Pulmonary Mechanics

The major function of the lungs is to exchange oxygen (O₂) and carbon dioxide (CO₂) between the atmosphere and the blood. The anatomy of the airways, mechanics of the respiratory muscles and rib cage, nature of the alveolar-capillary interface, pulmonary circulation, tissue metabolism, and neuromuscular control of ventilation all influence gas exchange.

Air enters the lungs when the pressure in the thorax is less than that of the surrounding atmosphere. During inspiration, negative intrathoracic pressure is generated by contraction and lowering of the diaphragm. The accessory muscles of inspiration (external intercostal, scalene, and sternocleidomastoid muscles) are not used during quiet breathing but are recruited during exercise or in disease states to raise and enlarge the rib cage. Exhalation is normally passive but, with active exhalation, the abdominal and internal intercostal muscles are recruited.

Airway resistance is influenced by the diameter and length of the conducting airways, the viscosity of gas, and the nature of the airflow. During quiet breathing, airflow in the smaller airways may be laminar (streamlined), and resistance is then inversely proportional to the fourth power of the radius of the airway. At higher flow rates, turbulent flow, especially in the larger airways, increases resistance. Relatively small changes in airway diameter can result in large changes in airway resistance. When mechanical forces acting on the lung are at equilibrium (at the end of a normal relaxed breath), the volume of gas in the lung is termed the functional residual capacity (FRC) (Fig. 133-1). This gas volume maintains exchange of O₂ during exhalation. Lung compliance (change in volume for a given change in pressure) is a measure of the ease with which the lung can be inflated. Processes that decrease lung compliance (surfactant deficiency, pulmonary fibrosis, pulmonary edema) may lead to decreases in FRC. Conversely, FRC may be increased in obstructive lung diseases (asthma and cystic fibrosis) secondary to gas trapping in the lungs.

FIGURE 133-1 Lung volumes and capacities. Vital capacity and its subdivisions can be measured by spirometry, but calculation of residual volume requires measurement of functional residual capacity by body plethysmography, helium dilution, or nitrogen washout.

(From Andreoli TE, Bennett JC, Carpenter CJ, et al [eds]: Cecil Essentials of Medicine, 4th ed. Philadelphia, WB Saunders, 1997, p 127.)

During normal tidal breathing, lung volumes are usually in the mid-range of inflation (see Fig. 133-1). Residual volume (RV) is the volume of gas left in the lungs at the end of a maximal exhalation, and total lung capacity (TLC) is the volume of gas in the lungs at the end of maximal inhalation. Vital capacity (VC) is the maximal amount of air that can be expelled from the lungs and is the difference between TLC and RV.

Alveolar ventilation is defined as the exchange of carbon dioxide between the alveoli and external environment. Normally, about 30% of each tidal breath fills the conducting (non-gas-exchanging) airways (anatomic dead space). Because the anatomic dead space is relatively constant, increasing the tidal volume may increase the efficiency of ventilation. Conversely, if tidal volume decreases, then the dead space/tidal volume ratio increases, and alveolar ventilation decreases.

Excessive airway secretions, bronchospasm, mucosal edema and inflammation, airway stenosis, foreign bodies, loss of airway wall integrity (as with bronchiectasis), and airway compression may all produce symptomatic airway obstruction. Asthma and bronchiolitis are common causes of airway obstruction. Restrictive lung disease is less common and is characterized by normal to low FRC and RV, low TLC and VC, decreased lung compliance, and relatively normal flow rates. Restrictive lung disease can result from neuromuscular weakness, an alveolar filling process (lobar pneumonia, pulmonary edema), pleural disease (pleural effusion, inflammation, or mass), thoracic narrowing/stiffness (scoliosis, severe pectus excavatum), and abdominal distention.

