Cardiovascular System
Re-entry loops and unidirectional block
Re-entry is an impulse running round a circuit of two connected pathways instead of dying out, so it keeps re-triggering the heart and sustains a fast abnormal rhythm.
Remember
Re-entry needs a one-way block (tissue that lets an impulse pass in one direction only) and slow conduction round a loop, so the impulse returns to tissue that has recovered and goes round again.
Usually an impulse that splits down two paths meets itself on the far side, and the two waves cancel. The heart then contracts evenly. Normally the impulse then dies out, since the tissue it has just passed through is refractory, meaning it cannot be excited again until it recovers.
Damage or a structural abnormality can disrupt the normal pathway of the electrical impulses. One pathway can develop a unidirectional block: the damaged tissue lets an impulse pass in one direction only. It will not carry the impulse in the forward direction, but it can still carry it backwards (retrogradely). A premature (early) impulse can arrive when one pathway is still refractory but the other is not, so it is blocked one way but can travel the other. The impulse goes down the working pathway, travels up the blocked one in reverse, and arrives back at its starting point after the tissue there has recovered. It excites the tissue again and keeps going round and round. Circus movement (an impulse going round a closed circuit) is the classic description of re-entry.
Slow conduction (slow travel of the impulse round the loop) is the second ingredient. It gives the blocked tissue time to recover before the impulse comes back. Each lap sends an impulse out to the rest of the heart, so the result is a sustained tachyarrhythmia (a fast abnormal rhythm). It continues independently of the sinoatrial node (the natural pacemaker) and often starts and stops abruptly. Ischaemia, scar and hypertrophy make a loop more likely, as do a long pathway and short refractory periods.
Examples are Wolff-Parkinson-White syndrome, where an accessory pathway (an extra route between the atria and ventricles) leads to a re-entry loop, and atrial flutter, a rhythm that can be caused by a loop in the atria.
Key facts
- The circuit
- two connected pathways that differ in how fast they conduct and how quickly they recover.
- The block
- one pathway will not carry the impulse forwards but still carries it backwards.
- Slow conduction
- gives the blocked tissue time to recover before the impulse returns.
- Pattern
- a regular rhythm that often starts and stops abruptly.
- Favouring factors
- ischaemia, scar and hypertrophy, a long pathway and short refractory periods.
- Examples
- Wolff-Parkinson-White syndrome and atrial flutter.
Exam traps
- Lengthening the refractory period does not sustain a loop. The returning impulse meets tissue that cannot fire again, so it tends to suppress re-entry.
- Slowing that is the same everywhere does not create a loop, because impulses simply die out when they meet. Slowing can help a loop persist, but only when a unidirectional block and unequal pathways already exist.
- Increased automaticity (the ability of a cell to fire spontaneously) of the sinoatrial node speeds up the normal pacemaker and gives sinus tachycardia, a fast rate that speeds up and slows down gradually. Re-entry needs a circulating impulse and often starts and stops abruptly.
- Colliding impulses cancelling is the normal safeguard that stops loops forming, because the tissue ahead is refractory. It prevents re-entry rather than causing it.
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