Physiological changes during pregnancy and uteroplacental circulation.

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Physiological changes during pregnancy and
uteroplacental circulation.
www.anaesthesia.co.in
email anaesthesia.co.in_at_gmail.com

• Presenter Dr. Anamika Sharma
• Moderator Dr. Rachna Wadhwa

University College of Medical Sciences GTB
Hospital, Delhi
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Contents

1. Body weight and composition during pregnancy.
2. Phases of pregnancy and labor.
3. Anatomical changes in the pregnancy.
4. System wise changes in the physiology during the
   pregnancy.
5. Uteroplacental circulation and the factors
   affecting.

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Anatomical changes in pregnancy

• Uterus an intrapelvic organ till 12 weeks of
  gestation.
• By 20 weeks- uterus reaches upto umbilicus.
• By 34 weeks it reaches the costal margin.
• Last 2 weeks fundal height decreases when fetal
  head descends in the pelvis.
• Increased lumbar lordosis.

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Pregnancy and labor

• 1st TM 1st 12 weeks
• 2nd TM 13-28 weeks
• 3rd TM 29-40 weeks.
• Stages of labor
• Stage 1a) dilatation and the effacement of the
  cervix. b) formation of the lower uterine
  segment.
• Stage 2 Complete dilatation of the cervix to
  the expulsion of the fetus.
• Stage 3 Phase of placental separation.

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BODY WEIGHT AND COMPOSITION

• Maternal weight gain-12kgs.
• Increased size of uterus(1kg)
• Amniotic fluid(1kg)
• Fetus and placenta(4kgs)
• Blood volume and interstitial fluid(4kgs)
• New fat and protein (2-4kgs)
• First trimester - 1-2 kgs
• Second trimester - 5-6 kgs
• Third trimester - 5-6 kgs

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Factors causing physiological changes in
pregnancy

• Hormonal alterations
• Mechanical effects of the gravid uterus.
• Increased metabolic and oxygen requirements.
• Metabolic demands of the fetoplacental unit.
• Hemodynamic alterations a/w placental circulation.

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Respiratory Changes during pregnancy

• AIRWAY
• Capillary engorgement of the mucosa - changes in
  the Mallampatti examination score.
• Exaggerated by mild URI, fluid overload or the
  edema associated with PIH.
• Intranasal manipulation can cause brisk
  epistaxis.
• Breast engorgement can interfere with
  laryngoscopy.

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Changes in the respiratory system at term
Variable Direction of change Magnitude
Minute ventilation(MV) ? 50
Alveolar ventilation ? 70
Tidal volume ? 40
Arterial PaO2 ? 10mm Hg(85-103mm)
Oxygen Consumption ? 20
Respiratory rate ? 15
Inspiratory lung capacity ? 5-15
Arterial pH ? 7.40-7.47
Vital capacity ?
Arterial PaCO2 ? 27-33.5 mm Hg
Serum bicarbonate ? 21-27 mEq/L
FRC ? -15-20
Expiratory reserve volume ? -20
Residual volume ? -20
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• During labor ? MV - ?300 ?development of
  maternal hypocarbia( PaCO2 20mm Hg) and
  alkalemia(pHgt7.53) ?left shift of ODC ? loss of
  consciousness ?hypoventilation.
• Obesity -?? vital capacity.
• Supine position--susceptibility to atelectasis
  and ? A-a gradient.
• Closing capacity may exceed FRC.
• Oxygen consumption -- ? 63.

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• Anaesthetic implications respiratory function
  changes
• rapidity of induction of inhaled anaesthetics-
  (?FRC and ?MV).
• MAC of the potent anaesthetic agents ? by
  24-40(?sensitivity to inhalational anaesthetic
  agents).
• Judicious dosing of the inhaled anaesthetics
  required ?loss of protective airway reflexes and
  cardiopulmonary depression at ? doses.
• Susceptibility to hypoxia ?, use of ? FiO2.

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Role of maternal preoxygenation

• Hypoxemia develops rapidly
• d/t ?FRC
• ? O2 consumption.
• FRC lt Closing capacity in supine position.
• During apneic periods rate of PaO2 fall, more
  than twice in parturient.
• After complete denitrogenation
• apneic period is 6-8 min. in non pregnant
• 2-3 min. in pregnancy.

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

• Plasma volume -? 40mL/kg - 70mL/kg(45)
• RBC volume -? 25ml/kg - 30ml/kg(30)?
  Physiological anemia of pregnancy.
• Leukocyte levels- ?12000-21000/mL.
• Platelet count- ?/?20 but no alteration in BT.
• Clotting factors hypercoagulable state, general
  ? in all the clotting factors.

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• Fibrinogen ?? to 400-650mg/dL(normal-200-450mg/dL)
  ? ?ESR value
• Factors VII, VIII and X ?? after the 3rd month of
  pregnancy.
• Factors II, V and XII ?
• Factors XI and XIII- slightly ?.
• Clotting factors return to normal within 2 weeks.
• .

