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.
We are excited to announce that the Cribsiders are now partnering with VCU Health Continuing Education to offer continuing education credits for physicians and other healthcare professionals. Check out cribsiders.vcuhealth.org and create your FREE account!
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.
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.
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.
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).
Ultimately, the diagnosis is contingent upon the EKG. The following are the most common EKG findings in retrograde AVRT:
Note: it is very difficult to discern AVRT from AVNRT on EKG. This is a job for an electrophysiologist.
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.
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.
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:
Note: most older children with previous episodes of SVT know works the best for them
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:
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.
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.
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.
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:
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.
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:
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.
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.
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.
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.
Listeners will learn to diagnose and manage supraventricular tachycardias in both the emergency department and outpatient settings.
After listening to this episode listeners will be able to…
Dr Fahey reports no relevant financial disclosures. The Cribsiders report no relevant financial disclosures.
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.
Got feedback? Suggest a Cribsiders topic. Recommend a guest. Tell us what you think.
We love hearing from you.
Yes, you can now join our exclusive community of core faculty at Kashlak Memorial Hospital along with all the perks:
Close this notice to consent.