Obstructive atelectasis is the commonest type of lung collapse resulting from a blockade at the level of the small or the large airways.
Patients of obstructive atelectasis are usually asymptomatic, with the development of clinical features dependent on the location and size of the affected pulmonary regions, the nature of occlusion and the presence of concomitant infection.
Hypoxemia is primarily responsible for the majority of symptoms seen in patients with obstructive collapse. Rapid blockade of the bronchi produces sudden dyspnea, cyanosis or even respiratory failure. A slow, less severe occlusion may be completely asymptomatic.
Pneumonia is also commonly seen in patients of atelectasis, presenting with symptoms such as cough with expectoration, fever, chest pain and breathlessness. Pleuritic pain may also originate from the underlying etiology responsible for lung collapse.
Overt clinical signs are often undetectable in cases of atelectasis. Large sections of lung collapse may lead to areas of dullness and decreased chest expansion. Breath sounds may also be reduced in such regions. Hypotension, tachycardia, and shock may also be seen in a few cases. Controversy exists regarding the association of atelectasis with early postoperative fever .
Hypoxemia resulting from obstructive atelectasis may be easily confirmed by an arterial blood gas analysis. A low partial pressure of oxygen in the arteries (PaO2) is often accompanied by a normal arterial carbon dioxide level (PaCO2) due to the compensatory increase in ventilation seen in such patients.
Imaging studies such as chest X-rays and computed tomography (CT) scans produce conclusive evidence of lung collapse    . Opacification of the affected lobes along with blurring/ displacement of the lung fissures are some of the direct signs seen in these studies. Some of the other signs that may indicate an atelectasis include: a shift of the mediastinal and hilar contents towards the side of the collapse, decreased rib spacing on the affected side, elevation of the ipsilateral dome of the diaphragm, increased translucency of the other normal lobes and loss of normal borders of the heart/ diaphragm (silhouette sign).
Atelectasis of an entire lung is characterized by whitening (opacification) of the hemithorax accompanied by some of the indirect signs of lung collapse listed above. Atelectasis resulting from thickening of the pleura is termed as rounded atelectasis  . This segmental anomaly is usually seen on imaging as a subpleural mass, with bronchovascular markings radiating out to the lung hilum . Pleural plaques are also commonly observed in such cases. Rounded atelectasis is commonly seen in the middle/ lower lobes.
Another investigation with therapeutic potential is flexible fibreoptic bronchoscopy that may help in determining the etiology of obstruction. Bronchoscopy may also afford clinical relief by clearing the respiratory tract of secretions. Obstructions distal to the subsegmental bronchi are however not easily visualized by this procedure.
Biopsies obtained from obstructive masses encountered during bronchoscopy must be evaluated for the histologic presence of infection, malignancy or any other possible etiologies of atelectasis.