Reactivity and the Normal FHR

The normal fetal heart rate is 110 to 160 bpm. This baseline (a "baseline" rate refers to a heart rate lasting > 10 minutes) normally has frequent periodic variations above and below termed accelerations (increases in HR) and decelerations (decreases in HR) (Figure 6-11).

TABLE 6-2. FHR Patterns

Early Decel

Late Decel

Variable Decel

Significance

Benign

Abnormal

Mild

Shape

U shaped

U shaped

Variable (often V or W shaped)

Onset

Gradual

Gradual

Abrupt

Depth

Shallow

Shallow

Variable

When

End with the uterine contraction

End after the uterine contraction

Variable

Why

Head compression

Uteroplacental insufficiency

Cord compression and occasionally head compression

Initial treatment

None required

O2, lateral decubitous position, Pitocin off

Amnioinfusion

1 Baseline i

Baseline

FHR

No periodic

No periodic

No periodic

FHR change

FHR change

FHR change

FIGURE 6-11. Fetal heart rate patterns.

(Reproduced, with permission, from Hon EH. An Atias of Fetal Heart Rate Patterns. Harty Press, 1968.)

A fetus < 28 weeks' GA is neurologically immature and thus is not expected to have a "reactive" FHR.

The lateral recumbent position (either side) is best for maximizing cardiac output and uterine blood flow. (In the supine position, the vena cava and aortoiliac vessels may be compressed by the gravid uterus.)

FIGURE 6-11. Fetal heart rate patterns.

(Reproduced, with permission, from Hon EH. An Atias of Fetal Heart Rate Patterns. Harty Press, 1968.)

A normally reactive fetal tracing has two accelerations of at least 15 bpm greater than the baseline, lasting for at least 15 seconds, in 20 minutes. It represents intact neurohumoral cardiovascular control mechanisms and indicates that the fetus is unstressed.

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