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) i s often used to monitor respiratory function in MND and is, in part, predic tive of survival time. However, such volitional tests are unreliable in man y patients, especially later in the disease, and access to hospital laborat ories can also be a problem for some disabled patients. We assessed the use of domiciliary venous serum chloride and bicarbonate measurements in evalu ating respiratory function in MND. Newly-diagnosed MND patients (n = 23) we re followed-up at home every 3 months for up to 15 months. Respiratory symp toms were measured using a questionnaire, and FVC was documented. Venous se rum chloride and bicarbonate were also measured. One patient had symptoms o f airway obstruction disease, and was excluded from the analysis. Ten patie nts developed abnormally low chloride (mean 95, range 88-97, reference inte rval 98-107 mmol/l) and an abnormally high bicarbonate (mean 33, range 31-3 7, reference interval 22-30 mmol/l) during follow-up, of whom eight died wi thin 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 h ad normal chloride and bicarbonate during follow-up: all were still alive a t 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 respirator y muscle weakness. Venous serum chloride and bicarbonate potentially can pr ovide useful information about respiratory status and prognosis in MND pati ents.