Mechanism of the induction of VF by the application of an electrical stimulus

1. Fibrillation caused by 50 Hz AC current [Eg: direct application of AC current via a stimulating electrode applied to myocardium]

a) The first cycle of stimulation causes depolarisation to spread in all directions leaving this myocardium refractory

b) After 0.25 sec, this depolarised muscle starts to come out of refractory phase — some portions of muscle coming out of refractoriness prior to other portions

c) New stimuli now depolarises only those portions of the heart that have ‘recovered’ from the previous stimuli — impulses travel in certain directions but not in all directions

d) Several events occur simultaneously eventuating in VF:

i) Block of impulses in some directions but transmission in other directions creates ‘circus movements’

ii) Rapid stimulation of myocardium:

a) Decreases velocity of conduction through heart allowing increased time for transmission of impulse around heart

b) Shortens refractory period of heart allowing reentry of an impulse into a previously excited heart

iii) Division of impulses: when a depolarisation wave reaches a refractory area, it divides and travels to both sides around this zone — multiplication of impulses

2. Non Uniform Cardiac recovery property

a) Exacerbated by:

i) Ischaemia

ii) Sympathetic stimulation