Basic Interpretation of ABG Values
1. Which of the following is the normal range for arterial pH?
A. 7.25 – 7.35
B. 7.35 – 7.45
C. 7.45 – 7.55
D. 7.30 – 7.40
Answer: B.
Explanation: The normal pH range for arterial blood is 7.35 – 7.45, indicating a balance between acid and base.
2. A normal arterial PaCO2 level falls within which range?
A. 25 – 35 mmHg
B. 35 – 45 mmHg
C. 45 – 55 mmHg
D. 55 – 65 mmHg
Answer: B.
Explanation: Normal PaCO2 levels are 35 – 45 mmHg, representing proper respiratory function.
3. Which ABG parameter is most useful in assessing metabolic disorders?
A. pH
B. PaCO2
C. HCO3-
D. PaO2
Answer: C.
Explanation: HCO3- (bicarbonate) reflects the metabolic component of acid-base balance.
4. What is the primary buffer system in the blood that regulates pH?
A. Carbonic acid-bicarbonate buffer
B. Phosphate buffer system
C. Protein buffer system
D. Renal buffer system
Answer: A.
Explanation: The carbonic acid-bicarbonate buffer is the primary system maintaining blood pH.
5. A patient has the following ABG results: pH = 7.48, PaCO2 = 32 mmHg, HCO3- = 24 mEq/L. What is the diagnosis?
A. Metabolic alkalosis
B. Respiratory acidosis
C. Respiratory alkalosis
D. Metabolic acidosis
Answer: C.
Explanation: A high pH (>7.45) and low PaCO2 (<35 mmHg) indicate respiratory alkalosis.
Assessment of Acid-Base Balance
6. What is the most likely cause of respiratory acidosis?
A. Hyperventilation
B. Hypoventilation
C. Diarrhea
D. Vomiting
Answer: B.
Explanation: Hypoventilation leads to CO2 retention, causing respiratory acidosis.
7. Which condition is commonly associated with metabolic acidosis?
A. Chronic obstructive pulmonary disease (COPD)
B. Diabetic ketoacidosis (DKA)
C. Hyperventilation syndrome
D. Pulmonary embolism
Answer: B.
Explanation: DKA leads to the accumulation of acidic ketones, causing metabolic acidosis.
8. What compensatory mechanism occurs in metabolic acidosis?
A. Hypoventilation
B. Hyperventilation
C. Increased renal excretion of hydrogen ions
D. Decreased renal reabsorption of bicarbonate
Answer: B.
Explanation: The body compensates for metabolic acidosis by increasing ventilation to blow off CO2.
9. A patient with severe vomiting may develop which acid-base disorder?
A. Respiratory acidosis
B. Metabolic acidosis
C. Metabolic alkalosis
D. Respiratory alkalosis
Answer: C.
Explanation: Vomiting causes loss of gastric acid (HCl), leading to metabolic alkalosis.
10. What is the most common cause of respiratory alkalosis?
A. Hypoventilation
B. Hyperventilation
C. Diabetic ketoacidosis
D. Renal failure
Answer: B.
Explanation: Hyperventilation results in excessive CO2 elimination, causing respiratory alkalosis.
Identification of Respiratory and Metabolic Disorders
11. A patient’s ABG results show pH = 7.28, PaCO2 = 52 mmHg, HCO3- = 24 mEq/L. What is the most likely disorder?
A. Respiratory alkalosis
B. Metabolic acidosis
C. Respiratory acidosis
D. Metabolic alkalosis
Answer: C.
Explanation: Low pH (<7.35) and high PaCO2 (>45 mmHg) indicate respiratory acidosis.
12. Which ABG abnormality is expected in a patient with an opioid overdose?
A. Respiratory alkalosis
B. Respiratory acidosis
C. Metabolic acidosis
D. Metabolic alkalosis
Answer: B.
Explanation: Opioids depress respiratory drive, causing hypoventilation and respiratory acidosis.
13. Which metabolic condition is associated with high anion gap metabolic acidosis?
A. Renal failure
B. Hyperaldosteronism
C. Chronic vomiting
D. COPD
Answer: A.
Explanation: Renal failure leads to accumulation of acids, increasing the anion gap.
14. A patient has a pH of 7.55 and HCO3- of 30 mEq/L. What is the most likely diagnosis?
A. Respiratory acidosis
B. Metabolic alkalosis
C. Metabolic acidosis
D. Respiratory alkalosis
Answer: B.
Explanation: A high pH and high HCO3- indicate metabolic alkalosis.
15. What is the primary compensatory response to metabolic alkalosis?
A. Increased renal bicarbonate excretion
B. Hyperventilation
C. Hypoventilation
D. Increased hydrogen ion excretion
Answer: C.
Explanation: The body hypoventilates to retain CO2 and lower pH.
Oxygenation and ABG Interpretation
16. A patient with PaO2 of 50 mmHg is experiencing which condition?
A. Normal oxygenation
B. Hyperoxia
C. Metabolic acidosis
D. Hypoxemia
Answer: D.
Explanation: PaO2 < 60 mmHg indicates hypoxemia.
17. Which ABG value is most useful in assessing ventilation?
A. PaO2
B. HCO3-
C. PaCO2
D. SaO2
Answer: C.
