Physiological changes in pregnancy are due to hormonal changes and fetal growth. Systems affected include respiratory (increased lung volumes), cardiovascular (increased cardiac output and stroke volume), hematologic (hemodilution), gastrointestinal (delayed gastric emptying), and renal (increased glomerular filtration rate). These changes result in implications for anesthesia like rapid denitrogenation and hypoxia during apnea due to decreased functional residual capacity. Spinal and epidural requirements are lower due to neural sensitivity changes. Changes revert to pre-pregnancy levels postpartum, generally within 6-12 weeks.

1. Physiological changes in
pregnancy
Dr Megha Aggarwal
University College of Medical Sciences & GTB Hospital,
Delhi
www.anaesthesia.co.in

2. Today’s seminar
1.
2.
3.
4.
5.
6.
Introduction
Why to know the changes during pegnancy
Systems affected
Anaesthetic implications
Changes during labour
Changes during puerperium

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Introduction
Changes occur in pregnancy to
1. Support the foetus
2. Prepare mother for delivery
Changes are due to
1. Hormonal changes
2. Increasing size of uterus and foetus
3. Anatomical changes

4. Why study these changes?
1.
2.
3.
4.
To differentiate normal from abnormal
To understand its anaesthetic implications
To make the process of delivery smooth
To anticipate and manage complications
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5. Systems affected
Body wt & metabolism
Respiratory
Cardiovascular
Hematopoietic
Gastrointestinal
CNS
Hepatobiliary
Renal
Endocrine
Pharmacological

6. Body wt. & metabolism
Wt GAIN = 17%
= 12 kg
T1 = 1-2 kg
T2 = 5-6 kg
T3 = 5-6 kg
BMR +15% at term
O2 consumption +35% (↑needs of fetus, uterus, placenta)
+ 40% in stage I of labour
+ 75% in stage II of labour

7. Respiratory
1. Anatomical
a) Rib cage and breast enlargement- laryngoscopy
difficult
b) Diaphragm pushed cranially- changes in lung vol
c) ↑ mucosal engorgement
nasal – epistaxis
nasal intubation difficult
oropharyngeal – smaller ETT
↑mallampatti score
d) ↓Chest wall compliance (lung compliance unaffected)
e) Subglottic airway dilatation (progesterone, cortisone,
S
relaxin) →↓pulmonary resistance (-50%)

9. Changes in lung vol and capacities
PARAMETER
CHANGE
1. TV
+45%
2. FRC
-20%
3. ERV
-25%
4. Dead space
+45%
5. RR
No change/+
6. MV
+45%
7. Alveolar ventilation
+45%
Note: change in MV is solely due to ↑in TV and not RR

10. Continued…

11. Continued…
2. Physiological changes
1. ↑MV → ↑ TV (RR unchanged)
1. Progesterone (↑CNS sensitivity to CO2)
2.↑CO2 production
alkalosis (compensatory but incomplete↓HCO3- →↑pH
.
by 0.02-0.06)
2. Breathing diaphragmatic > thoracic - advantage during
high regional blockade
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12. Continued…
3. Blood gases
a) Paco2_- ↓to 30 mm Hg by 30 wk, no further change
b) ∆ Paco2_- ETco2 = 0 (because no. of unperfused
alveoli i.e. DS ↓ due to ↑CO)
c) ↑ PaO2 to 107 mmHg but ↓when supine
d) ∆ AV O2
early gestation: ↑CO > ↑O2 consumption → ↑ ∆ AV O2
late gestation: ↑CO < ↑O2 consumption → ↓ ∆ AV O2
e) FRC < closing capacity → small airways close
during normal tidal ventilation → predisposes to hypoxia

13. Anaesthetic implications
PARAMETER
CONSEQUENCE
1. MV ↑
Faster denitrogenation
2. ↓FRC + ↑O2 consumption
Rapid hypoxia during
apnoea
3. ↑MV + ↓FRC
4. Mucosal engorgement
Faster inhalational induction
Faster emergence
Faster changes in depth
Difficult airway
5. Predominant
diaphragmatic breathing
High spinal does not affect
MV & PaCO2 much

14. Circulatory changes
Examination- 1.Apical impulse in 4th ICS & laterally
2.Loud S1
3.A2P2 changes less with respiration
4.S3 in 16% cases
5.Grade I - II early mid-diastolic murmur at
left sternal border.
6. Asymptomatic pericardial effusion
ECG – 1.Sinus tachycardia ( ↓PR & QT interval)
2.ST depression & T inversion in left precordial
leads
3.Left axis deviation (false)

15. Continued…
ECHO – 1. Enlargement of chambers
2. LVH
3. Annular dilatation of all valves except Aortic
(regurgitation)
4. ↑ LVEDV but no change in filling P(PCWP/CVP)
(because of cardiac dilatation & hypertrophy)
5. LVESV-unchanged
↑EF
Chest X Ray – 1. Apparent cardiomegaly
2. ↑ LA (lateral view)
3. ↑ vascular markings
4. Straightening of left heart border
5. Pleural effusion

