ECG Case 079

Rate:

• Ventricular Rate 60 bpm
• Atrial rate 90 bpm

Rhythm:

• Regular rhythm
• Regular atrial activity
• Pattern
  • P wave -> QRS -> P wave -> Pause -> Paced Complex + Buried P wave

Axis :

• Sinus Complexes – LAD (-37 deg)
• Paced Complexes – LAD (-80 deg)

Intervals – Sinus Complexes:

• PR – Normal (~180ms)
• QRS – Prolonged (120ms)
• QT – 440ms

Intervals – Paced Complexes:

• QRS – Prolonged (160ms)
• QT – 520ms 

Segments:

• Appropriate ST segment & T wave discordant changes in both Sinus & Paced Complexes

Additional:

• LBBB Morphology of Sinus Complexes
• Right Ventricle Pacing
  • Paced Complexes have LBBB Morphology
• Pacing Interval 60 bpm from Conducted QRS
• If no QRS sensed 1000ms after last QRS PM will pace
• Unknown if P wave following sinus QRS would have conducted with long PR or would have blocked
• P waves hidden in ST segment of Paced Complexes
  • Unlikely to be retrograde conduction
  • Likely Atrial Rate is isorhythmic to Pacing in 3:1 Pattern

Interpretation:

• High Grade AV Block with Escape Ventricular Pacing
• LBBB

Can you tell whether this PPM is DDD or VVI from this ECG?

• The PM is VVI.  
• A DDD pacemaker will be pacing the atrium if the sinus rate is below the lower rate limit only. Usually if the sinus rate is normal or above the lower rate, the way you know it’s a DDD, is that you have every P wave followed by a  paced QRS (A sense V pace) or a conducted QRS with a max set PR interval. (not common).  
• If there is a pattern to the P wave association with the QRS and it is not timing off a conducted beat like in this case, it must be dual chamber otherwise the PM can not coordinate an association.

Why is the PR Interval Normal for Sinus Complexes?

• We know we have significant conduction system disease present in this case, and yet the PR segment is normal for the conducted QRS complexes, why ?
• There are essentially two non-conducted P waves preceding every conducted QRS, this allows time for AVN & HIS systems to recover

• 2nd Degree AV Block
  • Mobitz I
  • Mobitz II
  • Fixed Ratio
  • High Grade
• Paced Rhythms