Relationship between intracranial pressure, mild hypothermia and temperature-corrected PaCO sub(2) in patients with traumatic brain injury

Objective: To study the effects of mild hypothermia and associated changes in temperature-corrected PaCO sub(2) (cPaCO sub(2)) on intracranial pressure (ICP), mean velocity of the middle cerebral artery (Vm), and venous jugular saturation in O sub(2) (SjvO sub(2)) 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: cPaCO sub(2) was 41 plus or minus 8 mmHg in periods 1 and 4, and 31 plus or minus 7 mmHg in periods 2 and 3. Core temperature was 37.1 plus or minus 0.8 degree C in periods 1 and 2, and 34.1 plus or minus 1.1 degree C in periods 3 and 4. End-tidal CO sub(2) and cPaCO sub(2) values showed no difference between periods 1 and 4 and periods 2 and 3. ICP and Vm were dependent on cPaCO sub(2) but independent of core temperature values. SjvO sub(2) was related to cPaCO sub(2) 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 cPaCO sub(2) 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.