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Chapter 180. Physiology of Pregnancy

Imre Rédai, MD, FRCA

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Overview of Physiological Changes during Pregnancy
Mean body weightIncreases by 17% on average
  • Parenchymal organ hypertrophy
  • Increase in muscle mass
  • Fetoplacental unit
Oxygen demand and CO2 productionIncreased by 30–40% at term
Total body fluids and electrolytes7 L water and 900 mEq sodium is gained by term
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Cardiovascular System
Cardiac outputBy 5th weekIncrease noticeableDue to increase in heart rate
By 12th week35–40% over nonpregnant valueStroke volume also increases
End of second trimester50% over nonpregnant valueAt term both heart rate and stroke volume increased by 25% compared with nonpregnant values
Ventricular volumes
  • LVED increases
  • LVES unchanged
ContractilityUnchanged throughout pregnancy
Filling pressuresCVP, PADP, PCWPAll unchanged compared with nonpregnant values
Systemic vascular resistanceReduced by 20% at term compared with nonpregnant values
Systolic blood pressureMinimally affected
  • Declines by 8% at midgestation
  • Returns to baseline at term
Diastolic blood pressureReduced by 20% at midgestation
Returns close to baseline at termDue to aortocaval compression
EKG changes
  • Sinus tachycardia
  • Shortened PR
  • QRS axis shifts initially to the right, and then to the left
  • ST depression, T-wave changes in lead III
  • New Q wave in leads III, aVF
Do not confuse with myocardial ischemia, pulmonary embolism
EchocardiographyBy 12th weekNoticeable LVH
By term50% increase in LV mass
Valve annular diameters increase (except aortic valve)
  • 94% patients have tricuspid regurgitation
  • 27% have mitral regurgitation
  • Aortic insufficiency is never normal

The degree of compression of the aorta and inferior vena cava by the gravid uterus depends on:

  • The gestational age
  • The position of the pregnant woman

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Effect of Gestational Age and Position on Aortocaval Compression
13–16 weeksFirst sign of IVC compression detectable
At termLateral decubitusPartial obstruction of IVC
No significant obstruction of aorta
SupineFull or close to full obstruction of IVC
  • RV filling pressure drops
  • Cardiac output reduced by 20% or more
Noticeable compression of the abdominal aorta
  • 20% decrease in uterine blood flow
  • 50% decrease in lower extremity blood flow
  • Increase in SVR
15° left lateral tiltReduces but does not eliminate compression of IVC and aorta
Supine hypotensive syndrome
  • Bradycardia
  • Severe hypotension
Occurs in about 8% women at term in supine positionCombination of reduced venous return and inadequate response of the autonomic nervous system
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Respiratory System
Nasopharynx and oropharynxVascular engorgementStarts as early as 7th week
Bony thoraxAP and transverse diameters increaseDue to emergence of uterus from pelvis
DiaphragmAt rest 4 cm higher at term compared with nonpregnant state
Tidal volumeIncreased by 20% in first trimesterDue to decrease of inspiratory reserve volumeProgesterone sensitizes the respiratory center to CO2...

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