You SHOULDN'T generally mix stimulants and beta-blockers. Not to treat....gah..well what happens is that with beta adrenergic receptors blocked that released noradrenaline/adrenaline is still there and it has to bind SOMEWHERE, so it can occur that you get an essentially unopposed alpha-adrenoreceptor stimulation leading to a paradoxically, increased vasoconstriction.
Generally more of a problem at higher doses but with a pre-existing heart problem...I'd talk to your doc about that.