This technique is based on the Doppler principle that states that, when sound waves are reflected from a moving structure, the frequency of the reflected wave is shifted to a higher or lower frequency (Doppler shift). In the case of blood vessels, the Doppler shift is caused by ultrasonic waves that reflect off moving red blood cells. Coronary flow velocity can be measured by several Doppler techniques: Doppler flow meter, Doppler catheter,174 Doppler guidewire,175 176 an epicardial probe,177 a transoesophageal probe,178 and a transthoracic Doppler probe.176 Guide wires as small as 0 36 mm in diameter, tipped with a 15 MHz piezoelectric ultrasound transducer, allow reliable velocity measurements even in poststenotic areas of coronary arteries.176 Intracoronary Doppler measurements can be affected by positioning of the wire, tortuous segments, and areas of varying luminal dimensions or configurations.179 Non-invasive flow velocity measurement using transthoracic Doppler echocardiography, under the guidance of colour Doppler flow mapping correlates well with measurements obtained invasively by an intracoronary Doppler catheter.176
The temporal resolution of this technique is ideal, allowing continuous online measurements of changes in velocity. The principal drawback of this technique is the fact that velocity rather than flow is measured. For the correct interpretation of results, it is crucial to have precise knowledge of the magnitude and direction of any change in coronary cross sectional area in response to a given intervention. Otherwise, statements regarding changes in absolute flow are not valid.
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