Fetal Supraventricular Tachycardia

Fetal SVT examples

More on postnatal electrocardiogram and basic review of neonatal arrhythmias here

Presentation on Fetal SVT by Dr Shahad Daali and Ms Laila Wazneh - October 2025

fetal SVT VB & TC Final 2.pdf

Top Diagram: 1:1 Conduction with Long VA

This diagram illustrates a normal or near-normal conduction pattern, even if the heart rate is fast.

A (Atrial): Represents the contraction of the atria.
V (Ventricular): Represents the contraction of the ventricles.
AV Conduction: The straight downward arrows show that for every atrial beat (A), an electrical signal is conducted down to the ventricles, causing them to beat (V). This is a 1:1 conduction.
Long VA Interval: The key feature here is the long time interval between a ventricular contraction (V) and the next atrial contraction (A). This is typical of a normal heart rhythm (sinus rhythm) where the atria initiate the beat and have ample time to fill with blood before the next cycle begins. In a fetus, this pattern during a tachycardia often points to conditions like sinus tachycardia or certain types of atrial tachycardia, where the atria are the primary driver of the fast rhythm.
Long VA interval is a surrogate for a long RP interval on the ECG in the post-natal setting.

Bottom Diagram: 1:1 Conduction with Short VA - This diagram illustrates a common type of abnormal heart rhythm called a re-entrant tachycardia.

1:1 Conduction: Like the top diagram, there is one ventricular beat for every atrial beat.
Abnormal Conduction Pattern: The arrows show the electrical impulse traveling from the atria to the ventricles (A ⟶ V) and then almost immediately traveling backward from the ventricles to the atria (V ⟶ A), shown by the wavy arrows. This backward (retrograde) conduction is abnormal.
Short VA Interval: Because the atria are activated by this abnormal backward signal almost as soon as the ventricles contract, the time between the ventricular beat (V) and the subsequent atrial beat (A) is very short. This "short VA" interval is a classic sign of atrioventricular nodal reentrant tachycardia (AVNRT), the most common cause of significant fetal arrhythmia. In AVNRT, an abnormal electrical "short circuit" in the center of the heart causes the signal to spin rapidly, activating both chambers in quick succession.
Short VA interval is a surrogate for a short RP interval on the ECG in the post-natal setting.

By using Doppler ultrasound to analyze blood flow patterns that correspond to atrial and ventricular contractions, doctors can measure these intervals in a fetus to distinguish between different types of arrhythmias and determine the best course of action for management and treatment.

This Doppler ultrasound image demonstrates fetal tachycardia using PW-Doppler in the aorta with a heart rate corresponding to 219 to 234 bpm. Fetal supraventricular tachycardia (SVT) is a common cause of sustained fetal tachyarrhythmia. If sustained, fetal SVT can lead to impaired ventricular filling, reduced cardiac output, and eventually hydrops fetalis due to high-output cardiac failure. Early recognition through Doppler echocardiography is essential, as transplacental antiarrhythmic therapy can often restore sinus rhythm and prevent complications.

In the context of fetal SVT (atrioventricular reentrant tachycardia, AVRT), the atrial and ventricular contractions occur nearly simultaneously (1:1 conduction). This mechanical fusion abolishes the normal atrial contribution to ventricular filling, leading to absent or reversed a-wave flow in the DV. The atrial contraction against a tricuspid valve can lead to reversal of flow in the ductus venosus, as outlined on this Doppler. This may lead to inadequate venous draining pressure and ascites. Similar a-reversal can be observed in the pulmonary veins when the left atrium contracts against a closed mitral valve.

Excellent image — this is a simultaneous pulsed Doppler tracing of the fetal left ventricular inflow (mitral valve) and outflow (aortic valve), commonly referred to as the Mitral–Aortic (M–A) Doppler pattern. It provides information about atrial and ventricular mechanical timing and is especially useful for assessing fetal arrhythmias and AV/VA intervals (as estimates of the PR and RP intervals).

