Emphysema Pulmonary bulla 2019-03-03T17:45:09Z Blebs and bullae are commonly subpleural and are mostly seen in both upper lobes in patients with coexisting centrilobular and paraseptal emphysema. Pulmonary bullae Pulmonary bullae are rounded focal regions of emphysema with a thin wall which measure more than 1 cm in diameter. They are often subpleural in location and are typically larger in the apices. In some cases, bullae can be very large and result in compression of adjacent lung tissue. A giant bulla is arbitrarily defined as one that occupies at least one third of the volume of a hemithorax. When large, bullae can simulate pneumothorax. The most common cause is paraseptal emphysema but bullae may also be seen in association with centrilobular emphysema.