Mrs S required mechanical ventilation as her blood gases had become deranged when her GCS decreased. Her initial blood gases indicated that she had hypoxemia and was hypercapnic, leading to respiratory acidosis. Once admitted to the ICU, her arterial blood gasses were assessed frequently to ensure her pulmonary gaseous exchange stayed within normal limits, promoting adequate cerebral oxygenation and preventing dilation of cerebral vasculature due to raised C 02 . Control of pC 02 is important because high levels of C 02 increase levels of hydrogen, causing vasodilation of cerebral vessels and an increase in cerebral blood flow that raises ICP . Conversely, low pCO, levels create vasoconstriction. Either process can precipitate ischaemia due to alterations in blood volume and oxygen delivery . Volume cycled ventilation was used with Mrs S so that her pC 02 could be closely controlled. Her tidal volumes were controlled at 5 ml/kg via airway pressures of volume cycled ventilation.