Indian Pediatrics 2003; 40:626-632 

Spirometry in Clinical Practice

D. Vijayasekaran
L. Subramanyam
A. Balachandran
So. Shivbalan

Correspondence to: D. Vijayasekaran, No 110/3, New Street, Mannady, Chennai 600 001, India.

In the past few decades, there is an increase in the incidence of respiratory illnesses including asthma throughout the world. This could be due to several factors like environmental pollution, passive smoking, modernization, increased awareness and early diagnosis of these conditions. Correct diagnosis is important for the proper management of respiratory ailments. Among the various investigation modalities available, pulmonary function tests (PFT) are an invaluable tool for the assessment of lung function. PFT serve as screening test, help to assess the respiratory sufficiency and provide a baseline data for future comparison. This will classify the diseases of the lungs into restrictive or obstructive types, confirm the clinical diagnosis, provide an objective and reproducible method to evaluate the disease and the response to therapy on followup(1,2). Hence, PFT for lungs is comparable to that of ECG for heart. However, PFT is not widely used in our country despite the above advantages. Therefore, physicians who are interested in the field of Pulmonology should have thorough knowledge and adequate practical training in PFT.

Pulmonary Functions

The lung maintains adequate oxygenation and removal of carbon dioxide from blood through ventilation, gas exchange and perfusion. Several tests are available to assess the adequacy of the above three processes. Ventilation can be assessed by spirometry, peak flow meter, helium dilution method and body plethysmography. Perfusion can be assessed by radioisotope technetium scan, cardiac catheterization, and pulmonary arteriography. Gas exchanges are measured by arterial blood gas, carbon monoxide diffusion capacity and 131xenon scan.

In majority of lung diseases (both obstructive and restrictive) ventilation is predominantly affected. Plethysmography and gas dilution techniques determine static lung volumes. Plethysmography is an ideal method to assess lung volume, but is costly and time consuming(3). Spirometry measures the combination of lung volumes and provides adequate information about the physiological derangement of the lung. Hence, in clinical practice spirometry is the investigation of choice for the overall assessment of pulmonary function and is equated with PFT in day to day practice. The knowledge about lung volumes and capacities will help in interpreting various lung diseases.

Lung volumes and capacities

As the knowledge about the conduction of electrical impulses is important in the interpretation of ECG, the knowledge about the lung volumes and capacities (combination of lung volumes) is important to understand the functioning of lung (Fig 1). The two opposite forces namely the "outer pulling force of thoracic cage" and the "inner recoiling force of the lungs", keep the lung distended at a volume called "functional residual capacity". Tidal breathing denotes the volume of air moving in or out of lung during normal respiration that is about 500 mL in adults and 5-6 mL/kg in infants. When respiratory muscles act additional reserve and additional volumes are accomplished (Table I). As diseases alter these lung volumes, measurement of it is helpful to diagnose pulmonary pathology.

Fig. 1. Lung volumes and capacities are shown by block diagram (left), spirographic tracing is depicted on the right.

Table I

Lung Volumes and Capacities

Tidal Volume (VT) - The volume of air either inspired or expired during tidal respiration.

Inspiratory Reserve Volume (IRV) -- The volume of air inspired by forcible inspiration after tidal inspiration.

Expiratory Reserve Volume (ERV) - The volume of air expired by forcible expiration after tidal expiration.

Residual Volume (RV) - The volume of gas remaining within the lungs at the end of maximal expiration.

Total Lung Capacity (TLC) - Gas contained within the lungs after a maximal inspiration

Inspiratory Capacity (IC) -- The volume of air inspired by deep inspiration

Forced Vital Capacity (FVC) -- Denotes the quantity of air expired forcefully after maximal inspiration

Functional Residual Capacity (FRC) - The volume of gas remaining in the lungs at the end of tidal exhalation

 

Spirometer

Spirometers are instruments to conduct spirometry tests. Earlier, spirometers were of either rolling model or bellows type but the recent ones are electronic and fully automated using digital turbine and flow sensors that require no calibration. Though spirometry does not measure the individual lung volumes, it measures the forced vital capacity (FVC), which is a combination of tidal volume (TV) expiratory reserve volume (ERV) and inspiratory reserve volume (IRV). The other indices like forced expiratory volume in one second (FEV1), the ratio of FEV1 with FVC (FEV1/FVC), forced expiratory flow 25% to 75% of forced vital capacity (FEF 25%-75%) are measured from FVC. The Spirometer cannot measure functional residual capacity (FRC) or total lung capacity (TLC) but these parameters are not important in routine assess-ment of common lung problems. Baseline spirometric values depend on various factors like race, sex, age, etc. The standing height is a satisfactory predictor of lung function(4).

