ABG Calculations

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Standard Base Excess:

BE_ECF = 0.93(HCO₃ - 24.4 + 14.84(pH -7.4))

From:

BE = (1 - 0.023Hb)(HCO₃ - 24.4 + (2.3Hb + 7.7)(pH - 7.4))

by substituting Hb = 3.1 mM (5 g/dL)

HCO₃ = 0.03 × pCO₂ × 10 ^{(pH - 6.1)}
(Simplified from Henderson-Hasselbalch equation)

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Story: Bedside Stewart

Na−Cl effect = (Na−Cl)−35
Lactate effect = 1−lactate
Albumin effect = 0.25×(42−albumin)
Other Ions effect = SBE−(sum of above effects)

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Schlichtig:

Metabolic change responding to change in PaCO2:
Acute deltaSBE = 0
Chronic deltaSBE = 0.4 x deltaPaCO2 

Respiratory change responding to change in SBE:
Acidosis deltaPaCO2 = 1.0 x deltaSBE 
Alkalosis deltaPaCO2 = 0.6 x deltaSBE

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“Six Rules”:

Rule 1	1 for 10 in Acute Respiratory Acidosis The HCO3^–will increase by 1 mmol/l for every 10mmHg in pCO2 over 40(mmhg) Expected HCO3^–= 24 + [actual pCO2 – 40]/10 Effectively the higher CO2 shifts the equilibrium towards the production of more HCO3^–
Rule 2	4 for 10 in Chronic Respiratory Acidosis The HCO3^– will increase by 4 mmol/l for every 10mmHg in pCO2 over 40 Expected HCO3^–= 24 + 4 [actual pCO2 – 40]/10 Renal compensation occurs over a few days.
Rule 3	2 for 10 in Acute Respiratory Alkalosis The HCO3^–will decrease by 2mmol/l for every 10mmgHg in pCO2 below 40 Expected HCO3^–= 24 -2 (40- Actual pCO2)/10 However you can normally not get a HCO3^–less than 18mmol/l because you cannot have negative values of PCO2.  So if your number here is less than 18, it suggests a co-existing metabolic acidosis.
Rule 4	The 5 for 10 Rule for a Chronic Respiratory Alkalosis HCO3^– will reduce by 5mmol/l for every 10mmHg decrease in pCO2 below 40mmHg. Expected HCO3^– = 24 -5 [40-Actual pCO2]/10 The limit of compensation is about 12-15 mmol/l Your answer can be +/-2
Rule 5	The One and Half plus 8 Rule for a metabolic acidosis Expected PCO2 = 1.5 x [HCO3-] + 8 The limit of PCO2 is about 10 mmHg  Your answer can be +/-2
Rule 6	The point Seven plus Twenty Rule for a metabolic alkalosis Expected pCO2 = 0.7 x [HCO3^–]  + 20 Your answer can be +/-5