Primary Lab Objectives and Goals:
Attempt an injection into the right subclavian vein.
Remove the anterior chest wall, including the sternum, ribs and intercostal muscles.
Examine the visceral surface of the ventral thoracic wall taking special note of the vessels on that surface.
Identify the 4 regions of the mediastinum and locate the phrenic and vagus nerves. It is important here to consider what is in the mediastinum and what is not.
Examine the pericardium and pericardial sinuses to understand the structures that surround and protect the heart, including their clinical relevance.
Examine the heart and great vessels in situ and consider the path of blood into and out of the heart.
Remove the heart and dissect each of the four chambers starting with the right atrium. Make note of the differences between chambers and the functional significance of each chamber.
1. SUBCLAVIAN VENIPUNCTURE
A common procedure used when rapid fluid resuscitation and/or rapid blood replacement therapy is indicated is the placement of a central venous line either in the internal jugular or subclavian vein. Subclavian central lines often have a lower risk of infection and are better tolerated by conscious patients. However, there is a risk of pneumothorax if the vein is missed and air is introduced into the pleural cavity. In this exercise, you will attempt to inject a small amount of paint into the right subclavian vein.
- Draw up a small bolus of paint.
- Locate the midpoint of the right clavicle (mid-way between the sternal notch and AC joint) and place the needle tip 1 cm lateral and inferior to the clavicle.
- Aim the needle medially in the direction of the suprasternal notch at a shallow angle (~15 degrees) relative to the skin and begin inserting the needle such that it passes just under the clavicle.
- Once the clavicle is passed, continue to advance in a plane almost parallel to the skin approximately 2-3 cm and then inject a small amount of paint.
2. ANTERIOR CHEST WALL
To remove the anterior chest wall:
- Clear away any remnants of the pectoral muscles adhering to the rib cage.
- Using either a hand or power saw, cut through the manubrium of the sternum inferior to the insertion of the sternal head of the sternocleidomastoid muscle (solid red line in figure below).
- Cut through the muscles of the first intercostal space, extending the cut laterally about 4 inches to where the ribs begin to turn posteriorly (solid black lines below).
- From this point, cut each rib along its lateral surface using either a saw or hand held garden shears.
- The ventral thoracic wall (still attached to the diaphragm and abdominal muscles) can now be folded down over the abdomen to expose the contents of the thorax.
HINT– you may find when you try to reflect the rib cage down that the parietal pleura is firmly adhered to the endothoracic fascia and the internal thoracic wall. You may need to use your fingers, a blunt probe, or even a scalpel to separate this tissue.
Saw along heavy black line, and fold rib cage down.
3. INTERIOR SURFACE OF THE ANTERIOR CHEST WALL.
- Examine the interior surface of the anterior chest wall.
- Identify the internal thoracic vessels, and the transversus thoracis muscle.
- Open a sternocostal joint and verify that these joints are usually synovial.
NOTE: the sternocostal joints tend to ossify over time, so they may no longer be synovial in your cadaver.
- On an articulated skeleton in the lab identify the articular surfaces of the costovertebral and costotransverse joints.
The space enclosed between the left and right pleural sacs, called the mediastinum, is divided into 4 regions: anterior, posterior, superior, and middle:
- The anterior mediastinum contains only a little extra-pleural fat and some lymph nodes associated with the internal thoracic vessels.
- Above this, in the ventral edge of the superior mediastinum, lies the thymus. In most adults, the thymus is reduced to a mass of fat containing small islands of thymic tissue. The superior mediastinum also contains the trachea, esophagus, and the great vessels of the heart.
- The middle mediastinum contains the heart, pericardium, and the roots of the great vessels.
- The posterior mediastinum contains the esophagus and thoracic aorta.
You should inspect all these spaces carefully and thoroughly when you have accessed them. The posterior mediastinum will need to wait until you remove the heart and lungs.
4. PHRENIC AND VAGUS NERVES
- Locate the phrenic and vagus nerves running through the middle mediastinum.
There are two very important nerves to examine in this region; the phrenic and the vagus nerves. The phrenic nerve supplies motor and sensory innervation to the diaphragm, along with sensory innervation to the pericardial sac. The vagus nerve is the 10th cranial nerve. It supplies parasympathetic autonomic innervation to all the thoracic organs, including the heart and lungs.
The phrenic nerves are the more lateral set of nerves, and run on the outer surface of the fibrous pericardial sac, trapped between this layer and the parietal pleura.
