ABG Study Series: Methemoglobinemia


(Example of a sample ABG printout, by author)

When we talk about airway and metabolic management, one of the first tools we think of to really get a good look into what’s actually going on with our patient is an Arterial Blood Gas (ABG). This, as we know, is a small-volume sample of arterial blood (most often taken from one of the radial arteries near the wrist) that is then analyzed to determine the type, amount, and concentration of the gases and particles in the blood directly relating to the overall respiratory and metabolic status of the patient in question.

Most of the time, the items on the resulting test levels that we’re most concerned with are overall pH level, pCO2 level, and HCO3- level. Simply by reading these three results we can quickly determine (using various methods ranging from rote memorization to mnemonics like ROME) whether or not the patient is acidotic or alkalotic, if the root cause of the dysfunction (if any) is metabolically driven or respiratory-induced, and if the attempts (if any) by the opposite system are having any effect (i.e. normal pH, uncompensated/partially compensated/fully compensated metabolic or respiratory acidosis/alkalosis). We can then use our super-duper respiratory management skills and/or medications to ramp up the compensatory mechanisms to get the patient back into a state of homeostasis (or as close as we can), or even better and obviously preferred, fix the underlying problem.

But what if we’re Provider-On-The-Spot with all the right answers and interventions, but the patient doesn’t play by the rules and start to improve or just straight up resolve? (I’m speaking strictly of respiratory emergencies, here… let’s not get crazy)

All other things being well in order, this patient should by all means be dramatically improving and hopefully will soon be off of the NPPV or extubated and dancing and singing our praises for our amazing fixy-uppy superpowers.

But they’re not. In fact, they’re worsening. They’re severely hypoxic. They’re severely cyanotic. They’re. Still. Dying. Why?? The tube’s perfect. Vent settings couldn’t get any prettier. Breath sounds are clear in all fields with fantastic bilateral chest rise and fall. The radiographs (PA CXR shown below) are picturesque as can be. What is going on?


(Image courtesy of Google Images public domain image search)

 Let’s enter Scenario World-

Local medics bring an intubated 56 year old female respiratory failure patient into your ED. Proper ETT placement verified by multiple assessment techniques including x-rays, and her vent settings and management strategy are both appropriate. Per EMS, pt was found to be in severe respiratory distress, which daughter states was rapid in onset following pt’s return home from her dermatologist’s office this afternoon. Cyanosis is noted to pt’s face and hands upon arrival to your ED, despite appropriate airway management pre-hospital. The patient has an SpO2 of 88% with an fiO2 of 100%. EtCO2 reading is 52mmHg. Medics increased the patient’s ventilations to 18bpm with no change in pt status. Patient is reported to have a history of COPD, atrial fibrillation, and hypertension, with no known allergies. Breath sounds are clear in all fields with equal chest rise and fall. You are informed that the patient’s daughter is in the waiting area of the ED and is a very thorough historian according to EMS.

Take a minute and reflect-

-What does your gut tell you?

-Why isn’t this patient responding to interventions?

-What pieces of information are you expecting to hear from the daughter?

-What other pieces of information are you looking for (Vitals, Labs, etc)?


Back to the ED-

Patient’s daughter arrives at bedside and says that the patient had a dermatology appointment this afternoon in which she had 6 places on her chest and arms biopsied. Daughter relates that the patient began complaining of increasing difficulty breathing about an hour after arriving back home. When patient’s home oxygen (that she usually only uses at night) didn’t seem to help, her daughter phoned 911. Latest vital signs are: B/P 158/94, HR 110, Respirations are provided via ventilator at 18/min. SpO2 has decreased to 80%, with an EtCO2 of 56mmHg. An ABG is ordered and taken, with the results as follows:

-pH 7.31

-pCO2 58mmHg

-HCO3 32mmol/L

-MetHb 42%

The patient’s blood was noted to have a marked dark brown color upon lab and ABG draw. In light of these findings, the patient is administered Methylene Blue at 1mg/kg IV over 5 minutes (repeatable up to a maximum of 7mg/kg) and has serial ABGs ordered.

Shortly following the Methylene Blue administration, the patient’s status slowly begins to improve. The patient is monitored closely, and her upward status trend continues as she’s transferred to ICU for ongoing care.

