1. ABG interpretation

2. Oxygenation Check the FiO2 Know your A-a gradient – A-a Gradient (at sea level) = PaO2 - FIO2 x (760 - 47) - (PaCO2/0.8) – Can be shortened to 150-(PaCO2/0.8) for room air at sea level

3. Acidaemia or alkalaemia This will tell you the primary pathology. – The body does not overcorrect pH – Beware that more than one problem can happen at once

4. Metabolic or respiratory Metabolic – pH, HCO3, PCO2 all move in the same direction Respiratory – pH moves in opposite direction from HCO3 and PCO2 Base excess measures metabolic component of acid/base. If negative then metabolic acidosis, if positive then metabolic alkalosis (normal +/-3)

5. Gaps – Anion gap (Na+K) – (Cl +HCO3) 3-12 is normal – Osmolar gap Measured – (2[Na]+[glucose]+[urea]+1.25x[ETOH]) Normal <10 Only do if given measured osmolality Positive in toxic alcohol ingestion as well as sorbitol and mannitol

6. Delta gap/ratio Ratio of the increase in anion gap to decrease in HCO3 < 0.4 - Hyperchloraemic normal anion gap acidosis 0.4 to 0.8 - Combined high AG and normal AG acidosis 1 - Common in DKA due to urinary ketone loss 1 to 2 - Typical pattern in high anion gap metabolic acidosis > 2 Check for either a co-existing Metabolic Alkalosis (which would elevate [HCO3]) or a co-existing Chronic Respiratory Acidosis (which results in compensatory elevation of [HCO3])

7. Compensation Metabolic acidosis – PaCO2 decreases 1.2mmHg per 1meq/L HCO3 fall – PaCO2=1.5x[HCO3] +8+/-2 Metabolic Alkalosis – PaCO2 increases 6 mmHg per 10 meq/L bicarbonate rise PaCO2 – PaCO2=0.7[HCO3+20+/-5 Acute Respiratory AcidosisRespiratory Acidosis – Bicarbonate increases 1 meq/L per 10 mmHg PaCO2 risePaCO2 Chronic Respiratory AcidosisRespiratory Acidosis – Bicarbonate increases 4 meq/L per 10 mmHg PaCO2 risePaCO2 Acute Respiratory AlkalosisRespiratory Alkalosis – Bicarbonate decreases 2 meq/L per 10 mmHg PaCO2 fallPaCO2 Chronic Respiratory AlkalosisRespiratory Alkalosis – Bicarbonate decreases 4 meq/L per 10 mmHg PaCO2 fallPaCO2

8. Other things Potassium goes up or down by 0.5 for every.1 pH in away from 7.4. lower pH=higher K – This is a real level: pH of 7.0 and K of 4 is not at risk but correct to pH7.4 and K will be 2 – then patient will be at risk Sodium goes down with hyperglycaemia: measured[Na] + {([glucose]-5) /3} = real Na. The measured level is not real