Impulse of left ventricular ejection is the rate of change of central arterial pressure with respect to time (dP/dt).
In cases with aortic root dilatation like Marfan syndrome, an increase in the impulse of left ventricular ejection can increase the chance of dissection. Same is true while treating severe hypertension. If blood pressure is reduced with vasodilators which do not decrease the impulse, but rather risks an increase of the impulse, chance of dissection does not decrease. This is the rationale of combining a beta blocker with vasodilators while treating severe hypertension, especially with aortic dissection. Reduction in left ventricular impulse by beta blockers reduces the chance of progression of aortic dissection.
Along similar lines beta blockers can be used to reduce the progression of aortic dilatation in Marfan syndrome. Trials have documented this effect right from 1994 .