Nature & mechanism of VF in relation to myocardial cell polarisation/depolarisation and its self sustaining nature

1. Higher risk of VF — lowered threshold for VF

a) Coronary artery disease

i) Myocardial ischaemia

ii) Myocardial infarction

2. Heart is especially vulnerable to VF during T-wave

a) Ectopic impulse interrupting cardiac cycle during this vulnerable period

i) Normal heart: well tolerated

ii) High risk heart: may provoke VF

3. Mechanism

a) Recovery of the ventricular muscle cells is not uniform or homogenous during repolarisation — not capable of uniformly responding

b) The response of the ventricles to a stimulus may become fragmented

i) Impulses may originate within the ventricle prior to the whole of the ventricle recovering from refractory period thereby creating pathways which are blocked in some areas by the refractory heart

a) Transmission of depolarisation waves occurs in only one direction thereby not being cancelled out by waves travelling in the opposite direction: circus movement

ii) Rapid stimulation of the heart further predisposes to circus movement by:

a) Decreasing velocity of conduction resulting in longer time interval for impulses to travel around heart

b) Shortening of the refractory period allowing reentry of impulses into a previously excited ventricle muscle in a much shorter time than normal

iii) Division of impulses

a) When an impulse reaches a refractory zone it divides and travels around both sides

b) Results in a chain reaction with an exponential increase in the number of travelling impulses

iv) Thus the continual irregular pattern of patchy refractory area are perpetuated with formation of a vicious cycle:

a) More & more impulses are formed

b) Causing more & more refractory muscle

c) Causing more & more division of impulses

d) As soon a region of muscle comes out of refractoriness, an impulse immediately stimulates it again