INTRODUCTION — Pulmonary function testing (PFT) in children plays an important role in the evaluation of the child with known or suspected respiratory disease. A basic approach to PFT for the primary care provider is presented here. The goal is to encourage pediatricians to obtain PFT to diagnose and monitor the pathophysiologic aspects of their patients' respiratory conditions, thereby improving management.
Conventional tests that are frequently performed in the evaluation of pediatric respiratory conditions include measurements that identify:
●Airway obstruction
●Restrictive lung, chest wall, and respiratory muscle defects
●Diffusion defects (those that impair diffusion of gas through the alveolar-capillary membrane)
●Respiratory muscle weakness
Measurements of flow and volume are most useful in the office setting. Spirometry, which provides both these measures, requires neither sophisticated technology nor expensive equipment, is easily interpreted, and is reliable when performed correctly. A brief discussion of the technique, clinical applications for and limitations of more sophisticated measurements of lung volume, diffusion capacity, and respiratory muscle function also is provided so that referrals for such testing can be made appropriately.
Readers interested in a more comprehensive review of PFT in children are referred elsewhere [1].
Interpretation of arterial blood gas analyses and exhaled nitric oxide, exercise testing, assessment of bronchial hyperresponsiveness, sleep studies, measurements of control of breathing, and PFT in adults are discussed separately. (See "Measures of oxygenation and mechanisms of hypoxemia" and "Exhaled nitric oxide analysis and applications" and "Exercise testing in children and adolescents: Principles and clinical application" and "Bronchoprovocation testing" and "Evaluation of suspected obstructive sleep apnea in children", section on 'Polysomnography' and "Control of ventilation" and "Overview of pulmonary function testing in adults" and "Pulmonary function testing in asthma".)
ROLE OF PULMONARY FUNCTION TESTING — PFT allows for assessment of the:
●Normal lung and airway growth
●Natural history of diseases (eg, bronchopulmonary dysplasia, cystic fibrosis [CF])
●Site and type of obstruction (central versus peripheral, intrathoracic versus extrathoracic, fixed versus variable)
●Impact of therapies (eg, bronchodilators, glucocorticoids, diuretics, mucolytics)
●Degree of impairment
●Impact of environmental factors (eg, tobacco smoke, toxic gases)
●Degree of airway reactivity
In addition, PFT can aid in preoperative assessment of the child with chronic lung disease or neuromuscular weakness and is also used for monitoring disease progression and prognosis. Forced vital capacity (FVC), for example, is a key parameter to monitor for progression of effects of scoliosis on the lungs, while forced expiratory volume in one second (FEV1) has prognostic value with regards to mortality in some diseases such as CF.
PFT is recommended by the National Asthma Education and Prevention Program (NAEPP) and the Global Initiative for Asthma (GINA) in the assessment and long-term monitoring of patients with asthma [2]. There is a growing need for PFT as the incidence of childhood asthma continues to rise. The development of compact and affordable instruments also enables pediatric lung function testing in the primary care setting.
ADVICE RELATED TO COVID-19 PANDEMIC — Spirometry and other PFT maneuvers can promote coughing and aerosol generation and could lead to spread of coronavirus disease 2019 (COVID-19; SARS-CoV-2) by infected patients. It is difficult to screen patients for active COVID-19 infection, particularly those with underlying respiratory symptoms, and infected but asymptomatic patients can shed the virus. Thus, we agree with expert recommendations that spirometry and other PFTs be limited to patients in whom results are essential to immediate management decisions [3]. Use of nebulizers to administer bronchodilators or methacholine for testing should be minimized.
Measures to prevent spread of COVID-19 should include hand hygiene and personal protective equipment (PPE; gloves, gown, face mask and shield) for staff and anyone else in the testing space (eg, interpreters). N95 masks or powered air purifying respirators (PAPR) are preferred over surgical masks. Patients should be brought to a testing room using an approach that avoids queuing or grouping individuals in a waiting area. Enhanced cleaning of the testing area should be performed between patients.
