Carly is a 9-month-old who was brought in by her parents because she had been unsettled and not feeding well for a day.
At triage, she has a pulse of 277 and is brought straight to resus.
What happens in a normal heart?
The sino-atrial (SA) node is the pacemaker of the heart. It is situated in the right atrium and generates an electrical impulse that conducts through the whole heart and causes it to contract. The impulses from the SA node travel to the atrioventricular (AV node). The AV node slows the current (0.1-second pause) and then allows it to pass through the ventricles (via the Bundle of His and the Purkinje fibres). This process allows the atria to contract fully and the ventricles to contract from base to apex.
From: https://www.apsubiology.org/anatomy/2020/2020_Exam_Reviews/Exam_1/CH18_Cardiac_Cycle.htm
There are two tracts through the AV node: fast and slow.
In a normal heart circuit, the fast pathway will reach the end of the AV node first (and then on to stimulate the Bundle of His), and as it starts to travel up the slow pathway, they cancel each other out. The result is a clear path conducting the impulse through the AV node, which produces a normal sinus rhythm.
What changes in SVT?
SVT is usually caused by a re-entrant circuit.
A second impulse is triggered before the normal impulse has been fully conducted through the AV node, and the tracts reset. This happens where there is increased automaticity in the atrium, causing impulses (from fever, stretch, stimulation, etc.). This stimulates the AV node when one part of the tract is refractory. The fast pathway has a longer refractory time, so while the slow pathway may work again, the fast pathway is in its refractory period.
The impulse goes down the slow tract and is blocked down the fast tract. By the time it reaches the end of the AV node, the refractory period has passed, and it can now travel back up the fast pathway.
This causes a retrograde pathway and results in a circuit. SV is an AV node re-entry circuit.
AVRT is similar to AVNRT, but it doesn’t involve the node. It involves an accessory pathway, and the idea is the same.
Treating SVT
The goal is to stop conduction through the AV node long enough for the SA node to take over as the primary pacemaker. In the ED, we can do this physiologically or medically.
Record a rhythm strip while you try to revert the ECG, as the cardiologists will want to see it.
The Valsalva Manoeuvre
Start with the Valsalva manoeuvre in older children. Get them to blow into a pipe or syringe. Patients can blow into a pipe attached to a sphygmomanometer to hit 40mmHg for 15 seconds or blow into a 10ml syringe to move the plunger. Both are effective.
The extra squeeze on the heart increases cardiac output and increases carotid and baroreceptor stimulation. This increases the blood pressure. As this raised pressure is sustained, blood volume is forced into the legs and head (causing neck vein distension). This leads to a drop in preload.
The drop in preload stimulates atrial baroreceptors, so the heart pumps harder and faster to compensate, and blood pressure drops.
At the end of the manoeuvre, there is a sudden release, and all the blood flows into the heart, causing a huge flow of blood into the carotid baroreceptors, leading to vagal inhibition.
Using a modified Valsalva works better – if you lift the patient’s legs in the air and lie them flat immediately after the release, you will increase the venous return and improve the chances of reversion. According to the REVERT study, this method reverted SVT in 43% (vs 17%) of (adult) patients.
The Diving Reflex
Young infants cannot perform a Valsalva manoeuvre (although it can happen when they cry).
The diving reflex causes vasoconstriction. It does this because your hands, feet, and face have ten times the heat and cold receptors of the rest of your body. They act to protect you.
When you jump into cold water, you vasoconstrict to preserve heat at your core. There is a massive shunt of blood to your central circulation that your heart has to pump into your lungs, and that causes you to take a deep breath.
When a baby is placed in a cold environment, it experiences profound vasoconstriction in the same way, and blood is shunted to the core. However, a well-functioning heart is necessary for this to work—all the blood that returns needs to be pumped out effectively. This is a potential problem in adults with heart failure.
There is a second pathway in children with heart failure: facial stimulation of cold blood to the hypothalamus. This causes the heart to slow down and decrease metabolism. Because of this, children with heart failure may still respond to the diving reflex (whereas adults will not).
Place a face mask dunked in ice-cold water on the baby’s face for 30-40 seconds. This is as effective as dunking them in a bucket of ice and a lot more gentle.
Medical treatment with Adenosine
Adenosine is our first-line medical treatment for SVT.
It briefly opens the potassium channels and depolarises the AV node, aiming to break the circuit and allow the SA node to take over.
It only has a half-life of 10 seconds.
Start with 100mcg/kg as the first dose, then increase to a max of 500mcg/kg (less in neonates).
One of FIVE things is then going to happen
Reversion
This is what we all hope for. The patient returns to sinus rhythm.
Often there is a recovery tachycardia due to a net deoxygenation from being tachycardic for so long.
All children with SVT need cardiology follow-up, even if they revert.
They can look for any underlying cause and discuss the need for prophylaxis.
Children (particularly infants) cannot tell us when they are in SVT, and they may present after several days. Prophylaxis aims to prevent them from presenting in collapse or failure.
No response
It’s happened to all of us. When the adenosine doesn’t work, consider the 5 Ds.
- Delivery: Remember that adenosine has a 10-second half-life. Give it through a cannula in a large vein, with the arm up in the air, using a three-way tap to ensure a fast bolus and a flush straight after the adenosine.
- Distance: Are you giving it through a cannula in the arm or the foot? If you are using a cannula further away from the heart, a higher dose may be needed, as we need the adenosine to be active by the time it reaches the heart.
- Dose: We are always told to start at 100mcg/kg, but the evidence shows that most children need 170-200mcg/kg to revert. So, if you use IO access or a cannula in the foot, consider starting at 200mcg/kg.
- Drugs: Theophylline competes at the adenosine receptor, so if you have a child on theophylline, you will need a higher dose.
- Diagnosis: Patients in fascicular VT or Lown-Ganong-Levine syndrome will not respond to adenosine.
Recurrence or retriggering
Sometimes, we see a response to adenosine where the child returns to sinus rhythm but then reverts within a minute or two.
In this case, there is no point in simply continuing with repeated adenosine doses. Something triggering the SVT and needs to be treated.
You probably need a second agent, and this patient should be discussed with a cardiologist.
Atrial Flutter
After the SVT corrects, it exposes an underlying atrial tachycardia.
This looks like SVT initially, but when you ablate the AV node, it reveals an atrial tachycardia.
The atrial tachycardia needs a second-line agent or another therapy (e.g. cardioversion, pacing).
Second-line agents can include amiodarone, though it is often difficult to get the right dose. If amiodarone is given slowly and at a low dose, it’s often ineffective, but given faster and at higher doses, it can cause hypertension, asystole, and collapse.
Sotalol cardioverts and rate controls, so it is an alternative second-line agent.
Unmasking congestive cardiac failure
This is where the child recovers from SVT, but then the BP starts to drop, and the child gets more tachycardic.
Up to 25% of children with congenital heart disease can present with SVT. However, children who have had SVT for more than 48 hours are often in heart failure when they present.
Prolonged tachycardia causes poor myocardial perfusion, so when they return to their normal rate, they have a floppy myocardium that struggles to recover. Myocarditis or myopathies may be unmasked.
Rarely, patients can deteriorate into VT or VF, so make sure you are ready for this.
You got access in a good vein in Carly’s arm, and although the ice to her face and then 100mcg/kg adenosine IV didn’t work, the next step of 200mcg/kg did.
Carly reverted and is stable.
You have contacted the cardiology team to arrange follow up.
