What are the diagnostic criteria for fetal tachycardia in a fetus in utero?

Diagnostic Criteria for Fetal Tachycardia

Fetal tachycardia is diagnosed when the fetal heart rate persistently exceeds 180 beats per minute. 1, 2

Definition and Threshold

• The diagnostic threshold for fetal tachyarrhythmia is a persistent fetal heart rate above 180 beats per minute (bpm). 1, 2
• This definition applies regardless of gestational age and represents a clear departure from the normal fetal heart rate range of 110-160 bpm. 1
• The key word is "persistent"—transient elevations above 180 bpm do not constitute tachycardia unless sustained. 2

Diagnostic Modalities

Fetal echocardiography is the primary modality for evaluating and confirming fetal tachycardia, allowing visualization of cardiac activity and precise calculation of heart rate. 1

Echocardiographic Techniques

• M-mode echocardiography provides simultaneous visualization of atrial and ventricular contractions, enabling determination of the atrial-to-ventricular relationship and precise heart rate calculation. 3, 1
• Pulsed Doppler is useful for evaluating fetal heart rhythm and calculating heart rate in cases of arrhythmia, particularly for measuring the relationship between atrial and ventricular contractions. 1
• Color Doppler can detect valvular regurgitation that may accompany tachyarrhythmias. 3

Common Pitfalls to Avoid

• Do not rely solely on standard fetal heart rate monitoring (such as NST) for diagnosing tachyarrhythmias, as these modalities cannot determine the mechanism or atrial-ventricular relationship. 4
• Distinguish between sustained tachycardia and transient accelerations—normal fetal heart rate accelerations of 15 bpm above baseline for 15 seconds are physiologic and do not represent tachycardia. 4
• Always extend evaluation beyond simple heart rate measurement to assess for signs of hemodynamic compromise including hydrops fetalis (pleural effusion, pericardial effusion, ascites, skin edema). 3

Types of Fetal Tachycardia

The most common fetal tachyarrhythmias requiring differentiation are:

• Supraventricular tachycardia (SVT) with 1:1 atrioventricular conduction—the most common type. 2, 5
• Atrial flutter—characterized by atrial rates of 300-500 bpm with variable atrioventricular conduction. 3, 2
• Atrial ectopic tachycardia—less common but important to identify. 2, 5
• Ventricular tachycardia—rare but carries different prognostic implications. 2, 5

Assessment of Hemodynamic Impact

Once tachycardia is diagnosed, assess for signs of cardiac compromise:

• Measure cardiothoracic ratio (normal 0.25-0.35 using area, <0.5 using circumference). 3
• Evaluate ventricular shortening fraction by M-mode (normal >28%). 3, 1
• Assess for atrioventricular or semilunar valve regurgitation. 3
• Check for abnormal ductus venosus Doppler patterns (absent or reversed A wave indicates diastolic dysfunction). 3
• Look for persistent umbilical vein pulsations, which indicate end-stage cardiac failure. 3
• The presence of hydrops fetalis in the setting of tachycardia is associated with 37.5% mortality and requires urgent intervention. 3, 5

Clinical Context

• Fetal tachycardias can cause ventricular dysfunction, hydrops, and intrauterine death if untreated. 2, 6
• Most fetal tachycardias presenting before 36 weeks can be treated successfully with transplacental antiarrhythmic therapy. 3, 5
• Fetuses presenting after 36 weeks of gestation can be managed with delivery and postnatal treatment. 5
• The long-term prognosis is excellent, with most arrhythmias resolving spontaneously during the first year of life. 5