A 35-year-old man is found unresponsive in a parking lot with syringes and drug paraphernalia next to him. Emergency medical personnel administer naloxone and intubate the patient for airway protection. On arrival at the emergency department, health care providers begin mechanical ventilation in assist control mode; tidal volume is 500 mL, respiratory rate is 10/min, fraction of inspired oxygen is 40%, and positive end-expiratory pressure is 5 cm H₂O. Physical examination reveals an unresponsive patient with sluggish pupils. His blood alcohol level is elevated, and urine drug screen is positive for opioids and benzodiazepines. CT scan of the head shows no acute findings. The patient is admitted to the intensive care unit for further management. An arterial blood gas obtained an hour later reveals the following: Which of the following is the most appropriate next step in management of this patient?

Question

Quizplus · Accepted Answer

Explanation: Alcohol, opioids, and benzodiazepines all cause central respiratory depression, and intoxication can lead to hypoventilation with resultant CO₂ retention. This patient with recent abuse of these substances has acute hypercapnic respiratory failure requiring intubation and mechanical ventilation. Arterial blood gas reveals primary respiratory acidosis (ie, low pH and high PaCO₂); as expected in the acute setting, there is minimal metabolic compensation by the kidneys (ie, minimal increase in HCO₃ level). ₂ ₂ ₃ Correction of hypercapnia requires increased ventilation to facilitate removal of excess CO₂. Given that minute ventilation = respiratory rate × tidal volume, increased ventilation can be accomplished via 2 main mechanisms: ₂ increased respiratory rate or increased tidal volume. These settings can be adjusted on the mechanical ventilator. Increased respiratory rate is often preferred because excessive tidal volume (eg, >6-8 mL/kg) increases the risk of pulmonary barotrauma. (Choice A) Decreased tidal volume would lower minute ventilation and worsen this patient's hypercapnia and acidosis. (Choices B and C) Increasing the fraction of inspired oxygen (FiO₂) or positive end-expiratory pressure (PEEP) are the 2 main methods of improving arterial oxygenation. In patients on mechanical ventilation, PEEP provides minimal pressure at end-expiration to help prevent alveolar collapse. This patient's PaO₂ is currently normal (eg, >75 mm Hg); therefore, increased FiO₂ or PEEP is not needed. ₂ ₂ ₂ (Choice E) Maintaining current ventilator settings is not appropriate as this patient has prominent respiratory acidosis in need of correction with increased minute ventilation. Educational objective: Alcohol, opioids, and benzodiazepines can all cause central respiratory depression, and intoxication can lead to hypoventilation and acute hypercapnic respiratory failure. Arterial blood gas findings will show a primary respiratory acidosis. Correction of the hypercapnia requires increased ventilation, which can be accomplished via increased respiratory rate or tidal volume.

A 35-Year-Old Man Is Found Unresponsive in a Parking Lot