1st Degree Heart Block

* PR interval is prolonged, ~ 360ms  (normal PR interval is 120 - 200ms or 3 - 5 small squares)

Causes of 1st degree heart block include: 

Coronary artery disease

Acute rheumatic carditis

Digoxin toxicity

Electrolyte disturbances

2nd Degree Heart Block

* Mobitz Type II --> PR interval remains constant, however one P wave is not followed by QRS complex

* Wenckebach / Mobitz Type I --> Progressive lengthening of PR interval, followed by one P wave not conducted, next conducted beat has shorter PR interval

* 2:1 / 3:1 --> 2 P waves followed by QRS complex or 3 P waves followed by QRS complex

P waves can be in the T waves as well.

 (2:1)

Causes of 2nd Degree Heart Block are the same as 1st Degree Heart Block

3rd Degree Heart Block / Complete heart block

- Atrial contraction is normal, however, none of the beats are conducted to the ventricles

- Could be due to fibrosis of the Bundle of His from previous myocardial infarction, or can occur during an acute myocardial infarction

** No relationship between P waves and QRS complexes. QRS complexes may be abnormally shaped as well due to the inconsistent spread of impulses in the ventricles. 

** the easiest way to see this is to draw a line to mark every P wave - make sure that the interval between each P wave is similar and constant, then see how the QRS complexes are not related to the P wave.

Right Bundle Branch Block

* has not much significance, may be a normal variant

* best seen in V1 as an M pattern - RSR1 / however in V6 there will be a flip - W pattern seen

Left Bundle Branch Block

* Best seen in V6 - M pattern, likewise at V1 it will be seen as a W pattern

* Causes: Aortic stenosis, ischemic disease

* When patient complains of chest pain (symptomatic), it could indicate an acute myocardial infarction

**** Left axis deviation + Right bundle branch block = SEVERE conducting tissue disease, may require pacemaker ****