Page 17 - ANZCP Gazette November 2021

Oxyhaemoglobin absorbs infrared light and deoxyhaemoglobin absorbs red light. Given that pulse oximetry measures only two wavelengths (oxyhaemoglobin and deoxyhaemoglobin) it cannot be used as a diagnostic tool. Co-oximetry can be used to determine methaemoglobin levels given that it measures four wavelengths: oxyhaemoglobin, deoxyhaemoglobin, carboxyhaemoglobin and methaemoglobin. It can be applied when there is a discrepancy between oxygen saturation and partial pressures of oxygen. Dissolved oxygen will remain unchanged therefore blood gas readings could show a high PaO2.
Allosteric interactions increase the affinity for oxygen at remaining binding sites therefore shifting the oxygen dissociation curve to the left (Figure 4).
When there are high levels of haemoglobin in this state it produces an anaemic hypoxia.
Case History
The patient (25 year old male of Indian ethnicity) presented to the emergency department (ED) with suicidal ideation. He had a long history of psychiatric mental health illness (bipolar affective disorder with anxiety) and several admissions to various EDs following suicide attempts. The patient was moved to the Behavioural Assessment Unit (BAU) and appeared to be ‘settled’. When the nurse went to check in on him they noticed him slumped and unconscious and a code blue was called. The ED and ICU teams responded as he was in cardiac arrest. Review of CCTV footage 12 hours later showed the patient had taken packets of an unknown powdered substance (it is not known how or where the patient acquired this substance) and unknown quantity.
Cardiopulmonary resuscitation (CPR) was commenced and four boluses of adrenaline given with sodium bicarbonate. An adrenaline infusion was started alongside continuation of CPR (resuscitative efforts continued for 60 minutes). The decision was made to put him on extra-corporeal membrane oxygenation (ECMO) with a veno-arterial (VA) configuration. ECMO (Maquet Rotaflow and Maquet PLS system) was started in BAU, but on starting the support it was noted the blood from the arterial side was dark brown/black. Initially it was thought that the oxygenator membrane was the problem. The perfusionist started a systematic check of all components and eventually connected a cylinder of oxygen directly to the oxygenator therefore eliminating a gas blender failure. This did not aid in changing the colour of the arterial blood and the decision was taken to move to theatre and commence cardiopulmonary bypass (CPB) in order to gain more control and to assess the oxygenator function of the ECMO circuit. Once on CPB, the same dark brown/black blood was seen and an arterial blood gas (ABG) was taken and showed a MetHb of 90%. It was at this stage, in conjunction with advice from the Poisons Department, that methylthioninium chloride (methylene blue) was commenced and a red cell exchange of 13 units of PRBCs were transfused. There was found to be no issue with the membrane of the initial oxygenator used.
The patient was successfully transferred to theatre and support from ECMO to conventional CPB was uneventful – as per unit protocols. The patient remained on bypass for 217 minutes. The decision was made to place the patient back on VA ECMO support and the patient was transferred with a MetHb of 10.7% to ICU post treatment with methylene blue.
The patient spent 36 hours on ECMO but transoesophageal echocardiography and neurologic investigation showed cardiac failure (heart was asystolic) and severe neurological damage (absent brain stem reflexes). It was also determined that there was irretrievable end-organ damage (liver transaminitis). The decision was made to terminate ECMO support based on clinical evaluation and family wishes.
Laboratory testing of the patient’s blood has yielded results indicating combined drug intoxication. Drugs found to be present in the blood and those prescribed are listed in Table 2:
NOVEMBER 2021 | www.anzcp.org 14