Arterial Blood Gas

Review of Normal Values

  • pH: 7.35–7.45
    • Too low: Acidosis – reduces cardiac output, decreases sensitivity to catecholamines.
    • Too high: Alkalosis – impairs oxygen delivery to tissues, disrupts neuro/muscular function.
  • PaCO2: 35–45 mmHg (regulated by RESPIRATORY system)
    • Increased: Respiratory acidosis or compensatory metabolic alkalosis.
    • Decreased: Respiratory alkalosis or compensatory metabolic acidosis.
  • HCO3: 22–26 mEq/L (regulated by KIDNEY – METABOLIC system)

Helpful Tips


Review the governing equation:

CO2 +H2O <–> H2CO3 <–> HCO3 + H+
Lungs <–> buffering <–> kidney

  • Key equation:
    CO2 + H2O ↔ H2CO3 ↔ HCO3 + H+
    • CO2 (carbon dioxide) combines with water (H2O) to form H2CO3 (carbonic acid), which breaks down into HCO3 (bicarbonate) and H+ (hydrogen ions, or acid).
  • Lungs ↔ Buffering ↔ Kidneys
    • Lungs: Regulate the CO2 levels, quickly adjusting the body’s pH by exhaling CO2.
    • Buffering: The bicarbonate system (HCO3) neutralizes excess acids in the blood.
    • Kidneys: More slowly regulate HCO3 and H+ levels, maintaining long-term pH balance.

Abnormal States

Respiratory Acidosis:

  • pH < 7.35, PaCO2 > 45
    • Causes: Pulmonary embolism, airway blockage, pneumothorax, severe COPD exacerbation.
    • CO2 builds up due to hypoventilation (excess acid).
    • Symptoms: CNS depression, fatigue, confusion.

Respiratory Alkalosis:

  • pH > 7.45, PaCO2 < 35
    • Hyperventilation leads to excess CO2 elimination (too little acid).
    • Symptoms: Dizziness, lightheadedness, tingling.
    • Causes: Anxiety, pain, fever, respiratory distress.

Metabolic Acidosis:

  • pH < 7.35, HCO3 < 22
    • Lack of bicarbonate (alkali).
    • Causes: Renal failure, diabetic ketoacidosis, starvation, alcohol intoxication, sepsis, diarrhea in infants.

Metabolic Alkalosis:

  • pH > 7.45, HCO3 > 26
    • Too much base or not enough acid.
    • Excess alkali: Overuse of antacids.
    • Acid loss: Vomiting, gastric suction, diuretic use.

Uncompensated ABGs

Process for Interpreting an ABG

  1. Assess the pH: Is it above or below the normal range? (Alkalosis vs. Acidosis)
  2. Identify the Primary Cause: Determine if the imbalance is metabolic or respiratory by matching the pH disturbance with either CO2 or HCO3.
  3. Check for Compensation: Analyze the opposing value (CO2 or HCO3) to see if the body is trying to restore normal pH.

Examples

Example 1:

  • pH: 7.2, CO2: 54, HCO3: 25
    1. pH Status: Low = Acidosis
    2. Primary Cause: CO2 is elevated (54), indicating excess acid = Respiratory cause
    3. Compensation Check: HCO3 is within normal limits (WNL) = No compensation
    • Conclusion: Uncompensated respiratory acidosis

Example 2:

  • pH: 7.82, CO2: 52, HCO3: 38
    1. pH Status: High = Alkalosis
    2. Primary Cause: HCO3 is elevated (38), indicating excess base = Metabolic cause
    3. Compensation Check: CO2 is also high (52), suggesting the body is retaining CO2 (acid) to compensate.
    • Conclusion: Partially compensated metabolic alkalosis

Example 3:

  • pH: 7.38, CO2: 20, HCO3: 20
    1. pH Status: Normal, but on the lower end = Compensated acidosis
    2. Primary Cause: HCO3 is low (20), meaning not enough base = Metabolic cause
    3. Compensation Check: CO2 is low (20), indicating the body is blowing off CO2 to balance the pH.
    • Conclusion: Fully compensated metabolic acidosis

Example 4:

  • pH: 7.39, CO2: 51, HCO3: 37
    1. pH Status: Normal, but leaning towards low = Compensated acidosis
    2. Primary Cause: CO2 is elevated (51), indicating excess acid = Respiratory cause
    3. Compensation Check: HCO3 is high (37), suggesting the kidneys are increasing bicarbonate to neutralize the acidity.
    • Conclusion: Fully compensated respiratory acidosis

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