Abstract
Hyperventilation is a well known method of reducing the raised ICP and has been used widely in clinical practice. The mechanism of reducing ICP is believed due to the cerebrovascular contraction resulting in the decrease of cerebral blood volume.
Little has been known about the dynamics of rCBF of the white matter during hyperventilation under the presence of mass lesion. The purpose of this study is to clarify the relationship between rCBF of the white matter with mass lesion and the various levels of PaCO2 produced by hyperventilation in dogs.
Epidural balloon compression of the brain was employed as simulating the model of intracranial mass lesion, which was placed in the right frontal area. Platinum electrode for hydrogen clearance determination of rCBF was stereotaxically inserted into the white matter of the left frontal lobe. ICP was monitored by subdural balloon technique, which was placed in the left occipital area. After the measurement of control value of rCBF, the epidural balloon was inflated and CIP was maintained at 40 mmHg for 77 minutes. Then, rCBF was measured in three different levels of PaCO2, normocapnia (40 mmHg), mild hypocapnia (30 mmHg) and severe hypocapnia (20 mmHg) produced by mild and extreme hyperventilation. rCBF measurements without compression were also performed for control group, under the same PaCO2 levels as in the epidural compression group.
Significant reduction of rCBF of the white matter (17.2 ± 3.2 to 12.6 ± 3.7ml/100 g/min) was recorded after the epidural balloon inflation. In the epidural compression group, rCBF of the white matter was decreased to 64% of control value at mild and 53% at extreme hyperventilation. rCBF in control group was decreased to 78% at mild and 69% at extreme hyperventilation. The raised ICP (40 mmHg) was reduced to 30.7 mmHg by mild and 28.6 mmHg by extreme hyperventilation.
It can be concluded that hyperventilation in a case with intracranial mass lesion may be a method of choice to reduce raised ICP, but rCBF of the white matter may easily decrease below the critical level. Extreme hyperventilation should be avoided, since the further reduction of ICP can not be expected and there is a danger to induce the ischemic insult of the white matter.
