RESPIRATORY MUSCLE FUNCTION IN AMYOTROPHIC LATERAL SCLEROSIS

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Abstract
Respiratory muscle function was studied in relation to indices of lung function in a group of patients with amyotrophic lateral sclerosis. Mouth and transdiaphragmatic pressures during maximal static inspiratory and expiratory efforts were related to total lung capacity and its subdivisions, maximal expiratory flow-volume curves, (with and without external resistive loads), lung resistance and compliance during quiet breathing and arterial blood gases. Based on the shape of the maximal expiratory flow-volume curve, 2 groups were defined. In 12 patients (group 1) the maximal expiratory flow-volume curve appeared to taper normally as residual volume was approached. In 15 patients (group 2), flow decreased with concavity toward the volume axis, giving the impression that flow had dropped off the flow-limiting envelope as residual volume was approached. Group 2 patients, as compared to group 1 patients, had lower maximal static expiratory pressures (P < 0.05), higher residual volumes (P < 0.01) smaller vital capacities (P < 0.001) and lower peak flows (P < 0.05). There were no differences between the 2 groups in maximal static inspiratory pressure, total lung capacity, lung resistance and compliance or arterial blood gases. Maximal expiratory flow-volume curves suggested that both groups achieved flow limitation over the mid-portion of the forced vital capacity. Relatively small external resistances caused a further increase in residual volume in group 2 patients. Maximal expiratory flow-volume relationships may serve to identify patients with more severe expiratory muscle weakness, who may be predicted to have a marked decrease in vital capacity and flow with small changes in resistance and may be at risk of developing respiratory failure during relatively minor respiratory illness.