Orthopnea in Obese Adult Patients: Can It Be Quantified From Lung Function Testing?

Obesity is an overwhelming public threat. It has been estimated that more than one third of the adult population is obese despite the fact that the prevalence has remained stable since 2004.1 The good news is that the prevalence of obesity has leveled off; the bad news is that the number remains high enough to pose a significant public concern with multiple health-related implications. With many of the obese adults getting older, it is very likely that an increasing number will be admitted to the hospital in the future. Obese patients present many clinical challenges in that they are more likely to develop sleep disordered breathing, diabetes mellitus, ischemic heart disease, hypertension, several different types of cancer, and gall bladder disease as well as potentially having a significant degree of respiratory compromise.2 In terms of respiratory compromise, obese patients experience a reduced expiratory reserve volume and a diminished functional residual capacity.3 In particular, a low functional residual capacity increases the risk of both expiratory flow limitation (EFL) and small airway closure. Thus, expiratory flow may be limited during tidal breathing in healthy obese individuals, and this may be even more pronounced when in the supine position.4 This is an important point because the basic physiologic principle in terms of functional residual capacity is that it should increase when a patient goes from the sitting to the supine position. However, physiologic changes occur in obese individuals that may be more profound in the supine position due to excess adipose tissue affecting the abdomen and chest wall. As a result of EFL and airway closure, there could be a ventilation/perfusion imbalance where some areas of the lungs might be underventilated and overperfused. Obese patients receiving mechanical ventilation are at even greater risk of deleterious effects because EFL promotes dynamic hyperinflation with a corresponding increase in the work of breathing to overcome the resultant intrinsic PEEP in the supine position.5 In this issue of RESPIRATORY CARE, Perino et al6 sought to examine the role of closing volume as an indicator of orthopnea in stable obese patients both in the sitting and supine position while, at the same time, looking at EFL.