#37: Out of Rhythm: SVT with Dr. Mike Fahey

October 27, 2021 | By Sam Masur

Summary

Does learning cardiology give you palpitations? Not to worry, Dr. Mike Fahey returns to address all your concerns about supraventricular tachycardia. He teaches us about cardiac conduction, Wolf-Parkinson-White, and more in this shocking episode.

Credits

• Producer, Writer, and Infographic: Sam Masur MD 
• Executive Producer: Max Cruz MD
• Cover Art: Chris Chiu MD
• Hosts: Justin Berk MD; Chris Chiu MD
• Editor:Justin Berk MD; Clair Morgan of nodderly.com
• Guest(s): Mike Fahey MD

SVT Pearls

1. The most common form of SVT in kids is Atrio-Ventricular Reentrant Tachycardia (AVRT), where there is an extra pathway that connects the atria and ventricles other than the AV node and His-Purkinje system.
2. The most common EKG findings in retrograde AVRT (where the signal travels from ventricles to atria) are: narrow complex QRS, extremely regular QRS, and no upright P waves in leads I or aVF.
3. The most effective vagal maneuver for children is often blowing into a partially occluded straw, which is a way to create a valsalva maneuver. For infants, Dr. Fahey recommends “ice to the face.”
4. Adenosine will break every reentrant rhythm that involves the AV node.
5. Adenosine should always be given with a rhythm strip running.
6. The diagnosis of Wolff-Parkinson-White (WPW) is made purely with EKG and only needs to be captured once.
7. Although rare, the risk of sudden cardiac death in WPW is due to an extremely fast electrical signal in atrial fibrillation. If the accessory pathway can conduct at high speeds, this atrial signal can cross to the ventricles, causing ventricular fibrillation.
8. Do not give adenosine in patients with WPW (anterograde conduction across an accessory pathway) and atrial fibrillation. 
9. When children weigh at least 25kg, an electrophysiologist can ablate the accessory pathway in WPW.

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SVT Notes 

Cardiac Conduction System

Normal Sinus Rhythm

Conduction begins from the sinus node in the high right atrium. The sinus node has the fastest automaticity and it doesn’t need any stimulus to depolarize. After depolarization, the electrical stimulus travels through the atria towards the ventricles, but cannot pass across due to the fibrous ring electrically insulating the structures. The AV Node is the crossing point, where the stimulus waits a split second, and then rapidly travels down the His-Purkinje system to depolarize the ventricles in a very short amount of time. The signal would eventually make its way back to the atria, but it cannot due to the fibrous ring. Once depolarized, every myocyte goes into its refractory state, and the process starts all over again. If you work someone to their limits, the fastest physiologic heart rate is considered to be 220 – age (years).

If you see a narrow QRS, you know the electrical stimulus came through the AV node and traveled down a functional His-Purkinje system. 

Tachycardia

The first step when approaching a tachycardic patient is to determine if the patient is sick or not sick, i.e. hemodynamically stable or unstable. This discussion focuses on the stable patient.

The second step is to determine if the QRS complex is wide or narrow. If it is narrow, that means it traveled through the AV node and His-Purkinje system, implying it started above the ventricles. Thus, the rhythm is termed Supraventricular Tachycardia (SVT). SVT is an umbrella term for every fast arrhythmia with a narrow QRS.

SVT Etiology

The most common form of SVT in kids is Atrio-Ventricular Reentrant Tachycardia (AVRT). In AVRT, there is an extra pathway (accessory) that connects the atria and ventricles other than the AV node and His-Purkinje system. Thus, electricity continues to loop through the atria and ventricles along the His-Purkinje system and accessory pathway. The prototypical cause of AVRT is Wolff-Parkinson-White Syndrome (WPW).

Another form of SVT that often gets confused with AVRT is Atrio-Ventricular Nodal Reentrant Tachycardia (AVNRT). In AVNRT, the reentrant loop happens within the AV node itself rather than through an extra pathway.

One of the rarest forms of reentrant SVT in pediatrics is Atrial Flutter, which is occasionally seen in infants. This reentry loop exists entirely within the atria. 

Although less prevalent than reentrant tachycardias, the most common form of automatic tachycardia is Ectopic Atrial Tachycardia (EAT). In EAT, there is an electrical signal coming from another part of the atria that beats faster than the sinus node.

Evaluation of Tachyarrhythmia

History and Physical

The most common symptoms in school aged children are chest pain and dizziness. According to Dr. Fahey, syncope is quite rare in SVT, and should be evaluated for an alternative rhythm. In an infant, the most common symptoms are unexplained irritability and color changes. If children are in SVT for a while, they will start to exhibit signs of congestive heart failure. 

In regards to family history, there are some hereditary arrhythmias, but not often forms of SVT. Most cases of WPW are sporadic, but there is a familial version with a slightly higher incidence (1.5-2x) in first degree family members (NEJM 2001).

