Junctional Rhythms and SVT
Key features of junctional escape rhythms, PJCs, and narrow-complex tachycardias like AVNRT.
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Questions Covered in This Set
10 cards to master
What is the intrinsic firing rate of the AV junction?
About 40–60 beats per minute — it acts as a backup pacemaker when the sinus node slows or fails.
Why is the QRS narrow in junctional rhythms?
The impulse still travels down the His-Purkinje system normally, so ventricular depolarization takes < 0.12 s.
What are the three possible P wave patterns in a junctional beat?
Inverted P just before the QRS (short PR < 0.12 s), no visible P (buried in QRS), or inverted P just after the QRS.
Why are P waves inverted in leads II, III, and aVF in junctional rhythms?
Retrograde atrial activation depolarizes the atria from bottom to top, flipping the P wave axis.
Name the junctional rhythms by rate.
Junctional bradycardia < 40; junctional escape 40–60; accelerated junctional 60–100; junctional tachycardia 100–180.
What causes accelerated junctional rhythm or junctional tachycardia?
Digoxin toxicity, ischemia, myocarditis, post-cardiac surgery, or catecholamine excess.
What is a PJC?
A premature junctional complex — an early narrow beat with an inverted or absent P wave; the junctional cousin of a PAC.
What is isorhythmic AV dissociation?
Sinus and junctional rates are nearly equal, so P waves drift in and out of the QRS; benign and common under anesthesia.
What is paroxysmal SVT and its most common mechanism?
A regular narrow-complex tachycardia at 150–250 bpm with abrupt onset/offset; ~60% is AVNRT (reentry in the AV node using slow and fast pathways).
How do you distinguish sinus tachycardia from AVNRT?
Sinus tach warms up and cools down gradually, rarely exceeds 220 minus age, and has upright P waves in lead II; AVNRT starts and stops like a light switch with hidden or retrograde P waves.
