Which of the following would create the highest P50?

a. pH = 7.40 c. PCO2 = 40 mm Hg
b. pH = 7.50 d. PCO2 = 60 mm Hg

ANS: D
A pH of 7.40 creates a normal oxyhemoglobin dissociation curve with a P50 of 27 mm Hg. A pH of 7.50 is indicative of alkalosis and will cause a left shift in the curve, lowering the P50 below 27 mm Hg. A PCO2 of 40 mm Hg is normal and will not cause a change in the P50 from the normal 27 mm Hg. However, a PCO2 of 60 mm Hg is acidotic and will shift the curve to the right, caus-ing the P50 to be greater than normal.