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Pulmonary function tests (PFTs) are used to diagnose, assess severity, and assess response to therapy in pulmonary disease. To assess reversible airway obstruction or bronchodilator responsiveness, the patient is given an inhaled bronchodilator and the test is repeated. Histamine, methacholine, exercise, isocapneic cold dry air, and hypertonic or hypotonic aerosol challenges may also help assess airway reactivity.
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Forced vital capacity (FVC): Volume that can be maximally forcefully exhaled after a complete inspiration
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FEV1: Volume of air that is forcefully exhaled in the first second following a complete inspiration
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FEV1/FVC: Ratio of FEV1 to FVC, expressed as a percentage
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FEV 25–75: Average forced expiratory flow over the mid portion of the FVC
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Peak expiratory flow (PEF): Peak expiratory flow rate during forced exhalation
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A volume is the amount that describes a compartment of the lung; when two or more volumes are added together, the new amount is called a capacity. All volumes except the residual volume (RV) can be measured by spirometry; any capacity that includes the RV requires specialized equipment for its measurement.
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Expiratory Reserve Volume (ERV): Volume that can still be exhaled following normal exhalation
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Functional residual capacity (FRC): Volume remaining in the lungs at the end of normal exhalation (RV + ERV)
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Inspiratory capacity (IC): Volume in the lungs at full inspiration (IRV + VT)
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Inspiratory reserve volume (IRV): Volume that can still be inhaled after normal inspiration
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Residual volume (RV): Volume remaining in the lungs after maximal expiration (FRC-ERV, or TLC-VC)
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Tidal volume (V T): Volume inhaled and exhaled during normal breathing
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Total lung capacity (TLC): Total volume of gas in the lungs at full inspiration (VC + RV, or IC + FRC, or IRV + VT + ERV + RV)
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Vital capacity (VC): Maximal volume that can be expired from a full inspiration (IRV + TV + ERV; TLC-RV)
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EQUIPMENT COMMONLY USED TO MEASURE LUNG FUNCTION
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Body plethysmograph: Apparatus for measurement of FRC, RV, TLC, and airway resistance. Fractional lung volumes can also be measured with dilution techniques using helium or nitrogen
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Spirometer: Apparatus for measuring lung volumes (except RV) and flow rates. Spirometry is used to plot a volume–time curve and a flow–volume loop
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ANTHROPOMETRIC MEASUREMENTS AFFECTING LUNG FUNCTION
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Height: Taller individuals have larger lung volumes and airways
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Age: Lung volumes change with increasing age in the pediatric population; RV and FRC increase, ERV decreases
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Sex: Males have larger lung volumes than females
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Ethnicity: TLC is generally lower in African Americans when compared to Caucasians because African Americans have smaller upper-to-lower body segment ratios
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OBSTRUCTIVE LUNG DISEASE
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Obstruction to airflow during expiration leads to gas trapping, increased RV, decreased, increased, or normal VC, and increased to normal TLC (Figure 27-1). Common causes include asthma, bronchiolitis, chronic bronchitis, CF, and bronchiectasis. Spirometry demonstrates a low FEV1 (<80% predicted), low FVC (<80% predicted), low FEV1/FVC (<75% predicted), decreased FEF 25–75, and an expiratory flow–volume curve that is concave to the volume axis (Figure 27-2B).
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Small airway obstruction is represented by low FEV 25–75
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Significant response to bronchodilators defined as >12% increase in the FEV1 and/or FVC
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Methacholine challenge that results in a decrease in FEV1 20% or greater from baseline at a dose less than 16 mg/dL, or exercise or cold dry air challenge that results in 15% or greater decrease in FEV1 from baseline are used to diagnose airway hyperreactivity
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RESTRICTIVE LUNG DISEASE
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Restrictive lung diseases cause reductions in lung volumes due to decreased lung compliance, decreased chest wall compliance, or muscle weakness. Etiologies include interstitial lung disease, neuromuscular diseases, and chest wall or spine abnormalities. TLC is decreased whereas flow rates are proportionally normal or slightly increased. Spirometry reveals a low FEV1 (<80% predicted), low FVC (<80% predicted), normal FEV1/FVC ratio, and “miniaturized” appearance to flow–volume curve (Figure 27-2C).
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VARIABLE EXTRATHORACIC OBSTRUCTION
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During inhalation, narrowing of the extrathoracic airway accentuates any obstruction, and airflow through the narrowed portion of the extrathoracic airway decreases. This results in flattening of the inspiratory flow–volume loop (Figure 27-2D). If the narrowing is not present during exhalation, the expiratory flow–volume curve is normal. Spirometry reveals a normal FVC and FEV1, but the ratio of forced expiratory to inspiratory flow at 50% of vital capacity (FEF50/FIF50) is usually greater than 1. Etiologies include vocal cord dysfunction or vocal cord paralysis.
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FIXED AIRWAY OBSTRUCTION
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There is no change in the caliber of the airway with a fixed intrathoracic or extrathoracic obstruction during the entire respiratory cycle. As a result, airflow limitation through the obstruction is independent of the phase of respiration and results in flattening of both inspiratory and expiratory flow–volume loops (Figure 27-2E). Etiologies include tumors, tracheal or subglottic stenosis, and foreign bodies in the airway.