Obesity and Lung Health

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Author(s):

	Kim Van Hoorenbeeck
	Kim Van Hoorenbeeck is trained as a pediatrician at the Antwerp University Hospital (UZA) and Zeepreventorium, Belgium. She did her PhD on sleep-disordered breathing and obesity in children.
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Introduction

Obesity, both in adults as in children, is known to affect lung health. There is an increased risk of respiratory complications with a higher degree of obesity. Respiratory complications may be grossly divided in upper airway disease and lower airway disease. The latter is addressed in this chapter. Upper airway problems are mostly affecting breathing during sleep and are described in the chapter on sleep disordered breathing. 

Pathophysiology 

Background on lung function testing 

Techniques most commonly used to perform lung function testing are spirometry, which is a measurement of the dynamic lung volumes, and body plethysmography, a method to establish static lung volumes and capacities (sum of specific volumes). Static lung volumes are not affected by airflow in contrast with dynamic volumes (1). The following parameters is a selection of variables that are most often reported on literature (1,2): 

• Dynamic volumes: measured during forced expiration after maximal inspiration to total lung capacity (TLC) 

• Forced vital capacity (FVC) = the volume of air exhaled 

• Forced expiratory volume in 1 second (FEV1) = the volume of air exhaled during 1 second 

• Tiffeneau index = the ratio of FEV1 over FVC (FEV1/FVC) 

• Maximal mid-expiratory flow (MMEF or FEF25-75) = the maximal mean expiratory flow between 25% and 75% of FVC expired 

• Static volumes and capacities: measured during quiet tidal breathing 

• Tidal volume (VT) = normal volume of air between inspiration and expiation during todal breathing 

• Inspiratory and expiratory reserve volume (IRV and ERV) = additional volume of air inhaled during maximal inspiration (IRV) and exhaled during maximal expiration (ERV) 

• Functional residual capacity (FRC) = volume of air that stays in the lung after normal expiration 

• Vital capacity (VC) = combination of the volume of air moved with maximal in- and expiration 

• Residual volume (RV) = volume of air that remains in the lung after maximal expiration 

• Total lung capacity (TLC) = maximal total volume of air, including VC and RV 

Figure: spirometry result of 15y old male with obesity shows obstructive lung function with incomplete reversibility. 

Effects of obesity on lung function without respiratory disease 

Both restrictive and obstructive components of lung disease are linked to obesity. Restriction is mostly caused by impaired respiratory mechanics. Excessive adipose tissue in the abdomen increases the intra-abdominal pressure on the diaphragm, whereas layers of excessive fat tissue in the chest reduces the lung and chest wall compliance. Together this reduces the lung volumes and capacities that is known from restrictive lung disease. Moreover, obesity is a systemic pro-inflammatory disease in which many cytokines and adipokines are being produced. These inflammatory agents may influence lung health directly or by altering the immune response. The process may promote airflow obstruction reflected by a reduced Tiffeneau index, although literature is still controversial (2,3). 

These findings are confirmed by a recent systematic review in which in slightly more than 50% of the studied papers a reduced FEVC1/FVC is shown in individuals with obesity reflecting an obstructive airway disorder. On the other hand, FEV1 as such was not associated with obesity, and data on MMEF were also conflicting. Therefore it seems that FEV1/FVC is the most suitable parameter to detect airflow obstruction in an obese population. In contrast with a previous analysis FVC did not show a restrictive pattern in most studies included. In contrast the measurements obtained through body plethysmography pointed towards lower values of most parameters obtained, indeed linked to restriction (2). 

Dysynapsis 

Dysynapsis is associated with obesity and means that there exists an incongruency between lung growth and airway length, and the airway caliber, as the lungs and airway length become larger and the airway caliber remains relatively small. This results in airflow obstruction. The mechanism is reflected by an increase in FVC versus a normal FEV1 resulting in a decreased FEV1/FVC ratio. The decrease of the Tiffeneau index with an increase in BMI suggest an imbalance between airflow and ventilation. Studies have shown that this effect of dysynapsis is more pronounced in children with asthma (see later) and obesity and might increase the risk of experiencing asthma attacks (3,4). 

Reduced exercise capacity 

Many children and adolescents with obesity may experience breathlessness and noisy breathing with physical activity. Most common, this is caused by deconditioning. If exercise-induced bronchoconstriction is suspected an exercise test may be performed to objectify exercise-induced airflow obstruction. If reversible bronchoconstriction is observed bronchodilators may be initiated or escalated. If the child shows a fixed airway obstruction reflecting dysynapsis without a positive bronchodilator response overmedication should be avoided.(3) 

Obesity and asthma 

Having obesity increases the risk of childhood asthma. Moreover, asthma control is more difficult to achieve in individuals with obesity leading to a lower quality of life and more frequent asthma attacks.(5) However, the link between obesity and asthma is complex and multifactorial. Asthma in obesity is probably not linked to type 2 inflammation but to other underlying inflammatory processes that are not associated with atopy.(3,5) Children and adolescents with obesity may also experience other comorbities that may contribute such as gastro-esophageal reflux disease (GERD), socioeconomic factors (e.g. exposure to tobacco smoke or vaping, less green space exposure), etcetera. This results in more difficult diagnosis of asthma as type 2 inflammation related markers are not useful and because of dysynapsis lung function testing is more difficult to interpret.(3-6) Treatment of asthma in patients with obesity relies on the same strategies as in healthy weight individuals with asthma mostly on inhaled corticosteroids as asthma control treatment.(6) They can improve the airway hyperresponsiveness and increase FEV1, but because of the increase of FVC (once again because of pronounced dysynapsis in obesity) the FEV1/FVC ratio may be underestimated, and it is more difficult to assess asthma severity. Treatment of asthma in individuals with obesity should also include a multidisciplinary approach to attain a healthy weight status as asthma control will benefit from this.(5-6) 

References 

1. Kieninger E, Salem Mahmoud Y, Fuchs O. Static and dynamic lung volumes. In: Eber E, Midulla F, eds. ERS handbook Paediatric respiratory medicine. 2nd edition. European Respiratory Society 2021:80-88. 

1. Ferreira MS, Marson FAL, Wolf VLW et al. Lung function in obese children and adolescents without respiratory disease: a systematic review. BMC Pulm Med 2020;20:281. 

1. Bush A, Pabary R. Effects of systemic and extrapulmonary conditions on the respiratory system. In: Eber E, Midulla F, eds. ERS handbook Paediatric respiratory medicine. 2nd edition. European Respiratory Society 2021:532-549. 

1. Huang L, Wang S-T, Kuo H-P, et al. Effects of obesity on pulmonary function considering the transition from obstructive to restrictive pattern from childhood to young adulthood. Obesity Rev 2021;22:e13327. 

1. Jessica Reyes-Angel, Parisa Kaviany, Deepa Rastogi, Erick Forno. Obesity-related asthma in children and adolescents. Lancet Child Adolesc Health 2022;6:713-724. 

1. Global Initiative for Asthma. Global strategy for asthma management and prevention, 2024. Updated May 2024. Available from www.ginasthma.org.