A 71-year-old man is brought to the ER after a witnessed tonic-clonic seizure. He is somnolent and intermittently combative on exam. No past medical history is available. His arterial blood gas (ABG) at room air is given below.
Which of the following best explains the acid-base disturbances in this patient?

Question

A 71-year-old man is brought to the ER after a witnessed tonic-clonic seizure.  He is somnolent and intermittently combative on exam.  No past medical history is available.  His arterial blood gas (ABG) at room air is given below.
 Which of the following best explains the acid-base disturbances in this patient?

Quizplus · Accepted Answer

This patient recently had a seizure and his arterial blood gases confirm an acidosis. The first step in determining whether he has a respiratory or a metabolic acidosis is to evaluate the pCO2. An elevated pCO2 is consistent with a respiratory acidosis due to hypoventilation. This is supported by hypoventilation-induced hypoxia. A low bicarbonate concentration, on the other hand, would argue for a metabolic acidosis. His hypoventilation is likely related to his post-ictal state. It can be explained by apnea/hypopnea during prolonged seizures or an aspiration event. (Choice A) The post-ictal state can be complicated by lactic acidosis from prolonged and forceful skeletal muscle activity. This will manifest as a metabolic acidosis with a decreased HCO3. However, the HCO3 in this patient is normal. (Choice C) The most common acid-base disturbance caused by a pulmonary embolism is respiratory alkalosis. This results from hyperventilation as a patient tries to overcome hypoxia and V/Q mismatching. An ABG in respiratory alkalosis would have a low pCO2. (Choice D) Renal failure commonly results in a metabolic acidosis due to inadequate excretion of metabolically-produced acids (sulfates, phosphates). This is a chronic problem often compensated by a mild respiratory alkalosis. A typical ABG in renal failure would have a low pCO2 and low bicarbonate level. (Choice E) Vomiting is not a very common feature during or after a tonic-clonic seizure. Moreover, vomiting induces a metabolic alkalosis due to loss of acid gastric secretions. This will cause a high pH and elevated serum HCO3 concentrations. Educational objective: Acidosis accompanied by hypercarbia and a normal or elevated serum bicarbonate is diagnostic of a respiratory acidosis. Hypoventilation is a major cause of respiratory acidosis and may occur in the post-ictal state.

A 71-Year-Old Man Is Brought to the ER After a Witnessed