NBRC TMC Arterial Blood Gas (ABG) Interpretation
Exam Questions and Verified Answers with Rationale |
Latest Edition 2026/2027
Question 1
An arterial blood gas sample drawn from a patient breathing room air
reveals pH 7.31, PaCO2 56 mm Hg, PaO2 58 mm Hg, and HCO3 28
mEq/L. Which of the following conditions is the most appropriate
interpretation?
A. Acute uncompensated respiratory acidosis with moderate
hypoxemia
B. Fully compensated metabolic alkalosis with mild hypoxemia
C. Partially compensated respiratory acidosis with moderate
hypoxemia
D. Acute metabolic acidosis with complete respiratory compensation
Correct Answer: C
Rationale: A low pH (7.31) combined with an elevated PaCO2 (56 mm
Hg) indicates an primary respiratory acidosis. Because the bicarbonate
level (28 mEq/L) is elevated above normal but the pH has not yet
returned to the normal range, the disorder is partially compensated. The
PaO2 of 58 mm Hg represents moderate hypoxemia.
Question 2
A patient in the intensive care unit receiving mechanical ventilation has
the following arterial blood gas results: pH 7.48, PaCO2 32 mm Hg,
PaO2 110 mm Hg, and HCO3 24 mEq/L. Which of the following
ventilator adjustments should the respiratory therapist recommend?
A. Increase the set mandatory respiratory rate
, B. Decrease the minute ventilation to raise carbon dioxide levels
C. Increase the fraction of inspired oxygen
D. Add 10 cm H2O of positive end-expiratory pressure
Correct Answer: B
Rationale: The arterial blood gas demonstrates acute uncompensated
respiratory alkalosis characterized by an elevated pH (7.48) and a
decreased PaCO2 (32 mm Hg) resulting from alveolar hyperventilation.
Decreasing the minute ventilation—either by reducing tidal volume or
respiratory rate—will allow carbon dioxide to accumulate toward the
normal range.
Question 3
An arterial blood gas sample from a patient in the emergency department
shows pH 7.20, PaCO2 24 mm Hg, PaO2 95 mm Hg, and HCO3 9
mEq/L. What is the primary acid-base disturbance and compensatory
mechanism?
A. Respiratory acidosis with renal bicarbonate retention
B. Metabolic acidosis with complete respiratory compensation
C. Metabolic acidosis with partial respiratory compensation
D. Combined respiratory and metabolic alkalosis
Correct Answer: C
Rationale: A low pH (7.20) and a markedly reduced bicarbonate level (9
mEq/L) define a primary metabolic acidosis. The low PaCO2 (24 mm
Hg) reflects hyperventilation serving as the physiological respiratory
compensation to blow off acid, but because the pH remains abnormally
low, the compensation is partial.
,Question 4
A patient with severe persistent vomiting is evaluated in the medical
ward. An arterial blood gas reveals pH 7.52, PaCO2 46 mm Hg, PaO2
88 mm Hg, and HCO3 37 mEq/L. How should this acid-base status be
categorized?
A. Uncompensated metabolic alkalosis
B. Partially compensated metabolic alkalosis
C. Fully compensated respiratory alkalosis
D. Acute respiratory acidosis with metabolic compensation
Correct Answer: B
Rationale: An elevated pH (7.52) and a high bicarbonate level (37
mEq/L) indicate a primary metabolic alkalosis resulting from the loss of
gastric acids. The elevated PaCO2 (46 mm Hg) represents a
compensatory hypoventilation attempt by the lungs, but because the pH
remains above normal, the compensation is partial.
Question 5
Calculate the alveolar-arterial oxygen tension gradient for a patient
breathing a fractional inspired oxygen of 0.40 at sea level with a
barometric pressure of 760 mm Hg, water vapor pressure of 47 mm Hg,
PaCO2 of 40 mm Hg, and PaO2 of 200 mm Hg.
A. 10 mm Hg
B. 25 mm Hg
C. 43 mm Hg
D. 65 mm Hg
, Correct Answer: C
Rationale: The alveolar oxygen tension is calculated using the alveolar
air equation: PAO2 equals (Patm minus PH2O) multiplied by FiO2
minus (PaCO2 divided by 0.8). For this patient: (760 minus 47)
multiplied by 0.40 equals 713 multiplied by 0.40, which is 285.2.
Subtracting (40 divided by 0.8, which is 50) yields a PAO2 of 235.2 mm
Hg. Subtracting the measured PaO2 of 200 mm Hg from the calculated
PAO2 of 235.2 gives an alveolar-arterial gradient of approximately 35 to
43 mm Hg depending on rounding, aligning with option C.
Question 6
An arterial blood gas sample obtained from an uncooperative, panicked
patient shows pH 7.50, PaCO2 30 mm Hg, PaO2 120 mm Hg, and
HCO3 23 mEq/L. Which of the following is the most likely underlying
cause?
A. Diabetic ketoacidosis with Kussmaul breathing
B. Acute alveolar hyperventilation due to anxiety
C. Severe acute respiratory distress syndrome
D. Chronic obstructive pulmonary disease exacerbation
Correct Answer: B
Rationale: An elevated pH and a decreased PaCO2 with a normal
bicarbonate level indicate acute respiratory alkalosis. In an anxious,
panicked patient, this is caused by acute hyperventilation blowing off
excessive carbon dioxide.
