Total respiratory resistance and reactance in ankylosing spondylitis and kyphoscoliosis

Ankylosing spondylitis and kyphoscoliosis both alter the function of the lung by modifying the mechanical properties of the thoracic cage. The purpose of the present study was to assess the changes in total respiratory resistance (Rrs) and reactance (Xrs) in these patients and to compare these data with conventional pulmonary function tests. In 16 patients with ankylosing spondylitis and seven with kyphoscoliosis we measured lung volumes, maximal flows, diffusing capacity, airway resistance, lung compliance and Rrs and Xrs between 2-26 Hz by means of the forced oscillation technique (FOT). In the patients with ankylosing spondylitis mean total lung capacity was 83% predicted (range 60-105%). Mean values of Rrs were normal; there was a small decrease in Xrs at the lowest frequency. In the patients with kyphoscoliosis mean total lung capacity (TLC) was 41% predicted for arm span (range 26-75%). Mean Rrs was elevated with a negative frequency dependence, and mean Xrs was decreased. The observed differences in Rrs and Xrs between the two groups of patients are related to differences in severity of the restriction. There is evidence that the changes in Rrs and Xrs in both groups are mainly attributable to an increase in chest wall resistance and a decrease in chest wall compliance, while in the patients with kyphoscoliosis an increase in airway resistance and a decrease in lung compliance also intervenes.