SPIROMETRY
Overview — Spirometry is the measurement of breathing over time. It includes measures of flow and volume generated by a forced and complete exhalation to residual volume (RV) after a full inspiration. The volume-time curve graphic presentation (figure 1) has been largely replaced in clinical practice by the flow-volume curve (figure 2 and figure 3), which provides a more immediate and intuitive perception of obstructive and restrictive disorders. However, the two graphic depictions are complementary and optimally should be analyzed together. Spirometry also includes the measurement of flow and volume with inspiration; however, inspiratory parameters are less commonly measured and are not discussed here. (See "Flow-volume loops".)
Production of a reliable, sustained expiration requires coaching from the clinician and both coordination and focus on the part of the subject. Thus, spirometry is usually not obtainable predictably from children younger than six years.
Spirometry is most useful for evaluation of common pediatric obstructive lung diseases, such as asthma, cystic fibrosis (CF), and the sequelae of bronchopulmonary dysplasia.
The following discussion of spirometry focuses on the measured parameters, interpretation, and clinical applications most relevant to the primary care provider. Methodology and interpretation of spirometry in children are discussed in greater detail in guidelines published by the American Thoracic Society (ATS) [4], accessed through the ATS website.
Measured parameters — The important parameters derived from spirometry include indices of flow:
●Forced expiratory volume in one second (FEV1)
●Flow between 25 and 75 percent of the vital capacity (FEF25-75%), also known as the maximal midexpiratory flow rate (MMEFR)
●Peak expiratory flow rate (PEFR)
These parameters also include indices of volume:
●Forced vital capacity (FVC)
●Slow vital capacity (SVC) performed with a submaximal expiratory effort
The air left in the lungs at the end of expiration (RV) cannot be measured by spirometry. Thus, any lung capacity measurement that includes RV as a component cannot be derived by spirometry.
The measured values for these parameters are compared with normative data and are reported as the percent of predicted value for subjects of similar age, height, gender, and race [5]. Children with chronic conditions can also be monitored by comparing their measured values with those obtained at previous visits [2].
Interpretation — The interpretation of spirometry depends upon which parameters are affected (table 1). As a general rule, obstructive disorders affect indices of flow, and restrictive disorders affect indices of volume.
FEV1, FEF25-75%, and PEFR are decreased in obstructive disorders. FEF25-75% represents flow in smaller conducting airways. It is less effort dependent than the other parameters, and it is a particularly useful measurement of milder intrathoracic airway obstruction. It can be reduced by 25 percent or more when the patient is symptom free and/or has a normal lung examination and in the presence of normal FEV1 and PEFR [6]. Such reduction in FEF25-75% can be visually noted as scooping of the flow-volume curve (figure 3).
FVC typically is decreased in restrictive disorders (figure 4) but may also be low due to gas trapping and hyperinflation in obstructive defects. In addition to affecting FVC, restrictive disorders can also lower parameters of flow. FEV1 is typically reduced since the volume at the initiation of the expiratory maneuver is lower. Thus, a restrictive disorder should be considered when flow indices are lowered to a similar extent as the reduced FVC. A normal FEV1/FVC ratio of >85 percent suggests that the observed decrease in flow is due to volume reduction rather than obstructed flow. In this setting, additional measurements of lung volume using methods unavailable in the primary care setting should be undertaken to confirm the restrictive respiratory disorder. (See 'Lung volume measurement' below.)
Many children have both restrictive and obstructive abnormalities in lung functions. These patterns can be difficult to interpret. A reduced FVC out of proportion to the reduced FEV1/FVC may indicate that an additional restrictive element exists. When in doubt, referral to a pulmonary function laboratory will be helpful.
Faulty technique will yield unreliable results, and many children are unable to perform the maneuvers adequately. The two major parameters to follow are reproducibility of the flow-volume curve and the duration of the expiratory maneuver. The latter is variable at young ages but should always exceed one second and plateau for reliable interpretation.
