Sino-atrial Block-Atrio-ventricular (A-V) Block- Incomplete A-V Block -

Arrhythmia Resulting from Conductive Conductive Block
Sino-atrial Block

In rare instances, SAN impulse is blocked before it enters the atrial muscle.

In ECG there is sudden cessation of P waves, with resultant standstill of the atria.

The ventricles pick up a new spontaneous rhythm, usually originating from the A-V node.

So the rate of the ventricular QRS-T complex is slowed down
Otherwise, ventricular depolarization (QRS) and repolarization (T) waves are not altered

Atrio-ventricular (A-V) Block
Conditions that can slow impulse conduction rate in bundle of His or block the impulse entirely are:

Ischemia of A-V node or A-V bundle fibers resulting from coronary insufficiency.

Compression of the A-V bundle by scar tissue or calcified portions of the heart.

Inflammation of A-V node or A-V bundle (resulting from different types of myocarditis, (diphtheria / rheumatic fever).

Extreme stimulation of the heart by the vagus nerves in rare instances blocks impulse conduction through the A-V node.

Incomplete A-V Block
Prolonged P-R Interval (First-Degree Block)
The usual lapse of time between beginning of the P wave and beginning of the QRS complex (P-R interval) is about 0.16 sec. 

When P-R interval > 0.20 sec, it is said to be prolonged & the patient is said to have first-degree incomplete heart block.

Thus, first-degree block is defined as a delay of conduction from the atria to the ventricles but not actual blockage of conduction.

P-R interval seldom increases > 0.35-0.45 sec, by that time, conduction through A-V bundle stops entirely.

One means for determining severity of some heart diseases (acute rheumatic heart disease) is to measure P-R interval


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Incomplete A-V Block
(Second-Degree Block)
When A-V bundle conduction is significantly slowed, sometimes AP is strong enough (& sometimes is not) to pass through the bundle.

In this instance, there will be atrial P wave but no QRS-T wave, & it is said that there are "dropped beats" of the ventricles.

This condition is called second-degree heart block.

At times, every other beat of the ventricles is dropped, so a "2:1 rhythm" develops.

At other times, rhythms of 3:2 or 3:1 also develop.