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Level 2 of 6Must-remember

Acid–base disorders: reading the gas and treating the cause

Assess, manage and stay safe — enough on its own

The card — assess, manage, caution

Assessment— look, ask, measure

  • Start from the principle: 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, and your job is to find that disease.
  • History to ask for: diabetes, chronic kidney disease, COPD, alcohol dependence, cirrhosis or hypoalbuminaemia, diuretic or metformin therapy, prolonged vomiting or diarrhoea, and any possibility of self-poisoning.
  • Look for 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.
  • Other features of metabolic acidosis: nausea, vomiting, abdominal pain, weakness, confusion and drowsiness, progressing in severe acidaemia to vasopressor-resistant hypotension, arrhythmia and coma; smell the breath for ketones, ethanol or a solvent.
  • Features of respiratory acidosis (hypercapnia): bounding pulse, warm peripheries, flapping tremor, headache, confusion, agitation and progressive drowsiness ending in CO₂ narcosis.
  • Features of alkalaemia: tetany, paraesthesiae, carpopedal spasm, apathy, confusion, drowsiness, weakness, cramps, arrhythmias and depressed respiration, with positive Trousseau's and Chvostek's signs.
  • Draw the samples before you treat: plasma electrolytes and blood gases simultaneously and before fluid, insulin, alkali or any change in inspired oxygen, and record the FiO₂ at the moment of sampling because a PaO₂ without an FiO₂ is uninterpretable.Not available at your setup — Arterial blood gas, Serum electrolytes.
  • Know the normal values: pH 7.35–7.45 ([H⁺] 35–45 nmol/L, pH 7.40 = 40 nmol/L), PaCO₂ 4.7–6.0 kPa (35–45 mmHg), HCO₃⁻ 22–26 mmol/L, base excess −2 to +2 mmol/L, lactate about 1 mmol/L with an upper limit of 2 mmol/L, and 1 kPa = 7.5 mmHg.Not available at your setup — Arterial blood gas.
  • Classify oxygenation first: PaO₂ on air is normally above 10.6 kPa (80 mmHg); below 8 kPa (60 mmHg) defines respiratory failure — type I with a normal or low PaCO₂, type II with a PaCO₂ above 6.0 kPa (45 mmHg) — and in type II failure the pH, not the PaCO₂, grades severity.Not available at your setup — Arterial blood gas.
  • Then apply the 7.40 rule: if the pH is below 7.40 the primary process is an acidosis (respiratory if PaCO₂ exceeds 5.3 kPa / 40 mmHg, metabolic if bicarbonate is below 24 mmol/L); if above 7.40 it is an alkalosis (respiratory if PaCO₂ is below 5.3 kPa, metabolic if bicarbonate is above 24 mmol/L).Not available at your setup — Arterial blood gas.
  • Calculate the anion gap on every acidosis: Na⁺ − (Cl⁻ + HCO₃⁻), normally 12 mmol/L (range 4–12) without potassium and 12–16 mmol/L with it, and add about 2.5 mmol/L for every 10 g/L (1 g/dL) that the albumin is below normal.Not available at your setup — Serum electrolytes, Liver function tests.
  • Measure the lactate: above 2 mmol/L is abnormal, above 2.4 mmol/L marks severity, above 4 mmol/L demands escalation and above 8 mmol/L carries extremely high mortality; established lactic acidosis is at least 4–5 mmol/L.Not available at your setup — Arterial blood gas.
  • The numbers that mean danger now: pH below 7.20 and especially below 7.10; pH above 7.60; a base deficit above 10 mmol/L; a lactate that will not fall with resuscitation (ischaemic bowel until proved otherwise); and a "normal" PaCO₂ in a severe metabolic acidosis, which is pre-terminal.Not available at your setup — Arterial blood gas.
  • 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), and a 12-lead ECG.
  • 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, and a respiratory alkalosis together with a high-gap metabolic acidosis is salicylate poisoning until proved otherwise.Not available at your setup — Serum electrolytes.
  • Venous or arterial: a venous gas is sufficient to judge the extent of an acidosis and to diagnose and stage diabetic ketoacidosis, but if there is hypoxaemia an arterial sample must be obtained, because venous PCO₂ runs higher and venous pH lower and respiratory failure cannot be diagnosed on venous blood.Not available at your setup — Arterial blood gas.

