ECG Update

In our ECG rounds at WCH this morning led by Dr. S. Chun, we reviewed several high-yield ECGs. Below I’ve outlined some key pearls with respect to each:

ECG#1- 39F with palpitations


Main findings: short PR interval with slightly widened QRS, most pronounced in leads I, V4-V6. Delta waves can be seen (A slurred upstroke to the QRS complex).


  • PR interval=beginning of P until the R
    • Represents conduction through the atria and AV node
    • Normal is 120-200 ms duration (3-5 small squares)
    • can see in rhythm strip above looks slightly < 3 squares
    • Visually, also looks like there is no space between the end of the P and beginning of QRS complex
    • Prolonged PR: 1st degree AV block
    • Shortened PR:
      1. Due to pre-excitation (presence of an accessory pathway)
        • conduction faster through accessory pathway, hence short PR
        • accessory pathway acts as re-entry site, enabling reentry tachyarrhythmias  (i.e. SVT)
        • Examples include:
          • WPW (Wolff-Parkinson-White) Syndrome
            • Delta Waves present
          • LGL (Lown-Ganong-Levine) Syndrome
            • Delta Waves absent
      2. AV nodal/junctional rhythm (i.e. rhythm originates close to/in the AV node)

ECG#2- 59 M with palpitations and dyspnea

Case 2.png

Main findings: Narrow complex, regular, tachycardia at a rate of ~150 BPM. Note that there are classic sawtooth P waves present in the rhythm strip respresented by lead II, consistent with atrial flutter. Specifically, this represents Atrial flutter with 2:1 AV conduction block.


  • Causes of regular, narrow complex tachycardias:
    • Sinus tachycardia
    • Atrial tachycardia
    • Atrial Flutter
    • AVNRT
    • AVRT
  • Sawtooth waves appear in inferior leads (II, III, aVF) typically
  • Strategies for how to recognize the pattern of conduction block in setting of flutter:
    1. Normally, atrial flutter firing occurs at ~300 BPM
      • In this case, the estimated rate is 150 BPM, indicating a 2:1 block
    2. Count the number of sawtooth waves between every R-R interval
      • In this case, 2 sawtooth waves for every QRS, indicating 2:1 block
    3. When  you see flutter waves, calculate the atrial rate and ventricular rate
      • In this case, atrial firing accurs at 300 BPM (one large box), while ventricular firing occurs at 150 BPM (2 large boxes), indicating a 2:1 block
  • The most common conduction delay pattern in atrial flutter is 2:1 block; higher degrees of block are usually seen in structural heart disease or medication-related

ECG #3- 69 M history of PCI x 2 for STEMI 3 years prior


Main findings: Less obvious here, but there are Q waves seen in lead I and aVL along with tall R waves in the septal leads (V2 in particular). This ECG is suspicious for a  posterior infarct.


  • Q waves
    • any negative deflection that precedes an R wave
    • Significance:
      • small ones may normally be seen in multiple leads
      • Deeper ones (i.e. 2 mm/2 small boxes)  in leads III and aVR might also be normal
      • Pathologic if:
        • >1 small box (40 ms) wide
        • > 2 small box (2 mm) deep
        • >25%  length in relation to QRS
        • any Q wave in V1-V3
      • In the above ECG, the Q waves seen are deep and wide (especially aVL)
  • Causes of Tall R waves in V1 and V2
    • If R>S in septal leads, more common finding might be right ventricular hypertrophy
    • However, another important etiology would a posterior infarct; ECG with R-sided leads should be undertaken to confirm this

ECG #4-54 M post-CABG, stable


Main findings: note the abnormal appearing P wave seen in Lead II. While at first glance, this looks to be in sinus rhythm, this is actually an ectopic atrial rhythm.


  • Criteria for sinus rhythm:
    • Regular (at 60-100 BPM)
    • P preceding QRS, QRS following P
    • Normal P wave axis (if originating in SA node)
      • positive in I, II, and negative in aVR
  • Inverted P wave causes:
    • P wave inversion may be indicative of a non-sinus P wave origin of which there are 2 possibilities:
      • When the PR interval is < 120 ms, the origin is in the AV junction (e.g. accelerated junctional rhythm)
      • When the PR interval is ≥ 120 ms, the origin is within the atria (e.g. ectopic atrial rhythm)

ECG #5-70 M with syncope and fatigue


Main findings: At first glance, there is likely a rate problem here. But might there  be a rhythm problem too?

When uncertain, start with the rhythm strip (lead II, at the bottom). Working your way through from left to right, you do, in fact, see a P wave, followed by a QRS, and T wave. This cycle repeats itself once before a long pause. Then, there are two cycles where a QRS-T  is NOT preceded by a P wave. Finally, there are two cycles of P-QRS-T before reaching the end of the strip.

Considering these findings, there is a baseline sinus rhythm followed by a prolonged sinus pause. You can tell that there is a sinus pause, rather than an AV block, because there are no P waves present prior to the pause (the pause follows a repolarization as indicated by the T wave). In high-grade AV block, you would see a P wave prior to the pause which is not present here.

When the sinus node pauses, and a subsequent beat originates from a non-SA node focus, you can determine the origin of the subsequent beat based on the QRS morphology. If the QRS is narrow, the triggering source of the beat must be supraventricular (ie. junctional); whereas if the QRS were wide, that would indicate a ventricular source.

Ultimately, this ECG would represent sinus rhythm with sinus pause as well as junctional escape beats.

Additional Resources:

Need more practice/teaching about ECGs? Check out the following helpful ECG websites:


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