Mastering ECGs Part 2: Heart Blocks and Conduction

Second post in the mastering ECG series. Read the first post here. In this post let’s explore conduction pathways and the various changes that we see in ECGs in case of heart blocks

Problems in the AV node and his bundle

 First Degree Heart Block

• Each wave of depolarisation that originates in the SA node is conducted to ventricles, but there is a slight delay because of which PR interval is prolonged i.e, greater than 5 small boxes (200ms)

Second Degree Heart Block

• Sometimes all the electrical excitations are not conducted to the ventricles. This happens intermittently. There are different variations of this phenomenon:
  • Mobitz type 1 or Wenckebach: Progressive lengthening of PR interval –> subsequent failure of conduction of electrical activity to the ventricles (dropped beat –> conducted beat with shortened PR interval. This cycle repeats.Mobitz Type 2: Most beats are conducted with a normal PR interval. Sometimes, the electrical activity isn’t conducted to ventricles, resulting in a missed beat.
  • 2:1, 3:1, 4:1 : Alternate conducted atrial beats, resulting in twice the number of P waves compared to QRS complexes (2:1). Can be 3:1 or 4:1 too

Third Degree Heart Block

• Atria and ventricles get excited independently of each other
• No relationship exists between P waves and QRS complexes.
• Abnormal-shaped QRS complexes because depolarization wave is originating from a ventricular focus. It is usually slow.

Conduction Problems in Right and Left Bundle Branches

• In the normal heart, the time taken by the wave of depolarisation to spread from the interventricular septum to the furthest ventricles is less than 120ms, represented by three small squares of ECG.
• If this conduction occurred through an abnormal pathway, the time taken is longer, resulting in the widening of QRS complexes. This means that widening can indicate a bundle branch block.
• Widening of the QRS complex can occur if the depolarisation begins within the ventricular muscle itself.
• How do we differentiate? In the case of sinus rhythm with bundle branch block, we can see the p wave. P wave is absent in those cases where the depolarisation starts from the ventricular muscle.
• What if both the branches (left and right) get blocked? It has same effect as block of His bundle (complete heart block)
• RBBB often indicates problem in the right side of heart. However RBBB ECG patterns can be seen in healthy people.
• LBBB patterns are always an indication of heart disease – usually left side.

• Septum is normally depolarised from left to right.
• The left ventricle has more mass. So, it exerts a greater influence on ECG pattern.
• A depolarization wave towards a lead causes an upward deflection in ECG.

Recap of first priciples before we move any furthur.

Right Bundle Branch Block

• No conduction occurs via the right bundle branch. However, the septum is depolarised from left to right as usual. This results in an upward deflection (R wave ) in V1 and a downward deflection (q wave) in lead V6.
• The left ventricle is depolarised before the right ventricle. This results in downward deflection in V1 (s wave) and upward deflection in V6 (R wave)
• The right ventricle is now depolarised, resulting in another positive deflection in V1 (R’ wave) and a deep negative deflection in V6 (deep S wave).
• This results in the “M” pattern in V1 and a “W” pattern in V6. Mnmemoniv MaRroW (M in V1, R for RBBB and W in V6)
• RSR’ pattern with a QRS complex of normal width is called partial RBBB and can be considered as a normal variant.

Left Bundle Branch Block

• No conduction occurs via the left bundle branch. The septum is depolarised from right to left. This results in a downward deflection (q wave) in lead V1 and an upward deflection (R wave ) in V6.
• The right ventricle is depolarised before the left ventricle. This results in upward deflection in V1 (R wave) and downward deflection in V6 (s wave)
• The left ventricle is depolarised, resulting in a positive deflection in V6 (R’ wave) and a deep negative deflection in V1 (deep S wave).
• This results in the “W” pattern in V1 and a “M” pattern in V6. Mnmemoniv WiLliaM (W in V1, L for LBBB and M in V6)
• W pattern with lead V1 may not always be present in LBBB.

Conduction problems in distal parts of left bundle branch

• The right bundle branch has no major division, and the left bundle branch has two major divisions – anterior fascicle and posterior fascicle. The cardiac axis depends on the average direction of depolarisation of the ventricles.
• Left Anterior Hemiblock: The anterior fascicle of the left bundle branch fails to conduct, so the posterior fascicle depolarises the left ventricle. The cardiac axis is rotated upwards – left axis deviation
• Left posterior hemiblock: ECG shows right axis deviation
• Right bundle branch block: features of RBBB and normal cardiac axis
• RBBB + Left anterior hemiblock: Bifascicular block – RBBB features + left axis deviation