Evaluation of a bedside monitor of regional CBF as a measure of CO2 reactivity in neurosurgical intensive care patients.

OBJECTIVE: Mild hyperventilation remains a key element in the management of elevated intracranial pressure. However, a harmful effect of hyperventilation on the development or deterioration of ischemic lesions has been shown in patients after severe head trauma. The objective of this study was to investigate the clinical feasibility and reliability of continuous monitoring of regional cerebral blood flow (rCBF) during mild hyperventilation using a thermodiffusion probe. CO2 reactivity was calculated. The measurement of the partial pressure of oxygen (PtiO2) in the cerebral tissue served as a reference parameter.

METHODS: An intraparenchymal intracranial pressure sensor, a multiparameter probe for determining the partial pressure of cerebral gases (pHti, PtiO2, PtiCO2), and a thermodiffusion probe for measuring rCBF were used in 10 intensive care patients. All patients were analgosedated and received pressure-controlled mechanical ventilation. Controlled mild hyperventilation was carried out on 2 consecutive days. CO2 reactivity was determined in relation to both CBF and PtiO2.

RESULTS: Controlled hyperventilation resulted in a rCBF reduction from 30+/-3 mL/100 g/min to 25+/-2.4 mL/100 g/min (-17%; P<0.05) on the first day of examination and 31+/-3.6 mL/100 g/min to 22+/-4.9 mL/100 g/min (-29%; P<0.05) on the second day. Likewise, mild hyperventilation resulted in a reduction of regional cerebral tissue oxygen partial pressure from 20+/-2.9 mm Hg to 15+/-4 (-25%; P<0.05) on the first day and 20+/-3.1 mm Hg to 14+/-1.5 mm Hg (-30%; P<0.05) on the second.

CONCLUSIONS: Continuous monitoring of regional CBF, using an intraparenchymally placed thermodiffusion probe, seems to be a simple and safe bedside technique. The promise of reliably monitoring and interpreting additional parameters such as PtiO2 and PtiCO2 warrants further investigation.