Pressure Discrepancies Between Angiography and Duplex Ultrasound in Iliac Artery Assessment
Angiographic pressure measurements during catheterization are more accurate than duplex ultrasound-derived pressure estimates for the iliac artery, as catheter-based measurements directly assess intraluminal pressure gradients while duplex ultrasound uses velocity-based calculations that can underestimate true hemodynamic significance.
Why Angiographic Pressures May Appear Higher
Direct Measurement vs. Calculated Estimation
Catheter angiography measures actual intraluminal pressure gradients directly, particularly when vasodilators like papaverine are administered to unmask hemodynamically significant lesions that may not be apparent at rest 1.
Duplex ultrasound derives pressure gradients indirectly using the simplified Bernoulli equation (ΔP = 4V²), which converts peak systolic velocity measurements into estimated pressure gradients 2.
The correlation between catheter-measured and duplex-derived pressure gradients is only moderate (r = 0.77-0.79), meaning duplex can miss or underestimate the true hemodynamic impact 2.
Technical Limitations of Duplex Ultrasound in the Iliac Region
Accuracy of duplex examination in the iliac arteries is significantly diminished when bowel gas or vessel tortuosity obscures visualization, which is common in this anatomic location 1.
Dense calcification can obscure flow assessment, particularly when flow velocity is already reduced, leading to underestimation of stenosis severity 1.
Duplex ultrasound has lower diagnostic accuracy for iliac disease (Cohen's κ agreement of 0.91) compared to CT angiography (Cohen's κ 1.0) when measured against digital subtraction angiography as the reference standard 3, 4.
Hemodynamic Factors During Angiography
Angiography can measure pressure gradients both at baseline and during provocative maneuvers, including hip flexion and following vasodilator administration, which reveals functional stenoses that may not be apparent on resting duplex examination 1.
The ability to assess pressure gradients across narrowings during various physiologic states provides superior hemodynamic information compared to static duplex velocity measurements 1.
Which Pressure Measurement Is More Accurate
Catheter-Based Pressure Measurement Is the Gold Standard
Direct intraarterial pressure measurement during angiography remains the reference standard for determining the hemodynamic significance of iliac artery stenoses 1, 2.
Studies comparing duplex-derived pressure gradients to catheter measurements show that while duplex provides "acceptable approximation," it is not equivalent to direct measurement 2.
Angiography has inconsistent correlation between morphologic appearance and hemodynamic effects, but direct pressure measurement resolves this limitation by providing functional assessment 1.
When Duplex Ultrasound May Underestimate Disease
In the presence of multiple sequential lesions, duplex ultrasound accuracy decreases significantly, as the cumulative hemodynamic effect is difficult to assess with velocity measurements alone 1.
Research demonstrates that angiography can underestimate stenosis severity when compared to pressure measurements, with some lesions showing hemodynamic significance on pressure measurement despite appearing less severe angiographically 5.
Duplex ultrasound showed poorer correlation with intraarterial pressure measurements compared to the correlation between angiography and duplex scanning 6.
Clinical Implications and Practical Approach
Algorithmic Decision-Making
For patients with suspected iliac artery disease requiring revascularization, obtain CT angiography as the primary anatomic imaging modality (sensitivity and specificity 90-100% for detecting significant stenoses) 1, 7.
Reserve catheter angiography with pressure measurement for cases where non-invasive imaging shows borderline stenosis or when the clinical presentation doesn't match imaging findings 1.
Use duplex ultrasound as an initial screening tool (sensitivity and specificity approximately 90-95% for iliac stenoses >50%), but recognize its limitations in the iliac region and obtain confirmatory imaging when intervention is planned 1.
Critical Pitfalls to Avoid
Do not rely solely on duplex ultrasound velocity criteria in the iliac arteries when planning revascularization, as bowel gas interference and vessel tortuosity commonly limit accuracy 1.
Do not assume that normal resting duplex examination excludes hemodynamically significant iliac disease, as ankle-brachial indices may be normal at rest in isolated iliac occlusive disease, requiring post-exercise ABI testing 1.
When duplex and clinical findings are discordant, proceed to CT angiography or catheter-based pressure measurement rather than assuming the duplex examination is definitive 1, 7.
Optimal Diagnostic Strategy
For diagnostic evaluation: Start with duplex ultrasound and ABI, proceed to CT angiography for anatomic detail and treatment planning 7.
For intervention planning: CT angiography provides superior anatomic detail for the iliac region compared to duplex ultrasound (Cohen's κ 1.0 vs 0.91) 3, 4.
For hemodynamic assessment: Direct pressure measurement during catheter angiography remains the gold standard when non-invasive testing is equivocal or intervention is planned 1, 2.