Interpret any blood gas in five steps (pH, primary process, compensation, anion gap with albumin correction, delta ratio), work up high and normal anion gap acidosis, metabolic alkalosis and respiratory disorders, recognise mixed disorders, and know when a venous gas is enough.
6 topics
- 01
Five-step blood gas interpretation
1) Acidaemia or alkalaemia? 2) Primary process (look at PaCO2 and HCO3-). 3) Is compensation appropriate? 4) Anion gap (correct for albumin). 5) If HAGMA, the delta ratio reveals hidden disorders. Then the clinical story.
6 stepsAlgorithmNew - 02
High anion gap metabolic acidosis
Lactate, ketones, uraemia and toxins cause most HAGMA. Measure lactate, ketones, creatinine, glucose and serum osmolality; an osmolal gap > 10-20 points to toxic alcohols. Treat the cause; bicarbonate only for selected severe cases.
5 stepsAlgorithm3 dosesNew - 03
Normal anion gap (hyperchloraemic) metabolic acidosis
NAGMA comes from bicarbonate loss (diarrhoea, fistula, ureteric diversion), renal tubular acidosis, or chloride loading (large volumes of 0.9% saline). The urine anion gap separates GI from renal causes.
4 stepsAlgorithmNew - 04
Metabolic alkalosis
Usually from vomiting/NG losses, diuretics or post-hypercapnia. Urine chloride separates chloride-responsive (< 20 mmol/L) from chloride-resistant causes. Treat with chloride, potassium and volume; acetazolamide if overloaded.
5 stepsAlgorithmNew - 05
Respiratory acidosis and alkalosis
Respiratory acidosis means alveolar hypoventilation: think drive (drugs, brain), pump (neuromuscular, chest wall, fatigue) or lungs (COPD, asthma). Respiratory alkalosis is hyperventilation from hypoxaemia, sepsis, pain, PE, liver failure, salicylates or pregnancy.
5 stepsAlgorithmNew - 06
Mixed disorders and when a VBG is enough
Suspect a mixed disorder when compensation is outside the expected range, the delta ratio is abnormal, or the pH is normal with abnormal PaCO2 and HCO3-. A venous gas is adequate for pH, HCO3-, lactate and electrolytes, but not for PaO2 or (often) PaCO2.
4 stepsAlgorithmNew
Drugs in this module
Doses written for this module's context. The same drug may read differently in another module.
Sodium bicarbonate
Alkalinising agent
Sodium bicarbonate
Alkalinising agent
- Indications
- Severe metabolic acidaemia (pH ≤ 7.20) with moderate-severe AKI (BICAR-ICU); hyperkalaemia with acidosis; sodium-channel blocker toxicity; NAGMA with ongoing bicarbonate loss; urinary alkalinisation (salicylate, rhabdomyolysis).
- Adult dose
- Bolus 50-100 mmol IV, or infusion titrated to pH > 7.30 (BICAR-ICU: 4.2% 125-250 mL over 30 min, max 1 L/24 h). Bicarbonate deficit (mmol) ≈ 0.5 × weight (kg) × (target − measured HCO3-); give half over hours and re-check.
- Preparation (India)
- 7.5% (0.9 mmol/mL) and 8.4% (1 mmol/mL) 10 mL and 25 mL ampoules; isotonic 1.4% can be made as 150 mL of 8.4% in 850 mL D5W/water (≈ 150 mmol/L).
- Cautions
- Hypernatraemia, volume overload, hypokalaemia, ionised hypocalcaemia, raised CO2 (needs ventilation to clear), tissue necrosis on extravasation.
Thiamine
Vitamin B1
Thiamine
Vitamin B1
- Indications
- Suspected thiamine deficiency (alcohol use, malnutrition, unexplained lactic acidosis, Wernicke's).
- Adult dose
- 200-500 mg IV three times daily for 3-5 days in suspected Wernicke's; 100-200 mg IV daily for prophylaxis.
- Preparation (India)
- 100 mg/mL ampoules (often 1 mL or 10 mL vials).
- Cautions
- Rare anaphylaxis; give before glucose in at-risk patients.
Fomepizole / ethanol
Alcohol dehydrogenase blocker
Fomepizole / ethanol
Alcohol dehydrogenase blocker
- Indications
- Methanol or ethylene glycol poisoning.
- Adult dose
- Fomepizole 15 mg/kg IV load, then 10 mg/kg every 12 h (increase during dialysis). If unavailable: oral or IV ethanol to blood level 100-150 mg/dL (loading ~0.8 g/kg).
- Preparation (India)
- Fomepizole is rarely available in India; ethanol (oral 40% spirits via NG tube or IV 10% in D5) is used.
- Cautions
- Ethanol: hypoglycaemia, sedation; needs frequent levels. Haemodialysis is definitive for severe poisoning.