In aortic dissection, the arm supplied by an artery that the dissection flap compresses or obstructs reads lower, not higher. There is no universal rule that the left arm or the right arm is always the elevated one. Which side drops depends entirely on where the tear in the aorta extends and which branch vessels it compromises. Because the anatomy of Type A dissections often involves the brachiocephalic artery feeding the right arm, the right arm frequently shows the lower reading, but this is far from guaranteed. The clinically meaningful finding is a large gap between the two arms, regardless of direction.
Why the Compromised Arm Reads Lower
The aorta branches into major arteries that supply each arm. On the right side, the brachiocephalic (innominate) artery splits into the right subclavian and right common carotid. On the left, the left subclavian artery comes directly off the aortic arch. When a dissection flap tears through the wall of the aorta and propagates along its length, it can partially block or narrow the opening of any of these branches. The arm downstream of the obstructed branch gets less blood flow and registers a lower blood pressure.
In a study of patients with acute aortic dissection, those who showed a left-minus-right systolic difference exceeding 20 mmHg all had Type A dissections, and imaging confirmed the dissection extended to the brachiocephalic artery just before it became the right common carotid artery.1PubMed. Characteristics of Inter-Arm Difference in Blood Pressure in Acute Aortic Dissection In those cases, the right arm was the low side because the brachiocephalic artery was compromised. But if a dissection extends to involve the left subclavian artery instead, the left arm reads lower. And if neither subclavian branch is obstructed, no significant arm-to-arm gap appears at all, even though the dissection is still present.
This is the core concept that trips people up: the higher arm is the one that still has normal flow. The lower arm is the one whose supply is being choked off. You are not looking for which arm is elevated above normal. You are looking for which arm has dropped below what it should be.
How Large a Difference Matters
A small gap between arms is normal and does not suggest anything dangerous. Healthy young adults typically show a systolic inter-arm difference around 5 to 6 mmHg on average, with roughly one in eight showing a gap exceeding 10 mmHg even under careful measurement.2PubMed Central. Inter-arm blood pressure differences in young, healthy patients That baseline variability matters, because it sets the floor for what counts as abnormal.
Research on patients presenting to emergency departments has identified specific thresholds. An inter-arm systolic blood pressure difference of 10 mmHg or more showed a sensitivity of about 62% and a specificity near 89% for acute aortic dissection in one study, meaning it correctly flagged roughly six in ten dissection patients while only falsely flagging about one in nine non-dissection patients.3PubMed Central. The Predictive Value of Inter Arm Blood Pressure Difference, Inter Leg Blood Pressure Difference and Ankle Brachial Index for Acute Aortic Dissection That same study found the average inter-arm systolic difference in dissection patients was about 15 mmHg, compared to roughly 4 mmHg in patients without dissection.
A higher threshold of 20 mmHg has also been studied. In one emergency-department cohort, a systolic difference exceeding 20 mmHg was significantly associated with acute aortic dissection, roughly tripling the odds that the patient had the condition.4PubMed. Bilateral blood pressure differential as a clinical marker for acute aortic dissection in the emergency department Both thresholds are used clinically, but neither is perfect on its own. A 10 mmHg cutoff catches more cases but generates more false alarms. A 20 mmHg cutoff is more specific but misses patients whose dissection has not severely obstructed a branch vessel.
Why a Missing Pulse Matters More Than the Numbers
Blood pressure difference gets a lot of attention in textbooks, but the evidence suggests that a missing or weakened pulse in one arm is actually a stronger sign of dissection. In the same emergency-department study, pulse deficit had a diagnostic odds ratio of nearly 29, compared to about 2.7 for a systolic blood pressure gap exceeding 20 mmHg.4PubMed. Bilateral blood pressure differential as a clinical marker for acute aortic dissection in the emergency department That is a striking difference. A pulse you cannot feel at the wrist is a much more ominous finding than a 25-point gap on two blood pressure cuffs.
This makes physiological sense. A dissection flap that merely narrows a branch vessel will reduce pressure somewhat but still allow enough flow to produce a detectable pulse. One that nearly or fully occludes the vessel wipes out both the pulse and the pressure. So pulse deficit signals a more severe obstruction and, in practice, carries more diagnostic weight. Clinicians checking for dissection generally palpate both radial pulses and both femoral pulses, not just strap on two cuffs.
The two findings are not interchangeable, though. Some patients have a notable blood pressure gap without pulse deficit, and others have a weakened pulse without a dramatic blood pressure difference. The combination of both strengthens suspicion, but interestingly the combined diagnostic odds ratio in the study cited above was only about 4.2, not the sum of both individual measures. The two overlap enough that stacking them doesn’t multiply their power the way you might expect.
