What are the potential causes of a significant difference in blood pressure readings between the right and left arms, with hypertension (150/100 mmHg on the right arm and 180/110 mmHg on the left arm) in a patient?

Causes of Significant Inter-Arm Blood Pressure Difference (30 mmHg)

A 30 mmHg systolic blood pressure difference between arms (150/100 right vs 180/110 left) is abnormal and requires urgent vascular evaluation for potentially life-threatening conditions, particularly subclavian artery stenosis, aortic dissection, or other large vessel pathology. 1, 2

Pathological Causes Requiring Urgent Investigation

Primary Vascular Pathology

• Subclavian or innominate artery stenosis is the most common pathological cause of inter-arm differences ≥15-20 mmHg, creating a pressure gradient across the narrowed vessel that results in lower blood pressure distal to the obstruction 2
• Aortic dissection must be urgently excluded, especially if accompanied by chest pain, back pain between shoulder blades, syncope, or neurological symptoms—this represents a medical emergency 2, 3
• Aortic coarctation should be considered, particularly in younger patients with consistently elevated inter-arm differences ≥20 mmHg 2
• Takayasu arteritis or other large vessel vasculitis can cause significant inter-arm differences through inflammatory arterial stenosis 2

Clinical Context

• Patients with peripheral artery disease have substantially increased risk for subclavian stenosis and should be evaluated more aggressively 2
• The 30 mmHg difference in this case far exceeds the threshold of 20 mmHg that mandates further investigation per international guidelines 1

Measurement-Related Causes (Must Be Excluded First)

Technical Errors

• Arm position errors can create artificial differences of 10 mmHg or more (2 mmHg for every inch above or below heart level) 2
• Inappropriate cuff sizing between arms produces false differences if cuffs are not properly sized for each arm circumference 1, 2
• Sequential rather than simultaneous measurement introduces variability, though a 30 mmHg difference would still be significant 2
• Isometric muscle contraction from unsupported arms raises blood pressure artificially 2

Normal Physiological Variation

• Approximately 20% of normal individuals have inter-arm differences >10 mmHg, but differences ≥20 mmHg are uncommon and warrant investigation 2, 4
• Mean inter-arm differences in healthy populations are only 1-3 mmHg systolic and approximately 1 mmHg diastolic 2
• A 30 mmHg difference far exceeds normal physiological variation and cannot be dismissed as benign 5, 4

Immediate Diagnostic Approach

Confirm the Finding

• Repeat measurements simultaneously in both arms using validated automated devices with appropriately sized cuffs to rule out measurement error 1, 2
• Ensure both arms are positioned at heart level with back and arms supported, after 5 minutes of seated rest 1, 2
• Take three measurements in each arm, 1 minute apart, and use the average of the last two readings 1

Vascular Examination

• Check for diminished or delayed pulses in the affected limb (the arm with lower pressure) 2
• Auscultate for bruits over subclavian and carotid arteries 2
• Assess for signs of peripheral vascular disease, including abnormal lower extremity pulses 2
• Evaluate for symptoms of vertebral artery steal, such as dizziness or syncope with arm use 2

Urgent Imaging

• Order duplex ultrasound of subclavian and axillary arteries as initial vascular imaging 2
• Consider CT angiography or MR angiography if clinical suspicion for aortic pathology (dissection or coarctation) exists 2, 3
• If acute aortic dissection is suspected with chest pain, syncope, or neurological symptoms, immediate emergency evaluation with CT angiography is warranted 2, 3

Management Implications

Blood Pressure Monitoring

• Always use the arm with the higher reading (left arm, 180/110 mmHg in this case) for all subsequent blood pressure measurements and hypertension management to avoid underestimating blood pressure and undertreating hypertension 1, 2
• This patient meets criteria for Grade 2 hypertension (≥160/100 mmHg) and requires immediate antihypertensive therapy regardless of the underlying cause of the inter-arm difference 1

Specialist Referral

• A 30 mmHg difference requires referral to vascular surgery or cardiology for further evaluation and management 2
• Patients with confirmed subclavian stenosis may need vascular intervention and should be evaluated for concomitant coronary artery disease 2

Critical Pitfalls to Avoid

• Do not dismiss large inter-arm differences as measurement error without proper confirmation with repeat simultaneous measurements 2
• Do not use different sized cuffs or different measurement techniques between arms, as this creates artificial differences 1, 2
• Do not fail to position both arms at heart level during measurement 1, 2
• Do not overlook the need for urgent evaluation when differences exceed 20 mmHg—this threshold signals potential serious vascular pathology 1, 2
• Do not measure blood pressure in arms with arteriovenous fistulas or after axillary lymph node dissection, as this creates artificial differences 2