Fetal Circulation and Heart Rate Basics

Fetal Circulation Connections

• Ductus venosus connects umbilical vein to inferior vena cava (IVC).
• Foramen ovale connects right atrium to left atrium.
• Ductus arteriosus connects pulmonary artery to aorta.

Fetal Heart Rate

• Normal fetal heart rate ranges between 110 to 160 beats per minute (BPM).
• Average baseline rate at 15 weeks gestation is 160 BPM.
• In a fetus with heart block, the average rate is 60 BPM.

Autonomic Nervous System and Fetal Response

• Autonomic nervous system fully develops around 32 weeks of gestation.
• Sympathetic nervous system increases heart rate and constricts peripheral blood vessels while dilating vital organs for better blood flow.

Indicators of Fetal Oxygenation

• Primary indicator of fetal oxygenation is the presence or absence of variability in heart rate.
• Location of chemoreceptors is in the aortic arch, responding to changes in oxygen (O2) and carbon dioxide (CO2) levels.
• Location of baroreceptors includes the aortic arch and carotid bodies.

Fetal Heart Rate Reactivity

• Maximal reactivity of fetal heart rate typically occurs late at night.
• Fetal nonreactivity may last for a maximum of 80 minutes.

Diagnostic Test Parameters

• Non-Stress Test (NST) requires two accelerations of 15 beats per minute lasting at least 15 seconds within a 20-minute period (may be extended to 40 minutes).
• Approximately 30% of contraction stress tests (CSTs) yield false positives.

Biophysical Profile (BPP) Components and Scoring

• BPP evaluates: fetal breathing movements, gross body movements, fetal tone, reactive fetal heart rate, and qualitative amniotic fluid volume.
• BPP score of 10 indicates normal conditions.
• BPP score of 8 suggests a normal infant with low risk of chronic asphyxia.
• Scores of 6 indicate a borderline result requiring re-evaluation within 24 hours.
• A score of 4 raises suspicion for chronic asphyxia, while a score of 0-2 strongly suspects it.

Fetal Acidosis and Influencing Factors

• Early signs of fetal acidosis include non-reactive NST and loss of fetal breathing movements (FBM).
• Fetal breathing movements increase with rising maternal glucose and decrease with maternal hypoglycemia.
• Maternal smoking decreases fetal breathing movements while corticosteroids may also lead to decreased FBM.

Maternal and Fetal Physiological Metrics

• Maternal arterial oxygen pressure (PAO2) at sea level approximates 105 mmHg.
• In pre-eclampsia, cardiac output (CO) and systemic vascular resistance (SVR) increase.
• Prolonged late decelerations occur in 71% of cases of uterine rupture.

Neonatal Health Indicators

• Significant neonatal morbidity occurs when 18 minutes or longer elapse from the onset of heart rate deceleration until delivery.
• Normal arterial pH for healthy vaginal deliveries is 7.28 ± 0.05.
• Normal ranges for arterial pO2 and pCO2 are 18.0 ± 6.2 and 49.2 ± 8.4 respectively, with a normal base deficit being less than 12.
• Acidosis is indicated when pH falls to 7.2 (or lower).

Heart Rate Dynamics

• Tachysystole is defined as more than five contractions within ten minutes.
• Baseline fetal heart rate (FHR) is the average rate rounded to the nearest 5 BPM over a 10-minute period, excluding periodic changes.
• A sinusoidal pattern in FHR typically indicates fetal anemia.

Congenital Heart Block Statistics

• Approximately 50% of infants with complete heart block have associated congenital cardiac malformations.
• Mortality rate for newborns with complete heart block is around 25%.
• Newborns with complete heart block, without underlying congenital heart disease, frequently exhibit neonatal lupus erythematosus.

Variability in Fetal Heart Rate

• Baseline variability consists of fluctuations in the FHR exceeding two cycles per minute, characterized by irregular amplitude and frequency.
• Minimal variability is defined as undetectable but can be greater than 25 BPM.

Test Performance Guidelines

• Fetal stimulation during decelerations or bradycardia is not appropriate; stimulation should occur when the FHR is at baseline.

Deceleration Characteristics

• Early decelerations are gradual decreases in FHR below baseline, with the nadir occurring at the peak of a contraction and taking at least 30 seconds from onset to nadir.
• Physiology of early decelerations is attributed to fetal head compression triggering a vagal reflex, often observed between 4-6 cm dilation.