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Unchanged, due to the fact that carbon monoxide (CO) is largely unbound and dissolved in plasma
1%
3/374
Shifted right, due to strong competition between CO and O2 for binding
14%
52/374
Shifted left, due to carbon monoxide binding
74%
276/374
Shifted left, due to metabolic acidosis in tissues
2%
6/374
Shifted right, due to decreased affinity for oxygen
7%
27/374
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The patient's clinical presentation is consistent with carbon monoxide (CO) poisoning, which causes the oxygen hemoglobin curve to shift to the left, due to tighter heme-O2 binding. CO poisoning may be caused by inhaled smoke, motor vehicle fumes, or furnace emissions. CO has 200-250x greater affinity for hemoglobin than oxygen, so the vast majority of heme in the blood quickly becomes bound by CO. Importantly, CO binding to heme increases the heme affinity for oxygen at the other three oxygen binding sites. This leads to decreased oxygen unloading at the tissue level, resulting in tissue hypoxia. Because a given oxyhemoglobin concentration/saturation can therefore be reached at a lower partial pressure of oxygen, the oxygen-hemoglobin curve is shifted to the left. Ernst and Zibrak discuss recognition and prevention of CO poisoning. They note that symptoms of CO poisoning are often non-specific, and include tachycardia, tachypnia, nausea, vomiting, and headache. The classical signs of retinal hemorrhages and cherry-red lips are rarely seen, and their absence should not rule out CO poisoning. The work-up includes measuring CO in exhaled air and measuring carboxyhemoglobin concentration in the blood. Treatment involves removing the patient from the exposure situation and administering oxygen. Henry et al. performed a prospective cohort study to evaluate myocardial injury in patients admitted to the hospital with CO poisoning. They found that 37% of patients had concurrent myocardial injury, and that those with injury had a 23% increased mortality rate over the study period. They conclude that myocardial injury is a strong, negative prognostic factor in patients with CO poisoning. Figure A depicts a patient with soot accumulation near the nares, suggesting likely inhalation injury. Illustration A shows the oxygen-hemoglobin curve, with several modifying factors. Illustration B shows a molecular model of oxyhemoglobin. Incorrect Answers: Answer 1: Carbon monoxide predominantly binds hemoglobin, and less than 1% of the absorbed gas is found in solution. Answer 2: Though CO and O2 compete for heme binding, the curve is shifted to the left because of increased affinity of hemoglobin for oxygen at sites not bound to CO. Answer 4: Metabolic acidosis causes the dissociation curve to shift right, not left Answer 5: CO poisoning actually causes greater heme affinity for oxygen, shifting the curve left.
4.3
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