• Sinoatrial Node (P and T Cells)
• 1st Degree SA Block
• ECG and 1st Degree SA Block

Sinoatrial Node (P and T Cells)

• The Sinoatrial (SA) node consists of 2 types of cells
• P Cells
• These are Pacemaker Cells (Pacemaker means "pace setter")
• They generate impulses in the SA node and determine the heart rate
• They have the property of automaticity
• Located in the central part of the SA node
• If P cells are impaired, there is a disruption of impulse generation:
• Sinus Pause (Sinus Arrest)
• T Cells
• These are Transport Cells
• Located around the SA node
• They transmit impulses from P cells to the atria and internodal pathways
• If the impulse does not pass through the T cells, no P wave is generated
• If T cells are impaired
• impulse generation remains intact (P cells)
• disruptions in impulse transmission occur:
• 1st Degree SA Block
• 2nd Degree SA Block - Type I (Wenckebach)
• 2nd Degree SA Block - Type II
• 3rd Degree SA Block (Complete SA Block)

ECG and 1st Degree SA Block

• The ECG shows a sinus rhythm (physiological P waves are visible)
• 1st Degree SA Block cannot be distinguished from sinus rhythm

Sinus Rhythm (1st Degree SA Block?)

• Physiological P waves
• P waves are a fundamental feature of sinus rhythm
• The beginning of the P wave occurs when the impulse passes from the SA node (from T cells) to the atria
• Sinus rhythm and 1st Degree SA Block have the same ECG appearance

First-Degree SA Block

• SA node (P cells) generates impulses regularly (green dots)
• Each impulse quickly reaches the T cells (S-A), where conduction is slowed
• Conduction in the (S-A) region is prolonged compared to the physiological SA node
• Then, the impulse from the T cells (S-A) moves to the atrium (A - start of the P wave)
• PP interval on the ECG is exactly the same as in sinus rhythm
• It is not possible to distinguish sinus rhythm from first-degree SA block based on the ECG
• Electrophysiological testing is used for diagnosis