The development of selective coronary angiography in the 1960s by Sones 1 offered a dramatic improvement in the management of ischaemic heart disease. However, decisions could only be made on ‘shadowgrams’ of coronary stenoses. Early invasive assessment of stenosis severity and coronary flow have been developed, based on the Doppler effect, using a piezoelectric crystal mounted at the tip of standard Sones catheters 2 or on intraluminal probes. 3However, partial obstruction of the coronary ostium by these devices limited their clinical use.
When Gruntzig performed his first percutaneous transluminal coronary angioplasty (PTCA) in 1977, the concept of physiological assessment of the results of percutaneous interventions (PCI) was already introduced. 4 The double-lumen dilatation catheters permitted balloon inflation on one side and the recording of the distal coronary pressure on the other side. Trans-stenotic pressure gradients were used to monitor the procedures – a residual trans-stenotic gradient less than 20mmHg was considered optimal. 5 However, with technical developments like the flexible-tipped guidewires introduced in the lumen, previously used to measure pressure, and the introduction of low-profile balloons, pressure recordings were more difficult to perform. Moreover, the relations between the measured pressure gradient, the diameter stenosis and the lesion length were imprecisely known, and were dependent on the presence of the catheter itself in the stenosis. 6 With the development of parameters to assess the functional significance of a stenosis from its geometry, 7 it was considered that the available anatomical information was sufficient. Nowadays, the angiogram is still considered by most physicians to be the ‘gold standard’ for defining coronary anatomy. However, its resolution is limited and numerous confounding factors (vessel tortuosity, overlap of structures, etc.) result in a marked disparity between the apparent severity of a lesion and its physiological effect. 8-10
Principles of Doppler Velocimetry
An observer moving towards a sound source will hear a tone with higher frequency than when stationary and an observer moving away from the source will hear a tone of lower frequency. This change in frequency is called the Doppler effect. This principle is applied in practice by mounting a piezoelectric crystal that emits and receives high-frequency sounds on the tip of an intravascular catheter. The blood flow velocity alters the return frequency, causing the Doppler shift. Electronic circuits performing spectral analysis of the received signal allow continuous determination of the Doppler shift, and of blood flow velocity, based on the following Doppler equation:
V=velocity of blood flow
F0 = transmitting (transducer) frequency
F1 = returning frequency
C = constant: speed of sound in blood
φ = angle of incidence.
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