Measurement of vascular reactivity in head injured patients

It is has been demonstrated that clinical outcome following head injury is correlated with the reactivity of the cerebrovasculature to carbon dioxide changes. Since CBF measurements are difficult to perform in these patients, a new technique is proposed utilizing the ICP response to capnic stimuli. In 40 head injured patients, the responses of ICP, pressure volume index (PVI) and middle cerebral artery velocities to hypocapnia and to hypercapnia were determined. Hypocapnia reduced ICP and MCA velocity while hypercapnia was followed by ICP and MCA velocity increases. Both changes were in the same magnitude supporting the concept the global ICP response reflects vascular reactivity. The fact that the velocity response to hypocapnia in lesioned hemispheres was less compared to the ICP response indicates the loss of ability to dilate in injured vessels and is consistent with earlier findings relating reduced reactivity to poor outcome.