Physiology of Pregnancy

OVERVIEW

• fetus considered viable at 24-25 weeks with a weight of > 750g

Airway

• progesterone mediated hyperaemia and oedema of mucosal surfaces -> down size tubes
• difficult -> higher incidence of failure to intubate

Breathing

• higher O2 consumption
• desaturate quickly
• increased aspiration risk
• increased risk of pulmonary oedema (increased blood volume and lower oncotic pressure)
• mechanical ventilation can be problematic
• difficult to implement protective ventilation -> hypercapnia produces fetal acidosis
• cephalad movement of diaphragm @ 37 weeks -> place chest drain higher than normal
• spirometry, flow volume loops and peak flow rates -> unchanged
• TLC decreases 5% c/o cephalad movement of diaphragm
• FRC decreases by 20% -> decreased ERV and RV
• MV increases by 50% from increase in TV -> chronic respiratory alkalosis (pH 7.4 to 7.47, PaCO2 30-32mmHg, slightly elevated PaO2, kidneys excrete HCO3- to compensate to keep HCO3- 18-21mmol/L, BE 3-4)
• O2 consumption increased 20%

Circulation

• tachycardia, low BP, increased Q and warm peripheries -> normal in pregnancy
• aortocaval compression
• PAC’s can be helpful in severe pre-eclampsia, APO and cardiac disease (non-invasive Q monitors are useless)
• uterine bed is sensitive to vasoconstrictors
• phenylephrine better than ephedrine -> less fetal acidosis
• no evidence favouring any particular inotrope
• diaphragm pushed upward and heart rotated to left -> changes Q waves inferiorly and gives TWI
• maternal blood volume increases 40%, red cell mass increases 25% -> relative anaemia of pregnancy
• normal ejection systolic murmur and third heart sound
• normal echo: increase in all cardiac chamber dimensions, increased LV thickness, small pericardial effusions, mild TR, mild MR

Coagulation

• x 5 increased in VTE -> prophylaxis is important

Gastrointestinal

• GORD -> aspiration
• hypoalbuminaemia from haemodilution
• increase in ALP
• appendicitis and cholecystitis happen

Genitourinary

• at 12/40 bladder becomes an abdominal structure -> susceptible to blunt trauma
• at 20/40 fundus at umbilicus -> susceptible to blunt or penetrating trauma
• GFR increases by 50% -> lower Cr, urea, uric acid

Placenta

• uterine blood flow @ term = 10% (600-700mL/min)
• under stress maternal blood flow will be maintained at the expense of the fetus
• oxygen consumption = 20mL/min -> can survive 10 min by shunting blood flow to vital organs and decreasing O2 consumption.

(TIME) -> its like the liver, kidney & the lungs all in one!

Transport

• gas exchange (O2 & CO2) – fetal Hb (higher concentration, greater affinity for O2)
• delivery of nutrients (glucose, aa, lipids)
• removal of wastes (urea, bilirubin)
• transport of other substances (drugs)
• heat transfer

Immunological

• protection of foetus from infection
• protection of foetus from rejection by mother (immunological barrier function)
• trophoblast cells do display Class I or II MHC (major histocompatibility complex) proteins thus they cannot present antigen to lymphocytes and cannot be recognised by activated cytotoxic T lymphocytes.

Metabolic

• synthesis of glycogen, cholesterol, FFA’s and enzymes

Endocrine

• synthesis of 4 main hormones: hCG, oestriol, progesterone, human placental lactogen
• synthesis of various other hormones & growth factors

Mother and Baby

• monitor for preterm labour
• think about placental transfer of drugs
• maintenance of placental perfusion and oxygenation