Respiratory Disease Manifesting as Respiratory Distress

Clinical examination is important in localizing the site of pathology (Chapter 365). Extrathoracic airway obstruction occurs anywhere above the thoracic inlet. Inspiratory stridor, suprasternal, chest wall, and subcostal retractions, and prolongation of inspiration are hallmarks of extrathoracic airway obstruction. By comparison, features of intrathoracic airway obstruction are prolongation of expiration and expiratory wheezing. Typical manifestations of alveolar interstitial pathology are rapid, shallow respirations, chest wall retractions, and grunting. The site of pathology can be localized and the differential diagnosis established on the basis of the clinical signs and symptoms (Tables 65-1 and 65-2).

Table 65-1 TYPICAL LOCALIZING SIGNS FOR PULMONARY PATHOLOGY

Table 65-2 EXAMPLES OF ANATOMIC SITES OF LESIONS CAUSING RESPIRATORY FAILURE

Respiratory Distress without Respiratory Disease

Although respiratory distress most commonly results from diseases of lungs, airways, and chest wall, pathology in other organ systems can manifest as “respiratory distress” and lead to misdiagnosis and inappropriate management (Table 65-3). Respiratory distress resulting from heart failure or diabetic ketoacidosis may be misdiagnosed as asthma and improperly treated with albuterol, resulting in worsened hemodynamic state or ketoacidosis.

Table 65-3 NONPULMONARY CAUSES OF RESPIRATORY DISTRESS

Pathophysiology of Respiratory Failure

Respiratory failure can be classified into 2 categories: (1) hypoxic respiratory failure (failure of oxygenation) and (2) hypercarbic respiratory failure (failure of ventilation). The two entities may coexist as a combined failure of oxygenation and ventilation. The main function of the respiratory system is to move atmospheric gases into the alveolar capillary units of the lung and to move alveolar gas back out into the atmosphere. Systemic venous (pulmonary arterial) blood is arterialized after mixing with the alveolar gas and being carried back to the heart by pulmonary veins. The arterial gas composition depends on the gas composition of the atmosphere and the effectiveness of alveolar ventilation, pulmonary capillary perfusion, and diffusion across the alveolar capillary membrane. Abnormality at any of these steps can result in respiratory failure.

Alveolar Gas Composition

When atmospheric gas enters alveoli, oxygen is exchanged for carbon dioxide. This exchange is not equimolar because the body’s average oxygen consumption is greater than carbon dioxide production, as represented by the respiratory quotient (R), which is CO₂ production ÷ O₂ consumption. Alveolar gas composition is roughly estimated by the following equation:

where PAO₂ is the expected partial pressure of oxygen in alveoli. Under normal metabolic conditions, R is assumed to be 0.8. PAO₂ is traditionally compared with PaO₂ to determine the extent of oxygenation abnormality; this comparison is referred to as the alveolar-arterial oxygen gradient (A-aO₂).

Alveolar Ventilation and Dead Space Ventilation

The amount of air breathed in a minute is termed minute volume, which is a product of tidal volume (VT) and respiratory rate. Part of the VT occupies conducting airways (anatomic dead space) and does not participate in gas exchange. Additionally, air that enters alveoli that are not perfused (e.g., from pulmonary embolism) or poorly perfused (e.g., because of decreased cardiac output) also does not contribute to gas exchange (physiologic dead space). Total dead space (VD) is a sum of anatomic and physiologic dead spaces. Alveolar ventilation (VA) therefore is calculated as follows:

At a constant level of CO₂ production, PaCO₂ is inversely proportional to alveolar ventilation. An increase in PaCO₂ from 40 torr to 80 torr is indicative of a 50% decrease in VA, and a decrease in PaCO₂ from 40 torr to 20 torr reflects doubling of VA. An elevated PaCO₂, which is indicative of alveolar hypoventilation, can occur from airway obstruction, weakness of respiratory muscles, or CNS dysfunction (hypoventilation).

