Chapter 366 Diagnostic Approach to Respiratory Disease
A careful history and physical examination are the critical components in determining a diagnosis in a child presenting with respiratory signs and symptoms. In some patients, additional diagnostic tests and modalities are required.
History
Beyond the narrative provided by the parent and in older patients, the history should include questions about respiratory symptoms (dyspnea, cough, pain, wheezing, snoring, apnea, cyanosis), chronicity, timing during day or night, and associations with activities such as exercise or food intake. The respiratory system interacts with a number of other systems, and questions related to cardiac, gastrointestinal, central nervous, hematologic, and immune systems may be relevant. Questions related to gastrointestinal reflux, congenital abnormalities (airway anomalies, ciliary dyskinesia), or immune status may be important in a patient with repeated pneumonia. The family history is essential and should include inquiries about siblings and other close relatives with similar symptoms or any chronic disease with respiratory components.
Physical Examination
Respiratory dysfunction usually produces detectable alterations in the pattern of breathing. Values for normal respiratory rates are presented in Table 62-1 and depend on many factors, most importantly, age. Repeated respiratory rate measurements are necessary because respiratory rates, especially in the young, are exquisitely sensitive to extraneous stimuli. Sleeping respiratory rates are more reproducible in infants than those obtained during feeding or activity. These rates vary among infants but average 40-50 breaths/min in the 1st few weeks of life and usually <60 breaths/min in the 1st few days of life.
Respiratory control abnormalities can cause the child to breathe at a low rate or periodically. Mechanical abnormalities produce compensatory changes that are generally directed at altering minute ventilation to maintain alveolar ventilation. Decreases in lung compliance require increases in muscular force and breathing rate, leading to variable increases in chest wall retractions and nasal flaring. The respiratory excursions of children with restrictive disease are shallow. An expiratory grunt is common as the child attempts to raise the functional residual capacity (FRC) by closing the glottis at the end of expiration. Children with obstructive disease might take slower, deeper breaths (Chapter 365). When the obstruction is extrathoracic (from the nose to the mid-trachea), inspiration is more prolonged than expiration, and an inspiratory stridor can usually be heard. When the obstruction is intrathoracic, expiration is more prolonged than inspiration, and the patient often has to make use of accessory expiratory muscles. Intrathoracic obstruction results in air trapping and, therefore, a larger residual volume and, perhaps, greater functional residual capacity.
Lung percussion has limited value in small infants because it cannot discriminate between noises originating from tissues that are close to each other. In adolescents and adults, percussion is usually dull in restrictive lung disease, with a pleural effusion, pneumonia, and atelectasis, but it is tympanitic in obstructive disease (asthma, pneumothorax).
Auscultation confirms the presence of inspiratory or expiratory prolongation and provides information about the symmetry and quality of air movement. In addition, it often detects abnormal or adventitious sounds such as stridor (a predominant inspiratory monophonic noise), crackles (or rales) (high-pitched, interrupted sounds found during inspiration and more rarely during early expiration, which denote opening of previously closed air spaces), or wheezes (musical, continuous sounds usually caused by the development of turbulent flow in narrow airways) (Table 366-1). Digital clubbing is a sign of chronic hypoxia and chronic lung disease (Fig. 366-1) but may be due to nonpulmonary etiologies (Table 366-2).
Figure 366-1 Finger clubbing can be measured in different ways. The ratio of the distal phalangeal diameter (DPD) over the interphalangeal diameter (IPD), or the phalangeal depth ratio, is <1 in normal subjects but increases to >1 with finger clubbing. The DPD/IPD can be measured with calipers or, more accurately, with finger casts. The hyponychial angle can be measured from lateral projections of the finger contour on a magnifying screen and is usually <180 degrees in normal subjects but >195 degrees in patients with finger clubbing. For bedside clinical assessment, the Schamroth sign is useful. The dorsal surfaces of the terminal phalanges of similar fingers are placed together. With clubbing, the normal diamond-shaped aperture or “window” at the bases of the nail beds disappears, and a prominent distal angle forms between the ends of the nails. In normal subjects, this angle is minimal or nonexistent.
