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General Formulas
The following are commonly used general formulas :
Alveolar gas equation : pA O2 = 150 – pa CO2/0.8
Minute ventilation �=tidal volume x respiratory rate (normal is 4-6 L/min)
Tidal volume =alveolar space + dead space
Anion gap = [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)
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