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Drift analysis of a novel device for measurement of regional cerebral blood flow


Bedside measurement of regional cerebral blood flow (rCBF) is a promising technology to detect vasospasm after aneurysmal subarachnoid hemorrhage [1]. The Bowman® rCBF monitor facilitates continuous and quantitative assessment of brain tissue perfusion in ml/100 g tissue/minute with a thermodiffusion microprobe. The probe is self-calibrating at user-definable time intervals from 1 to 120 minutes. To estimate the necessary recalibration frequency, we looked at the measurement drift between two calibration points.


In 32 patients with aneurysmal subarachnoid hemorrhage, a Bowman® rCBF probe was implanted in the vascular territory of the aneurysm-harboring vessel. According to previous experience, we performed automatic recalibration of the rCBF device every 30 minutes, thus yielding measurement periods of 25 minutes. CBF was recorded once per second. Data were averaged over 1 minute and all measurement cycles were pooled.


The mean analyzed monitoring time per patient was 6.9 days. The mean rCBF was 24.4 ml/100 g/min at the beginning and 25.6 ml/100 g/min at the end of the measurement cycles between two recalibration periods, thus representing a mean drift of 1.15 ml/100 g/min per 25-minute measurement period (P < 0.001). This drift was heterogeneous in different patients (range -3.67 to 12.0 ml/100 g/min). Patients number 24 to 32 who were monitored more recently showed a significant upward drift of 7.57 ml/100 g/min, whereas data from patients number 1 to 23 had a downward drift of -0.67 ml/100 g/min. Explanation for this phenomenon is a revision of the monitor software by the manufacturer that was performed after patient 23. This finding was verified with external data.


The current implementation of the Bowman® rCBF monitor shows a severe upward measurement drift that is clinically relevant. At present, the only solution is to perform recalibrations as frequently as possible.


  1. Vajkoczy P, et al.: Regional cerebral blood flow monitoring in the diagnosis of delayed ischemia following aneurysmal subarachnoid hemorrhage. J Neurosurg 2003, 98: 1227-1234.

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Wolf, S., Schürer, L., Horn, P. et al. Drift analysis of a novel device for measurement of regional cerebral blood flow. Crit Care 13 (Suppl 1), P86 (2009).

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