Lab Case 173 Interpretation

A 45 year old female presents to ED, after being found unconscious at home after being discharged form a MHU.

Primary process – respiratory alkalosis, pH 7.55 and pCO2 12 mmHg

Compensation – Expected HCO3 = 24-2(40-PCO2/10) =18.4. Actual HCO3 10mmol/l, therefore lower than expected

AG = Na-(Cl+HCO3) =18 therefore HAGMA

Delta Gap =(AG-12)/(24-HCO3) = 0.4

Expected A-a gradient –

Expected PAO2 =(713xFiO2)- (PCO2x1.25)= 270. Measured PaO2 =140mmHg

A-a gradient =130mmHg

This ABG shows a respiratory alkalosis and a combined HAGMA and NAGMA. There is a raised A-a gradient. The lactate is slightly raised with a normal BSL and Cr. The chloride is raised.

The respiratory alkalosis and HAGMA in this clinical context is suggestive of a OD, most commonly salicylates, but stimulants like theophylline and amphetamines need to be considered.

Recall other causes of respiratory alkalosis from Lab case 166

As this patient has raised A-a gradient, pulmonary causes of respiratory alkalosis need to be sought eg PE, pneumonia and sepsis. If the patient is intubated they could be hyperventilated.

The high chloride could be falsely raised in the setting of salicylate OD as some ABG machines read salicylates as chloride.

Further investigations – screening – ECG, BSL and paracetamol level

-Specific – Salicylate levels, ABG’s to monitor acidosis, potassium levels as they can drop and BSL’s as these can increase or decrease

Management – RRSIDEAD approach

  • If intubating the patient it is important to maintain hyperventilation to avoid the acidosis getting worse
  • Activated charcoal up to 8 hours post large ingestions and repeated doses if levels continue to rise. It is important to secure the airway first.
  • Enhanced elimination – urinary alkalinisation and haemodialysis as indicated

 

 

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