ABG analysis of a patient on ventilator, shows decreased pCO2, normal pO2, pH 7.5, diagnosis is -
First step

Determine whether academia or alkalemia is present by observing the pH.
pH is increased, indicating alkalosis

Second step

Inspect HCO3- /CO2 to determine the cause of academia or alkalemia
pCO2 is decreased indicating respiratory alkalosis.
Resp. acidosis
Metabolic alkalosis
Resp. alkalosis
Metabolic acidosis