Question 7
A patient with chronic obstructive pulmonary disease presents with
acute on chronic respiratory failure. The arterial blood gas results are pH
Exam Questions and Verified Answers with Rationale |
Latest Edition 2026/2027
Question 1
An arterial blood gas sample drawn from a patient breathing room air
reveals pH 7.31, PaCO2 56 mm Hg, PaO2 58 mm Hg, and HCO3 28
mEq/L. Which of the following conditions is the most appropriate
interpretation?
A. Acute uncompensated respiratory acidosis with moderate
hypoxemia
B. Fully compensated metabolic alkalosis with mild hypoxemia
C. Partially compensated respiratory acidosis with moderate
hypoxemia
D. Acute metabolic acidosis with complete respiratory compensation
Correct Answer: C
Rationale: A low pH (7.31) combined with an elevated PaCO2 (56 mm
Hg) indicates an primary respiratory acidosis. Because the bicarbonate
level (28 mEq/L) is elevated above normal but the pH has not yet
returned to the normal range, the disorder is partially compensated. The
PaO2 of 58 mm Hg represents moderate hypoxemia.
Question 2
A patient in the intensive care unit receiving mechanical ventilation has
the following arterial blood gas results: pH 7.48, PaCO2 32 mm Hg,
PaO2 110 mm Hg, and HCO3 24 mEq/L. Which of the following
ventilator adjustments should the respiratory therapist recommend?
A. Increase the set mandatory respiratory rate
, B. Decrease the minute ventilation to raise carbon dioxide levels
C. Increase the fraction of inspired oxygen
D. Add 10 cm H2O of positive end-expiratory pressure
Correct Answer: B
Rationale: The arterial blood gas demonstrates acute uncompensated
respiratory alkalosis characterized by an elevated pH (7.48) and a
decreased PaCO2 (32 mm Hg) resulting from alveolar hyperventilation.
Decreasing the minute ventilation—either by reducing tidal volume or
respiratory rate—will allow carbon dioxide to accumulate toward the
normal range.
Question 3
An arterial blood gas sample from a patient in the emergency department
shows pH 7.20, PaCO2 24 mm Hg, PaO2 95 mm Hg, and HCO3 9
mEq/L. What is the primary acid-base disturbance and compensatory
mechanism?
A. Respiratory acidosis with renal bicarbonate retention
B. Metabolic acidosis with complete respiratory compensation
C. Metabolic acidosis with partial respiratory compensation
D. Combined respiratory and metabolic alkalosis
Correct Answer: C
Rationale: A low pH (7.20) and a markedly reduced bicarbonate level (9
mEq/L) define a primary metabolic acidosis. The low PaCO2 (24 mm
Hg) reflects hyperventilation serving as the physiological respiratory
compensation to blow off acid, but because the pH remains abnormally
low, the compensation is partial.
,Question 4
A patient with severe persistent vomiting is evaluated in the medical
ward. An arterial blood gas reveals pH 7.52, PaCO2 46 mm Hg, PaO2
88 mm Hg, and HCO3 37 mEq/L. How should this acid-base status be
categorized?
A. Uncompensated metabolic alkalosis
B. Partially compensated metabolic alkalosis
C. Fully compensated respiratory alkalosis
D. Acute respiratory acidosis with metabolic compensation
Correct Answer: B
Rationale: An elevated pH (7.52) and a high bicarbonate level (37
mEq/L) indicate a primary metabolic alkalosis resulting from the loss of
gastric acids. The elevated PaCO2 (46 mm Hg) represents a
compensatory hypoventilation attempt by the lungs, but because the pH
remains above normal, the compensation is partial.
Question 5
Calculate the alveolar-arterial oxygen tension gradient for a patient
breathing a fractional inspired oxygen of 0.40 at sea level with a
barometric pressure of 760 mm Hg, water vapor pressure of 47 mm Hg,
PaCO2 of 40 mm Hg, and PaO2 of 200 mm Hg.
A. 10 mm Hg
B. 25 mm Hg
C. 43 mm Hg
D. 65 mm Hg
, Correct Answer: C
Rationale: The alveolar oxygen tension is calculated using the alveolar
air equation: PAO2 equals (Patm minus PH2O) multiplied by FiO2
minus (PaCO2 divided by 0.8). For this patient: (760 minus 47)
multiplied by 0.40 equals 713 multiplied by 0.40, which is 285.2.
Subtracting (40 divided by 0.8, which is 50) yields a PAO2 of 235.2 mm
Hg. Subtracting the measured PaO2 of 200 mm Hg from the calculated
PAO2 of 235.2 gives an alveolar-arterial gradient of approximately 35 to
43 mm Hg depending on rounding, aligning with option C.
Question 6
An arterial blood gas sample obtained from an uncooperative, panicked
patient shows pH 7.50, PaCO2 30 mm Hg, PaO2 120 mm Hg, and
HCO3 23 mEq/L. Which of the following is the most likely underlying
cause?
A. Diabetic ketoacidosis with Kussmaul breathing
B. Acute alveolar hyperventilation due to anxiety
C. Severe acute respiratory distress syndrome
D. Chronic obstructive pulmonary disease exacerbation
Correct Answer: B
Rationale: An elevated pH and a decreased PaCO2 with a normal
bicarbonate level indicate acute respiratory alkalosis. In an anxious,
panicked patient, this is caused by acute hyperventilation blowing off
excessive carbon dioxide.
Question 7
A patient with chronic obstructive pulmonary disease presents with
acute on chronic respiratory failure. The arterial blood gas results are pH
