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Fetal Heart Monitoring  


Electronic fetal heart monitoring is one of the most controversial diagnostic tools in modern obstetrics. The goal of monitoring, either by auscultation or by electronic means is to detect fetal stress and distress so that appropriate assistance can be taken to alleviate the cause(s) of the stress or distress.

Until the middle of the 1960's, care givers would palpate (or feel) the mothers abdomen or listen to the baby's heart beat through various stethoscopes including a fetoscope. Fear of litigation has made electronic fetal heart monitoring a staple item in today's birth atmosphere.

Please note: The information provided here is for informational purposes only.

External Monitoring
Electronic external monitoring of uterine contractions is accomplished by placing a tocodynamometer (or pressure transducer) over the area of greatest traceable activity during a contraction. The tocodynamometer (also called a tocotransducer) is placed over the fundus and secured with an elastic belt.

For external monitoring of the fetal heart, a Doppler ultrasound transducer is used to transmit high frequency sound waves into the fetus. These sound waves are then reflected back off of the moving heart. Frequency differences and reflected sound makes up the signal. The signal is then converted to an average rate, counted and displayed by the cardiotachometer. The transducer is placed over the fetal heart and periodic adjustments should be made to compensate for both maternal and fetal position changes.

Internal Monitoring
When electronic external monitoring of uterine contractions is not efficient to make a diagnosis or to adequately assess effectiveness of contractions, an intrauterine pressure catheter or IUPC may be used. Use of an IUPC is done if the cervix is partially dilated and membranes are no longer intact. Intrauterine pressure is transmitted from the amniotic fluid through the sterile water in the catheter to the pressure transducer. The IUPC can also be used to sample the amniotic fluid for the presence of meconium or flushing out meconium from the inside of the uterus.

Internal fetal monitoring is accomplished with a fetal spiral scalp electrode which is attached to the fetus and detects direct fetal electrocardiogram. While the wire is inserted under the scalp skin, care must be taken to be sensitive when describing the procedure to the parents.

Interpreting Values
The normal range for the fetal heart rate is 120 - 160 beats/minute. This normal range or baseline rate is the average fetal heart rate obtained between uterine contractions, changes and movements. The baseline can be affected by such factors as hypoxia and catecholamine production and certain medications On paper, the fetal heart rate is on the upper portion of the paper measured in beats/min. and the uterine activity is on the lower portion measured in mmg Hg.

Tracing

Pattern

Possible Cause(s)
Elevation in rate over 160/min for more than 10 min. Baseline tachycardia Maternal fever; fetal anemia; fetal tachyarrhythmia; dehydration; idiopathic maternal anxiety.
Rate below 120/min for 10 min. Fetal or baseline bradycardia Medications; maternal systemic lupus; hypoxia.
Fluctuations or oscillations in FHR for about 1 min. Long term variability Responsiveness of fetus' behavioral/wake state.
Rate counted from 1 R wave to the next in each cardiac cycle Beat-to-beat or short-term variability When variability is present, the fetus is thought to have an intact nervous system. Loss of variability causes a flat baseline and signals a compromised situtation.
FHR of more than 150 beats/min for more than 15 seconds. Accelerations Partial cord compression; may be non-specific.
Slowing of FHR in a pattern Early decelerations Head compression, commonly observed 4-6 cms. in mirror image of contractions.
"U" "V" or "W" shaped heart rate Variable decelerations Oligohydramnios; cord compression; nucal cord.
30 beats/min less than regular for up to 10 mins. Prolonged decelerations Maternal hypotension; abruptio placenta; uterine hyperstimulation; uterine rupture; cord compression; rapid fetal descent.
Repetitive undulation of baseline Sinusoidal RH isoimmunization; fetomaternal hemorrhage.
Slowing of FHR in a pattern- begins to fall after onset. Late decelerations Hypoxia; IUGR; maternal hypotension; uterine hypertonus; uteroplacental insufficiencies.

Fetal Scalp Sampling and Fetal Scalp Stimulation
When fetal heart monitor patterns are non-reassuring, a small amount of blood may be obtained from the fetal scalp (fetal scalp sampling) and the acid/base (pH) values may be assessed. Values above 7.25 are normal during labor. Values less than 7.20 is considered acidotic. Values 7.2 - 7.25 are equivocal and warrants repeating. Fetal scalp stimulation may be done by finger/digital stimulation during a vaginal exam. A fetal heart rate acceleration of 15 beats/min or more for more than 15 seconds is considered normal.

Read more online about fetal heart monitoring during labor:

University of Michigan Health System
Minnesota Department of Health

References:

  1. Bobak, I., Jensen, M. Maternity & Gynecologic Care: The Nurse and the Family. (1989) St. Louis: Mosby Publishers.
  2. Nichols, F., Humenick, S. Childbirth Education: Practice, Research & Theory (2000) Saunders & Co.
  3. Whitely, N. A Manual of Clinical Obstetrics (1985) Philadelphia: J.B. Lippincott Company.
  4. Reeder, S., Martin, L., and Koniak-Griffin, D. Maternity Nursing: Family, Newborn, and Women's Health Care. (1997) Philadelphia: J.B. Lippincott Company.



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