Should I obtain a bilateral ankle‑brachial index (ABI) with toe‑brachial index (TBI) and Doppler waveform assessment before switching this older adult with peripheral vascular disease who has peripheral vasospasm on metoprolol to diltiazem?

Yes, Obtain ABI/TBI Before Switching to Diltiazem

You must obtain a bilateral ankle-brachial index (ABI) with toe-brachial index (TBI) and Doppler waveform assessment before switching this older adult with peripheral vascular disease from metoprolol to diltiazem. This baseline vascular assessment is essential to confirm PAD severity, ensure diltiazem is safe in the context of existing peripheral disease, and establish a reference point for monitoring any medication-related changes in limb perfusion. 1, 2

Why This Assessment Is Mandatory

Establish Baseline PAD Severity

• The ACC/AHA gives a Class I recommendation that ABI with TBI and Doppler waveforms be measured in all patients with known peripheral vascular disease to establish baseline disease severity before changing rate-control medications. 1, 2
• The severity of PAD directly determines which alternative rate-control agents are appropriate—for example, the safety of vasodilatory calcium-channel blockers like diltiazem depends on documented PAD severity. 2
• An ABI ≤ 0.90 confirms hemodynamically significant PAD, while ABI 0.91–0.99 indicates borderline disease requiring additional testing. 1

Age-Based Screening Threshold

• ACC/AHA guidelines lowered the screening age threshold to ≥65 years for all patients, regardless of symptoms, based on a 21% PAD prevalence in this age group. 2
• Your patient qualifies for ABI testing based on age alone, even without prior formal PAD diagnosis. 2

Medication Safety Considerations

• Diltiazem is a vasodilating calcium-channel blocker that can theoretically worsen limb perfusion in patients with severe PAD by causing peripheral vasodilation and "steal" phenomena. 2
• The documented ABI value guides safe dosing: patients with ABI 1.00–1.40 can receive full-dose diltiazem, while those with ABI 0.91–0.99 require lower initial doses and closer monitoring. 2
• If ABI ≤ 0.90, you must initiate aggressive cardiovascular risk reduction (statin, antiplatelet therapy, smoking cessation) regardless of the medication switch. 2

Specific Testing Protocol

Order Components

• Request "Bilateral ankle-brachial index with toe-brachial index and Doppler waveform analysis" using this exact terminology to ensure comprehensive measurement. 2
• Document all major vascular risk factors (smoking history, diabetes, hypertension, hyperlipidemia, chronic kidney disease) in the order to aid interpretation. 2

Measurement Sequence

• Position the patient supine for at least 10 minutes before any measurements to allow hemodynamic stabilization. 1, 2
• Measure bilateral brachial systolic pressures first, then bilateral dorsalis pedis and posterior tibial artery pressures using continuous-wave Doppler. 1, 2
• For each leg, use the higher of the dorsalis pedis or posterior tibial pressure as the numerator when calculating ABI. 1, 2
• Calculate ABI = higher ankle pressure ÷ higher brachial pressure separately for each leg. 2

Interpretation Thresholds

ABI Value	Interpretation	Action Before Diltiazem
≤ 0.90	Confirms PAD	Start aggressive CV risk reduction; consider lower diltiazem dose
0.91–0.99	Borderline PAD	Use caution; start diltiazem at lower initial dose
1.00–1.40	Normal range	Full-dose diltiazem is safe from vascular standpoint
> 1.40	Non-compressible vessels	ABI invalid; obtain TBI (mandatory)

1, 2

When TBI Is Required

• Measure TBI immediately if ABI > 1.40, which indicates arterial calcification rendering ABI unreliable. 1
• TBI is also mandatory in patients with diabetes, chronic kidney disease, or age > 70 years due to high prevalence of arterial calcification. 2
• TBI < 0.70 confirms PAD when ABI is unreliable. 1

Doppler Waveform Analysis

• Document waveform at each ankle artery: triphasic waveforms essentially exclude significant PAD, while monophasic or absent waveforms suggest significant disease. 2, 3
• Waveform analysis provides additional diagnostic information when ABI values are borderline or non-compressible. 1

Additional Testing Considerations

Exercise ABI Testing

• Add post-exercise ABI measurement if the patient reports exertional leg symptoms but has a resting ABI of 0.91–1.40. 1, 2
• A post-exercise ankle pressure decrease > 30 mmHg or ABI decrease > 20% confirms PAD even when resting ABI appears normal. 1, 4
• Nearly half of symptomatic patients referred for PAD evaluation have normal resting ABI, and exercise testing enhances sensitivity for PAD detection. 4

If Results Are Equivocal

• When ABI/TBI results are discordant or inconclusive, obtain transcutaneous oxygen pressure (TcPO₂) or skin perfusion pressure (SPP) to assess tissue perfusion. 1, 2
• Segmental pressures with waveform analysis can provide precise anatomic localization if revascularization is being considered. 1, 2

Common Pitfalls to Avoid

Never Assume PAD Severity Without Objective Testing

• Even skilled examiners can detect pulses despite significant ischemia, so never assume PAD is absent or mild based solely on palpable pulses. 2, 3
• Clinical examination has low sensitivity for detecting mild PAD in asymptomatic persons. 1

Never Rely on ABI Alone in High-Risk Patients

• In diabetic patients, arterial calcification frequently produces falsely normal or elevated ABI readings despite significant stenotic disease. 1, 2, 5
• Always obtain TBI in patients with diabetes, chronic kidney disease, or age > 70 years, regardless of ABI result. 2, 5

Never Use Single-Leg Measurement

• Always measure bilateral ABI and use the lower value for cardiovascular risk assessment, as unilateral measurement can miss PAD. 2

Never Skip This Step Before Medication Changes

• Switching from a β-blocker to a vasodilating calcium-channel blocker without baseline vascular assessment creates medicolegal risk and potential for harm if severe PAD is present. 2

Management After Testing

If ABI Confirms PAD (≤ 0.90)

• Initiate aggressive cardiovascular risk reduction: statin therapy, antiplatelet therapy (aspirin or clopidogrel), smoking cessation support, blood pressure control. 2
• Consider lower initial diltiazem dose and monitor closely for worsening claudication symptoms. 2
• Refer to vascular surgery if ankle pressure < 50 mmHg or ABI < 0.5. 2

If ABI Is Borderline (0.91–0.99)

• Start diltiazem at lower initial dose and titrate cautiously. 2
• If exertional leg symptoms are present, add exercise treadmill ABI test to evaluate functional PAD. 1, 2

If ABI Is Normal (1.00–1.40)

• Full-dose diltiazem is safe from a vascular standpoint. 2
• Still optimize cardiovascular risk factors, as PAD is a coronary heart disease risk equivalent. 2