Hyperbaric Oxygen Therapy Indications Simplified

Indication #2: Carbon Monoxide Poisoning

It is estimated that in the United States, accidental CO poisoning occurs in about 50,000 people annually, resulting in at least 430 accidental deaths.¹ Fire-related smoke inhalation is responsible for most of the accidental CO poisoning cases reported. Carbon monoxide alarms can protect against CO poisoning.² However, there are close to 15,000 intentional CO poisonings annually, which account for close to two-thirds of the deaths related to CO poisoning.³

The diagnosis of CO poisoning is typically based on a high level of suspicion from the clinician, as CO poisoning typically presents with normal levels of oxygen saturation on SPO₂ monitoring. Standard SPO₂ monitoring is unreliable in the diagnosis of CO poisoning. SPO₂ monitors can measure what percentage of blood is saturated, but they are not able to differentiate between carboxyhemoglobin (COHb) and oxyhemoglobin.⁴ A thorough clinical history should be taken and effective interprofessional collaboration with emergency medical services (EMS) personnel can be critical for a prompt diagnosis. Suspected fire victims—along with patients who report flu-like symptoms with other members of the household and/or pets that are also exhibiting similar symptoms—should prompt the clinician to consider CO poisoning as a possible diagnosis.

CO’s Affinity for Hemoglobin and the Oxygen Hemoglobin Dissociation Curve

The oxyhemoglobin dissociation curve graphs the relationship between hemoglobin saturation with oxygen across different partial pressures of oxygen in the blood.
 
Under normal circumstances, oxygen is bound to hemoglobin and can dissociate to deliver oxygen to tissues. When present, CO will compete with oxygen for a binding site on hemoglobin. Carbon monoxide has 200–250 times greater affinity for hemoglobin than oxygen does,⁵ which will allow the CO to reversibly bind to hemoglobin even at small environmental concentrations. Once CO binds to hemoglobin, it does not unload easily, making those binding spots unavailable for O₂.

This carboxyhemoglobinemia will then shift the oxyhemoglobin dissociation curve to the left. This increases the affinity for O₂ to compete with CO, but in turn, also results in decreased unloading of O₂. The affinity of CO for myoglobin is even greater than that for hemoglobin, which can result in cardiac pathology.⁶

A Guide to the Signs and Symptoms of CO Poisoning

Signs and symptoms of CO poisoning can be vague and nonspecific. The most reported symptoms include headache, malaise, dizziness, weakness, nausea, confusion/fogginess, shortness of breath, and visual changes.^7,8 These symptoms can be confused for other common conditions like upper respiratory infections, flu-like syndromes, and acute gastroenteritis, making it even more important that clinicians remain vigilant for cases of CO poisoning.

The classic presentation historically described includes “cherry red lips” with peripheral cyanosis. These findings are rare and lack specificity and sensitivity and should not be relied on for a diagnosis of CO poisoning.⁹ 

Making a Diagnosis

Blood co-oximetry is the gold standard for confirming exposure to CO. However, diagnosis and management should be guided by the presentation of symptoms rather than the level of COHb because COHb levels are unable to reveal the degree of CO poisoning. There are tools available that can measure COHb and methemoglobin. These include pulse oximeters as well as infrared spectrometers coupled with smartphone technology.¹⁰ These should be used as screening rather than diagnostic tools but could play an important role in the rapid identification and intervention of cases of CO poisoning. Additionally, the clinician must complete a thorough assessment and consider concurrent exposures to other toxic agents such as drugs and poisons, particularly in the setting of intentional CO poisoning.¹¹

Complications and Long-Term Effects of CO Poisoning

The cardiovascular and central nervous systems are the most susceptible to injuries from CO poisoning and carry an additional risk of long-term complications. Hypoxic stress induced by CO results in cardiac injury, which can be irreversible and lead to death.¹² Myocardial injury occurs frequently in patients hospitalized for moderate to severe CO poisoning and is a significant predictor of mortality. Neurologic sequelae can include headaches, peripheral neuropathies, hearing loss, seizures, gait disturbances, movement disorders, motor weakness, and cognitive decline, to name a few.¹³ Weaver et al describe delayed neurologic sequelae as symptoms that can be attributed directly to CO poisoning and are evident immediately following poisoning.¹⁴ These can include mood disturbances, personality changes, and memory loss.