How Measurement Technique Skews the Gap
Whether you measure both arms at the same time or one after the other changes the result, and the difference is not trivial. When blood pressure is measured simultaneously in both arms using dual cuffs, the inter-arm gap is consistently smaller than when measured sequentially with one cuff moved from arm to arm. One study found the average absolute systolic difference was about 6 mmHg with simultaneous measurement versus nearly 8 mmHg with sequential measurement.5PubMed Central. Simultaneous compared with sequential blood pressure measurement results in smaller inter-arm blood pressure differences A separate study confirmed this directional effect, finding the sequential method produced significantly larger inter-arm differences for systolic pressure.6JAMA Internal Medicine. Consistency of Blood Pressure Differences Between the Left and Right Arms
The reason is straightforward. Blood pressure fluctuates from moment to moment with breathing, anxiety, and heart rate variability. When you measure both arms simultaneously, those fluctuations hit both readings at once and cancel out. When you measure one arm, move the cuff, and then measure the other arm a minute or two later, natural variability adds noise to the comparison. In a healthy person, that noise might push a 4 mmHg true difference up to 10 or 12 mmHg and trigger unnecessary concern. In an emergency setting where dissection is being ruled out, this distinction matters. Simultaneous bilateral measurement is more reliable for detecting a true pathological gap, and less likely to produce a false alarm from measurement artifact.
Most emergency departments do not routinely use dual-cuff simultaneous measurement, though. The standard workflow is to check one arm, then the other. If you are getting your blood pressure checked and notice a gap, understand that a few mmHg of that gap is likely just timing noise rather than a real anatomical difference.
Leg Blood Pressure and the Ankle-Brachial Index
The arm-to-arm comparison gets the most attention, but dissections can also compromise blood flow to the legs, and checking leg pressures adds diagnostic information. In the study that evaluated multiple bedside measures, a difference between legs of 13 mmHg or more in systolic pressure had a sensitivity of about 51% and specificity of 81% for acute aortic dissection. An ankle-brachial index at or below 0.9 showed similar numbers, with sensitivity of about 53% and specificity of 87%.3PubMed Central. The Predictive Value of Inter Arm Blood Pressure Difference, Inter Leg Blood Pressure Difference and Ankle Brachial Index for Acute Aortic Dissection
A low ankle-brachial index in the setting of sudden chest or back pain suggests the dissection has extended well beyond the aortic arch and into the descending aorta, potentially compromising the iliac arteries that feed the legs. This is a sign of more extensive disease and typically a worse prognosis. Organ malperfusion, where the dissection cuts off blood supply to the kidneys, gut, spinal cord, or limbs, is one of the strongest predictors of death in patients undergoing emergency surgery for Type A dissection.7PubMed. Impact of Hemodynamic Instability and Organ Malperfusion in Elderly Surgical Patients Treated for Acute Type A Aortic Dissection
Checking all four limbs, both arms and both legs, gives a more complete picture of how far the dissection has propagated and which vascular territories are compromised. In practice, emergency teams often combine these limb checks with urgent imaging, typically a CT scan with contrast, to map the tear precisely before surgery.
Left-Arm Pressure and Surgical Outcomes
An interesting wrinkle emerges when looking at outcomes after surgery for Type A dissection. Research on patients who underwent emergency repair found that the relationship between the left arm’s systolic blood pressure at admission and the risk of dying in the hospital followed a U-shaped curve. Both very low and very high left-arm pressures were associated with worse outcomes. Critically, this U-shaped pattern was not seen with the right arm’s pressure.8Heart, Lung and Circulation. Admission Left-Arm Systolic Blood Pressure and In-Hospital Mortality After Acute Type A Aortic Dissection Repair
Low left-arm systolic blood pressure was independently linked to higher in-hospital mortality. This finding has a logical explanation. In Type A dissections, the left subclavian artery sits close to the origin of the tear. A very low left-arm pressure may signal that the dissection has extended to involve the left subclavian, indicating a more extensive tear with more compromised branch vessels. Meanwhile, very high left-arm pressure could reflect severe, uncontrolled hypertension driving the dissection forward. The right arm, which branches off the aortic arch via the brachiocephalic artery, apparently does not track surgical outcomes in the same non-linear way.
For surgical and anesthesia teams, this means that monitoring the left arm specifically provides prognostic information that the right arm does not. During and after repair, a sudden drop in left-arm pressure could signal extension of the dissection or complications at the surgical site near the left subclavian origin.
