Physiological Changes In Pregnancy

Describes the various physicilogical changes in pregnancy

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Physiological changes in
Pregnancy & its Anaesthetic
implications
Dr. Shailendra.V.L. MBBS, DA, MD.
Specialist in Anaesthesia,
Bukariya General Hospital.

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Introduction
 Rapidly growing fetus
 Rising levels of progesterone,
oestrogen, prostaglandin & HCG
 Increasing size of uterus
 All systems undergo changes
 Placenta

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Changes in the pregnant
patient
 Changes due to uterine enlargement
 Changes to support the foetus

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Changes in the systems
 Cardiovascular System
 Respiratory System
 Haemopoietic System
 Hepatic System
 Renal System
 Gastro-intestinal System
 Metabolism & Nutrition
 Central Nervous System

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Changes in uterus
 Uterine blood flow increases from 50
ml/ min to 700 – 800 ml /min
 Uterine weight increases from 30-60
g to 1000 g

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Uterine blood flow
Uterine Blood Flow - UBF:
–Uteroplacental vascular bed is passive
capacitance bed
–Intervillous blood flow exhibits no
auto regulation
–UBF dependent on mean arterial
pressure, aorto-caval compression &
cardiac output

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Cardiovascular system
Changes in the cardiovascular system:
Intravascular fluid volume: + 35%

Plasma volume: + 45%
Erythrocyte volume: + 20%
Cardiac Output: + 40%
Stroke volume: + 30%
Heart rate: + 15%
Peripheral Circulation:
Systolic BP: no change
Diastolic BP: - 15%
Systemic Vascular Resistance: - 15%
Femoral venous pressure: + 15%

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Cardiovascular system
 Mean arterial pressure: MAP:
-Inspite of increase of cardiac
output, MAP is maintained due to
concomitant decrease in peripheral
resistance

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Cardiovascular system
Compression of Inferior Venacava: IVC:
-In supine position the gravid uterus
compresses the IVC and decreases the
CO without fall in the blood pressure
called as Concealed caval
compression.
-Reasons for no fall in blood pressure
are:
-Reflex vaso constriction
-Diversion of blood through
paravertebral venous plexus

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Cardiovascular system
 8 to 15% of pregnant women have Overt Caval
Compression (supine hypotensive syndrome)
Hypotension
Sweating
Bradycardia
Pallor
Nausea
Vomiting
 Prevention of SHS: (aim is to displace the
uterus)
 Providing left lateral tilt 15 degrees
 Placing wedge under the right buttock

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Cardiovascular system
 Poseiro Effect:
Uterine artery is compressed during
uterine contractions & by the presenting
part of the fetus reducing the UBF

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Cardiovascular system
Physiological Anaemia in Pregnancy:
 Total red cell mass increase by 30% (250-
450ml)
 Plasma volume increase by 50% (about
1250ml)
 Overall increase of 40% blood volume with
fall in haematocrit

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Cardiovascular system
 Heart rate: increases by about 15 beats
/min
 Arterial pressure: minimal change noted
 Heart Size: Gravid uterus pushes the
diaphragm cepahalad & displaces the
heart.
 ECG shows false left axis deviation
 Chest X ray shows (false) cardiac dilatation

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Respiratory system
Changes in the respiratory system:
Minute volume: + 50%
Tidal volume: + 40%
Breathing rate: + 10%
FRC: - 20%
Expiratory reserve volume: - 20%
Residual volume: - 20%
Oxygen consumption: + 20%

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Respiratory system
 Edematous mucosa of upper resp tract:
–Smaller Et tubes
–Gentle suctioning & larngoscopy
 Decreased FRC, ERV,RV:
 Increased O2 consumption:
–Pre-oxygenation prior to induced apnea
Rate of fall in PaO2 per minute of apnea
is 159mm hg in pregnant and 59 mm hg
in non-pregnant state

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Haemopoietic system
 Physiological anemia of pregnancy
 Fibrinogen, factors VII, VIII and X
increased
 Makes the blood hypercoaguable &
increases the risk of
thromboembolism
 This hypercoaguability along with
uterine inversion helps in reducing
blood loss after delivery

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Gastro-intestinal system
 Decrease in lower oesophageal tone
 Delay in gastric emptying:
-Pylorus is pushed upwards &
forwards making it non-dependent
-Relaxant effect on gastric smooth
muscle
 Increased intragastric pressure

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Hepatic system
 No alterations in the liver function
 Serum cholinesterase activity 
 Does not interfere with the
suxamethonium (Scoline)R
metabolism

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Renal system
 Renal plasma flow ↑
 Glowmerular filtration rate ↑
 Tubular reabsorbtion rate ↑
 Blood Urea Nitrogen 40% 

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Metabolism & Nutrition
 Basal metabolic rate: ↑ by 15 to 20%
 Weight gain upto 11 kgs
 Serum protein concentration ↓
 Albumin concentration ↓
 Drugs are less bound to serum proteins
thereby increasing the free drug
availability

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Nervous system
 ↓ in minimum alveolar concentrations
 exaggerated lumbar lardosis contribute
to cephalad spread of the local
anaesthetic
 engorged epidural plexus of veins will
decrease the amount of the local
anaesthetic in epidural blocks
 engorged epidural veins will block the
inter-vertebral foramina and prevent the
escape of the local anaesthetic

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Placental functions
 Metabolism
 Endocrine secretion
– Human chorionic gonadotrophin
– Human chorionic somatotrophin
– Progesterone
– Estrogen
 Molecular transfer

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Placental blood flow
 Placental Blood Flow = 500-700ml/min
( approximately 10% of the Cardiac output)
 Maternal blood pressure
 Maternal cardiac output
 Vasomotor tone of the uterine vessels
 Pathological changes of the placenta
 State of uterine contraction

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Placental drug transfer
 Passive-diffusion is the mechanism
Q/t = {k x A x (Cm-Cf)} / D
Q: quantity of free drug (non ionized &
non protein bound) crossing to placenta
t : per unit of time
k: diffusional coefficient of the drug
A: total area available for transfer
Cm-Cf: difference between maternal &
fetal drug concentrations
 D: distance across the membrane

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Placental transfer of drugs
Diffusion Coefficient depends on:
– molecular weight
– spatial configuration
– degree of ionization
– lipid solubility
Most anaesthesia drugs cross the
placental barrier except the muscle
relaxants because of their large size
molecule (quaternary ammonia
compounds)

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Anaesthetic implications
 Pre-anaesthetic considerations:
 Starvation
 Respiratory infection
 Prophylaxis against pulmonary
aspiration

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Anaesthetic implications
 General Anaesthesia:
 Posture
 Tracheal Intubation
 Placental transfer of drugs
 Pre oxygenation

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Anaesthetic implications
 Regional Anaesthesia:
 Posture
Technique
 Vasopressors
 Oxygen supplementation

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Summary
 Cardiovascular changes
 Respiratory changes
 Gastro-intestinal changes
 Haemopoietic changes
 Placental transfer of drugs

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Bibliography
 Anaesthesia & Co-existing diseases-Stoelting
 Short Practice of Anaesthesia – Churchill Davidson
 Refresher Course Lectures, Manipal, ISA 1989

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Thank you