- Trace the phrenic nerves down to where they enter the diaphragm. Note how they contact the outer surface of the pericardial sac. Leave them intact and pull them aside.
NOTE – Do not use your scalpel here – you will cut through the phrenic nerve. With your fingers, probe into the space between the pericardium and the parietal pleura and feel around for the nerve.
- In the superior mediastinum attempt to follow the vagus nerves down to where they give rise to recurrent branches that loop around the great vessels of the heart. The left recurrent laryngeal nerve loops around the aorta at the ligamentum arterosum. The right recurrent laryngeal nerve loops around the right subclavian artery. The recurrent laryngeal nerves travel superiorly into the neck to supply the larynx.
HINT - you will be able to see the recurrent laryngeal nerves much better once the pericardial sac has been opened and even better when the heart has been removed. To find the recurrent laryngeal nerve on the left side (the only side you can see it in the thorax because it hooks under the aorta) you will have to examine the space between the aorta and pulmonary trunk.
For now, just try to get a general sense of their location and course.
Note: the fibrous pericardium has been cut open to reveal the heart, which we will do in the following step.
5. PERICARDIAL SAC
- After moving aside the phrenic nerves, use sharp-pointed scissors to cut through the fibrous pericardium and the parietal layer of serous pericardium of the heart (the two layers will be effectively inseparable) PLAY MOVIE.
- Explore the inside of the pericardial sac with your hand.
6. THE HEART AND GREAT VESSELS IN SITU
- While the heart is still in situ, locate the superior and inferior vena cava.
- Identify the aortic arch and pulmonary trunk on the superior and left side of the heart as well as the other external features of the heart (PLAY MOVIE).
- Examine the ligamentum arteriosum connecting the pulmonary trunk and the arch of the aorta.
- Identify the left vagus nerve.
- Trace its left recurrent laryngeal branch around the ligamentum arteriosum. It passes dorsal to the ligament from lateral to medial.
- Continue to follow the left recurrent laryngeal branch superiorly towards the larynx following a path between trachea and esophagus.
NOTE -The visceral pericardium is reflected onto the parietal pericardium along two convoluted lines: one enclosing the outflow (pulmonary trunk and aorta), and one enclosing the inflow of the superior vena cava and pulmonary veins. The space that is created between the outflow and inflow is the transverse pericardial sinus. To explore this sinus, pass a probe behind the aorta but in front of the superior vena cava: the tip of the probe will be within the transverse sinus. The two lines of pericardial reflection surrounding the transverse pericardial sinus correspond to the primitive cranial and caudal ends of the pericardial sheath around the embryonic heart.
- Reflect the chest wall back into place and map out the sites commonly used to hear heart valve sounds:
- At the apex of the heart in the mid left 5th intercostal space for the mitral valve (left atrioventricular valve);
- At the left sternal edge in the 5th intercostal space for the tricuspid valve (right atrioventricular valve);
- At the left sternal border in the left 2nd intercostal space for the pulmonary valve;
- At the right sternal border in the right 2nd intercostal space for the aortic valve.
7. THE HEART AND ITS CHAMBERS
- Sever the aorta and pulmonary trunk just above the locations of the aortic and pulmoanry valves (PLAY MOVIE).
- Sever the superior vena cava just above where it enters the right atrium.
- Sever the inferior vena cava where it emerges from the diaphragm.
- Lift up on the apex of the heart to retract it forward and sever the four pulmonary veins at their attachment to the left atrium.
- Remove the heart, leaving the intact phrenic nerves behind.
HINT– You will need to make sure that you carefully cut through all vessels connecting the heart to the lungs and the great vessels. You may find it easiest to find the IVC first and cut this. There is only a very short course of the IVC above the diaphragm before it enters the heart. Find this short trunk and cut it cleanly. You can then work your hand over to each side to find and cut the pulmonary vessels. The ascending aorta should be cut above the valve and the coronary arteries. Then find and cut the SVC.
- Examine the removed heart.
- Identify the four chambers and locate the interventricular and atrioventricular grooves or sulci. For the most part the coronary arteries travel in these grooves to supply the heart muscle.
- Locate the inferior and superior vena cava. Review the parts of the body drained via each of these channels
- Pass a probe into the SVC into the right atrium.
- Cut a backwards "C"-shaped flap in the right atrium (cuts 1, 2, and 3 in the figure below) and study its inner surface.
In the right atrium:
- Wash out any clotted blood and identify the pectinate muscles with their roughened surface.
