Reading: Moore and Dalley pp. 114-141

THE PERICARDIUM AND HEART

I. PERICARDIUM

  1. A double-walled fibrous sac that encloses the heart and the roots of the great vessels. It occupies most of the middle mediastinum, and functions to maintain the position of the heart and protect it from overfilling.

  2. Parts

    1. Fibrous layer - a tough fibrous layer closed by attachment to the great vessels, blends with the central tendon of the diaphragm.

    2. Serous layer - a smooth inner sac with lubricated surfaces which allow movement
      1. parietal layer - lines the inner surface of the fibrous pericardium
      2. visceral layer - covers the entire surface of the heart
      3. The potential space between the parietal and visceral layers of the serous pericardium is the pericardial cavity.

    3. Clinical Correlations
      1. Pericarditis - inflammation of the pericardium. It causes substernal pain and produces pericardial effusion.
      2. Cardiac Tamponade - effusion which impairs cardiac filling, resulting in circulatory failure. Treated by pericardiocentesis.

II. HEART

  1. Review of Blood Flow

  2. Position

    1. The heart is situated obliquely in the middle mediastinum. The position and movement of the diaphragm are important factors that determine the position of the heart. Because the fibrous pericardium is attached to the diaphragm, the cardiovascular silhouette becomes longer and narrower during inspiration and shorter and broader during expiration.

  3. External Structure

    1. Described as having an apex, a base, and 3 surfaces, sternocostal, diaphragmatic and pulmonary. The base is located posteriorly and is formed mainly by the left atrium. Remember, the heart does not rest on its base.

    2. Anterior & posterior interventricular groove - divides ventricles

    3. Coronary or atrioventricular groove - separates atria from ventricles

    4. Surfaces
      1. Sternocostal - mostly right ventricle
      2. Diaphragmatic - 2/3 by left ventricle, 1/3 by right ventricle.
      3. Pulmonary - occupies the cardiac notch of the left lung.

  4. Atria

    1. Right atrium

      1. receives venous blood from the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus
      2. coronary sinus - returns much of the venous blood from the heart itself to the right atrium - lies in the posterior part of the coronary groove. The opening is guarded by a valve of the coronary sinus. It closes during contraction and prevents regurgitation.
      3. auricle - blind pocket, overlying the ascending aorta

    2. Interatrial septum

      1. fossa ovalis - oval depression where the septum is thin. This is the site of the foramen ovale in the fetus.

      2. clinical condition - atrial septal defect (ASD) - this can allow blood to shunt from the left to the right side of the heart.

    3. Left atrium

      1. forms most of the base of the heart
      2. receives the openings of the 4 pulmonary veins from the lungs.
      3. left auricle forms the superior part of the left border of the heart
      4. thrombi or blood clots form on the walls of the left atrium in certain types of heart disease.

  5. Ventricles

    1. Common structures

      1. trabeculae carnae - irregular bundles of muscle projecting on the inner surface
      2. atrioventricular valves (tricuspid and bicuspid)
      3. semilunar valves (aortic and pulmonary)

    2. Right ventricle

      1. forms most of the anterior surface, and almost all of the inferior border
      2. conus arteriosus - smooth part leading into pulmonary trunk
      3. atrioventricular or tricuspid valve
        1. cusps - 3 fibrous cusps are attached to the annulus fibrosis, a fibrous ring around both atrioventricular orifices.
        2. papillary muscles - conical projections into cavity. The papillary muscles contract prior to contraction of the right ventricle. They tighten the chordae tendenae and draw the cusps together by the time ventricular contraction begins.
        3. chordae tendinae - small tendinous bands which connect the cusps to the papillary muscles. They prevent inversion of the valve cusps into the RA during contraction of the RV.

      4. pulmonary valve - 3 semilunar cusps.

        1. clinical condition - pulmonary valve stenosis

    3. Interventricular septum

      1. position corresponds to interventricular grooves on the surface
      2. membranous part - thin upper part near atrium
      3. muscular part - thick, major part
      4. the membraneous part is the site of the most common congenital heart defect, a ventricular septal defect (VSD).

