Obesity has important effects on respiratory physiology and is associated with several pulmonary disorders, including asthma, obstructive sleep apnoea, obesity hypoventilation syndrome and pulmonary hypertension. It may also complicate the diagnosis and management of chronic obstructive pulmonary disease.
The respiratory consequences of obesity result from a combination of mechanical restriction, altered respiratory-muscle function, systemic inflammation, metabolic dysfunction and obesity-related comorbidities. The severity of impairment generally increases with the degree of obesity, particularly when fat is concentrated in the abdominal and thoracic regions.
Excess adipose tissue around the abdomen and chest wall reduces the compliance of the respiratory system. Abdominal fat pushes the diaphragm upwards, particularly in the supine position, and restricts its downward movement during inspiration.
Important physiological consequences include:
Total lung capacity is often relatively preserved in mild or moderate obesity but may decrease in severe obesity. Marked reductions in vital capacity or total lung capacity should not automatically be attributed to obesity; other pulmonary, neuromuscular or thoracic disorders should also be considered.
Spirometry may be normal or may show a restrictive pattern, with proportional reductions in forced expiratory volume in one second and forced vital capacity. Obesity alone does not typically produce the persistent post-bronchodilator airflow obstruction required for a diagnosis of COPD.
Obesity is an important risk factor for asthma and is associated with poorer symptom control, more frequent exacerbations and reduced quality of life. The relationship is multifactorial.
Potential mechanisms include:
Asthma symptoms in people with obesity may sometimes be overestimated because exertional breathlessness, reflux, deconditioning, sleep-disordered breathing and cardiac disease can produce similar symptoms. Objective confirmation of variable expiratory airflow limitation is therefore important before asthma treatment is intensified.
Weight reduction should form part of the treatment plan for patients with obesity and asthma. Structured dietary intervention combined with aerobic and resistance exercise can improve asthma control and general health. Asthma pharmacotherapy should otherwise follow standard guideline-based principles. (Global Initiative for Asthma – GINA)
COPD is defined by persistent airflow obstruction resulting from airway and alveolar abnormalities, usually after prolonged exposure to tobacco smoke, air pollution, occupational agents or other harmful particles and gases. Diagnosis requires post-bronchodilator spirometry.
The relationship between obesity and the development of COPD is complex. Observational studies have reported associations, but obesity is not considered a primary established cause of COPD in the same way as smoking or long-term inhalational exposure. Shared risk factors—including physical inactivity, socioeconomic factors, ageing, systemic inflammation and cardiometabolic disease—may partly explain the association.
In patients who already have COPD, obesity may:
Obesity can also mask hyperinflation by reducing resting lung volumes. Conversely, severe obesity may produce a restrictive ventilatory defect that is distinct from the obstructive defect of COPD.
Symptoms should therefore not be used to diagnose COPD without spirometric confirmation. Smoking status, occupational exposure, environmental exposure, asthma and other differential diagnoses must also be assessed. Current GOLD recommendations emphasise confirmation of persistent airflow obstruction and comprehensive management of comorbidities. (goldcopd.org)
Obesity is associated with chronic, low-grade systemic inflammation. Adipose tissue releases cytokines, adipokines and other mediators that may influence the airways, pulmonary circulation and respiratory muscles.
Inflammatory mechanisms may contribute to:
However, systemic inflammation associated with obesity should not be equated with the characteristic airway and parenchymal pathology of COPD. The biological pathways overlap in some respects, but the disorders remain clinically and pathophysiologically distinct.
Obesity is one of the most important modifiable risk factors for obstructive sleep apnoea. In OSA, repeated narrowing or collapse of the upper airway during sleep produces episodes of reduced or absent airflow despite continued respiratory effort.
Contributing mechanisms include:
Common manifestations include:
Some patients, particularly women and older adults, may present predominantly with fatigue, insomnia, mood symptoms or cognitive impairment rather than obvious sleepiness.
The severity of OSA is usually classified using the apnoea–hypopnoea index:
The diagnosis requires a sleep study using either polysomnography or an appropriate home sleep-apnoea test in selected patients. Questionnaires can identify increased risk but cannot independently establish or exclude the diagnosis.
Untreated OSA is associated with hypertension, atrial fibrillation, stroke, coronary disease, metabolic dysfunction, accidents related to sleepiness and impaired quality of life. Positive airway pressure therapy is the standard treatment for many patients, particularly those with moderate or severe disease or clinically important symptoms. Weight management is an essential parallel intervention. (American Thoracic Society)
Obesity hypoventilation syndrome is a serious form of obesity-related respiratory failure. It is defined by:
after other causes of hypoventilation have been excluded.
