M11ABG, acid-base disorders, lactate and ketoacidosis

Clinical overview

An arterial blood gas is valuable when it converts physiology into action. It can reveal dangerous oxygenation failure, ventilatory fatigue, a high-anion-gap acidosis, an inadequate compensatory response or a mixed disorder. It cannot tell you the diagnosis without clinical context. The same lactic acidosis can arise from haemorrhage, sepsis, seizure, cardiogenic shock or adrenergic stimulation; the response is to treat the mechanism, not the lactate number.

Pregnancy produces a chronic compensated respiratory alkalosis. PaCO₂ is normally lower, bicarbonate is modestly reduced and pH is slightly alkalinaemic/high-normal. Therefore a PaCO₂ that would be normal outside pregnancy may signal ventilatory failure, and a modestly low bicarbonate may be physiological only if the whole gas and clinical picture fit. Ketoacidosis can develop more rapidly and at lower glucose concentrations in pregnancy; starvation ketosis after vomiting or poor intake can also become severe.

Core knowledge

A reproducible gas sequence

1. Check identity, sample and oxygen/device. Arterial versus venous source, FiO₂, timing and air bubbles matter.
2. Assess oxygenation. Compare PaO₂/saturation with FiO₂, PEEP and clinical context.
3. Read pH. Determine acidemia or alkalemia.
4. Identify the primary process. PaCO₂ drives respiratory change; bicarbonate/base excess reflects the metabolic component.
5. Check compensation. An inappropriate response indicates a mixed disorder.
6. Calculate the anion gap. $\text{AG} = \text{Na} - (\text{Cl} + \text{HCO}_3)$, using values from the same sample.
7. Review lactate, glucose, ketones, electrolytes and renal/liver function.
8. State the diagnosis and action in one sentence.

Do not label a gas “metabolic acidosis” without naming likely cause and compensation. Do not report “compensated” when pH is normal without checking for two opposing primary disorders.

Compensation rules

For metabolic acidosis, Winter’s equation estimates expected PaCO₂ in mmHg:

$\text{expected PaCO}_2 = 1.5 \times \text{HCO}_3 + 8 \pm 2$

Convert to kPa by dividing by 7.5. A measured PaCO₂ higher than expected indicates an additional respiratory acidosis; lower indicates an additional respiratory alkalosis. Pregnancy baseline may shift interpretation, so use the calculation as a check, not an absolute law.

For acute respiratory acidosis/alkalosis, bicarbonate changes only modestly; larger changes suggest chronic renal compensation or a mixed metabolic process. In chronic pregnancy respiratory alkalosis, renal bicarbonate reduction is expected. If a breathless pregnant patient has a rising PaCO₂ and falling pH, respiratory reserve is failing regardless of whether the value has crossed a nonpregnant threshold.

Tap to enlarge

Figure M11.1 — A pregnancy-aware ABG sequence moves from sample and oxygenation through primary process, compensation and anion gap, while Winter’s equation exposes mixed respiratory components.