Fetal Heart Rate of 235 bpm at 28 Weeks: SVT, Flutter, or Something Else?
A 28-week-2-day fetus is scanned for a heart rate of 235 bpm recorded at a routine visit. On M-mode the atrial and ventricular contractions appear to be in a 1:1 relationship. The cardiothoracic ratio is 0.55, there is a small pericardial effusion, moderate tricuspid regurgitation, and a monophasic mitral inflow pattern. No ascites, pleural effusion, or skin edema. The four-chamber and outflow tracts appear structurally normal, though the atrial septum is incompletely seen. The mother is asymptomatic, euthyroid on recent labs, anti-Ro negative, and takes no stimulants. The referring physician asks whether this can be watched until delivery.
1. How do you establish the mechanism of the tachycardia, and how do you distinguish SVT from atrial flutter and sinus tachycardia at this rate?
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A 1:1 AV relationship on a single M-mode cursor is not proof of 1:1 conduction. With an atrial rate near 470, flutter with 2:1 block produces a ventricular rate of 235 - the same number. You must measure the atrial rate independently.
Technique:
- M-mode with a cursor through both an atrial wall and a ventricular wall (or through the aorta and atrium) so atrial and ventricular events are timed on the same tracing.
- Alternatively, simultaneous Doppler of the superior vena cava and aorta, or pulmonary artery and aorta: antegrade atrial flow (a-wave) versus ventricular ejection.
Rate-based reasoning:
- Sinus tachycardia is usually under 200 bpm with beat-to-beat variability and a normal AV interval; it is rare in a truly resting fetus.
- Supraventricular tachycardia is typically 220-260 bpm, 1:1, with abrupt onset and termination, and a short AV interval suggesting an accessory pathway.
- Atrial flutter shows an atrial rate of roughly 300-500 (often ~400-440) with variable block - 2:1 giving a ventricular rate in the low 200s, and often a sawtooth atrial pattern on M-mode.
Also look for a substrate: Ebstein anomaly (apical displacement of the septal tricuspid leaflet), cardiac rhabdomyoma (tuberous sclerosis), and any structural lesion that would change management. Re-image the atrial septum carefully.
2. Interpret the functional findings: how sick is this fetus, and what determines how urgently you act?
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This fetus is compensated but not normal.
- CTR 0.55 - cardiomegaly; a ratio above roughly 0.5 in a tachycardic fetus indicates volume loading and is a recognized early marker of impending hydrops.
- Small pericardial effusion plus moderate TR - early failure markers rather than hydrops itself. Hydrops requires two or more abnormal fluid collections (ascites, pleural effusion, skin edema, pericardial effusion).
- Monophasic mitral inflow - loss of the normal biphasic E/A pattern, a sign of impaired diastolic relaxation and elevated filling pressures.
- Together these describe a fetus tolerating the arrhythmia but trending toward decompensation.
What drives urgency:
- Sustained versus intermittent - a tachycardia present for the majority of a 10-15 minute observation is treated; brief, self-terminating runs can be observed.
- Hydrops - its presence shifts first-line drug choice and doubles the failure rate of therapy.
- Gestational age - at 28 weeks delivery is not a reasonable option; transplacental therapy is.
- Ventricular function - a falling shortening fraction or a rising Tei index argues for escalation. Serial measurements, not a single snapshot, guide this.
Answer to the referring physician: no, this should not simply be watched.
3. Outline your management plan and the counseling you would give, including what happens if therapy fails.
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Initiate transplacental antiarrhythmic therapy with fetal cardiology and maternal-fetal medicine co-management.
- Digoxin is a traditional first line for SVT without hydrops; it is limited in hydrops by poor placental transfer.
- Flecainide is often preferred, particularly with hydrops or ventricular dysfunction, and converts a high proportion of fetal SVT.
- Sotalol or amiodarone are second-line for flutter or refractory SVT; amiodarone carries thyroid and growth risks and is reserved for refractory cases.
Monitoring: maternal ECG and drug levels (particularly for flecainide), serial fetal echocardiograms every few days initially to document conversion, and surveillance for hydrops, function, and AV interval prolongation (a proarrhythmic risk with flecainide).
Counseling points:
- Explain that a structurally normal heart is likely but not yet proven - the atrial septum must be re-examined and a rhabdomyoma excluded, and a postnatal ECG is needed to look for pre-excitation.
- Give realistic expectations: transplacental conversion succeeds in the majority, and the presence of hydrops lowers that figure.
- If therapy fails, develops toxicity, or hydrops worsens after 35-36 weeks, delivery with postnatal cardioversion becomes the appropriate strategy - which is why gestational age matters here.
- Note that the earlier reassurance that the mother's thyroid, autoimmune status, and medications are unrevealing is useful, because it removes the reversible maternal drivers and focuses management on the fetus.