LABORATORY INVESTIGATIONS IN LUNG DISEASE

Important investigations in respiratory disorders include:

• Radioimaging, e.g. X-rays, CT/MRI, USG, etc. for anatomical localization of lesion and detection of fluids, etc.

• Pulmonary function tests and lung scintigraphy, for physiological assessment.

• Endoscopic studies, e.g. bronchoscopy or thoracoscopy, for direct visualization of pathology.

• Blood gas analysis for severity of disease (Ch 27.2)

• Microbiological or serological studies for etiological diagnosis.

Radioimaging studies: The choice of radioimaging study depends on clinically suspected pathology and site of lesion.

• Chest X-rays: Plain chest X-ray in PA and lateral view, preferably in erect position, is the baseline investigation for diagnosis and localization of pulmonary lesion. In addition, A decubitus film is indicated for suspected hydropneumothorax or fluid- filled cavity.

Thorough evaluation of chest X-ray should include: (a) soft tissue/bony cage shadows, (b) position of trachea, (c) crowding/widening of intercostal spaces, (d) location, shape and size of cardiac silhouette, (e) mediastinal widening and carinal angle, i.e. bifurcation angle between two major bronchi (N: ~70-80°), (f) diaphragmatic contours (normally right is higher than left), (g) obliteration of costophrenic or cardiophrenic angles, (h) subdiaphragmatic abnormality, and of course, (i) the status of lung parenchyma with/without air bronchogram.

Parenchymal lesions may be broadly classified as: (a) diffuse or localized (peripheral, perihilar, patchy, circular), (b) hyperlucency or opacity (homogenous or non-homogenous), and (c) special characteristics, e.g. shape, air-fluid levels, etc. (Table 16.3).

TABLE 16.3: Radiological D/D of lung shadows

• Diffuse, non-homogenous opacities

- Bronchopenumonia (large, confluent)

- Miliary mottling (small, discrete, uniform)

- Interstitial pneumonia

- Pulmonary edema

• Focal, homogenous opacities

- Consolidation (no tracheal shift)

- Collapse, fibrosis (trachea on same side)

- P.

• Solitary rounded shadows (with/without air)

- Tubercular cavity, lung abscess (thick walled)

- Lung cysts, pneumatoceles (thin walled)

- Encysted pyopneumothorax

• Diffuse/localized hyperlucency

- Pneumothorax (trachea on opposite side)

- Emphysema (no tracheal shift)

• Localized multi-cystic hyperlucency

- Pneumatoceles

- Bronchiectesis

- Diaphragmatic hernia

*also see X-rays in Figs 16.11 and 10.4.

Other X-rays, e.g. paranasal sinuses (recurrent sinusitis), mastoid (chronic otitis media), neck (acute croup) are indicated in selected cases.

• Ultrasonography (USG) is very useful to detect small pleural effusions as well as during thoracocentesis. It is also emerging as point of care imaging of choice to detect and monitor pneumonia in intensive care units.

• CT thorax is most informative investigation for localization and monitoring of pulmonary, pleural and mediastinal lesions and has almost replaced need for bronchography and diagnostic bronchoscopy. It is mainly indicated in—(a) chronic lung diseases or (b) acute lung diseases with abnormal course, e.g. unresolving pneumonia or suspected abscesses, (c) cysts or other congenital malformations, and (d) for guided biopsy or aspiration of mediastinal lesions. CT sinus/mastoid is indicated in suspected sinusitis/ mastoiditis.

• MRI is of limited value in parenchymal lung lesions, but more useful in suspected vascular, mediastinal or underlying spinal/bony lesions.

• Fluoroscopy, once frequently used to detect dia­phragmatic paralysis and non-opaque foreign bodies, is rarely used now, due to risk of excessive radiation and availability of superior tests.

Pulmonary function tests (PFT) are of limited value in children due to lack of cooperation, though provide valuable insight to altered pulmonary physiology in cooperative cases.

