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

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_DDH_peak% was 15% and the V_DEELV_peak% was 78%. Both were higher than those of the healthy controls. In the COPD group, the V_DDH_peak% and V_DEELV_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 (R² ≈ 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.