia alone. The relationship between cPaCO₂ and ICP might predict variations in ICP during changes in core temperature. Further studies are needed to confirm the cerebral metabolic effects of moderate hypothermia in TBI patients.