· Premature atrial beats- Premature atrial beats are common, accounting for 60—70% of fetal arrhythmias. They may rarely progress to SVT.
· Sinus tachycardia- The normal fetal heart rate is 120—170/mm. Sinus tachycardia occurswith beats above 170/mm. Fetal tachycardia may be secondary to
maternal disease, e.g. maternal pyrexia or thyrotoxicosis.
· Supraventicular tachycardia- SVT accounts for up to 15% of arrhythmias. Heart rates of 200— 300/mm are recorded, which, if they persist, may cause fetal heart failure ± fetal hydrops. Ten percent have underlying CHD. Maternal treatment with digoxin or flecainide.
· Atrial flutter- This results in cardiac contractions of up to 400/mm. Not all atrial beats are conducted. The rapid heart rate may result in heart failure, causing hydrops. Maternal treatment with digoxin may be effective. Rarely atrial flutter may be associated with CHD and intracardiac tumors.
· Artrial fibrillation- This has been recognized but is extremely rare.
· Venticular tachycadia- This has been recorded but appears to be extremely rare.
· Sinus bradycardia- Sinus bradycardia is uncommon but may be a sign of fetal distress, particularly in late pregnancy, in which case the fetus should be evaluated for other signs of fetal compromise.
· Atrial bigeminy- Blocked premature atrial contraction not conducted to the ventricles resulting in dropped beats. A common benign arrhythmia which usually resolves sOntaneously but may progress to SVT. The heart rate is usually below hO/mm.
· Atrioventricular block- Complete AV block produces a heart rate of 40—80/mm. The incidence of complete heart block is significantly higher in fetuses with maternal connective tissue disease. Fetal hydrops may occur in the third trimester. US reveals a slow ventricular rate with lack of correlation with atrial activity. Maternal anti-Rho antibodies should be assessed.
NB- M-mode echocardiography is a useful means of demonstrating the cause of a rhythm disturbance as it can simultaneously display atrial and ventricular motion. An M-mode section through the left ventricle shows ventricular wall motion and also the FAC wave of the mitral valve trace, i.e. atrial systole. Doppler can be used in a similar manner. Reviewing images of mechanical events in this way is less precise than ECG but the fetal ECG has almost no P waves, which can make the exact timing of atrial systole difficult to determine.
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