It is essential that you understand the electrical characteristics of the heart tissues. This will aid you in understanding the normal rhythm characteristics as well as the common dysrhythmias that you will encounter in your practice.
The heart is composed of two basic tissue types:
Tissue 1. The myocardium (muscle).
Tissue 2. The special electrical conduction system*.
Electrically speaking, the two tissue types are characterized as follows:
Tissue 1. The myocardium does not possess automaticity and subsequently will not depolarize until stimulated
by an external stimulus. This stimulus is usually the adjacent Purkinje Cell or an adjacent myocardial
cell in the active process of depolarizing.
Tissue 2. The special electrical conduction system has two important unique properties:
a) It possesses automaticity, i.e., it will spontaneously depolarize and repolarize without an external
stimulus.
b) It conducts the electrical signal very rapidly as compared to other tissues.
Tissues with Automaticity:
- SA Node (60-100 bpm)
- Internodal Pathway (Atria; 60-80 bpm)
- AV Junction Node (40-60 bpm)
- Bundle of HIS (& all branches; 40-60 bpm)
- Purkinje Cells (20-40 bpm)
Important properties of Automaticity Tissues:
The automaticity tissue of the heart is designed to provide an automatic electrical stimulus to ultimately depolarize
the myocardial tissue. The system is designed with a built in demand backup as its fail safe mechanism. The design
function is as follows:
If normal Sino-Atrial Node function…
SA Node discharges at a rate of 60-100 bpm. This signal will suppress all the other automaticity tissues.
If the SA Node fails…
The next in line automaticity tissue takes over as follows:
Atrial Internodal Pathways- discharge rate 60 – 80 bpm. (This signal will suppress all the lower hierarchy automaticity tissues.)
If the Atrial Tissue fails…
The next in line automaticity tissue takes over as follows:
Junctional (including Bundle of HIS and branches) – discharge rate 40-60 bpm. (This signal will suppress all the lower hierarchy automaticity tissues)
If the Junctional Tissue fails…
The next in line automaticity tissue takes over as follows:
Terminal Purkinje Fibers – discharge rate 20-40 bpm.
If the Purkinje Fibers fail…
Ventricle myocardium will receive no electrical signal. Flatline will follow, then death.
NonAutomaticity
This non-automaticity is also characteristic of the myocardial muscle cells.
Note that the resting potential will remain at rest until stimulated by an external signal (usually Purkinje cells or adjacent myocardial cell). In the abnormal environment this might be an ectopic discharge from anywhere along the electrical conduction pathway (atrial, junctional, Bundle of HIS or its branches), the Purkinje cell, or an artificial Electronic Pacemaker probe. An important conclusion from this electrical property is as follows: The myocardium will stay at the resting potential until stimulated by an outside voltage i.e. the heart will remain flat line.
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