Uses: PFT are valuable diagnostic aids to—(a) differentiate between obstructive vs. restrictive lung disease, (b) assess the severity of functional impairment, and (c) monitor the course of disease and response to therapy.

Fig.

TLC: Total lung capacity; FRC: Functional residual capacity; TV: Tidal volume; RV: Residual volume; IRV: Inspiratory reserve volume; VC: Vital capacity; ERV: Expiratory reserve volume.

Fig. 16.4: Kinetic lung volumes.

TLC: Total lung capacity; FVC: Forced vital capacity; FEV₁: Force expiratory volume in 1 second; RV: Residual volume.

Principle: PFT involves study of:

• Static lung volumes/capacities (Fig. 16.3), which are mainly altered in restrictive lung diseases, due to anatomical loss of parenchyma, reduced elastic recoil and/or restricted expansion of the lung. Functional vital capacity (FVC) is the most important static indicator of restrictive lung disorders.

• Kinetic airflow rates during different phases of breathing are mainly affected in obstructive airways diseases due to increased airway resistance. Two most important kinetic flow rates in practice include: Forced vital capacity (FVC) and Forced expiratory volume in first second of expiration (FEV₁) (Fig. 16.4).

• Flow-volume relationships for comparative assessment of parenchymal (restrictive) or airway (obstructive) pathology. These relationships are studied on graphic flow-volume curves provided directly on spirometry or by calculating the ratios like FEV₁/FVC.

Methodology: PFT are recorded by a manual or com­puterized spirometer that provides a graphic record of lung volumes and flow rates during various phases of respiration. However, spirometer cannot measure the Residual volume (RV), which needs to be assessed by gas dilution/wash-out methods.

While rarely used in practice, body plethysmography is an alternative and more accurate method to determine lung capacities (including residual volume and airway resistance), specially in uncooperative patients.

Interpretation: Primary value of PFT lies in differentiating obstructive airway diseases from restrictive lung diseases (Table 16.4).

Restrictive lung diseases include those with: (a) restric­tive chest-wall, e.g. scoliosis, neuromuscular disorders,

TABLE 16.4: PFT in obstructive vs restrictive lung diseases

FVC: Forced vital capacity, FEVj Force expiratory volume in 1 second.

etc., (b) restricted alveolar expansion, e.g. pneu­monia, pulmonary edema or high surface tension, e.g. ARDS, and (c) reduced elastic recoil, e.g. interstitial lung diseases or fibrosis. These diseases are characterized by:

• Predominant loss of lung volumes/capacities, i.e.

reduced TLC, VC, RV and FRC,

• Near-normal flow rates, e.g. FEV₁, and

• Normal or even increased FEV₁#8725;FVC.

Obstructive lung diseases due to narrowed airways and increased airway resistance during expiration, e.g. asthma, emphysema or chronic bronchitis, etc. are characterized by:

• Predominant decrease in FEV₁.

• Minimum decrease in lung volumes. FRC is even increased in asthma due to air-trapping.

• Marked reduction in FEV₁#8725;FVC.

Mid-maximal expiratory flow rate (MMEFR)—the average flow during the middle 50% of FVC, is more reliable indicator of mild airway obstruction before FEV₁ becomes abnormal, though difficult to record in children.

Asthma is further distinguished from other obstructive disorders by the reversibility, i.e. gt;15-20% increase in FEV₁ after salbutamol nebulization.

Peak flowmetry is a simple, reliable alternative to spirometry for regular monitoring of airway status in asthma, nearly comparable with FEV₁ (Ch 16.8 and 29.5).

Lung scintigraphy (radionuclide scan): Lung scinti­graphy is rarely used to assess ventilation and/or perfusion status in different parts of the lung, after administration of a radiotracer agent.

Perfusion scan involves IV administration of a radiotracer (^99mTc MHSA), which gets trapped into the pulmonary vascular bed in proportion to the regional blood flow. Subsequent radioimaging of different parts of lungs provides information regarding perfusion abnormalities.

