fetal premature atrial contractions

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Fetal premature atrial contractions are a type of extrasystoles that can be occasionally detected in fetal heart monitoring. They along with fetal premature ventricular contractions (PVC's) account for the majority of in utero rhythm disturbances.

Terminology

Premature atrial contractions may occur at a 1:1 ratio with sinus beats (a pattern of bigeminy), with every second sinus beat (trigeminy) or every third sinus beat (quadrigeminy). Conduction to the ventricles may or may not occur; in the latter instance, no ventricular systole will be observed following atrial systole, and the duration of the pause (non-compensatory, with the total length of the pause less than twice the expected interval between atrial contractions) may be used to distinguish PACs from ectopic beats of a ventricular origin.

Pathology

They may arise from immaturity in the fetal cardiac conduction system . The conductions arise in the atrium and can be either transmitted to the ventricles (conduced PACs) or blocked (non-conducted PACs).

Associations

in 1-2% of cases, they can be associated with a congenital cardiac anomaly

Significance

They are benign and usually self-limiting and do not generally compromise cardiac function. In most cases, they disappear spontaneously in utero.

Radiographic features

Ultrasound

M-mode interrogation of atrial and ventricular contractions may proceed with the placement of the M-mode line in a plane transecting atrial and ventricular walls simultaneously, displaying their respective contractions graphically over time. Alternatively, pulsed wave Doppler may be used to simultaneously insonate the left ventricular outflow tract and mitral inflow velocities, yielding diastolic "E" and "A" waves on one side of the baseline, and systolic "V" waves on the opposite side .

The presence of premature atrial contractions may be suspected when an atrial contraction (on M-mode) or an A wave (spectral Doppler) occurs earlier than would be expected based on the preceding intervals between atrial activity .

The PAC may be classified as non-conducted if it is not followed by a V wave or ventricular contraction. A non-compensatory pause should be noted following the PAC; blocked premature atrial contractions may mimic AV block (atrial rate > ventricular rate) but, unlike PACs, AV blocks maintain consistent intervals between atrial events .