M1Recognition, pregnancy physiology, triage and ICU systems

Clinical overview

The critically ill obstetric patient often declares herself through a trend before she declares herself through collapse. Respiratory rate rises, oxygen requirement appears, behaviour changes, urine output falls, pain becomes disproportionate, bleeding continues or a previously modest tachycardia accelerates. Blood pressure may remain apparently acceptable because pregnancy and youth provide cardiovascular reserve. Recognition therefore depends on knowing the altered baseline, measuring consistently, responding to trajectory and integrating bedside concern with a structured escalation system.

Critical illness is present when threatened or established organ dysfunction requires monitoring or support beyond ordinary ward capability. The diagnosis is functional: a patient needing escalating oxygen, vasoactive support, invasive monitoring, ventilatory assistance, renal replacement, active correction of coagulopathy or very frequent neurological observation is critically ill whether she is physically in a labour ward, high-care area, emergency centre or ICU.

Pregnancy and the puerperium create three diagnostic traps. First, normal physiological changes can resemble disease—mild tachycardia, lower blood pressure, physiological leucocytosis and respiratory alkalosis. Second, those same changes reduce reserve—lower functional residual capacity accelerates desaturation and a lower baseline creatinine makes an apparently normal value abnormal. Third, postpartum physiology can reverse quickly: autotransfusion from uterine contraction, resolution of aortocaval compression and mobilisation of extracellular fluid may expose cardiac or pulmonary failure after delivery.

Core knowledge

Cardiovascular physiology

Cardiac output rises during pregnancy through increases in stroke volume and heart rate, while systemic vascular resistance falls. Plasma volume expands more than red-cell mass, producing physiological haemodilution. Uterine blood flow becomes a large, relatively non-autoregulated component of cardiac output. These changes support placental perfusion but affect critical-care interpretation:

• A higher resting heart rate and lower vascular resistance mean that isolated tachycardia or a modest blood-pressure reduction is not diagnostic, but a new or progressive change is significant.
• Expanded blood volume may delay overt hypotension during haemorrhage. By the time pressure falls, substantial loss and impaired oxygen delivery may already exist.
• The gravid uterus can compress the inferior vena cava and aorta when supine, especially when the fundus is at or above the umbilicus. Manual left uterine displacement is preferred during chest compressions because it permits a firm supine surface.
• Labour increases cardiac output; immediately after delivery, uterine contraction and relief of caval compression return blood centrally. This postpartum load can precipitate pulmonary oedema in severe pre-eclampsia, valve disease or cardiomyopathy.

The shock index—heart rate divided by systolic blood pressure—can reveal a worsening relationship between rate and pressure, but it is a trend and risk signal rather than a stand-alone transfusion or theatre trigger. Pain, fever, medicines and baseline disease affect it. Use it with mental state, skin perfusion, urine output, bleeding, lactate and the response to treatment.

Respiratory physiology

Progesterone increases minute ventilation, mainly through tidal volume. The healthy pregnant patient therefore has a lower PaCO₂, usually about 3.7–4.3 kPa, a modestly reduced bicarbonate and a slightly alkalinaemic or high-normal pH. A PaCO₂ of 5.3 kPa may be normal outside pregnancy but can indicate ventilatory failure in a tired pregnant patient. Always interpret the gas against gestation, clinical work of breathing and the direction of change.

Functional residual capacity falls while oxygen consumption rises. Obesity, the supine position, anaesthesia, pulmonary oedema or pneumonia reduce reserve further. Preoxygenation is less forgiving, apnoea causes rapid desaturation and a small deterioration in lung disease may create a steep rise in oxygen requirement. Oxygen saturation is an important warning signal but does not assess ventilation; a patient receiving oxygen can retain CO₂ while maintaining saturation. Respiratory rate, effort, mental state, capnography and blood gas complete the picture.

Renal, fluid and metabolic physiology

Renal plasma flow and glomerular filtration increase, so serum creatinine normally falls. A creatinine within a nonpregnant laboratory reference interval may represent important renal impairment in pregnancy, particularly if it is rising from baseline. Urine output remains a useful perfusion and kidney signal, but oliguria is not synonymous with fluid deficiency. Pre-eclampsia, acute tubular injury, obstruction, sepsis, haemorrhage and drug toxicity require different treatment.

Pregnancy tends toward mild sodium reduction and lower measured osmolality. Labour adds risks from prolonged hypotonic fluid and the antidiuretic effect of oxytocin. Hyperemesis can produce hypokalaemia, hypochloraemia, alkalosis or starvation ketoacidosis. Diabetic ketoacidosis may occur at lower glucose concentrations than expected outside pregnancy. Glucose, ketones, electrolytes and a gas belong in the early assessment of an unwell patient with diabetes, vomiting, tachypnoea or unexplained acidosis.