Ventilation-Perfusion Mismatch, Venous Admixture, Intrapulmonary Shunt

For exchange of O₂ and CO₂ to occur, alveolar gas must be exposed to blood in pulmonary capillaries. Both ventilation and perfusion are lower in nondependent areas of the lung and higher in dependent areas of the lung. The difference in perfusion () is greater than the difference in ventilation (). Perfusion in excess of ventilation results in incomplete “arterialization” of systemic venous (pulmonary arterial) blood and is referred to as venous admixture. Perfusion of unventilated areas is referred to as intrapulmonary shunting of systemic venous blood to systemic arterial circulation. Conversely, ventilation that is in excess of perfusion is “wasted”; that is, it does not contribute to gas exchange and is referred to as dead space ventilation. Dead space ventilation results in return of greater amounts of atmospheric gas (which has not participated in gas exchange and has negligible CO₂) to the atmosphere during exhalation. The end result is a decrease in mixed expired PCO₂ (PECO₂) and an increase in the PaCO₂-PECO₂ gradient. The fraction of tidal volume that occupies dead space (VD/VT) is calculated as follows:

Normal VD/VT is around 0.33. VD/VT increases in states that result in decreased pulmonary perfusion, such as pulmonary hypertension, hypovolemia, and decreased cardiac output. Venous admixture and intrapulmonary shunting predominantly affect oxygenation, resulting in a PAO₂-PaO₂ (A-aO₂) gradient without elevation in PaCO₂. The reason is the greater ventilation of perfused areas, which is sufficient to normalize PaCO₂ but not PaO₂ because of their respective dissociation curves (Chapter 365). The relative straight-line relationship of hemoglobin-CO₂ dissociation allows for averaging of PCO₂ from hyperventilated and hypoventilated areas. Because the association between oxygen tension and hemoglobin saturation plateaus with increasing PaO₂, the decreased hemoglobin-O₂ saturation in poorly ventilated areas cannot be compensated for by well-ventilated areas where hemoglobin-O₂ saturation has already reached near-maximum. This results in decreased SaO₂ and PaO₂. Elevation of PaCO₂ in such situations is indicative of attendant alveolar hypoventilation. Examples of diseases leading to venous admixture include asthma and aspiration pneumonia, and those of intrapulmonary shunt include lobar pneumonia and acute respiratory distress syndrome.

Clinical Examination

Clinical observation is the most important component of monitoring. The presence and magnitude of abnormal clinical findings, their progression with time, and their temporal relation to therapeutic interventions serve as guides to diagnosis and management (Chapter 365). The child with respiratory distress or failure should be observed in the position of greatest comfort and in the least threatening environment.

Pulse oximetry is the most commonly utilized technique to monitor oxygenation. Noninvasive and safe, it is the standard of care in bedside monitoring of children during transport, procedural sedation, surgery, and critical illness. It indirectly measures arterial hemoglobin-O₂ saturation by differentiating oxyhemoglobin from deoxygenated hemoglobin using their respective light absorption at wavelengths of 660 nm (red) and 940 nm (infrared). A pulsatile circulation is required to enable detection of oxygenated blood entering the capillary bed. Percentage of oxyhemoglobin is reported as arterial oxyhemoglobin saturation (SaO₂); however, the correct description is oxyhemoglobin saturation as measured by pulse oximetry (SpO₂). This is because SpO₂ may not reflect SaO₂ in certain situations. It is important to be familiar with the hemoglobin-O₂ dissociation curve (Chapter 365) in order to estimate PaO₂ at a given oxyhemoglobin saturation. Because of the shape of the hemoglobin-O₂ dissociation curve, changes in PaO₂ above 70 torr are not readily identified by pulse oximetry. Also, at the same PaO₂ level, there may be a significant change in SpO₂ at a different blood pH value. In most situations, an SpO₂ value greater than 95% is a reasonable goal, especially in emergency situations. There are exceptions, such as in patients with single ventricle cardiac lesions, in whom the pulmonary and systemic circulations are receiving blood flow from the same ventricle (e.g., after Norwood procedure for hypoplastic left heart syndrome), or with large left-to-right shunts (e.g., ventriculoseptal defect [VSD] and patent ductus arteriosus). In these types of pathophysiologic situations, a lower SpO₂ is desired to avoid excessive blood flow to the lungs and pulmonary edema from the pulmonary vasodilatory effects of oxygen, and, in the patient with a single ventricle, diverting blood flow away from the systemic circulation. Because pulse oximetry recognizes all types of hemoglobin as either oxyhemoglobin or deoxygenated hemoglobin, it provides inaccurate information in the presence of carboxyhemoglobin and methemoglobin. Percentage of oxyhemoglobin is overestimated in carbon monoxide poisoning and methemoglobinemia. It should be recognized that dangerous levels of hypercarbia may exist in patients with ventilatory failure, who have satisfactory SpO₂ if they are receiving supplemental oxygen. Pulse oximetry should not be the only monitoring method in patients with primary ventilatory failure, such as neuromuscular weakness and CNS depression. It is also unreliable in patients with poor perfusion and poor pulsatile flow to the extremities. Despite these limitations, pulse oximetry is a noninvasive, easily applicable, and effective means of evaluating the percentage of oxyhemoglobin in most patients.

