Limitation in tidal volume expansion partially determines the intensity of physical activity in COPD

In patients with Chronic Obstructive Pulmonary Disease (COPD) reduced levels of daily physical activity are associated with the degree of impairment in lung, peripheral muscle and central hemodynamic function. There is however, limited evidence as to whether limitations in tidal volume expansion also, importantly determine daily physical activity levels in COPD. Eighteen consecutive COPD patients [9 active (FEV1:1.59±0.64 liters) with an average daily movement intensity >1.88 m/sec(2) and 9 less active patients (FEV1:1.16±0.41 liters) with an average intensity <1.88 m/sec(2)] underwent a 4-min treadmill test at a constant speed corresponding to each individual patient's average movement intensity, captured by a triaxial accelerometer during a preceding 7-day period. When chest wall volumes, captured by Optoelectronic Plethysmography, were expressed relative to comparable levels of minute ventilation (ranging between 14.5±4.3 to 33.5±4.4 liters/min), active patients differed from the less active ones in terms of the lower increase in end-expiratory chest wall volume (by 0.15±0.17 versus 0.45±0.21 liters), the greater expansion in tidal volume (by 1.76±0.58 versus 1.36±0.24 liters) and the larger inspiratory reserve chest wall volume (IRVcw: by 0.81±0.25 versus 0.39±0.27 liters). IRVcw (r(2)=0.420), expiratory flow (r(2) change=0.174) and Borg dyspnoea score (r(2) change=0.123) emerged as the best contributors accounting for 71.7\% of the explained variance in daily movement intensity. COPD patients exhibiting greater ability to expand tidal volume and to maintain adequate inspiratory reserve volume tend to be more physically active. Thus, interventions aiming at mitigating restrictions on operational chest wall volumes are expected to enhance daily physical activity levels in COPD.