Page 389 - ECG dr shamol_Slide

an SA block The electrical impulse is delayed or blocked on the way to the atria, thus delaying the

atrial beat.

The sino-atrial node continues to depolarise normally. However, some of the sinus
impulses are “blocked” before they can leave the SA node, leading to intermittent failure

of atrial depolarisation (dropped P waves).

First Degree SA block Delay between impulse generation and transmission to the atrium

This abnormality is not detectable on the surface ECG
Second Degree SA The P-P interval progressively shortens prior to the dropped P

block, Type I wave. This is also known as type I sinus exit block

(Wenckebach) gradual reduction in the P to P interval (the distance

between one P wave and the next). This results in a pause, the cycle
then continues

Second degree SA there is no shortening of the P to P interval but instead an

block Type II unexpected absence of a P wave and subsequent QRS complex The

pause is multiple of the P to P interval

A third degree Looks very similar to a sinus arrest.
sinoatrial block However, a sinus arrest is caused by a failure to form impulses. A

third degree block is caused by failure to conduct them.

It is followed by a long pause that is not a multiple of the P-R
interval.

The pause ends with a P wave, instead of a junctional escape