The ability to accurately obtain and interpret spirometry is essential for physicians caring for patients with asthma and inflammatory disease of the airway. This article reviews the basic equipment, setting, and personnel needed to obtain quality spirometric data. The fundamental measurements obtained in routine office spirometry and recommendations that are critical to obtaining high-quality reproducible test results are reviewed. The evaluation of flow-volume loops and normative data is discusses as well as criteria that define a quality measurement. Examples of normal spirometric data as well as data from different disease states are reviewed.
Key points
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Spirometry is useful in detecting and monitoring airway disease in patients with symptoms, risk factors or suspicion of airway disease.
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Spirometry should accurately measure forced expiratory volume in 1 second, forced vital capacity, or forced expiratory volume in 6 seconds, and it should be reported both as the absolute measurement and as a percentage of normative data.
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Spirometry should be used to diagnose disease as well as monitor response to therapy and progression of disease over time.
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The contour of the flow-volume loop provides additional information with regard to the location of obstruction.
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Most patients with the suspicion of, or being treated for, asthma should have a baseline spirometry test.
Introduction
The classic signs and symptoms of asthma, which include intermittent dyspnea, cough, and wheezing, are often nonspecific, making it difficult to distinguish asthma from other respiratory diseases. The intermittent nature of the disease also makes it difficult for both patient and clinician to monitor the efficacy of therapy. Tests of airflow limitation are critical tools in the diagnosis and monitoring of asthma. Office spirometry is the most frequently used basic tool used to detect, confirm, and monitor obstructive airway disease (eg, asthma, chronic obstructive pulmonary disease [COPD]). Spirometry plays an essential role in the management of patients with, or at risk for, respiratory dysfunction. Spirometry, in which a maximal inhalation is followed by a rapid and forceful complete exhalation into a spirometer, includes measurement of forced expiratory volume in the first second of expiration (FEV 1 ), forced vital capacity (FVC), and the relation of these two numbers (FEV 1 /FVC) and the representation of the effort graphically as a flow-volume loop and volume-time graph. These measurements provide information that is essential to the diagnosis and management of asthma.
This article discusses the use of spirometry in the office setting and discusses the primary measurements obtained and techniques used to obtain an accurate test. Issues related to equipment, performance of the forced expiratory maneuver, and interpretation of the data to obtain reliable and clinically useful information are discussed. Examples of normal spirometric data as well as spirometric data from disease states are briefly reviewed.
Introduction
The classic signs and symptoms of asthma, which include intermittent dyspnea, cough, and wheezing, are often nonspecific, making it difficult to distinguish asthma from other respiratory diseases. The intermittent nature of the disease also makes it difficult for both patient and clinician to monitor the efficacy of therapy. Tests of airflow limitation are critical tools in the diagnosis and monitoring of asthma. Office spirometry is the most frequently used basic tool used to detect, confirm, and monitor obstructive airway disease (eg, asthma, chronic obstructive pulmonary disease [COPD]). Spirometry plays an essential role in the management of patients with, or at risk for, respiratory dysfunction. Spirometry, in which a maximal inhalation is followed by a rapid and forceful complete exhalation into a spirometer, includes measurement of forced expiratory volume in the first second of expiration (FEV 1 ), forced vital capacity (FVC), and the relation of these two numbers (FEV 1 /FVC) and the representation of the effort graphically as a flow-volume loop and volume-time graph. These measurements provide information that is essential to the diagnosis and management of asthma.
This article discusses the use of spirometry in the office setting and discusses the primary measurements obtained and techniques used to obtain an accurate test. Issues related to equipment, performance of the forced expiratory maneuver, and interpretation of the data to obtain reliable and clinically useful information are discussed. Examples of normal spirometric data as well as spirometric data from disease states are briefly reviewed.
Indications: who should be tested?
Spirometry is an invaluable tool as a screening test of general respiratory health in the same way that blood pressure monitoring provides important information about the general health of the cardiovascular system. Data from spirometry are also important to help convince patients with asthma to be more attentive to their disease, particularly those patients with mild intermittent asthma, who often have accommodated to their disease by modifying their lifestyles and avoiding situations that might provoke symptoms. Spirometry lends objectivity to subjective symptoms, is used to determine control in treated patients, and can be a tool to convince patients to be more compliant. It is a simple validated tool, with the ability to be used in nearly any setting.
Spirometry measurements
Spirometry records the forced airflow from fully inflated lungs. Spirometry includes measurement of the FVC, the amount of air exhaled from the lungs from a maximal inhalation to a maximal exhalation, and the FEV 1 . Both FEV 1 (airflow) and FVC (air volume) can be compromised by airway narrowing, inflammatory and bronchospastic factors, and mucus plugging, which can obstruct or occlude some of the small (or even larger) airways. These values are typically reported in 2 ways: as a volume measurement (milliliters or liters of air), or as a percentage of the predicted normative or expected value for that patient’s age, height, gender, and race from data obtained in the National Health and Nutrition Examination Survey III (NHANES III).
