1. What is the pH?
- Identifies the primary acid base disturbance
- Acidaemia exists if pH <7.35
- Alkalaemia exists if pH > 7.45
- If pH is between 7.35 and 7.45 then there is either:
- No acid base disorder
- A compensated disorder (can never overcompensate)
- A mixed disorder
2. Is the primary process respiratory, metabolic or both?
- Check pCO2 and HCO3
- If pH < 7.35 and pCO2 > 44 mmHg it is respiratory acidosis
- If pH > 7.45 and pCO2 < 40 mmHg it is respiratory alkalosis
- If pH < 7.35 and HCO3 < 22 mmol/L it is metabolic acidosis
- If pH > 7.45 and HCO3 > 26 mmol/L it is metabolic alkalosis
3. Is there compensation?
- Rules for respiratory acid-base disorders:
- Acute respiratory acidosis: 1 for 10 rule
- Expected HCO3 = 24 + {(Measured pCO2 – 40)/10}
- HCO3 increases by 1 mmol/L for every 10 mmHg elevation in pCO2 above 40 mmHg
- In general, HCO3 cannot rise above 30 mmol/L
- Chronic respiratory acidosis: 4 for 10 rule
- Expected HCO3 = 24 + 4 x {(Measured pCO2 – 40)/10}
- HCO3 increases by 4 mmol/L for every 10 mmHg elevation in pCO2 above 40 mmHg
- In general, HCO3 may rise above 55 mmol/L
- Acute respiratory alkalosis: 2 for 10 rule
- Expected HCO3 = 24 – 2 x {40 – Measured pCO2)/10}
- HCO3 decreases by 2 mmol/L for every 10 mmHg decrease in pCO2 below 40 mmHg
- In general, pCO2 cannot fall below 16 mmHg
- Chronic respiratory alkalosis: 5 for 10 rule
- Expected HCO3 = 24 – 5 x {(40 – Measured pCO2)/10} (±2)
- HCO3 decreases by 5 mmol/L for every 10 mmHg decrease in pCO2 below 40 mmHg
- In general, pCO2 cannot fall below 16 mmHg
- Acute respiratory acidosis: 1 for 10 rule
- Rules for metabolic acid-base disorders:
- Metabolic acidosis: 1.5 plus 8 rule
- Expected pCO2 = 1.5 x HCO3 + 8 (±2)
- Winter’s formula
- In general, pCO2 cannot fall below 10 mmHg
- Metabolic alkalosis: 0.7 plus 20 rule
- Expected pCO2 = 0.7 x HCO3 + 20 (±5)
- In general, pCO2 cannot rise above 55 mmHg
- Metabolic acidosis: 1.5 plus 8 rule
- Over-compensation never occurs!!
4. Are there other clues to diagnosis?
- Metabolic acid-base disorders
- Anion gap = Na – (Cl + HCO3)
- Normal 12 ± 4
- In hypoalbuminaemia: add 2.5 per 10 g/L decrease in albumin below 40 g/L
- Corrected AG = AG + (40 – albumin)/4
- Expected potassium:
- Acidaemia: each 0.1 pH fall below 7.4 expect K+ to rise by 0.5 mmol (above 5 mmol/L)
- Expected K = 5 + (5 × (7.4 – pH))
- Alkalaemia: each 0.1 pH rise below 7.4 expect K+ to fall by 0.5 mmol (below 5 mmol/L)
- Expected K = 5 – (5 × (pH – 7.4))
- Acidaemia: each 0.1 pH fall below 7.4 expect K+ to rise by 0.5 mmol (above 5 mmol/L)
- In hyperglycaemia:
- Corrected Na = Na + ((glucose – 5) / 3)
- Delta Gap = (Anion gap – 12) ÷ (24 – HCO3) – Should be a 1:1 relationship between increase in anion gap above 12 and decrease in bicarbonate below 24
- Delta gap < 0.4 suggests a NAGMA
- Delta gap 0.4 – 0.8 suggests a coexisting HAGMA and a NAGMA
- Delta gap 0.8 – 2.0 is usual for a HAGMA
- Delta gap > 2 suggests a coexisting metabolic alkalosis, OR a pre-existing compensated respiratory acidosis
- Calculated Osmolality = (2 x Na) + urea + glucose + ethanol – Convert all units to mmol/L
- Ethanol (mmol/L) = %ethanol x 218
- Osmolar gap = Measured osmolality – calculated osmolality
- Normal osmolar gap < 10
- Lactate, Glucose, Electrolyte disturbances
- May be associated with predictable acid-base disorders
- Anion gap = Na – (Cl + HCO3)
- Respiratory acid-base disorders
- Expected PAO2 = (713 x FiO2) – (pCO2 x 1.25)
- At sea level, breathing room air (713 x 0.21) = 150
- Therefore PAO2 = 150 – (pCO2 x 1.25)
- A-a gradient = PAO2 – PaO2
- Where PAO2 equals the calculated alveolar pO2 and PaO2 equals the measured arterial pO2
- Normal = Age/4 + 4
- Expected PAO2 = (713 x FiO2) – (pCO2 x 1.25)
5. Formulate and confirm the acid – base diagnosis.
- Consider evidence from history, examination and investigations to formulate a complete acid – base diagnosis.
- In some cases, further biochemical testing may be useful (eg drug levels, urine ketones) for confirmation of the diagnosis.
