QRS #3 is the "giveaway"




Figure 1

This atrial fibrillation as seen by the fibrillatory waves. After QRS 1-3 are wide QRS complexes at regular intervals with a rate of about 60 bpm. Are these aberrant QRS complexes or ventricular ectopics (acceleated idioventricular rhythm or AIVR)?

QRS #3 is the "giveaway" in this case. This QRS complex  is intermediate in morphology between the sinus beat and the ectopic ventricular beat. This is because the ventricles are depolarized both by the ectopic ventricular impulse and a supraventricular impulse.  QRS#3 is a "fusion" beat. Thus, this wide QRS rhythm is AIVR.  

The Practical Value of a PVC in ECG Interpretation


Premature ventricular complexes (PVC) often are useful in ECG interpretation. Sometimes, in patient with long PR interval (first degree AV block) the P waves seems to merge/fuse with the T waves. If you look for a PVC, there you will see the distinct P wave and you are now convinced that there is sinus rhythm with long PRI (first degree AV block)



Image 1 - A 12-lead ECG misread by machine

Did You Know That: There are Two (2) Types of Mobitz I



Second degree AV Block Type I is also known as AV Wenckebach or Mobitz I. Did you know that there are two (2) types of AV Wenckebach? It could either be Typical or Atypical AV Wenckebach. This is how to recognize these ECG Patterns:


Second Degree AV Block Type I (Typical)

ECG Recognition:

  • ·       The P wave is normal.
  • ·       The PR interval progressively lengthens until a P wave is not followed by a QRS.
  • ·       As the PRI lengthens, there is shortening of the RR interval.
  • ·       The RR interval containing the dropped P wave is less than 2x of the shortest RR interval.
  • ·       The PRI of the first conducted P wave (may be normal or prolonged) is shorter than the PRI of the       last conducted P wave.
  • ·       The largest increment in the PRI is usually on the second conducted P wave.
  • ·       There is "group-beating" on the ECG.



Figure 1 – Sinus rhythm, second degree AV block type I (Mobitz I/Wenckebach) with 3:2 AV conduction. There are regular sinus P waves at a rate of about 100 bpm. There is group-beating of the QRS with a P to QRS ratio of 3:2. The first conducted P waves are marked with green arrows with a PR interval (PRI) of 0.20 seconds. The next conducted P waves are marked with a blue arrow with a PRI of 0.28 seconds. The non-conducted P waves are marked with red arrows. As can be seen there is prolongation of the PRI interval until a non-conducted P wave.  This is typical AV Wenckebach.


Complete Heart Block (CHB) ? Think Again


No clinical hx. Is this complete heart block (CHB)?


Image 1

This is sinus rhythm (SR) at about 88 bpm, first degree AV block, second degree advanced heart block or high-degree AV block, right bundle branch block.

It is very tempting to call this CHB. However, the clue that a sinus P wave is conducted is the irregularity of RR interval. There is sudden shortening of the RR interval (R4R5) vs the rest of the RR interval. If this is CHB the RR interval would be the same in all because in CHB the ventricles will be under the control of an ectopic junctional or ventricular focus. So, it would beat like clockwork. If there was shortening or difference in the RR interval, this means that a sinus impulse was conducted.

A simple trick to differentiate CHB from advanced heart block is the regularity of the RR interval. 



AT Will Look Like ST



Vignette: A 60 yo with h/o HTN, HLD, hyperthyroidism is admitted for sepsis. 

Cardiac telemetry showed event of tachycardia (~130 bpm).


Image 1

This will look like sinus tachycardia (ST). However, carrefull examination of initiation and termination would argue against ST. In ST thre is a gradual warm-up and cool-down in rate. In here there is "abrupt" or sudden increase and decrease in rate. Thus, this is paroxysmal atrial tachycardia (PAT)


Image 2


It is even better viewed by a heart rate histogram or trend.


Image 3