If lidocaine shortens phase 3 (which increases HR), why is it used in post-MI tachycardia?

Question

If lidocaine shortens phase 3 (which increases HR), why is it used in post-MI tachycardia?

Answer 1

Phase 3 is associated with Ca++ and K+.  Lidocaine influences phase 4, which is Na+ mediated. Lidocaine is a type Ib anti-arrythmic. It also increases threshold.  Lidocaine also works by having a higher affinity for sodium channels during giastole in ischemic tissue because this type of tissue remains partially depolarized and does not return to its resting state (-90mV). It is more effective in suppressing ventricular tachycardias than supreventricular tachycardias.