2025 – PAGE 121 – CARDIOLOGY

SUPRAVENTRICULAR TACHYCARDIA (SVT)

In supraventricular tachycardia (SVT), the heart rate is usually > 240 and is most commonly discovered in patients below 6 months of age. This may be noted incidentally during breastfeeding. Vagal maneuvers may be tried for 20 seconds, including water or ice to the face and Valsalva if the patient can cooperate. Carotid massage and orbital pressure are not universally recommended due to concerns for soft tissue injury, especially in children less than one year old. If vagal maneuvers do not work, try ADENOSINE (which may be repeated once at a higher dose and then escalated to amiodarone or procainamide). If the patient is unstable or becoming unstable, use synchronized DC electrical cardioversion. In children older than 12 months, you may also try VERAPAMIL, but it is generally contraindicated for children younger than 12 months. After the patient is back into sinus rhythm, repeat an EKG to look for WPW. Treat children with SVT with a chronic beta blocker (propranolol) for one year to prevent recurrence. If the patient is resistant to treatment, use radiofrequency ablation.

PEARLS: 

• SVT can cause heart failure due to ineffective filling times. In babies, a finding of HEPATOMEGALY is more common than crackles.
• For the boards, if you see a regular, narrow complex tachycardia that’s REALLY fast, pick adenosine if possible.

WOLFF-PARKINSON-WHITE SYNDROME (WPW) AND AV REENTRANT TACHYCARDIA (AVRT)

In Wolff-Parkinson-White Syndrome, an extra (accessory) pathway bypasses the AV node and connects the atrium directly to the ventricle. While a normal AV node would conduct slowly, providing the short delay between atrial and ventricular depolarization, the accessory pathway conducts more rapidly and “short-circuits” the delay. This “pre-excitation” through the accessory pathway results in a wave of depolarization that bypasses the AV node. The result is a short PR (less than 0.12s) and a widened QRS (greater than 0.10s). A DELTA WAVE is present due to fusion of the QRS and pre-excitation wave.

Children with WPW and similar disorders with accessory pathways may be prone to develop AV REENTRANT TACHYCARDIA (AVRT). Note that WPW refers to a static condition of having an accessory pathway, while AVRT refers to the result (tachycardia) that can occur. The reentrant tachycardia happens when the depolarization wave passes from the atria to the ventricles along one pathway but then continues back from ventricles to atria via the other pathway. Usually, the circle is “orthodromic” (“straight-racing” with a narrow complex tachycardia), with forward conduction through the AV node then retrograde from ventricle to atrium via the accessory pathway. The goal of treatment is to block conduction somewhere (usually the AV node) long enough to stop the cycle. If the circle is “antidromic” (rare), forward conduction is down the accessory pathway and then up the AV node and results in a wide complex tachycardia (low-yield for the boards).

Narrow-complex tachycardia in WPW is treated the same as any SVT, beginning with vagal maneuvers and adenosine. CALCIUM CHANNEL BLOCKERS and BETA BLOCKERS should be “avoided.” Furthermore, if a WPW patient has ATRIAL FIBRILLATION OR FLUTTER then AV NODAL AGENTS (BETA BLOCKERS, DIGOXIN, and VERAPAMIL) are CONTRAINDICATED because they increase the refractory period of the AV node and make forward conduction through the accessory pathway more likely. This can worsen the tachycardia because accessory pathways typically have a shorter refractory period than the AV node. Also, AV nodal agents do not affect the refractory period of the accessory pathways. This can lead to deterioration of the arrhythmia into ventricular fibrillation and death. For atrial fibrillation or flutter, use IBUTILIDE or PROCAINAMIDE (these increase the refractory period of the accessory pathway and may result in restoration of sinus rhythm) and prepare for possible electrical cardioversion or defibrillation if the arrhythmia does not terminate. If shown an EKG with delta waves, pick WPW and move on. Note, however, that the delta wave is not seen during episodes of reentrant tachycardia.

For the chronic treatment of WPW, options include beta-blockers, calcium channel blockers, and possibly a cardiac ablation.

IMAGE: www.pbrlinks.com/deltawave1

• NAME ALERT: The name alert is for AV NODE REENTRANT TACHYCARDIA (AVNRT) with “NODE” in the name.