Forced expiratory volume ratio

Assessment of airflow limitation through spirometry is the standard for diagnosing and monitoring COPD. The forced expiratory volume after 1 second (FEVj) is generally reduced except in very mild disease. The forced vital capacity (FVC) may also be decreased. The hallmark of COPD is a reduced FEVpFVC ratio to less than 70%. A postbronchodilator LEV, that is less than 80% of predicted confirms the presence of airflow limitation that is not fully reversible. 

A spirometry test is a breathing test in which a person takes as deep a breath as possible and blows out quickly and completely into a tube cormected to a spirometry machine (Table 3.1). Lung measurements obtained from fhis fesf include forced expiratory volume in one second (FEVi), fhe amounf of air blown ouf in one second forced vifal capacify (FVC), fhe fofal amounf of air blown ouf fhe FEVi/FVC ratio and fhe... 

The FVC, which represents the total amount of air than can be exhaled, can be expressed as a series of timed volumes. The forced expiratory volume in 1 second (FEVi) is the volume of air exhaled during the first second of the FVC maneuver. Although the FEVi is a volume, it conveys information on obstruction because it is measured over a known time interval. The EEVj depends on the volume of air within the lung and the effort during exhalation therefore, it can be diminished by a decrease in TLC or by a lack of effort. A more sensitive way to measure obstruction is to express the FEVi as a ratio of FVC. This ratio is independent of the patient s size or the TLC therefore, the FEVi/FVC is a specific measure of airway obstruction with or without restriction. Normally, this ratio is 75% or greater, and any value below 70% to 75% suggests obstruction. 

Asthma is a common chronic inflammatory disorder that affects the airway passages of the lungs. Primarily, asthma is characterized by reversible, episodic narrowing of the airway s smooth muscle secondary to hypersecretion of mucus, hyper-reactivity, and mucosal edema. The narrowing may be of sudden or prolonged onset, which accounts for the varying degrees of airway obstruction seen in this disease. The hallmark of airway obstruction is a reduction in the forced expiratory volume in 1 second (FEV and the ratio of FEVj to the forced vital capacity (FVC). 

Respiratory problems are diagnosed using a spirometer. The patient exhales as hard and as fast as possible into the device. The spirometer measures (1) the total volume exhaled, called the forced vital capacity (FVC), with units in liters (2) the forced expired volume measured at 1 second (FEV,), with units in liters per second (3) forced expiratory flow in the middle range of the vital capacity (FEV 25-75%), measured in liters per second and (4) the ratio of the observed FEVj to FVC X 100 (FEVj/FVC%). 

In adults and older children, lung volume and forced expiratory flows are lower in black than white subjects, based on values predicted from standing height (133,134). These discrepancies have been largely attributed to ethnic differences in trunk-leg ratio, since no such discrepancies are observed when respiratory function is related to sitting height. 

Severe mucus plugging in the peripheral airways may have an effect on lung volume, such as the residual volume to total lung capacity ratio or trapped gas volume. A smaller amount of mucus, not completely obstructing the airways, may have an effect on forced expiratory flow variables (5,16,18). 

---