16. Continued…
PARAMETER
CHANGE
1.CO
+40%
2. SV
+30%
3. HR
+15%
4. SBP
No change
5. DBP
-15%
6. SVR
-15%
7. Femoral venous P
+15%
Note: fall in DBP while SBP is unaffected

17. Continued…

18. Continued…
Blood pressure
Position
max. in supine
min. in lateral
Age
↑with age
Parity
nullipara> multipara
SV(↑)
SBP
SBP unaffected
vsl distensibility(↑compliance)
BP
↓PP
DBP
SVR(↓)
DBP ↓

19. Continued…
Aortocaval compression : starts at 13-16 wk
1.Concealed caval compression.
In supine position gravid uterus compresses IVC & ↓CO
without fall in the blood pressure.
Why no fall in blood pressure ?
1.Reflex vasoconstriction
2.Diversion of blood through paravertebral &
epidural venous plexus, ovarian veins – maintains
VR

20. Continued…
2.Overt caval compression (supine hypotensive
syndrome)
 Hypotension, sweating, bradycardia, pallor, nausea,
vomiting.
 Due to uncompensated ↓VR
Prevention of SHS: (aim is to displace the uterus)
1.Providing left lateral tilt 15 degrees beyond 28wk
2.Placing wedge under the right buttock
3. Oxford position

21. Compression of aorta & IVC in supine & lateral tilt position
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22. Anaesthetic implications
PARAMETER
1. ↓RA filling
CONSEQUENCE
↓SV & CO (25%)
2. Chronic partial IVC
Venous stasis, phlebitis,
obstruction
edema in lower limbs
Note: Adverse hemodynamic ↓ed spinal LA requirement
3. Epidural plexus engorged effects ↓ed after engagement of
fetal head.
4. Systemic hypotension +
Compromised uteroplacental
blood flow
↑ Uterine venous P

23. Hematology & Coagulation
PARAMETER
1. BV
CHANGE
+45%
2. Plasma volume + 55%
3. RBC volume
+33%
4. Hemoglobin
-17%
5. Hematocrit
35.5%

24. BV (%∆ from prepregnancy)
Table showing % change in RBC and plasma volume
Plasma
RBC
T1
T2
T3
1hr
1wk
6wk
Note: 1. Hemodilution - patency of uteroplacental vascular bed
2. Facilitates exchange of resp. gases, nutrients & metabolites
3. Reduces impact of maternal blood loss at delivery

25. Continued…
Plasma proteins:
1. ↓Total proteins - ↑unbound ( active) drug
2. ↓cholinesterase conc. (25%) but no change in duration
of action of Sch.
Immunity:
1. Leukocytosis – mainly PMN but function is impaired
(↓chemotaxis & adherence)
a) ↑ Infection
b) diagnosis difficult
c) ↓ s/s of autoimmune disorders
2. ↓Antibody titers to HSV, Measles, Influenza A

26. Continued…
Coagulation
Hypercoagulable,
↓AT III
↑coagulation factors
↑fibrinopeptide A
TEG
↓PT/PTTK
↑ fibrinolysis,
↑FDP
↑Plasminogen
↑platelet turnover
BT
unaltered

27. Gastrointestinal system
Anatomical
1. ↑Angle of GE junction
2. Cephalad displacement of
stomach & intestine
3. Vertical rather than horizontal
stomach
Physiological
1. Relaxed LES (progesterone)
↓barrier P.
2. Delayed gastric emptying
(narcotics, anticholinergics,
pain of labour)

28. Anaesthetic implications
Risk of aspiration pneumonitis
1. Ph < 2.5 (nearly all)
2. Gastric vol > 25 ml ( 60%)
3. ↓ LES tone + ↑ intragastric P + ↓ gastric emptying
4. Recent food intake prior to labour/ surgery
1. Consider gravida as FULL STOMACH beyond 1st trimester
2. Give aspiration prophylaxis
3. Regional anaesthesia / inhalational analgesia preferred
4. Plan RSI

29. Nervous system
Vertebral column
1. ↑ Lumbar lordosis - ↓vertebral interspinous distance
2. Distended epidural veins & ↓ CSF volume
3. Positive Lumbar epidural P (difficult identification)
4. CSF P unaffected (↑ during uterine contraction)

30. Continued…
1. ↑ pain threshold at term & ↑ endogenous neuropeptides
labour
2. ↓ MAC / ED95
1.Sedative effect of
progesterone
2. ↑ CNS serotonergic
activity
3.+ of endorphin system
Dependence on sympathetic nervous system ↑ progressively
a) counteracts adverse effects of aortocaval compresion
b) greater preloading during neuraxial blockade
c) pharmacological sympathectomy can cause marked ↓
in BP

