Physical Examination of the Respiratory System

Clinical assessment of respiratory function remains invaluable in the diagnosis and management of patients with respiratory failure despite all the technologic advances that have occurred in monitoring. Initial evaluation begins with assessment of the child’s comfort and activity level. Coexisting nonpulmonary conditions such as pain and anxiety may make this assessment difficult. Observation of the child’s body position, respiratory pattern, and body habitus provides important information as to his or her level of respiratory distress.

Often children in respiratory distress assume a body position that helps them breathe more comfortably. This position may be splinting of the chest in a patient with pneumonia or assuming the “sniffing position” to maintain an open airway in a child with an upper airway obstruction. An infant is less independent and therefore often is held in position by his or her caregivers, irrespective of comfort.

The respiratory pattern provides information regarding the work of breathing in a distressed child. The respiratory rate varies with age, but an early sign of distress is tachypnea. Additional signs of increased work of breathing include grunting or irregular respirations, nasal flaring, use of accessory muscles of respiration (strap muscles of the neck), and retractions.

Inspection of the shape of the chest wall may reveal abnormalities that affect pulmonary function. Increased anteroposterior diameter can be seen in conditions associated with hyperinflation (e.g., asthma or cystic fibrosis). Scoliosis in severe cases can cause a reduction in lung volume. Neuromuscular disorders may be associated with an “A-shaped” chest and lung hypoplasia/dysplasia. Thoracic asymmetry may be associated with neuromuscular or skeletal deformities, pneumothorax, or a paralyzed diaphragm. Fingers and toes should be examined for evidence of clubbing (painless enlargement of the connective tissues of the distal phalanges), which is nonspecific but may be indicative of chronic hypoxemia. Growth parameters and neurodevelopment should be obtained and compared with age-appropriate normal subjects to assist in evaluation of long-standing or associated diseases.

Detection of cyanosis centrally (lips, tongue) or peripherally (nail beds) may be difficult. Arterial oxygen tension must drop below 80 mm Hg before cyanosis can be detected clinically. Cyanosis may be absent in patients with severe anemia or missed when lighting is poor. It also may be intermittent and be seen only with exercise or change in position. Cyanosis that does not resolve with oxygen therapy may indicate right-to-left shunting of blood in the lungs or heart or the formation of methemoglobin or sulfhemoglobin following the ingestion of certain drugs.

Evaluation of breath sounds provides assessment of airflow through the tracheobronchial tree, the presence of fluid in or obstruction of the airways, and conditions outside the lung and pleural space. The child’s chest wall is thinner than that of the adult, which allows better access to breath sounds but impedes localization of the lesion because the breath sounds can be referred. Upper airway abnormalities may present with stridor or muffling of the voice. Bronchial breath sounds suggest consolidation, whereas wheezes result from narrowed airways. Crackles may be fine or coarse. They represent air bubbling through secretions and the reopening of closed airways. A friction rub may be heard when the inflamed surfaces of the pleura move against each other through the respiratory cycle. Breath sounds may be absent if there is a significant pleural effusion or complete lobar collapse resulting from a mucus plug or pneumothorax. Heart tones often are shifted away from the pneumothorax and toward the atelectasis because of complete airway obstruction. Assessment for pulsus paradoxus (i.e., exaggerated decrease in the pulse or systolic blood pressure with inspiration) should be made during the evaluation of severe airway obstruction or pulmonary embolus.

Although each assessment should include observation and a limited physical examination, it is recognized that interobserver repeatability of physical signs is poor and independent of the experience of the observer.² The assessments of clubbing, wheezes, friction rub, and crackles are the most reliable and reproducible.^2,³ The lack of accuracy of repeated physical examinations and the complexity of critically ill pediatric patients require adjunctive tests/assessments to monitor these patients.

