A 47-year-old woman is brought to the emergency department after being found unresponsive in her garage. The patient has a history of chronic pain, depression, and prior suicide attempts. Temperature is 35 C (95 F), blood pressure is 106/64 mm Hg, pulse is 108/min, and respirations are 22/min. Pulse oximetry is 96% on room air. The patient withdraws all extremities to painful stimuli but does not follow commands. Bilateral pupils are equal and reactive, and funduscopy shows no papilledema. Lung auscultation shows occasional wheezes. No heart murmurs are present. The abdomen is soft and nontender with decreased bowel sounds. There is no extremity edema. Laboratory results are as follows:
| Sodium | 144 mEq/L |
| Chloride | 108 mEq/L |
| Bicarbonate | 18 mEq/L |
| Creatinine | 0.8 mg/dL |
| Glucose | 120 mg/dL |
Endotracheal intubation followed by mechanical ventilation and other supportive measures are begun. A brain MRI obtained several days later is shown in the exhibit. Which of the following is the most likely cause of this patient's current condition?
Carbon monoxide poisoning | |
Epidemiology |
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Manifestations | Mild-moderate
Severe
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Diagnosis |
|
Treatment |
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ABG = arterial blood gas. | |
This patient most likely experienced acute carbon monoxide (CO) poisoning. This can result from inhalation of car exhaust in a closed space (eg, a garage), a situation often intentionally created in a suicide attempt.
CO causes toxicity by impairing oxygen (O2) delivery and usage in 3 ways:
Patients with CO poisoning typically develop clinical manifestations of cerebral hypoxia, including headache, dizziness, and confusion. Severe intoxication can cause myocardial ischemia, seizure, coma, and death. Vital signs are often largely unremarkable. Pulse oximetry is usually normal, because the oximeter cannot differentiate between oxyhemoglobin and carboxyhemoglobin. Laboratory results can show an anion gap metabolic acidosis (AGMA) due to lactic acidosis from peripheral tissue hypoxia. Permanent hypoxic brain injury can occur, as evidenced in this patient's MRI showing bilateral hyperintensity of the globus pallidus, an area highly sensitive to hypoxic conditions.
(Choice A) Acute salicylate toxicity typically causes AGMA as well as tachypnea (leading to a primary respiratory alkalosis and mixed acid-base disorder); however, hypoxic findings on brain MRI are not expected.
(Choice C) Opioid overdose causes respiratory depression and hypoventilation that may lead to hypoxic brain injury. However, this patient's elevated respiratory rate, normal O2 saturation on room air, and reactive (rather than pinpoint) pupils are not consistent with opioid overdose.
(Choice D) Severe hypothermia (ie, body temperature <28 C) is typically required to cause loss of consciousness. Bradycardia and a reduced respiratory rate, which are not present in this patient, are expected with such a degree of hypothermia.
(Choice E) Methanol or ethylene glycol intoxication can cause unresponsiveness and AGMA; however, these diagnoses are made less likely by absence of papilledema and normal renal function, respectively.
Educational objective:
Acute carbon monoxide poisoning can occur due to inhalation of car exhaust in a closed space, often performed intentionally in a suicide attempt. Toxicity results from impaired delivery and usage of oxygen, leading to clinical manifestations of cerebral hypoxia (eg, headache, confusion, seizure, coma). Laboratory results may demonstrate lactic acidosis, and permanent hypoxic brain injury can occur.