SVT
-#1 peds tachycardia
-1/2500 to 1/1000 kids w it
-90% of SVT involves a reentry circuit bn atria & ventricles, or AV Nodal reentry, or PJRT (persistent jctl reentry tachy)
-most (50-60%) present at <1yo
-% pts w AV reentry tachy decreases w age (85% of SVT is AV Reentry Tachy at <1yo, 82% at 1-5yo, 56% at 10yo
-most don't recur! - 15-41% of infants <6mo w SVT don't recur after 1yo
Wolff-Parkinson-White Syndrome
-Manifest Pathway
-a strand of atrial tissue (likely) crosses fibrous border bn atria & ventricles (AP =accessory pathway)
-Antegrade activation across the strand is seen on the ECG
-WPW = a manifest pathway; conducts antegrade at the AP
-stim's ventric b4 the path thru the AVN/HPS--> pre-excitation at ECG
-Incidence: low: 1.6/1000 to 4/100,000
-pre-excitation might be subtle- only see a lack of Q wave in lateral precordials, bc
-slow conductn in AP (so gets to ventric at about same time as the AVN/HPS path)
-AV nd accelerated conduction (so arrive at same time as the AP stim), common in kids
-the AP is a L Lateral pathway, in MV area; the #1 site for WPW AP, may only show intermtntly
-Concealed pathway is not WPW; it only conducts retrograde (unidirectional), so ECG is Nl!!
-WPW pts are more likely than concealed pts to recur.
-WPW is age related-
-may see pre-excitation only intermittently (espec if L Lat AP),
-may decr w growth, usually by 2yo.
-WPW more likely to recur if it presents at >1yo
-one study--> 94% recur if present at >1yo vs 29% if present at <1yo
-Assoc CHD
-most Nl (85-90%)
-ccTGA in
-Ebstein's anomaly
-cardiomyopathy
-if + CHD, incr risk of multiple APs (6-9%, but 43% if +Ebstein's)
-R sided AP- 63% are + CHD vs. Left sided AP- only 39% are+ CHD
Dx
-pre-excitation on ECG --> usually AVRT
-ID location by delta wave pattern
-if QRS transition (meaning R>S) is after v2 in precordias, it's likely R sided, if b4 V2 it's likely L sided...
-Best to have ECG during SVT
-retrograde P waves (inverted at II, III, aVF)
- R-P retrograde interval >65-70 msec in the T wave - seen often w AVRT SVT
-negative retrograde P wave in I, during narrow QRS tachy --> likely AVRT w retrograde conduction thru a L sided AP
AV Reentry Tachycardia Mechanism
-Orthodromic
-90% of AVRT
-antegrade Q thru AVN/HSP--> retrograde thru AP ==> narrow complex tachy
-concealed or manifest pathway when in sinus
-if manifest pathway, you don't see the ventric preexcitation (delta) during the SVT bc the antegrade Q is entirely thru the AVN/HSP, going rapidly, and the AP is retrograde rapidly
-Antidromic
-antegrade Q thru AP and retrograde Q thru HSP/AVN --> wide QRS bc it is going down the AP, and thus mimics VT.
AV & Mahaim Fibers
-Mahaim Fibers- atriofascicular AP bn lateral RA TV area to deep into R bundle, rather than AV groove
-may be manifest w delta and short PR
-if nodo-fascic fiber is fr AVN to RV--> LBBB pattern if engaged; PR varies by where it takes off
-if fascicular-ventric fiber fr His bundle to RV; Nl PR interval
(-may also act as a 'bystander' and provide a limb for an AVNRT AP upstream...)
