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Lab 4 - Mediastinum and heart

Pre-lab Exercise: Surface Anatomy of the Heart

Primary Lab Objectives and Goals:

  1. Remove the anterior chest wall, including the sternum, ribs and intercostal muscles.

  2. Examine the visceral surface of the ventral thoracic wall taking special note of the vessels on that surface.

  3. Locate the phrenic nerves.

  4. Examine the pericardium and pericardial sinuses to understand the structures that surround and protect the heart, including their clinical relevance.

  5. Examine the heart and great vessels in situ and consider the path of blood into and out of the heart.

  6. Locate the left vagus nerve.

  7. Map out the common sites to hear heart valve sounds.

  8. Remove the heart.

  9. Examine the coronary arteries.
  10. 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.

 

Procedure:

1. ANTERIOR CHEST WALL

To remove the anterior chest wall:

  1. Clear away any remnants of the pectoral muscles adhering to the rib cage.
  2. 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).
  3. 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). 
  4. From this point, cut each rib along its lateral surface using either a saw or hand held garden shears.
  5. 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.

Atlas Images:

 

 

2. INTERIOR SURFACE OF THE ANTERIOR CHEST WALL.

  1. Examine the interior surface of the anterior chest wall.
  2. Identify the internal thoracic vessels, and the transversus thoracis muscle.

NOTE: the sternocostal joints tend to ossify over time, so they may no longer be synovial in your cadaver.

c. On an articulated skeleton in the lab identify the articular surfaces of the costovertebral and costotransverse joints.

 

Atlas Images:

FUNCTIONAL ANATOMY: 

The space enclosed between the left and right pleural sacs, called the mediastinum, is divided into the superior and inferior mediastinum. The latter is further subdivided into the anterior, middle, and posterior mediastinum.

  • The anterior mediastinum lies between the sternal body and pericardium. It contains only a little extra-pleural fat and some lymph nodes associated with the internal thoracic vessels.
  • The superior and middle mediastinum can be found in the plane behind the anterior mediastinum. The superior mediastinum contains the thymus, trachea, esophagus, and great vessels of the heart. In most adults, the thymus is reduced to a mass of fat containing small amounts of thymic tissue.
  • The middle mediastinum contains the heart, pericardium, and the roots of the great vessels.
  • The posterior mediastinum contains the esophagus, thoracic aorta, azygous and hemiazygous veins, thoracic duct, and vagal trunks.

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.

 

Atlas Images:

 

 

 

 3. PHRENIC NERVES

  1. Locate the phrenic nerve running through the middle mediastinum. To find the phrenic nerve, use your fingers or a blunt probe to explore the fibrous pericardial sac next to the parietal pleura. The phrenic nerves can bbe found embedded in the pericardium.

    The phrenic nerve supplies motor and sensory innervation to the diaphragm, along with sensory innervation to the pericardial sac. 

  2. Trace the phrenic nerves down to the diaphragm. Leave them intact and pull them aside so they are not damaged when you remove the heart.

    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.

     

Note: the fibrous pericardium has been cut open to reveal the heart, which we will do in the following step.

Atlas Images:

 

 

 

4. PERICARDIAL SAC

  1. 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 along the incision lines indicated in the image below (the two layers will be effectively inseparable) PLAY MOVIE.
  2. Explore the inside of the pericardial sac with your hand.
 

 

 

 

5. THE HEART AND GREAT VESSELS IN SITU

  1. While the heart is still in situ, locate the superior and inferior vena cava.
  2. 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).
  3. Examine the ligamentum arteriosum connecting the pulmonary trunk and the arch of the aorta. 

Atlas images:

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.

 

6. THE VAGUS NERVES

  1. In the superior mediastinum attempt to find the vagus nerves where they give rise to recurrent branches that loop around the great vessels of the heart. You may need to free up the cut edge of the manubrium and reflect it superiorly in order to see these structures.
  2. Identify the left vagus nerve.
  3. Trace its left recurrent laryngeal branch around the ligamentum arteriosum. It passes dorsal to the ligament from lateral to medial.
  4. Continue to follow the left recurrent laryngeal branch superiorly towards the larynx following a path between trachea and esophagus.
  5. At this point, the right vagus nerve will be difficult to find because the right recurrent laryngeal nerve loops around the right subclavian artery. The recurrent laryngeal nerves travel superiorly into the neck to supply the larynx.

7. HEART VALVE SOUNDS

Reflect the chest wall back into place and map out the sites commonly used to hear heart valve sounds:

  1. At the apex of the heart in the mid left 5th intercostal space for the mitral valve (left atrioventricular valve);
  2. At the left sternal edge in the 5th intercostal space for the tricuspid valve (right atrioventricular valve);
  3. At the left sternal border in the left 2nd intercostal space for the pulmonary valve;
  4. At the right sternal border in the right 2nd intercostal space for the aortic valve.

