An aortic dissection is a severe medical event where the inner layer of the aorta tears, allowing blood to rush between the layers of the artery wall. This separation creates a new channel for blood flow, called a false lumen, which can rapidly lead to catastrophic consequences. A difference in blood pressure between the two arms is a highly specific finding associated with this condition, signaling compromised blood flow to the upper body. Recognizing this sign is urgent, as rapid diagnosis and treatment can be life-saving.
The Mechanical Cause of Unequal Blood Pressure
The aorta arches over the heart, and three major arteries branch off to supply blood to the head and upper limbs. These vessels include the brachiocephalic artery (feeding the right arm), the left common carotid artery, and the left subclavian artery (supplying the left arm). A dissection occurring in the ascending aorta or aortic arch can extend into the origin of these branch vessels.
When the tear extends, the resulting flap of tissue can physically obstruct the opening of one of these major arteries. Blood flowing into the false lumen can also compress the true lumen, narrowing the exit point into the branch vessel. This obstruction significantly reduces the volume and pressure of blood reaching the corresponding arm. The most common arteries involved are the brachiocephalic artery or the left subclavian artery.
This mechanism results in one arm receiving normal, forceful blood flow directly from the aorta, while the other receives diminished flow through a partially blocked vessel. This mechanical interference is the direct cause of the measurable pressure difference between the arms. The degree of obstruction can fluctuate, making the unequal pressure a dynamic and sometimes inconsistent finding.
Identifying the Significant Pressure Differential
The arm not affected by the dissection flap will register the higher blood pressure reading. This reading reflects the body’s true systemic blood pressure, which is often high due to the stress and pain of the aortic injury. The affected arm, whose subclavian artery is partially blocked, will register the lower reading. This lower reading is sometimes described as pseudohypotension because it does not accurately reflect the high pressure within the main circulation.
Medical guidelines consider a systolic blood pressure difference of 20 millimeters of mercury (mmHg) or more between the two arms a significant warning sign for aortic dissection. While this differential is a classic finding, it is not universally present. The presence of a pulse deficit—a noticeable difference in the strength or absence of a pulse in one arm compared to the other—may sometimes be a more accurate indicator than the pressure measurement alone. Healthcare providers must measure blood pressure in both arms to use the higher, more accurate reading to guide treatment.
Immediate Action and Emergency Evaluation
If sudden, severe, “tearing” or “ripping” chest or back pain is accompanied by a noticeable difference in pulse or blood pressure between the arms, immediate emergency medical attention is required. Aortic dissection is a time-sensitive medical emergency, and survival depends on rapid intervention. The first step for anyone experiencing these symptoms is to call emergency services immediately.
Emergency medical teams stabilize the patient by carefully controlling the heart rate and blood pressure to reduce stress on the damaged aorta. The goal is typically to lower the systolic blood pressure to a target range of 100 to 120 mmHg. Medications, such as intravenous beta-blockers, are often used to achieve this rapid control. The definitive diagnosis is confirmed through advanced imaging, such as a CT angiogram, which provides detailed pictures of the aorta and the extent of the tear.