Maximum forced expiratory flow

Abbreviations WTC, world trade center FDNY, Fire Department of the city of New York FEVi, forced expiratory volume in one second FVC, forced vital capacity MFEF, maximum forced expiratory flow RUDS, reactive upper airway dysfunction syndrome ESU, emergency service rmit GERD, gastroesophageal reflux dysfunction LLN, lower Umits of normal SLGPD, sarcoid-Uke granulomatous pulmonary disease NYC, New York City. 

Abbreviations G, confidence interval CMS, central nervous system EEG, electroencephaologram FVQ forced vital capacity FEV forced expiratory volume in 1 s PE F, forced expiratory flow, pg/ m micrograms per cubic meter NR, not reported ppb, parts per billion Raw, airway resistance SGaw, specific airway conductance VO MAX maximum oxygen uptake. 

Elevated peripheral eosinophil counts in asthmatics have correlated with decreases in specific airway conductance, forced expiratory volume (FEVi), maximum mid-expiratory flow rate (Horn et al., 1975), bronchial hyperreactivity (BHR) to histamine (H Taylor and Luksza, 1987) and clinical severity scores (Aas Bousquet et al., 1990). When circulating eosinophils were isolated fi om asymptomatic asthmatics, a large proportion of cells (35%) was recovered with centrifugal density less than 1.082 gml compared to normal subjects (10% Frick et al., 1989). Alteration of centrifugal density of eosinophils is one of the phenotypic responses to cellular activation (Hansel et al., 1990 Fukuda and Makino, 1992). Not only is the number of cells increased in asthma, circulating eosinophils may be activated intravascularly as well. 

The approach most commonly used to evaluate effects on distal airways in clinical and occupational medicine is the maximum forced expiratory maneuver, which allows measurement of airflows as a function of lung volume from total lung capacity to residual volume. Typically, the forced vital capacity (FVC) and the forced expiratory volume at 1 s (as a % of FVC) (FEVi) are measured. Peak expiratory flow is a frequently used measure since simple portable devices permit self-evaluation by patients with obstructive disease. Decreased airflow rates are seen with emphysema, chronic bronchitis, and following... 

Forced expiration is commonly used to assess pulmonary function in both healthy and impaired individuals. Static measures of lung volumes (TLC, Vj, FRC) fail to detect dynamic changes in pulmonary function that are attributable to disease (e.g., asthmatic airway constriction). Obtaining maximum expiratory flow-volume (MEFV) curves (Fig. 5.21) permits derivation of key parameters in detecting changes in lung function. 

Peak expiratory flow The maximum flow rate of air leaving the lungs upon forced exhalation. 

A second way of looking at forced expiration is with a maximum expiratory flow-volume (MEFV) curve, which describes maximum flow as a function of lung volume during a forced expiration (Fig. 12). In healthy human subjects, flow rates or flow-volume curves reach a maximum and will not increase with additional effort after the lungs have emptied 20-30% of their volume (Fry and Hyatt, 1960). This phenomenon of flow limitation is due to airway compression over most of the lung volume. Thus, flow rate is independent of effort and is determined by the elastic recoil force of the lung and the resistance of the airways upstream of the collapse point. In obstructive diseases of the lung this curve is shifted to the left, whereas restrictive diseases shift the curve in the opposite direction (also shown in Fig. 12). 

Clinical Trials In controlled clinical trials in patients with asthma, the onset of improvement in pulmonary function, as measured by maximum midexpiratory flow rate (MMEF), was within 30 minutes after a dose of albuterol tablets, with peak improvement occurring between 2 and 3 hours. In controlled clinical trials in which measurements were conducted for 6 hours, clinically significant improvement (defined as maintaining a 15% or more increase in forced expiratory volume in 1 second [FEVj] and a 20% or more increase in MMEF over baseline values) was observed in 60% of patients at 4 hours and in 40% at 6 hours. In other single-dose, controlled clinical trials, clinically significant improvement was observed in at least 40% of the patients at 8 hours. No decrease in the effectiveness of albuterol tablets was reported in patients who received long-term treatment with the drug in uncontrolled studies for periods up to 6 months. 

An experimental human exposure study titled Tetrachloroethylene Development of a biologic standard for the industrial worker by breath analysis, completed by Stewart and colleagues, was first published by NIOSH in 1974. This publication can now be obtained from the National Technical Information Service (NTIS) with a 1981 date, and is cited as Stewart et al. (1981) throughout this Profile. In this study, four male volunteers were sequentially exposed to 0, 20, 100, or 150 ppm tetrachloroethylene vapor for 7.5 hours/day, 5 days/week (Stewart et al. 1981). The men were exposed to each concentration for 1 week. Once each week, pulmonary function was assessed at both rest and during two levels of exercise with forced maximum expiratory flow measurements, while alveolar-capillary gas exchange was measured by single breath carbon monoxide diffusion. The exposures to tetrachloroethylene at these vapor concentrations and time intervals had no effect on the pulmonary function measurements. 

By the measurement of lung and forced expiratory volumes, nasal, lower, and total airway resistances, closing volume data, the phase III slope of the alveolar plateau, aud the maximum expiratory flow volume, peripheral airway dysfunction was confirmed in 24 adults with common colds. In a randomized, controlled trial, an aromatic mixture of meuthol, eucalyptus oil, and camphor (56%, 9%, and 35% w/w, respectively) were vaporized in a room where the subjects were seated. Respiratory function measurements were made at baseline, 20 and 60 min after exposure. After the last measurement, phenylephrine was sprayed into the nostrils and the measnrements taken again 5-10 min later to determine potential airway responsiveness. The control consisted of tap water. The results showed significant changes in forced vital capacity, forced expiratory volume, closing capacity, and the phase III slope after aromatic therapy as compared to the control. It was concluded that the aromatic inhalation favorably modified the peripheral airway dysfunction (Cohen and Dressier, 1982). 

The flow/volume loop plots exhalation (positive flow) upwards on the y-axis and inspiratory flow (negative flow) downwards. Volume is on the x-axis in liters (there is no time component in this plot, so the FEVi cannot be read from it). The time component is much faster at the beginning of exhalation, the peak usually being reached in less than 0.04 s. The maximum flow on a forced manoeuvre is the peak expiratory flow. Flow volume loops usually record flow in liters per second as here, whereas peak flow meters record in liters per minute. A normal expiratory limb has a near linear rise to peak expiratory flow and then a nearly straight line until vital capacity is reached. A reduction of flow below a straight line usually implies small airways obstruction. The normal inspiratory limb encloses about the same area on the plot as the expiratory limb but is semicircular. The inspiratory limb is flattened by reduced-compliance (stiffer) lungs, as seen in pulmonary fibrosis, by diaphragm muscle weakness and upper airways obstruction. 

---