**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**

- If pH < 7.35 and pCO2 > 44 mmHg it is

**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))**

- Acidaemia: each 0.1 pH
- 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