An investigation of factors limiting aerobic capacity in patients with ankylosing spondylitis

Ankylosing spondylitis (AS) has been shown to produce exercise limitation a nd breathlessness. The purpose of this study was to investigate factors whi ch may be responsible for limiting aerobic capacity in patients with AS. Twenty patients with no other cardio-respiratory disease performed integrat ive cardiopulmonary exercise testing (CPET). The results were compared to 2 0 age and gender matched healthy controls. Variables that might influence e xercise tolerance, including pulmonary function tests (body plethysmography ), respiratory muscle strength (MIP, MEP) and endurance (Tlim), AS severity assessment including chest expansion (CE), thoracolumber movement (TL), wa ll tragus distance and peripheral muscle strength assessed by maximum volun tary contraction of the knee extensors (Qds), hand grip strength and lean b ody mass (LBM), were measured in the patients with AS and used as explanato ry variables against the peak (V) over dot O-2 achieved during CPET. AS subjects achieved a lower peak (V) over dot O-2 than controls (25.2 +/- 1.4 vs. 33.1 +/- 1.6 mi kg(-1)min(-1), mean+/-SEM, P=0.001). When compared with controls, ventilatory response ((V) over dot(E)/(V) over dot CO2) in A S was elevated (P=0.01); however gas exchange indices, transcutaneous blood gases and breathing reserve were similar to controls. AS subjects develope d a higher HR/(V) over dot O-2 response (P<0.01) on exertion but without as sociated abnormalities in EGG, blood pressure response or anaerobic thresho ld. The AS group experienced a greater degree of leg fatigue (P<0.01) than controls at peak exercise. Although the breathlessness scores (BS) were com parable to controls at peak exercise, the slopes of the relationship betwee n BS and work rate (WR) [AS 0.054 (0.1), Controls 0.043 (0.06); P<0.05] and BS and % predicted oxygen uptake [AS 0.084 (0.18), Controls 0.045 (0.06); P<0.01] were steeper in the AS subjects. There was weak association between peak (V) over dot O-2 and vital capacity (r(2)% 12.0), MIP (11.8) but no association between Tlim, CE, Wall tragus distance or TL movement. The strongest association with aerobic capacity wa s between measurements of peripheral muscle strength (Qds; r =0.75; hand gr ip; r =0.47) accounting for 53% (P<0.001) and 23.5% (P < 0.01)of the total variance in peak (V) over dot O-2, respectively. The addition of LBM to Qds in the regression model significantly improved the explained variance to 7 8.3% (P<0.001). This study shows that peripheral muscle function is the most important dete rminant of exercise intolerance in AS patients suggesting that deconditioni ng is the main factor in the production of the reduced aerobic capacity. (C ) 1999 HARCOURT PUBLISHERS LTD.