It comes down to coronary perfusion.
Heart muscle needs blood and oxygen like any other muscle. In the case of the myocardium, coronary perfusion occurs during diastole, when the muscle relaxes. The pressure gradient between the aortic and left ventricular pressures drives blood forward through the coronaries. The coronary perfusion pressure, in the context of chest compressions, is that difference between at end-diastole, for example.
In a normal heart, the gradient is present throughout diastole, so the myocardium gets lots of good, oxygenated blood throughout that part of the cycle (Figure 1). In severe aortic insufficiency, though, the blood from the aorta quickly rushes back into the ventricle during diastole, dropping the gradient between aortic and ventricular pressures to zero (Figure 2). The myocardium, therefore, only has a short period of time, in early diastole, to get that oxygenated blood.
Beta blockade prolongs diastole and therefore prolongs the time that the coronaries aren’t being perfused. Based on that, some cardiologists (including where I trained) avoid beta blockers with the thought that they decrease coronary perfusion in severe AI and therefore promote ischemia. However, beta blockers also decrease myocardial oxygen demand.
So, as with most things in medicine, it’s a balance, and there are few good clinical studies.
Read more: Aortic Regurgitation in Chapter 283: Aortic Valve Disease, Harrison’s Principles of Internal Medicine 19e.
The above Wiggers diagrams are modifications of: adh30 revised work by DanielChangMD who revised original work of DestinyQx; Redrawn as SVG by xavax – Wikimedia Commons: Wiggers Diagram.svg, CC BY-SA 4.0