Lab 4 Pre-Lab Exercise:
1. Surface anatomy of the thorax
The ability to visualize anatomical structures in the thorax in relation to the surface features is fundamental to a physical exam. Palpable surface landmarks can be used to visualize the normal outlines of the heart and all other structures found in the mediastinum. With the help of your teammates and using the images below, find the following surface anatomy landmarks of the thorax to help you identify the content of the mediastinum, the borders of the hearts, and the locations to listen for heart sounds.
2. Surface anatomy of the mediastinum
The T4-T5 vertebral level is a transverse plane that passes through the sternal angle on the anterior chest wall and the intervertebral disc of T4-T5. Therefore at the sternal angle represents this very important T4-5 level where: the costal cartilage of 2 articulates with the sternum, the superior mediastinum is separated from the inferior mediastinum, the ascending aorta end and the arch of the aorta begins, the arch of the aorta ends and the thoracic aorta begins, and the trachea bifurcates. Superiorly, the trachea begins at the lower border of the cricoid cartilage, at the level of C6. It is approximately 10cm long with an external diameter of 2cm. It lies in the median plane and funs almost vertically.
A number of structures in the superior mediastinum in adults can be visualized based on their positions relative to the skeletal landmarks. On each side, the internal jugular and subclavian veins join to form the brachiocephalic veins behind the sternal ends of the clavicles near the sternoclavicular joints. The left brachiocephalic vein crosses from left to right behind the manubrium. The brachiocephalic veins unite to form the superior vena cava behind the lower border of the costal cartilage if the right first rib. The arch of the aorta begins and ends at the transverse plane between the sternal angle anteriorly and the vertebral level of T4-5 posteriorly. The arch may reach as high as the mid-level of the manubrium.
3. Surface anatomy of the heart
The heart rests on the diaphragm and is covered anteriorly by the body of the sternum and the 3rd-6th costal cartilages of both sides. More specifically, the upper limit of the heart reaches as high as the 3rd costal cartilage on the right side of the sternum and the 2nd intercostal space on the left side of the sternum (both 1.2cm from the sternal border). The right margin of the heart extends from the right 3rd costal cartilage to near the right 6th costal cartilage (at the 6th chondro-sternal junction). The left margin of the heart descends laterally from the 2nd intercostal space to the apex located near the midclavicular line in the 5th intercostal space. The lower margin of the heart extends from the sternal end of the right 6th costal cartilage to the apex in the 5th intercostal space near the midclavicular line (or 9cm from the midline).
The right border of the heart is formed by the right atrium. The vena cavae enter the heart at the upper and lower ends of the borders. The lower border of the heart is mainly the right ventricle but includes some left ventricle at the apex. It lies at the end of the xiphisternal joint. The lower 4/5 of the left border of the heart is formed by the left ventricle and the upper 1/5 by the left atrium. The upper border is formed by both atria. The superior vena cava enters the heart at its right end, the ascending aorta crosses the middle of the border, the pulmonary trunk bifurcates just above the left end, and the right pulmonary artery runs to the right just above the border.
The chambers of the heart can me marked out by drawing:
The coronary sulcus: separating the atria and the ventricles from the upper medial end of the 3rd left costal cartilage to the middle of the right 6th chondro-sternal joint.
The anterior interventricular sulcus: from the 3rd left intercostal space 2.5cm to the left of the midline to a point 1.2cm medial to the apex.
4. Surface anatomy of the orifices and where to listen for heart sounds
To listen for valve sounds, you should position your stethoscope downstream from the flow of blood through the valves. The tricuspid valve is almost vertical and centered at the 4th intercostal space just to the right of the midline. It can be heard just to the left of the lower part of the sternum near the 5th intercostal space. The mitral valve is oblique, running down and right, starting opposite the 4th costal cartilages and lying beneath the left side of the sternum. It can be heard over the apex of the heart in the left 5th intercostal space at the midclavicular line. The pulmonary valve is horizontal and centered at the 3rd left chondro-sternal joint. It is heard over the medial end of the left 2nd intercostal space. The aortic valve is oblique, running down and right, starting from the medial end of the 3rd left intercostal space. It can be heard over the medial end of the right 2nd intercostal space.