CARDIOVASCULAR PHYSIOLOGY IN PREGNANCY


I. Causes of the hemodynamic changes:

  1. Increased metabolic demands of new tissues
  2. New growth and increased caliber of vascular channels
  3. Increased mineralocorticoid levels

II. Blood changes

  1. Increased intravascular volume by 30-40%
  1. Quickest rise in wks 6-24; peaks at 32-34 wk and remains there (Kerr 1968)
  2. Subnormal increase correlates with poor obstetric outcome
  3. Returns to nl by 3-4wk pp
  1. Increased RBC mass secondary to EPO, hPL, PRL
  1. Increase < 1/3 of volume increase--apparent "dilution anemia"
  2. Avg 15% drop in HCT by term; lower limit nl at term Hg/HCT 11/32%
  3. Returns to nl by 8wk pp
  1. Decreased serum protein (by 1-1.5g/dl) due to dilution and less albumin synth
  2. Increased serum lipids by 50%

III. Cardiac changes: Begin early second TM, reverse rapidly postpartum. Normal changes often suggestive of heart disease.

  1. Position--anterior rotation, displaced superiorly and to left--unknown cause/sig
  2. Size--increased before uterus is big enough to push it, so prob some physiol. hypertrophy/dilation
  3. Cardiac output--greatly increased, more due to increased SV than HR
  1. Greatest overall rise in 1st TM but quickest rate of rise in wk 28
  2. Peaks in wk 28-32 at 30% above nl and stays there; vol. continues to rise
  3. Intrapartum
  1. Up in stage I (rises 30% during contractions--this may be due partly to the 250-300ml blood squeezed out of uterus when contr.; much less up in lat recum. pos.)
  2. And more in stage II
  3. LARGE incr. (29%) right after deliv as uterus autoxfuses pt then slowly declines
  1. Heart rate: slight tachy (10 BPM) developing between 14-30wks
  2. Blood pressure-Usually decreases by 10-15mm Hg in first trimester; gradually increases in 2nd trimester; usually returns to prepregnancy levels in 3rd trimester
  1. Pulse press rises 3rd TM, peaks wks 28-32
  1. Other
  1. 50% of pts have holosystolic murmurs representative of high flow
  2. Extrasystoles are common
  3. Venous pressure--us no change in UE unless in fluid overload, up in LEs
  4. Dyspnea is common (from decrease in residual volume from increased TV?)
  1. Vital capacity up 15% by wk 20 then drops 5% by term
  2. O2 consptn up 10-20% in preg but outstripped by CO incr. until 3rd TM