Safeurlives.com®
News update information

Antepartum Fetal and Heart Rate Testing Current Status

I. Objectives

A. Physiologic and technologic principles of antepartum fetal heart rate testing (AFHRT)

B. Application and interpretation of AFHRT

C. Areas of controversy and testing pitfalls

II. Fetal movement (FM) Counting

A. Physiologic background

1. Healthy fetuses move 20-30 times per hour

2. Fetal movement occupies 10-15% of 24h day

3. Active fetus has high likelihood of good result

4. Feud compromise may follow decreased FMs

B. Basis for maternal perception of FM

1. Inexpensive, simple No equipment needed

3. May be done in home, office, hospital

4. Patient is engaged in her care process

C. Application

1. Each fetus acts as own control

2. Baseline record of activity can be established

3. Clinical alerts: decreased or absent Fms

4. Note: same conditions for each session

D. Follow up

1. Supplement other forms of testing

2. Decreased FM 6 more intensive testing

3. NOTE: inform patient that absolute FM count may vary considerably between sessions

E. Limitations

1. Limited numbers of good clinical trials

2. Low sensitivity to prediction of acute distress

3. Normal “slow” or “hyperactive” fetus?

4. Variation in ability to educate the patient and compliance

III. Principles: Physiologic Bases

A. Fetal heart rate testing: applications

1. NST: office/hospital possibly home. 20-30 min to t-2 h

2. VAS' similar to NST. 10-20 min

3. ACTG: similar to NST CST: office/hospital. 20-30 min to 2-3 h

B. Nonstress test (NST)

1. What does the NST test?

a) Selected FHR baseline features

(1) Accelerations with FMs (<)

(2) Baseline rate and variability (?)

(3) Decelerations: spontaneous (<)

b) Physiologic

c) Pathophysiologic

d) Brainstem function

e) Hypoxia./acidosis

f) ANS/reflex control

g) Malnutrition

h) Maturation of FHR

i) Cord compression

j) Circadian rhythms

k) Placental insufficiency

1) Behavioral state

m) CV and CNS anomalies

2. Physiologic basis for NST

a) FM is normal, episodic phenomenon

b) Third trimester fetuses respond to FM with coupled accelerations (>90%)

c) Hypoxia, asphyxia, malnutrition reduce FMs. decrease coupling

fewer accelerations

3. NST interpretation

a) Reactive: accelerations + Fms

b) Nonreactive

(1) Accelerations present, too few

(2) Accelerations absent. FMs present

(3) Accelerations, FMs absent

4. Nonreactivity sequence

a) Decreased acceleration counts, amplitude

b) Decreased FM counts

c) Uncoupling accelerations and Fms

d) No accelerations or Fms

e) Spontaneous decelerations

5. Causes of nonreactive NST

a) Compromised fetus

b) Behavioral state

c) Immaturity

d) Maternal diet/drugs

e) Fetal anomalies

6. Testing conditions

a) Length of observation: 30'-60' needed for 1 acceleration m 95%. of

normal fetuses

b) Devoe, McKinzie, et al. Am JOb Gyn 1985

(1) Reactivity in 95% within 70 minutes

(2) Nonreactivity (>90') ~ abnl CST (95 %)

c) Corollary: prolonged NR NST at term --~ consider delivery

d) Effects of immaturity on FHR baseline

(1) Lower amplitude accelerations

(2) More frequent decelerations

(3) Less coupling

(4) Standard reactivity takes longer to obtain

e) Pathologic tracing as significant as in term

(1) Prolonged NR

(2) Late decelerations

(3) Absent variation

(4) Severe variables

f) Maternal drugs

(1) $-blockers, CNS depressants

(2) Nicotine, cocaine

g) Maternal diet

(1) Hypoglycemia: decreased FM only if profound

2) Hyperglycemia: more FBM. less FM,

7. Sequential nonstress tests

a) Concept of using each fetus as its own control

b) Devoe, et al. Am JOb Gyn. 1986

(1) 16 of 18 fetuses with eventual compromise showed abnormal trends in NSTs before tests fell

below threshold for normalcy

2) Recommend testing under same conditions

C. Contraction stress test (CST), oxytocin challenge test (OCT

1. Basis

a) Response of FHR base:line to reduced or spontaneous uterine contractions (Ucs)

b) FHR-UC association (see Figure 1)

c) Influences

(1) Contraction frequency, intensity

(2) Maternal buffering capacity, position

(3) Fetal oxygenation, acid-base levels

d) Sequence of events leading to positive test

2. Significance

a) CST reflection of 02, acid-base balance, placental reserve

b) Positive CST may reflect

(1) Fetal compromise

(2) Maternal hypoxia, hypotension

(3) Uterine hyperstimulatoin

(4) Umbilical cord vulnerability

V. Interpretive Criteria

Table 1 NST lnterperative Data

VI. Vibroacoustic Stimulation

A. Physiologic basis

1. Signal = broad-band (20-10K)

2. SPL = 82 db in air, 110 db in water

B. EAL provides two components

1. Vibrator'5' (+)

2. Acoustic (-)

C. Shortens testing time

D. Predictive accuracy is similar to standard NST

E May be useful intrapartum

F. No apparent adverse side-effects NST

G. Maybe useful intrapartum

H. No apparent adverse side effects in newborn

I. Normal fetal responses to VAS signal

1. 95% will become reactive post-VAS

2. 85 5 will have increased FHR baseline

10 bpm, >180 seconds

3. Mean onset: 7.5 sec

4. Mean duration: 600 sec

5. Modal duration: 300 sec

6. Median duration: 360 sec

J. Post-VAS responses reflect change in state

K. Post-VAS responses . spontaneous stimuli

L. Failed VAS does not exclude normal outcome

M. Safety remains to be firmly established

N. Efficacy trials are limited

VII. Actocardiotocography (ACTG)

A. Combined recording of Doppler-derived FHR and FMs in same time

B. Commercial units now available

C. Evaluated as potential for extending capability of NST

D. Studies are very limited

VIII. Test Selection/Diagnostic Values

A. NST vs CST

1. Contraindications

a) NST: none CST

(1) Third trimester bleeding

(2) Premature rupture of membranes

(3) Hypersensitivity

(4) Possible previous uterine surgery

2. Applications

a) NST: safer to perform

b) CST: potential hazard of UCs

3. Similar

a ) Test length: ff nipple-stimulation used

b) Specificity

c) Negative predictive value

4. Varies with interpretive criteria

a) Sensitivity

b) Positive predictive value

5. Note: few prospective studies of sufficient size exist to establish clear-cut advantage of either approach

6. Most recent study suggests that NST and nipple stimulation CST are virtually equivalent predictors of

outcome in similar obstetric populations managed in a similar manner

B. Diagnostic values

1. Specificity, negative predictive values are excellent with both tests

2. SensitMty varies with criteria, population tested (see Table 3)

3. False-negative test is uncommon and usually results from nonpredictable events (see Table 4)

4. False-positive rate is extremely variable but in most studies approximates 50%

5. Conclusions: both tests are most useful in determining health but. without the use of ancillary

information, fall short of good diagnostic methods for determining disease