(From Chernick V, Boat TF: Kendig’s disorders of the respiratory tract in children, ed 6, Philadelphia, 1998, WB Saunders.)
Table 366-1 LUNG SOUND NOMENCLATURE
| TYPE | SOUND |
|---|---|
| DISCONTINUOUS | |
| Fine (high pitch, low amplitude, short duration) | Fine crackles/rales |
| Coarse (low pitch, high amplitude, long duration) | Coarse crackles |
| CONTINUOUS | |
| High pitch | Wheezes |
| Low pitch | Rhonchi |
From Cugell DW: Lung sound nomenclature, Am Rev Respir Dis 136:1016, 1987, with permission; from Chernick V, Boat TF: Kendig’s disorders of the respiratory tract in children, ed 6, Philadelphia, 1998, WB Saunders, p 97.
Blood Gas Analysis
An arterial blood gas analysis is probably the single most useful rapid test of pulmonary function. Although this analysis does not specify the cause of the condition or the specific nature of the disease process, it can give an overall assessment of the functional state of the respiratory system and clues about the pathogenesis of the disease. Because the detection of cyanosis is influenced by skin color, perfusion, and blood hemoglobin concentration, the clinical detection by inspection is an unreliable sign of hypoxemia. Arterial hypertension, tachycardia, and diaphoresis are late, and not exclusive, signs of hypoventilation.
Blood gas exchange is evaluated most accurately by the direct measurement of arterial PO2, PCO2, and pH (Chapters 95.3 and 365). The blood specimen is best collected anaerobically in a heparinized syringe containing only enough heparin solution to displace the air from the syringe. The syringe should be sealed, placed in ice, and analyzed immediately. Although these measurements have no substitute in many conditions, they require arterial puncture and have been replaced to a great extent by noninvasive monitoring, such as capillary samples and/or oxygen saturation.
The age and clinical condition of the patient need to be taken into account when interpreting blood gas tensions. With the exception of neonates, values of arterial PO2 <85 mm Hg are usually abnormal for a child breathing room air at sea level. Calculation of the alveolar-arterial oxygen gradient is useful in the analysis of arterial oxygenation, particularly when the patient is not breathing room air or in the presence of hypercarbia. Values of arterial PCO2 >45 mm Hg usually indicate hypoventilation or a severe ventilation-perfusion mismatch, unless they reflect respiratory compensation for metabolic alkalosis (Chapter 52).
Transillumination of the Chest
In infants up to at least 6 months of age, a pneumothorax can often be diagnosed by transilluminating the chest wall using a fiberoptic light probe. Free air in the pleural space often results in an unusually large halo of light in the skin surrounding the probe. Comparison with the contralateral chest is often very helpful in interpreting findings. This test is unreliable in older patients and in those with subcutaneous emphysema or atelectasis.
Radiographic Techniques
Chest X-Rays
A posteroanterior and a lateral view (upright and in full inspiration) should be obtained except in situations in which the child is medically unstable. Portable films, although useful in the latter situation, can give a somewhat distorted image. Expiratory films can be misinterpreted, although a comparison of expiratory and inspiratory films may be useful in evaluating a child with suspected foreign body (localized failure of the lung to empty reflects bronchial obstruction). If pleural fluid is suspected, decubitus films are indicated. Films taken in a recumbent position are difficult to interpret if there is fluid within the pleural space or a cavity.
Upper Airway Film
A lateral view of the neck can yield invaluable information about upper airway obstruction and particularly about the condition of the retropharyngeal, supraglottic, and subglottic spaces (which should also be viewed in an anteroposterior projection). Knowing the phase of respiration during which the film was taken is often essential for accurate interpretation. Magnified airway films are often helpful in delineating the upper airways. Patients with suggested obstruction should not be unattended in the radiology department.
Sinus and Nasal Films
The general utility of roentgenographic examination of the sinuses is uncertain because of the large number of films with positive findings (low sensitivity and specificity, Table 366-3). Imaging studies are not necessary to confirm the diagnosis of sinusitis in children <6 yr. CT scans are indicated if surgery is required, in cases of complications due to sinus infection, in immunodeficient patients, and for recurrent infections that are not responsive to medical management.
