Expiratory flow-volume curves

Figure 2. Maximum (MEFV thin lines) and partial (PEFV heavy lines) expiratory flow-volume curves by a healthy 25 year old female subject.

FIGURE 25-5. Maximum expiratory flow-volume curves from patients with fixed obstruction, variable extrathoracic obstruction, and variable intrathoracic obstruction. RV = residual volume TLC = total lung capacity. 

The best way to determine the caliber of the intrathoracic airways is to utilize pulmonary tests measured at maximal flow rates such as forced expiratory volume in 1 second (FEVj) and maximal expiratory flow-volume curves. The forced expiratory volume maneuver requires a subject to inspire maximally and then exhale as hard as possible into a spirometer. The typieal... 

Fic. 12. Comparison of the typical maximal expiratory flow-volume curve in healthy subjects and those with obstructive or restrictive diseases. 

Green M, Mead J. Turner JM. Variability of maximum expiratory flow-volume curves. J Appl Physiol 1974 37 67-74. 

Prendiville A, Green S, Silverman M. Paradoxical response to nebulised salbutamol in wheezy infants, assessed by partial expiratory flow-volume curves. Thorax 1987 42 86-91. 

TABLE 6.6 Pulmonary Variables from the Maximal Expiratory Flow-Volume Curve... 

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. 

Clinical studies of cotton mill workers who had previously demonstrated a decreased expiratory flow measured by flow volume curves and FEV during cotton dust exposure showed an increase in WBC to 25.5% after 4 hours of exposure. Segmented neutrophils increased most (33%), while eosinophil mean counts did not change. The ratio of segmented neutrophils to epithelial cells from nasal mucosal swabs increased from 0.56 before to 1.84 after 4 hours of exposure. Peripheral blood and PMN counts increased upon exposure to cotton dust, and PMN were recruited to the nasal mucosa. Chest tightness and decreased flow were temporarily correlated with leukocyte recruitment following cotton dust exposure (2). 

Volume-Time and flow- Volume Curves The characteristics measured by the maximal expiration are forced vital capacity ( o, 1-s forced expiratory volume (FEv,), peak expiratofy flow rate (Vn x)> and flow rates at and 25% of the remaining fvc (Vj . 2 ) for partial and maximal flow-volume curves. These measurements give an easily obtained, relatively reproducible evaluation of overall pulmonary mechanical performance, but provide little information on the mechanisms responsible for an observed change. 

Tests of airway caliber are divided into two types those that are measured at submaximal flow rates (e.g., airway resistance) and those that are measured at maximal flow rates (e.g., force expiratory volume and flow-volume curves). 

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). 

Spirometry is the most widely available and useful PFT. It takes only 15 to 20 minutes, carries no risks, and provides information about obstructive and restrictive disease. Spirometry allows for the measurement of aU lung volumes and capacities except RV, FRC, and TLC and allows assessment of FEVi and FEF25%-7s%. Spirometry measurements can be reported in two different formats—standard spirometry (Eig. 25-2) and the flow-volume loop (Fig. 25-3). In standard spirometry, the volumes are recorded on the vertical (y) axis and the time on the horizontal (x) axis. In flow-volume loops, volume is plotted on the horizontal (x) axis, and flow (derived from volume/time) is plotted on the vertical (y) axis. The shape of the flow-volume loop can be helpful in differentiating obstructive and restrictive defects and in the diagnosis of upper airway obstruction (Fig. 25 ). This curve gives a visual representation of obstruction because the expiratory descent becomes more concave with worsening obstruction. 

Methodologies are currently available for measuring airway resistance and compliance in rodents and non-rodents either during spontaneous breathing (dynamic) or using a forced maneuver procedure (flow-volume or pressure-volume curves) that involves a controlled inflation and rapid deflation of the lung to evaluate forced expiratory flows and static or quasi-static compliance. The direct measurement of resistance and compliance t5q)ically requires an anesthetized model so that the airflows, pressures, and volumes can be monitored and controlled (Diamond and O DoimeU 1977 Costa et al. 1992 Mauderly 1989). 

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