Flow-volume curves

FIGURE 5.21 Representative spirogram (top) and flow-volume curve (bottom) during forced expiration. FEV o shown in the spirogram corresponds to the arrow in the flow-volume curve indicating forced expired volume in one second. 

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). 

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

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. 

In initial studies in which substance P was administered to subjects by intravenous infusion, no effect on airway caliber could be demonstrated. It is possible that potent cardiovascular side effects resulted in homeostatic reflexes with secondary effects on the airways that masked the effects of substance P (Fuller et al., 1987). However, when changes in the partial flow volume curve, a highly sensitive index of airway caliber, were used as the index of airway obstruction, inhaled NkA - and, to a lesser extent, inhaled substance P - caused airway obstruction in healthy people (Joos et al., 1987). NkA is approximately 10-100 times more potent as a bronchocon-strictor in asthmatic than in non-asthmatic subjects (Cheung et al., 1992, 1993) this ratio of potency is similar to that observed for other agonists. 

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. 

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). 

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... 

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). 

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. 

FIGURE 7.6 Flow-volume curve obtained from a spirometry maneuver. Solid line is a normal curve dashed line represents a patient with obstructive lung disease. 

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

Flow and volume are often plotted against one another as in Fig. 4.13. The flow-volume curves shown are for increasing levels of effort during expiration. The... 

FIGURE 4.13 Flow-volume curves for increasing effort level during expiration, including maximal effort Note eflbit-independent portion of curves. 

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