Chest Computed Tomography and Magnetic Resonance Imaging
Technical advances have greatly enhanced the utility of chest CT and MRI (three-dimensional reconstruction is often feasible) and have potentially decreased radiation exposure. CT scans are of particular importance in the evaluation of mediastinal and pleural lesions, solid or cystic parenchymal lesions, pulmonary embolisms, and bronchiectasis. Intravenous contrast material can be infused during the scan to enhance vascular structures, thereby allowing distinction of vessels from other soft tissue densities. MRI is an excellent procedure to delineate hilar and vascular anatomy associated with vascular rings or slings. MRI can be useful for mediastinal, but not parenchymal, structures and lesions.
Fluoroscopy
Fluoroscopy is especially useful for evaluating stridor and abnormal movement of the diaphragm or mediastinum. Many procedures, such as needle aspiration or biopsy of a peripheral lesion, are also best accomplished with the aid of fluoroscopy, CT, or ultrasonography. Videotape recording, which does not increase radiation exposure, can allow detailed study through replay capability during a brief exposure to fluoroscopy.
Barium Swallow
A barium swallow study, performed with fluoroscopy and spot films, is indicated in the evaluation of patients with recurrent pneumonia, persistent cough of undetermined cause, stridor, or persistent wheezing. The technique can be modified by using barium of different textures and thicknesses, ranging from thin liquid to solids, to evaluate swallowing mechanics, the presence of vascular rings, and tracheoesophageal fistulas, especially when aspiration is suspected. A contrast esophagram has been used in evaluating newborns with suggested esophageal atresia, but this procedure entails a high risk of pulmonary aspiration and is not usually recommended. Barium swallows are useful in evaluating suggested gastroesophageal reflux, but because of the high incidence of asymptomatic reflux in infants, the applicability of the findings to the clinical problem may be complicated.
Pulmonary Arteriography and Aortograms
Pulmonary arteriography has been used to allow detailed evaluation of the pulmonary vasculature; has been helpful in assessing pulmonary blood flow and in diagnosing congenital anomalies, such as lobar agenesis, unilateral hyperlucent lung, vascular rings, and arteriovenous malformations; and it is sometimes useful in evaluating solid or cystic lesions. Thoracic aortograms demonstrate the aortic arch, its major vessels, and the systemic (bronchial) pulmonary circulation. They are useful in evaluating vascular rings and suspected pulmonary sequestration. Although most hemoptysis is from the bronchial arteries, bronchial arteriography is seldom helpful in diagnosing or treating intrapulmonary bleeding in children. Real-time and Doppler echocardiography and thoracic CT with contrast are noninvasive methods that often reveal similar information and should be considered before arteriography is performed.
Radionuclide Lung Scans
The usual scan uses intravenous injection of material (macroaggregated human serum albumin labeled with 99mTc) that will be trapped in the pulmonary capillary bed. The distribution of radioactivity, proportional to pulmonary capillary blood flow, is useful in evaluating pulmonary embolism and congenital cardiovascular and pulmonary defects. Acute changes in the distribution of pulmonary perfusion can reflect alterations of pulmonary ventilation.
The distribution of pulmonary ventilation can also be determined by scanning after the patient inhales a radioactive gas such as xenon-133. After the intravenous injection of xenon-133 dissolved in saline, pulmonary perfusion and ventilation can be evaluated by continuous recording of the rate of appearance and disappearance of the xenon over the lung. Appearance of xenon early after injection is a measure of perfusion, and the rate of washout during breathing is a measure of ventilation in the pediatric population. The most important indication for this test is to demonstrate defects in the pulmonary arterial distribution that can occur with congenital malformations or pulmonary embolism. Spiral reconstruction CT with contrast medium enhancement is very helpful in evaluating pulmonary thrombi and emboli. Abnormalities in regional ventilation are also easily demonstrable in congenital lobar emphysema, cystic fibrosis, and asthma.
Pulmonary Function Testing
The measurement of respiratory function in infants and young children can be difficult because of the lack of cooperation. Attempts have been made to overcome this limitation by creating standard tests that do not require the patient’s active participation. Respiratory function tests still provide only a partial insight into the mechanisms of respiratory disease at early ages.