Spirometry Test

Children above 5 to 6 years of age can produce an acceptable FVC curve with adequate coaching. The environment for testing should be child friendly. Before attempting spirometry, it is important to make the child familiar with the laboratory, instruments and persons. The mouthpiece is given 1-2 days in advance for the child to practice at home, so that he/she will be comfortable and confident to perform the procedure that will definitely help in obtaining better results. The child’s torso and head should be erect during the procedure. Application of nose clips may yield better results. To achieve a good FVC the child should take a slow breath to full inhalation followed by a brief hold and then a sustained exhalation with maximum effort without coughing or quitting during the procedure. The child should be coached and encouraged during expiration to achieve a complete forced vital capacity i.e., "blowing" as long as possible for at least 3 seconds. FVC manoeuvre in spirometry is like that of balloon blowing and this example will make understanding of the technique easy.

The same person, preferably a doctor should perform and interpret spirometry. An ideal spirometry should include at least two reproducible curves with a difference of less than 5% and the best accepted curve is the one that has the largest sum of FEV1 and FVC(5). The child is allowed more than three attempts to achieve the above. The spirometer usually computes the largest value of FEV1 and FVC even if these values are from two different curves(6).

Forced vital capacity is the difference between total lung capacity and residual volume and is generated by maximum expiration after maximum inspiration. Normally it is reached within 3 to 4 seconds. The most important aspect of spirometry is to produce a good forced vital capacity curve for the specified period without quitting or coughing. During the FVC maneuver the expiratory volume is plotted against time and it is called as the time volume curve (Fig 2). Spirometry indices are reported comparing the individual’s value along with the predicted values. In normal individuals more than 80% FVC can be achieved in the first one second. The FEV1 denotes the fraction of forced vital capacity expired during the first second. The ratio of FEV1 to FVC is usually referred as forced expiratory ratio. Forced expiratory flow 25-75% is measured from FVC curve by excluding first 25% and last 25% of expiratory flow (FEF 25-75%) and sometimes referred as maximal mid expiratory flow rate (MMFR-25-75%), this mostly evaluates the small airways.

Fig. 2. Spirographic tracing of forced expiration and its components.

FVC will be diminished in both obstructive and restrictive diseases. In the early stages of obstruction, FVC may slightly increase due to air trapping. If the child quits before the end of the FVC manoeuvre, the FVC is underestimated. Consequently the FEV1/FVC ratio may be normal resulting in a wrong interpretation as restrictive lung disease instead of obstructive disease.

FEV1 signals airway obstruction. The FEV1/FVC ratio is decreased in obstructive diseases because the rate of airflow is slowed. In restrictive lung diseases, it is normal or even higher than normal because both FVC and FEV1 are reduced proportionally. FEF 25-75% is a more sensitive indicator of small airway obstruction than FEV1. In early or mild asthmatics because of air trapping, TLC will increase but FEV1 and FEV1/FVC ratio is deceptively normal. In such conditions, the measurement of FEF 25-75% may be diagnostic.

From FVC curve several indices can be derived which may be confusing for the beginner. Concentrating on the above discussed basic spirometry indices will fetch more than 75% of relevant information. The important indices that categorize the lung disease into obstructive and restrictive types are FVC, FEV1, FEV1/FVC ratio and FEF 25-75% (Table II).

Table II

	Spirometry Indices in Respiratory Diseases.

