Obesity can worsen the impact of asthma and may also mask its severity in standard tests, according to researchers in New Zealand, who studied lung function in asthmatic women with a range of Body Mass Indices (BMIs). This is the first prospective study to reveal a significant comparative difference in how the airways and lungs respond to a simulated asthma attack in obese and non-obese individuals.

The research is reported in the first issue for May of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society. It establishes a direct link between obesity and the development of a phenomenon known as “dynamic hyperinflation”—when air breathed into the lungs cannot be exhaled. This often occurs with acute asthma, but is more frequent in obese individuals.

“We have demonstrated significant differences in the changes in respiratory function that occur with asthmatic bronchoconstriction in relation to obesity,” said principal investigator, D. Robin Taylor, M.D., of the University of Otago in New Zealand.

The researchers recruited 30 asthmatic women and divided them into three groups by BMI: normal weight, overweight and obese. Each woman breathed nebulized methacholine to artificially induce an asthma-like attack, and was then assessed for changes in lung function on several measures, including how much air remained in her lungs after exhalation (functional residual capacity, or FRC) and how much air she could breathe in on her next breath (inspiratory capacity, or IC).

“After the methacholine challenge, the amount of bronchoconstriction was identical for each of the three groups, but the changes in FRC and IC were greatest in the obese group. This indicated to us that greater dynamic hyperinflation was occurring among obese individuals,” said Dr. Taylor.

With increasing BMI, FRC was higher, whereas IC was significantly decreased. “This means that among women with greater BMI, an asthma-like episode has the potential to cause greater breathing difficulties than in non-obese women,” said Dr. Taylor. “The greater dynamic hyperinflation means that obese individuals lose the ability to inhale as deeply or exhale as fully as normal weight individuals.”

Curiously, the group of obese individuals with asthma differed from their non-asthmatic counterparts in having a lower FRC before the methacholine challenge than the non-obese group, yet still recorded a greater increase in FRC after the methacholine challenge. “This is the surprising finding in our study. It is quite counterintuitive. You would expect individuals with a heavier chest wall not to develop hyperinflation quite so readily as those who are lighter. But that is not what happened,” said Dr. Taylor.

Perhaps most importantly, these findings point to fundamental differences in the way that obese individuals might experience shortness of breath if they have asthma. “We know that asthma in obese subjects is more likely to persist and is more likely to be perceived to be severe. These individuals often require more treatment to achieve asthma control. Our study provides an insight into why this might be happening–the same asthma “trigger” produces a greater effect in obese individuals.”

The study also showed that simple spirometry was inadequate to determine the level of pulmonary dysfunction which was occurring in obese individuals. “Our findings need to be explored further. We need to confirm that the differences in dynamic hyperinflation between obese and non-obese asthmatics are sufficient to explain the differences in symptoms between the two groups. Our study was not large enough to do this,” said Dr. Taylor.

About Obesity

Obesity is a condition in which the natural energy reserve, stored in the fatty tissue of humans and other mammals, exceeds healthy limits. It is commonly defined as a body mass index (BMI) (weight divided by height squared) of 30 kg/m² or higher.

Mortality is increased in obesity, with a BMI of over 32 being associated with a doubled risk of death.  Central obesity (male-type or waist-predominant obesity, characterized by a high waist-hip ratio), is an important risk factor for the metabolic syndrome, the clustering of a number of diseases and risk factors that heavily predispose for cardiovascular disease. These are diabetes mellitus, Type 2 diabetes mellitus, high blood pressure, high blood cholesterol, and triglyceride levels (combined hyperlipidemia).

Apart from the metabolic syndrome, obesity is also correlated with a variety of other complications. For some of these complaints, it has not been clearly established to what extent they are caused directly by obesity itself, or have some other cause (such as limited exercise) that causes obesity as well. The affected health conditions include but are not limited to:

• Cardiovascular: congestive heart failure, stroke, enlarged heart and its associated arrhythmias and dizziness, varicose veins, and pulmonary embolism
• Endocrine: type 2 diabetes mellitus, insulin resistance, polycystic ovarian syndrome (PCOS), menstrual disorders, and infertility
• Gastrointestinal: gastroesophageal reflux disease (GERD), fatty liver disease, cholelithiasishernia, and colorectal cancer
• Renal and genitourinary: erectile dysfunction, urinary incontinence, chronic renal failure, hypogonadism (male), breast cancer (female), uterine cancer (female), stillbirth
• Integument (skin and appendages): stretch marks, acanthosis nigricans, lymphedema, cellulitis, carbuncles, intertrigo
• Musculoskeletal: hyperuricemia (which predisposes to gout), immobility, joint pain, osteoarthritis, low back pain
• Neurologic: stroke, meralgia paresthetica, headache, carpal tunnel syndrome, dementia, idiopathic intracranial hypertension
• Respiratory: obstructive sleep apnea, obesity hypoventilation syndrome, asthma
• Psychological: depression, low self esteem, body dysmorphic disorder, social stigmatization
• Erectile Dysfunction 

About Asthma

Asthma occurs when the main air passages of your lungs, the bronchial tubes, become inflamed. The muscles of the bronchial walls tighten, and cells in the lungs produce extra mucus further narrowing your airways. This can cause minor wheezing to severe difficulty in breathing. In some cases, your breathing may be so labored that an asthma attack becomes life-threatening.

Asthma is a chronic but treatable condition. You can manage your condition much like someone manages diabetes or heart disease. You and your doctor can work together to control asthma, reduce the severity and frequency of attacks and help maintain a normal, active life.

Asthma signs and symptoms can range from mild to severe. You may have only occasional asthma episodes with mild, short-lived symptoms such as wheezing. In between episodes you may feel normal and have no difficulty breathing. Some people with asthma have chronic coughing and wheezing punctuated by severe asthma attacks.

Most asthma attacks are preceded by warning signs. Recognizing these warning signs and treating symptoms early can help prevent attacks or keep them from becoming worse. Warning signs and symptoms of asthma in adults may include:

• Increased shortness of breath. Due to bronchoconstriction, shortness of breath is a common asthma symptom. Shortness of breath is used to describe a feeling of breathlessness - a feeling that you cannot catch your breath. If you have asthma, you may feel breathless and struggle to get air in and out of your lungs.
• Wheezing. Wheezing sounds like a whistle or squeak when breathing in, out, or both. This is a common asthma symptom because asthma causes the bronchi to constrict (bronchoconstriction), which reduces the the air flow in and out of the lungs.
• Disturbed sleep caused by shortness of breath, coughing or wheezing
• Chest tightness or pain. Chest tightness is a squeezing feeling in the chest during respiration. It may feel as though someone is sitting on your chest.
• Rapid breathing. Rapid breathing is a common asthma symptom. When breathlessness occurs, you may try to breathe faster to try to get air in and out of your lungs.
• Increased need to use bronchodilators - medications that open up airways by relaxing the surrounding muscles.
• A fall in peak flow rates as measured by a peak flow meter, a simple and inexpensive device that allows you to monitor your own lung function.