Clear Threshold Value Research Articles

Improved Assessment of Coronary Stenosis Severity Using the Relative Flow Velocity Reserve

Myocardial fractional flow reserve (FFR) is based on pressure measurements. We have now sought to establish a Doppler-based concept of relative flow velocity reserve (RFVR) for the functional assessment of stenosis severity in epicardial coronary arteries. A clear threshold value to discriminate the functional severity of a coronary stenosis does not exist for coronary flow velocity reserve (CVR) based on intracoronary Doppler measurements. In contrast, the concept of FFR, which is based on intracoronary pressure measurements, has been extensively validated. An FFR value below 0.75 reliably indicates a significant stenosis. RFVR is calculated as the ratio between distal CVR in the stenosed target vessel and distal CVR in a nonstenotic reference vessel. In 21 patients, RFVR was determined in 24 target vessels by use of intracoronary adenosine and correlated to the FFR, determined as the ratio of mean poststenotic to aortic pressures, in the target vessel. Stenosis severity was classified according to quantitative coronary angiography analysis. Reference diameter was 3.0+/-0.4 mm (mean+/-SD), and area stenosis was 74+/-15% (range, 40% to 95%). CVRs in the target and reference vessels were 2.1+/-0.5 and 2.6+/-0.7, respectively. FFR ranged from 0.49 to 0.99 (mean, 0.81+/-0.15) and RFVR from 0.53 to 1.0 (mean, 0.82+/-0.13). Poststenotic CVR did not correlate with either percent area stenosis (r=0.27, P=NS) or FFR (r=0.33, P=NS). In contrast, FFR as well as RFVR showed a curvilinear relation to percent area stenosis (r=0.89, P<0.0001 and r=0.79, P<0.0001, respectively). There was a close linear correlation between FFR and RFVR (r=0.91, P<0.0001). RFVR correlates closely to FFR and to percent area stenosis, whereas the correlation of CVR with FFR and percent area stenosis is rather poor. RFVR is a promising new concept for assessment of coronary stenosis severity and clinical decision making based on Doppler measurements.

Vims Sea Carousel: A field instrument for studying sediment transport

Preliminary results from field experiments using a newly developed annular sea-bed flume, vims Sea Carousel, are presented to illustrate a method of in situ determination of critical threshold shear stress necessary for sediment entrainment. This flume has been deployed in the lower Chesapeake Bay and on the inner shelf of the Middle Atlantic Bight. We applied a series of controlled bed shear stresses by changing the stress sequentially, to examine the responses of nearly undisturbed natural sediment beds. An optical backscatter sensor mounted on the flume indicated the occurrences of resuspension of bed material. We identified clear threshold values of bed shear stress for resuspension. Generally, the sandy inner shelf sediment had a higher critical bed shear stress (0.22 Pa) for resuspension than that for silty sediments (0.1-0.19 Pa). Data from partially cohesive sediments in the lower Chesapeake Bay indicate significant seasonal differences in critical bed shear stress, 0.19 Pa at the beginning of summer and 0.11 Pa at the end of summer. These differences may be attributed to bioturbation, the seasonal difference in the microflora living on the grains, or spatial heterogeneity of bed.