TBL11 - Mediastinum and Pericardium

Question 1

Where is the mediastinum located in relation to the thoracic cavity? What is the mediastinum divided into?

Answer 1

1) The mediastinal part of the parietal pleura demarcates the mediastinum in the central part of the thoracic cavity
2) The mediastinum is divided into the superior mediastinum and inferior mediastinum by an imaginary transverse plane extending from the sternal angle to the disc at T4/T5

Question 2

What three parts is the inferior mediastinum subdivided by the heart into?

Answer 2

1) The inferior mediastinum is subdivided by the heart into three parts:
a) the anterior mediastinum between the sternum and heart
b) the middle mediastinum containing the heart and roots of the great vessels
c) the posterior mediastinum positioned between the bodies of vertebrae T5 to T12 and the heart

Question 3

What are common causes of mediastinal widening typically discovered in chest radiographs?

Answer 3

1) It is often observed after trauma resulting from a head-on collision, for example, which produces hemorrhage into the mediastinum from lacerated great vessels, such as the aorta or SVC
2) Frequently, malignant lymphoma (cancer of lymphatic tissue) produces massive enlargement of mediastinal lymph nodes and widening of the mediastinum
3) Hypertrophy (enlargement) of the heart (often occurring due to congestive heart failure, in which venous blood returns to the heart at a rate that exceeds cardiac output) is a common cause of widening of the inferior mediastinum

Question 4

What does fusion of the pleuropericardial membranes divide the thoracic cavity into? What do these membranes form after the division?

Answer 4

1) Fusion of the pleuropericardial membranes vertically divides the thoracic cavity into the pericardial cavity and bilateral pleural cavities
2) After the division, the pleuropericardial membranes form the fibrous pericardium

Question 5

What does the fibrous pericardium cover? What two layers make up the serous pericardium?

Answer 5

1) The tough fibrous pericardium covers the serous pericardium that, like the pleura of the lungs, consists of parietal and visceral layers
2) The parietal layer lines the inner surface of the fibrous pericardium and at the origin of the great vessels, the parietal layer reflects onto the heart as the visceral layer of the serous pericardium

Question 6

Where do the parietal and visceral layers of the serous pericardium join as one to resume continuity? What structure is the pericardial sac closely associated with? What does the pericardial sac fuse with?

Answer 6

1) The continuity of the parietal and visceral layers of serous pericardium occurs at the roots of the great vessels
2) The pericardial sac is closely associated with the diaphragm
3) The sac fuses with the central tendon

Question 7

What is the transverse pericardial sinus?

Answer 7

The transverse pericardial sinus is a pathway:

a) posterior to the intrapericardial parts of the aorta and pulmonary trunk
b) anterior to the intrapericardial parts of the superior vena cava (SVC) and pulmonary veins

Question 8

Why can cardiac tamponade be fatal and how is pericardiocentesis normally performed? (Can you explain #5?)

Answer 8

1) Cardiac tamponade (heart compression), is a potentially lethal condition because heart volume is increasingly compromised by the fluid outside the heart but inside the pericardial cavity
2) If extensive pericardial effusion exists, the
compromised volume of the sac does not allow full expansion of the heart, limiting the amount of blood the heart can receive, which in turn reduces cardiac output
3) Drainage of fluid from the pericardial cavity, pericardiocentesis, is usually necessary to relieve cardiac tamponade
4) To remove the excess fluid, a wide-bore needle may be inserted through the left 5th or 6th intercostal space near the sternum. This approach to the pericardial sac is possible because the cardiac notch in the left lung and the shallower notch in the left pleural sac leaves part of the pericardial sac exposed—the bare area of the pericardium
5) The pericardial sac may also be reached via the infrasternal angle by passing the needle superoposteriorly. At this site, the needle avoids the lung and pleurae and enters the pericardial cavity; however, care must be taken not to puncture the internal thoracic artery or its terminal branches

Question 9

How do cardiac surgeons use the sinus?

Answer 9

1) After the pericardial sac is opened anteriorly, a finger can be passed through the transverse pericardial sinus posterior to the ascending aorta and pulmonary trunk
2) By passing a surgical clamp or a ligature around these large vessels, inserting the tubes of a coronary bypass machine, and then tightening the ligature, surgeons can stop or divert the circulation of blood in these arteries while performing cardiac surgery, such as coronary artery bypass grafting