16- Acid base changes

• Normal ABG values


  • pH: 7.35-7.45
  • PaO2: (80-100) mmHg
  • PaCO2 (35-45) mmHg
  • HCO3 (22-26) mmol/L
• Respiratory acidosis


  low pH + high PaCO2
• Respiratory alkalosis


  high pH + low PaCO2
• Metabolic acidosis


  low pH + low HCO3
• Metabolic alkalosis


  high pH + high HCO3
• Primary Change


  1. Respiratory Acidosis: ↑ in CO2

  2. Metabolic Acidosis: ↓ in HCO3

  3. Respiratory Alkalosis: ↓ in CO2

  4. Metabolic Alkalosis: ↑in HCO3
• Compensatory Change


  1. Respiratory Acidosis: ↑ in HCO3

  2. Metabolic Acidosis: ↓ in CO2

  3. Respiratory Alkalosis: ↓ in HCO3

  4. Metabolic Alkalosis: ↑in CO2
• Compensation: when a primary acid-base disorder exists, the body attempts to return the pH to normal via the "other half" of acid base metabolism
• Steps to evaluate acid-base status


  1. Look at the pH

  2. Look at PaCO2 and HCO3
• Respiratory acidosis


  Caused by hypoventilation à CO2 retention
• Respiratory alkalosis


  Caused by hyperventilation
• Metabolic acidosis


  Due to gain of H+ or loss of HCO3
• Anion gap


  The difference between cations and anions in serum
• Normal AG: 10 ± 2
• High AG Metabolic acidosis


  There's addition of an Acid (not HCl)
• High AG Metabolic acidosis


  • DKA
  • Lactic acidosis
  • Salicylate (mixed respiratory alkalosis + metabolic acidosis)
  • Alcohol
  • Renal failure
  • Ethylene glycol
• Normal AG Metabolic acidosis


  There's loss of HCO3 or addition of HCl
• Normal AG Metabolic acidosis


  • Due to renal tubular acidosis
  • Due to diarrhea (GI loss)
• Normal AG Metabolic acidosis due to renal tubular acidosis


  • Defect in ability of acid excretion
  • Low Cl in urine
  • Normal volume status
  • +ve urine anion gap
• Normal AG Metabolic acidosis due to diarrhea


  • Intact ability of acid excretion
  • High Cl in urine
  • Low volume status
  • -ve urine anion gap
• Metabolic alkalosis


  Produced by: an excessive gain of HCO3ˉ in the ECF or loss of H+ from the ECF
• Check volume status to identify cause of Metabolic alkalosis


  1. If hypovolemia: check urine chloride

  2. If normovolemia: check renin/aldosterone
• Hypovolemic Metabolic alkalosis


  • Diuretic use (urine chloride > 20 mmol/L)
  • Vomiting (urine chloride < 10 mmol/L)
• Normovolemic Metabolic alkalosis


  • Renal artery stenosis (both renin/aldosterone high)
  • Renin-producing tumor (both renin/aldosterone high)
  • Malignant hypertension (both renin/aldosterone high)
  • Cushing syndrome (both renin/aldosterone low)
  • Exogenous mineralocorticoid use (both renin/aldosterone low)
  • Primary hyperaldosteronism (Conn's) (low renin, high aldosterone)