Possible alternative causes include severe obstructive or restrictive lung disease, neuromuscular disease, chest-wall deformity, hypothyroidism, sedative or opioid use and central hypoventilation disorders.
Most patients with obesity hypoventilation syndrome also have obstructive sleep apnoea, and many have severe OSA. A smaller group has predominantly non-obstructive sleep-related hypoventilation. (breathe.ersjournals.com)
The development of OHS is related to several interacting mechanisms:
The kidneys retain bicarbonate in response to chronic hypercapnia. Elevated serum bicarbonate may therefore be a useful screening clue, although it is not specific and does not replace arterial or appropriately obtained capillary blood-gas analysis when OHS is suspected.
Patients may present with:
OHS is frequently underdiagnosed until the patient is admitted to hospital with respiratory failure.
Evaluation may include:
An awake arterial carbon dioxide level of at least 45 mmHg is essential to the standard definition. Obesity with nocturnal hypoxaemia alone is not sufficient for a diagnosis of OHS.
Positive airway pressure treatment and sustained weight reduction are the principal therapies.
Continuous positive airway pressure is generally considered first-line treatment for stable ambulatory patients with OHS and coexisting severe OSA. Non-invasive ventilation with inspiratory pressure support may be required when hypoventilation persists, when OSA is absent or less prominent, or after an episode of acute hypercapnic respiratory failure.
Oxygen should not routinely be used as the only treatment for OHS because it does not correct hypoventilation and may worsen carbon dioxide retention in susceptible patients. Supplemental oxygen may be added when clinically necessary, but ventilation must be assessed and treated.
The American Thoracic Society recommends weight-loss interventions capable of producing a sustained loss of approximately 25–30% of body weight when resolution of OHS is the goal. Such weight loss may be more achievable with metabolic or bariatric surgery in appropriately selected patients. (American Thoracic Society)
Obesity alone is not a specific diagnostic category of pulmonary hypertension. However, obesity can contribute indirectly through:
Pulmonary hypertension is substantially more common in OHS than in uncomplicated OSA. It may result from chronic hypoxaemia, hypercapnia, pulmonary vasoconstriction, increased blood volume and coexisting cardiac dysfunction. Positive airway pressure treatment may reduce pulmonary pressures in patients whose pulmonary hypertension is associated with OSA or OHS. (ERS Publications)
Persistent pulmonary hypertension despite adequate treatment of sleep-disordered breathing and hypoxaemia requires further assessment to exclude left-heart disease, chronic thromboembolic pulmonary hypertension and other pulmonary vascular disorders.
Respiratory disease in patients with obesity should be evaluated in the context of common comorbidities, including:
These conditions may aggravate dyspnoea, reduce exercise tolerance and increase the risk of hospitalisation. They should be assessed and treated directly rather than assuming that all respiratory symptoms arise from obesity.
A comprehensive assessment may include:
Breathlessness in obesity should not automatically be attributed to body weight. Potentially treatable pulmonary, cardiovascular, haematological and neuromuscular causes must be considered.
Treatment should address both excess adiposity and the specific respiratory disorder.
Important measures include:
Weight loss may improve lung volumes, respiratory mechanics, asthma control, OSA severity, gas exchange and exercise capacity. Nevertheless, established respiratory disease often requires continued disease-specific treatment.
Obesity impairs respiratory mechanics primarily by reducing functional residual capacity and expiratory reserve volume and by increasing the work of breathing.
Obesity is strongly associated with asthma, obstructive sleep apnoea and obesity hypoventilation syndrome. Its relationship with the development of COPD is less direct and is influenced by shared risk factors and diagnostic overlap.
COPD must be confirmed by post-bronchodilator spirometry. Dyspnoea or reduced lung volumes in a patient with obesity are not sufficient to establish the diagnosis.
Obstructive sleep apnoea involves recurrent upper-airway obstruction during sleep. Obesity hypoventilation syndrome additionally requires awake daytime hypercapnia after other causes have been excluded.
Positive airway pressure therapy is central to the treatment of OSA and OHS. Oxygen alone is not an adequate treatment for obesity-related hypoventilation.
Weight management is a fundamental part of treatment, but significant respiratory symptoms must not automatically be attributed to obesity without an appropriate diagnostic evaluation.