Atelectasis refers to incomplete expansion or collapse of part of the lung. It may include a lung subsegment or the entire lung and is almost always a secondary phenomenon, with no sex or race proclivities; however, it may occur more frequently in younger children than in older children and adolescents. The direct morbidity from atelectasis is transient hypoxemia due to blood flowing through the lung, which does not have normal air flow. The blood does not pick up oxygen from the corresponding alveoli. This shunting results in transient hypoxemia.
Resorption or obstructive atelectasis due to intrinsic or extrinsic airway obstruction
Passive atelectasis from diaphragmatic dysfunction and hypoventilation
Compressive atelectasis from lung tissue compression and ineffective alveolar expansion from intra or thoracic forces
Adhesive atelectasis due to increased surface tension
Because the right middle lobe orifice is the narrowest of the lobar orifices and because it is surrounded by lymphoid tissue, it is the most common lobe to become atelectatic. This is referred to as right middle lobe syndrome.
Intrinsic airway obstruction is the most common cause of atelectasis in children, and asthma is the most common underlying disorder that predisposes patients to atelectasis. Other causes include bronchiolitis, aspiration due to a swallowing disorder, endobronchial tuberculosis, aspiration from gastroesophageal reflux, airway foreign bodies, cystic fibrosis, and increased or abnormal airway secretions for other reasons. Children younger than 10 years are less likely to have developed the interairway canals of Lambert or the interalveolar pores of Kohn. Thus, young children depend more on the feeding airways to move air into the alveoli. When their airways become obstructed, they are more likely to develop atelectasis than older children who have developed those communications.
Extrinsic compression on the airways is most likely to come from enlarged lymph nodes (such as those due to tuberculosis infection), lymphoma and other tumors in the chest, an enlarged heart that compresses the left main or left lower lobe bronchus, and left-to-right intracardiac shunts that increase blood flow through the pulmonary arteries.
In children with hypoventilation for a protracted period, the alveoli may collapse. This may occur in children with neuromuscular disease, those who have had recent thoracic or upper abdominal surgery, those on medications that decrease their minute ventilation (such as narcotics), and those with abnormally small or dysmorphic chest walls, which may be less compliant than the normal chest wall. Atelectasis is also often seen in children who undergo sedation for CT or MRI. A chest study often shows dependent atelectasis in these children. One study  suggested that children who are sedated with propofol infusion were less likely to develop atelectasis than children who have positive pressure ventilation anesthesia for MRI. However, the former group was older. Such children may also be predisposed to atelectasis because of poor clearance of airway secretions. An ineffective cough allows these secretions to obstruct the airway.
Atelectasis due to compressed lung tissue occurs most commonly when air, blood, pus, or chyle is present in the pleural space. Intrathoracic abdominal contents, chest wall masses, cardiomegaly, and an abnormal chest wall can all compress adjacent lung tissue. If a portion of lung enlarges, such as with congenital emphysema, or if focal overinflation occurs for any other reason, it may compress the adjacent lung, causing atelectasis.
Lack of surfactant coating on the alveolar surface from pus, fluid or cell, debris can cause increased surface tension and subsequent alveolar collapse. Some examples include respiratory distress of the preterm newborn, meconium aspiration, pneumonia, and acute respiratory distress syndrome.
Most of the morbidity and any mortality is due to the underlying disorder. The primary complication of atelectasis is hypoxemia, which is usually transient. Within 24-48 hours, the lung is able to decrease or shut off blood flow to the atelectatic area. This is probably caused by factors such as serotonin that reacts to the local hypoxia in the alveoli and causes an intense vasoconstriction. If the atelectasis is massive enough, it may cause enough hypoxemia acutely to require supplemental oxygen or ventilatory support.
Atelectasis is a suggested cause of fever; however, no known physiologic reason supports this. Recent data dispute this old dogma. A study of adults after open-heart surgery showed no correlation between atelectasis and fever and found that incidence of fever actually rose as the atelectasis was resolving.  Patients with temperatures of more than 38.5°C were less likely to have atelectasis on radiography findings than those patients who were afebrile and undergoing radiography as part of the postoperative routine.
Another concern is the likelihood of infection in the atelectatic portion of the lung. Although the clearance in this portion of the lung is abnormal, the lung is normally a sterile environment. In the otherwise healthy child with atelectasis, infection is unlikely. However, if the child has abnormal secretions or is prone to aspiration, secondary infection of the atelectatic lung may occur. In children with chronically infected lungs, the atelectatic portion is likely to be similarly infected, with decreased ability to clear the infection. This sets up the possibility of bronchiectasis developing in that portion of the lung. Children who remain on assisted ventilation with atelectasis are at risk of developing infection, including infection in the atelectatic portion of the lung. This portion has less intrinsic clearance, which increases the risk of significant infection if organisms enter this portion.
Atelectasis has no sex predilection.
Atelectasis is probably more common in children younger than 10 years because their airways are typically narrower and are more likely to become obstructed by secretions, airway inflammation, or both. In addition, these smaller airways are more easily compressed. These children are also less likely to have collateral ventilation.
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