Hemodynamic significance of coronary jet velocity in patients: Limitations of the Bernouilli equation in small conduits

The objective of this study was to assess the use of coronary stenosis velocity in the determination of translesional pressure gradients. In the physiologic assessment of coronary stenosis, the accelerated intracoronary flow velocity within a narrowing has correlated with minimal lesion cross-sectional area according to the continuity equation. In large conduits the jet velocity can determine pressure gradients when used in the Bernouilli equation. However, the use of intralesional flow velocity for calculation of translesional pressure gradients by the simplified Bernouilli equation ( P = 4V2) may be inaccurate in small (<5 mm diameter) conduits. Translesional pressure (2.2F catheter) and flow velocity (0.018-inch guidewire) were measured in a single coronary artery in 23 patients undergoing diagnostic angiography or angioplasty. The electronically determined mean of phasic proximal and distal pressure and planimetry of the instantaneous phasic pressure gradient were used and compared with the instantaneous velocity calculations of pressure by the simplified Bernouilli formula with both maximal jet velocity and a modified formula including proximal velocity. The mean measured translesional pressure gradient was 18 ± 13 mm Hg (range 0 to 50 mm Hg) and was equivalent to the instantaneous average pressure gradient by planimetry. The maximal jet velocity was 125 ± 40 cm/sec (range 63 to 250 cm/sec), yielding a calculated pressure gradient of 3 ± 3 mm Hg. The calculated pressure gradient by the simplified Bernouilli equation correlated poorly with the measured translesional gradient (r = 0.27, F = 1.63, P = 0.21). The correlation between stenosis cross-sectional area with flow velocity values and directly measured pressure gradients was weak and neared statistical significance (r = 0.421, F = 4.1, P = 0.057). No correlation was found between the Doppler-derived cross-sectional area and angiographic indexes of stenosis severity. These findings indicate that the intralesional jet velocity cannot be used to estimate translesional coronary pressure gradients.