Venous serum chloride and bicarbonate measurements in the evaluation of respiratory function in motor neuron disease.

Respiratory failure, with or without pneumonia, is the usual cause of death in patients with motor neuron disease (MND). Forced vital capacity (FVC) is often used to monitor respiratory function in MND and is, in part, predictive of survival time. However, such volitional tests are unreliable in many patients, especially later in the disease, and access to hospital laboratories can also be a problem for some disabled patients. We assessed the use of domiciliary venous serum chloride and bicarbonate measurements in evaluating respiratory function in MND. Newly-diagnosed MND patients (n=23) were followed-up at home every 3 months for up to 15 months. Respiratory symptoms were measured using a questionnaire, and FVC was documented. Venous serum chloride and bicarbonate were also measured. One patient had symptoms of airway obstruction disease, and was excluded from the analysis. Ten patients developed abnormally low chloride (mean 95, range 88-97, reference interval 98-107 mmol/l) and an abnormally high bicarbonate (mean 33, range 31-37, reference interval 22-30 mmol/l) during follow-up, of whom eight died within the next 5 (mean 2.2, range 0.5-5) months; two were still alive at the end of the study but had developed respiratory symptoms. Twelve patients had normal chloride and bicarbonate during follow-up: all were still alive at 15 months, all had a FVC of >50% predicted, and only one had respiratory symptoms at their last assessment. Raised bicarbonate and low chloride were associated with the presence of respiratory symptoms suggesting respiratory muscle weakness. Venous serum chloride and bicarbonate potentially can provide useful information about respiratory status and prognosis in MND patients.