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Fetal Heart Rate

Influences on Fetal Heart Rate Control

Physiology Effect on Fetal Heart Rate
Originates in medulla oblongata Parasympathetic Nervous System Decreases FHR
Vagus nerve (10th cranial innervates SA and AV nodes) Parasympathetic Nervous System With increasing gestational age, slow, gradual decrease in FHR and increase in FHR variability
Pathway for transmission of FHR variability Parasympathetic Nervous System Moderate variability indicates absence of metabolic acidemia
Nerve fibers widely distributed throughout myocardium at term Sympathetic Nervous System Increases FHR
Stimulation releases catecholamines (norepinephrine, epinephrine Sympathetic Nervous System With intermittent hypoxemia, initial normal fetal compensatory response is increase in FHR or brief tachycardia
CO=HR x SV Heart rate x stroke volume Formula for adult cardiac output
Blocking with propranolol results in approximately 10 bpm decrease in FHR In early gestation, sympathetic dominance results in slightly higher FHR and decrease in variability
Catecholamines may also cause fetal vasoconstriction and hypertension. Modulates baseline FHR with parasympathetic branch
Because the fetal heart appears to operate near the top of its cardiac function curve, SV does not fluctuate significantly. Hence, fetal CO is dependent on HR With fetal tachycardia greater than 240 bpm or bradycardia less than 60 bpm, fetal CO and umbilical blood flow can be significantly decrease
When arterial BP increases, baroreceptors quickly detect amount of stretch, sending impulses via vagus nerve to midbrain Abrupt decrease in FHR, CO, BP • Variable decelerations
When blood flow falls below threshold for normal respiratory gas exchange, increased PCO2 stimulates chemoreceptors to slow FHR Late decelerations • Variable decelerations resulting from umbilical cord occlusion coupled with hypoxemia • Prolonged deceleration coupled with hypoxemia
Head compression Early Deceleration
Cord compression Variable Deceleration
Utero placental Insufficiency Late Deceleration
Anemia Sinusoidal Pattern
Epinephrine and norepinephrine (adrenal medulla) Epinephrine and norepinephrine (adrenal medulla)
Renin-angiotensin system Maintains systemic arterial BP and umbilical placental blood flow
Prostaglandins Regulation of umbilical blood flow Maintains patency of fetal ductus arteriosus
Quiet sleep Normal baseline FHR, minimal variability, accelerations absent
FHR stable with narrow oscillation bandwidth Responds to external stimuli (vibroacoustic stimulation
Active (REM) sleep Moderate variability, accelerations present • Reactive NST • At term, duration of periods of active sleep are longer than quiet sleep
Created by: fwimsatt
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