Impulses can travel retrograde (3 to 2) but not orthograde.
An AP will travel down the branch 1, into the common distal path (br 3), then travel retrograde through the unidirectional block in branch 2.
When the AP exits the block, if it finds the tissue excitable, it will continue by traveling down (reenter) the branch 1.
Timing is critical –AP exiting the block must find excitable tissue to propagate.
If it can re-excite the tissue, a circular pathway of high frequency impulses (tachyarrhythmia) will become the source of APs that spread throughout a region of the heart (ventricle) or the entire heart.
Restore normal rhythm, rate and conduction or prevent more dangerous arrhythmias
Alter conduction velocity (SA Node or AV Node)
Alter slope 0 depolarisation or refractoriness
Alter excitability of cardiac cells by changing duration of Resting Potential (usually via changing AP Duration)
3. Suppress abnormal automaticity
Vaughan-Williams Classification Class Mechanism Example I Na channel blockers Membrane Stabilisers Lignocaine II Beta Blockers Metoprolol III K channel blockers Amiodarone IV Ca channel blockers Verapamil Other Digoxin. Adenosine. MgSO4. Atropine