Updated: 12/29/2021

Arterial Blood Gases

Review Topic
  • 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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(M2.PL.17.14) A 19-year-old male college student with no significant medical history presents to the emergency department with altered mental status. His girlfriend said that he drank multiple shots of vodka and gin throughout the weekend and was found slumped on the couch with some vomit on his shirt. He is afebrile, with a blood pressure of 128/60 mmHg, pulse of 100/min, respirations at 25/min, and SpO2 of 95% on room air. His pupils are equal and reactive to light bilaterally. He exhibits no tremors or myoclonus. The remainder of the physical exam is unremarkable. His basic metabolic panel is displayed below:

Na+: 138 mEq/L
Cl-: 90 mEq/L
K+: 4.0 mEq/L
HCO3-: 20 mEq/L
BUN: 30 mg/dL
Glucose: 100 mg/dL
Creatinine: 0.8 mg/dL

An arterial blood gas reveals the following:

pH: 7.32
pCO2: 34 mmHg
pO2: 89 mmHg

The girlfriend is concerned that the patient's breathing appears uncomfortable. How do you respond?

QID: 104288

His breathing is concerning for hepatic encephalopathy



His breathing requires evaluation for a pulmonary embolism



His breathing suggests that he needs some fluids



His breathing suggests that he has major depression disorder



You are not his healthcare proxy and I cannot speak to you



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(M2.PL.17.49) A 48-year-old woman is transferred from her primary care physician's office to the emergency department for further evaluation of hypokalemia to 2.5 mEq/L. She was recently diagnosed with hypertension 2 weeks ago and started on medical therapy. The patient said that she enjoys all kinds of food and exercises regularly, but has not been able to complete her workouts as she usually does. Her temperature is 97.7°F (36.5°C), blood pressure is 97/64 mmHg, pulse is 120/min, respirations are 15/min, and SpO2 is 94% on room air. Her physical exam is unremarkable. Peripheral intravenous (IV) access is obtained. Her basic metabolic panel is obtained below.

Na+: 135 mEq/L
Cl-: 89 mEq/L
K+: 2.2 mEq/L
HCO3-: 33 mEq/L
BUN: 44 mg/dL
Glucose: 147 mg/dL
Creatinine: 2.3 mg/dL
Magnesium: 2.0 mEq/L

What is the next best step in management?

QID: 104323

Obtain an electrocardiogram



Administer potassium bicarbonate 50mEq per oral



Administer potassium chloride 40mEq via peripheral IV



Administer isotonic saline with 40 mEq KCl 1 liter via peripheral IV



Obtain urine sodium and creatinine



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(M2.PL.14.4) A 60-year-old homeless man presents to the emergency department with an altered mental status. He is not answering questions. His past medical history is unknown. A venous blood gas is drawn demonstrating the following.

Venous blood gas
pH: 7.2
PaO2: 80 mmHg
PaCO2: 80 mmHg
HCO3-: 24 mEq/L

Which of the following is the most likely etiology of this patient's presentation?

QID: 104278

Aspirin overdose






Diabetic ketoacidosis



Ethylene glycol intoxication



Heroin overdose



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Evidence (5)
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