Respiratory Gas Exchange

Gas exchange depends upon alveolar ventilation, pulmonary capillary blood flow, and the diffusion of gases across the alveolar-capillary membrane. Exchange of CO₂ is determined by alveolar ventilation, while the exchange of O₂ is influenced primarily by the regional matching of ventilation (V) with pulmonary blood flow (Q) (V/Q matching). V/Q matching is maintained, in part, by hypoxic pulmonary vasoconstriction (local constriction of the pulmonary vessels in areas that are hypoventilated). There are five causes of hypoxemia (Table 133-1). Disorders resulting in V/Q mismatching are the most common causes of hypoxemia.

TABLE 133-1 Causes of Hypoxemia

Lung Defense Mechanisms

The lungs are constantly exposed to particles and infectious agents. The nose is the primary filter for large particles. The ciliated epithelium of the paranasal sinuses and nasal turbinates propagate filtered particles toward the pharynx. Particles less than 10 μm in diameter may reach the trachea and bronchi and deposit on the mucosa. Particles less than 1 μm may reach the alveoli. Ciliated cells lining the airways from the larynx to the bronchioles continuously propel a thin layer of mucus toward the mouth. Alveolar macrophages and polymorphonuclear cells engulf particles and pathogens that have been opsonized by locally secreted IgA antibodies or transudated serum antibodies.

Cough, important in protecting the lungs, is a forceful expiration that can clear the airways of debris and secretions. Cough may be voluntary or generated by reflex irritation of the nose, sinus, pharynx, larynx, trachea, bronchi, and bronchioles. The loss of the ability to cough results in poor secretion clearance and predisposes to atelectasis and pneumonia.

Imaging Techniques

Chest radiographs are useful in assessing respiratory disease in children. In addition to determining lung abnormalities, they provide information about the bony thorax (rib or vertebral abnormalities), the heart (cardiomegaly, pericardial effusion), and the great vessels (right aortic arch/vascular rings, rib notching). Chest radiographs should be obtained in both the posteroanterior (PA) and lateral projections. Estimation of lung hyperinflation based on a single PA view is unreliable, whereas flattened diaphragms and an increased AP diameter on a lateral projection indicates hyperinflation. Expiratory views and fluoroscopy may detect partial bronchial obstruction due to an aspirated foreign body resulting in regional hyperinflation because the lung or lobe does not deflate on exhalation. Routine chest radiographs should be obtained after a full inspiration. Crowding of the blood vessels with poor inspiration can be misinterpreted as increased markings or infiltrates. External skin folds, rotation, and motion may produce distorted or unclear images.

A barium esophagogram is valuable in diagnosing disorders of swallowing (dysphagia) and esophageal motility, vascular rings (esophageal compression), tracheoesophageal fistulas, and, to a lesser extent, gastroesophageal reflux. When evaluating for a tracheoesophageal fistula, contrast material must be instilled under pressure via a catheter with the distal tip situated in the esophagus (see Chapter 128).

A computed tomography (CT) scan of the chest is the imaging test of choice for evaluating pleural masses, bronchiectasis, and mediastinal lesions as well as delineating pleural from parenchymal lesions. CT scans with intravenous contrast material provide excellent information about the pulmonary and great vessel vasculature and pulmonary embolism. High-resolution CT scans are used to assess lung parenchyma (pulmonary fibrosis, interstitial fluid) and the airways (bronchiectasis). The speed of current CT scanners makes it possible to scan most children without sedating them. However, sedation may be required to decrease motion artifact. Magnetic resonance imaging (MRI), useful in visualizing cardiac anatomy, is less useful for evaluation of pulmonary lesions.

Ultrasonography can be used to delineate some intrathoracic masses and is the imaging procedure of choice for assessing parapneumonic effusion/empyema. It is useful when assessing diaphragmatic motion.

Measures of Respiratory Gas Exchange

Measurements of oxygenation (PO₂ and O₂ saturation) and ventilation (PCO₂) are important in the management of pulmonary diseases. A properly performed arterial blood gas analysis provides information about the effectiveness of both oxygenation and ventilation. However, arterial samples are more difficult to obtain, so capillary and venous blood samples are more commonly used. The PCO₂ from a capillary sample is similar to that from arterial blood. The PCO₂ in venous samples is approximately 6 mm Hg higher than arterial PCO₂. The ratio of the serum bicarbonate concentration to PCO₂ determines the pH. Capillary or venous samples should not be used to assess oxygenation.

