Page 5 - ANZCP GAZETTE DECEMBER 2023 TEASER

METHYLENE BLUE AND ECMO: A FALSE FLAG CASE REPORT
Cropper J. St Vincent’s Hospital Darlinghurst NSW
The light absorption spectrum of methylene blue is maximum at wavelengths of 668 nm. This is close to the 660 nm wavelength used in pulse oximetry for its strong absorption by deoxygenated haemoglobin. It is well documented that the presence of methylene blue in the circulation artificially raises the ratio of de-oxygentated haemoglobin:oxygenated haemoglobin thereby falsely lowering the measured percentage of saturated blood1. A similar phenomenon is reported with the use of near- infrared spectroscopy (NIRS) to measure cerebral blood saturation2.
In the case presented here methylene blue leached into the plastic tubing of the ECMO during its prolonged presence in the patient’s circulation. Consequently it acted as a blue light filter, absorbing non-blue wavelengths of light. This created the appearance of desaturated blood upon visual inspection. The phenomenon is worthwhile noting alongside the dyshaemoglobins, such as methaemoglobin, and their effect on the appearance of blood and blood oximetry measurements.
1. Chan ED, Chan MM, Mallory MM. Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations. Resp Med 2013;107:789-799
2. Mittnacht AJC, Fischer GW, Reich DL. Methylene Blue Administration Is Associated with Decreased Cerebral Oximetry Values. Anesthesia & Analgesia 2007;105:549-550
A 55 year old male (67 kg) underwent a heart transplant operation requiring a redo sternotomy and removal of his left ventricular assist device (Heartmate 3TM, Abbott). His post-operative course was complicated by global dysfunction of the donor heart and significant bleeding requiring multiple transfusions. Patients who have had an LVAD commonly have varying degrees of vasoplegia post- transplantation and in this patient it was profound, requiring ever increasing doses of multi-therapy vasopressors. In order to support the patient’s haemodynamics, veno-arterial (VA) ECMO (Cardiohelp System, Getinge) was instituted intraoperatively with a Bioline-coated 25Fr right femoral venous cannula and a 19Fr arterial cannula inserted into an 8 mm Gelweave graft (Terumo) which was then attached to the right axillary artery. Seventeen hours after admission to ICU the patient remained profoundly hypotensive therefore a methylene blue infusion was commenced based on our institutional protocol (0.5 mg/kg/hour for 6-7 hours). After a further 24 hours in ICU another 7 hour infusion of methylene blue at 0.5mg/kg/hour was given due to the profound hypotension.
On day 2 post-transplant there was rapid elevation of the trans-membrane pressure (TMP) gradient across the ECMO oxygenator and the perfusionist on-call was urgently called to assess the circuit. Upon initial examination it was noted that there was no discernible difference in the colour of the venous and arterial arms of the ECMO lines. In combination with the elevated TMP this indicated an acute failure of the ECMO oxygenator. An arterial gas was taken from the oxygenator to reveal a pO2 of >500 mmHg on an FiO2 of 1.0. A subsequent mixed venous sample revealed a SvO2 of 82%. It appeared that the blood was being sufficiently oxygenated by the ECMO oxygenator. However, in view of the ever increasing TMP and suspected acute thrombus formation within the oxygenator the decision was made to change the ECMO circuit. The circuit-change was made at the level of tubing attached to the cannulae, which remained in situ, via 3/8” connectors. When flow was initiated in the new circuit, it was observed that the arterial arm of the ECMO circuit now had the expected “bright” red oxygenated blood appearance and was noticeably different in colour to the desaturated blood of the venous limb. Interestingly, at the site of reconnection to the tubing at the arterial cannula, the blood distal to the connection remained dark. Closer inspection revealed a blue tinge to the plastic wall of the tubing that remained from the old circuit. We concluded that prolonged infusion of methylene blue had led to the plastic tubing of the ECMO circuit becoming impregnated with methylene blue.
Methylene Blue (Methylthioninium chloride) is a thiazine dye which is used medically for the treatment of severe (>30%) methaemoglobinaemia, in the treatment of septic shock and in vasoplegic syndrome. It was used in this case for the latter, utilising methylene blue’s inhibitory effect on the nitric oxide synthase pathway, reducing nitric oxide production and increasing vascular tone.
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