A 57-year-old man is transported to the emergency department by emergency medical services. The patient is unconscious and was emergently intubated in the field. The patient has a past medical history of major depressive disorder and post-traumatic stress disorder (PTSD). The patient wife reports he is an avid hobbyist and likes to rebuild engines on classic cars. Family history is unremarkable. The patient’s wife found her husband down when she returned home from work. She states that he was working on a car with several gas space heaters running. Vital signs are temperature is 37° C (98.6° F), blood pressure 78/57 mmHg, pulse 130 beats/minu, and respirations 33/min. The patient is 5’11 in and weighs 201 pounds. Arterial blood gas analysis is notable for a pH of 7.32, PCO2 of 33 mmHg, PO2 of 380 mmHg, bicarbonate of 16 mE1/L, and a carboxyhemoglobin level of 22.0% and is improving on continued oxygen therapy.
Which of the following forms of gas exchange governs the most likely etiology of this patient’s condition?
A) Diffusion-limited gas exchange
This patient presents with reduced level of consciousness, respiratory acidosis, and increased carboxyhemoglobin which are likely due to carbon monoxide (CO) poisoning. The use of gas space heaters in the winter months without adequate ventilation can result in a toxic buildup of CO. CO is an avid binder of hemoglobin and readily diffuses across the alveolar membrane saturating hemoglobin leading to diffusion limited gas exchange.
Answer Choice B: Early capillary perfusion-limited gas exchange, is incorrect. CO does not readily dissolve into the plasma, and the difference in alveolar and early capillary partial pressures are maintained throughout the length of the capillary.
Answer Choice C: End capillary perfusion-limited gas exchange, is incorrect. CO does not readily dissolve into the plasma, and the difference in alveolar and end capillary partial pressures are maintained throughout the length of the capillary.
Answer Choice D: Mixed diffusion-limited and perfusion-limited gas exchange, is incorrect. Carbon Monoxide gas exchange is solely diffusion limited.
Answer Choice D: Normal gas exchange, is incorrect. Carbon monoxide is not a gas that is involved in normal physiological gas exchange.
Key Learning Point
Carbon monoxide is a diffusion-limited gas. In diffusion limited gas exchange, diffusion across the alveolar-capillary barrier and binding of hemoglobin is the limiting factor. Another example of diffusion limited gas exchange is oxygen during strenuous exercise or in the presence of emphysema or pulmonary fibrosis. Perfusion-limited gas exchange is limited by blood flow into the alveolar-capillary unit and how quickly a gas dissolves into solution resulting in alveolar and capillary partial pressure equilibrium. Examples of perfusion-limited gas exchange are nitrous oxide, oxygen at rest, and carbon dioxide.
