Thermal anemometric assessment of coronary flow reserve with a pressure-sensing guide wire : an in vitro evaluation

Assessment of coronary flow reserve (CFR) with a commercially available pressure-sensor-tipped guide wire using the principle of thermal anemometry could provide major clinical benefits both in determining and in distinguishing between epicardial and microvascular coronary artery disease. In constant-temperature thermal anemometry, the electrical power required to maintain an element at a constant temperature is a measure for the local shear rate. Here, the feasibility of applying this thermoconvection method to a pressure-sensing guide wire is investigated using an in vitro model. A theoretical relation between electrical power and steady shear rate based on boundary layer theory was tested in an experimental set-up. In steady flow, a reproducible relation between electrical power and shear rate was obtained with an overheat temperature of 20 K, which was in good agreement with theory. The relation between shear rate and flow, however, depends on geometry of the artery and position of the guide wire inside the vessel. Although this means that this thermoconvection method is less useful for absolute flow measurements, CFR could be assessed even for unsteady flow using the steady calibration curve with a mean relative difference of (3 ± 5)% compared to CFR derived from the golden standard using an ultrasonic flow measurement device.