A quantitative analysis of plethysmographic loops

Contrary to healthy subjects, in whom the relation between alveolar pressure (Palv) and flow (V̇) is linear, Palv-V̇ loops are often seen in chronic obstructive pulmonary disease (COPD) patients. These loops contain potentially relevant information about the physiopathological phenomena that characterize this disease, but, at present, no quantitative data during spontaneous breathing are available. This study has characterized plethysmographic Palv-V̇ loops in terms of inspiratory (Ains), expiratory area (Aexp), and sense of rotation in 130 stable COPD patients at rest during spontaneous breathing. Relative to Ains, Aexp was larger (1.29±1.74 versus 0.39±0.33 cmH2O L s-1), and its sense of rotation was compatible with time-constants heterogeneity, air trapping, recruitment/derecruitment of lung units or expiratory flow-limitation. Aexp correlated with indexes of flow-limitation (RS= -0.274, P<0.01 with forced expiratory volume in one second) and gas trapping (RS= 0.321, P<0.01 with the ratio of residual volume to total lung capacity), but not with indexes of uneven distribution of ventilation as the alveolar volume measured during the single breath carbon monoxide test (RS= -0.081, P=0.36). These results suggest that in COPD patients breathing spontaneously at rest, expiratory looping appears primarily because of the presence of tidal expiratory flow-limitation and eventually gas trapping, rather than because of time-constants heterogeneity or recruitment/derecruitment.