G2ii: Abnormal conduction & arrhythmias
Arrhythmia = abnormality in rate, generation & propagation of an AP
2 major causes of arrhythmias are:
- Abnormal Automaticity
- Abnormal Conduction
Abnormal Automaticity
- Fast AP cells do not normally undergo spont. Depolarisation
- Can become “Fast → Slow Response” cells by blockage of Fast Na+ channel
- If fast Na+ channel blocked (drugs, hypoxia) the AP relies on Ca2+ channels to depolarise
- The AP looks more like a ‘slow AP’
- ∴producing arrhythmias as cell which is not a PM cell starts pacing
Triggered Automaticity → always caused by after-depolarisations
Fast AP cells can undergo spontaneous depolarisation in Ph 3-4 (relative refractory period)
- 2 types:
Abnormal Conduction
- Conduction block
- Reflection
- Re-entry
1) Conduction block
- An ectopic focus ‘escapes’ the SA Node to become the dominant PM
- There is a block between the ectopic focus & the SA Node, so the SA cannot control it
2) Reflection
- An adjacent cell that has already repolarised becomes stimulated by a cell that has not repolarised yet
- e. ventricular arrhythmias associated with long QT syndrome
3) Re-Entry
- When a cardiac impulse can re-excite some regions through which it has previously passed
- Responsible for most SVTs