Anesthetic Considerations
of Physiological Changes
During Pregnancy
Presented by:
Mona Abdelsamie
Assistant lecturer of Anesthesiology
Under Supervision of:
Prof. Dr. Hoda Omar
Professor of Anesthesiology & Intensive care
Anesthesiology Department
Ain Shams University
OBJECTIVES




Maternal physiology during
pregnancy.
Uteroplacental circulation.
Placental transfer of anesthetic
agents.
Effect of labor on maternal
physiology.
2
Anaesthesia for parturient
What is the difference?
Physiological
changes
2 Patients are cared
For simultaneously
Mother
Alter the usual
response
to anaesthesia
Fetus
3
Maternal Physiology during Pregnancy
CNS
1) Progressive MAC.
by 40% at term
Returns to normal by 3rd day
postpartum.
Progesterone increases
20 times normal
level at term
β- endorphin surge during
labor & delivery
4
Hormonally Mediated
2) ↑ Sensitivity to
Local Anesthetics.
Engorged Epidural
LA requirements
Venous Plexus
uring RA ↓ by 30%.
↓CSF Volume
↓Volume of
Epidural Space
↑Epidural space
Pressure
5
Respiratoryٌ
system
↑Oxygen consumption
20 – 40%
Progesterone
↑ CO2 Production
↑Minute Ventilation
40 – 50%
↑↑ VT & ↑ RR
↑PaO2 & ↑P50
(30 mmHg)
↓ PaCo2 (28-32 mmHg)
Compensatory ↓ HCo3ˉ
6
Volumes
2000
IRV
Lung volumes &
capacities at term
gestation in absolute
volumes & as the
percentage change
from non-pregnant
Values.
ml
+5%
VT
650ml
+45%
ERV
850ml
-25%
RV
1050
ml
-15%
Capacities
IC
VC
TLC
2650
ml
+15%
3500
ml
4550
ml
-5%
No
Change
FRC
1900
ml
-20%
7
↓ FRC + ↑O2 Consumption
=
Rapid desaturation during
periods of apnea.
☼Pre-oxygenation prior to GA is mandatory.
☼Parturient Should not lie flat without
supplemental oxygen.
↓FRC & ↑MV
☼ ↑Uptake of Inhalational
Anesthetics.
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Hormonal Changes
Capillary engorgement of
respiratory tract mucosa
1) ↑ Incidence of difficult intubation.
2) Trauma and bleeding during
endotracheal intubation.
☼ Use a small ETT (6 – 7 mm)
during GA
9
Hematological
Changes
I : ↑ Blood Volume ( up to 90ml/ Kg)
↑ by 1000 – 1500 ml at term.
Returns to normal 1 – 2 weeks postpartum.
+
↑ Plasma Volume > ↑ RBC mass
=
Dilutional anemia & ↓ blood viscosity
Facilitates maternal & fetal
exchange of respiratory gases,
nutrients & metabolites
↓ Impact of maternal blood
loss at delivery
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II : Hypercoagulable state
↑ Fibrinogen, factors VII, VIII, IX, X & XII
↓Factor XI
Risk Of DVT
One of the leading causes of maternal mortality
III : Other changes:
* Leucocytosis up to 21,ooo/µL.
* 10-20% ↓ in platelet count.
* Marked ↓ cell mediated immunity→ ↑susceptibility
to viral infection.
11
CVS
↑ COP by 40% at term
↑ HR 15 – 30%
↑ SV 30%
Returns to normal 2 weeks postpartum.
↓ SVR → ↓ SBP & ↓↓ DBP, the response to adrenergic and vasoconstr
agents is decreased.
CVP, PAP, PAWP → unchanged.
12
Supine Hypotension syndrome
COP ↓ in supine position after 28th week of gestation.
Occurs in 20% of women at term.
Aortocaval compression
Compression of IVC
↓ VR → ↓ COP by 24% at term.
Compression of lower aorta
↓ blood flow to kidneys,
uteroplacental circulation
&
lower extremeties
13
Compensatory mechanisms in
unanaesthetised Women
Venous Collaterals
Paravertebral
Venous plexus
Abdominal
wall
↑ SVR & HR
Reduced during general
or regional anesthesia.
