What does a slightly flattened waveform on an Ankle-Brachial Index (ABI) test indicate?

Interpretation of Flattened Waveforms on Ankle-Brachial Index Testing

Slightly flattened waveforms on an Ankle-Brachial Index (ABI) test indicate mild to moderate peripheral artery disease (PAD), even when the numerical ABI value may be within normal or borderline range.

Waveform Analysis in ABI Testing

Arterial Doppler waveforms provide critical diagnostic information beyond the numerical ABI value:

• Normal waveform: Triphasic pattern with sharp systolic upstroke, followed by early diastolic reversal and late diastolic forward flow
• Flattened waveform: Monophasic or biphasic pattern with reduced amplitude and loss of the normal triphasic configuration

Clinical Significance of Flattened Waveforms

Flattened waveforms indicate:

• Reduced arterial flow velocity
• Increased peripheral resistance
• Presence of proximal arterial stenosis
• Early PAD that may not yet be reflected in the ABI numerical value

Diagnostic Implications

According to the American Heart Association guidelines, waveform analysis should be considered alongside numerical ABI values 1:

• The presence of triphasic pedal Doppler arterial waveforms provides strong evidence for the absence of PAD 1
• Flattened waveforms may detect PAD even when the ABI is in the normal range (0.91-1.40) 1
• Waveform analysis is particularly valuable in patients with diabetes or chronic kidney disease who may have falsely elevated ABI due to noncompressible vessels 1

Clinical Correlation

When flattened waveforms are detected:

1. Consider additional testing:

   • Toe-brachial index (TBI) measurement (abnormal if <0.70) 1
   • Exercise treadmill ABI testing to unmask functional PAD 1
   • Segmental pressure measurements with pulse volume recordings (PVRs) to localize disease 1

2. Evaluate for cardiovascular risk factors:

   • Even mild PAD indicated by flattened waveforms is associated with increased risk of cardiovascular events 2
   • Patients with abnormal waveforms but normal ABI have approximately twice the risk of myocardial infarction and cardiovascular death compared to those with normal waveforms 2

Management Recommendations

For patients with flattened waveforms on ABI:

1. Comprehensive cardiovascular risk assessment 2

2. Initiate guideline-directed medical therapy including:

   • Antiplatelet therapy
   • High-intensity statin therapy
   • Blood pressure control
   • Diabetes management
   • Smoking cessation

3. Consider vascular specialist referral if:

   • Symptoms are present (claudication, rest pain)
   • ABI is borderline (0.91-0.99) with flattened waveforms
   • Patient has multiple cardiovascular risk factors 2

Common Pitfalls to Avoid

1. Relying solely on numerical ABI values without assessing waveform morphology
2. Dismissing borderline ABI values (0.91-0.99) when accompanied by flattened waveforms
3. Failing to consider alternative testing in patients with diabetes or renal disease who may have noncompressible vessels
4. Missing the cardiovascular risk implications of flattened waveforms even when ABI values appear normal

In conclusion, flattened waveforms on ABI testing represent an important early marker of PAD that warrants clinical attention, further diagnostic evaluation, and appropriate cardiovascular risk reduction measures, even when numerical ABI values may be within normal range.