Analysis of lung volume restriction in patients with respiratory muscle weakness.

TLDR

The study examined pulmonary mechanics in 25 neuromuscular disease patients aged 9–55 with varying respiratory muscle weakness. Patients exhibited markedly reduced inspiratory muscle force (average 39 % predicted) and a disproportionate loss of vital capacity (only 50 % predicted versus the expected 78 %), driven by decreased lung distensibility from microatelectasis and reduced chest wall recoil, indicating that vital capacity is the most informative metric for monitoring disease progression or treatment response.

Abstract

We investigated pulmonary mechanics in 25 patients, 9 to 55 years of age, with a variety of generalised neuromuscular diseases and variable degrees of respiratory muscle weakness. The average degree of inspiratory muscle force was 39.2% (range 8-83%) of predicted. The lung volume restriction far exceeded that expected for the degree of muscle weakness: the observed decrement in respiratory muscle force should, theoretically, decrease vital capacity to 78% of its control value, while the mean VC in our patients was only 50% of predicted. Analysis of lung pressure-volume curves indicated that the two principal causes of the disproportionate loss of lung volume were a reduction in lung distensibility probably caused by widespread microatelectasis, and a decrease in the outward pull of the chest wall. Because it reflects both direct (loss of distending pressure) and secondary (alterations in the elastic properties of the lungs and chest wall) effects of respiratory muscle weakness on lung function, we conclude that, in these patients, the vital capacity remains the most useful measurement to follow evolution of the disease process or response to treatment.