Which statement best describes the effect of labetalol in aortic dissection when blood pressure is low?

• A. It increases heart rate
• B. It reduces shear forces
• C. It has no effect on shear stress
• D. It raises blood pressure

Answer: (B)

In aortic dissection, the main therapeutic goal is to lessen the forces acting on the torn aortic wall. Labetalol achieves this by both lowering heart rate and lowering blood pressure, which together reduce the rate of pressure rise (dP/dt) and the overall wall stress on the aorta.

Labetalol is a dual-acting drug: it blocks beta receptors, which slows the heart rate and decreases contractility, and it blocks alpha-1 receptors, which lowers systemic vascular resistance. Slowing the heart reduces the force generated with each beat, and lowering the arterial pressure reduces the pressure pushing against the dissection. Even when blood pressure is already low, these effects decrease shear forces on the injured segment, helping prevent propagation of the dissection.

So the best description is that labetalol reduces shear forces on the aorta. It does not increase heart rate, it does not have no effect on shear stress, and it does not raise blood pressure.