The PR and RP intervals describe the temporal relationship between atrial and ventricular activation, helping identify which chamber is activated first (atria or ventricles) and how the reentrant circuit is organized. P wave (Atrial contraction) → represents atrial depolarization; R wave (Ventricular contraction) → represents ventricular depolarization

PR interval = time from the onset of atrial activation (P wave) → ventricular activation (R wave). → Reflects antegrade conduction through the AV node.
RP interval = time from the onset of ventricular activation (R wave) → next atrial activation (P wave). → Reflects retrograde conduction from the ventricle back to the atrium through the reentrant circuit.
In Doppler terms (fetal echo), PR ≈ A→V mechanical interval, and RP ≈ V→A mechanical interval (measured via mitral–aortic or venous–arterial Doppler).
Measuring VA (mechanical equivalent of RP) and AV (mechanical equivalent of PR) intervals allows the clinician to determine whether the arrhythmia is reentrant (short VA) or atrial-driven (long VA).
- Short RP (VA < AV) → reentrant tachycardia (AVRT/AVNRT)
- Long RP (VA > AV) → atrial tachycardia or flutter

M-Mode accross the Right Atrium and the Right Ventricular Apex. The top part of the panel represents the atrial wall contraction and relaxation, while the bottom line represents the ventricular contraction and relaxation.

V-A Sequence: The pattern is V-A-V-A, meaning the ventricles contract just before the atria.
Short VA Interval: The time from the ventricular beat (V) to the subsequent atrial beat (A) is very short.
Long AV Interval: The time from that atrial beat (A) to the next ventricular beat (V) is long.
This M-mode finding is the classic representation of the "1:1 with short VA" pattern demonstrated in the diagram above. It is highly suggestive of a re-entrant tachycardia, most commonly atrioventricular nodal reentrant tachycardia (AVNRT).
This is the echocardiographic equivalent of a short RP / long PR tachycardia on an electrocardiogram (ECG).

Fetal Pulsed-wave (PW) Doppler at SVC-Ao Junction. This Doppler simultaneously records blood flow in two adjacent vessels: the superior vena cava (SVC), which carries blood towards the heart, and the ascending aorta (Ao), which carries blood away from the heart.

Velocities obtained by PW-Doppler may be "negative" (below baseline), or "positive" (above baseline) depending on position of fetus regarding the probe at time of image acquisition, in the above example: majority of SVC flow is below the baseline and Aortic flow is above the baseline.
A-Wave (Atrial Kick): This is the wave seen above the baseline in the fetal example just before the pulsatile enveloppe of the Aortic flow . It represents the surge of blood velocities into the SVC caused by atrial contraction (atrial systole) against a closed tricuspid valve. The peak of this wave marks the mechanical activity of the atria.
Aortic Ejection Wave (Ventricular Ejection): This is the sharp, high-velocity waveform seen above the baseline. It represents the powerful ejection of blood from the ventricle into the aorta during ventricular contraction (ventricular systole).
How to Calculate AV and VA Timings: The diagram illustrates how these two key events on the Doppler tracing are used to measure the timing between atrial and ventricular contractions.
- AV (Atrioventricular) Interval: This measures the time delay between the atrial and ventricular contractions. You measure the time from the beginning of the A-wave (atrial contraction) to the beginning of the aortic ejection wave (ventricular contraction). This is the electro-mechanical equivalent of the PR interval on an ECG. It shows how long it takes for the signal to travel from the atria to the ventricles and cause them to pump.
- VA (Ventriculoatrial) Interval: This measures the time from a ventricular contraction to the next atrial contraction. You measure the time from the beginning of the aortic ejection wave (ventricular contraction) to the beginning of the next A-wave (atrial contraction). This interval is the surrogate for the RP interval. A short VA interval suggests a re-entrant tachycardia like AVRT/AVNRT, while a long VA interval is seen in normal sinus rhythm or other types of tachycardias.

Case of Fetal Supra-Ventricular Tachycardia

4-chamber with sweep. Towards the end of the clip, one may appreciate the higher rate (SVT)

Fetal SVT. The atriums and corresponding ventricles are contracting at the similar rate and rhythm.

M-Mode through the RVOT, Aorta and Left Atrium during a fetal SVT event.

M-mode with 1 to 1 conduction. one may follow the aortic valve opening and closure - indicating systolic ejection time of the left ventricle (in normal great vessels configuration). The left atrial m-mode indicate atrial contraction with 1:1 conduction (Aortic valve opening and RV contraction).