Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 108

Obstruction of a bronchus by a carcinoma, foreign body (e.g. inhaled peanut) or mucus plug causes loss of aeration with ‘loss of volume’ and collapse of the lung distal to the obstruction. Collapse of each individual lobe of the lung produces its own particular appearance on chest X‐ray (Figs 4.3 and 4.4) with shift of landmarks such as the mediastinum resulting from loss of volume. Obstruction of a main bronchus usually causes obvious asymmetry (Fig. 4.5). Compensatory expansion of other lobes may result in increased transradiency of adjacent areas of the lung.

Right upper lobe collapse is relatively easy to spot on a plain chest X‐ray; there is an area of increased density in the upper medial aspect of the right hemithorax with elevation of the horizontal fissure and right hilum. In right middle lobe collapse there may be little to see on a PA X‐ray apart from lack of definition of the right heart border. This is a useful sign that helps to distinguish it from right lower lobe collapse where the right border of the heart remains clearly defined. In right lower lobe collapse there is also a triangular opacity in the right lower zone (usually medially) pointing to the hilum and the medial aspect of the right hemidiaphragm is obscured. The left upper lobe collapses anteriorly, becoming a thin sheet of tissue up against the anterior chest wall. It appears like a veil, most obvious superiorly and fading inferiorly. Left lower lobe collapse is manifest as a triangular area of increased density behind the heart shadow, often with a shift of the heart shadow to the left and increased transradiency of the left hemithorax because of compensatory expansion of the left upper lobe (see Fig. 4.4). Collapse is a sinister sign often indicating an obstructing carcinoma that may be confirmed by bronchoscopy.

Figure 4.1 Diagram of chest X‐ray (PA view). The right hemidiaphragm is 1–3 cm higher than the left (a) and on full inspiration it is intersected by the shadow of the anterior part of the sixth rib (b). The trachea (c) is vertical and central or very slightly to the right. The horizontal fissure (d) is found in the position shown and should be truly horizontal. It is a very valuable marker of change in volume of any part of the right lung. The left border of the cardiac shadow comprises (e) aorta; (f) pulmonary artery; (g) concavity overlying the left atrial appendage; (h) left ventricle. The right border of the cardiac shadow (i) is formed by the right atrium, the superior vena cava entering superiorly and the inferior vena cava which is often seen at its lower margin.