I.c.p. increases with 50% nitrous oxide in oxygen in severe head injuries during controlled ventilation

In a randomized trial nitrous oxide 50% in oxygen (Entonox) or oxygen 100% was given during chest physiotherapy on 23 occasions to three mechanically ventilated patients with severe head injuries. Intracranial pressure (i.c.p.) increased by 22.7 mm Hg (SD 10.62) during chest physiotherapy with Entonox, compared with 10.5 mm Hg (SD 10.4) with oxygen 100% (P greater than 0.02). A further nine mechanically ventilated patients with severe head injuries were given Entonox without chest physiotherapy. There was a mean increase in i.c.p. of 3.8 mm Hg (SD 2.4) (P less than 0.001) when Entonox was given, and a mean decrease of 4.6 mm Hg (SD 2.8) when the nitrous oxide was withdrawn. End-tidal carbon dioxide concentration showed almost no change during nitrous oxide administration (decrease of 0--0.1%). We conclude that nitrous oxide causes an increase in i.c.p. in patients with severe head injuries and exacerbates the increases in i.c.p. occurring during chest physiotherapy.