CT sheds more light on relationship of obesity and emphysema

Obesity is blamed for many health issues including pulmonary disease, cardiovascular disease, and respiratory diseases that include asthma, sleep apnea and chronic obstructive pulmonary disease (COPD). As more research is conducted about the relationship between obesity and COPD, a paradox is emerging: obesity appears to reduce the severity of the disease.

A retrospective study of the CT examinations of 447 patients diagnosed with COPD conducted at the University of Pittsburgh determined that body mass index (BMI) is inversely associated with emphysema, and was independent of age, gender and smoking history. The study’s findings were published in the May issue of Clinical Radiology.

From a radiology perspective, the importance of the study suggests that the utilization of CT can determine the presence or absence of emphysema and may aid in quantifying the diseases. The study used CT imaging alone to quantify emphysema severity, providing a different perspective than a symptom-based diagnosis. “Unlike previous biochemical-based investigations, the present study quantitatively assessed the impact of tobacco exposure on the development of emphysema based on imaging,” wrote lead author S. Gu of the Department of Radiology’s Imaging Research Center and colleagues.

The study was of the first 447 participants over age 40 enrolled in a National Institutes of Health (NIH) sponsored specialized center for COPD at the university. Men represented 59%, at a mean age of 64 years with a mean smoking history of 57.7 years. Approximately half of the patients with COPD were either overweight or obese.

Emphysema depicted on CT images was quantified based on the application of a -950 HU threshold to the segmented lung. The extent of emphysema was defined as the percentage of low attenuation areas relative to the total lung volume. The severity of emphysema was quantified in the entire cohort and subgroups that included gender, age, smoking history and status, severity of airflow obstruction, and body mass index (BMI). In patients with COPD, gender, BMI and severity of obstruction were each independently associated with the extent of emphysema.

BMI was negatively associated with emphysema severity after adjusting for age, gender lung function and smoking status. The research team reported that a one unit increase in BMI was associated with a 0.93-fold change in emphysema for patients with COPD, and the estimated extent of emphysema in men was 1.75 times that in women. In the subgroup of patients who did not have airflow obstruction, the estimated extent of emphysema was 2.24 times that in women.

One hypothesis explaining the obesity/emphysema paradox is that obesity could be a protective mechanism against a decline in airflow obstruction in COPD. As yet this is unproven. The authors recommend that much more research be undertaken. In all likelihood, CT imaging will play a part.

REFERENCE

1. Gu S, Li R, Leader JK et al. Obesity and extent of emphysema depicted at CT. Clin Radiol. 2015; 70(5): e14-19.