Asymmetric Lower Extremity Blood Pressure in Type B Aortic Dissection
Asymmetric lower extremity blood pressures in type B aortic dissection occur due to dissection-related occlusion of aortic branch arteries (specifically the iliac arteries), where the dissection flap compromises true lumen flow to one or both lower extremities, resulting in erroneously low blood pressure readings in the affected limb. 1
Mechanism of Asymmetric Blood Pressures
The pathophysiology involves several key mechanisms:
Branch vessel occlusion: The dissection flap can directly obstruct iliac or femoral arterial flow by either compressing the true lumen or extending into the branch vessels themselves, creating differential perfusion pressures between limbs 1
Dynamic obstruction: The false lumen can compress the true lumen, particularly affecting branch vessels that arise from areas where the false lumen is distended, leading to variable degrees of malperfusion 1, 2
Malperfusion syndrome: Lower extremity malperfusion occurs in 5.5-6% of acute aortic dissections, with type B dissections showing rates ranging from 5.7% to 30% depending on the series 2, 3, 4
Clinical Implications and Assessment
Blood pressures must be measured in both arms and both legs to determine the highest central blood pressure, as relying on a single measurement site can lead to dangerous underestimation of true systemic pressure. 1
Key assessment points include:
Pulse deficits: Three-fold more common in patients with acute limb ischemia compared to those without 3
Neurologic deficits: Five-fold more common in patients presenting with limb ischemia 3
Rutherford classification: Most patients present with Rutherford IIB symptoms (threatened limb), though type A dissections more commonly present with Rutherford III (irreversible ischemia) 2
Critical Pitfall to Avoid
Never rely on blood pressure from a single extremity in suspected aortic dissection, as dissection-related branch vessel occlusion will give falsely low readings that can lead to inappropriate hypotensive management when the true central pressure remains dangerously elevated. 1 This is particularly important in type B dissections where 71% of patients have systolic blood pressure greater than 150 mmHg at presentation 1
Associated Morbidity
Lower extremity malperfusion carries significant prognostic implications:
Visceral ischemia: Acute mesenteric ischemia/infarction is 6.9-fold more likely in patients with limb ischemia (OR = 6.9; 95% CI 2.5-20) 3
Renal failure: 2.7-fold increased risk of acute renal failure (OR = 2.7; 95% CI 1.1-7.1) 3
Mortality: Limb ischemia is independently associated with death (OR = 3.5; 95% CI 1.1-10) 3
Multiple organ involvement: The number of organ systems with malperfusion is typically two-fold higher at aortography than suspected on initial clinical examination 3