Ventilation scan is performed after inhalation of a radioactive gas (¹³³Xe), followed by serial radioimaging to study its clearance (wash-out) from the lungs. Inadequately ventilated parts of the lungs show delayed appearance or clearance of radioactivity on serial images.

Combined ventilation-perfusion scan are obtained by IV administration of ¹³³Xe, followed by serial radio^­imaging to note the rate of appearance and disappearance of the tracer in the lungs. Appearance of radiotracer in the lung after injection is a measure of perfusion, whereas its clearance indicates ventilatory status.

Important lung scintigraphic abnormalities include:

• Predominantly perfusion abnormalities in pulmonary embolism, AV malformations and interstitial lung diseases, etc.

• Predominantly ventilatory abnormalities in foreign body, lung sequestration/hypoplasia, diaphragmatic hernia, etc.

• Combined ventilation-perfusion abnormalities in chronic obstructive airway disorders, e.g. asthma.

Diagnostic value of these lung scans is limited due to lack of specificity. However, new-generation lung scans, e.g. Gallium scan (⁶⁷gallium), positron emission tomography (PET) and single photon emission computed tomography (SPECT)-a scintigraphic analogue of CT scan, are useful in early diagnosis and monitoring of chronic, slowly progressive lung diseases.

Bronchoscopy is a very useful diagnostic as well as therapeutic procedure to: (a) visualize the trachea lumen and larger bronchi, (b) collect specimens for cytological and microbiological studies, e.g.

Indications for bronchoscopy include:

• Diagnostic

- Foreign body, congenital anomalies or mass lesions

- Persistent cough, stridor, wheeze or hemoptysis

- Recurrent/persistent pneumonia, collapse, emphysema

- Bronchoalveolar lavage and endobronchial biopsy

• Therapeutic

- Removal of foreign body or mucus plugs

- Dilatation of tracheo-bronchial strictures

- Resection of endobronchial mass lesions

- Bronchial toilet in suppurative lung diseases

- Endobronchial drug instillations

Methodology: Bronchoscopy is done under general or local anesthesia using a rigid or flexible fiberoptic bronchoscope. Rigid scope is preferred for foreign body removal, endoscopic mass resection and in presence of heavy hemoptysis, while flexible bronchoscopy is preferred in other cases due to ease of manipulation and more distal reach. Bronchoalveolar lavage (BAL) is collected by sequential instillation and withdrawal of sterile saline.

Complications of bronchoscopy include: (a) sudden reflex laryngospasm, (b) subglottic edema, (c) trauma, e.g. bleeding or pneumothorax, (d) nosocomial pneumonia, and (e) anesthesia-related complications.

Thoracocentesis is a common diagnostic/therapeutic procedure to access pleural fluid in effusions/empyema. A 16-18 g. Needle is inserted along the upper edge of lower rib in 7^th or 8^th ICS in posterior axillary line (sitting position) or under USG guidance with proper aseptic precautions and local anesthesia (Ch 32.6). Total aspirated volume in single sitting should not exceed gt;5% of body weight.

Common complications include—(a) pneumothorax, (b) infection, and (c) traumatic puncture of liver/spleen or bleeding, and rarely, (d) sinus/fistula formation (in tubercular effusion).

Percutaneous lung aspiration is rarely used, to collect parenchymal specimen for bacteriological studies in non-resolving pneumonia. After radiological localization of lesion, a needle attached to a saline-filled syringe is inserted and advanced to the site of interest gt; the saline is injected gt; re-aspirated quickly, to collect a few drops of parenchymal specimen, and gt; the needle is withdrawn. Pneumothorax is a common complication of this procedure.

Lung biopsy is rarely used in children to establish the diagnosis of chronic non-infectious lung disorders, e.g. interstitial lung disease or lymphocytic interstitial pneumonitis in HIV infected children. It may be performed via a bronchoscope, though USG-guided or thoracoscopic biopsies are preferred in young infants to ensure adequate sample size.

16.5