Effect of stenosis geometry on the Doppler-catheter gradient relation in vitro: a manifestation of pressure recovery.

OBJECTIVES: This study investigated the effect of stenosis geometry on the Doppler-catheter gradient relation.
BACKGROUND: Although gradient estimation by Doppler ultrasound has been shown to be accurate in various clinical and in vitro settings, there have also been reports of substantial discrepancies between Doppler and catheter gradients. These conflicting results may be due to differences in geometry and hemodynamic characteristics of flow obstructions.
METHODS: Stenoses of various geometry were simultaneously studied with continuous wave Doppler and catheter technique in a well controlled pulsatile flow model.
RESULTS: Doppler and catheter gradients correlated very well regardless of stenosis geometry and site of distal catheter measurement (r = 0.98 to 0.99, SEE = 1.8 to 5.3 mm Hg). When the catheter was pulled back through the stenosis, the highest gradients were found in or close to the stenosis. When these catheter gradients were compared with Doppler gradients, the agreement between the two techniques was excellent regardless of stenosis geometry (slope 0.97; mean difference 0.6 +/- 2.0 mm Hg). However, when distal pressures were measured 10 cm downstream from the stenotic segment, the slope of the regression line, and therefore the agreement between Doppler and catheter gradients, differed for the different stenosis types (slopes from 0.98 to 1.69). In stenoses with abrupt narrowing and abrupt expansion, agreement was acceptable. Doppler gradients were only slightly greater than catheter gradients (mean difference 4.5 +/- 5.2 mm Hg). In stenoses with a gradually tapering inlet and outlet, the Doppler-catheter gradient relation was dependent on the outflow angle. Good agreement was found for an angle of 60 degrees (mean difference 0.6 +/- 1.8 mm Hg). In stenoses with a 40 degrees outflow angle, Doppler gradients exceeded the catheter gradients by 13% on average; for stenoses with a 20 degrees outflow angle, Doppler gradients exceeded catheter gradients by 46 +/- 11.4%, with differences as great as 65 mm Hg. These results were identical for stenoses gradually tapering outward to the distal tubing diameter and those with abrupt expansion after 2 cm of gradual expansion. The results were also not affected by changing the inflow angle from 20 degrees to 60 degrees. However, an abrupt narrowing instead of a tapering inlet significantly altered the Doppler-catheter gradient relation (p < 0.001); Doppler gradients exceeded the catheter gradients by 34 +/- 10% for this stenosis type.
CONCLUSIONS: Doppler gradients accurately reflect the highest gradients across flow obstructions that occur in the vena contracta. However, these gradients may be significantly greater than catheter gradients that are measured farther downstream, as is usually the case in clinical catheterization studies. These discrepancies are due to pressure recovery. The magnitude of pressure recovery is highly dependent on the stenosis geometry, which therefore significantly affects the Doppler-catheter gradient relation. It is the outflow geometry that predominantly influences this relation, but the shape of the inlet may affect the results as well. Although pressure recovery occurs even in stenoses with abrupt narrowing and abrupt expansion, the phenomenon is most likely to become clinically relevant in stenoses with a gradually tapering inlet and outlet with an outflow angle < or = 20 degrees.