This patient presents to the Emergency Department with altered mental status.  This presenting symptom can be due to a large variety of etiologies, including hypoglycemia, sepsis, toxic ingestions, electrolyte abnormalities, stroke, and more.  The management and evaluation of a patient with altered mental status depends on the primary assessment of the patient (“ABCs”, or Airway, Breathing, Circulation) to identify any acute life-threatening conditions that need to be managed emergently, the history, and the physical examination.  One mnemonic that may help in remembering the many causes of altered mental status is “AEIOUTIPS”.  The table below outlines this mnemonic.

The first step in managing this patient should be to treat the hypoxia with supplemental oxygen.  Prolonged hypoxia is dangerous and if left untreated, can cause brain damage.  Hypoxia can cause altered mental status, however, when this patient’s hypoxia is resolved, she remains somnolent and altered.  This should raise concern over an alternative etiology for the patient’s condition.      

The arterial blood gas demonstrates a low pH (acidosis), normal paO2, elevated paCO2 (hypercarbia), and a normal HCO3 (no metabolic compensation for acidosis).  The final interpretation of the ABG would be an acute respiratory acidosis without metabolic compensation.  Acute elevations of pCO2 can manifest as somnolence and altered mental status as seen in this patient.  This is known as hypercarbic or hypercapnic respiratory failure (Choice A).  This condition is caused by the inability to exhale CO2.  Risk factors include obstructive lung diseases (i.e., COPD), obesity, and obstructive sleep apnea.  Treatment involves treatment of hypoxia with supplemental oxygen, non-invasive positive pressure ventilation (i.e., BIPAP, CPAP, High Flow Nasal Cannula), and treatment of the underlying cause.

The patient’s arterial blood gas does not show hypoxic respiratory failure (Choice B).  Since treatment of the patient’s hypoxia does not improve the patient’s mental status, hypercarbic respiratory failure is more likely the underlying cause of the patient’s condition.  Opioid overdose (Choice C) can cause a similar ABG and patient presentation.  The normal size pupils and absent history of drug abuse makes this diagnosis less likely. Sepsis (Choice D) can trigger changes in mental status and cause respiratory failure, however, the absence of infectious symptoms and the presence of obesity and COPD support hypercarbic respiratory failure as the more likely underlying cause. 

Correct Answer: A