Burns: CO poisoning Ddx

Advanced, Clinical Subspecialties, Critical Care

• Diagnosis

o History of exposure 

• Confirmation

o Co-oximeter

o Arterial or venous line samples

• CO levels are elevated for days

• Actual CO levels measured on arrival to the hospital correlates poorly with clinical status 

• Should not be used as sole criterion to determine need for treatment

o CO levels on exhaled air

• Pulse oximetry with only two light sources (wavelengths) will always show a normal oxygen saturation of >92%

o Always will be an overestimate of oxygen saturation 

o Carboxyhemoglobin (carbon monoxide bound to hemoglobin) absorbs more infrared (940 nm) than red (660 nm) wavelength of light just like oxygenated hemoglobin (HgbO2) → infrared to red light absorption ratio is the same as oxygenated hemoglobin (HgbO2)

o Hemoglobin’s affinity for carbon monoxide is much greater than it is for oxygen → increased carbon monoxide saturation of hemoglobin → less oxygen saturation of hemoglobin → decreased oxygen carrying capacity of blood

• Left shift in oxygen-hemoglobin dissociation curve

o Increased affinity of Hgb for oxygen → decreased oxygen release to tissue (tissue hypoxia)

o Lower p50 – less PaO2 required to saturate 50% of Hgb with oxygen

o Hemoglobin’s affinity for carbon monoxide is 200x that of oxygen → carboxyhemoglobin → carboxyhemoglobin has increased affinity for oxygen → left shift of hemoglobin dissociation curve 

• Treatment

o 100% oxygen

• Reverses carboxyhemoglobinemia within an hour 

o Hyperbaric oxygen

• Carboxyhemoglobin >25%

• Neurologic impairment

• Cardiac abnormalities

• Differential diagnoses

o Methemoglobinemia (met-Hb)

• Pulse oximetry always shows ~85%

• Causes falsely low oxygen saturations readings on pulse ox when SaO2 is actually greater than 85% and vice versa

• Methemoglobin absorbs more red (660 nm) than infrared (940 nm) wavelength light (opposite of oxygenated hemoglobin)

• Methemoglobin absorbs both red and infrared wavelengths of light equally – 1:1 ratio

• Left shift in oxygen-hemoglobin dissociation curve

• Increased affinity of Hgb for oxygen → decreased oxygen release to tissue (tissue hypoxia)

• Lower p50 – less PaO2 required to saturate 50% of Hgb with oxygen

o Alcohol intoxication

• Differentiae by blood ethanol levels

o Sedative hypnotic overdose

• Differentiate by urine toxicology screens

o Cyanide poisoning

• Differentiate by increased levels of serum cyanide 

Updated definition 2020:

Carbon monoxide (CO) poisoning should be considered in all patients with burn injuries, especially patients who present unconscious or mentally altered. Carbon monoxide binds hemoglobin with an affinity roughly 250 times greater than oxygen, shifting the hemoglobin dissociation curve to the left and preventing the unloading of oxygen to tissues.  Because conventional pulse oximetry cannot distinguish between hemoglobin saturated with O2 (HbO2) and hemoglobin saturated with CO (HbCO), the oxygen saturation will be falsely elevated using a conventional probe.  An arterial blood gas is needed to determine the concentration HbCO in the burn victim’s blood.  A HbCO < 10% is typically not clinically relevant, but a HbCO > 20% for a patient receiving high flow O2 supplementation roughly corresponds with a HbO2 <80% and is an indication for intubation with mechanical ventilation.  The conventional treatment for CO poisoning is 100% FiO2 as early as possible with the use of hyperbaric oxygen therapy when possible within 6 hrs of CO exposure.  Death from CO poisoning typically results with HbCO levels > 60%.

Several anesthetic considerations are important when treating burn patients who are suspected to have CO poisoning.  Temperature regulation becomes important as burn victims are unable to thermoregulate. Radiant air warming, forced air warmers, and fluid warmers should be implemented early in the care of these patients.  Volume resuscitation should be guided by the percentage of total body surface burned and titrated to urine output.  Using albumin instead of crystalloid solution within the first 24 hrs after injury may reduce the incidence of mortality and compartment syndrome for burn patients.  Indications for early intubation include voice changes, dyspnea, tachypnea and decreased consciousness.  An arterial blood gas should be obtained early to assess the HbCO level.  Succinylcholine can be used to facilitate a rapid sequence intubation within 48 hrs of injury, after which time elevated extra-junctional acetylcholine receptors may cause dangerous hyperkalemia when succinylcholine is administered.  Burn patients with pre-existing cardiac conditions are at greater risk of arrhythmias and myocardial infarction.  A balanced multimodal analgesia approach should be used for the long-term care of these patients to prevent opioid tolerance, and regional anesthesia can be used but may mask symptoms of compartment syndrome in the early post-burn period.

71%

Answered correctly

2020

Year asked

73%

Answered correctly

2017

Year asked

74%

Answered correctly

2014

Year asked

Author
Andrew Kim, MD and Brett Toimil, MD