Physiology of fetal heart function Definition Components Examples
The average baseline FHR in a healthy fetus at 20 weeks is 155 bpm (range 120-180 bpm) Controlled by : - sympathetic - parasympathetic As the parasympathetic matures with advancing gestational age, the resting heart rate ↓. At term, the average FHR is 140 beats per minute and the normal range is 120-160 bpm.
Vagus nerve - sympathetic - parasympathetic although the fetal heart is innervated by the sympathetic system as well, parasympathetic input maintains baseline heart rate. Parasympathetic stimulation becomes dominant over sympathetic input as the fetus develops which is why the FHR is initially faster when 1st detectable and slows as the fetal matures.
Vagal stimulation induces variability in the time interval between each beat secondary to influences on the vagus in the CNS : baroreceptors, chemoreceptors CNS influences the FHR via an intergrative center in the medulla oblongata where the vagus nerve originates. During fetal sleep, the oscillatory variation in the FHR diminishes and the variability around the baseline beat has less amplitude
commonly known as an electronic fetal monitor or external fetal monitor (EFM) or non-stress test (NST) measures simultaneously both the fetal heart rate and the uterine contractions, two separate disc-shaped transducers laid against the womans abdomen. - ultrasound transducer measures the fetal heartbeat. - pressure-sensitive transducer (tocodynamometer (toco) - measures the strength and frequency of uterine contractions
detect early fetal tendency to produce distress resulting from false-positive results fetal hypoxia and Increase LSCS statistic metabolic acidosis closer assessment of high-risk mothersBenefits Risks
normal FHR at term 110 – 160 bpm average fetal heart rate is considered to be the result of tonic balance between accelerator ( sympathetic ) and decelerator (parasympathetic) influences on pacemaker cells mediated via vagal slowing of heart rate Heart rate also is under the control of arterial chemoreceptors such that both hypoxia and hypercapnia can modulate rate.
More severe and prolonged hypoxia, with a rising blood lactate level and severe metabolic acidemia, induces a prolonged fall of heart rate due to direct effects on the myocardium.
Fetal tachycardia – baseline >160 bpm over 10 minutes or more - can be nonpathologic, considered a normal rate in the premature fetus Causes : - maternal : - chorioamnitis - other causes of infection causing fever - use of B-sympathomimetics - fetal - cardiac arrhythmias - fetal anemia - acute fetal blood loss - abnormal fetal conduction system
FIGURE 4. Fetal tachycardia that is due to fetaltachyarrhythmia associated with congenitalanomalies, in this case, ventricular septal defect.Fetal heart rate is 180 bpm. Notice the "spike"pattern of the fetal heart rate.
Fetal bradycardia – baseline heart rate < 110bpm for greater than 10 minutes. Rate : 100 - 119 beats/min, in the absence of other changes, usually is not considered to represent fetal compromise. Such low but potentially normal baseline heart rates also have been attributed to vagal response to head compression from occiput posterior or transverse positions, particularly during second-stage labor (Young and Weinstein, 1976).
Defined as fluctuations in the FHR baseline of 2 cycles/min or greater with irregular amplitude and inconstant frequency. The time interval between 2 heartbeats in a healthy fetus is seldom the same. Normal : 5 – 15 bpm This variability is secondary to the interaction of the sympathetic and parasympathetic reflexes
Causes of loss of variability : - fetal sleep - administration of drugs - narcotics, barbiturates, phenothiazines - MgSO4 - gestational age (28-30wks) - metabolic acidemia
Upward deflection in the baseline fetal heart rate of at least 15 bpm lasting for at least 15 seconds. In pregnancies of fewer than 32 weeks of gestation, accelerations are defined as having a peak 10 beats per minute or more above the baseline and duration of 10 seconds or longer.
