Physiological changes of pregnancy for critical care — structured oral station (12 minutes)

Opening prompt — "What do you make of these numbers?"

Model response — defend the pregnant reference: [1][3]

• "These numbers are not normal for a 36-week pregnant woman. Her PaO2 should be 101–105 mmHg — 72 mmHg is hypoxaemic respiratory failure. Her PaCO2 should be 28–31 mmHg — 34 mmHg is hypercapnia relative to her pregnant baseline. Her bicarbonate of 19 is at the lower edge of normal, but the lactate of 2.8 signals early metabolic acidosis on top."
• "She has type 1 respiratory failure with a rising PaCO2 trend and evolving sepsis. I would not observe — I would escalate to HDU, start high-flow oxygen, activate the sepsis bundle, and treat the pneumonia now."
• "The medical registrar is applying the non-pregnant reference range. The pregnant baseline is different — re-set the references or we will miss the deterioration."[1][3]

Examiner is listening for: the pregnant reference ranges verbatim, the recognition that PaCO2 of 34 in pregnancy is hypercapnia, and the confidence to defend a different threshold to a colleague. [3]

Probe 1 — "Walk me through the cardiovascular adaptations of pregnancy."

• "Cardiac output rises 30–50%, from about 4 to 6 L/min. About 75% of that rise happens by the end of the first trimester, and CO plateaus at 28–32 weeks. Stroke volume peaks at about 85 mL at 20 weeks, then heart rate takes over as the main driver."[1][2]
• "Heart rate rises to 90–100 bpm at rest in the third trimester — her HR of 122 is therefore significantly above the pregnant baseline, not 'borderline'."
• "Systemic vascular resistance falls by about 21%, and pulmonary vascular resistance by 34%, both mediated by progesterone, nitric oxide and prostaglandins. Blood pressure dips to a nadir at 20–24 weeks and climbs back to pre-pregnancy values by term."[1]
• "The implication is that she has less cardiovascular reserve — her baseline CO is already maxed, and any further demand from sepsis or hypoxia cannot be met by simply raising CO further."[1][2]

Probe 2 — "What about the respiratory system?"

• "Minute ventilation rises 30–50% via a 30–50% rise in tidal volume — respiratory rate is unchanged. The drive is progesterone-mediated sensitisation of the medullary respiratory centre to CO2."[1][3]
• "Functional residual capacity and total lung capacity fall by 10–20% — the diaphragm is pushed up 4–5 cm by the gravid uterus. Vital capacity is preserved."
• "The combined effect is the apnoeic cliff: she will desaturate in 60–90 seconds of apnoea, versus 3–4 minutes in a non-pregnant adult. Preoxygenation and a difficult-airway plan are essential."[3][5]

Probe 3 — "What about the haematological and renal systems?"

• "Plasma volume rises 40–50%, red cell mass rises 30% — physiological anaemia with Hb nadir 105–110 g/L at 30–32 weeks. Less reserve against haemorrhage."[1]
• "Hypercoagulable state: fibrinogen rises to 4–6 g/L in late pregnancy, clotting factors VII VIII IX X XII rise, PAI-1 rises 2–3 fold. She needs pharmacological thromboprophylaxis on ICU admission."[1]
• "GFR rises 50% from 14 weeks — her creatinine should be 44–62 micromol/L; any value above 75 is AKI. Send U&E now. Renal drug clearance rises 20–65% — antibiotics will need higher doses and shorter intervals."[1][4]

Probe 4 — "How does this change your resuscitation?"

• "Position first — 15-degree left lateral tilt or manual left uterine displacement above 20 weeks. Supine hypotension from aortocaval compression is iatrogenic."[5]
• "Preoxygenate rigorously, ramp the head, two large-bore cannulae, balanced crystalloid cautious in sepsis (avoid overload), high-flow oxygen to target SpO2 above 95%, early bloods including fibrinogen and crossmatch, lactate trend, sepsis bundle activated."[1][5]
• "Continuous CTG above 23 weeks. Multidisciplinary plan with obstetrics, anaesthetics and infectious diseases. HDU admission for ongoing respiratory support and the sepsis bundle."[5]

Probe 5 — "She is frightened and her partner is in the room. How do you communicate?"

This is a scored domain, not a courtesy. Demonstrate it out loud: [1]

• Move to her eye level, use her name, brief plainly: "You have a chest infection and your breathing is harder than we would like because of the pregnancy. We have a team here. We are giving you oxygen and antibiotics, and we are moving you to a higher-dependency area where we can watch you closely. I will keep telling you what is happening."
• Acknowledge fear, keep the partner informed, allocate a staff member to support them, and commit to a debrief afterwards.
• Address the baby explicitly: "We are watching the baby's heart rate continuously; the treatments we are using are safe in pregnancy."[1][5]