Summary: Dr. Hong Kim is back – this time to discuss two classic hemoglobinopathies. Do you you know the common iatragenic causes of methemoglobinemia? How about when to send a patient for hyperbaric oxygen for carbon monoxide exposure? If not, this is a can’t miss lecture.<br> Written summary by Ahmad Aswad<br> Carbon Monoxide Toxicity<br> <br> * In the U.S. common sources of exposure include residential fire (with cyanide as a potential co-exposure), engine exhaust, natural gas appliances, propane-powered engines, & paint stripper ingestion.<br> * Normal: Physiologic carboxyhemoglobin (COHb) levels: 1-2% in healthy individuals, up to 5-10% in smokers!<br> * Pathophysiology: Once inhaled/absorbed it binds to hemoglobin with 250 times higher affinity than oxygen causing “functional” anemia. Mitochondrial dysfunction also a major cause of clinical decompensation.<br> * Symptoms: At about 10%: Begins with headache, dizziness at level >10%, can progress to unconsciousness, death with levels above 50%<br> * Diagnosis<br> <br> * Lab: CO-oximetry<br> * Radiology: CT head shows basal ganglia lesions<br> <br> <br> * Treatment<br> <br> * Primary treatment: Removal from exposure, 100% FiO2 by non-rebreather mask<br> * t1/2: CO half-life is 5 hours<br> * Hyperbaric Oxygen (HBO) Therapy: causes dissociation of CO from Hb, myoglobin, and cytochrome oxidase in the mitochondria<br> <br> * Decreases half-life to 23 minutes<br> * HBO may also antagonize lipid peroxidation and decrease leukocyte adhesion, minimizing the inflammatory response.<br> * Survivors may develop delayed neuropsychiatric sequelae and cognitive impairment weeks after exposure.<br> * Risk factors of delayed reactions include LOC, age> 30 & duration of CO exposure.<br> * Case reports suggest that HBO initiated within 6 hours from exposure may prevent delayed neurologic sequelae<br> * Indications for HBO<br> <br> * Levels of CO Hb >25 %, ( >15 % in pregnant and pediatric population, expert opinion)<br> * Clinical symptoms: Chest pain, dizziness, syncope, seizures, coma, focal neurological findings, pregnancy<br> * <a href="http://www.ncbi.nlm.nih.gov/pubmed/21491385">Cochrane review</a>: insufficient evidence to support HBO use<br> * <a href="http://www.acep.org/workarea/DownloadAsset.aspx?id=33724">American college of emergency physicians (ACEP)</a>: HBO a therapeutic option, cannot be mandated<br> <br> <br> <br> <br> <br> <br> <br> Methemoglobin (MetHb)<br> <br> * Both hereditary & toxic forms of methemoglobinemia exist<br> * Normal levels are 1-3%<br> * Pathophysiology: Oxidizes iron moiety to Fe3+ in hemoglobin leading to inability to carry oxygen.<br> * Clinical findings<br> <br> * May not be symptomatic up until levels above 20%, with death occurring at levels above 70%<br> * Pulse oximetry shows hypoxemia with saturation levels <90%<br> * SpO2 usually stays around 85% as levels of MetHb increase<br> <br> <br> * Precipitants<br> <br> * Common drugs: Dapsone, benzocaine, phenazopyridine, amyl nitrites and anti-malarials.<br> * Chemicals such as aniline dye, organic nitrites, and nitrates food preservatives.<br> <br> <br> * Treatment: Methylene blue<br> <br> * Indication: Symptomatic patients and MetHb >25%. Response is seen within minutes and dose may be repeated once.<br> * Mechanism: Fe3+ oxidized to Fe2+ via NADPH dependent pathway<br> * Can cause hemolysis, decreased pulse-oximetry, and can potentially cause serotonin syndrome.<br> * Can be considered in patients with G6PD deficiency only in very life threatening cases, as hemolysis will often be seen 24 hours later.<br> * Alternative treatment options: Exchange transfusion, HBO, and ascorbic acid (slow response)<br> <br> <br> <br> <br> Suggested Reading<br> <br> <br> * Scheinkestel CD, Bailey M, Myles PS, et al.