There are both respiratory and metabolic causes of acidosis (see Chapter 37). In the presence of an alkalosis or acidosis, respiratory compensation (decreasing PCO₂ to maintain a normal pH) can occur within minutes, but renal compensation (increasing the serum bicarbonate level) may not be complete for several days.

Pulse oximetry measures the O₂ saturation of hemoglobin by measuring the blood absorption of two or more wavelengths of light. It is noninvasive, easy to use, and reliable. Because of the shape of the oxyhemoglobin dissociation curve, O₂ saturation does not decrease much until the PO₂ reaches approximately 60 mm Hg. Pulse oximetry may not accurately reflect true O₂ saturation when abnormal hemoglobin is present (carboxyhemoglobin, methemoglobin), when perfusion is poor, or if no light passes through to the photodetector (nail polish).

The measurement of PCO₂ is accomplished most reliably by blood gas analysis. However, there are noninvasive monitors that record end-expiratory PCO₂ (end-tidal CO₂), which is representative of alveolar PCO₂. End-tidal PCO₂ measurements are most commonly used in intubated and mechanically ventilated patients, but some devices can measure PCO₂ at the nares. Transcutaneous electrodes can be used to monitor PCO₂ and PO₂ at the skin surface, but are not particularly accurate. Noninvasive techniques of CO₂ measurement are best suited for detecting trends rather than for providing absolute values.

Pulmonary Function Testing

Measurement of lung volumes and airflow rates using spirometry are important in assessing pulmonary disease. The patient inhales to TLC and then forcibly exhales until no more air can be expelled. This test is often referred to as spirometry. During the forced expiratory maneuver the VC, the forced expired volume in the first second (FEV₁), and forced expiratory flow (FEF) rates are measured. The predicted values for lung functions are based on patient age, gender, and race, but rely mostly on height. Most children above 6 years of age can perform spirometry. It is possible to perform these tests in infants using sedation and sophisticated equipment.

Airway resistance, FRC, and RV cannot be measured with spirometry and require other techniques. Body plethysmography can be used to measure airway resistance and lung volumes. Helium dilution can measure TLC and RV by determining the magnitude of dilution of inhaled helium in the air within the lung.

Abnormal results on pulmonary function testing can be categorized as indicating obstructive airway disease (low flow rates and increased RV or FRC) or a restrictive defect (low VC and TLC, with relative preservation of flow rates and FRC). When the FEV₁ and flow rates are decreased to a greater extent than the VC, then airway obstruction is likely, but a proportional decrease in VC, FEV₁, and flow rates suggests a restrictive lung defect. The mean midexpiratory flow rate (FEF_25–75%) is a more sensitive measure of small airways disease than the FEV₁, but is also more variable. The VC, FEV₁, and FEF_25–75% can be obtained with a simple spirometer. Pulmonary function testing can detect reversible airway obstruction characteristic of asthma with a significant improvement in FEV₁ (>12%) or in FEF_25–75% (>25%) following inhalation of a bronchodilator. Spirometry is also useful for longitudinal patient management. The peak expiratory flow rate (PEFR) can be obtained with a simple handheld device and may be useful for home monitoring of older children with asthma. However, it is highly dependent on patient effort and values must be interpreted with caution. Inhalation challenge tests using methacholine, histamine, or cold, dry air are used to assess airway hyperreactivity, but require sophisticated equipment and special expertise and should be performed only in a pulmonary function laboratory with experienced technicians.

