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Acid-Base Disturbances           See also   Hemodynamic Data

The Henderson-Hasselbalch Equation:   pH = 6.1 + log [HCO3] / [0.0301 x pCO2]                                                                 [H+] = 24 x [pCO2/ HCO3]

pH  =  6.90   7.00   7.10   7.20   7.30   7.40   7.50   7.60
[H+]=  125    100    80      64      51      40      32       25

                              Primary change           pH          Compensatory response
Metab. acidosis     decreased HCO3      decreased     decreased pCO2
Metab. alkalosis     increased HCO3       increased      increased pCO2
Resp. acidosis        increased pCO2       decreased      increased HCO3
Resp. alkalosis       decreased pCO2       increased      decreased HCO3

Rules of thumb  for bedside interpretation of acid-base disorders:
For metabolic acidosis

• pCO2 should fall by 1 -1.5x the fall in plasma HCO3;  pCO2 should rarely be < 20 mmHg.
• Bicarbonate deficit (mEq/L) = [0.5 x BW(kg)] x (24 - HCO3)

For metabolic alkalosis

• pCO2 should rise by 0.25 -1 x the rise in plasma HCO3

For acute respiratory acidosis

• Plasma HCO3 should rise by about 1 mEq/L for each increment of 10 mmHg in pCO2  ( +/- 3  mEq/L)
• Acute change pH/pCO2 = 0.008

For chronic respiratory acidosis

• Plasma HCO3 should rise by about 4 mEq/L for each increment of 10 mmHg in pCO2 (+/- 4 mEq/L)
• Chronic change pH/pCO2 = 0.003

For acute respiratory alkalosis

• Plasma HCO3 should fall by about 1-3 mEq/L for each decrement of 10 mmHG in pCO2, but usually not to <18 mEq/L
• Acute change pH/pCO2 = 0.008

For chronic respiratory alkalosis

• Plasma HCO3 should fall by about 2-5 mEq/L for each decrement of 10 mmHg in pCO2, but usually not to <14 mEq/L
• Chronic change pH/pCO2 = 0.003

Anion gap = Na - [CL + HCO3]    (Normally =  12 +/- 4 mEq/L)

Normal ABG (arterial blood gases):  
pH 7. 36-7.44, pCO2  35-45 mmHg, HCO3  24-32 mEq/L, paO2 75-100 mmHg, arterial O2 sat SaO2 95-100%,
mixed venous pvO2 38-42 mmHg,

Alveolar-arterial O2 tension difference/gradient  (Normally 10-15 mmHg with room air):
       P(A-a) O2 =  (713 x FiO2  -  1.25 x pCO2)  -  PaO2

• For Room air FiO2 = 0.21,      remember 713 x 0.21 = 150     (Normal A-a gradient 10-15 mmHg)
• For 100% O2 FiO2 = 1.0  (Normal A-a gradient 100-150 mmHg);
  for each 5% AV shunt, decrease A-a gradient about 100 mmHg below 600 mmHg at FiO2= 1.0

REF:
Washington Manual 28th ed. 1995