Combining Dynamic Hyperinflation with Dead Space Volume during Maximal Exercise in Patients with Chronic Obstructive Pulmonary Disease

Physiological dead space volume (V(D)) and dynamic hyperinflation (DH) are two different types of abnormal pulmonary physiology. Although they both involve lung volume, their combination has never been advocated, and thus their effect and implication are unclear. This study aimed (1) to combine V(D) and DH, and (2) investigate their relationship and clinical significance during exercise, as well as (3) identify a noninvasive variable to represent the V(D) fraction of tidal volume (V(D)/V(T)). Forty-six male subjects with chronic obstructive pulmonary disease (COPD) and 34 healthy male subjects matched for age and height were enrolled. Demographic data, lung function, and maximal exercise were investigated. End-expiratory lung volume (EELV) was measured for the control group and estimated for the study group using the formulae reported in our previous study. The V(D)/V(T) ratio was measured for the study group, and reference values of V(D)/V(T) were used for the control group. In the COPD group, the DH(peak)/total lung capacity (TLC, DH(peak)%) was 7% and the EELV(peak)% was 70%. After adding the V(Dpeak)% (8%), the V(D)DH(peak)% was 15% and the V(D)EELV(peak)% was 78%. Both were higher than those of the healthy controls. In the COPD group, the V(D)DH(peak)% and V(D)EELV(peak)% were more correlated with dyspnea score and exercise capacity than that of the DH(peak)% and EELV%, and had a similar strength of correlation with minute ventilation. The V(Tpeak)/TLC (V(Tpeak)%), an inverse marker of DH, was inversely correlated with V(D)/V(T) (R(2) ≈ 0.50). Therefore, we recommend that V(D) should be added to DH and EELV, as they are physiologically meaningful and V(Tpeak)% represents not only DH but also dead space ventilation. To obtain V(D), the V(D)/V(T) must be measured. Because obtaining V(D)/V(T) requires invasive arterial blood gases, further studies on noninvasive predicting V(D)/V(T) is warranted.