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  pH = power of Hydrogen defined by the Henderson-Hasselbalch equation:

  
  
  pH=pK+log ([HCO3−]/0.03 PCO2)
  
  

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  Acidosis = decrease in arterial pH

  
  
  
  
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    Respiratory acidosis – caused by CO₂ retention. This increases the denominator in the Henderson-Hasselbalch equation and depresses the pH.

    
    
    
    
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      Hypoventilation and ventilatory failure can cause respiratory acidosis.

    
    

  
  
  
  
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    Metabolic acidosis – reduced pH not explained by increased PCO₂. Caused by a primary fall in the numerator [HCO3−] of the Henderson-Hasselbalch equation.

    
    
    
    
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      It is usually associated with an increased anion gap (see later).

    
    

  
  
  
  
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  Alkalosis = increase in arterial pH

  
  
  
  
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    Respiratory alkalosis – the decreased PCO₂ explains the increased pH.

    
    
    
    
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      Seen in alveolar hyperventilation.

    
    

  
  
  
  
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    Metabolic alkalosis – raised pH out of proportion to changes in PCO₂.

    
    
    
    
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      It is associated with hypokalemia, exogenous alkali administration, or volume contraction (e.g., severe prolonged vomiting) when the plasma bicarbonate concentration rises.

    
    

  
  
  
  
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  Anion gap: helps differentiate between acid gain and HCO3−loss.

  
  
  Anion gap=[Na+]−([Cl−]+[HCO3−])Normal gap 8−12 mEq/L
  
  

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  See Figure 22-1.

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  Anion gap acidosis: gap >12 mEq/L

  
  
  
  
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    Caused by decrease in HCO3−balanced by an increase in unmeasured acid ions, not by an increase in chloride.

    
    
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    Causes include salicylates, methanol, paraldehyde, ethylene glycol, lactic acidosis, ketoacidosis (from diabetes or starvation), and uremia.

  
  
  
  
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  Non–anion gap acidosis: gap 8 to 12 mEq/L

  
  
  
  
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    Caused by a decrease inHCO3− balanced by an increase in chloride.

    
    
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    Causes include renal tubular acidosis, carbonic anhydrase inhibitor, and diarrhea.

  
  
  
  
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  Table 22-1 gives normal values for infants and children.

  
  
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  Fencl-Stewart approach to understanding acid-base balance: looks for unmeasured anions, which could contribute to a pH disturbance.

  
  
  BEexplained=H2O+Cl−+albumin
  
  

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  H₂O contribution = 0.3 (Na⁻ + 140)

  
  
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  Cl⁻ contribution = 102 − (Cl⁻ × 140/Na)

  
  
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  Albumin contribution = 3.4 (4.5-albumin)

  
  
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  BE: base excess

*Get BE_measured from blood gas