Download Fetal Tachycardia

Maureen E. Sims, M.D.
Professor of Pediatrics, UCLA
Fetal Tachycardia
The normal fetal heart rate is 120-160 bpm at term. A sustained high heart rate beyond
160 bpm is an immense physiologic challenge for a fetus. Stroke volume is dependent on
the amount of blood returning to the heart (preload), strength of the myocardial
contractility (the pump) and the resistance against which the heart must pump (after
load). Fetal and neonatal cardiac output is more dependent on heart rate than on the force
of the contractions. The fetus in late gestation appears to have a limited ability to
maintain cardiovascular stability when tachycardia is persistent, mainly because of the
adverse impact the elevated heart rate has on preload. The fetal and neonatal myocardium
is very different from the older child or adult in several respects. Since the fetal heart is
composed of 60% of non-contractile elements, the myocardium is much stiffer, thereby
impairing cardiac relaxation and filling. Additionally, filling is reduced as a result of
ventricular constraint secondary to a compressed thoracic cavity, lungs and pericaridum.
And lastly, the preload is limited because there may be inadequate time for filling with a
high heart rate.
Several pathophysiologic processes may cause fetal tachycardia. Hypoxia is a
well-known cause of tachycardia. A high heart rate may be continuous or intermittent. In
the latter, it often appears after a deceleration probably as an attempt by the fetus to
increase perfusion by increasing cardiac output. Additionally a high heart rate may be due
to increased catecholamine activity from the adrenal medulla in response to the stress of
the hypoxemia. Fetal tachycardia may also be secondary to fetal anemia, typically from
hemolytic disease or chronic blood loss (fetal maternal transfusion or twin-to-twin
transfusion). Anemia from acute blood loss may occur secondary to a hemorrhage at or
near the time of delivery. These acute conditions include: blood loss from fetus to mother
or twin, abruptio placenta, placenta previa, vasa previa, or a torn cord (often
velamentously inserted). Fetal hypovolemia will coexist with the anemia if the
hemorrhage is acute and severe, but an equilibration time is needed before the true
hematocrit is reflected from the hemorrhage.
Fetal arrhythmias can cause fetal tachycardia as well, most often presenting with
heart rates over 200 bpm. Several maternally administered medications can increase the
fetal heart rate. Anticholinergic medications, such as Benadryl, and sympathommetic
medications, most notably terbutaline, can cause an elevated fetal heart rate. One of the
most common causes of fetal tachycardia is maternal fever. A wide variety of maternal
infections ranging from chorioamnionitis to pneumonia and sepsis can cause a fetal
tachycardia. The elevated fetal heart rate can be secondary either from the temperature
transmitted to the fetus from the mother, or from the fetus actually becoming infected.
Moreover, since the fetus normally has a half-degree higher temperature than the mother,
maternal fevers have a major impact on the homeostasis of her fetus. Fetal pain,
especially from twists, pulls and turns from assisted deliveries (vacuum or forceps) can
create tachycardia as well. One of the more unusual sources of fetal tachycardia is
maternal hyperthyroidism.
It is very important for the neonatologist to think outside the box. When faced
with a situation of fetal tachycardia, independently consider the risk factors that the
mother has and those that she and her obstetrician may not be aware of at the time of
labor. Doing so may provide valuable in-put to your obstetrical colleague. Co-morbidities
often exist. For example, a woman with pneumonia is entitled to a fever. This infection
surely can cause fever and fetal tachycardia. But it can also precipitate other illnesses that
were well in check during pregnancy. Perhaps the hyperthyroidism, which was well
controlled prenatally, may now develop into a state of thyrotoxicosis in labor because of
a newly acquired infection. This thyroid storm may be the cause of the tachycardia in the
fetus because of the thyroid stimulating immunoglobulin, which freely crosses the
placenta. This hyperthyroidism in the fetus may well continue in the newborn period and
beyond, requiring you to monitor the neonatal thyroid status and intervene with antithyroid medications.