Whether restrictive or obstructive, most forms of respiratory disease cause alterations in lung volume and its subdivisions (Chapter 365). Restrictive diseases typically decrease total lung capacity (TLC). TLC includes residual volume, which is not accessible to direct determinations. It must therefore be measured indirectly by gas dilution methods or, preferably, by plethysmography. Restrictive disease also decreases vital capacity (VC). Obstructive diseases produce gas trapping and thus increase residual volume and FRC, particularly when these measurements are considered with respect to TLC.
Airway obstruction is most commonly evaluated from determinations of gas flow in the course of a forced expiratory maneuver. The peak expiratory flow is reduced in advanced obstructive disease. The wide availability of simple devices that perform this measurement at the bedside makes it useful for assessing children who have airway obstruction. Evaluation of peak flows requires a voluntary effort, and peak flows may not be altered when the obstruction is moderate or mild. Other gas flow measurements require that the child inhale to TLC and then exhale as far and as fast as possible for several seconds. Cooperation and good muscle strength are therefore necessary for the measurements to be reproducible. The forced expiratory volume in 1 sec (FEV1) correlates well with the severity of obstructive diseases. The maximal midexpiratory flow rate, the average flow during the middle 50% of the forced vital capacity (FVC), is a more reliable indicator of mild airway obstruction. Its sensitivity to changes in residual volume and vital capacity, however, limits its use in children with more severe disease. The construction of flow-volume relationships during the FVC maneuvers overcomes some of these limitations by expressing the expiratory flows as a function of lung volume (Chapter 365).
A spirometer is used to measure VC and its subdivisions and expiratory (or inspiratory) flow rates (see Fig. 365-1). A simple manometer can measure the maximal inspiratory and expiratory force a subject generates, normally at least 30 cm H2O, which is useful in evaluating the neuromuscular component of ventilation. Expected normal values for VC, FRC, TLC, and residual volume are obtained from prediction equations based on body height.
Flow rates measured by spirometry usually include the FEV1 and the maximal midexpiratory flow rate. More information results from a maximal expiratory flow-volume curve, in which expiratory flow rate is plotted against expired lung volume (expressed in terms of either VC or TLC). Flow rates at lung volumes <~75% VC are relatively independent of effort. Expiratory flow rates at low lung volumes (<50% VC) are influenced much more by small airways than are flow rates at high lung volumes (FEV1). The flow rate at 25% VC (V25) is a useful index of small airway function. Low flow rates at high lung volumes associated with normal flow at low lung volumes suggest upper airway obstruction (Chapter 365).
Airway resistance (RAW) is measured in a plethysmograph, or, alternatively, the reciprocal of RAW, airway conductance (GAW), may be used. Because airway resistance measurements vary with the lung volume at which they are taken, it is convenient to use specific airway resistance, SRAW (SRAW = RAW/lung volume), which is nearly constant in subjects >6 yr old (normally <7 sec/cm H2O).
The diffusing capacity for carbon monoxide (DLCO) is related to oxygen diffusion and is measured by rebreathing from a container having a known initial concentration of carbon monoxide or by using a single-breath technique. Decreases in DLCO reflect decreases in effective alveolar capillary surface area or decreases in diffusibility of the gas across the alveolar-capillary membrane. Primary diffusion abnormalities are unusual in children; therefore, this test is most commonly employed in children with rheumatologic or autoimmune diseases and in children exposed to toxic drugs to the lungs (e.g., oncology patients) or chest wall radiation. Regional gas exchange can be conveniently estimated with the perfusion-ventilation xenon scan. Determining arterial blood gas levels also discloses the effectiveness of alveolar gas exchange.
Pulmonary function testing, although rarely resulting in a diagnosis, is helpful in defining the type of process (obstruction, restriction) and the degree of functional impairment, in following the course and treatment of disease, and in estimating the prognosis. It is also useful in preoperative evaluation and in confirmation of functional impairment in patients having subjective complaints but a normal physical examination. In most patients with obstructive disease, a repeat test after administering a bronchodilator is warranted.
