=reentry tachy that does revolve around the AV Nd (as oppose to AP associated SVT)
Sx
-palpitations
-+/- malaise, pallor, nausea, diaphoresis, = sympathetic discharge fr the tachy
-+/- SOB
-LOC is rare
-Triggers
-phys activity, emotl stress, abrupt change body position (e.g bending over)
-Trigger more influenced by incr symp tn than AP associated SVT
-thus less likely to a kid w SVT at rest to have AVNRT
-Unlike AP assoc SVT, AVNRT is less likely to be incessant (thus less assoc tachy CM)
Epi
-uncommon in small kids (3-13% of infant SVT), and incr w age, becoming more common than AVRT by teen years
--Thus if 1st SVT is <teens, likely AVRT, vs >teens likely AVNRT
ECG
-regular narrow tachy, constant rate w/o much variation at all
-rarely does it 'oscillate' w some irregularity noted...
-Typical AVNRT = "slow-fast AVNRT" = goes down slow path, up fast --> short RP tachy
-P waves buried in QRS so unseen, or are close after QRS
-Atypical AVNRT = "fast-slow AVNRT" = goes down fast, up slow --> long RP tachy
-see superior axis of P waves, retrograde
-+/- RBBB or LBBB that is rate related due to aberrancy, --> widen QRS, so must DDx VT
-Rate can be bn 120-300bpm, usually 180-250bpm
-Dual AV Nd Physiology
= 2 fxl conduction pathways exist bn atrium & penetrating His bundle
-e.g. antero-superior atrial connection to AV nd has a faster pathway, but longer refract pd than a slower post-inf pathway, which has a shorter refract pd ==> PAC at the right time can fall in the fast pathway refract pd and thus be conducted down the slow pathway alone, enter the His-Purkinje syst and activate the ventricles (--> QRS), but will also be able to up the fast pathway as it is no longer refractory by that pt
==> in the usual AVNRT, the atrium is activated retrogradely at the same time as ventricles antegradely = down the slow & up the fast; P wave buried in QRS...
-w each cycle, the stim goes down slow, ventricles are activated, then up fast...
-Demonstrate Dual AV Nd Physio in EP Study:
-Use PACs w shorter and shorter coupling interval of atrial extrasimulus, causing the AH interval to lengthen fr decremental conduction (at this pt it is still going down the fast path). Eventually, you hit the HPS during the refractory pd, so the stim must go down the slow pathway, --> thus you get a jump in the AH, and it increases suddenly by >50msec (after just a 10msec decr in PAC coupling interval). This demonstrates there are two pathways at the AV Nd, and that one is fast and one is slow.
-In this setting, when you put the pt in SVT, the stim is going Down Slow, and Up Fast (short RP)
-If pt has Atypical AVNRT, then it goes Down Fast, Up Slow, so long RP, and PR is closer to Nl duratn
-If pt has similar velocities at each pathway ("slow-slow" form) --> see retrograde P wave (in II, III, F) in the middle of the tachycardia cycle length (neither short or long RP...)
-It is possible to have 2:1 AVNRT bc the tissue in HPS & ventricles are independent of the circuit, so if block in the HPS, can have 2:1..., thus must DDx fr atrial flutter; do so by checking for Dual AVN Physio...
-Might also demonstrate Dual AVN Physio w stim at the ventricles w PVCs rather than as above, then see the PVC get blocked retrogradely in the fast pathway, and so only retro Q thru slow path ==> the AVNRT is usually a "fast slow" (atypical, long RP) AVNRT
Tx
-Indication for Rx d/o Sx, Sx duration, ability to stop w vagals, degree of intrusion, eg on sports
-b-blockers = 1st choice- well tolerated
--> reduce the trigger (PACs) or block the conduction of the PAC thru AVn
-xx = fatigue, malaise, exercise limitations, espec if older class like propranolol, but newer selective, longer acting B1 blockers- atenolol/nadolol cross BBB less so less xx
-Digoxin- less used than in the past, but it's cheap and does work
-CCBs- xx = fatigue, but good for ANRT especially if b-Blocker xx were too much
-Flecainide (Class I = Na Ch block), and Amiodarone (Class III) - if refract to other Rx if ablation is ng
-Ok to Tx Infants w/o Rx if infreq episodes, ASx, ?exact mech
-Sometimes by ECG you cannot DDx AVNRT fr concealed AP supported SVT
EP Study & Ablation
-ok to go straight to EPS/Ablation rather than Tx w Rx...
-c/s giving a dose of adenosine first, and see if there is AV block - if so then likely not AP mediated (thus likely AVNRT) bc APs are usually not adenosine responsive...
-note 30-50% of Nl kids (no SVT Hx) have dual AVN physio at EPS, and 1/3 of AVNRT pts have Nl physio when at baseline/not in SVT.
-can c/s isoproteronol to induce SVT...
-Intacardiac- during AVNRT SVT see near simultaneous A & V activation
RF & Cryo Ablation
...