A 28-year-old man is found unresponsive by his roommate and is brought to the emergency department. He has a history of injection drug use. On physical examination, the patient is obtunded and hypopneic. The pupils are pinpoint. Lung auscultation reveals decreased breath sounds bilaterally. Endotracheal intubation is planned for airway protection and arterial blood gas analysis is performed prior to the procedure while the patient is breathing room air. This patient's current acid-base status is best represented by which of the following points on the below graph?

Question

A 28-year-old man is found unresponsive by his roommate and is brought to the emergency department.  He has a history of injection drug use.  On physical examination, the patient is obtunded and hypopneic.  The pupils are pinpoint.  Lung auscultation reveals decreased breath sounds bilaterally.  Endotracheal intubation is planned for airway protection and arterial blood gas analysis is performed prior to the procedure while the patient is breathing room air.  This patient's current acid-base status is best represented by which of the following points on the below graph?

Quizplus · Accepted Answer

11ec160b_3b58_f650_9249_25a93e301d1c_MD0001_00 This patient's altered consciousness, pinpoint pupils, decreased breath sounds, and history of intravenous drug use strongly suggest acute opioid overdose. In addition to causing sedation, opioids cause respiratory depression by suppressing central respiratory drive in the medulla. The resulting hypoventilation leads to CO₂ retention and acute respiratory acidosis, characterized by low pH and high PaCO₂. To compensate for respiratory acidosis, the kidneys retain additional bicarbonate; however, metabolic renal compensation for an acid-base disturbance is a relatively slow process (taking place over several days) and is minimally evident in the acute setting. (Choice A) A high PaCO₂ with high serum bicarbonate and near normal pH is consistent with chronic respiratory acidosis with a compensatory metabolic alkalosis, such as occurs in severe chronic obstructive pulmonary disease. Although acute opioid overdose is expected to cause a rapid increase in PaCO₂, serum bicarbonate is near normal because there has not been time for metabolic compensation. (Choice C) Normal acid-base values include pH 7.4, PaCO₂ 40 mm Hg, and serum bicarbonate 24 mEq/L. Significant deviation from these values represents an acid-base disturbance. (Choice D) A high pH with low PaCO₂ and slightly low serum bicarbonate is consistent with acute respiratory alkalosis, such as occurs with the hyperventilation seen in acute pulmonary embolism or acute anxiety. Serum bicarbonate is only slightly low because the kidneys have had little time to compensate by increasing bicarbonate excretion. (Choice E) A low pH with low serum bicarbonate and low PaCO₂ is consistent with acute metabolic acidosis, such as occurs with lactic acidosis in sepsis. The lungs quickly compensate for the acidemia with hyperventilation to increase exhalation of CO₂ and help normalize pH. Severe metabolic acidosis can lead to respiratory failure if the lungs are unable to keep up with the need to exhale CO₂. Educational objective: Acute opioid overdose is characterized by altered level of consciousness, pinpoint pupils, and central respiratory depression. Patients are expected to have acute respiratory acidosis (low pH, high PaCO₂) due to hypoventilation. Serum bicarbonate is typically near normal as there is not time for metabolic compensation in the acute setting.

A 28-Year-Old Man Is Found Unresponsive by His Roommate and Is