These results are from the arterial blood gas report of a 41-year-old female ventilated in ICU for three weeks with H1N1 influenza and ARDS

Parameter Patient value     Normal range
FiO2                       0.6  
pH                               7.5* 7.35 – 7.45 
PO2      79.0 mmHg (10.5 kPa)  
PCO2               48.0* mmHg (6.3 kPa)         35 – 45 (4.6 – 6.0)
HCO3                                   36* mmol/L   22 – 27 
Base excess              12* -2.0 – +2.0
Sodium                      138 mmol/L 135 – 145
Potassium                 5.0 mmol/L 3.5 – 5.0
Chloride                     97 mmol/L 95 – 105                 

a) Describe this acid-base picture

b) What is the likely cause of the acid-base disturbance?

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College Answer

Metabolic alkalosis
A-a DO2 295

Resolution of respiratory acidosis with delayed correction of metabolic compensation
Diuretic therapy


This is another one of these "interpret an ABG" questions.

Widened A-a gradient? yes there is; by the standard equation,

(713 x 0.6) – (48 x 1.25) – (79) =288.8

On FiO2 of 60%, the PaO2 should be about 378.

There is also a metabolic alkalosis with normal chloride and normal potassium. The hint is that this patient has been ventilated for 3 weeks. ARDS ventilation typically involves "permissive hypercapnea", which leads to a gradual renal compensation, with the retention of bicarbonate. As the respiratory acidosis resolves, an alkalaemia develops because the increase in the rate of bicarbonate excretion is delayed.


For all your acid-base needs, visit

Williamson JC. Acid-base disorders: classification and management strategies. Am Fam Physician 1995 Aug; 52(2) 584-90.