Re-entrant Supra Ventricular Tachycardia
MECHANISM OF ACTION
Wolff-Parkinson-White syndrome: WPW (preexcitation)
syndrome is the most common accessory pathway SVT,
occurring in about 1 to 3/1000 people.
In classic (or manifest) WPW syndrome, antegrade conduction
occurs over both the accessory pathway and the normal
conducting system during sinus rhythm. The accessory
pathway, being faster, depolarizes some of the ventricle early,
resulting in a short PR interval and a slurred upstroke to the
QRS complex
In concealed WPW syndrome, the accessory pathway does not
conduct in an antegrade direction; consequently, the above
ECG abnormalities do not appear. However, it conducts in a
retrograde direction and thus can participate in reentrant
Two pathways connect the same points. Pathway A has slower tachycardia.
conduction and a shorter refractory period. Pathway B conducts
normally and has a longer refractory period.
In manifest Wolff-Parkinson-White (WPW) syndrome,
I. A normal impulse arriving at 1 goes down both A and B antegrade conduction occurs over the accessory pathway. If
pathways. Conduction through pathway A is slower and finds atrial fibrillation (AF) develops, the normal rate-limiting
tissue at 2 already depolarized and thus refractory. A normal sinus effects of the atrioventricular (AV) node are bypassed, and the
beat results.
resultant excessive ventricular rates (sometimes 200 to 240
II. A premature impulse finds pathway B refractory and is blocked, beats/min) may lead to ventricular fibrillation and sudden
but it can be conducted on pathway A because its refractory period death. Patients with concealed WPW syndrome are not at risk
is shorter. On arriving at 2, the impulse continues forward and because in them, antegrade conduction does not occur over the
retrograde up pathway B, where it is blocked by refractory tissue at accessory connection.
3. A premature supraventricular beat with an increased PR interval
results.
 Sinoatrial node reentrant tachycardia (SANRT) is caused
III. If conduction over pathway A is sufficiently slow, a premature
by a reentry circuit localised to the SA node, resulting in a
impulse may continue retrograde all the way up pathway B, which
normal-morphology p-wave that falls before a regular,
is now past its refractory period. If pathway A is also past its
narrow QRS complex. It is therefore impossible to
refractory period, the impulse may reenter pathway A and continue
distinguish on the ECG from ordinary sinus tachycardia.
to circle, sending an impulse each cycle to the ventricle (4) and
It may however be distinguished by its prompt response
retrograde to the atrium (5), producing a sustained reentrant
to Vagal manoeuvres.
tachycardia.
 AV nodal reentrant tachycardia (AVNRT) Involves a
TREATMENT
1. Give O2 and gain IV access
2. Vagal manouvers (caution in digoxin toxicity, acute
ischaemia or carotid bruits)
3. ADENOSINE 6mg followed by 12mg then 12mg (2min
interval) – ECG recording at all times, and warn about SE
4. Seek expert help
5. Other medications:
a. Verapamil 5-10mg IV
b. Beta Blocker
c. Amiodarone
6. DC cardioversion if compromised.
TYPES OF SVT
Under certain conditions, typically precipitated by a premature
beat, reentry can produce continuous circulation of an activation
wavefront producing a tachyarrhythmia. Normally, reentry is
prevented by tissue refractoriness following stimulation. However,
3 conditions favor reentry: shortening of tissue refractoriness (eg,
by sympathetic stimulation), lengthening of the conduction
pathway (eg, by hypertrophy or abnormal conduction pathways),
and slowing of impulse conduction (eg, by ischemia).
The reentry pathway is within the atrioventricular (AV) node in
about 50%, involves an accessory bypass tract in 40%, and is
within the atria or sinoatrial (SA) node in 10%.

reentry circuit forming just next to or within the AV node.
The circuit often involves two tiny pathways one faster
than the other. Because the AV node is immediately
between the atria and the ventricle, a retrogradely
conducted p-wave is buried within or occurs just after the
regular, narrow QRS complexes.
Atrioventricular reentrant tachycardia (AVRT) also
results from a reentry circuit, physically larger than
AVNRT. One portion of the circuit is usually the AV
node, and the other, an abnormal accessory pathway from
the atria to the ventricle. WPW with bundle of kent being
an example
o In orthodromic AVRT, atrial impulses are conducted
down through the AV node and retrogradely re-enter
the atrium via the accessory pathway. A
distinguishing characteristic of orthodromic AVRT
can therefore be a p-wave that follows each of its
regular, narrow QRS complexes, due to retrograde
conduction.
o In antidromic AVRT, atrial impulses are conducted
down through the accessory pathway and re-enter the
atrium retrogradely via the AV node. Because the
accessory pathway initiates conduction in the
ventricles outside of the bundle of His, the QRS
complex in antidromic AVRT is often wider than
usual, with a delta wave.