Differential Diagnosis

The given arterial blood gas (ABG) values are: PO2 of 90, PCO2 of 28, and pH of 7.16 on room air. Let's break down the differential diagnosis into categories:

• Single Most Likely Diagnosis

  • D. Hyperventilation: The low PCO2 (28 mmHg) and low pH (7.16) suggest a respiratory alkalosis, which is consistent with hyperventilation. The PO2 of 90 on room air is within a relatively normal range, not indicating severe hypoxemia that would be expected in conditions primarily causing hypoventilation or severe respiratory failure.

• Other Likely Diagnoses

  • C. Prolonged Nasogastric Suctioning: This can lead to hyperventilation due to the stimulation of the respiratory center or due to the loss of gastric acid, which can cause a metabolic alkalosis but in the context of respiratory compensation or mixed disorders, could present with these ABG findings.
  • B. Alveolar Hypoventilation: Although the primary issue here appears to be hyperventilation, some forms of hypoventilation can present with complex ABG patterns, especially if there's a component of respiratory muscle weakness or an underlying chronic condition. However, the low PCO2 argues against pure hypoventilation.

• Do Not Miss Diagnoses

  • A. Hypovolaemic Shock: While not directly indicated by the ABG values, hypovolaemic shock can lead to respiratory alkalosis due to hyperventilation as a compensatory mechanism for the metabolic acidosis that often accompanies shock. It's crucial to consider this diagnosis due to its high mortality if untreated.
  • Other conditions that could lead to hyperventilation and thus should not be missed include pulmonary embolism, severe anxiety or panic attacks, and certain toxic ingestions.

• Rare Diagnoses

  • High-Altitude Exposure: Individuals at high altitudes may hyperventilate due to the lower oxygen levels, leading to a respiratory alkalosis.
  • Certain Endocrine Disorders: Such as hyperthyroidism, which can cause an increase in respiratory rate leading to hyperventilation.
  • Neurological Conditions: Like central nervous system lesions or infections that affect the respiratory centers in the brain, leading to abnormal breathing patterns.

It's essential to consider the clinical context, including the patient's symptoms, physical examination findings, and other diagnostic tests, to narrow down the differential diagnosis and guide management.