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CVSHemodyanamics in pregnancy.

CO Starts to increase by 5 wks.
CO End of the 1st TM 30-40
CO end of the 2nd TM 50
2ND Stage of labor 40
With each uterine contraction 15-20
Immediately post partum 75-80 above prelabor values.
Returns to pre pregnant levels by 2 weeks post partum.
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CVS Hemodynamicscont.
SV 20-50
Contractility 10
Systemic vascular resistance -20
Pulmonary vascular resistance -30
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CVS-Examination

• Innocent grade I/II systolic heart murmur.
• Doppler and echo studies- ?end diastolic chamber
  size ? in total LV wall thickness, asymptomatic
  pericardial effusion, trivial MR and TR.
• Diaphragmatic rise shifts the position of the
  heart leftwards, enlarged appearance on CXR.
• ECG shows increase in benign dysrhythmias,
  reversible ST, T and Q wave changes and some LAD.
  

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• The findings indicating heart disease
• overtly symptomatic patient
• systolic murmurgtgrade III
• any diastolic murmur
• severe arrythmias
• unequivocal cardiac enlargement on chest X
  ray.

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• Maternal BV markedly ?during pregnancy.
• Immediate post-partum-- 35-40 ?in BV, due to
  auto transfusion of 300-500 ml blood.
• Healthy parturient - tolerate up to 1500 mL of
  blood loss.
• Women with cardiac and pulmonary disease remain
  at risk after the delivery due to ?CO.
• Epidural block beneficial -cardiac diseases.
• Normal nonpregnant blood volume is reached by
  7-14 days post partum.

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CVSMeasurement of BP.

• Auscultatory better than oscillometric.
• Diastolic BP represents Phase V(disappearance)
  Korotkoffs not Phase IV(muffling).
• Position Dependent left arm in the left
  decubitus .
• SV(?)
• SBP
  ?SBP unaffected
• vsl distensibility(?complianc
  e)
• BP
• DBP SVR(?)
  DBP ?

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• Despite ?CO, BP remains static during pregnancy
  because of 21 and 34 ? in systemic and
  pulmonary vascular resistance and an ? aortic
  compliance.
• Oxytocin with a free water infusion can cause
  volume overload.
• High Hb. level(gt14gm) indicates a low volume
  state caused by preeclampsia, HT or inappropriate
  diuretics

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• AORTOCAVAL COMPRESSION
• 15 pregnant patients near term develop signs of
  shock, hypotension, pallor, sweating, nausea,
  vomiting when they assume supine position.
• This syndrome was described by Howard et al as
  supine hypotension syndrome now known as
  aortocaval compression syndrome.
• Symptoms are because of the lack of the venous
  return to the heart.
• Compression of the IVC is most common in the late
  pregnancy.

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Compression of aorta IVC in supine lateral
tilt position
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• Pooling of the venous blood and ?venous pressure
  in the lower torso and lower extremities cause ?
• tendency towards phlebitis and venous
  varicosities.
• Blood from the obstructed IVC ?
  paravertebral(epidural) veins ? azygos system.
  Venodilatation ? accidental epidural vein
  puncture.
• Prevention Left uterine displacement(LUD).
• Left tilt of the table by 15 degrees or wedge or
  bag to elevate the right buttock and back by
  10-15 cms.
• Avoid trendlenburg position without LUD.

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NERVOUS SYSTEM CHANGES

• ?MAC near term.
• MAC returns to normal values within 3-5 days
  after delivery.
• ?maternal beta- endorphins.
• Increased intra abdominal pressure ? Engorgement
  of epidural veins? ? spinal CSF volume
  accidental intravascular injection in epidural or
  caudal block.
• ? lumbar lordosis.
• Apex of thoracic kyphosis at a higher level.

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• 30 reduction in the LA requirements for SAB.
• Swelling of the epidural veins ??CSF
  volume.
• Labor? ?CSF pressure.
• ?neurosensitivity to LA.
• ? cephalad spread caused by widening of the
  pelvis when the injections are made in the
  patients in the lateral position.

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Pelvic widening resultant head down tilt
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GIT Changes

• LES tone? d/t upward gastric displacement by the
  gravid uterus, muscle relaxation by the
  progestins.
• Gastric emptying may be prolonged during
  pregnancy but certainly during labor(? by opiates
  and anticholinergics).
• Gastrin secreted from placenta??acid, chloride
  content, volume and enzyme levels of the stomach.
• Increased chances of aspiration d/t ? LES tone,
  nausea, obesity, drugs, ketosis, recent food
  ingestion.

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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)
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• At risk for Mendelsons syndrome due to acid
  material.
• Solid material a/w atelectasis, lung abscess and
  mechanical obstruction.

