Narrow complex tachycardias

see also:

• cardiac arrhythmias
• Broad Complex Tachycardias and the DDx
• Supraventricular tachycardia (SVT) for Mx of acute paroxysmal SVT

• tip: measure rate by counting number of QRS on the rhythm strip section of ECG and multiply by 6
• if QRS duration is < 120ms, then it is almost always a SVT rather than a VT
• in SVTs, the P waves will be retrograde and thus hidden in T waves causing a spiky T wave appearance (this can also occur in sinus tachy but the P waves precede the QRS)
• check V1 for atrial flutter P waves which should have a constant P-P interval (may need to flip ECG to see the inverted P waves better)
• if RR interval is irregular, then consider AF, or AT or flutter with variable block (flutter waves), or multifocal atrial tachycardia (MAT - no flutter waves even in compensatory pause regions)
• if regular and rate is 150 (ie. R-R is 2 large squares) then consider:
  • atrial flutter with 2:1 block ¹⁾
  • AV-nodal reentry tachycardia (“SVT”)
  • Orthodromic AV reentry tachycardia in WPW
    • NB. Pseudo-R’ waves in V1-2 = retrograde P waves superimposed on the terminal QRS causing peaking of the J-point may occur in ANRT or orthodromic
  • NB. regular narrow complex tachy with long R-P intervals suggest either:
    • if negative P wave axis in inferior leads then either:
      • Atypical AVNRT
      • Permanent junctional reciprocating tachycardia (PJRT)
    • else an abnormal P wave axis here with an upright P wave in lead II would suggest Focal atrial tachycardia (FAT)
  • Sinus tachycardia
    • should see P waves but may be hidden in the T waves (e.g. with concurrent 1st degree AV block).
    • there should also be some HR variability compared to the other 3 rhythms
    • note maximum sinus rate for sinus tachycardia is generally 220-age in years
  • see https://lifeinthefastlane.com/ecg-library/100-ecgs/quiz-ecg-016/
• If no P waves or evidence of atrial activity is apparent and the RR interval is regular, then AVNRT is most commonly the mechanism.
• If P waves visible and atrial rate > ventricular rate then atrial flutter or AT.
• P-wave activity in AVNRT may be only partially hidden within the QRS complex and may deform the QRS to give a pseudo–R wave in lead V1 and/or a pseudo–S wave in inferior leads
• If a P wave is present in the ST segment and separated from the QRS by 70 ms, then AVRT is most likely.
• If a P wave is present in the ST segment and separated from the QRS by 70 ms, then AVRT is most likely. In tachycardias with RP longer than PR, the most likely diagnosis is atypical AVNRT, permanent form of junctional reciprocating tachycardia (PJRT) (ie, AVRT via a slowly conducting accessory pathway), or AT.
• see DDx tree diagram
• Responses of narrow QRS-complex tachycardias to iv adenosine or carotid massage may aid in the differential diagnosis:
  • no change in rate:
    • inadequate dose/delivery or VT (fascicular or septal in origin)
  • gradual slowering then reacceleration:
    • sinus tachycardia
    • focal AT (atrial tachycardia)
    • nonparoxysmal junctional tachycardia
  • sudden termination:
    • AVNRT (atrioventricular nodal reciprocating tachycardia)
    • AVRT (atrioventricular reciprocating tachycardia)
    • sinus node re-entry
    • focal AT
    • NB: Termination of the tachycardia with a P wave after the last QRS complex favors a diagnosis of AVRT or AVNRT
    • NB: Tachycardia termination with a QRS complex favors AT, which is often adenosine insensitive.
  • persisting atrial tachycardia with transient high grade AV block:
    • AF or flutter
    • AT
    • NB. Continuation of tachycardia with AV block is virtually diagnostic of AT or atrial flutter, excludes AVRT, and makes AVNRT very unlikely