- Healthcare Professionals
AV Reentrant Tachycardia (AVNRT) is caused by a macro-reentry circuit involving the AV node and an abnormal second bridge between the atria and the ventricles, called an “accessory pathway” that uses a cardiac muscle fiber that breeches the usually non-conducting tricuspid or mitral annulus.
AVRT can be formed in two different ways: Orthodromic or Antidromic reentrant loops. Orthodromic AVRT is the most common by far and occurs when a conduction signal travels through the AV node; then retrogradely up the accessory pathway into the atria and back into the AV node perpetuating the loop. This type of AVRT like AVNRT (please see AVNRT) is often initiated by a PAC but may also begin with a PVC.
Often it occurs during sinus tachycardia when conduction times in the AV node and accessory pathway allow for capture of the AV node by the reentrant wavefront rather than by the sinus node wavefront. With Orthodromic AVRT the QRS complexes tend to be narrow since the AV node passes the signal to the ventricles instead of the accessory pathway (receives signal retrogradely). On occasion, at the outset of AVRT, either left or right functional bundle branch block can occur prior to normalizing the QRS when the His-Purkinje system “warms up”.
Orthodromic AVRT and AVNRT can closely mimic one another, but there is several key differences between them.
Orthodromic AVRT will typically have a long R-P interval (inverted p wave post QRS) usually in the ST segment or upstroke of the T-wave and AVNRT will generally present with an inverted p wave immediately after the QRS or even buried by the QRS. AVNRT also has the hallmark of being by a PAC with a long PR interval in >90% of cases. PVCs also are not typically seen conducting retrogradely into the atria with AVNRT but are common in AVRT.
Antidromic AVRT occurs when a supraventricular impulse first travels through an unimpeded accessory pathway (bypassing the AV node); then retrogradely travels backward through the AV node. Because the signal does not follow normal conduction pathways, when it reaches the ventricles over the accessory pathway, it is passed via cell-to-cell transmission and thus the QRS tends to be slurred and wide, sometimes being confused with VT. Antidromic AVRT is usually seen with obvious WPW. There are many manifestations of this type of arrhythmia and beyond the scope of this discussion. Suffice to say that it should be suspected in a patient with WPW who appears to have VT.
4-year-old male with no known structural heart disease other than Wolff-Parkinson-White (WPW). This patient had 29 episodes of AVRT. Longest run 1.5 hours at rates from 154 bpm – 220 bpm. Fastest run 21 minutes at rates from 186 bpm – 242 bpm. Occasional, spontaneous antegrade block over the accessory pathway resulted in the onset of. See strip below.
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Example Strip #23: AVRT onset (green arrows) after spontaneous antegrade block in the accessory pathway, Sinus with pre-excitation showing a delta wave (yellow arrow), retrograde P-wave (light blue arrow).
Example Strip #27: AVRT Offset (red arrows), retrograde P-wave (light blue arrow), Sinus with pre-excitation (yellow arrow).
7-year-old male with palpitations. Predominant Rhythm: NSR – 5 brief episodes of AVRT, totaling 2 minutes and 15 seconds, at heart rates up to 230 bpm; Longest lasting 30 seconds. Most episodes, including echo couplets, ended with an RP interval >120ms. Two runs of AVRT showed LBBB to normal QRS transitions consistent with the absence of a left sided bypass tract that can be softly inferred by the absence of a longer AVRT cycle length during LBBB of more than 35ms. No PVCs or PACs
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Example Strip #2: NSR with Echo Pair (red arrow).
Example Strip #17: AVRT with Left Bundle Branch Block (LBBB) onset (red arrows), Sinus Tachycardia (yellow arrow). Note retrograde p-waves post QRS complexes (light blue arrow).
Example Strip #18: Continuation of the previous AVRT onset strip with normal QRS complexes (yellow arrows).
Example Strip #19: Termination of AVRT (red arrows), Sinus Bradycardia with PAC (yellow arrow). Note this episode does not terminate with a retrograde p-wave. Block occurs in the accessory pathway rather than the typical location of the AV node. This patient had block occur both ways.
Example Strip #33: ST with Echo Pair (red arrow)