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Acid-Base Regulation

General

  • Normal pH of normal saline = 5.5
    • Salting Out
    • Exposure of atmospheric CO₂ which diffuses into H₂O i.e. ionizes H₂O
  • Administration of normal saline causes acidosis
    • dilution of buffers such as bicarbonate to form more carbonic acid
    • NaCl results in more hydrogen ions → pushes equation to the left toward carbonic acid

Approach

  • Step 0: Required elements
    • ABG -- pH, PaCO₂, HCO3-
    • Electrolytes -- Na+, Cl-, CO₂ (total = bicarbonate + total dissolved CO₂)
    • Albumin
    • Clinical History
  • Step 1: Establish internal consistency
    • H+ = 24 x (PaCO₂/HCO3-); range +/- 2
    • For pH range 7.25-7.55
      • Change in pH by 0.01 equals change in H+ of 1 nmol/L in the opposite direction
      • pH 7.4 = 40 nmol/L
    • ABG nearly always consistent and if not internally consistent then needs to be repeated
  • Step 2: Evaluate pH
  • Step 3: Determine the primary process
  • Step 4: Calculate Anion Gap
  • Step 5: Adjust Anion Gap for Albumin
    • Albumin = single largest contributor to the normal anion gap
    • When albumin is low -- decrease AG by 3 mmol/L for every 1 g/dL decrease in albumin from normal of 4 g/dL
  • Step 6: Calculate Delta-Delta Gap
  • Step 7: Evaluate Osmolar Gap
  • Step 8: Compensation & Finding Complex Acid-Base Disorders
    • Rules of thumb
      • Tends to normalize pH, never overcorrects
      • Requires normal kidney and lung function
      • Metabolic compensation takes 12-24 hours
    • In simple disorders, PaCO₂ and HCO3- change in the same direction
    • A change in pH = 0.08 in the opposite direction for every 10 mmHg change in PaCO₂ from 40 mmHg
    • Compensation
      • Metabolic acidosis -- expected PaCO₂ = (1.5 x HCO3-) + 8 +/- 2
      • Metabolic alkalosis -- expected PaCO₂ = (0.7 x HCO3-) + 20 +/- 1.5
      • Acute Respiratory Acidosis -- expected HCO3- = 1 decrease in HCO3- for every 10 increase in PaCO₂
      • Acute Respiratory Alkalosis -- expected HCO3- = 2 increase in HCO3- for every 10 decrease in PaCO₂
      • Chronic Respiratory Acidosis -- expected HCO3- = 4 decrease in HCO3- for every 10 increase in PaCO₂
      • Chronic Respiratory Alkalosis -- expected HCO3- = 5 increase in HCO3- for every 10 decrease in PaCO₂
    • Tip-offs to complex acid-base disorders
      • Normal pH but abnormal PaCO₂ and HCO3-
      • PaCO₂ and HCO3- move in opposite directions
      • pH change opposite from primary disorder
    • Finding Mixed Disorders
      • Discern the primary disorder
      • Evaluate compensation
      • Evaluate Delta-Delta Gap

Metabolic Acidosis

Metabolic Alkalosis

Respiratory Acidosis

Respiratory Alkalosis

Mixed Acid-Base Disorders

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