Mini 3 - Michigan Questions: Superior Mediastinum And Lungs

There are several reasons why a bronchoscope or inhaled objects will be more likely to enter the right lung instead of the left lung. First, the carina, a ridge-like structure at the bifurcation of the trachea, is set a little towards the left. So, there is a more direct path for objects to fall to the right. Also, the right bronchus is shorter, wider, and more vertical than the left bronchus. All of these factors mean that an inhaled object will usually enter the right main bronchus.

The carina is a keel-shaped cartilage lying at the tracheal bifurcation--it separates the right main stem bronchus from the left main stem bronchus. The carina is a little to the left of the tracheal bifurcation, so if there is an inhaled body the carina will tend to divert foreign objects to the right main bronchus. The cricoid cartilage is the inferior and posterior cartilage of the larynx. The costal cartilages prolong the ribs anteriorly and contribute to the elasticity of the thoracic wall. They increase in length through the first seven and then gradually decrease. The pulmonary ligament is a fold of pleura located below the root of the lung. Tracheal rings are the cartilagenous structures which support the trachea and keep it patent.

Because of the sharp angle of the oblique fissures, the posterior surfaces of both the right and left lungs are almost entirely comprised of the inferior lobes. And remember--the middle lobe of the right lung is not part of the posterior surface of the lung. So, a stab wound halfway between the apex and diaphragmatic surface of the right lung would result in injury to the inferior lobe. The cardiac notch is an indentation on the left lung which helps to form the lingula. The lingula is part of the superior lobe of the left lung, which forms the anterior and superior sides of that lung. The superior lobe (along with the middle lobe) forms the anterior surface of the right lung. The superior lobe also forms the superior aspect of both lungs.

The cardiac notch is only found on the left lung, which makes sense since the heart is located on the left side of the mediastinum. The horizontal fissure is a deep groove that separates the middle lobe from the upper lobe of the right lung. The left lung does not have a horizontal fissure. The oblique fissure is found in both lungs. It separates the upper lobe from the lower lobe in both lungs and the middle lobe from the lower lobe in the right lung. Both lungs also have a superior lobar bronchus leading to their superior lobes. Finally, the right lung has three lobes while the left lung has two lobes.

The superior segmental bronchus to the lower lobe of the right lung branches posteriorly off of the intermediate bronchus or the inferior lobe bronchus. So, it is the segmental bronchus most likely to receive the fluid or foreign bodies that enter the right main bronchus. This segment of the lung is even more likely to accumulate fluid when the patient is supine

The superior segmental bronchus of the inferior lobe branches posteriorly off of the intermediate bronchus or the inferior lobe bronchus, so it is the most likely segmental bronchus to receive foreign bodies or fluids that enter the right bronchus. This is even more likely to occur if the patient is in the supine position.

There are several reasons why inhaled objects will be more likely to enter the right lung instead of the left lung. First, the carina, a ridge-like structure at the bifurcation of the trachea, is set a little towards the left. So, there is a more direct path for objects to fall to the right. Also, the right bronchus is shorter, wider, and more vertical than the left bronchus. All of these factors mean that an inhaled object will enter the right main bronchus. It would be almost impossible for a bead to get lodged in the terminal bronchiole of the right lung--that is a very small space!

Remember--the thymus is a very superficial structure found in the anterior mediastinum. The left brachiocephalic vein courses through the mediastinum to join the right brachiocephalic vein and form the superior vena cava on the right side of the thorax. Since the left brachiocephalic vein is fairly superficial, it travels just deep to the thymus. So, it might be compressed by the tumor. See Netter Plate 206 for a picture of the thymus and the left brachiocephalic vein.

The left pulmonary vein, left bronchial vein, and right pulmonary arteries are deep structures that enter and exit the lung at its root--they are not near the thymus. The right superior intercostal vein drains intercostal spaces 2-4. It drains into the arch of the azygos vein and is not associated with the thymus.

When inspiring fully, the lingula of the left lung might partially fill the costomediastinal recess. The lingula, a tongue-like projection of the left lung below the cardiac notch, is right next to the costomediastinal recess. If the apex of the lung was highly inflated, it might expand to fill the cupola. The hilum is the part of the lung where the structures forming the root of the lung--the main bronchus, pulmonary vessels, bronchial vessels, lymphatics, and nerve--enter and leave the lung. It does not expand on inspiration. The middle lobe of the lung may expand to fill the costomediastinal recess, but remember that the question is specifically asking about a left lung structure, and the middle lobe is only in the right lung!

Remember: Inhaled material tends to go into the right bronchus because it is bigger and more vertically orientated than the left! The superior segmental bronchus branches posteriorly off of the intermediate bronchus or the inferior lobe bronchus, so it is the segmental bronchus most likely to receive the foreign bodies that enter the right main bronchus.

The left brachiocephalic vein joins with the right brachiocephalic vein to form the superior vena cava on the right side of the body. So, the left brachiocephalic vein must course across the mediastinum to reach its destination. The left subclavian artery and vein are lateral to the mediastinum, while the left jugular and common carotid travel vertically.

