the chest wall or damage to the lung itself, or occasionally because microorganisms in the pleural space produce gas. once air enters the pleural cavity, the intrapleural pressure increases resulting in the difference between the intrapulmonary pressure and the intrapleural pressure (defined as the transpulmonary pressure) to equal zero which cause the lungs to deflate in contrast to a normal transpulmonary pressure of ~4 mm hg. chest-wall defects are usually evident in cases of injury to the chest wall, such as stab or bullet wounds ("open pneumothorax"). in secondary spontaneous pneumothoraces, vulnerabilities in the lung tissue are caused by a variety of disease processes, particularly by rupturing of bullae (large air-containing lesions) in cases of severe emphysema. areas of necrosis (tissue death) may precipitate episodes of pneumothorax, although the exact mechanism is unclear. primary spontaneous pneumothorax (psp) has for many years been thought to be caused by "blebs" (small air-filled lesions just under the pleural surface), which were presumed to be more common in those classically at risk of pneumothorax (tall males) due to mechanical factors. in psp, blebs can be found in 77% of cases, compared to 6% in the general population without a history of psp. as these healthy subjects do not all develop a pneumothorax later, the hypothesis may not be sufficient to explain all episodes; furthermore, pneumothorax may recur even after surgical treatment of blebs. it has therefore been suggested that psp may also be caused by areas of disruption (porosity) in the pleural layer, which are prone to rupture. smoking may additionally lead to inflammation and obstruction of small airways, which account for the markedly increased risk of psps in smokers. once air has stopped entering the pleural cavity, it is gradually reabsorbed. tension pneumothorax occurs when the opening that allows air to enter the pleural space functions as a one-way valve, allowing more air to enter with every breath but none to escape. the body compensates by increasing the respiratory rate and tidal volume (size of each breath), worsening the problem. unless corrected, hypoxia (decreased oxygen levels) and respiratory arrest eventually follow. diagnosis the symptoms of pneumothorax can be vague and inconclusive, especially in those with a small psp; confirmation with medical imaging is usually required. in contrast, tension pneumothorax is a medical emergency and may be treated before imaging – especially if there is severe hypoxia, very low blood pressure, or an impaired level of consciousness. in tension pneumothorax, x-rays are sometimes required if there is doubt about the anatomical location of the pneumothorax. chest x-ray a plain chest radiograph, ideally with the x-ray beams being projected from the back (posteroanterior, or "pa"), and during maximal inspiration (holding one's breath), is the most appropriate first investigation. it is not believed that routinely taking images during expiration would confer any benefit. still, they may be useful in the detection of a pneumothorax when clinical suspicion is high but yet an inspiratory radiograph appears normal. also, if the pa x-ray does not show a pneumothorax but there is a strong suspicion of one, lateral x-rays (with beams projecting from the side) may be performed, but this is not routine practice. anteroposterior inspired x-ray, showing subtle left-sided pneumothorax caused by port insertion lateral inspired x-ray at the same time, more clearly showing the pneumothorax posteriorly in this case anteroposterior expired x-ray at the same time, more clearly showing the pneumothorax in this case chest x-ray showing a pneumothorax on the right (left in the image), where the absence of lung markings indicates that there is free air inside the chest chest x-ray showing the features of pneumothorax on the left side of the person (right in image) it is not unusual for the mediastinum (the structure between the lungs that contains the heart, great blood vessels, and large airways) to be shifted away from the affected lung due to the pressure differences. this is not equivalent to a tension pneumothorax, which is determined mainly by the constellation of symptoms, hypoxia, and shock. the size of the pneumothorax (i.e. the volume of air in the pleural space) can be determined with a reasonable degree of accuracy by measuring the distance between the chest wall and the lung. this is relevant to treatment, as smaller pneumothoraces may be managed differently. an air rim of 2 cm means that the pneumothorax occupies about 50% of the hemithorax. british professional guidelines have traditionally stated that the measurement should be performed at the level of the hilum (where blood vessels and airways enter the lung) with 2 cm as the cutoff, while american guidelines state that the measurement should be done at the apex (top) of the lung with 3 cm differentiating between a "small" and a "large" pneumothorax. the latter method may overestimate the size of a pneumothorax if it is located mainly at the apex, which is a common occurrence. the various methods correlate poorly but are the best easily available ways of estimating pneumothorax size. ct scanning (see below) can provide a more accurate determination of the size of the pneumothorax, but its routine use in this setting is not recommended. not all pneumothoraces are uniform; some only form a pocket of air in a particular place in the chest. small amounts of fluid may be noted on the chest x-ray (hydropneumothorax); this may be blood (hemopneumothorax). in some cases, the only significant abnormality may be the "deep sulcus sign", in which the normally small space between the chest wall and the diaphragm appears enlarged due to the abnormal presence of fluid. computed tomography ct with the identification of underlying lung lesion: an apical bulla on the right side a ct scan is not necessary for the diagnosis of pneumothorax, but it can be useful in particular situations. in some lung diseases, especially emphysema, it is possible for abnormal lung areas such as bullae (large air-filled sacs) to have the same appearance as a pneumothorax on chest x-ray, and it may not be safe to apply any treatment before the distinction is made and before the exact location and size of the pneumothorax is determined. in trauma, where it may not be