Background It is common for patients with exertional dyspnoea to be referred to Cardiology for investigation. This case highlights a rare but important cause of cyanosis, diagnosed after extensive investigations.
Case Presentation A 73-year-old was referred to the Respiratory Medicine team with limiting exertional dyspnoea. Initial standard investigations proved unremarkable, with normal spirometry, CT pulmonary angiogram, V/Q scan, 6-minute walk test and normal PaO2 on arterial blood gas sampling. The patient was then referred to Cardiology for consideration of a cardiac cause of her symptoms. Her past medical history included rheumatoid arthritis, retinal vein thrombosis and idiopathic right arm thrombosis. After her initial review by Cardiology, a decision was made to perform both a coronary angiogram and right heart catheterisation. The coronary arteries showed no significant flow limiting lesion and the pulmonary pressures were not significantly elevated with no evidence of a shunt During the procedure, it was noted that the patient’s blood was dark brown in colour (figure 1), with low systemic (66%) and pulmonary artery (48.6%) oxygen saturation as measured on the cath lab oximeter, with normal respiratory rate (12/minute). Arterial blood was mixed in a syringe with 100% oxygen, which produced no change in the colour. A full blood gas analysis was therefore undertaken, which identified a disparity between the measured PaO2 (12.0kPa) and oxygen saturations. A haemoglobinopathy was therefore suspected, which was felt most likely to be a case of methaemoglobinaemia. However, it proved not possible to measure this on the multiple point of care ABG machines, nor at the main biochemistry laboratory. The patient was subsequently investigated extensively by Haematology, ultimately identifying diagnosis of sulfhaemoglobinaemia. The source for the sulphated haemoglobin was due to an Epsom salt (MgSO4) containing tonic the patient was using for constipation. The patient was treated with exchange transfusions under the care of the haematology team alongside stopping the Epsom salt tonic.
Discussion Sulfhaemoglobinaemia (SulfHb) is a rare haemoglobinopathy which arises due to the sulphation of haemoglobin, this results in a significantly reduced oxygen carrying capacity due to reduced affinity for oxygen (Lu et al., 1998); it is often reported that the source of exogenous sulphur is from medications, bowel pathology or occupational exposures. This case highlights the importance of right heart catheterisation as an investigation to help differentiate the causes of exertional dyspnoea and the importance of considering full blood gas analysis when the numbers do not add up.
Conclusion Sulfhaemoglobinaemia is a rare cause of exertional dyspnoea and should be considered in patients with a clear disparity between saturations measured by pulse oximetry and atrial blood sampling.
Conflict of Interest None
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