Conduction speed of cardiac action potential

Factors influencing conduction speed

Depends on

• Amplitude of the AP
• Rate of change in phase 0
• Level of RMP

i.e. higher amplitude, faster upstroke, and more negative RMP, all lead to increased conduction speed

Thus,
Conduction speed slow in pacemaker tissues

Conduction speed in various cardiac tissues

• SA node - 0.05 m/s
• Atrial - 1 m/s
• AV node - 0.05 m/s
• Bundle of His - 1 m/s
• Purkinje system - 1-4 m/s
• Ventricular muscle - 1 m/s

Order of excitation

1. Interventricular septum (except for basal portion) and papillary muscles

• Rigid septum serves as anchor point
• Papillary muscle contraction prevents eversion of AV valves

2. From endocardium to epicardium

• Because RV is thinner, right epicardium is depolarised earlier than left epicardium

3. Last to be excited: posterior basal epicardial regions and a small area in basal portion of interventricular septum

NB:

• Electrical activation:
  * RA --> LA
  * LV --> RV
• But because pressure is lower in pulmonary artery
  --> RV ejection is before LV ejction
  * [WG21:p568]
• During inspiration: aortic valve close before pulmonary valve
• During expiration: AV and PV close about the same time
• Last part to be depolarised is the epicardial surface of the left venticular wall at the base of the heart
  * [KB2:p87]