tof cardio physio
beta blockers block beta 1 receptors on the heart, decreasing heart rate, and contractility.
During Tet spells (hyper cyanotic spell) acute changes occur near pulmonary valve in the infundibulum,leading to more rt ventricular outflow obstruction.
The pulmonary vascular resistance increases further. deoxygenated blood from RV flows more into the more welcoming systemic vasculature.(SVR

VR is already lower than normal).
So the venous return has to be increased ,to increase the volume of blood pumped into the lungs..fill up the pulmonary capillary beds,increase oxygenation of blood.
beta 1 blockers will prolong the time in which blood can be oxygenated, so it helps tet spells?
and decreased contractility will decrease right to left shunt too?