During pregnancy, physiological changes alter the response to anesthesia. The respiratory system adapts to increased oxygen consumption through higher minute ventilation and respiratory drive. Cardiovascular changes include increased blood volume, heart rate, and stroke volume. Supine hypotension can occur due to compression of the inferior vena cava. Anesthetic agents readily cross the placenta and can depress the fetus, so doses must be carefully titrated. Labor further increases oxygen demand and the risk of supine hypotension due to uterine contractions displacing blood from the uterus into central circulation.

1. Anesthetic Considerations
of Physiological Changes
During Pregnancy
Presented by: Dr.zikrullah mallick

2. 2
OBJECTIVES
 Maternal physiology during
pregnancy.
 Uteroplacental circulation.
 Placental transfer of anesthetic
agents.
 Effect of labor on maternal
physiology.

3. 3
Anaesthesia for parturient
What is the difference?
Physiological
changes
Alter the usual
response
to anaesthesia
2 Patients are cared
For simultaneously
Mother
Fetus

4. 4
Maternal Physiology during Pregnancy
1) Progressive MAC.
by 40% at term
Returns to normal by 3rd day
postpartum.
CNS
Progesterone increases
20 times normal
level at term
β- endorphin surge during
labor & delivery

5. 5
2) ↑ Sensitivity to
Local Anesthetics.
LA requirements
uring RA ↓ by 30%.
Hormonally Mediated
Engorged Epidural
Venous Plexus
↓CSF Volume
↓Volume of
Epidural Space
↑Epidural space
Pressure

6. 6
ٌ
Respiratory
system
↑Oxygen consumption
20 – 40%
↑Minute Ventilation
40 – 50%
↑↑ VT & ↑ RR
& ↑P50 (30 mmHg)
↑PaO2
↓ PaCo2 (28-32 mmHg)
Compensatory ↓ HCo3ˉ
Progesterone
↑ CO2 Production

7. 7
TLC
4550
ml
-5%
VC
3500
ml
No
Change
IC
2650
ml
+15%
2000
ml
+5%
IRV
650ml
+45%
VT
FRC
1900
ml
-20%
850ml
-25%
ERV
1050
ml
-15%
RV
Volumes Capacities
Lung volumes &
capacities at term
gestation in absolute
volumes & as the
percentage change
from non-pregnant
Values.

8. 8
↓ 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.

9. 9
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

10. 10
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

11. 11
II : Hypercoagulable state
↑ Fibrinogen, factors VII, VIII, IX, X & XII
↓Factor XI
III : Other changes:
* Leucocytosis up to 21,ooo/µL.
* 10-20% ↓ in platelet count.
* Marked ↓ cell mediated immunity→ ↑susceptibility
to viral infection.
Risk Of DVT
One of the leading causes of maternal mortality

12. 12
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.

13. 13
Supine Hypotension syndrome
COP ↓ in supine position after 28th week of gestation.
Occurs in 20% of women at term.
Compression of IVC Compression of lower aorta
Aortocaval compression
↓ blood flow to kidneys,
uteroplacental circulation
&
lower extremeties
↓ VR → ↓ COP by 24% at term.

14. 14
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

15. 15
No woman in late pregnancy should lie supine without shifting
the uterus off the great abdomino-pelvic vessels.
Left lateral decubitus
Tilting the table
Left side down
Rigid wedge under
The right hip
Fluid preloading before neuroaxial anesthesia
It does not completely avoid maternal hypotension but
it↑ maternal COP → preserve uteroplacental
blood flow.

16. 16
GIT
☼ Upward displacement of the stomach by the
uterus → Incompetence of gastroesophageal
sphincter → Gastroesophageal reflux & esophagitis.
The parturient should be considered a full stomach patient
during most of gestation
☼ ↑ Progesterone → ↓ tone of gastroesophageal sphincter.
☼ Placental Gastrin → Hypersecretion of gastric acid.
☼ Gastric emptying → Delayed with labor.

