Pregnancy physiology shifts
What changes by 3rd trimester
Mnemonic device
Up · Down · Anatomic
Mapping
Up
↑ Cardiac output 30-50%, ↑ blood volume 30-40%, ↑ minute ventilation 50%, ↑ FRC eaten by gravid uterus, ↑ aspiration risk, ↑ thyroid binding globulin, ↑ factors VII/VIII/X (hypercoag)
Down
↓ FRC 20% supine, ↓ residual volume, ↓ albumin (dilutional), ↓ pseudocholinesterase 25% (succ effects mostly unchanged), ↓ MAC 25-40% (volatile)
Anatomic
Difficult airway 8x more common (engorged mucosa, larger breasts, edema, weight gain)
Aortocaval
After 20 wk — LEFT lateral tilt 15° to prevent supine hypotension; aortocaval compression by gravid uterus
Clinical note: These shifts mean: full-stomach precautions always, RSI default for general anesthesia, ↓ induction doses, anticipate difficult airway, lateral tilt always, expect hypotension with neuraxial.