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However, the physiological V D refers to ventilation not involved in gas exchange and involved in unperfused or underperfused alveoli and includes anatomical and alveolar V Ds. In the alveolar dead space (V D) of the three component (Riley) model, if alveolar V D exists, residual volume is expected to increase, potentially causing air trapping and hyperinflation of the lung. Because obtaining V D/V T requires invasive arterial blood gases, further studies on noninvasive predicting V D/V T is warranted. To obtain V D, the V D/V T must be measured. 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. The V Tpeak/TLC (V Tpeak%), an inverse marker of DH, was inversely correlated with V D/V T (R 2 ≈ 0.50). 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.

Both were higher than those of the healthy controls. After adding the V Dpeak% (8%), the V DDH peak% was 15% and the V DEELV peak% was 78%. In the COPD group, the DH peak/total lung capacity (TLC, DH peak%) was 7% and the EELV peak% was 70%. 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. 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. Demographic data, lung function, and maximal exercise were investigated.

Forty-six male subjects with chronic obstructive pulmonary disease (COPD) and 34 healthy male subjects matched for age and height were enrolled. 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). Although they both involve lung volume, their combination has never been advocated, and thus their effect and implication are unclear. Physiological dead space volume (V D) and dynamic hyperinflation (DH) are two different types of abnormal pulmonary physiology.