17. 17
Pharmacological prophylaxis against aspiration.
No positive pressure ventilation before intubation
Rapid sequence induction.
Sellick’s maneouvre
For GA:

18. 18
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.

19. 19
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.

20. 20
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→ ↑ T3 & T4
↑ TBG → Free T3, T4 & TSH remain normal

21. 21
Uteroplacental Circulation
At term: uterine blood
flow is 10% of COP
≈ 600 – 700 ml/min.
80% to placenta

22. 22
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.

23. 23
3 major factors ↓ uterine blood flow during pregnancy
Systemic
Hypotension
Uterine
Vasoconstriction
Uterine
Contractions
♦ Aortocaval compression.
♦ Hypovolemia.
♦ Sympathetic block
with regional anesthesia.
♦ stress-induced endogenous
Catecholamines during labor.
♦ α-adrenergic agonists.
♦ Local anesthetic agents.
♦ Hypertensive disorders
→ generalized vasoconstriction.
♦ Labor.
♦ Oxytocin infusions.
♦ Extreme hypocapnia
PaCO2 < 20 mmhg.
♦ Barbiturates &
Propofol.

24. 24
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.

25. 25
Limited effects if <
1MAC & delivery within
10 min. of induction
Cross placenta
freely
Inhalational Agents
Intravenous Agents:
Thipental, ketamine
& propofol
Limited fetal effects
in usual induction
doses
(drug distribution,
metabolism & placental
uptake)
Variable effects.
Cross placenta
freely
Opioids
Most significant respiratory depressant
effects
Morhine
Significant respiratory depression peaking 1-
3 h after administration.
Meperidine
Minimal effect if < 1µg/Kg.
Fentanyl
Minimal effects on
The highly ionized
property impedes
Muscle Relaxants

26. 26
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
diffuse poorly across the placenta.
Chloroprocaine has the least placental transfer as it is rapidly
broken down by plasma cholinestrase in the maternal circulation.

27. 27
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.

28. 28
Effect of labor on maternal physiology
Stages of labor
1st stage 2nd stage 3rd stage
Starts with true labor
pains, ends by full
cervical dilation.
Starts with full cervical
dilation, fetal descent
occurs, ends with complete
delivery of fetus.
Extends from birth of the
baby to delivery of the
placenta.
Latent phase Active phase
Progressive cervical effacement
& minor dilataton (2 – 4 cm).
Progressive cervical dilatation
up to 10 cm.
8 – 12 h in nulliparous
5 – 8 h in multiparous.
Contractions are 1.5- 2 min
apart, last 1 – 1.5 min
15 – 120 min.
15 – 30 min.

29. 29
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.

30. 30
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.

31. 31
Discussion

32. 32
Questions

33. 33
Fetal blood concentrations of lidocaine following
maternal administration would be higher than
expected:
1. If administered during uterine contraction.
2. In the presence of umbilical cord compression.
3. In the presence of maternal acidosis.
4. In the presence of fetal acidosis.
5. In the presence of increased maternal
metabolism.
X
X
X
√
X

34. 34
1. Total peripheral resistance decreases.
2. Hb concentration decreases.
3. Plasma cholinestrase concentration increases.
4. Blood glucose concentration increases.
5. Functional residual capacity increases.
During pregnancy:
√
√
X
X
X

35. 35
The dose of bupivacaine required for spinal
anesthesia is reduced in the pregnant patient at
term because of decreased :
1. CSF volume.
2. Spinal cord blood flow.
3. Metabolism of bupivacaine.
4. CSF pressure.
5. Turnover of CSF.
√
X
X
X
X

36. 36
1. Maternal arterial pH.
2. Fetal cerebral blood flow.
3. Maternal cerebral blood flow.
4. Maternal uterine artery flow.
5. Fetal arterial PO2.
Maternal hyperventilation produces a decrease in:
X
X
√
X
√

37. 37
The following substances transfer freely across the
placenta:
1. Neostigmine.
2. Insulin.
3. Pancuronium.
4. Atropine.
5. glycopyrolate.X
√
X
X
√

38. 38
Thank you