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Updated: Dec 29 2021

Arterial Blood Gases

  • Snapshot
    • A 62-year-old man presents to the emergency department for increased confusion and a productive cough. He was found on the couch by his children and was only oriented to person but not place or time. Medical history is significant for chronic obstructive pulmonary disease and hypertension. He smokes approximately 1.5 packs of cigarettes a day for the past 35 years. His temperature is 101.4°F (38.5°C), blood pressure is 157/98 mmHg, pulse is 110/min, respirations are 17/min, and oxygen saturation is 83% on room air. An arterial blood gas is performed and demonstrates a pH of 7.21, PaCO2 of 99 mmHg, and PaO2 of 51 mmHg. (Chronic obstructive pulmonary disease exacerbation)
  • Introduction
    • Definition
      • an arterial blood test that effectively determines the patient's pH, PaCO2, PaO2, and HCO3
        • an arterial blood gas (ABG) should be performed in the following circumstances
          • determining and monitoring a patient's acid-base status
          • assessing treatment response (e.g., treated diabetic ketoacidosis with insulin)
    • Procedure
      • blood can be obtained via
        • a percutaneous needle to the artery (e.g., radial artery, femoral artery, and brachial artery)
        • an indwelling arterial catheter
    • Interpretation
      • normal values
        • pH: 7.35-7.45
        • PaCO2: 35-45 mmHg
        • HCO3: 21-27 mEq/L
        • PaO2: > 80 mmHg
        • SaO2 > 95%
      • respiratory acidosis
        • characterized by a PaCO2 > 45 mmHg and a pH < 7.35
          • typically due to alveolar hypoventilation
            • the patient retains CO2 because the patient is not adequately expelling it
        • causes
          • sedative overdose (e.g., opiates and benzodiazepines)
          • obesity hypoventilation
          • brainstem disease
          • chronic obstructive pulmonary disease
          • amyotrophic lateral sclerosis
          • kyphoscoliosis
          • asthma
          • heroin overdose
      • respiratory alkalosis
        • characterized by a PaCO2 < 45 mmHg and a pH > 7.45 (this is true in acute cases, in chronic cases the pH can be close to normal due to renal compensation)
          • typically due to alveolar hyperventilation
            • the patient is effectively expelling too much CO2
        • can serve as a method to compensate for a metabolic acidosis
        • examples
          • anxiety and panic attacks
          • fever
          • hyperventilation syndrome
          • pulmonary embolism
          • pneumonia
          • pregnancy
          • hyperthyroidism
          • aspirin overdose (recall that it causes both a respiratory alkalosis and metabolic acidosis)
      • metabolic acidosis
        • characterized by a pH < 7.35 and a serum bicarbonate < 22 mEq/L
        • examples
          • diabetic ketoacidosis
          • lactic acidosis
          • diarrhea
          • type 1, 2, and 4 renal tubule acidosis
      • metabolic alkalosis
        • characterized by a pH > 7.45 and a serum bicarbonate > 28 mEq/L
        • examples
          • primary mineralocorticoid excess
          • loop or thiazide diuretics
          • calcium-alkali syndrome
          • bicarbonate ingestion
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