The costomediastinal recess is an area right next to the cardiac notch, which is an indentation in the superior lobe of the left lung. This is where the medial area of the superior lobe of the left lung would tend to expand if it became very inflated. The lung would not enter the anterior mediastinum, which is an area between the two pleural sacs, bounded anteriorly by the sternum and posteriorly by the pericardium. The anterior mediastinum contains areolar tissue, sternopericardial ligaments, lymph vessels and nodes, but no lung tissue. The costodiaphragmatic recess is the recess at the inferior border of a lung. This is the space into which the inferior lobe of the lung would expand following deep inhalation. The cupola is the serous membrane lining the pleural cavity which extends above the level of the 1st rib into the root of the neck. The most superior portion of the superior lobe might expand into this space. The pulmonary ligament is the fold of pleura located below the root of the lung where the visceral pleura and the mediastinal parietal pleura are continuous with each other.

Because of the sharp angle of the oblique fissure, the posterior surface of the left lung is almost entirely comprised of the inferior lobe. So, a stab wound halfway between the apex and diaphragmatic surface of the lung would result in injury to the inferior lobe. The hilum is the point at which the structures forming the root of the lung enter the lung. The lingula is part of the superior lobe, which forms the anterior and superior sides of the lung. And remember, the middle lobe isn't relevant here because it is on the right lung. (Besides, it doesn't even contribute to the posterior surface of the right lung.)

Because of the sharp angle of the oblique fissures, the posterior surfaces of both the right and left lungs are almost entirely comprised of the inferior lobe--the middle lobe of the right lung is not part of the posterior surface of the lung. So, a stab wound halfway between the apex and diaphragmatic surface of the lung would result in injury to the inferior lobe. The lingula is part of the superior lobe of the left lung, which forms the anterior and superior sides of that lung. The superior lobe (along with the middle lobe) forms the anterior surface of the right lung. The superior lobe also forms the superior aspect of both lungs. The cupola is the serous membrane lining the pleural cavity which extends above the level of the 1st rib into the root of the neck--it is not part of the lung!

The horizontal fissure cuts across the right lung in such a way as to separate the middle lobe from the upper lobe. The oblique fissure separates the lower lobe from both the middle and upper lobes. Remember that the lingula is only on the left lung, and it is part of the superior lobe.

For a thoracocentesis, the needle needs to be inserted below the level of the lungs, in the costodiaphragmatic recess. At the midclavicular line, the recess is between rib spaces 6 and 8, at the midaxillary line it is between 8 and 10, and at the paravertebral line between 10 and 12. Additionally, the needle needs to be inserted at the top of the rib (or the bottom of the intercostal space). This is essential for avoiding damage to the neurovascular bundles that are found below the ribs, running in the costal groove. Taken together, these two pieces of information point to the conditions listed in D as the only appropriate ones for a thoracocentesis.

Remember that the ascending aorta is the short part of the aorta emerging from the heart before the aortic arch. The left brachiocephalic vein is the only vein listed which is anterosuperior to that part of the aorta. It crosses horizontally through the mediastinum to join with the right brachiocephalic vein and form the superior vena cava. The right brachiocephalic vein stays on the right side of the chest and would not be affected by the aortic aneurysm. The azygos vein is also on the right side of the chest, and it lays deep in the thoracic cavity. The internal thoracic vein lies on the interior surface of the anterior wall of the chest. Although it drains into the brachiocephalic vein, it would not be affected by the aneurysm. The left superior intercostal vein crosses the aortic arch laterally, but its blockage would only affect drainage of intercostal spaces 2-4 on the left side.

The oblique fissure cuts across the right lung in such a way as to separate the lower lobe from both the middle and upper lobes. So what does the horizontal fissure do? It separates the middle lobe from the upper lobe. Remember that the lingula is only on the left lung, and it is part of the superior lobe.

The tracheobronchial nodes are at the tracheal bifurcation. There are three groups of these nodes: right superior, left superior, and inferior. The aorta loops over the left bronchus, near the tracheal bifurcation. So, it is reasonable to assume that any nerve that is closely associated with the aorta might be irritated if the tracheobronchial lymph nodes became inflamed. This means that the correct answer is the left recurrent laryngeal nerve, which loops under the aorta to ascend to the larynx.

The right and left phrenic nerves are lateral and would not be irritated by the inflammation. The right recurrent laryngeal nerve loops around the right subclavian artery and is not close enough to this area. Finally, the right vagus is not closely associated with the aorta.

The structures at the root of the lung have different relationships in the right and left lungs. On both sides, the pulmonary veins are anterior and inferior while the bronchus is posterior. The difference between the two sides involves the pulmonary arteries. On the right side, the arteries are anterior to the bronchus, while on the left side the arteries are superior to the bronchus. The right lung is slightly larger than the left lung, and the lingula is found in the left lung only. Neither lung is in the mediastinum--the mediastinum is the space between the two pleural sacs. Finally, the phrenic nerve passes anterior to the root of the lung--on both the left and right sides.

The right bronchus receives blood from a single right bronchial artery. This artery may branch from one of the left bronchial arteries or it may branch from the right 3rd posterior intercostal artery, the first intercostal artery that arises from the descending aorta. Damaging this artery might stop the blood supply to the main bronchus. The intercostal arteries to the first and second intercostal spaces are derived from the highest intercostal artery, so the blood supply to either of these spaces would not be disrupted. The left bronchus is supplied by two left bronchial arteries which branch directly from the descending aorta. The fibrous pericardium is a fibrous sac that contains the pericardial cavity and the heart. Its blood supply is not a major concern