Intrapartum Assessment

Chapter 14. Intrapartum Assessment

1st year resident, Sang-Yun. Oh. M.D.

- Advantages of continuous electronic fetal heart rate monitoring

Provided accurate information

Be of value in diagnosing fetal distress

Can intervene to prevent fetal death or morbidity
Be superior to intermittent methods

Internal Electronic Fetal Heart Rate Monitoring

The fetal heart rate may be measured by attaching a bipolar spiral electrode directly to the fetus (F14-1)

Electrical cardiac complexes detected by the fetal scalp electrode include those generated by the mother

External Electronic Fetal Heart Rate Monitoring

- Membrnae rupture and uterine invasion may be avoided

- Not provide the precision

- The easiest technique employs the ultrasound Doppler principle

I. Fetal Heart Rate Patterns

Typical scaling factors employed in the United States are 30 beats/min per vertical cm (range, 30 to 240 beats/min) and 3 cm/min chart recorder paper speed

1. Baseline Fetal Heart Activity

1) Rate

During the third trimester, the normal average baseline fetal heart rate - 120~160 beats/min

The accelerator influence - sympathetic system

The decelerator factor - parasympathetic system

Other controller of heart rate - arterial chemoreceptors

More sever and prolonged hypoxia -> rising blood lactate -> severe metabolic acidemia ->

direct effects on the myocardium -> decrease heart rate
Bradycardia

Baseline fetal heart rate - under 120 beats/min, last 15 minutes or longer

Degree ( for 3 minutes or longer )

- Mild : 100~119 beats/min

- Moderate : 80~100 beats/min

- Severe : < 80 beats/min

Mild bradycardia without deceleration or acceleration is not necessarily evidence for fetal compromise

Causes

- Umbilical arterial blood acidemia

- Congenital heart block

- Placenta abruption

- Maternal hypothermia

- Severe pyelonephritis
Tachycardia

Degree

- Mild : 161~180 beats/min

- Severe : > 181 or more

Causes

- Maternal fever from amnionitis - the most common

- Fetal compromise

- Cardiac arrhythmias

- Administration of parasympathetic (atropine) or symphthomimetic (terbutalilne) to the mother

2) Beat-to-Beat Variability

- Important index of cardiovascular function

- Be regulated largely by the autonomic nervous system
Short-term variability

Reflect the instantaneous change in fetal heart rate from one beat to the next

A measure of the time interval between cardiac systoles

Long-term variability

The oscillatory changes that occur during the course of 1 minute and result in the waviness of the baseline

The normal frequency of such waves is three to five cycles per minute

Causes of increased variability

- Fetal breathing

- Fetal body movements

- Advancing gestation

Causes of decreased variability ( < 2 cyclic changes per minute of long-term variability)

- Severe fetal acidemia					- Severe maternal acidemia

Metabolic acidemia causes depression of the fetal brainstem or the heart itself

Analgesic drugs : narcotics, barbiturates, phenothiazines, diazepam, general anesthetics,

? ? ? ? ? ? ? ? ? ? ? ? ? ? meperidine, fentanyl, magnesium sulfate

3) Sinusoidal Heart Rates

Causes

- Serious fetal anemia - ex. D-isoimmunization, reptured vasa previa, fetomaternal hemorrhage

- twin to twin trnasfusion

- Drugs - meperidine, morphine, alphaprodine, butophanol

- Amnionitis, fetal distress, umbilical cord occlusion

2. Periodic Fetal Heart Rate

- The pathophysiological events

Early deceleration - head compression
Late deceleration - uteroplacental insufficiency
Variable deceleration - cord compression

1) Accelerations

An increase in the fetal heart rate of at least 15 beats/min, usually of 15 to 20 seconds duration

Causes

- Fetal movement

- Stimulation by uterine contractions

- Umbilical cord occlusion

- Fetal stimulation during pelvic examination

- Fetal scalp blood samplling

- Acoustic stimulation

- Labor

Accelerations during the first and/or last 30 minutes was a favorable sign for fetal well being

2) Early Deceleration

There was a drop in heart rate with uterine contraction and that this was related to cervical dilatation

Freeman and co-authors(1991) defined early decelerations as those generally seen in active labor between 4~7 cm dilatation.

The degree of deceleration is generally proportional to the contraction strength and rarely falls below 100 to 110 beats/min or 20 to 30 beats/min below baseline

Not associated with fetal hypoxia, acidemia, or low Apgar scores

3) Late Deceleration

A symmetrical decrease in fetal heart rate beginning at or after the peak of the contraction and returning to baseline only after the contraction has ended

Uniform in shape and typically begin 30 seconds or more after the onset of the contraction

Descent and return of the fetal heart rate are gradual and smooth

The magnitude is rarely more than 30 to 40 beats/min below baseline and typically not more than 10 to 20 beats/min in intensity.

Usually not accompanied by accelerations

The lag period was directly related to basal fetal oxygenation

The length of the lag phase was predictive of the fetal Po2 but not fetal PH

The lower the fetal Po2 prior to contractions, the shorter the lag phase to onset of late decelerations

In late decelerations, there are two pathophysiological pathways
Chemoreceptor-mediated vagal reflex
Hypoxic myocardial depression

Causes of the late deceleration

- Maternal hypotension - from epidural analgesia

- Excessive uterine activity - from oxytocin

- Placenta dysfunction - maternal diseases such as hypertension, diabetes, collagen-vascular disorders,

- severe chronic maternal anemia

- Placental abruption

4) Variable Decelerations

The most common deceleration patterns encountered during labor

Attributed to umbilical cord occlusion, usually by release of amnionic fluid and fetal descent

One fourth of fetuses have one or more loops of cord wound around the neck. Short (< 35 cm) and long

( > 80 cm ) cords are found in 6 percent of births and are associated with variable decelerations

Melchior and Bernard (1985) identified variable decelerations in 40 % of over 7000 monitor tracings when labor had progressed to 5 cm dilatation and in 83 % by the end of the first stage

Two types of variable decelerations

A - complete cord occlusion

B - partial cord occlusion -> complete -> partial (shoulder)

Variable decelerations are vagally mediated and the vagal response may be due to chemoreceptor or baroreceptor

activity or both, these reflexes "physiological" rather than pathophysiological.

The American College of Obstetricians and Gynecologists(1995) has defined significant variable decelerations

as those decreasing to less than 70 beats/min and lasting more than 60 seconds

5) Saltatory baseline heart rate (F14-26)

Be linked to umbillical cord complications during labor

In the absence of other fetal heart rate findings, these do not signal fetal compromise

6) Mixed cord compression pattern

Consisting of an acceleration immediately followed by a deceleration associated with abnormal cord positions at delivery

This acceleration-deceleration combination - the Lambda pattern

Attributed to fetal movement

Not associated with adverse outcomes

7) Prolonged deceleration

Be defined as isolated decelerations lasting more than 60 to 90 seconds, however this description does not

define the maximum duration.

Incidence

- During first-stage labor - unclear

- During 2nd -stage lavor - one third of labor

The significance of the amplitude of prolonged deceleration is also unclear

Causes

- Cervical examination

- Uterine hyperactivity

- Cord entanglement

- Maternal supine hypotension

- Epidural, spinal, or paracervical analgesia

- Maternal hypoperfusion or hypoxia due to any cause

- Placental abruption

- Umbilical cord knots or prolapse

- Maternal seizures

- Application of fetal scalp electrode

- Impending birth

- Maternal valsalva maneuver