Researchers Recommend Additional RV/TLC Testing for Patients with Longer Duration Asthma

A new study determined that residual volume/total lung capacity (RV/TCL) ratios have a significantly stronger association with longer duration asthma than more commonly used forced expiratory volume in 1 s/forced vital capacity (FEV1/FVC) ratios.

The researchers at the University of California San Francisco-Fresno, and East Carolina University in Greenville, NC led by Anupama Tiwari, MD, explained that while this can diagnose airway obstruction, it cannot identify the severity or duration of symptoms in patients with persistent asthma.

According to the study, lung volume measurements like the RV/TCL should be conducted in addition to spirometry tests like the FEV1 and FVC to "detect changes related to airway obstruction in patients with longer duration of asthma." Although Tiwari pointed out that the FEV1/FVC ratio is currently considered "the gold standard” to diagnose airflow obstruction, this ratio has not been shown to correlate with severity or symptoms of asthma. The study concluded that the RV/TLC ratio might serve as a better diagnostic tool for patients with longer duration asthma.

The study included 93 patients with diagnosed persistent asthma symptoms recruited from university clinics that were given a questionnaire during clinic visits and via telephone interviews.

Lung volume testing, pulmonary function tests and lung volume measurements (via plethysmography) were completed for each patient, and patient charts were reviewed for additional information. Tiwari and colleagues reported that although there was an observed trend associating FEV1/FVC ratio "and the duration of asthma, this finding was not statistically significant."

Tiwari said that many asthma symptoms could overlap with the symptoms of COPD in patients; therefore using spirometry tests to determine the presence of lung obstruction caused by airway inflammation (like the FEV1/FVC testing) is crucial to diagnosing asthma. However; there are previous studies that have determined that spirometry indices like the FEV1/FVC ration "do not correlate with symptoms or severity of asthma”.

RV/TLC ratios, which determine abnormal lung volume and abnormal residual lung volume, may serve as better indicators of total Functional Residual Capacity (FRC) in patients with longer duration of asthma as a means of diagnosing airflow obstruction. The study explored "the relationship between RV/TLC ratio and duration of asthma" seeking to enumerate "the proportion of subjects with abnormal spirometric and lung volume abnormalities."

The study; however, did make new observations about the association between the RV/TLC ratio and duration of asthma. RV/TLC ratios increased progressively in patients with longer duration of asthma even when FEV1/FVC ratios indicated no abnormality or when that ratio declined. Tiwari surmised that the progressive increase in RV/TLC ratio in patients who experienced longer duration of asthma could be associated with chronic inflammatory changes affecting small airway, leading to airway closure even in those taking inhaled steroids to control asthma patients.

This discovery of the association between RV/TLC and longer duration asthma has several clinical implications. According to Tiwari, "detecting an elevated RV/TLC ratio in a clinically stable asthmatic should alert the clinician about the possibility of obstruction related to the chronicity of asthma despite a normal FEV1/FVC ratio." The research also suggested that RV/TLC ratios are a better diagnostic tool for early detection of small airway closures in patients with longer duration, persistent asthma, and as such should be used in addition to spirometry for diagnosis of airflow obstruction.

The article, “Longer Duration of Asthma is Significantly Associated with Increased RV/TLC Ratio,” was published in the March 2017 issue of Respiratory Medicine.

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