Reductions in fetal heart rate of at least 15bpm lasting for at least 15 seconds 4 types : - Type 1 (early)- physiological - Type 2 (late)- pathological - variable - prolonged
consists of a gradual decrease and return to baseline associated with a contraction. Result of a physiologic chain of events that begins with head compression during a uterine contraction the degree of deceleration is generally proportional to the contraction strength and rarely falls below 100 to 110 beats/min or 20 to 30 beats/min below baseline.
early decelerations are not associated with fetal hypoxia, acidemia, or low Apgar scores
FIGURE 5. Early deceleration in a patient with anunremarkable course of labor. Notice that the onset and thereturn of the deceleration coincide with the start and theend of the contraction, giving the characteristic mirrorimage.
smooth, gradual, symmetrical decrease in fetal heart rate beginning at or after the peak of the contraction and returning to baseline only after the contraction has ended. The magnitude of late decelerations is rarely more than 30 to 40 beats/min below baseline and typically not more than 10 to 20 beats/min. Late decelerations usually are not accompanied by accelerations.
Inconsistent time of onset when compared to uterine contraction The onset of deceleration commonly varies with successive contractions . The duration is less than 2 minutes. represent fetal heart rate reflexes that reflect either blood pressure changes due to interruption of umbilical flow or changes in oxygenation
significant variable decelerations are those decreasing to less than 70 beats/min and lasting more than 60 seconds. Causes : - Umbilical cord entanglement - Eg: - Umblilical around body or neck - True knot in the umbilical cord - Prolapsed umbilical cord
isolated deceleration lasting 2 minutes or longer but less than 10 minutes from onset to return to baseline. Causes :1) Total umbilical cord occlusion (cord prolapse)2) Maternal hypotension3) Uterine hypertonia4) VE or artificial ruptured of membrane
regular, smooth, undulating form typical of a sine wave that occurs with a frequency of 2- 5/minute and an amplitude range of 5-15 bpm also characterized by a stable baseline heart rate of 120 to 160 bpm and absent beat-to- beat variability Occurs in severe fetal anemia, as occurs in cases of Rh disease or severe hypoxia
Saltatory pattern : - rapidly recurring couplets of acceleration and deceleration causing relatively large oscillations of the baseline fetal heart rate - sympathetic stimulation overrides parasympathetic dominance in response to acute but temporary hypoxemia ( umbilical cord compression ) - almost exclusively seen during labour
FIGURE 2. Saltatory pattern with widevariability. The oscillations of the fetalheart rate above and below thebaseline exceed 25 bpm.
Accelerations: absent for >40 minutes-first to become apparent, and any of the following Baseline heart rate bradycardia<110 bpm Tacycardia>150 bpm Baseline variability:<10 bpm lasting for> 40 min, greater significant if < 5 bpm Decelerations: variable decelerations without ominous features
Accelerations: absent for>40 min and any of the following Baseline heart rate: abnormal Baseline variability:less than 5 bpm lasting for >90 min Deceleration Repetitive late decelerations Variable deceleration with ominous features( duration >60s; beat loss>60 bpm;late recovery; late deceleration component;poor baseline variability btwn and/or during deceleration
Baseline FHR = 130 bpm Variabilitiy = 5- 15 bpm Have Acceleration No deceleration No contraction CTG reactive IMP: Normal Fetal Heart Rate
BHR= 120 bpm Variability 5 to 10 bpm Prolonged deceleration Contracting 2-3 in 10 minutes,varying in strenght Deceleration occurs after VE;variability normal before and after deceleration
1) BL=145-150 bpm2) Variability < 5bpm3) Early deceleration ( type 1 )4) Contracting 5 in 10 minutes, lasting 90 s on average5) Head compression6) Mx Change maternal position Reduce pitocin infusion Continue observe trace for further abnormalities
1) Baseline FHR 1) Twin i=140-155 bpm 2) Twin ii=150-160bpm2) Variability 1) Twin i 5-10bpm 2) Twin ii 5-10bpm3) No deceleration4) Contracting 3-4 in 10 minutes5) Normal CTG for both twins.
Baseline FHR = 130 bpm Poor Beat to Beat Variability < 5 bpm Have Acceleration No Deceleration Suspicious CTG MX= Left lateral Position of the mother and repeat CTG
BHR = 160 bpm Poor beat to beat variability < 5 bpm No acceleration Prolonged deceleration until 140 bpm and occur more than 3 min. No contraction CTG not reactive Imp: Acute fetal distress.
BHR = 155 to 160 bpm Poor Beat to Beat < 5 bpm No acceleration in 20 min No Deceleration CTG not reactive Acute Fetal distress