Epidemiology, Etiology, And Pathophysiology
It is obvious to any practicing emergency physician that respiratory emergencies are a common reason children present to the ED. Subjective criteria for the diagnosis of specific respiratory illnesses can affect the accuracy of epidemiological studies. For example, criteria for the diagnosis of pneumonia may differ between providers. A child with fever, wheezing, and hypoxia may be diagnosed as pneumonia by one provider, bronchiolitis by another provider, and asthma with an upper respiratory infection by a third provider. Despite this incongruity, some epidemiologic statistics have been collected. One study found that 17% of all ED visits are for children with respiratory distress.39 It is also the most common reason children under 4 years old are admitted to the hospital. Asthma, bronchiolitis, croup, and pneumonia account for a large number of discharge diagnoses from the ED and the hospital.40
Respiratory emergencies are also a leading cause of EMS activations. One study found that 38% of pediatric EMS activations were for respiratory emergencies.41 The true incidence of specific respiratory emergencies is again hampered by the subjective nature of diagnosing respiratory distress.
Recent trends show that despite the advances made in medicine, asthma continues to grow in prevalence, severity, and mortality. It is uncertain why there has been an increase in the prevalence and mortality of pediatric asthma. Multiples theories exist, including environmental reasons, immunology, and increased disease recognition. The overall rise in the morbidity and mortality in children's asthma has become a national focus. The National Heart, Lung, and Blood Institute's Asthma Education program has published guidelines for the management of acute asthma in the ED as well as multiple recommendations for the use of preventative therapy.
There are a large number of illnesses with the final common pathway of respiratory distress, yielding a broad differential diagnosis. (See Table 1.) In general, conditions that result in upper or lower airway obstruction, airway inflammation, or respiratory mechanical derangement (congestive heart failure, neuro-muscular weakness) will result in respiratory distress. Obstruction can be mechanical in nature (foreign body) or structural (laryngomalacia). Airway inflammation is a common cause of difficulty breathing. The symptoms of croup and asthma are secondary to inflammation from presence of inflammatory mediators like cytokines, histamines, and bradykinin. The mainstay of therapy for these illnesses is the use of antiinflammatory medications. Systemic illnesses can also frequently have respiratory symptoms in children. In response to a systemic disease and increased metabolic demand, children have a limited ability to increase pulmonary functional residual capacity and therefore increase their imnute ventilation primarily by breathing faster rather than taking deeper breaths. In addition, young children have less respiratory reserve and tend to show symptoms of respiratory distress earlier in the course of a disease processbreathing difficulty.
Anatomic differences in children make them more susceptible to respiratory difficulty than adults. Relative to their size, the tongue is large, and the upper airways are narrow. The chest wall is also more pliable, which makes the generation of negative inspiratory pressure more difficult. Untreated respiratory difficulty can lead to respiratory failure, which is the inability to provide adequate oxygenation and ventilation to pulmonary tissues. Grunting is uniquely seen in infants and is thought to represent a form of self-induced auto-PEEP (positive end expiratory pressure), which allows infants to keep their smaller airways and alveoli open. Respiratory failure may manifest as periods of slowed breathing and apnea. Perfusion and cyanosis will worsen, and complete respiratory arrest will follow.
Etiology / Differential Diagnosis
Due to the broad nature of this subject, it is helpful to discuss the differential diagnosis by sorting this topic into multiple categories. A logical approach to the differential diagnosis of difficulty breathing in children entails dividing the categories based on mechanical etiology. (See Table 1.) In this manner, the etiology of respiratory distress may stem from the upper airways, the lower airways, chest wall conditions, cardiac, central nervous system, gastrointestinal, or metabolic causes.
Upper Airway Etiologies
As previously mentioned, the anatomy of children makes them more susceptible to airway difficulties from infection or obstruction. The resistance to airflow is inversely related to the radius of the airway. Thus, any obstruction at the level of the nose, mouth, larynx, or trachea can cause significant obstruction. Upper airway obstructions also tend to produce stridor, in contrast to lower airway disorders, which tend to manifest with wheezing, rales, or rhonchi. Nasal congestion alone in a small infant can result in significant symptoms of distress. Oropharyngeal conditions such as a peritonsillar or retropharyngeal abscess or macroglossia are important considerations. Tracheo-laryngeal inflammation from disease states such as croup, epiglottitis, anaphylaxis, or foreign body aspirations can become lifethreatening.
Lower Airway Etiologies
The vast majority of children whose respiratory illness produces significant distress have lower airway etiologies. The conditions causing lower airway pathology induced symptoms come from inflammation, bronchoconstriction, obstruction, mucous plugging, or a combination of the factors. Asthma produces airway inflammation, bronchoconstriction, and mucous plugging. Bronchiolitis also is characterized by airway inflammation and mucous plugging. Prior to assuming a lower respiratory tract etiology, it is important to consider other etiologies, such as congenital heart disease.
Chest Wall / Pleural Cavity
A complex physiologic process that generates negative thoracic pressure is responsible for inspiration. Any condition that mechanically interferes with this process can ultimately result in respiratory difficulties. The relatively higher compliance of a child's chest wall means that even small amounts of air, blood, effusion, or chyle in the pleural space can significantly interfere with normal respiratory mechanics.
This subgroup of illnesses is often overlooked, since the vast majority of children have a healthy cardiovascular system. Nevertheless, cardiac disease can result in respiratory difficulty in children by a variety of mechanisms. Children with cyanotic congenital heart disease will appear cyanotic secondary to shunting of deoxygenated blood to the systemic circulation. These patients often present in the neonatal period, when the ductus arteriosus begins to close. Often their cyanosis will not be accompanied by an increased work of breathing and will not be responsive to supplemental oxygen therapy. Children with acyanotic congenital heart disease may also present with difficulty breathing. The pathophysiology for their difficulty breathing is often congestive heart failure. These children will present with tachypnea and possibly wheezing. The wheezing is not responsive to bronchodilators, and occasionally the tachypnea is not associated with an increased work of breathing. Acquired heart disease such as myocarditis and cardiomyopathy may similarly present with wheezing and tachypnea. These patients usually present with hepatomegaly, poor perfusion, and difficulty breathing not responsive to bronchodilator therapy.
Central Nervous System
Difficulty breathing may be a symptom of a primary neurologic disease. A child with a central nervous system malformation such as schizencephaly may present with central apena. Spinal cord disease such as spinal muscle atrophy may present with progressive respiratory insufficiency or frequent lower respiratory tract infections from inability to handle secretions. Peripheral neuropathies affecting the neuromuscular junction or muscles of respiration, such as myasthenia gravis, botulism, or muscular dystrophy, can also result in difficulty breathing.
Due to poor lower esophageal sphincter tone and the frequency of being in the supine position, infants are predisposed to gastroesophageal reflux. This can often cause a reflex bronchoconstriction that can be mistaken for asthma. Significant abdominal distension can also cause difficulty breathing in children due to impaired diaphragmatic movement.
Occasionally, respiratory distress can be a symptom of another systemic disorder. Metabolic acidosis, sepsis, and dehydration will result in increased work of breathing as compensation for acidosis. Many toxicologic syndromes that result in metabolic acidosis will also cause respiratory distress. It is prudent to keep this fact in mind, especially when patients are not responding appropriately to the therapeutic interventions.