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

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). Prospective, observational study. Intensive care unit. Severe TBI patients mechanically ventilated, sedated and paralyzed. 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. 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 degrees C in periods 1 and 2, and 34.1 +/- 1.1 degrees 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). 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.