o_1977r8vv9vk1ts2ms0kd8pksa.pdf

502 D. Maulik
Fig. 33.23. Pulmonary arterial Doppler
waveforms show a premature cardiac
contraction followed by a compensatory
pause (arrows). Note the relative
increase in the peak velocity of the
post-compensatory wave, confirming
the existence of the Frank-Starling
mechanism in the fetal heart
sion. The mechanism of reentrant tachyarrhythmia
consists of circus electric activity encompassing atrial
and ventricular conduction systems, most frequently
involving an accessory bypass tract. The atrial impulse
first is conducted to the A-V node and then
into the His-Purkinje system accessory pathway and
ventricles. The accessory pathway transmits the impulse
from the ventricle to the atrium, thereby completing
the reentry circuit (Fig. 33.24). This pattern
comprises the most common type of reentrant SVT
encountered in the fetus and neonate. When the accessory
pathway transmits the impulse in a downward
direction (antegrade), Wolff-Parkinson-White
syndrome results. For the A-V nodal reentry, the slow
tract within the node (a pathway) conducts the impulse
antegradely and the fast track (b pathway) conducts
the impulse retrogradely, which then completes
the ring-like conduction pathway of the circus movement.
Although there are other reentry mechanisms,
they are infrequently seen in the fetus. Reentrant SVT
is more amenable to therapeutic intervention.
The automatic SVT is caused by the presence of
ectopic atrial foci that stimulate the tachyarrhythmic
activity of the ventricles. Atrial fibrillation and flutter
are related to the atrial circus movement, which produces
atrial tachyarrhythmia, a variable transmission
of which results in ventricular tachyarrhythmia.
These classes of SVT are rare in the fetus and are associated
with a worse prognosis than the reentrant
variety.
It is noteworthy from the perinatal perspective that
fetal SVT is often associated with cardiac malformations,
including Ebstein's anomaly, tricuspid atresia,
corrected transposition, and cardiomyopathy,
although most infants are without any such complications.
In addition, viral infections, specifically cytomegalovirus
and coxsackie B, have been implicated.
Fig. 33.24. Electrophysiology of the reentrant supraventricular
tachyarrhythmia involving the accessory bypass tract.
SA sinoatrial, AV atrioventricular
The condition is usually recognized during fetal
heart rate auscultation or electronic monitoring. The
definitive diagnosis is achieved during fetal echocardiographic
examination. Duplex imaging-guided M-
mode sonography remains the most important modality
permitting determination of the atrial and ventricular
rates. Figure 33.25 presents a color M-mode
demonstration of the atrial contribution to the tachyarrhythmic
process. M-mode echocardiography
may also be useful for assessing ventricular hypertrophy
or dilation and spectral Doppler interrogation for
establishing atrial and ventricular rates by the simultaneous
interrogation of either the A-V flow and ventricular
outflow of the aorta and inferior vena cava
(see above). Finally, the importance of a detailed 2D
echocardiographic examination cannot be overemphasized,
as it allows targeted evaluation of the struc-