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.
- If QRS is < 0.12s then the AV block is located in AV node or Bundle of His (better prognosis)
- Broad QRS indicates distal blocks (further than bifurcation of Bundle of His)
- Conduction system degrading or ischemia
- Idiopathic Fibrosis of conduction system
- Vagal stimulation
- Hyperkalaemia
- Medications (Beta Blockers, Digoxin, Amiodarone, Verapamil etc.)
- Lyme Disease (Borreliosis)
- Hypothermia
| 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.
- Impulse spread to left atrium via Bachmann Bundles and towards ventricles via AV node
- In the ventricles, impulse travel along Bundle of His then
- The impulse then bifurcates (divide) into left & right bundle branches
- These branches terminate in a network of conduction tissues (Purkinje Fibres)
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
- When there is a conduction delay (widened QRS) without LBBB or RBBB – Can be observed after a large MI event where necrotic areas can cause conduction disturbances – Some other changes can be due to myocardial fibrosis, amyloidosis, cardiomyopathy or hypertrophy.
- 1. > 110 ms (2.5 to 3 small boxes or more) in adults OR;
- 2. > 90ms (2 to 2.5 small boxes or more) in children 8 to 16 years OR;
- 3. > 80ms (2 small boxes or more) in children < 8 years AND;
- 4. Do not met criteria for RBBB or LBBB.
- When the conduction through the right bundle is obstructed (delaying right ventricular depolarisation) – Depolarisation of right ventricle will rely on the impulse from the left ventricle.
- QRS complex > 0.10 (wide)
- rSR’ / rsr’ / rsR’ complex pattern in V1 & V2
- Second R-wave (R’) is larger than 1st R-wave and S-waves does not reach the baseline – Basically QRS displays “M” shape.
- ST depression & T-wave inversion in V1 & V2
- Deep & Broad S-Wave in Lead I, aVL, V5-V6
- Secondary ST-T changes (ST-T segment opposite to QRS)
TIP: Compare V1 & V6
- V1 produces an “M” shape pattern
- V6 has a broad & deeper S wave
Example 1:
Example 2:
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.
- Idiopathic fibrosis or RBB degeneration
- Heart disease
- Coronary Artery Disease
- Pulmonary Embolism
- COPD
- Cardiac surgery or PCI may cause RBBB
- Cardiomyopathy
- When the conduction through the left bundle is obstructed (delaying left ventricular depolarisation) – Depolarisation of left ventricle will rely on the impulse from the right ventricle.
- QRS complex > 0.10 (wide)
- Lateral QRS has a broad / notched R-wave with prolonged upstroke in Lead I, aVL, V5-V6
- ST depression & T-wave inversion in Lead I, aVL, V5-V6
- V1 & V2 has asymmetrical, broad, & deep QS complexes.
- V1-V3 may have ST elevation (< 5mm) & positive T-waves
- Left axis deviation may be present (due to dominant ventricle)
TIP: Compare V1 & V6
- V1 produces rS wave (V-shape QRS)
- V6 mini ‘M’ on the peak of QRS
Example 1:
Example 2:
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.
- Ischemia & infarction (may be masked by LBBB)
- Idiopathic fibrosis or LBB degeneration
- Heart disease
- Coronary Artery Disease
- Cardiomyopathy
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.
- Can occur in healthy individuals, or those suffering from conditions such as LBBB related symptoms, or ventricle hypertrophy, & medications. Can be progressive to other blocks but rare.
Due to the block, the impulse backtracks and move down to the left posterior fascicle (LPF) causing left axis deviation.
- Can mask symptoms of anteroseptal infarction
- rS complex in Lead II, III, & aVF can mask Q-wave from prior inferior infarction
- QRS 0.10 – 0.12s in V5 & V6
- Left axis deviation (-60°)
- No other cause of LBBB
- Larger branch (greater blood supply); due to the block, the impulse backtracks and move down to the left anterior fascicle causing right axis deviation – similar causes as left anterior fascicular block (LAFB; unusual in healthy people, and overall rare)
- Can mask inferior & lateral infarction
- T-wave inversion may occur in inferior leads showing similar to post-ischemic T-wave
- QRS 0.10 – 0.12s in V5 & V6
- Left axis deviation (+120°)
- No other cause of LBBB
- No right ventricle hypertrophy
Bifascicular Block
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.
- RBBB present
- Left axis deviation or right axis deviation present
Page contributors:
 | Thanh Bui, AP60825 Event Medic, Emergency Medical Technician & Volunteer Development Officer
|
 | Andrew Moffat, AP16790 |
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