EKG

Ultimately, the diagnosis is contingent upon the EKG. The following are the most common EKG findings in retrograde AVRT:

• Narrow QRS complex
• Extremely regular QRS complex 
• There is no discernable P wave coming from the sinus node (i.e. upright in lead I and aVF)

Note: it is very difficult to discern AVRT from AVNRT on EKG. This is a job for an electrophysiologist.

Other Testing

The remainder of testing is often completed once the arrhythmia has been treated (see below). Echocardiogram is recommended, as patients with SVT have a higher incidence of congenital heart disease (JAMA 2009). Dr. Fahey recommends checking a TSH in patients with atrial fibrillation, atrial flutter, or rarer forms of pediatric SVT.

Treatment of SVT

Why Should We Treat?

SVT is often not immediately dangerous, unless at very high heart rates (above maximum physiologic heart rate). The concern is directly proportional to the heart rate. In general, Dr. Fahey becomes worried about decreased cardiac output when a child has been in SVT for 10-20 hours or more. This would manifest as signs and symptoms of decompensated heart failure as above. For those children who find themselves with decreased cardiac output for a prolonged period of time, they can develop residual ventricular dysfunction, called tachycardia-induced cardiomyopathy.

Vagal Maneuvers

The vagus nerve works to slow sinus node automaticity and AV nodal conduction. Once AV nodal conduction is slowed, re-entrant loops involving the AV node will hit a new refractory period, causing the electrical signal to stop. Once that circuit breaks, the next electrical impulse will come from the automatic sinus node, the area with the most automaticity. Vagal maneuvers are exercises that can be performed to stimulate the vagus nerve and increase vagal tone. The following are commonly used maneuvers:

• Blow as hard as possible through a partially occluded straw. This mimics a valsalva maneuver, and is often the most effective according to Dr. Fahey
• For infants, “ice to the face” is one of the most common maneuvers. Grab a bag of ice, pour water in to create a slurry, and then place the bag on the upper portion of face with gentle pressure
• For infants, stimulate a gag reflex
• For infants, rectal stimulation
• For older children able to follow commands, the modified valsalva maneuver (blow into a syringe and invert legs) can be used (REVERT Trial 2015). 

Note: most older children with previous episodes of SVT know works the best for them

Adenosine

Adenosine works by shutting down the AV node completely. Any reentrant tachyarrhythmia that utilizes the AV node as part of the circuit should be broken with adenosine. The following must be performed when giving adenosine:

• Must be given push IV with a 3-way stopcock.
• Give as close as possible to the heart due to its extremely short half life.
• Run a rhythm strip or EKG while giving it to identify any changes once the adenosine has been pushed.
• Adenosine should not be given to high-risk patients with WPW in atrial fibrillation due to the risk of shunting the fibrillating signal directly down the accessory pathway (see below).

If adenosine does not work, it is unlikely that there is a reentrant loop that includes the AV node. This often happens in atrial flutter or automatic tachycardias, such as EAT or atrial fibrillation.

Dr. Fahey’s decision to give more adenosine is based on the rhythm the patient is in. This is why the concurrent rhythm strip is so important. He will likely try one extra dose of the full adenosine just in case there was an unlucky aberrant beat that put the patient right back into the tachyarrhythmia. 

Other Medications

If adenosine is ineffective, the conduction system of the heart must be changed in a more systemic manner. This is often called chemical cardioversion. Dr. Fahey’s next antiarrhythmic is often a beta blocker. Other types of antiarrhythmic agents are often chosen by local cardiologist preference, as there is little data comparing these medicines head-to-head in children. Examples include procainamide, sotalol, or flecainide. All antiarrhythmics are also pro-arrhythmic, so the more powerful the antiarrhythmic, the higher potential for triggering other nastier arrhythmias.

Electrical Cardioversion

In general, any hemodynamically unstable child in a supraventricular tachycardia should be electrically cardioverted as soon as possible. Electrical cardioversion works by delivering an electrical signal to every myocyte, causing them all to depolarize at the same time. Thus, every myocyte enters a refractory period, and then the most automatic signal in the heart starts back up again – the sinus node. 

Electricity, in contrast to adenosine, will work for every reentrant rhythm as it affects every myocyte in the heart. In the stable patient, electrical cardioversion is often reserved for patients who can’t get IV access, but this is very rare.

Wolff-Parkinson-White (WPW)

Definition

A type of AVRT where the accessory pathway (extra pathway connecting atria to ventricles) can conduct electricity from atria to ventricles and/or ventricles to atria. The ability to conduct from the atria to ventricles is what makes WPW both unique and dangerous. It is also how WPW gets its hallmark EKG findings:

• Short PR interval – instead of pausing at the AV node, some of the electrical signal quickly crosses to the ventricles across the pathway, shortening the PR interval
• Delta wave – instead of the entire electrical stimulus traveling through the His-Purkinje system, some of the signal crosses the accessory pathway and directly depolarizes ventricular myocytes, initiating an early sloped QRS that eventually meets up with the remaining signal traveling the His-Purkinje system.

