A patient is breathing air at sea level and has a respiratory exchange ratio of 1.0. The aerial blood values are: PO2 90 mm Hg ;PCO2 20 mm Hg and pH 7.30. These indicate that the:: (A) Alveolar-aerial PO2 difference exceeds 20 mm Hg., (B) Plasma bicarbonate concentration is low, (C) Patient is hyperventilating, (D) All of the above
Answer is D. The alveolar PO2 is found from the alveolar gas equation: PAO2 = PIO2 - PACO2/R We can assume that the inspired PO2 is the sea level normal value of 149 mm Hg. Therefore, neglecting the small correction factor, the alveolar PO2 is 149 - 20/1 or 129 mm Hg. Thus the alveolar-aerial PO2 diffusion is 129 - 90 = 39 mm Hg. The PCO2 of 20 mm Hg means that the patient is hyperventilating. The combination of the low PCO2 and low pH means that plasma bicarbonate concentration is reduced. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 35. Gas Transpo & pH. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.