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*Note: Commonly used conversions are listed separately and may be referred to as needed in the “Conversions of Units” section of this chapter.


A - a  gradient = PAO 2 PaO 2

PAO 2 = F i O 2 ( P atm P H 2 O ) ( PaCO 2 / R )

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)

P H 2 O = 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(years)+10]/4

  • Normal A-a gradient ranges from 7 to 14 mm Hg when breathing room air; 20–65 mm Hg when on 100% O2. As the gradient value increases, it is reflective of a worsening condition

  • Gradient is affected by ventilation/perfusion (V/Q) or diffusion abnormalities but unaffected by hyper- 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


ANC = WBC × ( Polys / 100 + Bands / 100 )

WBC = White blood cell count [cells/mm3]

Polys = Percentage of polymorphonuclear neutrophils

Bands = Percentage of band forms

  • Neutropenia generally defined as an ANC less than 1500 cells/mm3

  • Severe neutropenia (ANC <500 cells/mm3) is associated with a high risk of infection


Serum  AG = [ Na + ] ( [ Cl ] + [ HCO 3 ] )

AG = Anion Gap

[Na+] = Serum concentration of sodium ions [mEq/L]

[Cl] = Serum concentration of chloride ions [mEq/L]

[ HCO 3 ] = Serum concentration of bicarbonate ions [mEq/L]

  • Aids in classifying metabolic acidosis

  • Normal anion gap is 8–12 mEq/L

  • Elevated anion gap is caused by an increase in unmeasured anions which may be any of the following:

    • “MUD PILES”: M...

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