Diagnostic ultrasound ( PDFDrive )

1296 PART IV Obstetric and Fetal Sonography
beat is also early. PVCs have an early ventricular contraction
that is not preceded by an atrial contraction. he atrial pacemaker
is not reset, and the next normal beat occurs when expected, as
if the rhythm were regular. hus PVCs are compensatory, allowing
the preexisting rhythm to continue, whereas PACs are generally
less than compensatory. PACs and PVCs may be distinguished
by M-mode or spectral Doppler sonography (see Fig. 37.15) by
placing the M-mode cursor or spectral Doppler sample volume
simultaneously through an atrial and ventricular structure. PACs
followed by ventricular contraction are described as “conducted,”
whereas a PAC that is not followed by a ventricular contraction
is “nonconducted” or “blocked.” hese must be diferentiated
LV
RV LA
RA
FIG. 37.43 Thoracic Ectopia Cordis. The heart is located outside
the thorax. LA, Left atrium; LV, left ventricle; RA, right ventricle; RV,
right ventricle; SP, spine.
SP
from A-V block, in which a PAC does not occur. Conducted
PACs can be distinguished from PVCs by noting that a PAC
precedes the early ventricular beat and that, in the majority of
cases, PACs are less than compensatory whereas PVCs are generally
compensatory.
Premature contractions are benign arrhythmias in most
cases. Most disappear in utero or in the early neonatal period.
From 1% to 2% of PACs may evolve into sustained
tachyarrhythmia. 228
Tachycardia
Fetal tachycardia is a heart rate greater than 180 beats/min.
Supraventricular tachycardias (SVTs) are more common in the
fetus than ventricular tachycardias. Of cases of SVT, 5% to 10%
are associated with CHD. 229
SVTs are classiied as follows:
• Paroxysmal supraventricular tachycardia: atrial rate of 180
to 300 beats/min and a conduction rate of 1 : 1 (Fig. 37.44).
• Atrial lutter: atrial rate of 300 to 400 beats/min, frequently
associated with heart block, and a conduction rate of 2 : 1 to
4 : 1, yielding a ventricular rate of 60 to 200 beats/min.
• Atrial ibrillation: atrial rate of greater than 400 beats/min
and an irregular ventricular response at a rate of 120 to 160
beats/min.
Ventricular tachycardia is deined as a rapid heart rate
associated with three or more consecutive premature ventricular
systoles.
Fetal cardiac rhythm disturbances are usually irst suspected on
the basis of auscultatory indings. M-mode and pulsed Doppler
echocardiography are useful in identifying and characterizing
tachycardias by observing the atrial and ventricular rates.
he M-mode tracing is obtained through the heart to allow
independent assessment of atrial and ventricular wall motion.
Further information is obtained from simultaneous recordings
of the atrial and ventricular walls or A-V and semilunar valve
motion. Spectral Doppler ultrasound evaluation can demonstrate
A
B
FIG. 37.44 Supraventricular Tachycardia. (A) M-mode tracing through the left ventricle and the right atrium shows a fetal heart rate of 273
beats/min and a 1 : 1 conduction. (B) Spectral Doppler ultrasound through the aortic valve in the same fetus shows a heart rate of 283 beats/min.