Observe the Atrioventricular (AV) Node in the upper-mid portion of the heart, and the Left and Right Bundle Branches at the intraventricular septum
Heart block type that originate from a group of arrythmias raised from failed or delayed transmission above the atrioventricular (AV) node to the ventricles - Classified into 1st, 2nd & 3rd degree AV blocks.
|Type of AV block||Definition||Clinical Features|
|1st Degree AV block||Difficulty transmitting impulse through AV node (where SA node is normal).||Due to this delay, the PR interval is longer – Once this travels through AV node, the impulse travels normally.|
|2nd Degree AV Block / Mobitz-Type I (Wenchkebach)||Represents a delayed transmission impulse through the AV node – However the time it takes to reach the ventricles increase for every impulse. |
This results in gradual PR interval increase. This continues until SA node intervenes with a sinus beats which resets the cycle again to prevent a fall in blood pressure.
The RR interval shortens with each beat of the cycle.
The P-P interval remains constant
|2nd Degree AV block / Mobitz-Type II||Severe 2nd degree block resulting in a dropped beat, due to SA node impulse not being transmitted to the ventricles. |
If 2 or more impulses are blocked at the AV node before reaching the ventricles, it is called 2:1 or 3:2 block.
Can develop into 3rd degree AV block or ventricular standstill.
|3rd Degree AV block / Complete Heart Block||No communication through AV node resulting in separate atrial & ventricle transmission by their own branches. The SA node fires at 60-100 where the ventricles are at about 20-40bpm from nearby impulses from AV node or; within the ventricles.|
Perfusion is through junctional or ventricular escape rhythm – Can result to ventricle standstill.
Hypotension is displayed due to inability of atria to intervene resulting in diminished cardiac output.
Normal PQRST wave means good normal conduction – Conduction is initiated by the sinoatrial node (SA node) in the right atrium.
Conduction is very fast – In event where conduction branch is impaired, the impulse finds a way past the obstruction by transmitting impulses from cell-to-cell until it can re-join the fast conduction system again. Hence the process is delayed (due to added time due to added distance as result of the blockage) reflected by a wider QRS complex.
Examine QRS width & configuration for right (V1 / V2) & left (V5 / V6) ventricles
TIP: Compare V1 & V6
The impulse from left ventricle will be slow as it needs to travel partly or entirely outside of the conduction system resulting in slow & abnormal activation of right ventricles creating an abnormal and prolonged QRS complex. Therefore, the left ventricle will activate before the right ventricle; therefore, displaying its own vector.
TIP: Compare V1 & V6
The impulse from right ventricle will be slow as it needs to travel partly or entirely outside of the conduction system resulting in slow & abnormal activation of left ventricles creating an abnormal and prolonged QRS complex.
Left bundle branch contains three (3) fascicular left, right and septal branch – Hemiblock is a block to one (1) fascicular branch only, therefore is not a complete LBBB. A normal conduction is when the impulse move down AV node to left bundle, then split into both fascicles where the Purkinje fibres merge - A hemiblock rely on impulses from other parts of the ventricles where fascicle is intact.
Due to the block, the impulse backtracks and move down to the left posterior fascicle (LPF) causing left axis deviation.
Combination of either left anterior or left posterior hemiblock with RBBB – Dangerous condition as only one fascicle of left bundle branch is supplying the heart with electrical activity.
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|Thanh Bui, AP60825|
Event Medic, Emergency Medical Technician &
Volunteer Development Officer
Andrew Moffat, AP16790