Physiological and clinical relevance of exercise ventilatory efficiency in COPD

Abstract

Exercise ventilation (') relative to carbon dioxide output (' ) is particularly relevant to patients limited by the respiratory system, those with chronic obstructive pulmonary disease (COPD). High'-' (poor ventilatory efficiency) has been found to be a key physiological abnormality in symptomatic patients with largely preserved forced expiratory volume in 1 s (FEV). Establishing an association between high '-' and exertional dyspnoea in mild COPD provides evidence that exercise intolerance is not a mere consequence of detraining. As the disease evolves, poor ventilatory efficiency might help explaining "out-of-proportion" breathlessness (to FEV impairment). Regardless, disease severity, cardiocirculatory co-morbidities such as heart failure and pulmonary hypertension have been found to increase '-' In fact, a high '-' has been found to be a powerful predictor of poor outcome in lung resection surgery. Moreover, a high '-' has added value to resting lung hyperinflation in predicting all-cause and respiratory mortality across the spectrum of COPD severity. Documenting improved ventilatory efficiency after lung transplantation and lung volume reduction surgery provides objective evidence of treatment efficacy. Considering the usefulness of exercise ventilatory efficiency in different clinical scenarios, the '-' relationship should be valued in the interpretation of cardiopulmonary exercise tests in patients with mild-to-end-stage COPD.

Ventilation efficiency to exercise in patients with cystic fibrosis

“…Upon maximal exercise in these diseases a shallow breathing pattern is noted with elevated minute ventilation (VE) and increased ventilatory equivalents for CO 2 and ventilation efficiency index . V′E/V′CO 2 is also elevated in COPD as Neder et al point, however no data exist on the structural correlates of the ΔVE/ΔVCO 2 slope with further studies in this direction needed both in COPD and other disease entities.…”

“…The respiratory compensation point (RCP) was determined visually as the point when V′E increased out of proportion of V′CO2. Linear regression was used to estimate the ΔV′E/ΔV′CO 2 slope from the data obtained during incremental exercise until the RCP occurred . The V′E – V′CO 2 ‐relationship intercept (V′Eint) was also calculated.…”

“…Steeper ΔV′E/ΔV′CO 2 slope has been associated with a higher ventilation perfusion mismatch mostly due to inadequate perfusion and has been related to poorer prognosis . In COPD ΔV′E/ΔV′CO 2 slope is also elevated in concordance to increased dead space ventilation and the V′E–V′CO 2 nadir yields prognostic information in addition to resting assessment of hyperinflation …”

Are the “critical” inspiratory constraints actually decisive to limit exercise tolerance in COPD?

“…Even considering the differences in patients' severity, it is noteworthy that we found in other studies that the CIC may indeed happen at exercise intensities well below the (estimated) MBC [9]. This is particularly true when the MBC is a high value and the patients do not present with severe ventilatory inefficiency [15]. In other words, a combination of a high MBC with relatively low submaximal V′ E -CO 2 output may give more time for the CIC to emerge during a ramp-incremental CPET.…”

Preface

“…More recently, several studies have demonstrated that a high ventilatory inefficiency is an early sign of impaired gas exchange efficiency (increased "wasted" ventilation) and/or excessive afferent stimuli to ventilation (as recently reviewed in [28]) as it is related to a high respiratory neural drive and dyspnoea in symptomatic smokers [29,30] and patients with mild COPD [31,32], heart failure [8], heart failure-COPD [33,34], ILD [35] and pulmonary hypertension (PH) [36,37]. Thus, the use of different indices of the V′E-V′CO 2 relationship has proved useful in the interpretation of CPET responses in patients with unexplained or disproportionate exertional dyspnoea [38].…”