Severe Hypotension
Profound Fetal Hypoxia
14
No woman in late pregnancy should lie supine without shifting
the uterus off the great abdomino-pelvic vessels.
Left lateral decubitus
Rigid wedge under
The right hip
Tilting the table
Left side down
Fluid preloading before neuroaxial anesthesia
It does not completely avoid maternal hypotension but
it↑ maternal COP → preserve uteroplacental
blood flow.
15
GIT
The parturient should be considered a full stomach patient
during most of gestation
☼ Upward displacement of the stomach by the
uterus → Incompetence of gastroesophageal
sphincter → Gastroesophageal reflux & esophagitis.
☼ ↑ Progesterone → ↓ tone of gastroesophageal sphincter.
☼ Placental Gastrin → Hypersecretion of gastric acid.
☼ Gastric emptying → Delayed with labor.
16
For GA:
Pharmacological prophylaxis against aspiration.
No positive pressure ventilation before intubation
Rapid sequence induction.
Sellick’s maneouvre
17
Renal
System
♦ RBF & GFR ↑ by 50% at 1st trimester but returns to
normal in 3rd trimester.
♦↑ Renin & Aldosterone → Na+ retention.
♦ Sr. Creatinine & BUN may ↓ to 0.5 – 0.6mg/dL &
8 – 9 mg/dL respectively.
♦↓ Renal tubular threshold for glucose & amino acids → mild
glycosuria (1-10g/d) & proteinuria (< 300mg/d).
♦ Plasma osmolality ↓ by 8 – 10 mosm/Kg.
18
Hepatic
Effects
♦Hepatic function & hepatic blood flow→ unchanged.
♦Minor ↑ in Sr. Transaminases & LDH in 3rd trimester.
♦↑ Sr. Alkaline phosphatase (placental).
♦ Mild ↓ in Sr. albumin (dilutional).
♦ 25 – 30% ↓ in pseudocholine estrase activity.
♦↑ Progesterone levels→ inhibit release of cholecystokinin→ incomp
emptying of gall bladder→ altered bile acid composition→ formation
of cholesterol stones.
19
Metabolic
Effects
Pregnancy is Diabetogenic
Human Placental lactogen→ relative insulin resistance.
Starvation like state
↓ Blood Glucose & Amino Acid levels.
↑ Free Fatty Acids, Ketones & triglycerides.
↑ Estrogen levels→Thyroid gland hypertrohy→ ↑ T 3 & T4
↑ TBG → Free T3, T4 & TSH remain normal
20
Uteroplacental Circulation
At term: uterine blood
flow is 10% of COP
≈ 600 – 700 ml/min.
80% to placenta
21
aximally dilated uterine vasculature with absent autoregulation.
Uterine Blood Flow
Directly proportional to difference between
uterine arterial and venous pressure.
Inversely proportional to uterine
vascular resistance.
Abundant α-adrenergic
& some β-adrenergic receptors.
Previously , vasoconstrictor agents with predominant β-adrenergic activity
(e.g. Ephedrine) were of choice for hypotension during pregnancy.
Recent studies show that α-adrenengic drugs (e.g.Phenylephrine) have
better effects.
22
3 major factors ↓ uterine blood flow during pregnancy
Systemic
Hypotension
Uterine
Vasoconstriction
Uterine
Contractions
♦ Aortocaval compression. ♦ stress-induced endogenous
♦ Labor.
Catecholamines during labor.
♦ Hypovolemia.
♦ Oxytocin infusions.
♦ α-adrenergic agonists.
♦ Sympathetic block
♦ Local anesthetic agents.
with regional anesthesia.
♦ Barbiturates &
Propofol.
♦ Hypertensive disorders
→ generalized vasoconstriction.
♦ Extreme hypocapnia
PaCO2 < 20 mmhg.
23
Placental transfer of anesthetic agents
Placental transfer of drugs depends on:
1: Molecular weight : < 500 Da cross easily.
2: Protein binding.
3: Lipid solubility: Highly ionized substances have poor lipid solubi
4: Maternal & fetal pH : affect ionization of the drug.
