 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
• 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
•  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))
• 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
• 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

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. A Systematic Approach to Blood Gas Interpretation
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