What ABG finding is expected in respiratory alkalosis?

What ABG finding is expected in respiratory alkalosis?

• A. PaCO2 < 35 mm Hg and pH > 7.45
• B. PaCO2 > 45 mm Hg and pH < 7.35
• C. PaCO2 40 mm Hg and pH 7.40
• D. HCO3- < 22 mEq/L

Answer: (D)

In respiratory alkalosis the primary abnormality is a low PaCO2 from hyperventilation, which raises the blood pH. The body then compensates by the kidneys excreting bicarbonate, lowering HCO3- to help bring the pH back toward normal. So, while the immediate, direct sign of respiratory alkalosis is a low PaCO2 with a high pH, the compensatory response shows up as a decreased bicarbonate level. An ABG reflecting compensated respiratory alkalosis would commonly have HCO3- below the normal range (less than 22 mEq/L), with the PaCO2 still low and the pH elevated but moving toward normal depending on the degree of compensation. The exact drop in bicarbonate depends on whether the alkalosis is acute or chronic, but a bicarbonate below 22 mEq/L is a classic compensatory finding.