The FEV 1 is the most important spirometric measurement for assessment of the severity of airflow obstruction. The highest FEV 1 from the 3 acceptable forced expiratory maneuvers is used for interpretation, even if it does not come from the maneuver with the highest FVC.
In patients with asthma, the FEV 1 declines are in direct and linear proportion with clinical worsening of airway obstruction. FEV 1 has been shown to increase with successful treatment of airway obstruction. The FEV 1 should be used to determine the degree of obstruction (mild, moderate, or severe) and for serial comparisons when following patients with asthma. The measured FEV 1 is usually expressed as a percentage of the predicted value for determination of normality. The reference values from the NHANES III study (recently expanded to preschool children) are recommended for use throughout North America. The lower limit of normal FEV 1 is more accurately defined by the fifth percentile of healthy never-smokers, instead of the traditional 80% of predicted.
The FVC (also known as the forced expiratory volume) is the maximal volume of air exhaled with a maximally forced effort from a position of full inspiration and is expressed in liters. The highest FVC from the 3 acceptable forced expiratory maneuvers is used for interpretation.
The FVC may be reduced by suboptimal patient effort, airflow limitation, restriction (eg, from lung parenchymal, pleural, or thoracic cage disease), or a combination of these. In general, a moderately or severely low FVC needs further evaluation with a more complete battery of pulmonary function tests.
The forced expiratory volume in 6 seconds (FEV 6 ) is a term that is sometimes used to describe the forced expiratory volume in 6 seconds of maximal exhalation, obtained by stopping the expiratory effort after 6 seconds rather than at cessation of airflow. This surrogate for the FVC is acceptable. The advantages of the FEV 6 compared with FVC include less frustration by the patient after repeated attempts at a prolonged and forceful blow and by the technician trying to achieve an end-of-test plateau. Additional advantages of an FEV 6 compared with the FVC include a smaller chance of syncope, shorter testing time, and better repeatability, without loss of sensitivity or specificity.
Together, the FEV 1 and FVC (or FEV 6 ) are considered the most readily available and most useful components of spirometry and the most reflective of an obstructive disease state such as asthma.
Another important relationship shown by spirometry is the ratio between the FEV 1 and FVC: the FEV 1 /FVC ratio is the fraction of FVC that can be exhaled in the first second. It is the most important parameter for detecting airflow limitation in diseases like asthma. However, once established, the ratio has little value in predicting progression of disease because typically both the FVC and FEV 1 decline with progression of disease. The threshold for an abnormal FEV 1 /FVC ratio is the fifth percentile lower limit of normal.
Additional lung functions that can be measured during spirometry include the forced expiratory flow of the midexpiratory phase of forced expiration. This middle 50% of the total FVC is called the forced expiratory flow 25% to 75% (FEF 25%–75% ), and has been thought to reflect the reactivity in the mid to small airway. Although asthma is a disease of the small to midsized airway, the clinical value of this value as a single meaningful number has recently been questioned. Although of use in the overall picture of airway reactivity, therapeutic decisions should not be based exclusively on this number.
Graphic representation of data
Flow-Volume Loops
This approach to the data yields useful additional information beyond that obtained by analysis of FEV 1 and FVC measurements. Information generated during the office spirometry can be analyzed by plotting the data, creating a graph of the test known as the flow-volume loop (also called a spirogram). The flow-volume relationship or loop is created by plotting flow against volume during the FVC (forced expiratory) maneuver. The flow-volume loop is a plot of inspiratory and expiratory flow in liters per second (on the y-axis) against volume (on the x-axis) during the performance of maximally forced inspiratory and expiratory maneuvers ( Fig. 1 ). Analysis of this loop provides rapidly recognizable patterns that permit the clinician to differentiate bronchial asthma from airflow limitations with other causes, such as vocal cord dysfunction or a fixed obstruction. The flow-volume loop can also show reversibility of the disease process, differentiate an obstructive pattern of airflow from a restrictive pattern, and easily separates a quality maximal effort from a suboptimal test.
The normal expiratory portion of a well-performed flow-volume loop is characterized by a rapid increase to the peak flow rate, followed by a nearly linear decrease in flow as the patient exhales toward residual volume. Less-than-optimal effort, early glottic closure, and coughing are some of the variables that can influence the expiratory curve ( Figs. 2–4 ). The expiratory curve should be examined on every test as a key component of the test interpretation.
In contrast, the inspiratory curve is a symmetric, saddle-shaped curve. The flow rate at the midpoint of exhalation (between total lung capacity and residual volume) is normally approximately equivalent to the flow rate at the midpoint of inspiration.
Changes in the contour of the loop can aid in the diagnosis, type, and localization of airway disease and are critical components of spirometry interpretation. Characteristic flow-volume loop patterns are also often found in certain forms of restrictive disease, although flow-volume studies are not considered primary diagnostic aids in the evaluation of these disorders.
A flow-volume loop representative of an obstructive disorder (asthma, COPD) typically shows a departure from the linear slope of the flow curve and instead is a scalloped or scooped-out concavity ( Fig. 5 ). Lung volumes are typically normal in isolated obstructive disease.