Atlas Images:

 

8. REMOVE THE HEART

  1. Cut the aorta and pulmonary trunk just above the locations of the aortic and pulmonary valves (PLAY MOVIE).
  2. Cut the superior vena cava just above where it enters the right atrium.
  3. Cut the inferior vena cava where it emerges from the diaphragm.
  4. Lift up on the apex of the heart to retract it forward and cut the four pulmonary veins (2 on each side) at their attachment to the left atrium.
  5. Remove the heart.

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.

Atlas Images:

 

9. THE CORONARY VESSELS

a. Examine the removed heart.

b. 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.


c. To see the coronary vessels, use blunt dissection to remove fat surrounding the heart and mobilize the vessels.


d. Start your dissection of the coronary arteries from the aortic valve by identifying the right, left, and posterior semilunar cusps.


e. Insert the tip of a probe into the opening of the left coronary artery in the left aortic sinus. Palpate the tip of the probe, which should now be between the aorta and the pulmonary trunk, to find the short stem of the left coronary artery.

The left coronary artery divides to form:

  1. The anterior interventricular (left anterior descending, LAD) artery, which descends obliquely in the anterior interventricular sulcus, supplying mainly the left ventricle and most of the interventricular septum. It then circles around the apex of the heart towards the posterior interventricular sulcus to anastomose with the posterior descending artery (a branch of the right coronary artery).
  2. The circumflex artery, which enters the left atrioventricular sulcus and circles around the left margin of the heart onto the diaphragmatic surface. It mainly supplies the left atrium and left ventricle.

Both the LAD and circumflex arteries are accompanied by the great cardiac vein, which becomes the enlarged coronary sinus on the diaphragmatic surface of the heart. The coronary sinus returns the blood into the right atrium via an opening medial to the IVC.

f. Find the opening of the right coronary artery in the right aortic sinus. Insert the tip of a probe into its opening and palpate the tip on the surface of the heart.


 

The right coronary artery runs in the right atrioventricular sulcus, giving off branches to the sinoatrial (SA) node (circling behind the SVC) and to the chambers on either side (right atrium and right ventricle). Its main branches are:

  1. The right marginal branch at the inferior (acute) border of the heart, accompanied by the small cardiac vein, for the apical portions of the right ventricle.
    The right coronary artery circles around the inferior border of the heart towards the diaphragmatic surface. Here it anastomoses with the circumflex artery, which branches from the left coronary artery.
  2. The posterior interventricular branch courses along the posterior interventricular sulcus to the apex of the heart, where it anastomoses with the anterior interventricular branch of the left coronary artery. The posterior interventricular branch is accompanied by the middle cardiac vein.
  3. The artery to the atrioventricular (AV) node also arises from the right coronary artery where the posterior interventricular sulcus meets the coronary sulcus (diaphragmatic surface).

Common variation: Note that a heart with a posterior interventricular branch from the right coronary artery is said to be "right coronary dominant." Although this is the most common pattern (about 70% of the population), sometimes the posterior interventricular artery is a branch of a larger circumflex artery (branch of the left coronary artery). In these cases, the heart is then "left coronary dominant." Try to determine if your cadaver has a right or left coronary dominant artery.

 

10. THE HEART AND ITS CHAMBERS

  1. Locate the inferior and superior vena cava.  Review the parts of the body drained via each of these channels  
  2. Pass a probe through the SVC into the right atrium and out through the IVC. 
  3. Cut a backwards "C"-shaped flap in the right atrium (cuts 1, 2, and 3 in the figure below) and study its inner surface. 

Atlas Images:

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.

Atlas Images:

d. 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 interventricular sulcus. 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, make sure to avoid the left anterior descending artery).

 

In the right ventricle:

  • Wash out any clotted blood and locate the papillary muscles and septomarginal trabecula.
  • Note how the chordae tendineae of the papillary muscles attach to the three cusps of the right atrioventricular (tricuspid) valve
  • The cordae tendineae 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.

Atlas Images:

FUNCTIONAL ANATOMY:

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.

 

f. 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

Atlas Images:

g. 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.

Atlas Images:

FUNCTIONAL ANATOMY:
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.

 

 Readiness Assessment.

  1. Cutting through the rib cage can require effort and can be dangerous because you can cut yourself on the cut-edge of ribs.  It is important to be prepared.  What part of the sternum will you cut through and where will you cut the ribs?

  2. The phrenic nerve is a very important nerve to dissect out.

    2a. What muscle does it innervate? 

    2b. What surface will you find it crossing? 

    2c. Should you use your scalpel while dissecting it?

 

 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. 

  1. What are the spinal cord contributions to the phrenic nerve (where does it come from)? 
  2. 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.

  1. What does the right recurrent laryngeal nerve loop around? 
  2. What does the left recurrent laryngeal nerve loop around?
  3. 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. 

  1. Why is the left ventricular wall of the heart thicker than the right?
 

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