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Third Degree SA Block

3rd degree Sinoatrial (SA) block

SA Node (P and T Cells)
SA Block III Degree
ECG and Third-Degree SA Block

SA Node (P and T Cells)

The 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)

Carry impulses from P cells to the atrium and internodal pathways
If the impulse does not pass through T cells, then P wave does not occur

SA Block III Degree

The SA node generates impulses regularly

P cells are intact

The issue is at the impulse exit from the SA node (in T cells)

On the ECG, a PP pause occurs (no P waves are present)
PP pause must be > 2 seconds
If the pause is shorter, it is SA Block II Degree - Type II

ECG and Third-Degree SA Block

No P waves are present on the ECG

A backup rhythm is activated, most commonly junctional
Junctional rhythm is activated after approximately 3 seconds of asystole

If the sinus rhythm reactivates, then

PP pause is X times the sinus PP interval
When a third-degree SA block occurs, all impulses are blocked (P waves disappear)

ECG 3rd degree sa block, junctional rhythm

Third-Degree SA Block

Junctional rhythm
P waves are absent
Third-degree SA block is often indistinguishable from sinus arrest (SA node arrest) on the ECG

Third-degree SA block
- The SA node generates impulses, but all are blocked at the exit in T cells
- PP pause is > 2 seconds
- No P waves are present on the ECG
- PP pause is X times the PP interval
Sinus arrest (SA node arrest)

The SA node does not generate impulses
No P waves are present on the ECG
PP pause is not X times the PP interval

ECG 3rd degree complete sa block, PP pause

Third-Degree SA Block and Sinus Rhythm

Initially, I see 2 P waves (sinus rhythm)
Then follows a PP pause

It could be either a third-degree SA block or sinus arrest

After approximately 3 seconds, the sinus rhythm reactivated

Because the T cells in the SA node stopped blocking impulses

PP pause of 2880ms is X times the PP interval (4x720 = 2880)

Therefore, on the ECG, it is a third-degree SA block

Laddergram SA node depolarisation, sa node, sa junction, atria

Sinus Rhythm

Laddergram illustrates 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 the T cells (S-A) ascends to the atrium (A - beginning of the P wave)
PP interval is the same on the ECG

Laddergram, sa node, sa junction, atria, 3rd degree sa block, pp pause

Third-Degree SA Block

The SA node (P cells) generates impulses regularly (green dots)
However, the impulses are blocked at the S-A junction (T cells), resulting in a PP pause
If the function of the T cells is restored after a third-degree SA block

Then the PP pause is X times the PP interval without a pause

sinoatrial node depolarisation, SA block

Sinoatrial Rhythm

Heart Rate: 60/min.
Physiological P wave

Positive (I, II)
Negative (aVR)

PQ interval (0.18s)
Normal QRS complex (0.1s)
SA Block I and Sinoatrial Rhythm have the same ECG appearance

SA Block I is a very rare diagnosis

and every healthy person has a sinoatrial rhythm

Definitive diagnosis is made using electrophysiological examination

ECG 3rd detree sinoatrial (SA) block and ventricular escape rhythm

Third-Degree SA Block

Ventricular Rhythm (QRS > 0.12s)

Heart Rate: 25/min.
QRS has the appearance of left bundle branch block
Ectopic ventricular focus is in the right ventricle

P waves are not present
There is no sinoatrial PP interval on the ECG, nor is there a PP interval with a pause

Therefore, it could also be a sinus arrest

ECG second degree sinoatrial SA block, type 2

Second-Degree SA Block - Type II

PP interval is the same (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
If the PP pause > 2 seconds, it would indicate a Third-Degree SA Block

which is almost always long-lasting

Third-Degree SA Block

Initially, there are 2 sinus P waves
Then follows a pause (without P waves)
The pause is exactly 4 times the PP interval

During the pause, the SA node generates impulses

but they are blocked (no P waves are seen)