5: Maternal drug concentration: affected by dose given
and route of administration.
6: Timing of administration.
24
Limited effects if <
1MAC & delivery within
10 min. of induction
Inhalational Agents
Intravenous Agents:
Thipental, ketamine &
propofol
Cross placenta
freely
Limited fetal effects in
usual induction doses
(drug distribution,
metabolism & placental
uptake)
Opioids
Cross placenta freely
Morhine
Most significant respiratory depressant effects
Meperidine
Significant respiratory depression peaking 1- 3
h after administration.
Fentanyl
Minimal effect if < 1µg/Kg.
Muscle Relaxants
The highly ionized
property impedes
placental transfer.
Variable effects.
Minimal effects on
fetus.
25
Local anesthetics → Placental transfer depends on:
1: pKa.
2: Maternal & fetal pH : Fetal acidosis → higher fetal to
maternal
drug ratios . Binding of hydrogen ions to the nonionized form →
trapping
of
local anesthetic in fetal circulation
3: Degree of protein binding : highly protein bound agents
Chloroprocaine has the least placental transfer as it is rapidly
diffuse down
poorly
placenta.
broken
by across
plasmathe
cholinestrase
in the maternal circulation.
26
Most of anesthetic agents show significant
placental transfer
Fetal effects of drugs administered to parturient depend on:
1: Maturity of fetal organs, substantial fetal hepatic
uptake of many drugs.
2: Dilution of the umbilical venous blood by venous blood
from lower half of fetal body → modify fetal
drug distribution.
27
Effect of labor on maternal physiology
Stages of labor
1st stage
2nd stage
3rd stage
Starts with true laborStarts with full cervical
dilation, fetal descent
pains, ends by full
occurs, ends with complete
cervical dilation.
delivery of fetus.
Extends from birth of the
baby to delivery of the
placenta.
Contractions are
1.5-phase
2 min
Active
8Latent
– 12 phase
h in nulliparous
apart, last 1 – 1.5 min
5 – 8 h in multiparous.
15 – 30 min.
Progressive cervical effacement
& minor dilataton (2 – 4 cm).
Progressive cervical dilatation
up to 10 cm.
15 – 120 min.
28
Intense painful contractions
Maternal
hyperventilation
MV ↑ up to 300%.
↑ O2 consumption 60%
above 3rd trimester values
PCo2 < 20 mmHg
Uterine VC → Fetal acidosis
+
Periods of hypoventilation → transient
maternal & fetal hypoxemia in between
Contractions.
29
Each contraction
Displaces 300 – 500ml blood from
uterus to central circulation.
COP ↑ 45% above 3rd trimesteric value.
Maximum strain on the heart occurs immediately
after delivery.
Uterine intense involution→ sudden relieve of IVC
→ ↑ COP 80% above prelabor values.
30
Discussion
31
Questions
32
Fetal blood concentrations of lidocaine following
maternal administration would be higher than
expected:
X
1.
If administered during uterine contraction.
2.
3.
4.
5.
In the presence of umbilical cord compression.
X
In the presence of maternal acidosis.
X
√
In the presence of fetal acidosis.
In the presence of increased maternal
metabolism.
X
33
During pregnancy:
1.
2.
3.
4.
5.
Total peripheral resistance decreases.
√
Hb concentration decreases. √
Plasma cholinestrase concentration increases.X
Blood glucose concentration increases. X
Functional residual capacity increases.X
34
The dose of bupivacaine required for spinal
anesthesia is reduced in the pregnant patient at
term because of decreased :
1.
2.
3.
4.
5.
CSF volume. √
Spinal cord blood flow.X
Metabolism of bupivacaine. X
CSF pressure. X
Turnover of CSF. X
35
Maternal hyperventilation produces a decrease in:
1.
2.
3.
4.
5.
Maternal arterial pH.X
Fetal cerebral blood flow.X
Maternal cerebral blood flow.√
Maternal uterine artery flow.
X
Fetal arterial PO2.√
36
The following substances transfer
placenta:
1.
2.
3.
4.
5.
freely across the
Neostigmine. √
Insulin.X
Pancuronium. X
Atropine. √
glycopyrolate.X
37
Thank you
38