Flow-volume curve — The flow-volume curve is a graphic representation of individual forced expiratory maneuvers. It is included in most spirometry reports; only the expiratory limb is typically displayed (figure 2). Obstructive defects are portrayed by scooping of the descending limb of the curve (figure 3), a change that can be easily detected by the experienced observer. The graphic display of the flow-volume curve also provides the best measure of quality control by permitting assessment of the reproducibility of repeated maneuvers. Reliable measurements should have at least three flow-volume curves that can be closely superimposed.
Examination of the inspiratory flow-volume curve is helpful when flow obstruction derives from the extrathoracic airways, most commonly in the larynx or upper trachea. These often useful parameters are not always included in the flow-volume curves reported by standard spirometers. Flow-volume patterns in upper-airway obstruction are discussed in detail separately. (See "Flow-volume loops".)
Use in asthma — Primary care providers should consider the use of office spirometry in the evaluation and management of their patients with asthma. Spirometry can be used to support a diagnosis of asthma by demonstrating reversible airflow obstruction. This is particularly helpful in children with isolated symptoms (eg, persistent cough, exercise intolerance) or atypical presentations. In such patients, spirometry may also guide the clinician to an alternative diagnosis (eg, restrictive lung disease). (See "Asthma in children younger than 12 years: Initial evaluation and diagnosis".)
Once a diagnosis of an obstructive disorder has been made, the reversibility of the obstruction can be assessed by measuring FEV1 before and after inhalation of a bronchodilating agent (eg, albuterol). As a general rule, individuals without bronchial hyperreactivity have a <5 percent increase in FEV1 after inhalation of a bronchodilator [7]. A postbronchodilator increase in FEV1 of >12 percent constitutes a reversible obstructive lung defect and supports a diagnosis of asthma. However, this definition for bronchodilator response (BDR) positivity was established primarily in adults. An increase in FEV1 of ≥8 percent may be a better definition for BDR in children, although it remains an insensitive test [8-10]. The degree of reversible obstruction can vary from one visit to the next. Some children may have technically normal flows that are still less than 100 percent predicted. Bronchodilators can produce improvement in such children who may have flows of >110 percent predicted as their normal values. A bronchodilator challenge in this circumstance may also produce enough improvement to confirm reversible airway obstruction and indicate asthma. Other features of asthma, such as clinical response to bronchodilators, must be taken into account in making the diagnosis in children with mild asthma, rather than reliance on changes in lung function after a bronchodilator alone.
Monitoring of asthma with spirometry, used in conjunction with daily symptoms and exacerbation frequency, is also useful in evaluating response to therapy and changes in asthma severity over time in children with asthma [11]. Performance of spirometry at the start of treatment and during the first six months of follow-up were both associated with a decreased risk of hospitalization in the subsequent year in 27,193 Danish children aged 6 to 14 years with asthma [12]. Regular measurements in children with uncomplicated asthma are recommended by multiple asthma guidelines, such as the National Asthma Education and Prevention Program (NAEPP) and Global Initiative for Asthma (GINA) guidelines.
The midexpiratory flows measure (FEF25-75%), while highly variable in repeat measurements, is sometimes a sensitive parameter to indicate an obstructive defect since it may be reduced in patients who are without symptoms and have normal FEV1 and PEFR [6].
Barriers to performance of spirometry in the primary care setting include lack of time and training, particularly in interpretation of results [13].
Peak expiratory flow rate — The PEFR measurement (often referred to as the peak flow measurement), unlike full spirometry, requires only a short expiratory blast, without the subject having to sustain a prolonged expiratory effort, and is therefore feasible for younger children four to six years of age. Typically, the highest of three PEFRs is reported. The portability and ease of use of inexpensive versions of the Wright Peak Flow Meter have made this device commonplace in the primary care setting and the home.
