Methaemoglobinaemia in a patient scheduled for double valve replacement surgery - A case study

Dear Editor,

The occurrence of methaemoglobinaemia as a cause of desaturation in cardiac surgery patients is quite rare. We present a case of methaemoglobinaemia that was incidentally detected in a patient scheduled for double valve replacement. Written informed consent was obtained from the patient for publication of the case including the photograph.

A 41-year-old female patient was scheduled for double valve replacement surgery. Preoperative evaluation revealed no positive clinical signs other than cardiac murmurs correlating with the diagnosis. The pulse oximetry showed an oxygen saturation of 84%–86%. An arterial blood gas analysis was done at bedside and showed a pH of 7.47, partial pressure of oxygen (PaO₂) of 103 mmHg, and an arterial oxygen saturation (SaO₂) of 98%, despite the sample looking dark brown like venous blood. Since the patient was asymptomatic from a respiratory point of view (with normal PaO₂)and an echocardiogram had revealed no shunt lesions to explain the low saturation levels, we decided to proceed with the scheduled surgery.

In the operation theatre, the patient was preoxygenated with 100% oxygen for three minutes, which failed to improve the saturation. An uneventful anaesthetic induction followed by sternotomy revealed a dark brown pericardium. Marrow bleeding was also dark, unlike the bright red blood normally seen. After the establishment of cardiopulmonary bypass (CPB), it was observed that the blood in both the aortic and venous cannulae was of the same dark colour [Figure 1], in spite of 100% oxygenation through the CPB machine. An arterial blood gas with co-oximetry showed a PaO₂ of 510 mmHg, an SaO2 of 99%, and a methaemoglobin level of 28%—higher than the maximum acceptable level of 3%—thus confirming the diagnosis. The surgery was uneventful, and the patient was weaned off the CPB with minimum inotropic supports. Nitroglycerin was not used because it could precipitate methaemoglobinaemia. However, oxygen saturation (SpO₂) continued to remain at 85%–90%. In the ICU, repeat co-oximetry showed a persisting high methaemoglobin level of 23%, with PaO₂ measuring 160 mmHg, SaO₂ measuring 99%, and pH measuring 7.38. Hence, we decided to treat the patient with an intravenous methylene blue. Seventy-five milligrams was administered as slow intravenous infusion over a period of 15 minutes. Saturation immediately improved to 98%–100%. Fraction of inspired oxygen (Fi_O2) was gradually reduced from 0.6 to 0.4. Co-oximetry now showed a methaemoglobin level of 0.3%. The treatment was continued twice a day for the next two days and was supplemented with 500 mg of ascorbic acid once a day. The patient continued to maintain normal levels of saturation without supplemental oxygen and was discharged on the fifth postoperative day.

Figure 1.

Darkly coloured blood observed in both the aortic and venous cannulae

Methaemoglobinaemia can be either congenital[1] or be acquired due to exposure to oxidising agents.

The physiological level of methaemoglobin in the blood is 0%–2%. Patients become symptomatic at methaemoglobin levels higher than 25%–30%. Cyanosis starts when methaemoglobin is around 15%; the chocolate brown blood, as seen in our case, appears when the level is 15%–30%. Dyspnoea develops between 30% and 50%. Severe manifestations such as metabolic acidosis, arrhythmias, coma, and convulsions occur at 50%–70%. Methaemoglobin levels of 70% or higher may be fatal.[2]

Routine pulse oximetry and arterial blood gas analysis failed to detect the presence of methaemoglobin. A co-oximeter measures absorbance at over 100 wavelengths to construct a continuous absorption spectrum from 450 to 700 nm. A peak absorbance of light at 630 nm characterises methaemoglobin and is considered to be diagnostic.[3]

Methylene blue is the agent of choice to treat methaemoglobinaemia. It interacts with the enzyme NADPH diaphorase to form leucomethylene blue and reduces methaemoglobin to Hb. Leucomethylene blue is excreted in the urine which turns green or blue upon stasis, as was seen in our case [Figure 2]. It may also be excreted via the bile, resulting in blue stool. Some adverse effects associated with methylene blue infusion include anaphylaxis, haemolysis, and encephalopathy. Patients with glucose-6-phosphate dehydrogenase deficiency are at particular risk of overdose and haemolytic anaemia.[4] Vitamin C or ascorbic acid and dextrose can be used as adjunctive treatments, although efficacy has not been proven. Exchange transfusion may be used in patients with severe life-threatening methaemoglobinaemia, with methaemoglobin levels exceeding 70%.[5]

Figure 2.

Green-coloured urine after the administration of methylene blue

A saturation gap between arterial blood gas analysis and pulse oximetry also helps in the diagnosis when the methaemoglobin level is not available.[6]

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understands that her name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.