Coronary blood flow velocity measurements by conventional intravascular catheter-based Doppler devices are well known to be affected by catheter size. Moreover, it is of clinical importance that the assessment of maximum vasodilator capacity, i.e., the coronary reserve, might be considerably affected by a shape change of the velocity profile under hyperemia. Therefore, the present in vitro study aimed to assess the impact of a small-size Doppler guidewire on the velocity profiles interrogated in tubes with diameters corresponding to the epicardial coronary arteries at clinically relevant flow rates. A 0.014" guidewire was inserted into four serially connected silicone tubes of known diameter, which were perfused with discarded human whole blood by means of a roller pump. In order to determine the effect of the Doppler guidewire on the velocity profile antegrade and retrograde perfusion were carried out in each vessel segment. Vm, the true mean velocity, was calculated from the time collected flow divided by the corresponding vessel cross-sectional area. Average peak velocity (APV) measurements were obtained by pulling back the Doppler device across each of the four vessel segments with given flow rates ranging from 1.14-5.88 ml/s in antegrade and retrograde direction. The shape factor of the recorded velocity profile (fp) is defined as vm/APV and is generally assumed to be close to 0.5. Antegrade perfusion: APV = 1.63 vm + 5.35 (R2 = 0.98); fp = 0.001 vm + 0.51. Retrograde perfusion: APV = 1.71 vm + 3.33 (R2 = 0.98); fp = 0.001 vm + 0.52. Due to the constant relationship between APV and vm, velocity profiles within vessels of epicardial coronary artery size are not substantially disturbed by the presence of the Doppler guidewire. The slight, but significant increase of experimental fp with increasing flow is at variance with the theoretically expected fp values.