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Fetal Heart Rate Abnormalities: Minimal Variability and Heart Rate of 100

Mar 2014 – by A. Sandlin, MD and P. Wendel, MD

Fetal bradycardia is defined as a baseline fetal heart rate less than 110 beats per minute.1 Fetal bradycardia can occur due to poor uterine perfusion, maternal hypotension (e.g. post-epidural), umbilical cord prolapse or occlusion, rapid fetal descent, tachysystole, placental abruption, or uterine rupture.2 Other causes include congenital fetal heart abnormalities or conduction defects (e.g. congenital heart block).2 Minimal fetal heart rate variability can be associated with fetal sleep cycles, maternal medication (e.g. opioids, magnesium sulfate), decreased fetal oxygenation, or fetal neurologic abnormalities (e.g. anencephaly). Minimal fetal heart rate variability does occur intermittently under normal circumstances but should return to moderate variability.

Treatment and management of fetal bradycardia will depend on gestational age, underlying cause and overall clinical status of the patient and fetus. When persistent fetal bradycardia with minimal variability is detected antenatally and remote from term, sonographic examination to assess for structural anomalies and growth restriction should be performed. If a cardiac defect or conduction abnormality is suspected, then a fetal echocardiogram is indicated. Maternal laboratory evaluation for collagen vascular disease, specifically SS-A and SS-B antibodies, should be performed if fetal congenital heart block is suspected. Umbilical artery Doppler studies are recommended if fetal growth restriction is present, and antenatal steroids may be warranted if there is likelihood for preterm delivery. Antenatal surveillance is recommended in the third trimester.

Intrapartum fetal bradycardia with absent or minimal fetal heart rate variability needs to be evaluated to identify the underlying cause. Steps should be taken to determine whether this low baseline fetal heart rate indicates suboptimal uterine perfusion which might be improved with intrauterine resuscitative measures such as maternal lateral positioning, maternal oxygen administration, or intravenous fluid bolus administration. Digital scalp or vibroacoustic stimulation should be performed and the fetal heart rate monitored for an appropriate response. According to ACOG, “continued minimal variability (in the absence of accelerations or normal scalp pH) that cannot be explained or resolved with resuscitation should be considered as potentially indicative of fetal acidemia and should be managed accordingly.”2 Intrapartum fetal bradycardia which does not resolve should result in prompt delivery.

References:

1. ACOG Practice Bulletin #106: Intrapartum Fetal Heart Rate Monitoring: Nomenclature, Interpretation, and General Management Principles. Obstet Gynecol 2009:114:192-202.

2. ACOG Practice Bulletin #116: Management of Intrapartum Fetal Heart Tracings. November 2010. Reaffirmed 2013. Obstet Gyencol 2010;116:1232-40