The local anesthetics (Na channel blockers) are subdivided into groups depending on their affinity for the Na channel and their ability to dissociate from the channel. What is the effect of drugs with a low affinity on normal and high heart rates?
Do not alter Na conduction at normal heart rate, only at high heart rate. This is because at a normal heart rate they dissociate from the channel before the next action potential. At high rates of channel opening, they are still bound to the channel when the next action potential is initiated and block Na conduction.