Endoscopic Evaluation of the Airways

Endoscopic evaluation of the upper airways (nasopharyngoscopy) is performed with a flexible fiberoptic nasopharyngocope to assess adenoid size, patency of the nasal passages, and abnormalities of the larynx. It is especially useful in evaluating stridor and assessing vocal motion/function. Endoscopic evaluation of the intrathoracic airways can be done with either a flexible or rigid bronchoscope. Bronchoscopy is useful in identifying airway abnormalities (stenosis, malacia, endobronchial lesions, excessive secretions) and in obtaining airway samples for culture (bronchoalveolar lavage), especially in immunocompromised patients. Rigid bronchoscopy is the method of choice for removing foreign bodies from the airways and performing other interventions, and flexible bronchoscopy is most useful as a diagnostic tool and for obtaining lower airway cultures. Bronchoscopy requires deep sedation, but flexible nasopharyngoscopy can be performed without anesthesia. There are few absolute contraindications to bronchoscopy. Relative contraindications include bleeding diatheses, thrombocytopenia (<50,000/cm³), and clinical conditions too unstable to tolerate the procedure.

Examination of Sputum

Sputum specimens may be useful in evaluating lower respiratory tract infections, but they are difficult to obtain in young children. In addition, an expectorated specimen may not provide a representative sample of lower airway secretions. Specimens containing large numbers of squamous epithelial cells are either not from the lower airways or are heavily contaminated with upper airway secretions and may yield misleading results. Sputum in patients with lower respiratory tract bacterial infections often contains polymorphonucleated leukocytes and one predominant organism on culture. If sputum cannot be obtained, then bronchoalveolar lavage specimens (obtained by bronchoscopy) may be used for microbiologic diagnosis in selected situations. In patients with CF who cannot produce sputum, specially processed throat cultures are often used as surrogates for lower airway cultures.

Lung Biopsy

When less invasive methods fail to provide diagnoses in patients with pulmonary disease, a lung biopsy may be required. Although transbronchial lung biopsy through a bronchoscope is useful in adults, it is seldom done in children. Either a thoracoscopic procedure or a thoracotomy is preferred in children. Thoracotomy allows the surgeon to inspect and palpate the lung, which aids in choosing the best site for biopsy, but it is more invasive than thoracoscopy. In most cases, infants and children tolerate lung biopsy well.

Oxygen Administration

Any child in respiratory distress should be treated with supplemental O₂ to maintain normal O₂ saturation levels. A nasal cannula is the easiest way to provide supplemental O₂, but the oxygen concentration provided is variable and affected by the child’s own respiratory pattern. Supplemental O₂ may also be delivered by a variety of facemask systems ranging from a simple facemask, which can provide 30 to 40% O₂, to a nonrebreather mask with reservoir that can provide nearly 100% O₂. For long-term administration of O₂, a nasal cannula is the most widely used device, as it enables patients to eat and speak unhindered by the O₂ delivery system.

The concentration of administered O₂ should be high enough to relieve hypoxemia. Inspired O₂ concentrations less than 40% are usually safe for long-term use. Patients requiring supplemental O₂ should be monitored with pulse oximetry, either intermittently or continuously, or with arterial blood gas measurements of PO₂ to allow titration to the lowest possible O₂ concentration.

The acceptable O₂ saturation depends on the patient and clinical situation. Generally, supplemental O₂ should be administered to achieve a goal saturation level above 90%. Normal oxygen saturation is greater than 95%. It is unnecessary to achieve 100% saturation, especially if this requires potentially toxic levels of inspired O₂ for extended periods of time.

Aerosol Therapy

Delivering therapeutic agents to the lower respiratory tract can be accomplished by inhalation of aerosol forms of the agents via dry powder inhalers (DPIs), metered dose inhalers (MDIs), or nebulizers. All of these devices are designed to generate relatively small particles that can bypass the filtering action of the upper airway and deposit in the lower airways. Many factors influence drug deposition including patient technique, the device used, age of the child (cooperation, inspiratory flow, and tidal volume), and breathing pattern. Plastic holding chambers (spacers) should be used with MDIs. Dry powder inhalers require a single rapid deep inhalation for optimal drug delivery, which is difficult for children under 6 years of age. MDIs and nebulizers can be used in children of all ages and are equally effective in delivering medications. The drugs most often delivered as aerosols are bronchodilators (albuterol, levalbuterol, ipratropium) and inhaled corticosteroids. Occasionally antibiotics (tobramycin) are delivered by aerosol.

