Pulmonary function tests are an important method for evaluating and managing respiratory disease in children. New, low- resistance, lowvolume equipment has opened the door for the evaluation of young children. Pulmonary function tests may be quite useful in all children with suspected pulmonary disease, and in particular for children with known cardiac disease prior to operative procedures; in children who have developed symptoms that may be unrelated to, or unexplained by their cardiac disease, and children with chest wall deformities.
SPECIAL CONSIDERATIONS AND INDICATIONS FOR TESTING CHILDREN
There are special considerations for conducting and interpreting pulmonary function studies in children. Since accurate assessment is dependent on the child's ability and willingness to cooperate, attempts should he made to minimize the child's anxiety and maximize his attention span. When possible, children should not be evaluated in an adult -oriented laboratory where noise, equipment, and interruptions can distract the child's attention. Children should be introduced to the laboratory in a reassuring and informative fashion by a technician who has experience in working with young children. The testing equipment should be demonstrated to the child before a test is attempted. Nose clips and mouthpieces may be placed on the child's finger to demonstrate that they are not painful. Since the child must sustain a forced expiratory effort, it is helpful to blow against the child's hand, and have the child blow against the technician's hand to demonstrate both the force and duration necessary for a successful test. The need for patience and innovation in testing young children cannot be overemphasized. These procedures can make the difference between a valid and an invalid test.
Since a child's lungs are growing and developing, the interpretation of results must be based on guidelines for the child's age, size, sex, and race. Hsu et al1 have demonstrated clear racial and ethnic differences in ventilatory function in normal children. Therefore, normal predicted values for children are based on the child's age, sex, height, and race. All of these factors must be taken into account when choosing reference data for the interpretation of test results.
There are several indications for performing pulmonary function tests in children:
1 . To investigate symptoms: cough, dyspnea, exercise intolerance.
2. To define the nature and determine the magnitude of physiologic impairment. Pulmonary function tests are not diagnostic of a specific disorder, but can characterize the nature of the functional disorder (restrictive, obstruction, mixed defect) and demonstrate reversibility. They can therefore be extremely useful in making a specific diagnosis.
3. To follow the course of pulmonary disease. In many instances these tests provide more sensitive and quantitative information about changes in lung function than physical examination or chest xray.2
4. To evaluate the response to therapy. Pulmonary function tests are particularly useful in determining the effectiveness of bronchodilator therapy in obstructive airway disease.
5. To assist in preoperative evaluation. Pulmonary function tests can provide useful and important information about the likelihood of postoperative respiratory complications. Froese3 has demonstrated the usefulness of the inspiratory capacity in predicting the ability of patients to maintain adequate ventilation postoperatively. An inspiratory capacity of 15 mL/kg is necessary to sustain ventilation. In thoracic or abdominal surgery, pain may reduce the inspiratory capacity by 50%. Therefore, an inspiratory capacity of at least 30 mL/kg is necessary to ensure adequacy of ventilation postoperatively. Pulmonary function rests should be performed in all patients with neuromuscular weakness and in all patients with known respiratory disease prior to abdominal or thoracic surgery.
PULMONARY FUNCTION TESTS
Pulmonary function tests measure several broad categories of lung function: lung volume, airway function, and gas exchange.
Lung volume, when the lung is fully inflated, is the total lung capacity. This measurement gives the best estimate of lung size and is the most universal index of a restrictive defect.4 Lung volumes may be reduced due to intrinsic pulmonary parenchymal disease, abnormalities of the chest wall, or pleural or neuromuscular weakness.
There are two techniques for measuring lung volume. The helium dilution method measures only those parts of the lung which communicate with the tracheobronchial tree. Lung cysts or poorly ventilated areas may not be measured by this method, and an artificially low total lung capacity (TLC) may be observed. Thoracic gas volume is determined by body plethysmography. This method measures both communicating and noncommunicating portions of the lung and is the more accurate of the two methods.