So what just happened? What did the findings tell you? Why did the Methylene Blue work? The answer is a mouthful: Methemoglobinemia secondary to topical Benzocaine application during her biopsies. Let’s dive in a bit deeper-

Methemoglobinemia is a process that occurs in the presence of increased Methemoglobin in the blood stream. Methemoglobin is formed when the iron content in heme oxidizes into Ferric Iron. Oxygen does not bind to Methemoglobin like it does Hemoglobin, leading to hypoxic hypoxia, cyanosis, and respiratory distress progressing to respiratory failure and death. These symptoms will not necessarily respond to standard airway management techniques, as in this case. Normal MetHb is 0-1.5%. Patients will most likely begin to show symptoms of Methemoglobinemia at or above 15-20%, to include the symptoms listed above as well as a ‘chocolate’ or dark brown coloration to the blood. There are two actual types of Methemoglobinemia, Acquired (Due to exposure to certain drugs or toxic substances) and Congenital (most often passed from one or both parents). The different sub-types of Methemoglobinemia are listed below:

-Type I, which is congenital with one or both of the parents passing the trait on to the patient. This type is typically benign in nature and most often presents with only slight to moderate distal cyanosis which can be treated via medication and/or cosmetic procedures.

-Type II, which is also congenital, but is malignant in nature and is usually discovered at birth as the newborn has noted cyanosis and other associated symptoms and/or failure to thrive. Most infants do not survive the first year of life.

-Hemoglobin M is a genetic disorder that may occur even in the absence of familial history. Hemoglobin M is benign and typically does not present with any signs or symptoms

-Acquired Methemoglobinemia is the most common type of Methemoglobinemia and usually develops after use or exposure to one of several drugs or toxins. Some of the drugs known to cause Methemoglobinemia are those used topically for numbing either the skin (as in our case study patient) or the mouth.These drugs are also known to have caused rebound Methemoglobinemia following Methylene Blue administration. Rebound symptoms have been known to occur as late as 12 hours post-treatment. One example of a toxic substance known to have caused Methemoglobinemia is Trinitrotoluene, or TNT. Trinitrotoluene is one member of the family of nitrates, which is known to be one of the most common chemicals to cause Acquired Methemoglobinemia. It has been shown that TNT-induced Methemoglobinemia has been found in patients exposed to the chemical through manufacturing, mining processes, and in people who have consumed water that contained runoff from TNT manufacturing areas. Studies have even found that blast victims in which TNT was used have acquired Methemoglobinemia as a result of exposure to the chemical. Patients with a COPD/Asthma/Cardiac Disease history are at an increased risk of Acquired Methemoglobinemia.

Treatment of Acquired Methemoglobinemia can be varied. The first-line intervention used most often is IV Methylene Blue administration. While this treatment is the traditional primary method, it may not work in patients with Congenital Methemoglobinemia, regardless of whether or not they are also suffering from acute Acquired Methemoglobinemia. These patients, along with those whose symptoms are refractory to Methylene Blue treatment (without previous history of Methemoglobinemia) may receive further treatment in the form of blood transfusion. There are several other medications/treatments that are currently undergoing study for efficacy in Methemoglobinemia treatment, but Methylene Blue and blood transfusion appear to be the gold standard treatments for the time being. In fact, in a study done in the year 2000, Methylene Blue (given at a dose of 10microM) was noted to reduce the overall half-life of Nitric Oxide-induced methemoglobin from an average of 356 minutes in the control group to just 5 minutes. In the same study the other medications that were tested (N-Acetylcisteine and Riboflavin) had very little to no effect on the overall half-life of the methemoglobin, with High-Dose Riboflavin (120 microM dose) having a more favorable effect than N-Acetylcisteine with a reduction to 168 minutes vs no reduction from the N-Acetylcisteine administration. There have also been studies done to evaluate the efficacy of hyperbaric oxygen administration to treat Methemoglobinemiawith promising preliminary results.


After spending a day in the ICU for observation due to the potential for rebound symptoms, the patient was successfully extubated and moved to a Medical Surgical floor where she was discharged symptom-free a few days later. The patient and her daughter are now aware of the patient’s sensitivity to Benzocaine and updated her Emergency Medical Information accordingly.



D. G. Dempsey is a Flight Paramedic based in North Dakota. Originally from Central Florida, he has been involved with emergency services since he began as a volunteer firefighter in 1999. He has worked for local government (both stand-alone and fire-based) agencies in voluntary and career EMS and Fire roles, as well as in private air medical and ED clinical settings. Dempsey is an avid student of Free Open Access Medical Education (FOAMed) and Evidence Based Medicine (EBM).

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