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Know All About : NON-REASSURING FETAL STATUS

Non-reassuring fetal status is a term used to describe suspected fetal hypoxia and is meant to replace the more ubiquitous term “fetal distress.” Fetal distress, defined as progressive fetal hypoxia and/or acidemia secondary to inadequate fetal oxygenation, is a term that is used to indicate changes in fetal heart patterns, reduced fetal movement, fetal growth restriction, and presence of meconium stained fluid.

Non-reassuring fetal status is not an adverse event per se, but rather an indicator of an underlying condition resulting in temporary or permanent oxygen deprivation to the fetus which may lead to fetal hypoxia and metabolic acidosis. Since fetal oxygenation is dependent upon maternal oxygenation and placental perfusion, perturbations of maternal oxygenation, uterine blood supply, placental transfer or fetal gas transport may lead to fetal hypoxia and non-reassuring fetal status. Conditions commonly associated with non-reassuring fetal status include maternal cardiovascular disease, anemia, diabetes, hypertension, infection, placental abruption, abnormal presentation of the fetus, intrauterine growth restriction and umbilical cord compression, among other obstetric, maternal or fetal conditions.

The fetus experiences three stages of deterioration when oxygen levels are depleted: transient hypoxia without metabolic acidosis, tissue hypoxia with a risk of metabolic acidosis, and hypoxia with metabolic acidosis.

Fetal response to oxygen deprivation is regulated by the autonomous nervous system, mediated by parasympathetic and sympathetic mechanisms. The fetus is equipped with compensatory mechanisms for transient hypoxia during labor, but prolonged, uninterrupted fetal hypoxia may lead progressively to acidosis with cell death, tissue damage, organ failure and potentially death.

In response to hypoxia, fetal compensatory mechanisms include 1) a decrease in heart rate; 2) a reduction in oxygen consumption secondary to cessation of nonessential functions such as gross body movements; 3) a redistribution of cardiac output to preferentially perfuse organs, such as the heart, brain, and adrenal glands; and 4) a switch to anaerobic cellular metabolism. Prolonged fetal hypoxia is associated with significant perinatal morbidity and mortality with particular concern for short- and long-term complications including encephalopathy, seizures, cerebral palsy, and neurodevelopmental delay. The fetal heart rate changes markedly in response to prolonged oxygen deprivation, making fetal heart rate monitoring a potentially valuable and commonly used tool for assessing fetal oxygenation status in real time. Non-reassuring fetal heart rate patterns are observed in approximately 15% of labors .

The two most common methods of monitoring fetal heart rate are cardiotocography (CTG) and intermittent auscultation. In high resource settings, continuous electronic fetal heart rate monitoring, via cardiotocography is the most prevalent method. Continuous CTG involves monitoring the fetal heart rate and uterine contractility simultaneously to detect fetal heart rate patterns associated with deficient fetal oxygen supply.

Normal CTG tracings are characterized by 1) stable baseline fetal heart rate (FHR) of 120–160 beats per minute (bpm), 2) FHR variability between 5 and 25 bpm above and below baseline FHR, and 3) periodic changes in the baseline FHR (accelerations above baseline or decelerations below baseline). While accelerations are associated with fetal well-being, decelerations, especially prolonged bradycardia, late decelerations, and severe variable decelerations are indicative of fetal stress and should prompt the clinician to evaluate and initiate intrauterine resuscitation with consideration for delivery of the fetus as indicated. Abnormal fetal heart rate patterns have high sensitivity, but low specificity and low predictive value to discriminate between neonates with or without metabolic acidosis. While a normal fetal heart rate pattern is usually indicates reassuring fetal status, an abnormal fetal heart rate pattern does not necessarily equate with hypoxia or acidosis.In settings where CTG is unavailable, intermittent auscultation is recommended for all laboring parturients .

Intermittent auscultation (IA) involves assessing the fetal heart rate at predetermined intervals with either a fetal stethoscope, or handheld Doppler. Abnormal heart rate findings by IA indicative of non-reassuring fetal status include prolonged fetal tachycardia or bradycardia, presence of repetitive or prolonged decelerations, and uterine tachysystole (more than 5 uterine contractions in a 10 min period). There is no evidence that IA performs worse than CTG in reducing morbidity and mortality associated with fetal acidosis. Studies comparing CTG to IA show no reduction in the risk of perinatal death or cerebral palsy.

Intermittent auscultation, characterized by low cost and low technology equipment, is more feasible than CTG in low resource settings. However, it requires a high level of training and skill, frequent interaction between patient and health care provider, and does not provide as sophisticated a level of information that may be needed in high risk populations.

Case definition of non-reassuring fetal status:

Level 1 of diagnostic certainty

  • Category III fetal heart rate tracings detected via continuous cardiotocography as defined by NICHD

∘ Absent baseline fetal heart rate variability AND any of the following:

– recurrent late decelerations

– recurrent variable deceleration

– bradycardia (<110 bpm)

OR

∘ Sinusoidal pattern

AND

  • Umbilical cord blood analysis consistent with metabolic acidosis (pH < 7.0 and Base deficit >12 mmol/L)

Level 2 of diagnostic certainty

  • Category III fetal heart rate tracings detected via continuous cardiotocography as defined by NICHD

∘ Absent baseline fetal heart rate variability AND any of the following:

– recurrent late decelerations

– recurrent variable deceleration

– bradycardia (<110 bpm)

OR

∘ Sinusoidal pattern

Level 3 of diagnostic certainty

  • Fetal heart pattern detected via intermittent auscultation suggestive of fetal hypoxia

∘ Baseline FHR <110 bpm or >160 bpm

∘ Presence of repetitive or prolonged (>3 min) decelerations

∘ More than 5 contractions in a 10 min period

Major and minor criteria used in the case definition of non-reassuring fetal status

Major criteria

Cardiovascular

CTG:

Category III heart

Rate tracing

  • Absent baseline fetal heart rate variability AND any of the following:

– recurrent late decelerations

– recurrent variable deceleration

– bradycardia < 110 beats/min

OR

  • Sinusoidal pattern

IA:

Abnormal

Findings

  • FHR <110 bpm OR >160 bpm
  • Presence of repetitive or prolonged decelerations
  • More than 5 contractions in a 10 min period

Minor criteria

Laboratory

  • Cord blood pH ≤7.0
  • Cord blood base deficit ≥12 mmol

 

 

 

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