Capnography (end-tidal CO₂ measurement) is helpful in determining the effectiveness of ventilation and pulmonary circulation. This method is especially useful for monitoring the level of ventilation in intubated patients. It should be kept in mind that diseases that increase dead space or decrease pulmonary blood flow lead to decreases in end-tidal CO₂ and an overestimation of the adequacy of ventilation.

Blood Gas Abnormalities in Respiratory Distress and Respiratory Failure

(See Chapters 52.7 and 365.)

Assessment of Oxygenation and Ventilation Deficits

Indicators for following clinical progress and for determining the prognosis in patients with defects in oxygenation or ventilation include:

PaO₂/FIO₂ is calculated by dividing arterial PO₂ by FIO₂. In hypoxic respiratory failure, a PaO₂/FIO₂ value <300 is consistent with acute lung injury, and a value <200 is consistent with acute respiratory distress syndrome. Although the intent is to measure mismatch, intrapulmonary shunt, and diffusion defect, the status of alveolar hypoventilation could have a significant impact on PaO₂/FIO₂.

PAO₂/PAO₂ is determined by dividing arterial PO₂ by alveolar PO₂. The level of alveolar ventilation is accounted for in the calculation of PAO₂. Therefore, PaO₂/PAO₂ is more indicative of mismatch and alveolar capillary integrity.

Oxygenation index (OI) is aimed at standardizing oxygenation to the level of therapeutic interventions such as mean airway pressure (MAP) and FIO₂, which are directed toward improving oxygenation. None of the previously mentioned indicators of oxygenation account for the degree of positive pressure respiratory support. OI is calculated as follows:

Ventilation index (VI) is aimed at standardizing alveolar ventilation to the level of therapeutic interventions (such as peak inspiratory pressure [PIP] and ventilator rate) directed toward lowering PaCO₂. VI is calculated as follows:

Oxygen Administration

Supplemental oxygen administration is the least invasive and most easily tolerated therapy for hypoxemic respiratory failure. Nasal cannula oxygen provides low levels of oxygen supplementation and is easy to administer. Oxygen is humidified in a bubble humidifier and delivered via nasal prongs inserted in to the nares. In children, a flow rate <5 L/min is most often used because of increasing nasal irritation with higher rates. A common formula for an estimation of the FIO₂ during use of a nasal cannula in older children and adults is as follows:

The typical FIO₂ value using this method is between 23 and 40%, although the fraction of inspired oxygen varies according to the size of the child, the respiratory rate, and the volume of air moved with each breath. In a young child, because typical nasal cannula flows are a greater percentage of total minute ventilation, significantly higher FIO₂ may be provided. Alternately, a simple mask may be employed, which consists of a mask with open side ports and a valveless oxygen source. Variable amounts of room air are entrained through the ports and around the side of the mask, depending on the fit, size, and minute volume of the child. Oxygen flow rates vary from 5 to 10 L/min, yielding typical FIO₂ values between 0.30 and 0.65. If more precise delivery of oxygen is desired, other mask devices should be used.

A Venturi mask delivers preset fractions of oxygen through a mask and reservoir system by entraining precise amounts of room air into the reservoir with high-flow oxygen. The amount of room air entrainment and subsequent FIO₂ are determined by the adapter at the end of each mask reservoir. The adapter can be chosen to provide between 30 and 50% oxygen concentrations. Oxygen flow rates of 5-10 L/min are recommended to achieve desired FIO₂ and to prevent rebreathing. Partial rebreather and nonrebreather masks utilize a reservoir bag attached to a mask to provide higher fractions of oxygen. Partial rebreather masks have two open exhalation ports and contain a valveless oxygen reservoir bag. Some exhaled gas can mix with reservoir gas during exhalation, although most exits the mask via the exhalation ports. Through these ports, room air is also entrained during inspiration. A partial rebreather mask can provide up to 0.6 FIO₂ as long as oxygen flow is adequate to keep the bag from collapsing (typically 10-15 L/min). As with nasal cannulas, smaller children with smaller tidal volumes entrain less room air, and their FIO₂ values will be higher. Nonrebreather masks include two one-way valves, one between the oxygen reservoir bag and the mask and one on one of the two exhalation ports. This arrangement minimizes mixing of exhaled and fresh gas and entrainment of room air during inspiration. The second exhalation port has no valve, a safeguard to allow some room air to enter the mask in the event of disconnection from the oxygen source. A nonrebreather mask can provide up to 0.95 FIO₂. The use of a nonrebreather mask in conjunction with an oxygen blender allows delivery of fractions of oxygen between 0.50 and 0.95. When supplemental oxygen alone is inadequate to improve oxygenation, or when ventilation problems coexist, additional therapies may be necessary.