- Locate the opening of the coronary sinus and pass a probe into it
- Palpate the probe in the coronary sinus on the posterior (basal) surface of the heart.
- Locate the crista terminalis.
- Examine the fossa ovalis, and probe to see if it is still patent.
- To open the right ventricle of the heart make a short transverse cut just below the pulmonary trunk (cut 4). The cut should span across the right ventricle from the lower margin of the A-V sulcus to the inteventricular suclus. Next, cut along the lower margin of the A-V sulcus (cut 5) and along the right ventricular margin of the interventricular sulcus (cut 6, keeping to the right of the anterior descending coronary artery).
In the right ventricle:
- Wash out any clotted blood and locate the papillary muscles and septomarginal trabecula (which transmits a fascicle of the atrioventricular bundle to the right wall of the ventricle and the anterior papillary muscle).
- Note how the chordae tendineae of the papillary muscles attach to the three cusps of the right atrioventricular (tricuspid) valve.
- Visualize how this arrangement prevents the cusps, which are flexible flaps of smooth muscle and connective tissue, from being forced into the right atrium during ventricular contraction.
- Make a longitudinal cut in the pulmonary trunk to open and inspect the three cusps of the pulmonary valve.
The pulmonary and aortic valves are not attached to papillary muscles or chordae tendineae. Some other mechanism is needed to prevent expelled arterial blood from forcing its way back into the expanding ventricles. The saccular shape of the cusps in the aortic and pulmonary semilunar valves accomplishes this task. As the expelled blood attempts to return to the ventricles it fills the cusps of the valve, thus pressing the edges of the three cusps tightly together, closing the valve.
- Turn the heart to the right, identify the left atrium and probe the openings of the pulmonary veins. There are normally 4 veins, two from each lung. Open the left atrium by making a U-shaped cut (as shown below) along the margins where the pulmonary veins enter the left atrium.
In the left atrium:
- Wash out any clotted blood and examine where the pulmonary veins enter the wall of the left atrium
- Examine the fossa ovalis in the interatrial wall
- To open the left ventricle, make a longitudinal incision through the aorta (in between the left and right cusps of the aortic valve) into the left ventricle, and extend this cut along the left ventricular margin of the interventricular sulcus (i.e. keeping to the left of the anterior descending coronary artery) to the apex of the heart (cut 7 in the figure below). A transverse cut (cut 8) just below the aorta and extending toward the left margin of the heart will help to open the wall of the ventricle.
In the left ventricle and aorta:
- Compare the right and left ventricles and the thickness of the walls
- Locate the papillary muscles and anterior and posterior cusps of the mitral valve.
- Open the aorta and identify the left and right coronary arteries arising from the left and right aortic sinuses.
Both ventricles contain internal trabeculae, papillary muscles, and chordae tendineae. The apex of the left ventricle bears more trabeculae and its wall is considerably thicker compared to the right ventricle. In addition, the left atrioventricular valve has only two cusps. These two cusps (which vaguely resemble a bishop’s miter) give the left atrioventricular valve its two interchangeable names, the bicuspid or mitral valve.
Clinical Questions to consider
1. The phrenic nerve runs from multiple ventral rami of the spinal cord to the diaphragm. If a patient suffers a neck injury he or she can sustain damage to those ventral rami.
- What are the spinal cord contributions to the phrenic nerve (where does it come from)?
- What happens when an accident victim's spinal cord is severed above C4?
2. When patients suffer a stroke that affects their vagus nerve (Cranial Nerve X, which emerges from the brain and travels through the thorax and into abdomen) this can lead to weakness in speech because the recurrent laryngeal nerve loops back up (cranially) to innervate the larynx (hence its name). Patients can also suffer from difficulty moderating speech if the vagus nerve or its recurrent branch are damaged by tumors or trauma in the area of the thorax where the recurrent branch can be found branching away from the vagus. Thus it is important this location in the thorax. But it differs on the left and right sides of the body for embryological reasons.
- What does the right recurrent laryngeal nerve loop around?
- What does the left recurrent laryngeal nerve loop around?
- What is the embryological basis for the courses of the right and left recurrent laryngeal nerves?
3. Cardiac output is a function of stroke volume and muscle force. Patients who experience diminished cardiac output may suffer from weakness of the heart muscle, fluid in the pericardium that restricts, blood flow or resistance along paths of blood out of the heart. The right and left ventricle have to send blood to different areas with different levels of resistance.
- Why is the left ventricular wall of the heart thicker than the right?