Formulas, General

The following are commonly used general formulas ^[1] :

• Alveolar gas equation ^[2] : p_A O₂ = 150 - p_a CO₂/0.8

• Minute ventilation ^[3] = tidal volume x respiratory rate (normal is 4-6 L/min)

• Tidal volume = alveolar space + dead space

• Anion gap ^[4] = [Na⁺] - ([Cl^-] + [HCO₃⁻]) (normal is 12 ± 2)

• Delta-delta = (anion gap - 12) / (24 - HCO₃)

• Osmolal gap = measured osmoles - calculated osmoles (normal is < 10)

• Osmoles = (2 x [Na⁺]) + (glucose/18) + (BUN/2.8) + (ethanol/4.6)

• Creatinine clearance = (140 – age) x weight (kg) / serum Cr x 72

• Fractional excretion of sodium = (urine Na / plasma Na) / (urine Na / plasma Na)

• Corrected calcium = Ca + (normal albumin - albumin) x 0.8

• Body surface area = Square root of (height [cm] x weight [kg] / 3600)

• Sensitivity = a / (a + c)

• Specificity = d / (d + b)

• Prevalence = (a + c) / (a + b + c + d)

• Positive predictive value = a / (a + b)

• Negative predictive value = c / (c + d)

• Accuracy = (a + d) / (a + b + c + d)