Atelectasis
Atelectasis is a condition in which all or part of a lung becomes airless and contracts.
Atelectasis may be an acute or chronic condition. In acute atelectasis, the lung has recently collapsed and is primarily notable only for airlessness. In chronic atelectasis, the affected area is often characterized by a complex mixture of airlessness, infection, widening of the bronchi (bronchiectasis (see Section 4, Chapter 47)), destruction, and scarring (fibrosis). People who smoke have a greater risk of developing atelectasis.
Causes
The most common cause of atelectasis is an obstruction of a large bronchus (one of the two main branches of the trachea leading directly to the lungs). Smaller airways can also become blocked. The obstruction may be caused by a plug of mucus, a tumor, or an inhaled foreign object inside the bronchus. Alternatively, the bronchus may be blocked by something pressing from the outside, such as a tumor, enlarged lymph nodes, or a significant amount of fluid (pleural effusion) or air (pneumothorax) in the pleural space (see Section 4, Chapter 52). When an airway becomes blocked, the air in the small air sacs of the lung (alveoli) beyond the blockage is absorbed into the bloodstream, causing the alveoli to shrink and retract. The collapsed lung tissue commonly fills with blood cells, serum, and mucus and becomes infected.
Atelectasis can occur in jet fighter pilots when the high forces generated by high-speed flying close small airways. Atelectasis under these circumstances, sometimes described as acceleration atelectasis, leads to collapse of the alveoli in much of both lungs.
Additionally, atelectasis can result if there is a deficiency in the amount or effectiveness of the liquid substance (surfactant) that coats the lining of the alveoli. Normally, this liquid prevents the alveoli from collapsing.
Acute Atelectasis: Acute atelectasis is a common postoperative complication, especially after chest or abdominal surgery. Acute atelectasis may also occur with an injury, usually to the chest (such as that caused by a car accident, a fall, or a stabbing). Atelectasis following surgery or injury, sometimes described as massive, involves most alveoli in one or more regions of the lungs. In these circumstances, the degree of collapse among alveoli tends to be quite consistent and complete. Large doses of opioids or sedatives, tight bandages, chest or abdominal pain, abdominal swelling (distention), and immobility of the body increase the risk of acute atelectasis following surgery or injury, or even spontaneously. Certain neurologic conditions and chest deformities are additional factors that can limit chest movement, lead to shallow breathing, cause bronchial secretions to accumulate, preclude the lung from expanding fully, and suppress the cough reflex.
In acute atelectasis that occurs because of a deficiency in the amount or effectiveness of surfactant, many but not all alveoli collapse, and the degree of collapse is not uniform. Atelectasis in these circumstances may be limited to only a portion of one lung, or it may be present throughout both lungs. When premature babies are born with surfactant deficiency, they always develop acute atelectasis that progresses to neonatal respiratory distress syndrome (see Section 23, Chapter 264) unless they are treated with replacement surfactant. Adults can also develop acute atelectasis from excessive oxygen therapy and from mechanical ventilation, because of decreased effectiveness of surfactant. Another cause of acute atelectasis resulting from decreased effectiveness of surfactant is acute respiratory distress syndrome (see Section 4, Chapter 56).
Chronic Atelectasis: Chronic atelectasis may take one of two forms--middle lobe syndrome or rounded atelectasis. In middle lobe syndrome, the middle lobe of the right lung contracts, usually because of pressure on the bronchus from enlarged lymph glands and occasionally a tumor. The blocked, contracted lung may develop pneumonia that fails to resolve completely and leads to chronic inflammation, scarring, and bronchiectasis.
In rounded atelectasis (folded lung syndrome), an outer portion of the lung slowly collapses as a result of scarring and shrinkage of the membrane layers covering the lungs (pleura). This produces a rounded appearance on x-ray that doctors may mistake for a tumor. Rounded atelectasis is usually a complication of asbestos-induced disease of the pleura, but it may also result from other types of chronic scarring and thickening of the pleura.
Symptoms
The loss of functioning lung tissue leads to shortness of breath. The persistence of blood flow through the collapsed area leads to a decrease in the blood oxygen level--the heart rate increases, and sometimes the person may look bluish (a condition called cyanosis).
The severity of symptoms depends on how rapidly the bronchus is blocked, how much of the lung is affected, what the precipitating factor was, and whether infection is present. When blockage happens quickly and a lot of lung tissue is affected, a person may become blue or ashen in color, have sharp pain on the affected side, and have sudden and extreme shortness of breath. The person may also experience shock (see Section 3, Chapter 24) with a severe drop in blood pressure; a rapid heart rate; and fever if infection develops.
Widespread atelectasis resulting from deficient or ineffective surfactant produces shortness of breath; rapid, shallow breathing; a low blood oxygen level; and other symptoms depending on the cause of the acute lung injury (for example, fever and low blood pressure from sepsis) and any accompanying effects of low blood oxygen (such as abnormal heart rhythms) on organs other than the lung.
Slowly developing atelectasis may cause no symptoms or only minor ones, such as shortness of breath or an increased heart rate. People with middle lobe syndrome and rounded atelectasis may have no symptoms, although some people with middle lobe syndrome have a hacking cough or develop pneumonia that resolves slowly or incompletely.
Diagnosis
Doctors suspect atelectasis based on a person's symptoms, the physical examination findings, and the setting in which the symptoms occurred. A chest x-ray that shows the airless area confirms the diagnosis, but the x-ray may appear normal even when the person is feeling breathless. When bronchial obstruction is suspected, computed tomography (CT), bronchoscopy, or both these tests may be performed to find the cause, especially when the collapse persists despite usual treatment measures.
Prevention and Treatment
People who smoke can decrease their risk of atelectasis after surgery by stopping smoking 6 to 8 weeks before an operation. After an operation, all people should be encouraged to breathe deeply, cough regularly, and move about as soon as possible. The use of breathing devices to encourage voluntary deep breathing (incentive spirometry) and certain exercises, including changing position to increase the drainage of lung secretions, may help to prevent atelectasis.
People with chest deformities or neurologic conditions that cause shallow breathing for long periods may benefit from mechanical devices that assist their breathing. One method is continuous positive airway pressure, which delivers oxygen through a nose or face mask to help ensure that the airways do not collapse, even at the end of a breath. Sometimes additional respiratory support is needed with a mechanical ventilator (see Section 4, Chapter 55).
The primary treatment for acute massive atelectasis is correction of the underlying cause. A blockage that cannot be removed by coughing or by suctioning the airways often can be removed by bronchoscopy (see Section 4, Chapter 39). Antibiotics are given for an infection. Chronic atelectasis often is treated with antibiotics because infection is almost inevitable. In certain cases, the affected part of the lung may be surgically removed when recurring or chronic infections become disabling or bleeding is significant. If a tumor is blocking the airway, relieving the obstruction by surgery, radiation therapy, chemotherapy, or laser therapy may prevent atelectasis from progressing and recurrent obstructive pneumonia from developing.
In treatment of atelectasis due to deficient or ineffective surfactant, attention is directed at treating the low blood oxygen (often with mechanical ventilation or positive end expiratory pressure) and its effects promptly and at identifying and treating the underlying condition. Treatment with a surfactant drug is lifesaving for premature babies with a surfactant deficiency. Such therapy is experimental in adults with the acute respiratory distress syndrome who have reduced surfactant activity.
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