Most tests require some cooperation and understanding by the patient, and interpretation is greatly facilitated if the test conditions and the patient’s behavior during the test are known. Infants and young children who cannot or will not cooperate with test procedures can be studied in a limited number of ways, which often require sedation. Flow rates and pressures during tidal breathing, with or without transient interruption of the flow, may be useful to assess some aspects of airway resistance or obstruction and to measure compliance of the lungs and thorax. Expiratory flow rates can be studied in sedated infants with passive compression of the chest and abdomen with a rapidly inflatable jacket. Gas dilution or plethysmographic methods can also be used in sedated infants to measure FRC and RAW.
Microbiology: Examination of Lung Secretions
The specific diagnosis of infection in the lower respiratory tract depends on the proper handling of an adequate specimen obtained in an appropriate fashion. Nasopharyngeal or throat cultures are often used but might not correlate with cultures obtained by more-direct techniques from the lower airways. Sputum specimens are preferred and are often obtained from patients who do not expectorate by deep throat swab immediately after coughing or by saline nebulization. Specimens can also be obtained directly from the tracheobronchial tree by nasotracheal aspiration (usually heavily contaminated), by transtracheal aspiration through the cricothyroid membrane (useful in adults and adolescents but hazardous in children), and in infants and children by a sterile catheter inserted into the trachea either during direct laryngoscopy or through a freshly inserted endotracheal tube. A specimen can also be obtained at bronchoscopy. A percutaneous lung tap or an open biopsy is the only way to obtain a specimen absolutely free of oral flora.
A specimen obtained by direct expectoration is usually assumed to be of tracheobronchial origin, but often, especially in children, it is not from this source. The presence of alveolar macrophages (large mononuclear cells) is the hallmark of tracheobronchial secretions. Nasopharyngeal and tracheobronchial secretions can contain ciliated epithelial cells, which are more commonly found in sputum. Nasopharyngeal and oral secretions often contain large numbers of squamous epithelial cells. Sputum can contain both ciliated and squamous epithelial cells.
During sleep, mucociliary transport continually brings tracheobronchial secretions to the pharynx, where they are swallowed. An early-morning fasting gastric aspirate often contains material from the tracheobronchial tract that is suitable for culture for acid-fast bacilli.
The absence of polymorphonuclear leukocytes in a Wright-stained smear of sputum or bronchoalveolar lavage (BAL) fluid containing adequate numbers of macrophages may be significant evidence against a bacterial infectious process in the lower respiratory tract, assuming that the patient has normal neutrophil counts and function. Eosinophils suggest allergic disease. Iron stains can reveal hemosiderin granules within macrophages, suggesting pulmonary hemosiderosis. Specimens should also be examined by Gram stain. Bacteria within or near macrophages and neutrophils can be significant. Viral pneumonia may be accompanied by intranuclear or cytoplasmic inclusion bodies visible on Wright-stained smears, and fungal forms may be identifiable on Gram or silver stains.
Exercise Testing
Exercise testing (Chapter 417.5) is a more-direct approach for detecting diffusion impairment as well as other forms of respiratory disease. Exercise is a strong provocateur of bronchospasm in susceptible patients, so exercise testing can be useful in the diagnosis of patients with asthma that is only apparent with activity. Measurements of heart and respiratory rate, minute ventilation, oxygen consumption, carbon dioxide production, and arterial blood gases during incremental exercise loads often provide invaluable information about the functional nature of the disease. Often a simple assessment of the patient’s exercise tolerance in conjunction with other, more static forms of respiratory function testing can allow a distinction between respiratory and nonrespiratory disease in children.
Sleep Studies
The sleep state has an important influence on respiratory function at all ages. Polysomnographic studies are often helpful for diagnosing airway obstruction during sleep, when abnormalities of central respiratory control, muscular disorders, or respiratory complications from gastroesophageal reflux (GER) are suspected. Polysomnography is now considered the gold standard test for obstructive sleep apnea or hypoventilation during sleep. pH probe studies are indicated and are added to such sleep studies when GER is suspected. In these studies, a pH probe is placed in the esophagus and prolonged (usually over several hours) monitoring is undertaken (Chapter 315). These studies, which usually include the simultaneous assessment of ventilatory effort, airway gas flow, gas exchange, and sleep state, are also useful in the diagnosis and management of airway and respiratory control disorders and nocturnal hypoxemia and hypercapnia in children with chronic respiratory disease (Chapter 365).