PEFR meters are not "the poor man's spirometer" and have several important limitations [6,14,15]. The measurements obtained by PEFR meters are highly effort dependent and can be manipulated by children [16]. In addition, intrapersonal variability can be substantial and is particularly affected by circadian rhythms, although diurnal variability may also indicate poor control. Finally, because mini peak flow meters are not precise tools, wide variation of recorded PEFR can be observed between devices, even of the same brand. Thus, measurements performed by devices other than those customarily used by the patient should be interpreted with caution.
Peak flow meters have a limited role in establishing the diagnosis of asthma in the office. However, they are useful in gauging the severity of asthma exacerbations, both by comparing PEFR measurements with population-specific normative data (table 2) [17] and, more importantly, to preestablished baseline ("personal best") values. Such PEFR measurements are commonly used to assist in determining levels of interventions according to predetermined asthma management plans. The optional use of PEFR has also been incorporated into the NAEPP guidelines for asthma management [2]. The use of PEFR monitoring in asthma is discussed in detail separately. (See "Peak expiratory flow monitoring in asthma".)
Low-cost electronic handheld spirometers that measure FEV1 and PEFR are now available for use in the home setting [18-21]. The addition of FEV1, a more reliable spirometric parameter, to PEFR improves individual monitoring without significantly increasing cost.
Bronchial challenge tests — Bronchial challenge, or bronchoprovocation, tests are designed to provoke an asthmatic response in children suspected of having bronchial hyperreactivity. Methacholine, exercise, and/or cold air may be used as the challenge agent. Bronchial challenge tests are performed when there is uncertainty about the diagnosis of asthma. Exercise challenges are particularly useful to uncover exercise-induced airway reactivity, a common and often underdiagnosed condition in childhood asthma. Bronchial challenge testing should be performed only in specialized centers [2,22]. In addition, because it has the potential to induce severe bronchospasm, bronchial challenge testing requires the presence of staff who are experienced in managing acute airway obstruction. Bronchial challenge tests are covered in greater detail separately. (See "Bronchoprovocation testing".)
Spirometry in the care of cystic fibrosis — FEV1 is a key outcome measure in CF. It is routinely used to monitor the rate of progression of the lung disease, the need for interventions such as the administration of antibiotics or new drugs, and the effectiveness of such interventions. (See "Cystic fibrosis: Clinical manifestations of pulmonary disease", section on 'Pulmonary function testing'.)
LUNG VOLUME MEASUREMENT — Lung volume measurement is typically undertaken in specialized pulmonary function laboratories with adult-based services, which limits use of this measurement in children. The measurement of lung volumes is important in clinical conditions in which a restrictive lung defect and/or air trapping may be present. It is also important when addressing possible diffusion capacity defects. The partitioning of lung volumes is depicted in the figure (figure 1). (See 'Diffusion capacity measurement' below.)
Methods — There are two conventional methods of measuring lung volumes in children: whole-body plethysmography and gas dilution (usually with helium and, less frequently, by nitrogen washout). Both methods measure the functional residual capacity (FRC), which is the residual air in the lung at the end of exhalation during tidal breathing (figure 1). The thoracic gas volume (TGV) measured equals FRC because the measurement is begun at end expiration, after a stable baseline end-expiratory volume is achieved. The FRC is unobtainable by spirometry since it includes the residual volume (RV). The use of spirometry together with measures of FRC allows division of lung volumes, as depicted in the figure (figure 1). Demonstration of reduced total lung capacity (TLC) is the gold standard for the diagnosis of restrictive respiratory disease in both adults and children.
Plethysmography — Plethysmography involves placing the child inside a whole-body plethysmograph or "body box," a sealed structure similar in appearance to a telephone booth. This is typically done with the child alone, although the measurements can be performed with the child seated on the lap of a parent. The plethysmograph is tightly sealed, and the child is asked to breathe normally at tidal volume (TV) through a mouthpiece.
The TV is measured, and the mouthpiece is briefly occluded by a shutter at end expiration (FRC). With the child panting against the closed shutter, pressure oscillations are simultaneously measured at the mouthpiece and within the plethysmograph. This maneuver results in alternating compression and decompression of the intrathoracic gas, which is measured at the mouth and within the box. By employing Boyle's law, which states that the product of pressure and volume remains constant in a closed system (P1V1 = P2V2), the FRC is calculated.