The diagnosis of WPW is successfully made with purely the EKG findings (and it only needs to be captured once!) Importantly, these findings will only be seen when the patient is out of SVT, as most patients are in retrograde AVRT.

Risks

With an accessory pathway that can conduct anterograde (atria to ventricles), there will be some uninhibited electrical signal to the ventricles. In normal sinus rhythm, this is harmless. But for patients who develop atrial fibrillation, this can be dangerous. And for unknown reasons, those with WPW are at increased risk of developing atrial fibrillation.

In general, the AV node protects the ventricles from beating too fast. In atrial fibrillation, the AV node protects the heart from ventricular fibrillation and sudden death. If the accessory pathway can conduct at rates bordering on fibrillating speeds, the electrical signal could jump directly to the ventricles, causing V-fib and sudden cardiac death. 

According to Dr. Fahey, this is a rare complication as the patient must both develop atrial fibrillation AND have a pathway that can conduct at fibrillating speeds. For these patients, the EKG will look like the following:

• Irregularly irregular rhythm due to atrial fibrillation
• Mix of narrow and WIDE QRS complexes due to signal traveling anterograde across the accessory pathway, causing direct myocyte to myocyte conduction.

Prior to a formal EP study, the accessory pathway can be evaluated by exercise testing. With leads connected, patients with WPW can be placed on a treadmill to increase their heart rates. If the shortened PR interval and delta wave disappear at a higher heart rate, then the accessory pathway has a max speed well within physiologic rates.

Lastly, although very rare, there could be some ventricular dysfunction based on the physical location of the accessory pathway. In the rare instance there is ventricular myocyte dyssynchrony, this can eventually lead to ventricular dysfunction.

Treatment

AVRT

It is safe to treat patients in AVRT with adenosine or beta blocker as above. Dr. Fahey says you should feel confident that the patient is not in atrial fibrillation if the QRS is dead regular and narrow complex. If you are unsure, this is a great time to reach out to a cardiologist. Otherwise, the patient can get electrically cardioverted.

Prevention

For a first episode of SVT in an older child, Dr. Fahey will often watch and wait before starting preventative medicines. That is because the child can report if he/she/they develops symptoms again. For infants or repeat episodes, Dr. Fahey will start children on preventative beta blockers. With babies, Dr. Fahey will start with short-acting beta blockers, such as short-acting propranolol 2mg/kg/day, due to risk of overdose. When kids are large enough to handle pills, Dr. Fahey will switch to long-acting beta blocker such as metoprolol succinate, long-acting propranolol, or atenolol. Pill-in-pocket technique (no daily therapy, but abortive therapy) can be considered for patients with very infrequent episodes.

For athletes, Dr. Fahey does not often restrict children from sports. Nevertheless, this needs to be shared decision making with the family when discussing the risks of sudden cardiac death. Most importantly, although preventative medicines decrease the incidence of SVT, they do not decrease the risk of sudden death in association with atrial fibrillation.

Ablation

When children are large enough, an electrophysiologist can perform a catheter ablation to destroy the accessory pathway. Although techniques are constantly improving, patients must be a minimum of 25kg to undergo the procedure. The patients who are referred for ablation are high-risk (accessory pathway can conduct at above physiologic heart rates), documented ventricular tachycardia or ventricular fibrillation, or unexplained syncope.

Links

Dr. Fahey’s Favorite Books

• A Game of Thrones by George RR Martin
• Lonesome Dove by Larry McMurtry
• Sophie’s Choice by William Styron
• The Power of One by Bryce Courtenay

Goal

Listeners will learn to diagnose and manage supraventricular tachycardias in both the emergency department and outpatient settings. 

Learning objectives

After listening to this episode listeners will be able to…  

1. Recognize signs of supraventricular tachycardia in infants and children
2. Describe EKG findings seen in reentrant supraventricular tachycardias
3. Explain how to use vagal maneuvers in outpatient practice and adenosine in the emergency department to treat SVT.
4. List the diagnostic criteria for Wolff-Parkinson-White
5. Discuss risks and preventative treatment strategies for supraventricular tachycardias.

Disclosures

Dr Fahey reports no relevant financial disclosures. The Cribsiders report no relevant financial disclosures. 

Citation

Masur S, Fahey M, Cruz M, Chiu C, Berk J. “#37: Out of Rhythm: SVT with Dr. Mike Fahey”. The Cribsiders Pediatric Podcast. https:/www.thecribsiders.com/ October 27, 2021.