To assess the relationship between thoracoabdominal motion during quiet breathing and standardised indices of disease severity in patients with ankylosing spondylitis (AS); also to evaluate whether thoracoabdominal motion improves after institution of biological agents in these patients.
Displacement of the rib cage (RC) and abdomen (Abd) during quiet breathing in the sitting, standing and supine position were recorded by impedance plethysmography in 60 patients (mean (SD) age 41 (10) years, 56 men) and 21 healthy men (mean (SD) 36 (7) years). x-y plots of RC versus Abd displacement during quiet breathing were constructed, and the angle of the slope of the RC-Abd loop was calculated and averaged for five consecutive breaths. In 13 patients treated with anti-tumour necrosis factor alpha (TNFalpha), measurements were made before and at 3, 6 and 12 months after the start of treatment.
In the entire AS group, the angle of the slope of the RC-Abd loop correlated with Bath Ankylosing Spondylitis Functional Index (BASFI) in the sitting (R = -0.50, p<0.0001), standing (R = -0.36, p = 0.004) and supine (R = -0.47, p = 0.0001) position, but not with Bath Ankylosing Spondylitis Disease Activity (BASDAI), Bath Ankylosing Spondylitis Metrology Index (BASMI) or the modified Schober's test. In 13 patients treated with anti-TNFalpha, the angle of the RC-Abd slope improved significantly (35-69% over baseline at 3 months) in all body positions and in a nearly parallel fashion with the improvements in standardised clinical measurements.
The pattern of thoracoabdominal motion during quiet breathing correlates with BASFI, and its response to anti-TNFalpha treatment is large. This variable may be an appropriate target for evaluating potential usefulness in monitoring thoracic spine involvement and response to treatment in AS.