What are the causes and management of unequal blood pressure in both limbs?

Causes of Unequal Blood Pressure Between Limbs

An inter-arm blood pressure difference >15-20 mmHg is abnormal and most commonly indicates subclavian or innominate artery stenosis, particularly in patients with peripheral artery disease, while differences between upper and lower limbs suggest aortic coarctation. 1

Upper Limb (Arm-to-Arm) Differences

Pathological Causes

Subclavian or innominate artery stenosis is the primary pathological cause when systolic blood pressure differs by >15-20 mmHg between arms. 1

• Atherosclerotic disease: Patients with confirmed peripheral artery disease (PAD) are at significantly increased risk for subclavian artery stenosis 1
• Takayasu arteritis or giant cell arteritis: These vasculitides can cause occlusive disease in major arteries to all four limbs, making blood pressure measurement unreliable in any extremity 1
• Thoracic aortic dissection: Dissection-related occlusion of aortic branch arteries results in erroneously low blood pressure readings in the affected limb, requiring measurement in both arms and sometimes both legs to determine the highest central blood pressure 1

Clinical Significance of Arm-to-Arm Differences

Measuring blood pressure in both arms is essential for accurate diagnosis and management. 1

• Identifies the arm with highest systolic pressure, which is required for accurate ankle-brachial index (ABI) measurement 1
• Allows more accurate blood pressure measurement for hypertension treatment (subsequent measurements should use the arm with higher readings) 1
• In asymptomatic patients (no arm claudication or vertebral artery steal symptoms), no further imaging or intervention is warranted even with >15-20 mmHg difference 1

Normal Physiological Variation

While pathological differences are >15-20 mmHg, smaller differences are common and usually benign:

• Mean inter-arm differences in healthy individuals are typically 1-2 mmHg systolic 2
• However, absolute individual differences of 5-10 mmHg occur in 15-21% of people and are clinically significant enough to affect hypertension diagnosis 3, 4
• Differences >10 mmHg systolic occur in approximately 20% of individuals, and >20 mmHg in 3.5% 4

Upper-to-Lower Limb Differences

Aortic Coarctation

An increased non-invasive gradient between upper and lower limbs (decreased ABI) with hypertension suggests coarctation of the aorta. 1

• Diagnostic criteria for significant coarctation: 1

  • Non-invasive right arm-to-leg gradient >35 mmHg peak or >20 mmHg mean
  • Blood pressure gradient >20 mmHg between arms and legs

  • 50% narrowing on any imaging modality

• Clinical presentation: Symptoms reflect pre-stenotic hypertension (headache, nosebleeds) and post-stenotic hypoperfusion (abdominal angina, claudication) 1

• Management approach: Blood pressure measurements at both arms and one lower extremity are required in patients with suspected coarctation 1

Normal Physiological Amplification

Systolic blood pressure and pulse pressure are physiologically amplified from the aorta toward peripheral arteries, increasing with distance from the heart. 1

• In young subjects, ankle pressure can be 30% higher than arm pressure 1
• In older subjects, ankle and arm pressures tend to equalize 1
• The reference value of 140 mmHg systolic is valid only for brachial pressure; lower limb reference values are essentially unknown 1

Management Algorithm

Initial Assessment

1. Measure blood pressure in both arms simultaneously or sequentially at the first clinical encounter 1

   • Use proper technique: patient seated quietly for ≥5 minutes, feet on floor, arm supported at heart level 1
   • Ensure appropriate cuff size (bladder encircles ≥80% of arm) 1, 5

2. If difference >10 mmHg systolic is found: 3, 4

   • Use the arm with higher pressure for all subsequent measurements
   • Document the difference in the medical record

3. If difference >15-20 mmHg systolic: 1

   • Suspect subclavian/innominate artery stenosis
   • Assess for symptoms of arm claudication or vertebral artery steal
   • If symptomatic or patient has known PAD, consider vascular imaging
   • If asymptomatic, no further workup needed but continue using higher-pressure arm

Special Populations

Patients with PAD: Must have bilateral arm blood pressure measurement as part of initial assessment, given increased risk of subclavian stenosis 1

Suspected aortic dissection: Measure blood pressure in both arms AND both legs to determine highest central pressure, as branch vessel occlusion can give falsely low readings 1

Suspected coarctation: Measure blood pressure in both arms and one lower extremity; pursue imaging if upper-to-lower limb gradient suggests significant stenosis 1

Common Pitfalls to Avoid

• Using too small a cuff in obese patients leads to overestimation of blood pressure and potential misclassification as hypertensive 1
• Measuring blood pressure in only one arm can miss clinically significant inter-arm differences, leading to misdiagnosis of hypertension or inadequate blood pressure control 3, 4
• Confusing post-mastectomy status with lymphedema: Blood pressure can be measured in mastectomy limbs unless active lymphedema is present 5
• In patients with arrhythmias (especially atrial fibrillation), single automated measurements are frequently inaccurate; measure several times and average the values 1