Airway Visualization and Lung Specimen–Based Diagnostic Tests
Laryngoscopy
The evaluation of stridor, problems with vocalization, and other upper airway abnormalities usually requires direct inspection. Although indirect (mirror) laryngoscopy may be reasonable in older children and adults, it is rarely feasible in infants and small children. Direct laryngoscopy may be performed with either a rigid or a flexible instrument. The safe use of the rigid scope for examining the upper airway requires topical anesthesia and either sedation or general anesthesia, whereas the flexible laryngoscope can often be used in the office setting with or without sedation. Further advantages to the flexible scope include the ability to assess the airway without the distortion that may be introduced by the use of the rigid scope and the ability to assess airway dynamics more accurately. Because there is a relatively high incidence of concomitant lesions in the upper and lower airways, it is often prudent to examine the airways above and below the glottis, even when the primary indication is in the upper airway (stridor).
Bronchoscopy and Broncheoalveolar Lavage
Bronchoscopy is the inspection of the airways. BAL is a method used to obtain a representative specimen of fluid and secretions from the lower respiratory tract, which is useful for the cytologic and microbiologic diagnosis of lung diseases, especially in those who are unable to expectorate sputum. BAL is performed after the general inspection of the airways and before tissue sampling with a brush or biopsy forceps. BAL is accomplished by gently wedging the scope into a lobar, segmental, or subsegmental bronchus and sequentially instilling and withdrawing sterile nonbacteriostatic saline in a volume sufficient to ensure that some of the aspirated fluid contains material that originated from the alveolar space. Nonbronchoscopic BAL can be performed, although with less accuracy and, therefore, less-reliable results, in intubated patients by instilling and withdrawing saline through a catheter passed though the artificial airway and blindly wedged into a distal airway. In either case, the presence of alveolar macrophages documents that an alveolar sample has been obtained. Because the methods used to perform BAL involve passage of the equipment through the upper airway, there is a risk of contamination of the specimen by upper airway secretions. Careful cytologic examination and quantitative microbiologic cultures are important for correct interpretation of the data. BAL can often obviate the need for more-invasive procedures such as open lung biopsy, especially in immunocompromised patients.
Indications for diagnostic bronchoscopy and BAL include recurrent or persistent pneumonia or atelectasis, unexplained or localized and persistent wheeze, the suspected presence of a foreign body, hemoptysis, suspected congenital anomalies, mass lesions, interstitial disease, and pneumonia in the immunocompromised host. Indications for therapeutic bronchoscopy and BAL include bronchial obstruction by mass lesions, foreign bodies or mucus plugs, and general bronchial toilet and bronchopulmonary lavage. The patient undergoing bronchoscopy ventilates around the flexible scope, whereas with the rigid scope, ventilation is accomplished through the scope. Rigid bronchoscopy is preferentially indicated for extracting foreign bodies, for removing tissue masses, and in patients with massive hemoptysis. In other cases, the flexible scope offers the advantages that it can be passed through endotracheal or tracheostomy tubes, can be introduced into bronchi that come off the airway at acute angles, and can be safely and effectively inserted with topical anesthesia and conscious sedation.
Regardless of the instrument used, the procedure performed, or its indications, the most common complications are related to sedation. The relatively more common complications related to the bronchoscopy itself include transient hypoxemia, laryngospasm, bronchospasm, and cardiac arrhythmias. Iatrogenic infection, bleeding, pneumothorax, and pneumomediastinum are rare but reported complications of bronchoscopy or BAL. Bronchoscopy in the setting of possible pulmonary abscess or hemoptysis must be undertaken with advance preparations for definitive airway control, mindful of the possibility that pus or blood might flood the airway. Subglottic edema is a more common complication of rigid bronchoscopy than of flexible procedures, in which the scopes are smaller and less likely to traumatize the mucosa. Postbronchoscopy croup is treated with oxygen, mist, vasoconstrictor aerosols, and corticosteroids as necessary.
