Continuous monitoring of regional cerebral blood flow: experimental and clinical validation of a novel thermal diffusion microprobe

Object. Current clinical neuromonitoring techniques lack adequate surveilla nce of cerebral perfusion. In this article, a novel thermal diffusion (TD) microprobe is evaluated for the continuous and quantitative assessment of i ntraparenchymal regional cerebral blood flow (rCBF). Methods. To characterize the temporal resolution of this new technique, rCB F measured using the TD microprobe (TD-rCBF) was compared with rCBF levels measured by laser Doppler (LD) flowmetry during standardized variations of CBF in a sheep model. For validation of absolute values, the microprobe was implanted subcortically (20 mm below the level of dura) into 16 brain-inju red patients, and TD-rCBF was compared with simultaneous rCBF measurements obtained using stable xenon-enhanced computerized tomography scanning (sXe- rCBF). The two techniques were compared using linear regression analysis as well as the Bland and Altman method. Stable TD-rCBF measurements could be obtained throughout all 3- to 5-hour s heep experiments. During hypercapnia, TD-rCBF increased from 49.3 +/- 15.8 m1/100 g/min (mean +/- standard deviation) to 119.6 +/- 47.3 ml/100 g/min, whereas hypocapnia produced a decline in TD-rCBF from 51.2 +/- 12.8 ml/100 g/min to 39.3 +/- 5.6 ml/100 g/min. Variations in mean arterial blood press ure revealed an intact autoregulation with pressure limits of approximately 65 mm Hg and approximately 170 mm Hg. After cardiac arrest TD-rCBF decline d rapidly to 0 m1/100 g/min. The dynamics of changes in TD-rCBF corresponde d well to the dynamics of the LD readings. A comparison of TD-rCBF and sXe- rCBF revealed a good correlation (r= 0.89; p < 0.0001) and a mean differenc e of 1.1 +/- 5.2 ml/100 g/min between the two techniques. Conclusions. The novel TD microprobe provides a sensitive, continuous, and real-time assessment of intraparenchymal rCBF in absolute flow values that are in good agreement with sXe-rCBF measurements. This study provides the b asis for the integration of TD-rCBF into multimodal monitoring of patients who are at risk for secondary brain injury.