Physiological changes of pregnancy for critical care — structured SAQ (15 marks)

(a) Pregnant ABG references and the interpretation error (4 marks)

One mark per reference range, with the interpretation. Source: Costantine 2014 (Front Pharmacol) and Hegewald 2011. [1][3]

• PaO2 in normal late pregnancy: 101–105 mmHg. Her PaO2 of 76 mmHg is therefore hypoxaemic respiratory failure, not normal — the pregnant baseline is higher than the non-pregnant.[1]
• PaCO2 in normal late pregnancy: 28–31 mmHg. Her PaCO2 of 33 mmHg is therefore mild hypercapnia in the pregnant patient — she has lost 5 mmHg of CO2 clearance versus her own baseline. The ED registrar's call of 'normal' applies the non-pregnant reference and misses this.[1][3]
• pH in normal late pregnancy: 7.40–7.45. Her pH of 7.42 sits at the lower edge of the pregnant reference — but combined with a rising PaCO2 and a normal-to-low bicarbonate, she is moving from her compensated respiratory alkalosis toward a mixed acid-base picture (the lactate of 2.6 confirms a metabolic component).[1]
• Bicarbonate in normal late pregnancy: 18–21 mmol/L. Her bicarbonate of 19 is at the lower edge of normal — but the lactate of 2.6 mmol/L signals early metabolic acidosis, and the pregnant baseline of reduced bicarbonate means she has less buffer reserve to absorb it.[1]

Reasoning mark: "This woman has type 1 respiratory failure with a rising PaCO2 trend and a metabolic acidosis on a background of compensated respiratory alkalosis — she needs high-flow oxygen, escalation to HDU, and active treatment of the pneumonia now. Observation against non-pregnant references will miss the deterioration."[3]

(b) Cardiovascular adaptations with headline numbers (4 marks)

One mark per system adaptation, with the verbatim number. Source: Costantine 2014, Sanghavi 2014. [1][2]

• Cardiac output rises 30–50% (from ~4 to ~6 L/min); 75% of the rise occurs by the end of the first trimester, and CO plateaus at 28–32 weeks. Stroke volume peaks at about 85 mL at 20 weeks; thereafter the heart-rate rise carries the CO.[1][2]
• Heart rate rises throughout pregnancy to 90–100 bpm at rest in the third trimester — a tachycardia threshold above 100 (not 110 as in the non-pregnant) is the appropriate trigger.[1]
• Systemic vascular resistance falls by about 21%, and pulmonary vascular resistance by about 34%, both mediated by progesterone, nitric oxide and prostaglandins.[1]
• Blood pressure dips to a nadir at 20–24 weeks and climbs back to pre-pregnancy values by term — her BP of 102/58 is at the nadir limb of the curve; her booking pressure should be checked before interpreting this as 'normal for pregnancy'.[1]

(c) Haematological and renal adaptations (4 marks)

One mark per adaptation with the implication. Source: Costantine 2014, Conrad 2014. [1][4]

• Plasma volume rises 40–50%, red cell mass rises 30% — the mismatch produces the physiological anaemia of pregnancy (Hb nadir 105–110 g/L at 30–32 weeks). Her reserve against haemorrhage is less than a non-pregnant patient's.[1]
• Fibrinogen rises to 4–6 g/L in late pregnancy, and clotting factors VII, VIII, IX, X, XII and von Willebrand factor rise; PAI-1 rises 2–3 fold, suppressing fibrinolysis. The hypercoagulable state means she needs pharmacological thromboprophylaxis on ICU admission unless contraindicated.[1]
• GFR rises 50% from 14 weeks and renal blood flow rises 50% — a serum creatinine above 75 micromol/L (0.85 mg/dL) is abnormal in pregnancy. Send U&E and creatinine now; do not interpret against the non-pregnant range.[1][4]
• Renal drug clearance rises 20–65% in pregnancy — antibiotics for this pneumonia (e.g., beta-lactams) will need higher doses and shorter dosing intervals; monitor levels where relevant. Lithium clearance doubles in the third trimester.[1]

(d) Modified resuscitation and disposition (3 marks)

One mark per step, with the disposition decision. [1][6]

• Position first: 15-degree left lateral tilt or manual left uterine displacement above 20 weeks (she is 34 weeks). Supine hypotension from aortocaval compression is iatrogenic; correct it before reaching for vasopressors.[6]
• Preoxygenate rigorously (the reduced FRC will desaturate her in 60–90 seconds of apnoea), two large-bore cannulae, balanced crystalloid cautious in this woman with pneumonia and possible evolving sepsis (avoid overload), high-flow oxygen to target SpO2 above 95%, early bloods including fibrinogen and crossmatch, lactate trend, sepsis bundle activated (cultures, antibiotics within the hour).[6]
• Disposition: HDU. She has type 1 respiratory failure with rising PaCO2, metabolic acidosis, lactate 2.6 and sepsis — she meets the criteria for HDU admission for ongoing single-organ respiratory support and the sepsis bundle. Continuous CTG above 23 weeks. Multidisciplinary plan with obstetrics, anaesthetics and infectious diseases.[1][6]