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Chapter 181. Physiology of Labor and the Postpartum Period

Imre Rédai, MD, FRCA

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Respiratory System
Minute ventilation during labor75–150% increase in the first stageOxygen consumption may increase by 45%Epidural anesthesia prevents changes in the first stage
150–300% increase in the second stage
  • Oxygen consumption may increase by 75%
  • Paco2 may fall to 10–15 mm Hg
  • Serum lactate rises
Epidural anesthesia does not prevent changes in minute ventilation, oxygen consumption and serum lactate in the second stage
PostpartumFRC increases after birth but remains below normal for 1–2 weeksO2 consumption, minute ventilation, and tidal volume remain elevated till at least 6–8 weeks postpartum
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Cardiovascular System
Cardiac output in labor (between uterine contractions)10% increase in early first stageIncrease in SV (HR unchanged)Epidural analgesia reduces but does not fully eliminate the increases in cardiac output
25% increase in late first stage
40% increase in second stage
Systolic and diastolic blood pressuresElevated from the late first stageProgressive activation of the sympathetic nervous system during laborLabor epidural analgesia attenuates these changes
Aortocaval compression (in the supine position)
  • 20% reduction in uterine blood flow
  • 50% decrease in lower extremity blood flow
About 8% of women may develop bradycardia and significant hypotension when lying supine (supine hypotensive syndrome)
Uterine blood flow and uterine contractions during laborIncreases to 600–900 mL/min on average (50–190 mL/min preconception)Uterine contractions augment CO and SV by an additional 15–25% (300–500 mL of blood displaced with each contraction)
Compression of the aorta by the uterus increases with contractionsReduces uterine filling and increases afterload
Immediate postpartum hemodynamicsCVP rises and cardiac output increases up to 75% of predelivery valuesRelative hypervolemia and increased venous return in this period is a result of relief of caval compression and reduction of vascular capacitance, which exceeds the blood loss of labor (autotransfusion)
Further postpartum hemodynamics
  • During the first hour postpartum term, CO decreases to 30% above the prelabor value; it reaches the prelabor value about 48 h postpartum
  • CO 10% above prepregnant value at 2 weeks, returns to baseline at 12–24 weeks postpartum
  • HR normalizes in 2 weeks
  • Stroke volume takes much longer and is still 10% above baseline at 24 weeks
  • LV hypertrophy regresses gradually and is still appreciable at 24 weeks
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Blood Volume
Blood volume94 mL/kg (prepregnant value is 76 mL/kg)
  • Average blood loss during vaginal delivery is about 600 mL
  • Average blood loss during Cesarean section is 1,000 mL
General anesthesia using volatile anesthetics results in a somewhat larger blood loss than regional anesthesia
Plasma volume69 mL/kg (prepregnant value is 49 mL/kg)
Postpartum changes
  • Blood volume falls to 125% of prepregnant value by the end of the first postpartum week and declines to 110% at 6–9 weeks
  • Hb and hematocrit fall in the first 3 days, and then rise rapidly (plasma volume contraction) and reach prepregnant values ...

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