31. Continued…
↓Spinal anaesthetic dose requirement
(25%)
1.↑ Neural suseptibility to LA
2. Epidural plexus engorgement
3. CSF changes a)↓CSF protein (↑unbound drug)
b)↑ CSF pH (↑ unionised drug)
4. Pelvic widening & resultant head down tilt in
lateral position
5. Apex of thoracic kyphosis higher

32. Pelvic widening & resultant head down tilt

33. Anaesthetic implications
SPINAL
EPIDURAL
1. ↓ Segmental dose
S
1. ↑ Dural puncture
2. Rapid onset & longer
duration
2.↓Sensitivity of hanging
drop technique (+epidural P)
3. Requirement normalise at 3.Unintentional i.v. injection
3.
24-48 hr PP
4. ↑ Rostral spread (esp.
during uterine contraction)
4. ↓Segmental dose (small
doses) (↑neural sensitivity)
5. Same spread with large
doses (unaltered
extravascular epidural vol)

34. Hepatobiliary system
Progesterone →↓ cholecystokinin→↓GB emptying
Altered bile composition
 Serum bilirubin & liver enzymes
↑upto upper limit of normal range
Gall
stones

35. Renal
Progesterone + estrogen → +RAAS → Na & H2O retention
CHANGE
CONSEQUENCE
1. Renal plasma flow↑(70%)
GFR ↑
+
Plasma expansion
Renal indices < normal
(creatinine ↓0.5-0.6)
BUN ↓ 8-9)
2. ↑GFR + ↓absorption
threshold
Mild glycosuria(1-10g/dl)
Proteinuria(<300mg/d)
3. Ureter & renal pelvis dilate
Pyelonephritis

36. Continued…
 ↑ Kidney size → normal at 6 wk postpartum
 ↑ creatinine clearance →normal at 8-12 wk postpartum
 ↑ frequency of micturition6-8wk → resetting of osmoregulation (polyuria + polydipsia)
late pregnancy → P on bladder by presenting part

37. Endocrine
ensure continuous
GLUCOSE METABOLISM
Estrogen, progesterone
Hpl, prolactin,
cortisol, FFA
glucose supply
to foetus
4
contrainsulin factors
hyperinsulinemia (resistance)
lipogenesis, hyperlipidemia, hyperketonemia
Fasting hypoglycemia (foetal consumption)
PP hyperglycemia& hyperinsulinemia

38. Continued…
LIPID METABOLISM
↑HDL, LDL, TG
Hyperlipidemia of pregnancy is not atherogenic
PROTEIN METABOLISM
+ nitrogen balance

39. Continued…
THYROID
Thyromegaly due to ↑ placental HCG (↓TSH )
↑ T3 + T 4
↑TBG (estrogen)
Free T3/T4
unchanged
Euthyroid

40. Pharmacological
1. Sch. - ↓pseudocholinesterase (-25%) but no effect on
duration of action
2. NDMR - Rapid & prolonged effect
3. ↓Chronotropic response to isoproterenol & epinephrine
(downregulation of β rec. )
4. Pressor response – inconsistent
refractory
5. LA toxicity – unaffected

41. Changes during labour
RESPIRATORY SYSTEM
Stage I
MV
Stage II
+75-150% +150-300%
O2 need +40%
+75%
O2 requirement > consumption → Anaerobic metabolism

42. Continued…
CARDIOVASCULAR SYSTEM
↑sympathetic activity
↑cardiac contractility, SVR, VR(↑CVP)
↑CO (+10,+25,+40 in stage I,II,III)
(+15-25% during each contraction)

43. Changes in puerperium
Cardiovascular
Relative hypervolemia
(autotransfusion)
+
TIME
↑VR (↑CVP)
CO
Immediate
PP
+75%
D-2
Just below predelivery
2 wk
+10%
12-24 wk
= Prepregnant
Nervous system
Spinal LA dose requirement reaches prepregnant level at 24-48 hr

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Continued…
Respiratory
PARAMETER
PREPREGNANT
LEVEL AT
FRC
1-2 wk
O2 consumption
6-8 wk
TV
6-8 wk
MV
6-8 wk
Alveolar PCO2
6-8 wk
Mixed venous
PCO2
6-8 wk

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Continued…
600 ml –vaginal
delivery
1L – caesarean
section
Same for RA/GA
1st wk = 25%
6-9 wk = +10%
Hb
6 wk
Protein
Blood loss
PREPREGNANT AT
BV
Hematological
PARAMETER
6 wk
TLC
D-1 = 15000
6 wk >prepreg.
Fibrinolysis
Immediate postpartum
Clotting
+ at placental
separation
Fibrinogen & platelet
count
↑ D3 – D5
Thrombosis

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References
1. Obstetric anaesthesia – principles and practiceDavid H Chestnut
2. Anaesthesia & Co-existing diseases-Stoelting
3. Millers anaesthesia
4. Short Practice of Anaesthesia – Churchill Davidson
5. Textbook of obstetrics- DC Dutta

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