LAUSR

Introduction Most people with motor neurone disease (MND) die from ventilatory failure due to respiratory muscle weakness. Once PaCO2 is greater than 6 kPa, survival is measured in weeks but repeated blood gas estimates (ABG’s) can be difficult to organise and painful for the patient. Based on expert opinion symptoms and oxygen saturation measured with a pulse oximeter (SpO2) are proposed as a screening tool for ventilatory failure in MND care (NICE 2016). In our MND clinic we routinely record daytime and night time SpO2 and ABG’s in patients not using home ventilation. We asked the question, could the NICE screening method safely reduce the number of ABG’s performed? Methods Data, consisting: age, sex, relevant symptoms (orthopnoea, headaches, sleepiness), SpO2 at rest in the day and daytime PaCO2 on air were extracted from the records of a retrospective, consecutive series of patients seen in the clinic. If a patient was using non-invasive ventilation (NIV) the last ABG taken before starting NIV was recorded (with contemporaneous symptoms and SpO2). The data were examined for sensitivity and specificity of relevant symptoms and/or SpO2 ≤94% for triggering an ABG revealing a PaCO2 >6 kPa. Results The records of a convenience sample of 100 patients were examined, from the 3 months from January to March 2018. The mean age was 67.1 (11.97) years and 42 of the patients were women. The number of patients with a PaCO2 >6 kPa at that visit, or from a previous visit if using home NIV, was 25. At these same visits 25 patients had SpO2 ≤94% of whom just 7 had a PaCO2 >6 kPa giving a sensitivity of 28% and specificity of 76%. Relevant symptoms were present in 22 patients and increased the number of cases found to 13 (a sensitivity of 52%) while lowering the screening specificity to 60%. Conclusion The screening method recommended in the NICE guidance risks missing half of patients who have developed ventilatory failure, patients who could benefit from increased longevity and quality of life if initiated on NIV. Until a better screening paradigm is developed regular ABG’s are still required.