Indices (Units)	Obstruction	Restriction	Normal
FVC (lts)	↓	↓	100 ± 20%
FEV1(lts)	↓	↓	100 ± 20%
FEV1/FVC% (Ratio)	↓	Normal	> 80%
FEF 25-75% ml/Sec	↓	↓	100 ± 35%

 

Modern Spirometry Curves

Time volume curves are conventionally used to measure forced vital capacity in all spirometers. Spirometry is a growing field and the forced vital capacity curves are also getting metamorphosed. In addition to time-volume curve, modern spirometry machine presents new plots like flow volume curve and flow volume loops (Fig 3). In a time-volume curve the time is plotted against volume, whereas if the expiratory flow is plotted against lung volume it is called as Flow volume curve which is another way of visualizing the time volume curve. In a flow volume loop the expiratory flow rate is recorded against the expired volume. In this study, the patient performs the FVC manoeuvre and at completion, he or she is requested to perform full inspiration. Though the new curves do not add any additional information, graphic illustration gives a quick assessment of the disease pattern. Since all the modern spirometers depict flow volume loops, knowledge about flow volume loop is mandatory for the interpretation of modern spirometry.

Fig. 3. Depiction of spirometry curves in different methods. Whereas in time volume and flow volume curves expiration is measured, in flow volume loop both expiration (upper half) and inspiration (lower half) are measured.

Flow Volume Loop

In flow volume loop, upper half of the curve represents expiration while the lower half inspiration, forming a loop and the direction of the loops is clockwise(7) (Fig. 4). Reduction of vertical axis (Y-axis) represents reduction of flow representing obstructive lung problems. Reduction of horizontal axis (X-axis) denotes reduction of lung volume thereby indicating restrictive lung diseases. Obstructive lung disease alters the shape of the loop while restrictive lung disease alters the size of the loop. Thus, flow volume loop provides a graphic picture to classify pulmonary disease and to locate the site of obstruction. In restrictive lung disease, all lung volumes and flows (inspiratory and expiratory) are reduced resulting in a small loop without any change in the shape (Fig. 4).

Fig. 4. Normal flow volume curve and curves in different pulmonary conditions.

Intrathoracic obstruction reduces all expiratory flows and as the flow reduction becomes more severe, there is concavity or "scoop" in the expiratory limb of loop. In distal obstruction (e.g., asthma) flows are most affected at low volumes and in proximal obstruction (e.g., tracheal pathology) flows are affected at high volumes. In extra thoracic obstruction there is flattening of the inspiratory limb of the loop (lower portion of the loop) without altering the expiratory limb.

In clinical conditions with mixed airway obstruction, the ratio of maximum expiratory flow (Vmax50%) to maximum inspiratory flow (MIF 50%) is used. In normal individual, the ratio is equal to 1. (Vmax 50%/MIF 50% = 1). In variable intra thoracic obstruction the ratio is <1 and in variable extra thoracic obstruction the ratio is >1.

Spirometry Indications

The indications of spirometry have been increasing over the years. Though etiological diagnosis is not possible it is used to assess the functional derangement in many lung diseases. Spirometry plays a significant role in the management of many chronic lung conditions like asthma, chronic obstructive airway disease (COPD), and interstitial lung disease and as a preoperative assessment before cardiopulmonary surgery.

Lung Disease

The most important indication for spirometry is to differentiate lung diseases into obstructive and restrictive for effective management. The common obstructive lung disease in children is asthma and other obstructive disorders are bronchiectasis, cystic fibrosis, chronic obstructive airway disease (COPD). Restrictive lung diseases include structural diseases of the chest wall (kyphoscoliosis), neuromuscular problems and interstitial lung diseases (ILD). Obstructive diseases are characterized by low flows with near normal volumes whereas restrictive lung diseases by small volumes and normal flows.

Asthma

The urgent issue in asthma management strategy is early diagnosis. Under diagnosis of asthma leads to under treatment that causes progressive remodeling in the airway mucosa. Recent studies have demonstrated (on the basis of broncho alveolar lavage) inflam-matory changes even in mild persistent asthma(8). Though asthma can be diagnosed on clinical grounds, poor compliance and difficulty in monitoring are the impedence in successful management. Recent studies indicate that upto 70% of patients with asthma do not comply with treatment(9). So all older children should be subjected to spirometry in the initial evaluation of the disease. In the majority, the demonstration of the objective deviation of the observed value (>20%) from the predicted value by spirometer confirms asthma. When spirometry values are normal and asthma is strongly suspected the response to bronchodilator aerosol is measured (Bronchodilator challenge test). Reversible airway obstruction characterized by a rise in the FEV1 and/or FVC by atleast 12% (from pre to post bronchodilator), is characteristic of asthma(10). Airway obstruction due to fixed anatomic obstruction may not respond.

