An abnormally low heart rate. When referring to the fetal heart rate (FHR) tracing in labor, abnormally low heart rates can signal problems with the fetus before delivery. The FHR is often monitored for abnormalities of the heart rate. Fetal bradycardia episodes are sometimes called FHR decelerations. Some types of FHR decelerations are common during labor. Others suggest fetal stress and demand further evaluation or intervention. Sometimes the FHR not only dips down, it stays down and does not return to its usual level. This is an ominous situation that demands immediate delivery. FHR decelerations or bradycardia episodes come in three types.
1. Early decelerations are normal and common. These decelerations are called early because they occur early during a uterine contraction. These FHR decelerations usually occur after labor is well established (4-7 cm of cervical dilatation). The FHR rarely goes below 100 beats per minute. The cause of these decelerations is head compression during uterine contractions.
2. Late decelerations cause more concern. They are called late because they first appear at or after the peak of the uterine contraction. The FHR improves only after the contraction has stopped. These FHR decelerations may be mild or severe based on how low the FHR goes and how long it takes for the FHR to recover. It is thought to be caused by reduced blood flow to the uterus and placenta during a contraction.
3. Variable decelerations are a common type of FHR deceleration in labor and are caused by umbilical cord compression. Up to 80% of fetuses will have variable decelerations during labor. The significance of these decelerations depends on how low the heart rate drops and how long the episode lasts.
When referring to a newborn baby bradycardia is usually associated with apnea or cessation of breathing. Apnea and bradycardia spells are most common in prematurely born infants. During these spells the infant will stop breathing for at least 15 seconds and the heart rate will start to slow. Gentle touching or other stimulation almost always restarts the breathing and increases the heart rate. Medications (theophylline or caffeine) are sometimes used to treat these spells in newborn babies. Apnea of prematurity does not cause sudden infant death syndrome (SIDS)
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