The Effect of Lesion Length on Doppler Velocities Used Routinely to Determine Carotid Stenosis Cross-Sectional Severity

Abstract

Background/Objective: Transcutaneous Doppler ultrasound is a fundamental tool in evaluating carotid stenosis cross-sectional severity (CS-CSS) in clinical practice because peak-systolic and end-diastolic velocities (PSV, EDV) increase with angiographic diameter stenosis. We tested the hypothesis that lesion length (LL) may affect PSV and EDV. Methods: CARUS (Carotid Artery IntravasculaR Ultrasound Study) prospectively enrolled 300 consecutive patients (age 47-83 years, 64.3% men, 63.3% symptomatic) with carotid stenosis ≥50% by Doppler ultrasound considered diagnostic (corelab analyst). We correlated stenosis LL (mm) and minimal lumen area (MLA, mm²) with PSV and EDV. Results: IVUS imaging (20 MHz Volcano/Philips) was uncomplicated. As IVUS probe forward/backward movement with systole/diastole ("jumping"-related artifact superimposed on motorized pullback) restrained LL (but not MLA) determination, LL measurement was angiographic. Final data set included 293 patients/stenoses (applicable to seven angiograms unsuitable for LL measurement). Irrespective of CS-CSS, a significant LL effect on PSV and EDV occurred with LL ≥ 7 mm (n = 224/293, i.e., 76.5% study patients/lesions; r = 0.38 and r = 0.35); for MLA irrespective of CS-CCS the coefficients were r = 0.49 (PSV) and r = 0.42 (EDV); p < 0.001 for all. For LL and MLA considered together, the correlations were stronger: r = 0.61 (PSV) and r = 0.54 (EDV); p < 0.0001 for both. Combined LL and MLA effect was represented by the following formulas: PSV = 0.31 × LL/MLA + 2.02 [m/s]; EDV = 0.12 × LL/MLA + 0.63 [m/s], enabling to correct the PSV (EDV)-based assessment of CS-CSS for the LL effect. Conclusions: This study provides, for the first time, systematic evidence that the length of carotid stenosis significantly affects lesional Doppler velocities. We established formulas incorporating the contribution of both stenosis length and its cross-sectional severity to PSV and EDV. We advocate including stenosis length measurement in duplex ultrasound reports when performing PSV (EDV)-based assessment of carotid cross-sectional stenosis severity.

Keywords: carotid stenosis; cross-sectional stenosis; doppler ultrasound; duplex ultrasound; lesion length; stenosis severity; stenotic segment length.

Figure 1

Schematic presentation of the study hypothesis. We hypothesized that carotid lesion length may affect the measured Doppler flow velocities in clinical practice; an effect on flow velocities beyond that of cross-sectional stenosis severity. If the hypothesis is valid, an increase in lesion length (LL2 > LL1) would correlate with a change (to maintain flow, likely—an increase) in flow velocity. This might be relevant to clinical practice as majority of DUS reports usually only include measurements of “peak systolic” (PSV) and “end-diastolic” velocity (EDV). D1—minimal lumen diameter of the stenotic segment in Lesion 1, D2—minimal lumen diameter of the stenotic segment in Lesion 2, LL—length of the lesion, V—flow velocity.

Figure 2

Quantitative measurements of the lesion length and IVUS evaluation of minimal lumen area. (A) Carotid artery catheter angiography in a symptomatic lesion. (B) Quantitative angiography measurement of the total lesion length and stenosis severity by % diameter stenosis. (C) Quantitative angiography measurement of the length of stenotic segment with cross-sectional severity of at least 50% diameter stenosis (DS). (D) IVUS evaluation (in this case, under proximal neuroprotection by transient flow reversal) of the minimal lumen cross-sectional area. Arrows indicate, respectively, total lesion length (B) and the length of stenotic segment with cross-sectional severity of ≥50% diameter stenosis (C). RD is the reference diameter. Diameter stenosis (%) was calculated according to the NASCET method [43]; cf., Supplementary Figure S1.

Figure 3

Carotid lesion length affects flow velocities irrespective of cross-sectional stenosis severity. Graphs present relationship between the stenotic segment length (total lesion length in (A,B) and the length of stenotic segment with ≥50% DS ((C,D); cf., Figure 2) and PSV (A,C) or EDV (B,D). PSV—peak systolic velocity, EDV—end-diastolic velocity, DS—(angiographic) diameter stenosis, n—number.

Figure 4

Stenotic segment length cutoff for the effect on lesional flow velocities ((A)—PSV; (B)—EDV). Note that the effect on flow velocities is statistically significant for carotid lesion lengths (LL) of at least 7 mm. In the present consecutive patient sample, only a minority of lesions (n = 69; 23.5%) showed LL < 7 mm, consistent with the LL effect on flow velocities applicable to a majority (>75%) of carotid stenotic lesions. LL—total lesion length, PSV—peak systolic velocity, EDV—end-diastolic velocity, n—number.

Figure 5

Combined effect of lesion length and cross-sectional stenosis severity on flow velocities in carotid stenotic lesions. Graphs on the left (A,C,E) show a combined effect of LL and MLA on PSV whereas those on the right (B,D,F) demonstrate a combined effect of LL and MLA on EDV. In (A,B) the LL is indexed to MLA by IVUS; in (C,D) the MLA is estimated from conventional quantitative angiography measurements, whereas in (E,F) the MLA is automatically determined from the minimal contrast column density in the stenotic segment in relation to that in the reference segment. Note that the correlation coefficients in (E,F) nearly reach those in (A,B), consistent with a greater accuracy of MLA determination via the automatic densitometric measurement rather than estimation from a conventional (planar) contrast angiography. See text for details. PSV—peak systolic velocity, EDV—end-diastolic velocity, LL—total lesion length, IVUS-MLA—intravascular ultrasound minimal lumen area. QA-MLA—minimal lumen area estimation from conventional (planar) quantitative angiography; QA_DENSITOM-MLA—quantitative angiographic densitometric (contrast column density-based) minimal lumen area, n—number.

Figure 6

Relationship between planar angiographic and IVUS measurement of D_min and cross-sectional stenosis severity. Histogram (A) and Bland–Altman presentation (B) demonstrate systematic underestimation of D_min by angiographic measurement, with mean absolute difference (Abs D) of −0.39 mm and mean relative difference (Rel D) of −22.5%. This, along with inability of the planar angiogram–based estimation to correct for any non-circular lumen (example in Figure 2D), results in a systematic overestimation of cross-sectional stenosis severity (mean Abs D in area stenosis of 13.4%; (C) with a rather wide data scatter on Bland–Altman analysis (D). AS_CIRC—circular estimation of cross-sectional carotid stenosis severity based on angiographic measurement of reference diameter and minimal lumen diameter, AS_IVUS—intravascular ultrasound (IVUS) measurement of area stenosis (AS), QA—quantitative angiography, D_min—minimal lumen diameter.