Level 1 of 6Core
Acid–base disorders: reading the gas and treating the cause
The core to-do list — diagnose and manage, at a glance
Diagnose— recognise it
- An acid–base abnormality is a sign, never a diagnosis — it is the biochemical shadow of shock, sepsis, ketoacidosis, poisoning, renal failure, ventilatory failure or ischaemic bowel. Kussmaul respiration (deep sighing air hunger): a tachypnoeic patient with a clear chest, a normal chest radiograph and a normal ECG has a metabolic acidosis until proved otherwise.
- Draw the samples before you treat: plasma electrolytes and blood gases simultaneously, before fluid, insulin, alkali or any change in inspired oxygen, and record the FiO₂ at the moment of sampling. Normal values: pH 7.35–7.45, PaCO₂ 4.7–6.0 kPa (35–45 mmHg), HCO₃⁻ 22–26 mmol/L, base excess −2 to +2 mmol/L, lactate upper limit 2 mmol/L; PaO₂ below 8 kPa (60 mmHg) on air defines respiratory failure (type II if PaCO₂ above 6.0 kPa / 45 mmHg — the pH, not the PaCO₂, grades severity).Not available at your setup — Arterial blood gas, Serum electrolytes.
- Apply the 7.40 rule: pH below 7.40 → primary acidosis (respiratory if PaCO₂ exceeds 5.3 kPa / 40 mmHg, metabolic if bicarbonate below 24 mmol/L); above 7.40 → alkalosis. Calculate the anion gap on every acidosis: Na⁺ − (Cl⁻ + HCO₃⁻), normally 12 mmol/L (range 4–12), adding about 2.5 mmol/L for every 10 g/L the albumin is below normal.
- Bedside tests in every case: capillary glucose within the first minute, ketones (capillary β-hydroxybutyrate ≥ 3.0 mmol/L or urine ketones ≥ 2+ defines significant ketosis), a 12-lead ECG, and the lactate — above 2 mmol/L abnormal, above 4 mmol/L demands escalation, above 8 mmol/L extremely high mortality.
- The numbers that mean danger now: pH below 7.20 (especially below 7.10); pH above 7.60; base deficit above 10 mmol/L; a lactate that will not fall with resuscitation (ischaemic bowel until proved otherwise); and a "normal" PaCO₂ in severe metabolic acidosis, which is pre-terminal — the patient should be hyperventilating and is tiring.
- Two patterns to name instantly: a raised anion gap with a raised osmolar gap (normal below 10 mOsm/kg) is methanol or ethylene glycol until proved otherwise; a respiratory alkalosis together with a high-gap metabolic acidosis is salicylate poisoning until proved otherwise.
Manage now— do this, in order
- Airway and oxygen first: secure the airway and target SaO₂ 94–98%, or 88–92% where chronic type II respiratory failure is suspected. Obtain IV access with simultaneous blood sampling and check the capillary glucose within the first minute, treating hypoglycaemia at once.
- Fluid in the shocked adult: crystalloid bolus 500 mL over 15 minutes, reassessing after each; shocked child 10–20 mL/kg over 5–10 minutes (10 mL/kg aliquots in known cardiac disease, severe malnutrition or suspected ketoacidosis). Prefer a balanced crystalloid (compound sodium lactate, Plasma-Lyte) to 0.9% sodium chloride, which contains chloride 154 mmol/L.
- Cover the likely cause: take blood cultures and give empirical antibiotics wherever the cause of a lactic acidosis is not apparent; give thiamine intravenously before any glucose load in alcohol dependence or malnutrition; and sequence potassium before alkali — correcting acidosis drives potassium into cells and can precipitate fatal hypokalaemia.Not available at your setup — Blood culture.
- Treat the disorder you have named: lactic acidosis type A — maximise oxygen delivery and treat the shock, sepsis, haemorrhage or ischaemic bowel; type B — withdraw metformin, nucleoside analogues or β-agonist infusions and replace thiamine, following lactate clearance at about 2 hours and again at 4–6 hours. Diabetic ketoacidosis — fluid, fixed-rate IV insulin and potassium (bicarbonate seldom indicated); alcoholic ketoacidosis — 5% dextrose in 0.9% sodium chloride with parenteral thiamine first.Doctor / Nurse
- Respiratory acidosis: controlled oxygen through a Venturi mask at 24% or 28%, targeting SaO₂ 88–92% or PaO₂ above 8 kPa (60 mmHg); repeat the gas 30–60 minutes after any change in oxygen delivery. Non-invasive ventilation is indicated when the acidosis is not corrected within one hour of optimal medical therapy including controlled oxygen.Doctor / NurseNot available at your setup — Arterial blood gas, Mechanical ventilator. No ventilator on site: give controlled oxygen at 24–28%, repeat the gas at 30–60 minutes and arrange urgent transfer before the patient tires.
- Bicarbonate for the extremes only — pH below 6.9: 1.26% (150 mmol/L) IV over 2–3 hours with pH and electrolyte monitoring (8.4% only in an emergency); paediatric 1 mmol/kg IV slowly and diluted, only with adequate ventilation. Tricyclic poisoning with arrhythmia, broad QRS or QT prolongation: 50 mmol (50 mL of 8.4%) IV repeated, even without acidosis. Recheck the gas, potassium and ionised calcium 30–60 minutes after any bicarbonate.Doctor / NurseNot available at your setup — Arterial blood gas.
- Never give bicarbonate to a patient who cannot increase minute ventilation; never give IV sodium lactate; never mix bicarbonate with calcium or run it down the same line without a saline flush; never add bicarbonate to 0.9% sodium chloride; do not give it in diabetic ketoacidosis at pH 7.1 or above, and never to a child with ketoacidosis.
- Arrange ventilatory support before the patient tires: a rising PaCO₂ with a falling pH, or a falling respiratory rate without improvement, means impending ventilatory failure — and in toxic alcohol poisoning start the antidote (fomepizole, or ethanol where unavailable) without waiting for levels.Doctor / Nurse
Normal arterial values to carry in your head
| Measurement | Normal |
|---|---|
| pH | 7.35–7.45 |
| PaCO₂ | 4.7–6.0 kPa (35–45 mmHg) |
| PaO₂ on air | > 10.6 kPa; failure < 8 kPa (60 mmHg) |
| HCO₃⁻ | 22–26 mmol/L |
| Lactate | ≈ 1; upper limit 2 mmol/L |
| Anion gap | 12 mmol/L (4–12) without K⁺ |
| Osmolar gap | < 10 mOsm/kg |
Refer / escalate
Escalate immediately for ventilatory support or critical care if the pH is below 7.20, the PaCO₂ is rising with a falling pH or the respiratory rate is falling without improvement, the lactate is above 4 mmol/L or will not clear, the base deficit exceeds 10 mmol/L, or there is a raised osmolar gap, suspected salicylate, methanol or ethylene glycol poisoning, or an acidosis needing haemodialysis.
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