Airway obstruction is commonly determined by the measurement of a maximal forced expiration. Peak expiratory flow rate is the simplest of these measurements. Peak expiratory flow measures muscle strength and effort as well as lung function, and so may be a relatively insensitive measurement of lung function. The best way to use this test is to compare the child's measurements on a longitudinal basis.
The other more sensitive measurements are derived from forced expiratory maneuvers. The child must be able to maintain a forced expiratory effort for at least six seconds to conduct an accurate test. It is helpful to have the child practice this technique before the actual test is performed. The young child may be asked to pretend to blow out all the candles on a birthday cake in a single breath. The best of three tests is recorded.
After 25% of the vital capacity has been expired, the maximal flow that can be achieved at any lung volume is dependent on the state of the airways and the elastic recoil of the lungs, and is unaffected by patient effort. This method, therefore, provides more accurate measurements of airway function than does the peak expiratory flow rate. The results are expressed as expiratory flow rate versus time (the spirogram) or as expiratory flow rate as a function of lung volume (the expiratory flow-volume loop).
Legend for Table and Figures
PATTERNS IN OBSTRUCTIVE AND RESTRICTIVE DEFECTS
A decrease in the volume of air expired in the first second (FEV1) correlates well with the clinical severity of most types of lung disease.4 In obstructive airway disease, the FEV1 is reduced because of airway obstruction and decreased flow rates. The FEV/forced vital capacity (FVC) ratio is also decreased. In restrictive defects, airway function is normal, and most of the small vital capacity is expired in the first second. The FEV1 may be reduced or normal. The ratio of FEV1/ FVC is normal or increased.
The maximal midexpiratory flow rate (MMEF)25,75 is the average flow over the middle 50% of the forced vital capacity. It is a more sensitive indicator of mild airway obstruction than is the FEV1. The different patterns in obstructive and restrictive defects are shown in the Table.
Gas exchange is determined by the measurement of arterial blood gases. The arterial oxygen tension (PAO2) is the single most sensitive test of lung function abnormalities in children.5 The lower limit of normal PAO2 is 85 mmHg. The PAO2 may be decreased in some children with lung disease in the absence of symptoms or abnormalities on standard pulmonary function testing.6 It is a sensitive index of small airway involvement.
Arterial puncture can be painful. The procedure itself may cause hyperventilation which will affect baseline results. It is critical that a local anesthetic agent such as 1% lidocaine be used prior to performing an arterial puncture.
The following examples have been selected to illustrate the usefulness of pulmonary function tests in children with cardiac disease.
Case 1: Evaluation of Lung Function in Children with Known Skeletal Abnormalities
A 13-year-old girl with pulmonic stenosis and scoliosis was evaluated prior to surgery for scoliosis. She had no respiratory symptoms.
Her total lung capacity was reduced consistent with a restrictive deficit. Her flow rates decreased in proportion to the decrease in lung volumes. The ratio increased as would be anticipated in pure restrictive disease. There was no evidence of abnormalities in airway function. Her inspiratory capacity exceeded 30 mL/kg.
This patient tolerated scoliosis surgery well and was discharged after an uneventful hospital course. Studies allowed the physicians to plan her pre- and postoperative course (Figure 1).
Case 2: Symptoms Unrelated to or Unexplained by Primary Disease
Case 2 involves a 15-year-old girl with a history of a ventricular septal defect, which closed spontaneously at 4 years of age. She had been in her usual state of good health when, after an otherwise mild upper respiratory tract infection, she developed symptoms of chest tightness with exercise. Prior to the onset of these symptoms she had been able to participate fully in sports, but now was unable to do so because of chest tightness and feelings of dyspnea. Her cardiologist could not find any cardiac reason for her symptoms, her chest x-ray was negative, and she was referred for pulmonary function testing.
Pulmonary function tests, done at rest, revealed normal lung volumes, normal flow rates, and a flow volume loop. Exercise testing was then performed (Figure 2A).
