## NOTE

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

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

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

## ABSOLUTE NEUTROPHIL COUNT (ANC)

$ANC=WBC×(Polys/100+Bands/100)$

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

## ANION GAP (SERUM)

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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