4.31: Toxicology (CO Poisoning)

The laboratory received three venous blood samples for carboxyhemoglobin (HbCO) analysis. The samples, drawn from three individuals at a fire, had the following results:

Patient	% HbCO
DJ (>30 years, pregnant female)	41.4%
AJ (approx. 7-8 years, female)	32.4%
EW (>40 years, female)	33.9%

Because the results were highly elevated, the medical technologist repeated the entire analysis and obtained similar results. The QC (Hi and Lo) in both sets of analyses were within normal range.

Upon receiving these results, the Emergency Unit physician called and asked to speak to a supervisor. The physician stated that he believed the results were inaccurate since the patients appeared normal. He mentioned that a blood gas specimen drawn on DJ while she was breathing room air had been recently sent, and asked if the lab would repeat the HbCO analysis on it.

The specimen was retrieved and the repeat HbCO on this sample was 42.8%. The blood gas values were: pH = 7.40, PCO₂ = 32, PO₂ = 110; TCO₂ = 21.

QUESTIONS

1. Are the HbCO results at all compatible with the physical condition of the patients?
2. Approximately 120 minutes later, a second set of specimens was received in the lab for HbCO analysis and the following results were obtained: AJ = 0%; DJ = 5.6%; EW = 5.4%. Can you explain the differences between the initial and final CO levels and the results of the blood gas analysis?

Questions to Consider

1. What is the biochemical mechanism for CO toxicity?
2. What is the physiological mechanism for CO toxicity?
3. What factors affect the degree of CO toxicity?
4. What is the treatment for CO poisoning?
5. Is there anything additional the lab could do to confirm the elevated CO levels?

Answer

1. On the face of it, these HbCO values are highly elevated. HbCO levels this high are usually associated with severe symptoms of CO poisoning: severe headache, dizziness, vomiting and even coma. So these very elevated levels of HbCO are not consistent with the observed condition of the patients. Moreover, an even severer reaction would be expected in the pregnant female and the child.
2. The physician was concerned about the validity of the results because, although the three patients had been exposed to CO in a fire, they did not demonstrate the typical signs of CO toxicity associated with the reported values (headache, dizziness, vomiting). The reason for the discrepancy was obvious once the clinical chemist and medical technologist pieced together the order of events. The three patients were treated with 100% oxygen while they were being brought to the hospital. However, the blood samples were drawn at the time that the oxygen therapy was initiated. Thus, the analysis accurately reflected the degree of CO exposure and poisoning of the three individuals at that point in time. However, by the time the results were returned to the emergency unit, enough time had elapsed since the exposure for the O₂ to replace most of the CO from the blood and reduce the symptoms.
   
   The bright cherry-red color of the retrieved blood gas sample did confirm the initial HbCO analysis and convince the laboratory that the results were valid. The apparent discrepancy between the high % HbCO and the high PO2 is explained by the fact that the oxygen electrode does not measure total O₂ being carried by blood, but only that portion actually dissolved in the plasma portion of blood. Usually there is a direct releationship between the Hb binding of O₂, total O₂ carried in blood and the PO₂ of blood. However, any impairment of the ability of Hb to bind O2 will cause a dissociation between O₂ capacity and the PO₂ of blood. This dissociation can occur in CO poisoning and certain hemoglobinopathies which involve defective hemoglobin molecules.

Answers to Questions to Consider

3. The most important factors are those that would increase the body’s oxygen requirements. Thus, preexisting cardiac failure, fever, pregnancy, or low hematocrit in an individual each would exacerbate the toxicity shown at any given level of CO in the blood. In addition, young children have higher rates of metabolism than adults and thus show a greater degree of toxicity at a given level of CO. Some Web sites on the toxicity of CO include: ash.xanthia.com/co.html, www.health.state.mn.us/divs/eh/indoorair/co/
4. The primary treatment for CO poisoning is the displacement of the CO from the hemoglobin molecule by increasing the PO₂ of the blood (see p 1011). This patients with CO poisoning are treated with 100% oxygen. In very severe cases, patients can be treated with hypebaric oxygen, that is, oxygen at 2-4 atmospheres of pressure. Hyperbaric oxygen treatment not only reduces the HbCO concentration faster, but it raises the PO₂ of plasma so that more can be carried in the dissolved state. The half-life of HbCO with O₂ treatment is approximately 30-40 minutes, greatly reduced from the 180-240 minute half-life when only room air is inspired. For additional information on hyperbaric treatment, see these Web sites: http://www.pulmonaryreviews.com/feb0...yperbaric.html
   www.hyperchamber.com/co_poisoning/
5. To confirm the presence of CO the medical technologists could inspect the color of the blood gas specimen that was retrieved. HbCO has a characteristic bright cherry-red color. In fact, at high levels of CO poisoning, the tips of fingers can be noticeably brighter red and CO poisoning is readily diagnosed by pathologists at autopsy by the bright red color of tissues.