What is the management plan for a patient with triphasic and biphasic waveforms on bilateral arterial Doppler ultrasound except for a left popliteal monophasic waveform?

Management of Monophasic Left Popliteal Waveform with Otherwise Normal Bilateral Arterial Doppler

The finding of an isolated monophasic waveform in the left popliteal artery requires urgent vascular imaging (ultrasound, CTA, or MRA) to exclude popliteal artery aneurysm, thrombosis, or significant proximal stenosis, as this represents hemodynamically significant arterial disease even when other segments appear normal. 1, 2

Immediate Diagnostic Priorities

Rule Out Popliteal Artery Aneurysm

• Ultrasound imaging is the most rapid means to confirm or exclude a popliteal aneurysm, which commonly presents with thromboembolism rather than rupture 1, 2
• Approximately 50% of popliteal aneurysms are bilateral, so check for a prominent popliteal pulse in the contralateral leg as a diagnostic clue 1, 2
• Popliteal aneurysms ≥2.0 cm in diameter require surgical repair to prevent thromboembolic complications and limb loss 1, 2
• Up to 50% of asymptomatic popliteal aneurysms become symptomatic within 2 years and 75% within 5 years 1, 2

Assess for Acute or Subacute Thrombosis

• A monophasic waveform indicates loss of the normal retrograde diastolic flow component, suggesting significant upstream obstruction or downstream resistance 3, 4
• If acute ischemia is present (rest pain, absent pulses, pallor, paresthesias), initiate immediate unfractionated heparin and arrange urgent catheter-directed thrombolysis or mechanical thrombectomy to restore distal runoff 2
• Thrombosed popliteal aneurysms account for approximately 10% of acute arterial occlusions in elderly men and are commonly mistaken for embolic events 1

Evaluate for Proximal Iliac or Femoral Disease

• A poor monophasic waveform at the common femoral artery has 92% positive predictive value for significant aortoiliac obstructive disease 3
• Perform segmental leg pressures with pulse volume recordings (PVR) and/or Doppler waveforms to delineate the anatomic level of disease 1
• The absence of triphasic waveforms does not exclude PAD, but their presence (ABI 0.9-1.3 with triphasic pedal waveforms) largely excludes significant disease 1

Structured Diagnostic Algorithm

Step 1: Obtain Ankle-Brachial Index (ABI) and Toe Pressures

• Measure resting ABI on both legs; an ABI ≤0.90 is abnormal and confirms PAD 1
• If ABI >1.40 (noncompressible vessels), obtain toe-brachial index (TBI) with waveforms; TBI <0.70 is abnormal 1
• A monophasic waveform with normal ABI suggests isolated popliteal pathology rather than multilevel disease 3, 5

Step 2: Urgent Duplex Ultrasound of Left Popliteal Artery

• Assess for aneurysm (diameter >2.0 cm), mural thrombus, or luminal stenosis 1, 2
• Evaluate distal runoff vessels for evidence of prior emboli (tibioperoneal occlusions) 1
• Screen the contralateral popliteal artery and abdominal aorta, as 50% of popliteal aneurysms are bilateral and 50% are associated with abdominal aortic aneurysms 1, 2

Step 3: Advanced Imaging if Ultrasound is Inconclusive

• CTA or MRA provides superior anatomic definition for surgical planning if revascularization is being considered 1
• Consider catheter angiography if endovascular intervention is planned 1

Management Based on Findings

If Popliteal Aneurysm ≥2.0 cm Identified

• Proceed with elective surgical repair even if asymptomatic, as the risk of thromboembolic complications is 36-70% over 5-10 years of observation 1, 2
• Saphenous vein bypass provides superior long-term patency and limb salvage rates compared to prosthetic grafts 2
• If acute thrombosis with absent runoff, perform catheter-directed thrombolysis or mechanical thrombectomy first to restore distal vessels, then definitive bypass 1, 2

If Isolated Stenosis Without Aneurysm

• Consider vascular imaging and revascularization if symptoms are functionally limiting or if the patient has a foot ulcer that does not improve within 6 weeks despite optimal management 1
• Initiate guideline-directed medical therapy (GDMT) including antiplatelet agents, statin therapy, and cardiovascular risk factor modification 1

If No Structural Abnormality Found

• Consider popliteal artery entrapment syndrome in young, athletic patients with exertional symptoms 6
• Perform dynamic MRA or ultrasound with plantar flexion maneuvers to assess for functional compression 6

Critical Pitfalls to Avoid

• Do not assume a monophasic waveform is benign simply because other segments show triphasic or biphasic flow—this represents a focal hemodynamic abnormality requiring explanation 3, 5
• Do not delay imaging in the presence of rest pain or tissue loss, as toe pressure <30 mmHg or TcPO2 <25 mmHg indicates critical limb-threatening ischemia requiring urgent revascularization 1
• Do not miss bilateral popliteal aneurysms—always examine the contralateral leg and abdominal aorta 1, 2
• Monophasic waveforms increase particle residence time and eliminate the protective "washout" effect of retrograde diastolic flow, raising thrombosis risk 4
• Normal triphasic waveforms at the common femoral artery do not exclude iliac stenosis (sensitivity 86%, specificity 57%), so abnormal distal waveforms warrant further investigation 5