Insights on the Treatment of CO Poisoning

After prompt removal from the source of CO, immediate evaluation in the emergency department is warranted. While diagnostic testing with a COHb level is ideal, this should not delay management.

Oxygen has the capacity of shortening the half-life of COHb as it will compete for binding sites on hemoglobin; therefore, high-flow O₂ via a non-rebreather mask is a standard first step. Hyperbaric oxygen therapy (HBOT) has been shown to treat carbon monoxide poisoning since the 1800s. Since then, we have used HBOT for CO poisoning treatment. Therefore, patients should be transported to a facility with a hyperbaric oxygen chamber, all while monitoring the cardiovascular and respiratory status of the patient closely.

The decision to administer HBOT should be taken by an experienced clinician. The mechanism of action of HBO therapy is related to its ability to decrease the half-life of COHb from 4–6 hours to 15–30 minutes.⁸ HBOT should be administered as soon as possible if the patient is stable enough to enter the chamber.

Randomized trials demonstrating improved outcomes have offered two protocols:

1. Initial compression to 3 ATA, then 2 ATA for 140 minutes followed by two HBO sessions at 2 ATA for 90 minutes (with 5-minute air breaks used periodically to reduce oxygen toxicity) in 6–12 hours.15  

2. Initial compression to 2.8 ATA, then 2 ATA for 120 minutes, without further HBOT.¹⁶ HBOT has been associated with improved outcomes and decreased short and long-term mortality after CO poisoning.¹⁵ In patients with acute, symptomatic CO poisoning who received three HBOT sessions within a 24-hour window, the rates of cognitive sequelae were reduced.¹⁷

What You Should Know About Pregnancy and CO Poisoning

Pregnancy has long been thought to be an absolute contraindication to HBOT, but in the case of CO poisoning, it can be extremely beneficial and improve fetal outcomes. Fetal hemoglobin differs from adult hemoglobin in that it has 2 alpha and 2 gamma subunits, rather than 2 alpha and 2 beta subunits. This makes HBOT’s affinity to both oxygen and CO particularly high. HBOT should be considered in all pregnant patients with CO poisoning.^18,19

A Word to the Wise

·      Maintaining a high level of suspicion is critical for a prompt diagnosis of CO poisoning.

·      Standard SPO₂ monitors will not aid in the diagnosis of CO poisoning.

·      Normobaric and hyperbaric oxygen therapy are the standard treatment for CO poisoning.

·      CO poisoning is the exception to the rule when it comes to administering HBOT to pregnant patients.

·      Clinicians should always discuss possible cardiovascular and central nervous system sequelae with CO-poisoning patients. Appropriate referrals should be done.

·      Portable generators were the most common source of carbon monoxide exposure after a storm that resulted in power losses; car exhaust was the most frequent source of exposure after an extensive snowstorm.

·      Carbon monoxide alarms can protect against CO poisoning.

Denise Nemeth is a second-year medical student at the University of the Incarnate Word School of Osteopathic Medicine in San Antonio, TX. Formerly a general and vascular surgery PA in a rural community, Ms. Nemeth aspires to become a general surgeon. She is certified wound specialist with the American Board of Wound Management. Her interests include rural health, wound healing, colorectal surgery, and minimally invasive surgery.  

Jayesh B. Shah is Immediate Past President of the American College of Hyperbaric Medicine and serves as medical director for two wound centers based in San Antonio, TX. In addition, he is president of South Texas Wound Associates, San Antonio. He is also the past president of both the American Association of Physicians of Indian Origin and the Bexar County Medical Society and Current of Board of Trustees of Texas Medical Association.

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References
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