It is noteworthy that this reduced risk
of mortality was not observed in obese patients with milder
COPD [25] and that subgroups of COPD patients with more
severe obesity are at a greater risk of death due to respiratory
failure than normal weight COPD [26]. There is an exponential
relation between increasing BMI and decreasing ERV [27].
This volume reduction effect occurs across all severity stages
of airway obstruction and is seen even as BMI increases from
normal weight to the overweight range. TLC and RV are relatively
less affected by the increasing weight in COPD.
Importantly, as in health, the resting IC increases in response
to increasing BMI across all severity stages, reflecting the
greater reduction in end expiratory lung volume relative to
TLC [28]. Obesity and COPD have various influences on respiratory
physiology, some are similar and some are opposite.
The relationship between BMI and either functional residual
capacity or expiratory reserve volume is not affected by the
presence of airflow obstruction [21].