Combination of:
Bradycardia
Renal failure
AV node blocker: beta-blocker, verapamil or diltiazem- typically adherent with medication but an overdose
Shock
Hyperkalemia
Vicious cycle: in setting of medications, hyperkalemia, renal failure
Often initiated by an episode of hypovolemia, hypoperfusion, or uptitration of antihypertensives or K sparing diuretics
Renal failure causes hyperkalemia (+/- accumulation of AV node blockers), hyperkalemia synergizes with AV node blockers to cause bradycardia and hypoperfusion, hypoperfusion worsens renal failure 
Bradycardia
Renal failure
AV node blocker: beta-blocker, verapamil or diltiazem- typically adherent with medication but an overdose
Shock
Hyperkalemia
Brash syndrome
Cardiac
Scleroderma
Inferior MI 
DiGeorge syndrome
Sick sinus syndrome
Sjögren syndrome
Neurocardiogenic/reflex-mediated