    4. Left ventricle

      1. wall thickness - about 2-3 times thicker because the left ventricle contracts against greater resistance than the RV.

      2. atrioventricle or mitral valve - two large sets of papillary muscles attach to the cusps via chordae tendinae. The mitral valve is the most frequently diseased of the heart valves.

      3. aortic valve - located in the right posterosuperior part of the LV.

    5. Auscultation of heart sounds

      1. Areas of maximum audibility

        1. pulmonary valve: left second intercostal space
        2. aortic valve: right second intercostal space
        3. mitral valve; apical region
        4. tricuspid valve: lower left sternal border

  6. Blood Supply

    1. Right coronary artery - originates in the right aortic sinus, runs in the coronary groove to the back of the heart where it gives off the posterior interventricular artery, which anastomoses with the anterior interventricular branch of the left coronary artery.

      1. branches to right atrium and right ventricle
      2. sinus node artery - most common site or origin (60%)
      3. right marginal branch - runs toward the apex of the heart
      4. in 85% of the cases the right coronary supplies the AV node

    2. Left coronary artery and branches - arises from the left aortic sinus and gives off circumflex branch and anterior interventricular artery. It typically supplies most of the LV, LA, interventricular septum, and some of the RV.

    3. Venous drainage

      1. anterior cardiac veins drain directly into the right atrium

      2. coronary sinus - receives most veins of the heart, runs in the posterior part of the coronary groove and ends in the right atrium
        1. great cardiac vein the main tribituary of the coronary sinus. Travels with the anterior interventricular artery.
        2. middle cardiac vein - travels with the posterior interventricular artery
        3. small cardiac vein - travels with the marginal branch of the right coronary

    4. Clinical Conditions

      1. Although the coronary arteries anastomose with each other at the arteriolar level, they are functional end arteries. A sudden block thus leads to necrosis of the cardiac muscle (myocardial infarction).

      2. Coronary atherosclerosis results in slow narrowing of the lumen of these arteries.

      3. Angina pectoris is a clinical syndrome characterized by substernal discomfort resulting from myocardial ischemia.

      4. A common surgical treatment for severe coronary atherosclerosis is a coronary artery bypass graft (CABG).
  7. Nerve Supply

    1. Autonomic and sensory fibers from the vagus nerve and sympathetic trunk via the cardiac plexus. Sympathetic supply arises from the cervical and upper thoracic part of the sympathetic trunk. Postganglionic sympathetic fibers terminate on the SA and AV nodes and on the coronary arteries. Postganglionic parasympathetic fibers also terminate on the SA and AV nodes and coronary arteries. Stimulation of the sympathetic nerves increases the heart rate and contractility.

    2. Referred cardiac pain - usually to left shoulder, and medial side of left arm, forearm and hand.

  8. Conducting System - consists of specialized muscle fibers and conducting fibers which are not nervous tissue. Conducts impulses that connect certain pacemaker regions of the heart with cardiac muscle fibers. Both atria contract together as do the ventricles, but atria contract first.

    1. Sinoatrial (SA) node

      1. it is the natural pacemaker of the heart
      2. location - junction of superior vena cava and right atrium
      3. the impulse for contraction begins at the SA node and is conducted throughout the atria by ordinary atrial mycardial fibers, arriving at the AV node.
      4. The SA node is supplied by both divisions of the ANS

    2. Atrioventricular (AV) node

      1. location - above the opening of the coronary sinus in the atrial septum.

    3. Atrioventricular bundle - passes from the AV node, divides into right & left limbs.

      1. This collection of specialized muscle fibers, often called Purkinje fibers conduct impulses from AV node to ventricular myocardium.

    4. Clinical considerations
      1. Artificial pacemakers are used to correct arrhythmias, abnormalities in cardiac conduction rhythms.
      2. The passage of impulses over the heart can be amplified and recorded as an electrocardiogram (ECG).