Other Conditions That Cause an Arm-to-Arm Gap
Aortic dissection is the most dramatic cause of a large inter-arm blood pressure difference, but it is far from the only one. A persistent, reproducible gap between arms in someone without acute symptoms more often points to a different problem altogether.
Subclavian artery stenosis, a narrowing of one of the arteries leading to an arm, is probably the most common chronic cause. It is typically related to atherosclerosis, the same plaque buildup that causes heart attacks and strokes. The affected arm reads lower, and the gap tends to be stable over time rather than appearing suddenly with tearing chest pain.
Coarctation of the aorta is a congenital narrowing of the aorta itself, usually located near the origin of the left subclavian artery.9PubMed Central. Coarctation of the aorta: a secondary cause of hypertension Depending on the exact location of the narrowing relative to the left subclavian takeoff, it can produce a difference between arms, between the arms and legs, or both. This condition is usually detected in childhood or adolescence and is a recognized secondary cause of high blood pressure in young people.
Thoracic outlet syndrome, where the blood vessels or nerves passing between the collarbone and the first rib become compressed, can also produce a lower blood pressure on the affected side.10Thoracic Outlet Syndrome. Thoracic Outlet Syndrome: Public Education The arterial form of this condition is uncommon, but when it occurs, it can mimic some of the arm-level findings of more serious vascular problems.
Context is what separates these from a dissection. A sudden, large inter-arm difference in someone with acute tearing chest or back pain is an emergency. A persistent 15 mmHg gap discovered incidentally in someone who feels fine warrants investigation but is far more likely to be atherosclerotic narrowing than a tear in the aorta.
When the Inter-Arm Difference Is Absent
A common and dangerous misconception is that a normal inter-arm blood pressure comparison rules out aortic dissection. It does not. Many dissections, including some large and life-threatening ones, do not involve the branch vessels that supply the arms. A Type B dissection originating below the left subclavian artery, for example, can tear along the descending aorta, compromise the kidneys and gut, and produce absolutely no arm-to-arm blood pressure difference because neither subclavian artery is affected.
Even in Type A dissections, the flap does not always extend into the brachiocephalic or left subclavian arteries. When it stays within the ascending aorta or involves the coronary arteries instead, the arms read identically. The sensitivity figures bear this out: a 10 mmHg threshold catches only about 62% of dissections, meaning nearly four in ten are missed by this sign alone. A 20 mmHg threshold misses even more. Relying on inter-arm blood pressure difference as a screening test would leave a substantial number of dissection patients undiagnosed.
This is why clinical decision rules for aortic dissection combine multiple features rather than depending on any single one. Sudden severe pain, its tearing or ripping character, a history of hypertension or connective tissue disease, widened mediastinum on chest X-ray, pulse deficits, and inter-arm blood pressure differences are all pieces of a puzzle. No individual piece is sensitive enough to stand alone, and emergency physicians treat the overall clinical picture, not any one bedside test, as the trigger for definitive imaging.
How Blood Pressure Is Managed During a Dissection
Once aortic dissection is suspected or confirmed, controlling blood pressure becomes the immediate therapeutic priority. The goal is to reduce the shearing force on the aortic wall to prevent the tear from extending further. Standard practice targets a systolic blood pressure below 120 mmHg and a heart rate below 60 beats per minute, using intravenous medications that lower both simultaneously.
The arm-to-arm discrepancy creates a practical challenge for monitoring. If one arm reads 160 mmHg and the other reads 110 mmHg, which number do you use to guide treatment? The general approach is to use the higher reading as the target for blood pressure reduction, since that number better reflects the pressure the aortic wall is actually experiencing. The lower-reading arm is the one with obstructed flow, so its number underestimates the true aortic pressure.
In the operating room, invasive arterial lines are placed directly into an artery, most commonly the right radial artery, to provide continuous beat-to-beat pressure monitoring during surgical repair. However, if the right subclavian is compromised by the dissection, that line will read falsely low and mislead the surgical team. Knowing which arm is affected before placing the arterial line, based on the earlier bilateral blood pressure check, prevents this error. Some centers place arterial lines in both arms or add a femoral line to cross-reference. The finding from the study on left-arm pressure and surgical mortality reinforces why bilateral awareness matters throughout the case, not just at the initial assessment.8Heart, Lung and Circulation. Admission Left-Arm Systolic Blood Pressure and In-Hospital Mortality After Acute Type A Aortic Dissection Repair
After surgery, bilateral monitoring continues because the repaired aorta remains fragile, and new dissection flaps can develop. A sudden change in the inter-arm difference during recovery can be one of the earliest bedside signs that something has gone wrong at or beyond the repair site, sometimes before imaging reveals it.