Management— 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.
  • Access and glucose: obtain intravenous access with simultaneous blood sampling, and check the capillary glucose within the first minute, treating hypoglycaemia at once.Doctor / Nurse
  • Fluid in the shocked adult: a crystalloid bolus of 500 mL over 15 minutes, reassessing after each bolus; prefer a balanced crystalloid (compound sodium lactate, Plasma-Lyte) to 0.9% sodium chloride, which contains chloride 154 mmol/L.Doctor / Nurse
  • Fluid in the shocked child: 10–20 mL/kg over 5–10 minutes, using 10 mL/kg aliquots in known cardiac disease, severe malnutrition or suspected ketoacidosis.Doctor / Nurse
  • Sepsis cover: take blood cultures and give empirical antibiotics wherever the cause of a lactic acidosis is not apparent.Doctor / NurseNot available at your setup — Blood culture.
  • Thiamine intravenously before any glucose load in alcohol dependence or malnutrition.Doctor / Nurse
  • Sequence potassium before alkali: acidosis drives potassium out of cells, so correcting the acidosis reverses this abruptly — where acidosis and hypokalaemia coexist, potassium repletion takes precedence.Not available at your setup — Serum electrolytes.
  • Lactic acidosis: in type A maximise oxygen delivery by protecting the airway and improving breathing and circulation and treat the shock, sepsis, haemorrhage or ischaemic bowel; in type B withdraw the offending drug (metformin, nucleoside analogues, β-agonist infusions), treat hepatic failure, malaria or seizures and replace thiamine; follow lactate clearance, repeating at about 2 hours and again at 4–6 hours, and consider cyanide after exposure to burning vinyl or plastic.Doctor / NurseNot available at your setup — Arterial blood gas.
  • Ketoacidoses: diabetic ketoacidosis needs fluid, fixed-rate intravenous insulin and potassium replacement, and bicarbonate is seldom indicated; alcoholic ketoacidosis is treated with 5% dextrose in 0.9% sodium chloride with parenteral thiamine first; starvation ketosis resolves with feeding.Doctor / NurseNot available at your setup — Infusion pump.
  • Normal-gap and uraemic acidosis genuinely respond to alkali: give oral sodium bicarbonate, Shohl's solution or intravenous bicarbonate to raise the plasma bicarbonate slowly to a target of 22 mmol/L, considering replacement once it falls below 20–23 mmol/L; for diarrhoeal loss use an isotonic solution of 150 mmol sodium bicarbonate in 1 L of 5% dextrose.Doctor / NurseNot available at your setup — Serum electrolytes.
  • Metabolic alkalosis: treat the chloride-responsive form with intravenous 0.9% sodium chloride plus potassium supplements, always replacing potassium because its depletion maintains the alkalosis; stop or reduce diuretics and nasogastric aspiration, consider a proton pump inhibitor if drainage must continue, and use acetazolamide in the volume-overloaded patient.Doctor / NurseNot available at your setup — Serum electrolytes.
  • Respiratory acidosis: give controlled oxygen through a Venturi mask at 24% or 28%, targeting SaO₂ 88–92% or a PaO₂ above 8 kPa (60 mmHg) without worsening the acidosis, aiming for a safe PaO₂ above 7.0 kPa (52 mmHg); repeat the gas 30–60 minutes after any change in oxygen delivery.Not available at your setup — Arterial blood gas.
  • Escalate ventilation on the clock: non-invasive ventilation is indicated when the respiratory acidosis is not corrected within one hour of optimal medical therapy including controlled oxygen, and invasive ventilation where the acidosis deteriorates despite optimal NIV, the interface cannot be tolerated or the airway is unprotected.Doctor / NurseNot available at your setup — Mechanical ventilator, Endotracheal intubation kit.
  • Bicarbonate for the extremes only — pH below 6.9: 1.26% (150 mmol/L) intravenously over 2–3 hours with pH and electrolyte monitoring, using 8.4% only in an emergency because it causes volume expansion; the paediatric dose is 1 mmol/kg intravenously slowly and diluted, only with adequate ventilation.Doctor / NurseNot available at your setup — Serum electrolytes, Mechanical ventilator.
  • Bicarbonate for specific poisonings: methanol or ethylene glycol — 250 mL of 1.26% intravenously repeated as necessary (child 1 mmol/kg repeated); metformin-associated lactic acidosis — 250 mL of 1.26% or 50 mL of 8.4%; tricyclic antidepressant poisoning with arrhythmia, broad QRS or QT prolongation even without acidosis — 50 mmol (50 mL of 8.4%) intravenously repeated; cardiac arrest with persisting acidosis after defibrillation and adequate ventilation — 1 mmol/kg.Doctor / NurseNot available at your setup — Defibrillator, Mechanical ventilator.
  • Start the antidote without waiting for levels: in toxic alcohol poisoning give an alcohol dehydrogenase blocker — fomepizole, or ethanol where fomepizole is unavailable — and give desferrioxamine immediately in iron poisoning with coma, shock, acidosis or haemolysis.Doctor / Nurse
  • Recheck the gas, the potassium and the ionised calcium 30–60 minutes after any bicarbonate, and recheck the gas after every change in oxygen or ventilation.Not available at your setup — Arterial blood gas, Serum electrolytes.
  • Arrange ventilatory support before the patient tires: a patient holding a pH of 7.15 with a respiratory rate of 40 and a PaCO₂ of 2.0 kPa is doing enormous work, and when they tire the PaCO₂ rises and the pH falls precipitously.Doctor / NurseNot available at your setup — Mechanical ventilator.

