Flow-volume loops

You should be able to draw the following loops as examples of various respiratory system pathologies. 

Obstructive disease reduces peak expiratory flow rate (PEFR) and increases RV via gas trapping. The TLC may also be higher although this is difficult to demonstrate without values on the x axis. The important point to demonstrate is reduced flow rates during all of expiration, with increased concavity of the expiratory limb owing to airway obstruction. The inspiratory limb is less affected and can be drawn as for the normal curve but with slightly lower flow rates. 

In contrast to obstructive disease, restrictive disease markedly reduces TLC while preserving RV. The PEFR is generally reduced. Demonstrate these points by drawing a curve that is similar in shape to the normal curve but in which the flow rates are reduced. In addition, the left-hand side of the curve is shifted to the right, demonstrating a fall in TLC. 

An intrathoracic obstruction is more likely to allow gas flow during inspiration as the negative intrathoracic pressure generated helps to pull the airways open. As such, the inspiratory limb of the curve may be near normal. In contrast, the positive pressure generated during forced expiration serves only to exacerbate the obstruction, and as such the expiratory limb appears similar to that seen in obstructive disease. Both TLC and RV are generally unaffected. 

This curve is seen where a large airway has a fixed orifice through which gas is able to flow, such as may be seen in patients with tracheal stenosis. The peak inspiratory and expiratory flow rates are, therefore, dependent on the diameter of the orifice rather than effort. The curves should be drawn almost symmetrical as above, with both limbs demonstrating markedly reduced flow. The TLC and RV are generally unaffected. 

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Spirometry, flow-volume loops, plethysmography Bronchodilators... 

Patel, M.H., Kolhatkar, V.P., Potdar, V.P., Shekhavat, K.L., Shah, H.N., Kamat, S.R. (1987). Methyl isocyanate survivors of Bhopal sequential flow volume loop changes observed in eighteen month follow-up. Lung (India) 2 59-65. 

Lodrup Carlson KC, Halvorsen R, Ahlstedt S, Carlsen KH Eosinophil cationic and tidal flow volume loops in children 0-2 years of age. Eur Respir J 1995 8 1148-1154. (II)... 

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. 

FIGURE 25-3. Normal flow-volume loop. Flows are measured on the vertical (y) axis, and lung volumes are measured on the horizontal (x) axis. FVC can be read from the tracing as the maximal horizontal deflection. Instantaneous flow (Vmax) at any point in FVC also can be measured directly. FVC = forced vital capacity. 

FIGURE 25-4. A. Flow-volume loop depicting mild obstruction characterized by decrease flow at low lung volumes. B. Moderate airflow obstruction characterized by a more concave curve. C. Variable intrathoracic obstruction in which peak flow is decreased at higher lung volumes with normalization of curve at lower lung volumes. D. Restrictive lung disease with a curve that is decreased in width but with a normal shape. 

Another test used to distinguish upper airway obstruction from COPD and asthma is the FEVi/FEVq.s (FEV at 1 second/FEV at 0.5 second). This ratio is usually greater than 1.5 in patients with upper airway obstruction. This is so because the FEV0.5 is proportionately more reduced in upper airway obstruction because forced expiration measured at 0.5 second better reflects obstruction at high lung volumes. The abnormahty seen on the flow-volume loop has been referred to as straightening of the curve during early expiration. 

FIGURE 7.7 Typical flow-volume loops, (a) Normal flow-volume loop, (b) Flow-volume loop of patient with obstructive lung disease. 

A unique presentation of airway obstruction in RA is due to cricoarytenoid arthritis. The cricoarytenoid joint is the only true joint in the larynx that can be involved with RA. When the vocal cords are unable to adduct because of arthritis, severe inspiratory stridor develops with a flow volume loop typical of variable extrathoracic upper airway obstruction. Cricoarytenoid arthritis is also seen in gout, systemic lupus erythematosus (SLE), and Reiter s syndrome but is more common in RA. The symptoms of cricoarytenoid arthritis may be subtle... 

Three anatomic patterns of TBA exist, each with distinct pulmonary function test characteristics (i) proximal trachea, ii) mainstem bronchi, and (Hi) distal airways. Proximal disease limits expiratory airflows, producing flow-volume loop changes consistent with extrathoracic upper airway obstruction. Mainstem bronchial disease affects large airways flow, decreasing FEVl/FWC ratio. In contrast, distal airway involvement results in decreased small airway or FEF 25 to 75 flows (56). Bronchoscopically, TBA appears as submucosal plaques or diffuse infiltration in 44% cases, nodular disease in 28%, and circumferential lesions in 28% (58). 

Figure 2 Lung volume recruitment. Flow volume loops. The MIC is maintained (higher expired flow) in a patient with ALS over a period of one year despite complete loss of measurable, spontaneous (lower flow) respiratory muscle function. Abbreviations MIC, maximum insufflation capacity ALS, amyotrophic lateral sclerosis.

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. 

Fig. 3.3.10a,b The flow volume loops from worker 3 (a) and worker 1 (b) from the same manoeuvre as the volume/time plots in Figure 3.3.7... 

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