ALVEOLAR-ARTERIAL OXYGEN GRADIENT (A-a GRADIENT)
PAO2 = Alveolar partial pressure of O2 [mm Hg]
PaO2 = Arterial partial pressure of O2 [mm Hg]
FiO2 = Fraction of inspired O2 (0.21 in room air)
Patm = Atmospheric pressure (about 760 mm Hg at sea level)
PH2O = Partial pressure of water vapor (about 47 mm Hg at 37°C)
PaCO2 = Arterial partial pressure of CO2 [mm Hg]
R = Respiratory quotient, reflecting basal metabolic rate describing amount of CO2 production for a given O2 consumption (ranges from 0.7 to 1; usually 0.8 is used)
Measure of the efficiency of the oxygenation of blood
The normal A-a gradient varies with age. A helpful calculation is [age(in years)/4] + 4
Normal A-a gradient ranges from 5–10 mm Hg when breathing room air and 20–65 mm Hg when on 100% O2. A higher A-gradient reflects impaired transfer of oxygen from the alveoli to the peri-alveolar capillaries
Gradient is affected by V/Q or diffusion abnormalities but unaffected by hyperventilation or hypoventilation.
If PaO2 corrects with supplemental O2, consider asthma or other conditions associated with V/Q mismatch.
If PaO2 does not correct with supplemental O2, consider shunt.
ABSOLUTE NEUTROPHIL COUNT
WBC = White blood cell count [cells/mm3]
PMN = Percentage of polymorphonuclear neutrophils
Bands = Percentage of band neutrophils
[Na+] = Serum concentration of sodium ions [meq/L]
[Cl−] = Serum concentration of chloride ions [meq/L]
 = Serum concentration of bicarbonate ions [meq/L]
Aids in classifying metabolic acidosis
The normal anion gap is 8–12 meq/L.
An elevated anion gap is caused by an increase in unmeasured anions, which may be any of the following:
“MUD PILES”: methanol; uremia; diabetic ketoacidosis; paraldehyde or phenformin; iron, infection, or isoniazid; lactic acidosis; ethanol or ethylene glycol; and s...