Plethysmography requires cooperation on the part of the subject and is therefore difficult to obtain in children younger than six years. Reliance on complex maneuvers, such as panting against a closed shutter, is the major limitation. In addition, some children find enclosure within the plethysmograph frightening.
Plethysmography measures the total TGV, including areas not communicating with the central airways (eg, cysts, areas distal to airway obstruction), while gas dilution measures only those areas of the lungs in direct communication with the measurement apparatus. Dilution techniques, therefore, will underestimate FRC in conditions in which there is significant air trapping. Thus, plethysmography is preferred in obstructive conditions, in which air trapping may occur [22].
Gas dilution — The gas dilution methods of lung volume determination are based upon the assessment of helium or nitrogen concentrations and can be done either as a single breath measurement or, more commonly, with multiple breaths. The helium dilution method is more commonly used. Gas dilution methods require less cooperation from the child compared with plethysmography. These methods are a good alternative for patients who cannot physically fit in a plethysmograph (eg, children confined to wheelchairs) or those who feel too claustrophobic when sitting in the body box. Gas dilution using a sealed mask applied to the face can also be used in children whose buccal musculature is too weak to maintain a seal around a mouthpiece.
●In the multiple breath helium dilution method, the subject inhales a gas mixture containing a known concentration of helium in a known volume; the subject continues to inhale and exhale into a closed circuit until equilibrium is obtained. FRC is derived from the proportionate change of the helium concentration before and after equilibrium is reached (diluted by the gas inside the chest).
●Multiple breath nitrogen washout is performed via an open circuit measurement by having the child breathe 100 percent oxygen for several minutes until the nitrogen content of the exhalate is less than 1 percent, at which point virtually all the nitrogen in the lung has been exhaled into the spirometer. A rapid nitrogen analyzer measures the exhaled nitrogen in each exhalate as it is "washed out" with the 100 percent oxygen being inhaled. The volume of exhaled nitrogen is measured throughout the procedure. FRC can be calculated by dividing the total volume of nitrogen obtained by the difference in concentrations obtained.
Clinical application and interpretation
Restrictive disease — Restrictive lung defects are defined by reduction in functional lung volumes and can only be confirmed by such measurements. In these conditions, the TLC is reduced to below 80 percent of that predicted by age, height, and gender. The RV remains unchanged with hypotonia or even increased in cases of chest wall disease. Thus, the RV/TLC ratio is increased in children with these disorders, which mainly affect inspiratory, or vital capacity (VC).
The most common pathologic conditions in which lung volume determination is useful include:
●Intrinsic lung diseases, such as interstitial pneumonias
●Chest wall pathologies (eg, scoliosis)
●Neuromuscular diseases (eg, Duchenne muscular dystrophy)
In restrictive defects caused by intrinsic pulmonary disease, the RV is often reduced as well, resulting in a normal RV/TLC ratio. In restrictive lung defects caused by neuromuscular weakness and chest and spine deformities, the RV remains almost normal, and the RV/TLC ratio is increased (table 1).
Obstructive disease — Lung volume measurements are not needed to define obstructive defects, although such defects can lead to air trapping, defined as an increased RV/TLC ratio in the setting of airway obstruction (a reduced FEV1/FVC ratio). When air trapping is present, the TLC can be normal or increased, the VC is normal or decreased, and the RV is increased. The latter changes can be the earliest parameters detected in early airway pathologies, such as cystic fibrosis (CF). Reduced VC and/or symmetrically reduced VC and FEV1 should prompt lung volume measurement.
