Can hyperventilation cause cerebral ischaemia?
Hyperventilation reduces ICP acutely, but this potentially beneficial effect is at the expense of cerebral perfusion. Whether or not hyperventilation can actually result in cerebral ischaemia in head-injured patients is controversial. Several studies suggest that excessive hyperventilation after severe head injury can be a secondary ischaemic insult. In one prospective study where SjvO2 was monitored in 116 patients with severe head injury [4], hypocapnia was the second most common cause of jugular venous desaturation, accounting for 36% of the episodes. The degree of reduction in CBF is related to the level of PCO2 reduction. The relationship of hyperventilation and global cerebral blood flow has been examined in a series of 171 head-injured patients during the first 10 days after injury [5]. Of 1212 CBF measurements in these patients, 132 (11%) were less than 25 ml 100g-1 min-1. Of the 132 low CBF values, 71 (54%) were appropriately reduced relative to the lower CMRO2, while 61 (46%)
