CHAPTER 4: Calculations

NOTE

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)

P_atm = 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)

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...