Mechanism of large scale cell depolarisation by the application of a powerful electrical current

1. Heart may be stimulated by either intrinsic impulses originating within SA node (or ectopic pacemaker) OR to an external stimulus

2. > 100 mA applied to skin of chest

a) heart may fibrillate

3. Large enough current delivered to chest

a) Depolarisation of the majority of myocardial cells

b) All abnormal pacemakers are depolarise

c) Remain in a state of depolarisation as long as the depolarising current continues

4. Upon ceasing of depolarisation:

a) Heart’s normal intrinsic pacemakers regain control

5. Critical mass

a) Defibrillator acts by producing a current which is strong enough to depolarise a large mass of cardiac cells

b) When the current is removed, if a critical mass (» 75%) of the myocardial cells are in the same phase (recovery) defibrillation occurs

c) The depolarisation of a variable mass of myocardium may reduce the amount of excitable myocardium to that it is insufficient to perpetuate fibrillation