Fetal Tachycardia Assessment at 36+4 Weeks
A nonstress test (NST) is the best initial assessment for this fetus with tachycardia at 36+4 weeks gestation, as it provides immediate evaluation of fetal oxygenation and acid-base status while being non-invasive and easily repeatable. 1
Rationale for NST as First-Line Test
The NST has replaced stress testing as the preferred method for heart rate-based fetal assessment because it is:
- Non-invasive and does not require intravenous access 1
- Less time-consuming and easier to interpret 1
- Readily repeatable if needed 1
- Excellent at predicting fetal well-being with high negative predictive value 2, 3
At 36+4 weeks with maternal illness (flu), this represents a high-risk scenario requiring fetal surveillance. 2, 1
Specific Assessment Protocol
Initial NST should evaluate:
- Presence of ≥2 fetal heart rate accelerations in 20 minutes (defines reactive NST) 1
- Baseline heart rate variability and pattern 4
- Any decelerations associated with contractions or fetal movement 5
Simultaneously assess amniotic fluid volume to create a modified biophysical profile, which is the recommended approach rather than NST alone:
- Maximum vertical pocket (MVP) ≥2 cm is normal 1
- Oligohydramnios may indicate uteroplacental insufficiency 1
Interpretation and Next Steps
If NST is reactive (normal):
- This is highly reassuring with excellent predictive value for fetal well-being 6, 3
- The reactive NST is as predictive of good outcome as a negative contraction stress test 4
- No further immediate testing needed unless other abnormalities detected 1
If NST is nonreactive (abnormal):
- Proceed to full biophysical profile (BPP) including fetal breathing movements, body movements, tone, and amniotic fluid 1, 7
- Consider umbilical artery Doppler if growth restriction suspected 1
- BPP score ≤6 warrants hospitalization; score <4 with fetal maturity may indicate delivery 7
Critical Context for This Case
The fetal tachycardia (190 bpm with variability 160-190) in the setting of maternal flu illness likely represents a physiologic response to maternal fever or stress, even though mother is currently afebrile. 2 The presence of variability is reassuring, but formal NST assessment is still warranted to:
- Document reactivity (accelerations with movement) 4
- Rule out fetal compromise 6
- Establish baseline for potential repeat testing 1
Important caveat: No antenatal test can predict acute events like cord accidents or abruption, so ongoing surveillance may be needed if maternal illness persists. 2, 1
Why Not Stress Testing
Contraction stress testing (oxytocin challenge test) is not indicated as initial assessment because:
- It is more invasive, time-consuming, and requires IV access 1
- NST provides equivalent predictive value for fetal well-being 4
- At 36+4 weeks with suspected maternal illness, NST gives immediate information without the risks of induced contractions 2
The nonreactive NST group shows significantly increased cesarean section rates and perinatal mortality, making proper initial assessment critical. 6