DIFFUSION CAPACITY MEASUREMENT — Diffusion capacity is typically measured in specialized centers because of the cost and complexity of the required equipment. The diffusing capacity of the lungs for carbon monoxide (DLCO) and alveolar volume (VA) are measured in conditions in which impairment of gas diffusion across the alveolar-capillary membrane or a reduction in the alveolar-capillary surface area is suspected (eg, in children with exercise intolerance unrelated to bronchospasm, especially when associated with oxyhemoglobin desaturation, unexplained dyspnea or hypoxemia, known interstitial lung disease, or pulmonary fibrosis). The DLCO is used diagnostically to detect the presence of diffusion abnormalities when diseases of the pulmonary interstitium or vasculitis are suspected. It is used therapeutically to track effects of interventions for those diseases or side effects of therapies known to cause pulmonary fibrosis.
DLCO and DLCO/VA are reduced in intrinsic lung disease, such as the interstitial lung diseases and idiopathic pulmonary fibrosis as well as in pulmonary fibrosis secondary to cytotoxic or radiation therapy. Less frequently, pulmonary edema and pulmonary vascular diseases, including vasculitis associated with rheumatologic disease, can also reduce the DLCO. The DLCO is increased above the normal range in pulmonary hemorrhage. Diffusion capacity testing is not frequently performed in children; the technical complexity of its performance limits its use to the older child. The DLCO test is discussed in greater detail separately. (See "Diffusing capacity for carbon monoxide".)
RESPIRATORY MUSCLE PRESSURE MEASUREMENTS — Determinations of respiratory muscle pressures are typically measured in specialized centers. These measurements evaluate the global strength of the inspiratory muscles (maximal inspiratory pressure [PImax]) and expiratory muscles (maximal expiratory pressure [PEmax]). The tests consist of a forceful inhalation and exhalation into tubing connected to a pressure manometer. The values obtained are compared with normative data for subjects of similar height, age, and gender.
These tests are useful in determining whether decreases in expiratory flows or lung volumes are caused by weakness of various respiratory muscles. They are particularly valuable to assess progression of muscle weakness in children with progressive neuromuscular disorders, such as Duchenne muscular dystrophy, since they can help determine when therapies such as assistance with coughing should be instituted [23]. These tests are discussed in greater detail separately. (See "Tests of respiratory muscle strength" and "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation" and "Duchenne and Becker muscular dystrophy: Clinical features and diagnosis".)
EXHALED NITRIC OXIDE — Attention has focused on biomarkers that reflect pathologic processes, such as airway inflammation. The most widely studied of these measures is fractional exhaled nitric oxide (FeNO or eNO) [24,25]. Increased levels of this gas reflect the degree of eosinophilic airway inflammation in asthma and atopy. Altered levels of exhaled FeNO are also seen in other diseases. FeNO measurement is easily performed, even in preschool-age children. Measurement of nasal FeNO is useful for the diagnosis of primary ciliary dyskinesia, but FeNO is not a reliable tool for assessing inflammation in cystic fibrosis (CF). The measurement of FeNO and its role in the diagnosis and management of asthma are discussed in greater detail separately. (See "Exhaled nitric oxide analysis and applications".)
EVALUATION OF PULMONARY FUNCTION IN EARLY CHILDHOOD AND INFANCY — Spirometry is usually difficult to obtain in children younger than six years of age. However, in one study, technically acceptable and reproducible spirometry was performed by experienced pediatric pulmonary function technicians in more than 80 percent of 307 children between the ages of three and six years [26]. Technical standards and reference data differ from those used for older children and adults; age-specific standards must be used [27]. In this young age group, forced expiratory volume in one second (FEV1) is often more than 95 percent of predicted. An alternative index to follow that is more sensitive is the forced expiratory volume at 0.5 seconds (FEV0.5). The advent of interactive software for incentive spirometry may facilitate performance of spirometry in preschool children [28].
Alternative measurements that require less patient cooperation have been developed for use in young children because of the difficulties in obtaining reliable forced expiratory maneuvers in this age group [29,30]. Of these, respiratory system resistance (Rrs) is the most commonly evaluated and can be assessed by whole-body plethysmography, the interrupter technique (R[int]) [27], or the forced oscillation technique [27,31-33]. None of these measurements are widely available. The uses and limitations of techniques are reviewed in a statement by the American Thoracic Society (ATS) [27].