Chest Physiotherapy and Clearance Techniques

When disease processes impair clearance of pulmonary secretions, airway clearance techniques may help maintain airway patency. One method is chest percussion, which moves secretions toward the central airways, from which they can be expectorated. Chest physiotherapy can also be performed effectively with the flutter valve, acapella device, and pneumatic vests. Chest physiotherapy is most beneficial in children with chronic airway secretions, especially those with CF. Children who are too weak to generate an effective cough benefit from the use of a mechanical cough assist device, used in conjunction with chest physiotherapy. Chest physiotherapy is not generally beneficial for patients with asthma or pneumonia, and its effectiveness in patients with atelectasis has not been clearly established.

Intubation

If the upper airway is obstructed or mechanical ventilation is needed, it may be necessary to provide the patient with an artificial airway. This is best done by placing an endotracheal tube via the mouth or nose into the trachea (intubation). Intubation alters the physiology of the respiratory tract in many ways, not all of which are beneficial. It interferes with the humidification, warming, and filtration of inspired air and prevents phonation. Intubation also stimulates secretion production. However, intubation with an endotracheal tube can be lifesaving.

Endotracheal tubes can damage the larynx and the airways if the tubes are of improper size and are not carefully maintained. The cricoid ring is the narrowest segment of a child’s airway and is completely surrounded by cartilage, which makes it vulnerable to damage, leading to subglottic stenosis. If the pressure created by the tube against the airway mucosa exceeds capillary filling pressure (roughly 35 cm H₂O), mucosal ischemia develops, leading to necrosis. Therefore, a small air leak should be maintained around the endotracheal tube to minimize the risk of mucosal damage.

Artificial airways must be kept clear of secretions as mucous plugs in artificial airways can be fatal. Providing adequate humidification of inspired air and appropriate suctioning of the tube reduce the probability of occlusion by secretions. In addition to endotracheal tubes, the laryngeal mask can provide an artificial airway. This device consists of a tube with a soft mask at the distal end. The mask is placed over the larynx, creating a seal and allowing mechanical ventilation without the trachea being instrumented.

Tracheostomy

Tracheostomy is the surgical placement of an artificial airway into the trachea below the larynx. If prolonged intubation is anticipated, elective tracheostomy may be used to prevent laryngeal trauma, obviate the danger of accidental extubation, increase patient comfort, and facilitate nursing care. No clear guidelines are available as to how long patients can be intubated without sustaining airway damage or when a tracheostomy is indicated.

Children with severe chronic upper airway obstruction or those requiring long-term mechanical ventilation may benefit from tracheostomies. Because the tracheostomy tube hampers the ability to phonate and communicate, the child must be monitored carefully at all times. As with endotracheal tubes, tracheostomy tubes must be kept clear. Occlusion of the tube with secretions or accidental dislodgment of the tube can be fatal. Many children with tracheostomy tubes can be cared for at home provided the caregivers are well trained and adequately equipped.

Mechanical Ventilation

Patients who are unable to maintain adequate gas exchange may require mechanical ventilation. Most modes of mechanical ventilation involve inflation of the lungs with gas using positive-pressure ventilators. The inspiratory phase is active (air is pushed in) and exhalation is passive.

Positive-pressure ventilation often requires endotracheal intubation or tracheostomy, though it can be provided noninvasively via nasal or full facemasks. Noninvasive ventilation is particularly useful in patients with neuromuscular disease, but it can also be used to assist ventilation in patients in acute respiratory failure from a variety of causes.

No method of mechanical ventilation truly simulates natural breathing. All methods have their drawbacks and complications. Positive pressure is transmitted to the entire thorax and may impede venous return to the heart during inspiration. The airways and lung parenchyma may be damaged by inflation pressures and high concentrations of inspired O₂. In general, inflation pressures should be limited to those necessary to provide sufficient lung expansion for adequate ventilation and the prevention of atelectasis. Pressure-cycled and volume-cycled ventilators (conventional ventilation) are the most widely used modalities in pediatrics, but high-frequency jet ventilation and high-frequency oscillation ventilators may be used in patients with severe lung disease who are failing conventional mechanical ventilation.