Resting Dead Space Fraction as Related to Clinical Characteristics, Lung Function, and Gas Exchange in Male Patients with Chronic Obstructive Pulmonary Disease.

Background: Measures of forced expired volume in one second % predicted (FEV%), residual volume to total lung capacity ratio (RV/TLC) and diffusing capacity for carbon monoxide measurements (DCO) are the standard lung function test for evaluating patients with chronic obstructive pulmonary disease (COPD). The dead space fraction (V/V) has been shown to be a robust marker of gas exchange abnormality. However, the use of V/V has gradually become less common. As V/V measured at rest (V/VR) has been successfully used in non-COPD conditions, it was hypothesized that in COPD the V/VR was more sensitive than the standard lung function test in correlation with clinical characteristics and gas exchange. This study aimed to test the hypothesis and to identify the variables relevant to V/VR.

Methods: A total of 46 male subjects with COPD were enrolled. Clinical characteristics included demographic data, oxygen-cost diagram (OCD), and image studies for pulmonary hypertension. The standard lung function was obtained. To calculate V/V, invasive arterial blood gas and pulmonary gas exchange (PGX) were measured. The variables relevant to V/VR were analyzed by multiple linear regression.

Results: Compared to lung function, V/VR was more frequently and significantly related to smoking, carboxyhemoglobin level, pulmonary hypertension and PCO (all p <0.05) whereas FEV% was more related to lung function test, PO and OCD score. V/VR and FEV% were highly related to resting gas exchange but RV/TLC and DCO% were not. Cigarette consumption, the equivalent for CO output, arterial oxyhemoglobin saturation, and the product of tidal volume and inspiratory duty cycle were identified as the parameters relevant to V/VR with a power of 0.72.

Conclusion: Compared to lung function test, V/VR is more related to clinical characteristics and is a comprehensive marker of resting gas exchange. Further studies are warranted to provide a noninvasive measurement of V/VR.

Registration Number: MOST 106-2314-B-040-025 and CSH-2019-C-30.