Respiratory morbidity is frequent in children younger than two years, making evaluation of pulmonary function all the more important in these children. Significant advances in the assessment of spirometry and plethysmography in this age group have been made since the 1990s [34]. These techniques, however, are beyond the scope of this review since the need for sedation, expensive equipment, and a high level of training limits them to a few specialty centers.
PREOPERATIVE EVALUATION — Lung function testing, including maximal respiratory pressures and lung volumes in patients who have known restrictive lung disease and including spirometry in patients with chronic obstructive lung diseases, such as cystic fibrosis (CF), can help anticipate which patients may have a more difficult time with extubation and/or require prolonged ventilatory assistance. (See "Evaluation of perioperative pulmonary risk".)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Asthma in children" and "Society guideline links: Pulmonary function testing".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topic (see "Patient education: Breathing tests (The Basics)")
SUMMARY
●Pulmonary function tests (PFTs) can measure obstructive, restrictive, and diffusion defects and respiratory muscle function. Measurements of flow and volume are most useful in the office setting. Spirometry, which provides both these measures, requires neither sophisticated technology nor expensive equipment, is easily interpreted, and is reliable when performed correctly. (See 'Introduction' above.)
●The important parameters derived from spirometry include indices of flow, including forced expiratory volume in one second (FEV1); flow between 25 and 75 percent of the vital capacity (FEF25-75%), also known as the maximal midexpiratory flow rate (MMEFR); and peak expiratory flow rate (PEFR) (table 1). Indices of volume are also measured, including forced vital capacity (FVC). (See 'Measured parameters' above.)
●FEV1, FEF25-75%, and PEFR are decreased in obstructive disorders. FVC typically is decreased in restrictive disorders and in obstructive diseases where air trapping is substantial. An FEV1/FVC ratio >85 percent suggests that the observed decrease in flow is due to volume reduction rather than airway obstruction and requires direct lung volume assessment for confirmation. (See 'Interpretation' above.)
●Spirometry can be used to support a diagnosis of asthma by demonstrating reversible airflow obstruction. Spirometry is also helpful in monitoring the response to long-term therapy and changes in the degree of obstruction over time. PEFR, as measured by a peak flow meter, may also be used to gauge the severity of asthma exacerbations, provided that caretakers and providers understand the limitations of peak flow meters (table 2). (See 'Use in asthma' above.)
●Lung volume measurement is typically undertaken in specialized centers. The measurement of lung volumes is important in clinical conditions in which a restrictive lung defect and/or air trapping may be present. It is also important when addressing possible diffusion capacity defects. There are two conventional methods of measuring lung volumes in children: whole-body plethysmography and gas dilution. Both methods measure the functional residual capacity (FRC), which is the residual air in the lung at the end of exhalation during tidal breathing (figure 1). This value is unobtainable by spirometry. Restrictive lung defects are defined by reduction in functional lung volumes and can only be confirmed by such measurements. In these conditions, the total lung capacity (TLC) is reduced to below 80 percent of that predicted by age, height, and gender. (See 'Lung volume measurement' above.)
●Additional types of PFTs that are typically performed at specialized centers include measurement of diffusing capacity of the lungs for carbon monoxide (DLCO), respiratory muscle pressure, and fractional exhaled nitric oxide (FeNO). (See 'Diffusion capacity measurement' above and 'Respiratory muscle pressure measurements' above and 'Exhaled nitric oxide' above.)
●Alternative measurements that require less patient cooperation have been developed for use in young children because of the difficulties in obtaining reliable forced expiratory maneuvers in this age group. Of these, respiratory system resistance (Rrs) is the most commonly evaluated and can be assessed by whole-body plethysmography, the interrupter technique (R[int]), or the forced oscillation technique. (See 'Evaluation of pulmonary function in early childhood and infancy' above.)
