Middle Mediastinum

In this lecture we are going to talk about the middle mediastinum . si This is the bifurcation of the pulmonary trunk , but here the pulmonary trunk has been removed si 1- Pericardium This diagram summarizes the embryogenesis of the pericardium :- 1- imagine that the heart is invaginating the pericardial sac BUT never entering the pericardial cavity (as if you are putting your finger (the heart ) inside a balloon ( the pericardium ) without your finger reaching the air inside the balloon ) 2- as the heart invaginates it takes with it this layer and we would call this layer the visceral layer 3- and what has been left here we would call it the parietal layer Both the Parietal layer and the the visceral layer are called Serous pericardium ( the two layers are two serous membranes ) D 4- As the heart gets more and more inside this balloon , the space between the visceral and parietal layer ( pericardial cavity ) is decreasing until these two layers reach a potential space { it is not a true space ,if there is an inflammation this space will be filled with exudate or pus and then a cardiac tamponade would develop ( to be discussed later on ) } , so if we dissect the pericardium we would see that it is adherent to the heart. So , we have a visceral layer which adherent to the heart and the parietal layer and between them we have a potential pericardial cavity which contains a small amount of serous fluid secreted by these two serous membranes (lubricating fluid ; to reduce the friction between them ) . This concept (having a parietal and visceral layer ) is true for many organs in our body ( e.g. the intestines , the lungs , and even it is true for the BRAIN if we consider the dura matter and Pia matter are two layers of fibrous tissue to protect the brain itself ) 5- And because the heart is beating all the time , the friction with nearby structures has to be minimized to its lowest possible range , and this is achieved by having another sac that surrounds the Parietal layer and this sac is called The Fibrous Pericardium ( sac ) { a tough fibrous tissue } In dissecting the pericardium we will notice that the outer layer of the pericardium ( the fibrous pericardium ) is rough , however the inner side ( the parietal layer of the serous pericardium ) is smooth , so when we open the fibrous sac we will be inside the pericardial cavity immediately ( the Fibrous pericardium is completely adherent to the parietal layer of the serous pericardium !!) It is considered part of the heart . So we can call it the Visceral layer of the serous pericardium Or Epicardium ( as it is a very thin layer covering the outer surface of the heart ) * we give the heart a very smooth protection from the serous balloon (serous pericardium : The Parietal and Visceral layers ) and the heart moves protected by the pericardial cavity between these two layers in one ballon * Now the balloon and the heart are protected by the tough fibrous pericardium and we can connect this fibrous sac by nearby structures , so the fibrous is the one that gets fixed to nearby structures and inside it is the heart moving . So this what makes the pericardium move without friction and also to be fixed to in this area . - g ) We will talk about them later on in the embryology of the heart , but simple :The heart in the embryogenesis was a tube ( not pyramidal in shape as we are familiar with ) with two ends (arterial end superiorly and it is fixed to the aorta and the venous end inferiorly ) , as the heart gets larger and larger in size the lower part would be free to move and fold , as a result , part of the balloon which covers the heart ( Serous sac ) gets trapped between the arterial and venous end forming the Transverse sinus and another sinus The Oblique sinus . So , anterior to the surgeon’s finger ( his finger in the Transverse sinus ) is the Arterial end and behind his finger is the Venous end Whenever we are talking about nervous supply of an internal organ we have to consider facts about these organs ; do you need these organs to feel somatic pain ? Do you need these organs to be totally autonomic ( muted ) ? And in each system you will see different presentations of these options , example the Pericardium here has two sacs :1- The Parietal peritoneum ( Fibrous pericardium ) ; should be similar to the somatic representation in you body . 2- The visceral Pericardium ( Serous pericardium ) ; which is attached to the heart , so what comes from this area is similar to the organ itself . Now lets differentiate between the Somatic pain and the Visceral pain :A- Somatic pain : localized pain Like a patient comes to you with his finger indicating the source of the pain B- Visceral pain : it is a vague pain , the patient talks about an area of pain he will nerve be able to exactly define this pain (VIP concept to evolve the right diagnosis ) ① 2- another example of referred pain along the supraclavicular nerve from the phrenic nerve is Cholecystitis Explanation : The diaphragm has two surfaces: thoracic and abdominal. The thoracic diaphragm is in direct contact with the lungs (pleura) and pericardium, while the abdominal diaphragm is in direct contact with the liver, stomach, and spleen The diaphragm is shaped as two domes, with the right dome positioned slightly higher than the left because of the liver and the gallbladder is touching the diaphragm Sensory innervation (pain and proprioception) at the central tendinous part is innervated by the phrenic nerves. d Now , if we have Cholecystitis ( the inflammation of the gallbladder ) and this inflammation irritated the peritoneum around the the diaphragm , the pain can travel thorugh the phrenic nerve to its origin which is the same origin of the supraclavicular nerve that supplies the skin over the shoulder region . So , if a patient came to you with a right shoulder pain , think of these possibilities :1- True right shoulder pain , so we examine the shoulder , if it is fine we should think about another two conditions :2- Referred pain from the pericardium ( Pericarditis ) 3- Referred pain from the Gallbladder ( Cholecystitis ) Hint , if a fat female patient in her 40s came with a right shoulder pain it is mainly referred pain from the gallbladder ( Cholecystitis ) If a patient came to you with a left shoulder pain ,in addition to true shoulder pain consider Pericarditis only . h Why the Cardiac Tamponade is dangerous ? Because the heart is a pump needs to contract and relax , while in this case it is surrounded by fluids within a very compressed compartment both ventricles won’t be able to contract .