Martijn A. Oudijk, MD, PhD*, Gerard
H.A. Visser, MD, PhD* and Erik J. Meijboom, MD, PhD¶
*Department of Obstetrics and Gynecology,
University Medical Center Utrecht,The Netherlands
¶Division of Pediatric Cardiology,
Central Hospital University of Vaud, Lausanne, Switzerland
Address for correspondence: Martijn
A. Oudijk, Department of Obstetrics, University Medical Center (UMC), KE
04.123.1/P.O.Box 85090, 3508 AB, Utrecht, The Netherlands. Email: m.oudijk@azu.nl
Introduction
Fetal tachycardia,
first recognized in 1930 by Hyman et al
1,
is a condition occurring in approximately 0.4-0.6% of all pregnancies
2. A subset of these cases with more sustained
periods of tachycardia is clinically relevant. The necessity of therapeutic
intervention in this condition is still a matter of discussion focused
on the natural history of the disease. The spectrum of opinions varies from
non-intervention
3,4,5 based on a number of cases in which the tachycardia
subsided spontaneously
6, to aggressive
pharmacotherapeutic intervention
7,8 based on reports of deterioration of
the fetal condition ultimately ending in significant neurological morbidity
9,10,11, or fetal demise
12,13,14.
Prenatal treatment through indirect, maternally administered drug therapy
seems to be the preference of most centers
15,16,17,18,19,20,21.
This matter will be discussed further in
Fetal Tachyarrhythmia, Part
II, Treatment.
The choice
of the specific pharmacotherapeutic agents and the chances on success
of therapy depend largely on the type of tachycardia. The determination
of the type of tachycardia is therefore of the utmost importance
20,22, however the available
diagnostic armamentarium is limited. The most widely used method of diagnosis
of fetal tachycardia, M-mode echocardiography, provides a time related
documentation of function of the various cardiac structures. The current
subdivision into supraventricular tachycardia (SVT), atrial flutter (AF)
and ventricular tachycardia (VT) derived from M-mode echocardiography is
not sufficient enough for differentiation according to the electrophysiologic
mechanism. Several attempts have been made in the last years to increase
the accuracy and reliability of M-mode echocardiography by measurement of
atrioventricular (AV) and ventriculoatrial (VA) intervals
23,24, and the
addition of Doppler echocardiography
25,26. New methods registering the actual electrophysiologic
events are in development including noninvasive techniques such as magnetocardiography(MCG)
27. Characteristics of the most common types
of fetal tachycardia are described and examples of M-mode echocardiography
and FMCG are presented.
Definitions of tachycardia
The normal
fetal heart rate ranges are approximately 120-160 bpm at 30 weeks and 110-150
bpm at term
28,29.
Frequencies up to 170 bpm are considered mildly abnormal, whereas overt
tachycardia is usually defined as a heart rate exceeding 170 bpm
28 or 180 bpm
22.
These rhythm abnormalities of the fetus are usually noticed at routine
prenatal visits. Fetal echocardiography is used to exclude structural cardiac
defects and used to position the M-mode sampling line to intercept both
the atrial and ventricular walls. The relationship between peak atrial
and ventricular systolic excursions currently provides a division into
SVT, AF and VT.
Fetal magnetocardiography,
a technique recording the magnetic field generated by the electrical activity
of the fetal heart
27,30,
offers a more precise delineation of the fetal electrophysiology. Detection
occurs non-invasively by sensors cooled by liquid helium positioned several
centimeters above the maternal abdomen in a magnetically shielded room.
As the maternal heart generates magnetic activity as well, a maternal ECG
is recorded simultaneously and substracted from the fetal MCG. This way,
an averaged one lead fetal MCG is obtained and allows for a more detailed
differentiation of the type of tachycardia.
Premature Atrial Contractions
Premature
atrial contractions (PAC’s) do not qualify as a form of fetal tachycardia,
and are associated with good outcome. However, in approximately 0.4 % of
cases, it may progress to runs of tachycardia and even become persistent.
It is therefore recommended that these patients are monitored weekly by
doptone to exclude the presence of runs of tachycardia
12,16. M-mode echocardiography
will show premature atrial contractions not followed by a ventricular
contraction and a subsequent ‘drop’ in heart rate (
Figure 1).
Figure 1: M-mode echocardiography of a premature
atrial contraction (PAC).
The FMCG will show a premature P-wave not followed by a QRS complex
(
Figure 2).
Figure 2: FMCG of a PAC (black arrow).
Atrial Flutter
AF is a condition
that accounts for approximately 21 – 50 % of fetal tachycardia and may
be associated with structural abnormalities
14,
21,31. It is defined
as an atrial rate ranging from 250 up to 500 bpm with a fixed or variable
AV block, as the AV-node is not able to conduct every contraction of the
atrium which results in a 2:1 or 3:1 conduction to the ventricles. Rarely
in some cases paroxysmal 1:1 conduction is seen. It may be either paroxysmal
or incessant in nature and is reported to be associated with fetal hydrops
in 7 – 43 % of cases
14,18,31,32. The most common electrophysiologic
mechanism in AF is a re-entry circuit confined to the atrium. M-mode echocardiography
will show typical atrial contractions that are followed by ventricular
contractions every 2 or 3 atrial contractions (
Figure 3).
Figure 3: M-mode echocardiography of AF. Flutter contractions
of the atrium are indicated by the small arrows; atrioventricular conduction
is 2:1 and ventricular contractions are indicated by the large arrows.
FMCG will show typical flutter waves (P-waves) followed by QRS complexes
every 2-3 P-waves (
Figure 4.1).
Figure 4.1: Raw FMCG trace of atrial flutter
with 2:1 conduction.