Explanation: PaCO2 directly reflects alveolar ventilation status.
18. What is the normal range for arterial oxygen saturation (SaO2)?
A. 70 – 80%
B. 80 – 85%
C. 90 – 95%
D. 95 – 100%
Answer: D.
Explanation: Normal SaO2 is 95 – 100%.
19. Which of the following conditions leads to respiratory failure Type I?
A. Hypercapnia with normal oxygenation
B. Hypoxemia without hypercapnia
C. Hypoxemia with hypercapnia
D. Normal PaO2 and PaCO2
Answer: B.
Explanation: Type I respiratory failure involves hypoxemia (low PaO2) without hypercapnia (normal PaCO2).
Compensatory Mechanisms and Clinical Application
20. Which of the following best describes the compensatory response in respiratory acidosis?
A. The lungs increase ventilation to blow off CO2
B. The kidneys excrete hydrogen ions and retain bicarbonate
C. The kidneys excrete bicarbonate and retain hydrogen ions
D. The lungs decrease ventilation to retain CO2
Answer: B.
Explanation: In respiratory acidosis, the kidneys compensate by excreting H+ and retaining HCO3- to buffer the excess CO2.
21. A patient’s ABG results show pH = 7.29, PaCO2 = 38 mmHg, HCO3- = 18 mEq/L. What is the most likely disorder?
A. Respiratory acidosis
B. Respiratory alkalosis
C. Metabolic acidosis
D. Metabolic alkalosis
Answer: C.
Explanation: The low pH and low HCO3- indicate metabolic acidosis, with a normal PaCO2 suggesting no respiratory compensation.
22. Which ABG finding is most commonly seen in chronic obstructive pulmonary disease (COPD)?
A. Low pH, high PaCO2, high HCO3-
B. High pH, low PaCO2, low HCO3-
C. Normal pH, high PaCO2, high HCO3-
D. Normal pH, low PaCO2, low HCO3-
Answer: C.
Explanation: In chronic respiratory acidosis due to COPD, the kidneys retain bicarbonate (HCO3-) to partially compensate for elevated CO2.
23. A patient with a severe asthma attack is likely to exhibit which ABG result initially?
A. pH = 7.30, PaCO2 = 50 mmHg
B. pH = 7.50, PaCO2 = 30 mmHg
C. pH = 7.35, PaCO2 = 40 mmHg
D. pH = 7.25, PaCO2 = 25 mmHg
Answer: B.
Explanation: In early asthma attacks, hyperventilation occurs, leading to respiratory alkalosis (high pH, low PaCO2).
24. A patient with renal failure and a bicarbonate level of 16 mEq/L is most likely to have which condition?
A. Respiratory acidosis
B. Metabolic acidosis
C. Respiratory alkalosis
D. Metabolic alkalosis
Answer: B.
Explanation: Renal failure leads to accumulation of metabolic acids, causing metabolic acidosis with a low HCO3-.
Oxygenation and Clinical Cases
25. A PaO2 value of 58 mmHg in an ABG sample indicates which condition?
A. Normal oxygenation
B. Hyperoxia
C. Mild hypoxemia
D. Severe hypoxemia
Answer: C.
Explanation: Mild hypoxemia is defined as PaO2 between 60 – 79 mmHg, moderate hypoxemia as 40 – 59 mmHg, and severe hypoxemia as <40 mmHg.
26. Which ABG value is most important in assessing ventilation efficiency?
A. PaO2
B. HCO3-
C. PaCO2
D. SaO2
Answer: C.
Explanation: PaCO2 is the primary indicator of alveolar ventilation. High PaCO2 suggests hypoventilation, while low PaCO2 suggests hyperventilation.
27. A patient is given excessive oxygen therapy and develops a PaO2 of 180 mmHg. What is this condition called?
A. Hypercapnia
B. Hypoxia
C. Hyperoxia
D. Respiratory acidosis
Answer: C.
Explanation: PaO2 above 100 mmHg indicates hyperoxia, which can lead to oxygen toxicity.
28. Which condition is characterized by a normal PaCO2 but a low PaO2?
A. Type 1 respiratory failure
B. Type 2 respiratory failure
C. Metabolic acidosis
D. Metabolic alkalosis
Answer: A.
Explanation: Type 1 respiratory failure is hypoxemic respiratory failure, where PaO2 is low, but PaCO2 is normal or low.
29. What is the primary cause of respiratory alkalosis in critically ill patients?
A. Hyperventilation
B. Hypoventilation
C. Metabolic acidosis
D. Ketoacidosis
Answer: A.
Explanation: Hyperventilation leads to excessive CO2 elimination, causing respiratory alkalosis. This can be due to anxiety, pain, fever, or mechanical ventilation.
30. Which ABG pattern is expected in a patient with prolonged diarrhea?
A. Low pH, low HCO3-, normal PaCO2
B. High pH, high HCO3-, high PaCO2
C. Normal pH, normal HCO3-, high PaCO2
D. Low pH, high HCO3-, normal PaCO2
Answer: A.
Explanation: Diarrhea leads to the loss of bicarbonate, causing metabolic acidosis (low pH, low HCO3-).