Physiologic changes of pregnancy
Stem
A 30-year-old G1P0 at 36 weeks, 92 kg, needs urgent cesarean delivery for breech presentation in labor. She is healthy. Her room-air arterial blood gas, drawn for an unrelated reason, shows pH 7.44, PaCO2 31 mmHg and bicarbonate 20 mEq/L. Hemoglobin is 10.8 g/dL and creatinine 0.6 mg/dL. The resident asks whether anything in her physiology changes the anesthetic.
Question
Which physiologic changes of pregnancy matter for your anesthetic, and how does each change what you do?
Ideal answer
HeadlineI organize by what changes my anesthetic: harder airway, fast desaturation, aortocaval compression, full stomach, lower neuraxial doses. Her gas is normal for pregnancy.
- Airway: smaller tube, video laryngoscope, because edematous friable mucosa, enlarged breasts and weight gain make failure roughly seven times more likely at term.1
- Respiratory: preoxygenate thoroughly, because FRC falls about 20% and oxygen consumption rises;1 minute ventilation rises up to 50%,2 so normal PaCO2 is about 30 mmHg, exactly hers.1
- Cardiovascular: left uterine displacement always, because output rises about 50%, higher in labor, systemic vascular resistance falls, and compression after 20 weeks drops venous return.1
- Full stomach: sodium citrate, for reduced lower esophageal sphincter tone.
- Neuraxial: reduced spinal dose, for engorged epidural veins and increased nerve sensitivity.
Watch for: hypotension after the block, the output surge at delivery.
Breaking down the answer
- Aortocaval compression — supine compression of the vena cava cuts preload and of the aorta cuts uterine flow; every positioning or arrest answer includes it.
- Apneic time is roughly halved — so I preoxygenate to an end-tidal oxygen above 0.9 and add nasal oxygen for apneic oxygenation.
- Full-stomach precautions — progesterone relaxes the sphincter and the uterus displaces the stomach; up to 80% of term parturients have reflux;1 rapid sequence induction for general anesthesia from mid-pregnancy onward.
- Interpret the blood gas — a PaCO2 of 40 mmHg in a term patient is relative hypercarbia, a warning of respiratory fatigue.
- Hematologic — dilutional anemia explains her hemoglobin, and she is hypercoagulable with a high fibrinogen.
- Drug doses — MAC falls by about 30–40%, and less local anesthetic is needed per dermatome;2 plasma cholinesterase falls about 25%,1 but succinylcholine duration is barely prolonged because volume of distribution rises.
- Cardiac output peaks immediately after delivery — autotransfusion of 500 mL as the uterus contracts and relief of caval compression: the dangerous moment for stenotic valve lesions and pulmonary hypertension.
Follow-up question
Her spinal does not reach a surgical level, and you induce general anesthesia. She desaturates to 85% despite good preoxygenation, and the first look shows a swollen airway with bleeding. What do you do?
Follow-up answer
HeadlineA difficult airway with rapid desaturation, explained by her small FRC and high oxygen consumption; I declare the difficulty.
Ramped, left tilt, two-hand mask ventilation with an oral airway while a supraglottic airway is prepared; maternal oxygenation is fetal oxygenation. Once the airway is secured, no hyperventilation, because maternal hypocapnia reduces uterine flow, and mean pressure within 20% of baseline until delivery. I ask the obstetrician to hold the incision until she is oxygenated and we have agreed whether to proceed on a supraglottic airway or wake her.
Pitfalls that cost points
- Reciting a list of numbers without saying what each changes in the plan.
- Calling a PaCO2 of 40 mmHg normal in a term patient.
- Assuming she has a normal coagulation reserve because the INR is normal; it is the fibrinogen that matters in hemorrhage.
- Stating that succinylcholine is dangerously prolonged in pregnancy.
Rapid-fire
Q: Why is the normal hemoglobin lower? A: Plasma volume rises about 45–50% and red cell mass only about 20–30%, a physiologic dilutional anemia.1
Q: Why does the bicarbonate fall? A: Renal compensation for the chronic respiratory alkalosis from progesterone-driven hyperventilation.
Q: Why is GFR higher? A: Renal blood flow rises about 50%, so a creatinine of 1.0 mg/dL is abnormal in pregnancy.
Q: When do these changes resolve? A: Hematologic changes have mostly reverted by 2 weeks postpartum;1 full return of all systems takes up to 6 months.
Numbers to know
| Item | Value |
|---|---|
| Cardiac output | ~+50% at term; higher in labor and immediately after delivery1 |
| Plasma volume / red cell mass | +40–50% / +20–30%2,1 |
| FRC | −20% |
| Minute ventilation | +45–50%; PaCO2 ~30 mmHg |
| MAC | −30–40%2 |
| Fibrinogen | 400–600 mg/dL at term |
| Normal creatinine | ~0.5–0.8 mg/dL (50–73 µmol/L)1 |
Links
- General anesthesia for cesarean delivery, Non-obstetric surgery during pregnancy, Maternal cardiac arrest and resuscitative hysterotomy, aspiration, Cardiac disease in pregnancy, The obese parturient
- ASA Practice Guidelines for Obstetric Anesthesia; ACOG Committee Opinion on nonobstetric surgery during pregnancy
Sources
- 1.Changes in Maternal Physiology During Pregnancy (Update in Anaesthesia 21) · WFSA · resources.wfsahq.org
- 2.Obstetric Regional Anesthesia · NYSORA · nysora.com
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.