Thermal Diffusion Flowmetry

Cerebral blood flow measurement by thermal diffusion technique was originally conceived by Gibbs in 1933 [23]. He used a heated thermo-coupler to measure flow though the internal jugular vein. However, by nature of its design, it could accurately reflect relative changes but not absolute values. Brawley extended the technique by incorporating a Peltier stack to improve the stability of the recording probe, thereby making it possible to measure cerebral blood flow over extended periods of time, and the measurements were quantitative [24]. Carter eliminated the Peltier stack to miniaturize the recording probe so that it could more easily used post-operatively in a wide variety of cases [25].

The probe has two gold plates in a thin 3-mm Silastic sheath, one heated and one non-heated. The temperature difference between these plates is monitored constantly, measured by computer, and the resultant difference is converted to CBF in ml/100 g/min. The following is the mathematical formula, derived to quantify cerebral flow by thermal diffusion:

CBF = K(1/V - 1/Vo), where CBF is the cortical blood flow in ml/100 g/min, K is the conductivity constant of brain tissue, V is the voltage difference between the two plates and Vo is the voltage difference between the two plates at zero flow. The depth range of the measurement of CBF by thermal diffusion in brain tissue is about 1.5 mm. Care must be taken not to place the probe on any major surface vessel. The probe must be in contact with the tissue surface in order to provide valid temperature measurements. Because the placement of the probe must be visually inspected in relation to the cortex, it may be difficult to install the probe at a bedside setting.

The measurement of CBF by thermal diffusion has been used post-operatively to monitor patients following aneurysm clipping and resection of cerebral arteriovenous malformations. It has also been used to monitor patients with temporal lobe epilepsy and after severe head injury. The advantage of measuring CBF by thermal diffusion is to be able to continuously monitor cerebral blood flow in real time. The disadvantage is that this technique measures small volumes of tissue, resulting in possible inaccuracies because of the heterogeneity of blood flow distribution.

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