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A COPD patient on long term oxygen therapy developed breathlessness since 4 hrs and was brought to ER on examination HR : 110bpm, RR : 40 bpm. Chest : B/L wheeze present.

ABG : pH : 7.6,PCO2: 13 mmHg, PO2 :95 mmHg, Bicarbonate : 13.5 mEq/l, Lactate : 3.4 mmol/l, Chloride : 113 mEq/l

Step 1 : Alkalosis (pH >7.45)

Step 2 : Respiratory Alkalosis (PCO2 is decreased and towards alkalotic side)

Step 3 : Skipped

Step 4: It's a Acute Respiratory Alkalosis

• Expected pH in acute respiratory alkalosis = 7.4 + (0.008 *[40- PaCO2] )

• Expected pH = 7.4 + (0.008 * 40 – 13)

• Expected pH = 7.6

Step 5 : There is no third acid base disorders, as the Expected pH is the same as the actual Ph.

Answer: ABG shows a Acute Compensated Respiratory Alkalosis due to Acute Exacerbation of COPD.

Note : One has to note that since the patient had a episode acute breathlessness even though ABG is showing respiratory alkalosis, is a sign of impending respiratory arrest . In a COPD patient we expected a type 2 failure, but here the PCO2 has been washed out due to tachypnea & lactate of 3.4 shows the fatigue of respiratory muscle. This stresses the importance of clinical history while interpreting ABG.

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