mechanisms or the distribution of alveolar collapse; resorption, compression, microatelectasis and contraction atelectasis. Relaxation atelectasis (also called passive atelectasis) is when a pleural effusion or a pneumothorax disrupts the contact between the parietal and visceral pleurae. Risk factors associated with increased likelihood of the development of atelectasis include: type of surgery (thoracic, cardiopulmonary surgeries), use of muscle relaxation, obesity, high oxygen, the lower lung segments. Factors also associated with the development of atelectasis include: age, presence of chronic obstructive pulmonary disease or asthma, and type of anesthetic. In the early 1950s, in UK aviation medicine, the condition acceleration atelectasis was given the name "Hunter lung" due to its prevalence in pilots of the transonic fighter jet, the Hawker Hunter, which used a 100% oxygen supply. Diagnosis Clinically significant atelectasis is generally visible on chest X-ray; findings can include lung opacification and/or loss of lung volume. Post-surgical atelectasis will be bibasal in pattern. Chest CT or bronchoscopy may be necessary if the cause of atelectasis is not clinically apparent. Direct signs of atelectasis include displacement of interlobar fissures and mobile structures within the thorax, overinflation of the unaffected ipsilateral lobe or contralateral lung, and opacification of the collapsed lobe. In addition to clinically significant findings on chest X-rays, patients may present with indirect signs and symptoms such as elevation of the diaphragm, shifting of the trachea, heart and mediastinum; displacement of the hilus and shifting granulomas. Classification 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), destruction, and scarring (fibrosis). Absorption (resorption) atelectasis The Earth's atmosphere is mainly composed of 78 vol. % nitrogen and 21 vol. % oxygen (+ 1 vol. % argon and traces of other gases). Since oxygen is exchanged at the alveoli-capillary membrane, nitrogen is a major component for the alveoli's state of inflation. If a large volume of nitrogen in the lungs is replaced with oxygen, the oxygen may subsequently be absorbed into the blood, reducing the volume of the alveoli, resulting in a form of alveolar collapse known as absorption atelectasis. Compression (relaxation) atelectasis It is usually associated with accumulation of blood, fluid, or air within the pleural cavity, which mechanically collapses the lung. This is a frequent occurrence with