The postexercise study revealed significant airway obstruction with a 51% decrease in FEV1 and a 69% decrease in FEF25^75 (forced expiratory flow). These striking decreases in flow rates can also be seen in the flow volume loop. The obstruction was reversible with inhaled bronchodilators.
The patient was then placed on inhaled cromolyn sodium four times a day and reevaluated 8 weeks later. Inhaled cromolyn is highly effective in managing exercise-induced bronchospasm. Her baseline spirometry was again normal, as was her postexercise study (Figure 2B).
In this instance, pulmonary function testing provided data that led to a specific diagnosis, that of exercise- induced bronchospasm. The patient had neither cough nor wheezing. In addition, it allowed the physician to monitor the patient's response to a specific therapeutic modality and provided objective data which correlated with the patient's symptomatic improvement.
Case 3: Pulmonary Function Testing in a Child with Neuromuscular Weakness
Case 3 involves a 15-year-old female with myasthenia gravis who was scheduled for a thytnectomy. She had been on pyridostigmine, an orally active cholinesterase inhibitor, and pulmonary function testing was performed prior to surgery. Initial studies were done prior to receiving a scheduled dose of pyridostigmine. A follow-up study was done 50 minutes after receiving medications. This young woman's flow rates and lung volumes were normal and remained unchanged after medication was given. Her inspiratory capacity of 44 mL/kg was adequate to sustain postoperative ventilation. She tolerated the thymectomy well and was discharged after an uneventful hospital course (Figure 3).
Figure 1. Preoperative pulmonary function test of 13-year-old girl with pulmonic stenosis and scoliosis.
Figure 2A. Pre- and postexercise pulmonary function tests of a 15-year-oki girt with history of ventricufar septai detect
Figure 2B. Pre- and postexercise pulmonary function test results of a 15-year-old girl following inhaled cromolyn sodium therapy.
These cases illustrate the clinical importance of pulmonary function testing in children.
The first case demonstrated a significant restrictive defect in a child with pulmonic stenosis and scoliosis. Although this child had no respiratory symptoms, it was important to document the degree of functional impairment to plan for her postoperative course.
The second case demonstrated the way in which pulmonary function tests can lead to a specific diagnosis and provide important information about response to therapy. This patient had distressing symptoms which limited her ability to participate in sports. Her chest x-ray revealed no significant abnormality and she had no auscultatory finding. Pulmonary function tests defined both the nature and severity of her problems and provided objective information about her response to therapy.
Figure 3. Pulmonary function testing in a child with neufomuscufar weakness.
In the third case, one might have anticipated decreases in flow rates and lung volumes because of neuromuscular weakness. That was not evident on testing and the patient had an uneventful postoperative recovery.
Pulmonary function testing provides invaluable information about the nature and severity of functional impairment in children with known or suspected pulmonary disease. New equipment and individuals specifically trained to work with children now permit the evaluation of even young children. Pulmonary function testing can expand our knowledge of disease processes and improve our therapeutic strategies.
1. Hsu KH. JenltinsDE, HsI BP. et al· VeniiUiory functions in normal children and young adults: Mexican Americans, whiles and blacks. I. Spirometry. ) fVtuoir 1979; 95:13.
2. Wilson A (ed): Pulmón^ FwuxwTcsü^Ind&iiíoní and Interjmtatiani. Florida. Gruñe & Stratum, 1985.
3. Froese AB; Pre -operati ve evaluation of pulmonary function. Pediatr Clin North Am 1979; 26:3.
4. McBride JT, Wohl ME: Pulmonary function test». ftAotr CIm North Am 1979; 26:3.
5. West JB: Pulmonary fítthofhysÍology - The Essentials. Baltimore, Williams &WiUcins Co. 1978.
6. Lamarre A. Reilly J. Btyan AC, et al: : Early detection of pulmonary function abnormalities in cystic fibrosis. ftditBrics 1972; 50:291.
Legend for Table and Figures
PATTERNS IN OBSTRUCTIVE AND RESTRICTIVE DEFECTS