If the SA node temporarily stopped generating impulses (sinus arrest)

the pause would not be a multiple of the PP interval

The pause lasts for 4 seconds, after which the sinus rhythm is reactivated

Usually, after 3 seconds of asystole, a junctional rhythm is activated
If the pause were < 2 seconds (blocking only 1 P wave)

it would indicate a Second-Degree SA Block - Type II

Third-Degree SA Block is long-lasting

this is a rare EKG where the sinus rhythm reactivated

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

Home /

Third Degree SA Block

3rd degree Sinoatrial (SA) block

SA Node (P and T Cells)
SA Block III Degree
ECG and Third-Degree SA Block

SA Node (P and T Cells)

The 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)

Carry impulses from P cells to the atrium and internodal pathways
If the impulse does not pass through T cells, then P wave does not occur

SA Block III Degree

The SA node generates impulses regularly

P cells are intact

The issue is at the impulse exit from the SA node (in T cells)

On the ECG, a PP pause occurs (no P waves are present)
PP pause must be > 2 seconds
If the pause is shorter, it is SA Block II Degree - Type II

ECG and Third-Degree SA Block

No P waves are present on the ECG

A backup rhythm is activated, most commonly junctional
Junctional rhythm is activated after approximately 3 seconds of asystole

If the sinus rhythm reactivates, then

PP pause is X times the sinus PP interval
When a third-degree SA block occurs, all impulses are blocked (P waves disappear)

Third-Degree SA Block

Junctional rhythm
P waves are absent
Third-degree SA block is often indistinguishable from sinus arrest (SA node arrest) on the ECG

Third-degree SA block
- The SA node generates impulses, but all are blocked at the exit in T cells
- PP pause is > 2 seconds
- No P waves are present on the ECG
- PP pause is X times the PP interval
Sinus arrest (SA node arrest)

The SA node does not generate impulses
No P waves are present on the ECG
PP pause is not X times the PP interval

Third-Degree SA Block and Sinus Rhythm

Initially, I see 2 P waves (sinus rhythm)
Then follows a PP pause

It could be either a third-degree SA block or sinus arrest

After approximately 3 seconds, the sinus rhythm reactivated

Because the T cells in the SA node stopped blocking impulses

PP pause of 2880ms is X times the PP interval (4x720 = 2880)

Therefore, on the ECG, it is a third-degree SA block

Sinus Rhythm

Laddergram illustrates 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 the T cells (S-A) ascends to the atrium (A - beginning of the P wave)
PP interval is the same on the ECG

Third-Degree SA Block

The SA node (P cells) generates impulses regularly (green dots)
However, the impulses are blocked at the S-A junction (T cells), resulting in a PP pause
If the function of the T cells is restored after a third-degree SA block

Then the PP pause is X times the PP interval without a pause

Sinoatrial Rhythm

Heart Rate: 60/min.
Physiological P wave

Positive (I, II)
Negative (aVR)

PQ interval (0.18s)
Normal QRS complex (0.1s)
SA Block I and Sinoatrial Rhythm have the same ECG appearance

SA Block I is a very rare diagnosis

and every healthy person has a sinoatrial rhythm

Definitive diagnosis is made using electrophysiological examination

Third-Degree SA Block

Ventricular Rhythm (QRS > 0.12s)

Heart Rate: 25/min.
QRS has the appearance of left bundle branch block
Ectopic ventricular focus is in the right ventricle

P waves are not present
There is no sinoatrial PP interval on the ECG, nor is there a PP interval with a pause

Therefore, it could also be a sinus arrest

Second-Degree SA Block - Type II

PP interval is the same (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
If the PP pause > 2 seconds, it would indicate a Third-Degree SA Block

which is almost always long-lasting

Third-Degree SA Block

Initially, there are 2 sinus P waves
Then follows a pause (without P waves)
The pause is exactly 4 times the PP interval

During the pause, the SA node generates impulses

but they are blocked (no P waves are seen)

If the SA node temporarily stopped generating impulses (sinus arrest)

the pause would not be a multiple of the PP interval

The pause lasts for 4 seconds, after which the sinus rhythm is reactivated

Usually, after 3 seconds of asystole, a junctional rhythm is activated
If the pause were < 2 seconds (blocking only 1 P wave)

it would indicate a Second-Degree SA Block - Type II

Third-Degree SA Block is long-lasting

this is a rare EKG where the sinus rhythm reactivated

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