Differential Diagnosis

The given arterial blood gas (ABG) values show a PO2 of 90 torr, PCO2 of 28 torr, and pH of 7.16 on room air. These values indicate a state of respiratory alkalosis with a low PCO2 and a slightly low pH, suggesting some degree of compensation or a mixed disorder. Here's the differential diagnosis categorized for clarity:

• Single Most Likely Diagnosis

  • D. Hyperventilation: This is the most likely diagnosis given the low PCO2 (28 torr) and the low-normal pH (7.16). Hyperventilation leads to excessive CO2 elimination, resulting in a decrease in PCO2 and an increase in pH (respiratory alkalosis). The PO2 of 90 torr on room air is within a relatively normal range, considering the context of hyperventilation.

• Other Likely Diagnoses

  • C. Prolonged Nasogastric Suctioning: This can lead to metabolic alkalosis due to the loss of gastric acid, but it might not directly explain the low PCO2 unless the patient is also hyperventilating. However, it's less directly related to the primary ABG findings.
  • B. Alveolar Hypoventilation: Typically presents with hypercapnia (elevated PCO2), which is the opposite of what is seen in this patient. However, early or mild cases might not yet show significant hypercapnia, but this would not typically be the first consideration with a PCO2 of 28 torr.

• Do Not Miss Diagnoses

  • A. Hypovolaemic Shock: While hypovolaemic shock itself does not directly cause the ABG abnormalities seen here, severe shock can lead to respiratory alkalosis due to hyperventilation (either due to pain, anxiety, or the body's attempt to compensate for metabolic acidosis). It's crucial to consider this diagnosis because of its high mortality if not promptly addressed.
  • Panic Attack or Anxiety: Similar to hyperventilation, these conditions can cause significant respiratory alkalosis due to hyperventilation. They are important to recognize as they require different management approaches.

• Rare Diagnoses

  • High-Altitude Exposure: Can lead to hyperventilation due to lower oxygen levels, resulting in respiratory alkalosis. However, this would typically be considered in the context of recent travel to high altitudes.
  • Salicylate Toxicity: Can cause a mixed disorder with both respiratory alkalosis (due to central hyperventilation) and metabolic acidosis. This diagnosis is critical to consider due to its potential for severe outcomes if not treated promptly.

Each of these diagnoses should be considered in the context of the patient's clinical presentation, history, and other diagnostic findings to determine the most appropriate course of action.