Airway Adjuncts

Maintenance of a patent airway is a critical step in maintaining adequate oxygenation and ventilation. Artificial pharyngeal airways may be useful in patients with oropharyngeal or nasopharyngeal airway obstruction and in those with neuromuscular weakness in whom native extrathoracic airway resistance contributes to respiratory compromise. An oropharyngeal airway is a stiff plastic spacer with grooves along each side that can be placed in the mouth to run from the teeth along the tongue to its base just above the vallecula. The spacer prevents the tongue from opposing the posterior pharynx and occluding the airway. Because the tip sits at the base of the tongue, it is usually not tolerated by patients who are awake or whose gag reflex is strong. The nasopharyngeal airway, or nasal trumpet, is a flexible tube that can be inserted into the nose to run from the nasal opening along the top of the hard and soft palate with the tip ending in the hypopharynx. It is useful in bypassing obstruction from enlarged adenoids or from contact of the soft palate with the posterior nasopharynx. Because it is inserted past the adenoids, a nasopharyngeal airway should be used with caution in patients with bleeding tendencies.

Inhaled Gases

Helium-oxygen mixture (heliox) is useful in overcoming airway obstruction and improving ventilation. Helium is much less dense and slightly more viscous than nitrogen. When substituted for nitrogen, helium helps maintain laminar flow across an obstructed airway, decreases airway resistance, and improves ventilation. It is especially helpful in diseases of large airway obstruction in which turbulent airflow is more common, such as acute laryngotracheobronchitis, subglottic stenosis, and vascular ring. It is also used in patients with severe status asthmaticus. To be effective, helium should be administered in concentrations of at least 60%, so associated hypoxemia may limit its use in patients requiring more than 40% oxygen. Nitric oxide (NO) is a powerful inhaled pulmonary vasodilator. Its use may improve pulmonary blood flow and mismatch in patients with diseases that elevate pulmonary vascular resistance, such as persistent pulmonary hypertension of the newborn (PPHN), primary pulmonary hypertension, and secondary pulmonary hypertension due to chronic excess pulmonary blood flow (e.g., VSD) or collagen vascular diseases. NO is administered in doses ranging from 5 to 20 parts per million (ppm). Although administration of NO to unintubated patients is possible, it is usually administered to patients receiving mechanical ventilation through endotracheal tubes, because of the need for precision in NO dosing.

Positive Pressure Respiratory Support

Noninvasive positive pressure respiratory support is useful in treating both hypoxemic and hypoventilatory respiratory failure. Positive airway pressure helps aerate partially atelectatic or filled alveoli, prevent alveolar collapse at end exhalation, and increase functional residual capacity (FRC). This improves pulmonary compliance and hypoxemia and decreases intrapulmonary shunt. In addition, positive pressure ventilation is useful in preventing collapse of extrathoracic airways by maintaining positive airway pressure during inspiration. Improving compliance and overcoming airway resistance also improves tidal volume and therefore ventilation. A high-flow nasal cannula delivers gas flow at 4-16 L/min, providing significant continuous positive airway pressure (CPAP). The amount of CPAP provided is not quantifiable and varies with each patient, depending on the percentage of total inspiratory flow that is delivered from the cannula, airway anatomy, and degree of mouth breathing. In small children, the relative amount of CPAP for a given flow is usually greater than in older children and may provide significant positive pressure. The FIO₂ can be adjusted by provision of gas flow through an oxygen blender. For delivery of high-flow air or oxygen, adequate humidification is essential and is achieved with use of a separate heated humidification chamber. CPAP can also be provided through snugly fitting nasal prongs or a tight-fitting facial mask attached to a ventilator or other positive pressure device. Noninvasive CPAP is most useful in diseases of mildly decreased lung compliance and low FRC, such as atelectasis and pneumonia. Diseases of extrathoracic airway obstruction in which extrathoracic negative airway pressures during inspiration lead to airway narrowing (e.g., laryngotracheitis, obstructive sleep apnea, postextubation airway edema) may also benefit from CPAP.

Bilevel positive airway pressure (BiPAP)

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