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Second Degree SA Block - Type II
2nd degree Sinoatrial (SA) block, type II (SA exit block)
SA Node (P and T Cells)
- SA Node consists of 2 types of cells
- P Cells (Pacemaker Cells)
- Generate impulses in the SA node and determine the heart rate
- T Cells (Transport Cells)
- Transmit impulses from P cells to the atrium and internodal pathways
- If the impulse does not pass through T cells, then a P wave will not be generated
SA Block II Degree - Type II
- The SA node generates impulses regularly
- The disturbance occurs at the exit of impulses from the SA node (in T cells)
- It is an intermittent block of impulses originating from the SA node (in T cells)
- The blocked impulse does not produce a P wave
- If the absence of P waves lasts more than 2 seconds, it is considered SA block III degree
ECG and SA Block 2nd Degree - Type II
- On the ECG, there is a sinus rhythm (we see physiological P waves)
- PP interval is constant because the SA node (P cells) generates impulses regularly
- However, some impulses are blocked upon exit from the SA node (no P wave is generated), resulting in a PP pause
- PP pause is exactly double the PP interval without the pause
- PQ interval remains unchanged (the AV node is intact)
SA Block 2nd Degree - Type II
- PP interval remains unchanged (720ms)
- The impulse is blocked upon exit from the SA node, creating a pause of 1440ms
- The impulse was generated in the SA node but was not conducted to the atria
- PP interval with pause (1440ms) is exactly double the PP interval (2 x 720 = 1440)
Sinus Rhythm
- Laddergram shows the propagation of impulses through the conduction system
- S - P cells (SA node), S-A - T cells (Sinoatrial junction), A - atrium
- In sinus rhythm, the SA node (P cells) generates impulses regularly (green dots)
- Each impulse quickly reaches the T cells (S-A), where conduction slows down
- Then the impulse from T cells (S-A) ascends to the atrium (A - beginning of the P wave)
- PP interval is the same on the ECG
SA Block II Degree - Type II
- SA node (P cells) generates impulses regularly (green dots)
- Impulse is blocked at the SA junction (S-A) and a PP pause occurs
- PP interval with pause is exactly double the PP interval without a pause
Sinoatrial Rhythm
SA Block Degree II - Type II
- PP interval is constant (970ms)
- PP interval with a dropped P wave (1940ms) is
- Exactly double the PP interval without a pause (2 x 970 = 1940)
- PQ interval is constant
SA Block Degree II, Type I (Wenckebach)
- PP interval shortens (1000ms...820ms)
- PP interval with a dropped P wave (1580ms) is shorter
- than double the shortest PP interval (2 x 820 = 1640ms)
- PQ interval is constant
- SA Block Degree II - Type I (Wenckebach)
- Conduction of the impulse through T cells in the SA node
- gradually prolongs until the impulse is blocked
SA Block Degree II - Type II and Junctional Extrasystoles
- Arrows indicate depolarization of the SA node
- Blue - conducted impulses (P waves)
- Black - non-conducted impulses (no P waves)
- PP interval with pause is exactly double any given PP interval
- RR interval with pause
- IS NOT exactly double any given RR interval
SA Block Degree II - Type II and Junctional Extrasystoles
- Note the continuous lead II
- PP interval with a pause is exactly double any given PP interval
- RR interval with a pause is NOT double any given RR interval
- Interpretations are on the ECG above (record with blue and black arrows)
- On 12-lead ECG, for diagnosing SA blocks
- the continuous lead II is used
SA Block Degree II - Type II
- Laddergram
- Illustrates impulse propagation through the conduction system
- S-A - SA junction, A - atrium, A-V - AV junction, V - ventricles
- Note the continuous lead V6 (lowest)
- PP interval with a pause is exactly double any given PP interval
- 2x PP interval without a pause (190) = PP interval with a pause (190)
SA Block Degree II - Type II
- Laddergram
- Illustrates impulse propagation through the conduction system
- SA node generates impulses regularly S1-S6 (670ms)
- Impulse transmission to the atrium through the SA junction is intermittently blocked
- PP pause (1340ms) is exactly double the PP interval (2 x 670 = 1340ms)
Sources
- ECG from Basics to Essentials Step by Step
- litfl.com
- ecgwaves.com
- metealpaslan.com
- medmastery.com
- uptodate.com
- ecgpedia.org
- wikipedia.org
- Strong Medicine
- Understanding Pacemakers