Figure 4.2: Averaged FMCG trace of atrial
flutter with 2:1 conduction.
Supraventricular tachycardia
SVT is reported
to account for 47 – 68 % of cases of fetal tachycardia
14,18,21
and is associated with a low percentage of structural abnormalities in
2 % of cases. It is defined by a 1:1 atrioventricular conduction in which
the atrial contraction precedes the ventricular contraction. Heart rates
in SVT most commonly range from 200-300 bpm, is either paroxysmal or incessant
in nature and associated with fetal hydrops in 36 - 64 %
14,33. The condition has
several different underlying electrophysiologic mechanisms, such as re-entry
using an accessory atrioventricular connection (AVRT), which may be either
apparent or concealed. Other possibilities include primary atrial tachycardias
and re-entry within the AV node
34.
The diagnosis
of these specific types of fetal tachycardia is difficult, if not impossible,
with the most widely used current technology, M-mode echocardiography.
As mentioned in the introduction, several studies have been conducted to
allow for a more specified diagnosis using Doppler and M-mode echocardiography.
These techniques are based on the relationship in time between atrial and
ventricular wall excursions on M-mode, and flow patterns over the AV and
semilunar valve orifices. In most re-entrant tachycardias, conduction through
the accessory connection is fast, as in the Wolff-Parkinson-White syndrome
(WPW). This results in a short RP interval on the postnatal ECG, comparable
to a short VA interval on M-mode (
Figure 5.1). In several patients,
slow conduction is observed in the accessory pathway, as in Persistent Junctional
Reciprocating Tachycardia (PJRT) and Atrial Ectopic Tachycardia (AET)
26, resulting in a long RP interval on the
ECG, comparable to a long VA interval on M-mode echocardiography (
Figure
5.2). In the literature there has been one case diagnosed as Junctional
Ectopic Tachycardia (JET) by Doppler echocardiography
35. It is extremely rare and we have not encountered
JET in the prenatal situation at our center.
Figure 5.1: M-mode echocardiography of a SVT with
a short VA interval. First white line placed on the peak excursion of the
left atrium wall, second line placed on the peak excursion of the right
ventricle wall, third line on the consecutive peak atrial wall excursion.
The AV interval is markedly longer than the VA interval.
Figure 5.2: M-mode echocardiography of a SVT
with a long VA interval.
The detailed
electrophysiological events however, are irretrievable by M-mode echocardiography.
Specific types of fetal tachycardia have been detected and published using
FMCG
36,37.
A more detailed configuration, comparable to the postnatal ECG is obtained.
An example of this category of fetal SVT’s is shown in
Figure 6.
Figure 6: FMCG of a fetal SVT
The electrophysiologic
events are clearly defined in this situation, however, a precise diagnosis
is still debatable. The SVT shown in
Figure 6 for instance, may
be AVRT using a concealed accessory pathway, but could also be attributed
to represent a PJRT.
Ventricular Tachycardia
VT in the
prenatal situation is rare and has only been reported occasionally in
the international literature
5,16. It is usually paroxysmal in nature and
outcome depends on the electrophysiologic mechanism. Of the utmost importance
is the diagnosis of the congenital Long QT syndrome (LQTS). The origin
of this arrhythmia lies in malfunctioning myocardial ion channels as a
result of mutations in genes encoding these ion channels. It is characterised
by prolongation of the QT interval and the occurrence of polymorphic ventricular
arrhythmia (such as Torsade de Pointes). Patients with the LQTS are predisposed
to ventricular fibrillation and sudden death
38,39. The prenatal diagnosis is focused on
fetuses of mothers with prolonged QT syndrome to document the possible
presence of this syndrome in the fetus. In addition, fetuses presenting
with (baseline) bradycardia or intermittent ventricular tachycardia of
the torsade de pointe type, the diagnosis of LQTS must be considered as
well, as spontaneous mutations have been reported. The suspicion of fetal
LQTS may be raised in case M-mode echocardiography shows (baseline) bradycardia
varying from 60-110 bpm and intermittent ventricular tachycardia in which
there is atrioventricular dissociation. FMCG shows a prolonged QT interval
(
Figure 7), and the possible presence of intermittent VT
40,41.
Figure 7: The QT interval is 319 ms, RR interval is 430 ms, QTc
interval is 486 ms.
Conclusions
Fetal tachycardia
is a rare disorder, in which difficulties are encountered in the diagnosis
of the exact underlying electrophysiological mechanism and is therefore
probably underreported. The spectrum of pathologic symptomatology varies
from an infrequent paroxysmal tachycardia to a persistent form, which may
deteriorate in fetal hydrops, neurological morbidity and even fetal demise.
The presence of fetal tachycardia therefore deserves more attention and
requires a specialized evaluation to define its electrophysiologic diagnosis.
Fetal atrial
flutter is a frequently encountered form of fetal tachycardia and can be
well diagnosed by either M-mode echocardiography or FMCG. Fetal SVT remains
the most complicated form of fetal tachycardia, both in diagnosis as in
treatment. The difficulty in delineating an accurate diagnosis of the specific
form and the relative therapy resistance of the various subtypes of this
tachycardia, for instance PJRT, makes the development of an optimal treatment
protocol complicated.
The most infrequent
encountered form of tachycardia is fetal VT, however, if present, it has
very serious implications for the fetus. In utero management is complicated
and yields a limited success but it is also predictive for an uncertain
future after birth. Improved prenatal diagnostic techniques may support
a better understanding of the pathophysiology, but does not necessarily
result in an improved outcome. Studies on these new techniques will hopefully
result in the ultimate goal of reduced morbidity and mortality in the field
of fetal tachycardia.
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