possible to perform an upright film, chest radiography may miss up to a third of pneumothoraces, while ct remains very sensitive. a further use of ct is in the identification of underlying lung lesions. in presumed primary pneumothorax, it may help to identify blebs or cystic lesions (in anticipation of treatment, see below), and in secondary pneumothorax, it can help to identify most of the causes listed above. ultrasound ultrasound is commonly used in the evaluation of people who have sustained physical trauma, for example with the fast protocol. ultrasound may be more sensitive than chest x-rays in the identification of pneumothorax after blunt trauma to the chest. ultrasound may also provide a rapid diagnosis in other emergency situations, and allow the quantification of the size of the pneumothorax. several particular features on ultrasonography of the chest can be used to confirm or exclude the diagnosis. treatment the treatment of pneumothorax depends on a number of factors and may vary from discharge with early follow-up to immediate needle decompression or insertion of a chest tube. treatment is determined by the severity of symptoms and indicators of acute illness, the presence of underlying lung disease, the estimated size of the pneumothorax on x-ray, and – in some instances – on the personal preference of the person involved. in traumatic pneumothorax, chest tubes are usually inserted. if mechanical ventilation is required, the risk of tension pneumothorax is greatly increased and the insertion of a chest tube is mandatory. any open chest wound should be covered with an airtight seal, as it carries a high risk of leading to tension pneumothorax. ideally, a dressing called the "asherman seal" should be utilized, as it appears to be more effective than a standard "three-sided" dressing. the asherman seal is a specially designed device that adheres to the chest wall and, through a valve-like mechanism, allows air to escape but not to enter the chest. tension pneumothorax is usually treated with urgent needle decompression. this may be required before transport to the hospital, and can be performed by an emergency medical technician or other trained professional. the needle or cannula is left in place until a chest tube can be inserted. critical care teams are able to incise the chest to create a larger conduit as performed when placing a chest drain, but without inserting the chest tube. this is called a simple thoracostomy. if tension pneumothorax leads to cardiac arrest, needle decompression or simple thoracostomy is performed as part of resuscitation as it may restore cardiac output. conservative small spontaneous pneumothoraces do not always require treatment, as they are unlikely to proceed to respiratory failure or tension pneumothorax, and generally resolve spontaneously. this approach is most appropriate if the estimated size of the pneumothorax is small (defined as <50% of the volume of the hemithorax), there is no breathlessness, and there is no underlying lung disease. it may be appropriate to treat a larger psp conservatively if the symptoms are limited. admission to hospital is often not required, as long as clear instructions are given to return to hospital if there are worsening symptoms. further investigations may be performed as an outpatient, at which time x-rays are repeated to confirm improvement, and advice given with regard to preventing recurrence (see below). estimated rates of resorption are between 1.25% and 2.2% the volume of the cavity per day. this would mean that even a complete pneumothorax would spontaneously resolve over a period of about 6 weeks. there is, however, no high quality evidence comparing conservative to non conservative management. secondary pneumothoraces are only treated conservatively if the size is very small (1 cm or less air rim) and there are limited symptoms. admission to the hospital is usually recommended. oxygen given at a high flow rate may accelerate resorption as much as fourfold. aspiration in a large psp (>50%), or in a psp associated with breathlessness, some guidelines recommend that reducing the size by aspiration is equally effective as the insertion of a chest tube. this involves the administration of local anesthetic and inserting a needle connected to a three-way tap; up to 2.5 liters of air (in adults) are removed. if there has been significant reduction in the size of the pneumothorax on subsequent x-ray, the remainder of the treatment can be conservative. this approach has been shown to be effective in over 50% of cases. compared to tube drainage, first-line aspiration in psp reduces the number of people requiring hospital admission, without increasing the risk of complications. aspiration may also be considered in secondary pneumothorax of moderate size (air rim 1–2 cm) without breathlessness, with the difference that ongoing observation in hospital is required even after a successful procedure. american professional guidelines state that all large pneumothoraces – even those due to psp – should be treated with a chest tube. moderately sized iatrogenic traumatic pneumothoraces (due to medical procedures) may initially be treated with aspiration. chest tube a chest tube placed on the right for a pneumothorax a chest tube (or intercostal drain) is the most definitive initial treatment of a pneumothorax. these are typically inserted in an area under the axilla (armpit) called the "safe triangle", where damage to internal organs can be avoided; this is delineated by a horizontal line at the level of the nipple and two muscles of the chest wall (latissimus dorsi and pectoralis major). local anesthetic is applied. two types of tubes may be used. in spontaneous pneumothorax, small-bore (smaller than 14 f, 4.7 mm diameter) tubes may be inserted by the seldinger technique, and larger tubes do not have an advantage. in traumatic pneumothorax, larger tubes (28 f, 9.3 mm) are used. when chest tubes are placed due to either blunt or penetrating trauma, antibiotics decrease the risks of infectious complications. chest tubes are required in psps that have not responded to needle aspiration, in large ssps (>50%), and in cases of tension pneumothorax. they are connected to a one-way valve system that allows air to escape, but not to re-enter, the chest. this may include a bottle with water that functions like a water seal, or a heimlich valve.