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Anaesthetic implications

• Risk of aspiration pneumonitis
• Ph lt 2.5 (nearly all)
• Gastric vol gt 25 ml ( 60)
• ? LES tone ? intragastric P ? gastric
  emptying
• 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
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RENAL CHANGES

• ?RPF- 40-90
• ?GFR- 50-60 by 4th month of gestation and
  return to normal by 3rd TM.
• ?RPF and GFR ??creatinine clearance, ?upper limit
  of normal BUN and creatinine by 40.
• Normal pregnancy- BUN- 6-9mg/dl and
  creatinine-0.4-0.6 mg/dl.
• ? RAAS activity??total body sodium and water.
• Threshold for ADH secretion reset??plasma
  osmolality

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• ?angiotensin but ?sensitivity
• Glucosuria of 1-10 gms and proteinuria of 300
  mg/day common.
• Renal calyces, pelvis and ureters dilate after
  3rd month of gestation progesterone and
  compression of ureters at the pelvic brim.
• Urinary stasis?frequent UTIs

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Hepatic and Endocrine

• ?SGOT, LDH, ALP, GGT and cholesterol levels.
• Abnormal bromsulphthalein excretion test.
• Serum bil.- unaltered.
• Total protein and AG ratio-?, ?ser. albumin.
• Unbound plasma cortisol-2.5X
• Serum cholinesterase activity-?24 before
  delivery and by 33 at 3 days post partum, normal
  by 2-6 weeks post partum.

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• ?bound T3 and T4 by 50, free hormones are normal
• ? Calcium dependent nitric oxide synthase
  activity.
• ?ANP
• ?insulin sensitivity in the last half of
  pregnancy d/t placental insulinase production.
• ?resting blood glucose levels and ?post prandial
  blood glucose levels.

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Endocrine
ensure continuous
glucose supply to foetus
GLUCOSE METABOLISM
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Estrogen, progesterone Hpl, prolactin,
contrainsulin factors cortisol,
FFA
hyperinsulinemia (resistance)


lipogenesis, hyperlipidemia, hyperketonemia

Fasting hypoglycemia (foetal consumption) PP
hyperglycemia hyperinsulinemia
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Uteroplacental circulationPlacental anatomy.

• Placenta - villous haemochorial type.
• At term wt- 500 gm, diameter 20cms, 3 cm thick.
  Normal fetal to placental weight ratio 61
• Fetal maternal blood is separated by 3
  structures
• cytotrophoblast.
• syncytiotrophoblast(metabolically active), has
  got endocrine function.
• connective tissue.

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• The placenta - consist of a basal and a chorionic
  plate.
• a semipermeable membrane -interface for the
  maternal and fetal circulation.
• The intervillous space separates the plates and
  is subdivided by decidual tissue.
• Chorionic villi and spiral arteries protrude into
  this intervillous space.
• Maternal blood flows into the intervillous
  space(80) from the spiral artery.

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Placental Anatomy cont.
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Mechanisms of exchange.

• Substances are exchanged across the placenta by 5
  mechanism
• Diffusion ( FA.CO2.respiratory gases,Na)
• Active transport.(AA.Ca,Fe,water sol vitamin) a.
  Primary active transport. b. Secondary active
  transport.
• Bulk flow.
• Pinocytosis ( Fe).
• Breaks

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Uterine Blood Flow

• ? throughout the pregnancy, at term700ml/min10
  of the CO
• Blood flow? and resistance ?
• Maximally dilated under normal conditions, no
  autoregulation, capable of VC by alpha adrenergic
  action, unresponsive to ? gas tensions.
• Flow proportional to mean perfusion pressure.

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Factors causing decreased uterine blood flow

• Uterine contractions
• Hypertonus abruptio placentae, tetanic
  contractions, overstimulation with oxytocin.
• Hypotension sympathectomy, hypovolemic shock,
  aortocaval compression.
• Hypertension essential, pre-eclamptic.
• Vasoconstriction endogenous sympathetic
  discharge, adrenal medullary activity.
• Vasoconstrictors exogenous most
  sympathomimetics, exception is ephedrine(B
  adrenergic)

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Summary

• CO increases during pregnancy as a result of ?SV
  and HR.
• Pregnant women should not lie supine after 20
  weeks of gestation to prevent aortocaval
  compression.
• Increased blood volume of pregnancy allows the
  parturient to tolerate blood loss of delivery
  with minimal hemodynamic perturbation.
• Oxygen demand and delivery both are increased.

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• Higher sympathetic tone in pregnancy.
• Pre-oxygenation mandatory.
• Increased risk of failed intubation.
• Sensitivity to inhaled anesthetics and local
  anesthetics-?.
• Analgesia for labor requires epidural block-
• T10-L1- 1st stage of labor.
• T12-S4- 2nd stage of labor.

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References

• Obstetric anaesthesia principles and practice-
  David H Chestnut
• Obstetric anaesthesia- Schneider, 2nd edition.
• Anaesthesia Co-existing diseases-Stoelting
• Millers anaesthesia- 7th edition.
• Clinical anesthesiology by Morgan et al - 4th
  edition
• Textbook of obstetrics- DC Dutta

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