Thoracoscopy
The pleural cavity can be examined through a thoracoscope, which is similar to a rigid bronchoscope. The thoracoscope is inserted through an intercostal space and the lung is partially deflated, thus allowing the operator to view the surface of the lung, the pleural surface of the mediastinum and diaphragm, and the parietal pleura. Multiple thoracoscopic instruments can be inserted, allowing endoscopic biopsy of the lung or pleura, resection of blebs, abrasian of the pleura, and ligation of vascular rings.
Thoracentesis
For diagnostic or therapeutic purposes, fluid can be removed from the pleural space by needle. Generally, as much fluid as possible should be withdrawn, and an upright chest roentgenogram should be obtained after the procedure. Complications of thoracentesis include infection, pneumothorax, and bleeding. Thoracentesis on the right may be complicated by puncture or laceration of the capsule of the liver and, on the left, by puncture or laceration of the capsule of the spleen. Specimens obtained should always be cultured, examined microscopically for evidence of bacterial infection, and evaluated for total protein and total differential cell counts. Lactic acid dehydrogenase, glucose, cholesterol, triglyceride (chylous), and amylase determinations may also be useful. If malignancy is suspected, cytologic examination is imperative.
Transudates result from mechanical factors influencing the rate of formation or reabsorption of pleural fluid and generally require no further diagnostic evaluation. Exudates result from inflammation or other disease of the pleural surface and underlying lung and require a more complete diagnostic evaluation. In general, transudates have a total protein of <3 g/dL or a ratio of pleural protein to serum protein <0.5, a total leukocyte count of fewer than 2,000/mm3 with a predominance of mononuclear cells, and low lactate dehydrogenase levels. Exudates have high protein levels and a predominance of polymorphonuclear cells (although malignant or tuberculous effusions can have a higher percentage of mononuclear cells). Complicated exudates often require continuous chest tube drainage and have a pH <7.2. Tuberculous effusions can have low glucose and high cholesterol content.
Lung Tap
Using a technique similar to that used for thoracentesis, a percutaneous lung tap is the most direct method of obtaining bacteriologic specimens from the pulmonary parenchyma and is the only technique other than open lung biopsy not associated with at least some risk of contamination by oral flora. After local anesthesia, a needle attached to a syringe containing nonbacteriostatic sterile saline is inserted using aseptic technique through the inferior aspect of an intercostal space in the area of interest. The needle is rapidly advanced into the lung; the saline is injected and reaspirated, and the needle is withdrawn. These actions are performed as quickly as possible. This procedure usually yields a few drops of fluid from the lung, which should be cultured and examined microscopically.
Major indications for a lung tap are infiltrates of undetermined cause, especially those unresponsive to therapy in immunosuppressed patients who are susceptible to unusual organisms. Complications are the same as for thoracentesis, but the incidence of pneumothorax is higher and somewhat dependent on the nature of the underlying disease process. In patients with poor pulmonary compliance, such as children with Pneumocystis pneumonia, the rate can approach 30%, with 5% requiring chest tubes. Bronchopulmonary lavage has replaced lung taps for most purposes.
Lung Biopsy
Lung biopsy may be the only way to establish a diagnosis, especially in protracted, noninfectious disease. In infants and small children, thoracoscopic or open surgical biopsies are the procedures of choice, and in expert hands, there is low morbidity. Biopsy through the 3.5 mm diameter pediatric bronchoscopes limits the sample size and diagnostic abilities. As well as ensuring that an adequate specimen is obtained, the surgeon can inspect the lung surface and choose the site of biopsy. In older children, transbronchial biopsies can be performed using flexible forceps through a bronchoscope, an endotracheal tube, a rigid bronchoscope, or an endotracheal tube, usually with fluoroscopic guidance. This technique is most appropriately used when the disease is diffuse, as in the case of Pneumocystis pneumonia, or after rejection of a transplanted lung. The diagnostic limitations related to the small size of the biopsy specimens can be mitigated by the ability to obtain several samples. The risk of pneumothorax related to bronchoscopy is increased when transbronchial biopsies are part of the procedure; however, the ability to obtain biopsy specimens in a procedure performed with topical anesthesia and conscious sedation offers advantages to the select population for whom this procedure offers a reasonable diagnostic yield.
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