Fetal Tracing
Normal Ranges:
baseline
110 – 160bpm // (mean FHR in 10mins)
median arterial pH
baseline tachycardia
>165bpm
median venous pH
baseline bradycardia
<110bpm for 10 minutes
pH suggesting acidemia
variability
moderate variability
6-25bpm
marked variability
>25bpm
minimal
2-5 bpm
>15bpm FHR increase within 30 seconds
Lasts >15s
early
gradual decrease – occurs with contraction
late
gradual decrease – occurs after contraction
variable
abrupt decrease >15bpm from baseline lasting 15s-2mins
prolonged
abrupt decrease >15bpm from baseline lasting 2m-10m
4 contractions / 10 minutes
acceleration
deceleration
max contractions
Abnormals
minimal/absent variability
causes fetal sleep
maternal narcotics
fetal hypoxia
fetal brain damage
fetal cardiac arrhythmia
decelerations
normal response to
stress/contractions/fetal head
compression
hypoxia
cord compression
base deficit - arterial ECF
base deficit – venous ECF
BD suggesting acidemia
baseline bradycardia
epidural
aortocaval compression
maternal hypotension
tachysystole
cord compression
7.26
7.05 – 7.38 (95% range)
7.35
7.17 – 7.48 (95% range)
<7.20 – FBS
<7.05 – cord blood arterial
<7.10 – cord blood venous
2.3
Range 2-9
3
Range 1-9
>12 arterial
>10 venous
baseline tachycardia
maternal
fever (tachy is mild)
drugs (such as ritodrine)
fetal
sepsis
hypoxia (reactive tachycardia – stress
induced catecholamine response)
fetal blood loss
anemia
cardiac arrhythmia
preterm
preterm fetus has higher baseline
mgmt
minimal variability
observe for 1hr
may be normal (fetal sleep). but if
prolonged, suspect fetal hypoxia
absent variability
very serious, need quick response
marked variability
may be normal
suspect acute fetal
hypoxia/hypoxemia
Beware shallow decelerations with
loss of variability
-
change maternal position
FBS if mild
deliver ASAP if
severe/persistent
1st stage of labor
If brief
no immediate intervention
change mom’s position
maternal O2
consider tocolytic (if contractions frequent)
late decel –
If persistent
FIRST: correct any causes (maternal hypotension,
variable decal –
-
likely uteroplacental insufficiency
likely umbilical cord compression
tachysystole)
@ 5 mins: prepare for C/S – start IV, call for assistance
@ 10 mins: decision for C/S needs to be made &
implement promptly
//perisistent bradycardia >10m in 1st stage is indication for C/S
2nd stage of labor
@ 5 min: instrumental forceps or vacuum delivery
Heart physiology review


heart rate determined by
o intrinsic heart rate
o heart’s nerve supply (vagus)
o circulating catecholamines (via adrenal glands)
o CNS activity
note that these are in turn influenced by
o changes in fetal BP
o changes in fetal blood gases (pCO2, pO2, pH)
Acid/base physiology review




fetus uses O2 for glucose production
impaired gas exchange leads to
o co2 retention
o reduced O2 supply
CO2
o co2 is a weak acid. if retained, blood pH will fall → respiratory acidemia
O2
o if O2 reduced = hypoxemia, fetus adapts well
o if O2 reduction prolonged → hypoxia, fetal tissues can’t metabolize →
anaerobic metabolism → lactic acid → pH falls → metabolic acidemia
o
so note: in blood gas analysis, blood O2 may be low, but if the pH is
normal, it means the fetus is adapting well and this is reassuring.
interpreting 2nd stage tracings
decel’s are common. 2nd stage is more stressful for feus. Fetal pH falls more rapidly too
mgmt for poor quality tracing
o
scalp electrode
o
maternal internal transducer
despite decel’s variability and baseline should be maintained
Fetal blood gas
Fetal blood sample
technique
mom in left lateral position
when is it inappropriate
severe tracing – FBS may delay CS
2cm dilation and stable presenting part required
2nd stage – an abnormal tracing in 2nd stage is indication for
instrumental delivery, NOT FBS
obtain sample from fetal scalp
obtain quickly – prolonged exposure of blood to air
-
allows CO2 diffusion
avoid bubbles – again, CO2 diffusion
FBS interpretation
pH
Technically difficult – small dilation/ unstable presenting part
Maternal/fetal conditions
HIV, Hepatitis (vertical transmission)
fetal hemophilia suspected
7.25 = normal
7.20 – 7.25 = borderline; repeat in 30-60min, continue labor
7.20 = indication for delivery
keep in mind
maternal hyperventilation --> high maternal pH --> high fetal pH
may hide a compromise
get a maternal venous sample to check if suspected
Cord blood gas
do you collect arterial or venous blood? Both – arterial better represents fetal condition, but both collected to make sure that the arterial is actually arterial, and not venous
arterio-venous difference is usually 0.08 units; <0.03 means likely the same vessel was sampled
large a-v difference points to an acute problem (an issue in 2nd stage labor)
Analysis – values suggesting acidemia:
pH
 pH-arterial < 7.05
 pH-venous < 7.10
base deficit
 arterial BD-ECF > 12mmol/L
 venous BD-ECF > 10mmol/L

// metabolic acidemia of this degree is associated with higher fetal morbidity



low pH & large A-V difference suggests?
o acidemia due to acute event, or cord compression
low pH & small A-V difference suggests?
o acidemia due to chronic condition
low pH & very small (<0.3) A-V difference suggests?
o both samples are from same vessel
Note: base deficit is a measure of how much buffer in blood is used up. The more free acid is being produced (metabolic academia), the more buffers are used up. In respiratory academia, buffer is not
being used up (tho it is involved).