The ‘Vulnerable Period’ of a cardiac cycle

1. Relative Refractory Period

a) Period during which the membrane becomes capable of being stimulated again though it would require a stronger stimulus than it does at - 90 mv

b) The membrane becomes refractory once phase 0 of the action potential ascends to approximately -50 mv. It remains absolutely refractory until the phase 3 of the action potential descends to - 55 mv. From this point the membrane becomes relatively refractory until the resting potential is restored. At this point, which indicates the commencement of Phase 4, the refractory period ends and the membrane becomes normally responsive again.

2. Vulnerable Period

a) Coincides with the middle & terminal phases of the T-wave (last part of phase 3 and the first part of phase 4)

b) During this period the heart is particularly susceptible to the induction of VF by an electrical stimulus

c) Represents the time when the ventricles are most likely to exhibit slow conduction & unidirectional block

3. Cardioversion

a) An electrical current used to depolarise all abnormal pacemakers thus allowing the SA node to resume initiation of the heart rhythm.

b) The impulse is synchronised with the R wave of the ECG to avoid the ‘vulnerable period’ of ventricular repolarisation