Caution— what harms

  • Never give bicarbonate to a patient who cannot increase minute ventilation — bicarbonate raises CO₂ production and will correct the acidosis only if the extra CO₂ can be cleared, and the CO₂ generated enters cells more readily than bicarbonate, worsening intracellular acidosis.
  • Never give intravenous sodium lactate, never mix bicarbonate with calcium or run it down the same line without a saline flush, and never add bicarbonate to 0.9% sodium chloride, which produces a markedly hypertonic solution.
  • Do not give bicarbonate in diabetic ketoacidosis at a pH of 7.1 or above, and never give it to a child with ketoacidosis — it has been implicated in cerebral oedema.
  • A "normal" PaCO₂ in severe metabolic acidosis is pre-terminal: the patient should be hyperventilating and is tiring; a rising PaCO₂ with a falling pH, or a falling respiratory rate without improvement, means impending ventilatory failure.Not available at your setup — Arterial blood gas.
  • Induction of anaesthesia in a profoundly acidotic patient is hazardous, because the apnoeic period removes the respiratory compensation that is keeping the pH survivable.Doctor / NurseNot available at your setup — Endotracheal intubation kit.
  • Compensation never overshoots and never fully corrects the pH: a pH restored exactly to 7.40 with a grossly abnormal bicarbonate and PCO₂ is a mixed disorder, not successful compensation, and a normal pH, PaCO₂ and bicarbonate together do not exclude a disorder.Not available at your setup — Arterial blood gas.
  • An uncorrected anion gap is falsely reassuring in exactly the septic, cirrhotic and malnourished patients most likely to have a disorder — every 10 g/L fall in albumin lowers the measured gap by about 2.5 mmol/L.Not available at your setup — Serum electrolytes, Liver function tests.
  • Urine ketone sticks miss the sickest patients: nitroprusside sticks detect acetoacetate and, less well, acetone, but not β-hydroxybutyrate, so they under-call severity and paradoxically appear to worsen during recovery — track recovery on blood ketones.
  • Sampling errors invent disease: excess liquid heparin falsely lowers bicarbonate and PCO₂, air bubbles raise PaO₂ and lower PaCO₂, delay in analysis lowers pH and PaO₂ and raises lactate, and gases should not be corrected for temperature.Not available at your setup — Arterial blood gas.
  • Large-volume 0.9% sodium chloride produces an iatrogenic hyperchloraemic acidosis: a closing anion gap with persistent acidaemia and a rising chloride means the acidosis you are now measuring is the saline you gave, not the original disease.Not available at your setup — Serum electrolytes.
  • Do not rebreathe into a bag for presumed hyperventilation — it is dangerous when the diagnosis is wrong, and hypoxaemia excludes the diagnosis; hyperventilation syndrome may be diagnosed only with hypocapnia of 15–30 mmHg, no hypoxaemia and normal investigation.Not available at your setup — Arterial blood gas.
  • Uncontrolled high-flow oxygen in chronic type II respiratory failure worsens hypercapnia and acidosis — but withholding oxygen from a hypoxaemic patient is more dangerous still, because hypoxia kills faster than hypercapnia.
  • Correct chronic hypercapnia gradually towards the patient's own baseline: rapid correction provokes arrhythmias, reduced cerebral perfusion and seizures, and in ARDS or severe obstructive disease permissive hypercapnia is deliberate and should not be "corrected".Not available at your setup — Mechanical ventilator.
  • Watch the potassium in both directions: acidosis raises the serum potassium and hyperkalaemia may itself be fatal, while correcting the acidosis drives potassium intracellularly and can precipitate a dangerous hypokalaemia.Not available at your setup — Serum electrolytes.

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, if the lactate is above 4 mmol/L or will not clear, if the base deficit exceeds 10 mmol/L, or if there is a raised osmolar gap, suspected salicylate, methanol or ethylene glycol poisoning, or an acidosis needing haemodialysis.

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