Radiography

Portable chest x-ray films are the most common films taken in the PICU. The technical quality of the chest radiograph affects the interpretation; therefore, it is important to assess the film for adequacy of penetration, degree and symmetry of lung inflation, and degree of chest rotation. Chest x-ray films can be used to evaluate for cardiac, vascular, bone, and lung abnormalities, to assess for device placement (e.g., an endotracheal tube), and to determine the need for intervention.⁴ Lateral decubitus films help identify and quantify pleural effusions, pneumothorax, and the position of chest tubes/lines. Cross-table lateral films also may be used for these purposes but are harder to interpret. A chest computed tomography (CT) scan is used when details on the plain film are obscured by the superimposition of structures or an opaque hemithorax. A chest CT scan also can be used to guide drainage of fluid collections and assist in obtaining biopsy specimens. A high-resolution chest CT scan examines 1- to 1.5-mm slices at 10-mm intervals; therefore it can be used to illustrate lung parenchymal details better than a conventional CT scan, which examines 7- to 10-mm slices at 10-mm intervals.⁵ This detail can be helpful in distinguishing the pathologic process causing diffuse lung diseases that appear as diffuse lung shadowing on chest x-ray films. Chest ultrasound, ventilation/perfusion scanning, spiral CT, and magnetic resonance imaging may be useful adjutants depending on the disease process.

Evaluation of Gas Exchange

Episodic hypoxemia is common in critically ill adults and has been associated with increased mortality. The frequency and impact of episodic hypoxemia in pediatrics has not been studied. Additionally, whether monitoring and early treatment of episodic hypoxemia will improve patient outcome remains unanswered, but continuous monitoring of oxygen saturation in the PICU has become the standard of care.

Noninvasive Respiratory Monitoring

Transcutaneous Oxygen and Carbon Dioxide Monitoring

Transcutaneous measurements reflect both gas exchange and skin perfusion. In this technique, a probe composed of a heater, an electrode, and a thermistor is applied to the patient’s skin. The skin is warmed and softened to improve diffusion and permeability. This step also causes capillaries to dilate, resulting in better approximation of arterial oxygen values. Several disadvantages limit the use of transcutaneous monitoring to the newborn population. Skin thickness increases with age, making transcutaneous measurements less predictable. Frequent electrode site changes are required to prevent local burns. Relatively frequent calibration and comparison with arterial blood gases are necessary. Because of these limitations and the ease of application of pulse oximetry and end-tidal carbon dioxide monitoring, transcutaneous monitoring has nearly been replaced by other monitoring methods.

Pulse Oximetry

Pulse oximetry is considered a significant technologic advance that has improved patient safety.^1,⁶^–⁹ Its ease of application and accuracy have resulted in widespread use. Pulse oximetry is commonly used to detect hypoxemia and to wean the oxygen concentration in patients undergoing mechanical ventilation.

Pulse oximetry is based on the principles that (1) the pulsatile absorbance detected is arterial blood and (2) oxyhemoglobin and reduced hemoglobin have different absorption spectra.⁶ Red (660-nm) and infrared (940-nm) wavelengths of light are used to determine the ratio of oxygenated to deoxygenated blood. Deoxygenated blood absorbs more red light, whereas oxygenated blood absorbs more infrared light. The two wavelengths are passed through an arterial bed, and the ratio of infrared and red light transmitted to the photodetector is determined. The ratio is calibrated against measurements of arterial oxygen saturations from human volunteers and their absorbance ratios.

Several factors may affect the accuracy of pulse oximetry. Pulse oximetry measures oxygen saturation (SaO₂). SaO₂ and PaO₂ are not linearly related; the oxyhemoglobin dissociation curve is sigmoid in shape (Figure 39-1). Large changes in PaO₂ at high levels of oxygen, the upper flat portion of the oxyhemoglobin dissociation curve, may occur with little change in saturation. Additionally, a reduction in oxygenation on the steep portion of the curve may not be appreciated as significant because only a small change in saturations will have occurred.^6,⁷ The accuracy of pulse oximetry falls with arterial oxygen saturations less than 70%.^1,⁶ At arterial oxygen saturations below 70%, pulse oximetry may be more appropriate for showing trends.⁶

Figure 39–1 Hemoglobin-oxygen (Hb-O₂) dissociation curve shows the percentage saturation of hemoglobin at each PO₂. When the hemoglobin concentration is known, the content of oxygen can be calculated. The total content includes the small additional content of oxygen in solution, which becomes significant at high levels of PO₂. The saturation scale on the left applies only to the Hb-O₂ line. The scale on the right shows content values for a normal hemoglobin level of 15 g/100 mL blood.

(Modified from Hlastala MP: Blood gas transport. In Culver BH, editor: The respiratory system, Seattle, 1997, ASUW Publications. Redrawn in Albert RK, Spiro SG, Jett R, editors: Clinical respiratory medicine, ed 2, St. Louis, 2004, Mosby Elsevier.)

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