--> pre-excitation w LBBB pattern & wide complex tachy (bc you hit RV first)
-R sided atriofascicular fiber (sometimes also called Mahaim fiber)
-in R post AV groove
-antegrade only usually; --> LBBB pattern
-inserts deep in RV, near distal fascicle of R bundle (in moderator band)
-has decremental conduction
--->s antidromic tachycardia w a wide QRS (LBBB pattrn) w SVT (antegrade thru the AP, retro thru HPS)
DDx:
in SVT:
-Concealed: 200-300bpm if <1yo, 160-250 if kids
-regular rhythm
-P: absent/neg & within T wave of II & aVF
-QRS: Nl, wide if aberrant conduction
-when in sinus--> Nl, w/o pre-excitation
-WPW Orthodromic (down AVN, up AP): 200-300bpm if <1yo, 160-250 if kids
-regular rhythm
-P: absent/neg & within T wave of II & aVF
-QRS: Nl, wide if aberrant conduction
-when in sinus--> + pre-excitation & short PR
-WPW Antidromic (down AP, up AVN): 200-300bpm if <1yo, 160-250 if kids
-regular rhythm
-P: absent
-QRS: wide, similar to or wider pattern than the sinus rhythm
-Mahaim Atriofascicular: 200-300bpm if <1yo, 160-250 if kids
-regular rhythm
-P: absent
-QRS: wide, LBBB QRS pattern
-Mahaim Nodo- or fasciculo-ventricular: 200-300bpm if <1yo, 160-250 if kids
-regular rhythm
-P: absent/neg, buried w/in T wave in II
-QRS: wide, with a QRS pattern
Dx Testing
-Exercise Stress Test
-if pre-excitation abruptly disappears, --> = AP has an effective refract pd that is long (360-390msec) --> low risk (of transmission of a-flutter etc)
-xx - less diagnostic if L sided pathway bc of enhanced AVN conduction time fr incr adrenergic tone, --> masks the pre-excitation thru the L sided pathway. & hard to read bc of artifact...
-EP Study
-check baseline HV
-HV is <40 or negative (if ventric activated b4 His)
-if manifest AP--> see location of ventric insertion- see earliest area of ventric activation, prior to surface QRS onset
-check baseline atrial & ventric pacing thresholds
-check antegrade EP props of AVN & the manifest AP
-atrial pacing at a cycle length a bit shorter than sinus, and graduallydecr by 10-20msec increments, until no longer 1:1 AV conduction (Wenckebach cycle length)
-w manifest conduction, may achieve antegrade block in AP much before you block in AV nd, or block in AV nd before you block in the AP --> localize it.
-then determine AV nd & AP antegrade refract pd by delivering an extrastimuli after an 8-10 beat drive cycle length. Decrement the S2 stim by 10-20msec, and watch for an abrupt change in the HV interval and ventric activation, that occurs when the AP is blocked (AP effective refract pd), or when you lose AV conduction (AVN effective refract pd)
-do same for atrial effective refract pd
-if WPW, the antegrade AP ERP is used for risk stratification
-short RR interval during rapid afib (RR <220msec) -->high risk
-then give adenosine to block AVN & do ventric pacing, cycle length 500msec to ensure it's faster than the reflex sinus tachy after giving adenosine, to check for AP's...
-adenosine usually blocks retrograde Q thru AVN but not the AP
-check wenckebach cycle length, and ERPs w retro conduction...
-may need isoproterenol to induce SVT if unable w pacing alone...
.......
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Treatment
-Observation
-AVRT is only rarely life threatening
-only rarely causes syncope (if orthodromic)
-obsv ok if only infreq Sx, espec in small kids
-just use vagal maneuvers, & if tachy continues then see MD
-Acute Tx
-esoph overdrive pacing in infants mainly
-adenosine 90-100% effective w 25-30% recurrence
-be ready for a-fib, that might occur after adenosine admin
-may degenerate to v-fib
-rare to have WPW+a-fib (1% of pts who present w narrow complex tachy), but have a defibrillator available.
-Verapamil- 80-95% effective
-c/i xx if infant w CHF bc CCBs are a negative inotrope ...--> hypotension
-if pt tolerates SVT but vagals done help, can use a betablocker PRN or CCB PRN to slow/terminate the SVT
-Antiarrhythmic Meds
-Digoxin, beta-blocker
-if WPW, no digoxin bc it can shorten the AP ERP
-Na & K ch blocker are effective
-amio can be effective, but not for longer than 18-24 months
-no CCBs if <1yo
-flecanide + sotolol como can be effective for refractory cases
-Ablation
-effective...
Prognosis
-not common to have spontaneous resolution of AVRT SVT if it continues beyond 1-2 years
-Indication for ablation - pt preference
-ablation success rate 90-95%, best for L sided pathways, lowest if para-Hisian pathway
-Risks
-Hrt Block- 1.2%, 10% w midseptal ablation
-WPW sudden death- said to be 3-6%, likely fr a-fib that --> VF, ; RR interval <220msec during a-fib has 100% Sn for sudden death, so do prophylactic ablation