What could cause an unrecordable blood pressure in the right arm when the left arm reads 120/70 mm Hg?

Unrecordable Right Arm Blood Pressure with Normal Left Arm Reading

An unrecordable blood pressure in the right arm with a normal left arm reading of 120/70 mmHg most likely indicates complete or near-complete occlusion of the right subclavian or brachiocephalic (innominate) artery, requiring urgent vascular imaging to rule out critical arterial obstruction, aortic dissection, or large vessel vasculitis. 1, 2

Primary Pathological Causes

The inability to obtain any blood pressure reading in one arm while the other arm is normal represents an extreme form of inter-arm blood pressure difference and suggests:

• Subclavian artery occlusion or severe stenosis is the most common cause, where complete or near-complete obstruction prevents adequate blood flow to generate measurable pressure in the affected limb 1
• Brachiocephalic (innominate) artery occlusion should be considered, particularly when the right arm is affected, as this vessel supplies the right subclavian and right common carotid arteries 1
• Acute aortic dissection must be ruled out emergently, especially if accompanied by chest pain, back pain, syncope, or neurological symptoms, as dissection can completely obstruct branch vessels 2, 3
• Takayasu arteritis or giant cell arteritis can cause progressive large vessel occlusion, particularly in younger patients (Takayasu) or those over 50 years (giant cell arteritis) 1, 2

Clinical Evaluation Algorithm

Immediate Assessment

First, confirm the finding is real and not technical error:

• Verify proper cuff size for the right arm circumference, as an inappropriately small cuff on a large arm can make pressure unrecordable 1, 2
• Ensure the right arm is positioned at heart level (mid-sternum/fourth intercostal space), as improper positioning can affect readings 1, 4
• Attempt measurement with the patient's arm fully supported and relaxed, avoiding isometric muscle contraction 1
• Try measuring at the forearm over the radial artery if the upper arm is difficult to cuff 1

Second, perform focused vascular examination:

• Palpate the right brachial, radial, and ulnar pulses—absent or severely diminished pulses confirm arterial obstruction 1
• Check for pulse delay between arms and compare pulse quality 1
• Auscultate for bruits over the right supraclavicular/infraclavicular area, which suggests subclavian stenosis 1
• Examine for signs of right upper extremity ischemia: pallor, coolness, or muscle atrophy 1

Third, assess for emergency conditions:

• Ask about acute chest pain, back pain (especially between shoulder blades), syncope, or neurological symptoms—these suggest aortic dissection requiring immediate imaging 2, 3
• Check for symptoms of posterior circulation ischemia (dizziness, vertigo, ataxia, diplopia) that worsen with right arm use, suggesting subclavian steal syndrome 1
• Evaluate for symptoms of large vessel vasculitis: fever, weight loss, jaw claudication (giant cell arteritis), or constitutional symptoms 1, 2

Urgent Diagnostic Workup

Obtain vascular imaging immediately:

• Duplex ultrasound of subclavian and axillary arteries as the initial non-invasive test to identify stenosis or occlusion 1, 2
• CT angiography or MR angiography of the aortic arch and great vessels to definitively visualize the anatomy, identify the level of obstruction, and rule out aortic dissection 1, 2
• If acute dissection is suspected based on symptoms, proceed directly to emergency CT angiography without delay 2, 3

Additional testing:

• Inflammatory markers (ESR, CRP) if vasculitis is suspected 1
• ECG and cardiac evaluation, as patients with subclavian disease often have concomitant coronary artery disease 1

Management Implications

For ongoing blood pressure monitoring:

• Use the left arm (with the recordable pressure) for all subsequent blood pressure measurements and hypertension management 1, 4, 3
• Document clearly in the medical record that the right arm has unrecordable pressure and the reason 4

Referral and intervention:

• Immediate vascular surgery or interventional cardiology referral is mandatory for evaluation and potential revascularization 2, 3
• Asymptomatic chronic subclavian occlusion may be managed conservatively with medical therapy if collateral circulation is adequate, but this decision requires specialist evaluation 1
• Symptomatic patients (arm claudication, subclavian steal syndrome, or planned coronary bypass requiring internal mammary artery) require revascularization via endovascular or surgical techniques 1

Critical Pitfalls to Avoid

• Do not dismiss this as a measurement error without thorough confirmation—an unrecordable pressure is never normal and demands investigation 2, 4
• Do not delay imaging if acute symptoms are present—aortic dissection is immediately life-threatening and requires emergency diagnosis 2, 3
• Do not use different cuff sizes or techniques between arms during confirmation, as this introduces artificial differences 1, 2
• Do not overlook the need to evaluate for coronary artery disease—patients with subclavian occlusion have high rates of concomitant atherosclerotic disease in other vascular beds 1
• Do not assume chronic occlusion is benign—even asymptomatic patients may develop symptoms with increased arm use or may have unrecognized subclavian steal syndrome 1