Most of the asthmatics exhibit exercise intolerance and this is used for diagnosis of asthma (Exercise testing). Child is made to excercise for 6 minutes. Tread mill or bicycle ergometer are preferred methods of testing in children to provide exercise-induced broncho-spasm to diagnose asthma. Flow rates should be measured 3,10 and 15 minutes following exercise. A drop in FEV1 of 10% or more is taken as positive test(11). Measuring other parameters of obstruction such as FEF 25-75% of FVC and PEFR increases the sensitivity of the test.

Though spirometry forms the cornerstone of asthma diagnosis, some individuals with asthma may have near normal spirometry and may not show significant bronchodilator response. In such cases challenge testing with inhaled histamine or methacholine may help to make a diagnosis before starting empirical therapy. Increasing dose of histamine (0.06, 0.12, 1.00, 2.50 mg/mL) or methacholine (25 mg/mL) is administered and FEV1 measured before and after the test. A 20% decrease in FEV1 when compared to the baseline value is a positive response. However, in routine practice, challenge tests are rarely required. In addition to diagnosis, comparison of the curves/loops during therapy will evoke interest among parents and involve them in the partnership management of asthma. Peak flow meters and asthma diary may further increase patient compliance.

Chronic obstructive pulmonary disease (COPD)

Adolescents and older children addicted to the habit of smoking, those who are exposed to high environmental pollution or bio fuel smoke are potential candidates of COPD. Since clinical presentation of COPD is invariably cough, the majority may ignore this problem for a long period of time. Subjecting this group for early spirometry will alert them about this progressively fatal disease. In COPD unlike in asthma, bronchodilator reversibility test may be negative and FEV1 may progressively fall.

Interstitial lung disease (ILD)

Interstitial lung disease is a heterogenous group of disorders of varied etiology with common clinicoradiological presentation. Since majority present with cough and exertional dyspnea they will be treated as asthma or tuberculosis for prolonged periods. Though lung biopsy is the confirmatory test, spirometry plays a significant role in the diagnosis and monitoring the course of the diasese. Restrictive spirometry pattern is typically observed in ILD due to reduction of the static lung volumes. Reduction of FVC is greater than FEV1 resulting in normal or supernatural FEV1/FVC ratio(12).

Lung surgery

All children planned for lobectomy or pneumonectomy require spirometry tests to assess whether there will be adequate lung sufficiency after lung resection. Children contemplating for pneumonectomy should have FVC of more than 2 liters.

Preoperative work up

Spirometry plays a major role in the preoperative work up of structural anomalies of chest wall diseases. If the preoperative inspiratory capacity is less than 30 mL/kg the child may require assisted ventilation in the postoperative period.

Peak Flow Meter

Though Spirometry is a gold standard test to diagnose asthma, peak (expiratory) flow meter (PEF) can be a useful alternative to predict underlying asthma when spirometry is not available. Peak flow meter records peak expiratory flow rate (PEFR). Peak expiratory flow rate is the greatest flow obtained on forced expiration after complete inspiration using Peak flow meter. Peak flow rates are effort dependent and measure mostly large airway function. Though PEFR correlate well with FEV1, it is not a substitute for spirometry. A difference of 20% or more between morning and night values is considered a good predictor of asthma. Since early asthma can be missed by PEFR measurement, spirometry should be preferred to diagnose asthma. However, PEFR plays a major role in asthma follow up. A sudden fall of PEFR may be an early warning of impending attack of asthma(2). PEFR plays a major role to monitor asthma therapy and serial recording will reflect the prognosis of the disease as well as outcome of therapy. Further, it provides an objective assessment of lung function in those children with asthma who are unable to do a FVC procedure.

Respiratory diseases are at an upsurge globally. Early objective evidence of pulmonary disease will ensure adequate compliance and successful management. In the era of consumerism, evidence based approach is in demand and spirometers play a significant role in management strategy of many respiratory illnesses, especially asthma. Basic Knowledge about PFT will help practitioners in early identification of respiratory problem and its successful management.

Contributors: DV and LS conceived and designed the article. AB and SS drafted the manuscript. All the authors were involved in literature search and final approval.

Funding: None.

Competing interest: None stated.

 

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