A 101-year-old healthy woman with no major medical problems except for osteoarthritis has a fainting episode. She is taken from a nursing home to the emergency department. Vital signs show T 37 C, P 80/min, RR 18/min, and BP 135/90 mm Hg. Her lungs are clear to auscultation and heart rate regular with no murmurs. A chest radiograph shows a normal cardiac shadow, no infiltrates, but there are a few small left upper lobe bright densities, brightness of the aortic arch, and brightness of small muscular arteries in the lower neck region. An arterial blood gas shows pH 7.42 pO₂ 79 mm Hg, pCO₂ 40 mm Hg, and O₂ saturation 90% on room air.

Questions:

7.1 Calculate her A-a (alveolar - arterial) oxygen gradient. What key factors affect this measurement?

The PAO₂ (the amount of oxygen in the air inspired into the alveoli) is calculated as:

PAO₂ = FIO₂ x (PB - PH2O) - Pa-CO₂/R

The FIO₂ is .21 on room air. At a standard sea level environment, the PB is 760 millibars and the PH₂O is 47 mm Hg with fully saturated inspired air at body temperature of 37 C. R is the respiratory quotient (the ratio of CO₂ production to O₂ consumption) and is typically assumed to be 0.8. Thus the equation simplifies to:

PAO₂ = 150 - 1.25 x Pa-CO₂

For a normal Pa-CO₂ of 40 mm Hg, then

PAO₂ = 150 - 1.25 x 40 mm Hg

PAO₂ = 100 mm Hg

The Pa-O₂ and the Pa-CO₂ are typically measured with an arterial blood gas. Ordinarily, the PaO₂ is around 95 to 100 mm Hg. Thus, in young, healthy persons, the A-a gradient is normally <15 mm Hg. In older patients, the A-a gradient may reach 30 mm Hg.

7.2 If the A-a gradient is normal but the PaCO₂ is increased, what is suggested?

Hypoventilation with decreased respirations, or neuromuscular disease, is suggested.

7.3 If the A-a gradient is increased, and the PO₂ decreased, what is the explanation?

Intrinsic pulmonary disease is suggested. However, remember to adjust for age.

7.4 If the A-a gradient is normal, but the PO₂ is decreased, what is the explanation?

You are on top of a very high mountain. On Mt. Everest, without supplemental oxygen, the maximum pO₂ is only 47 mm Hg.