Atrial Fibrillation/Mitral Stenosis Case File

Atrial Fibrillation/Mitral Stenosis Case File

Eugene C. Toy, MD, Lawrence M. Ross, MD, PhD, Han Zhang, MD, Cristo Papasakelariou, MD, FACOG

CASE 13

A 35-year-old Hispanic woman comes to your office tired and complaining of shortness of breath and fatigue. Her history is unremarkable except for a vague history of fever and joint pain as a child in Mexico. She notes some recent fatigue and difficulty sleeping that she attributes to job-related stress. On examination, her heart rate is 120 beats/min, and the rhythm has no discernible pattern (is irregularly irregular). Auscultation of the heart indicates a systolic murmur (during left ventricular ejection of blood) that is harsh in character.

⯈ What is the most likely diagnosis?

⯈ What is the underlying etiology?

ANSWER TO CASE 13:

Atrial Fibrillation/Mitral Stenosis

Summary: A 35-year-old Hispanic woman complains of fatigue. She had fever and joint pain as a child in Mexico. On examination, her heart rate is 120 beats/min and irregularly irregular. Cardiac examination shows a harsh systolic murmur.

• Most likely diagnosis: Atrial fibrillation due to left atrial enlargement

• Underlying etiology: Mitral stenosis due to rheumatic heart disease

CLINICAL CORRELATION

This 35-year-old woman most likely has atrial fibrillation with tachycardia that is irregularly irregular. The electrical impulse originating from the sinoatrial (SA) node of the right atrium does not depolarize both atria in a regular, orderly manner; instead, this patient’s atria receive constant electrical stimulation, leading to almost continual atrial contraction that visually resembles a bag of worms. The irregular character of the pulse is the result of inconsistent transmission of the electrical impulse to and through the atrioventricular (AV) node and then onto the two ventricles. One common cause of atrial fibrillation is left atrial enlargement. In this patient, the history of childhood fever and joint pain likely is the result of streptococcally caused rheumatic fever. If untreated, the microorganism can cause inflammation of the mitral valve, leading to mitral stenosis. After 3-5 years, the mitral stenosis is likely to worsen, leading to atrial enlargement, fibrillation, and pulmonary edema with intolerance to physical exertion. Treatment in this patient would focus on decreasing her heart rate with an agent that acts on the AV node such as digoxin. Oxygen and diuretics would relieve her pulmonary symptoms. An ultimate goal will be conversion of her cardiac contractions to a normal sinus rhythm. Anticoagulation is often warranted in the face of long-term atrial fibrillation because of the likelihood of intracardiac thrombus and the possibility of emboli after conversion to sinus rhythm, called the “atrial stunning” effect. Surgical correction of the mitral stenosis is also important.

APPROACH TO:

Cardiac Conduction System

OBJECTIVES

1. Be able to describe the type of tissue that makes up the cardiac conduction system

2. Be able to describe the locations and functions of the SA node, the AV node, the AV bundle (of His), and the right and left bundle branches

3. Be able to describe the nature of sinus rhythm and the influence of the divisions of the autonomic nervous system on this rhythm

4. Be able to describe the anatomy of the four cardiac valves

DEFINITIONS

MURMURS: Soft or harsh abnormal heart sounds, often caused by turbulent blood flow, and described in relation to the phase of the cardiac cycle in which they are heard

ATRIAL FIBRILLATION: Rapid, uncoordinated muscular twitching of the atrial wall

TACHYCARDIA: A heart rate of at least 100 beats/min

DISCUSSION

Cardiac Conduction System

The conduction system of the heart is composed of specially modified cardiac muscle cells. It initiates and rapidly conducts cardiac impulses throughout the heart to produce cardiac muscle contraction. The system ensures the simultaneous contraction of both atria, followed by a similar coordinated contraction of both ventricles.

The SA node, composed of these modified cardiac muscle cells, lies within the atrial wall on the right side of its junction with the superior vena cava (SVC). This can be located at the superior end of the external landmark, the sulcus terminalis. The SA node spontaneously depolarizes to initiate the cardiac conduction impulse and thus is often referred to as the heart’s pacemaker. The impulse generated by the SA node spreads through the atrial wall to converge on the AV node and produces simultaneous atrial contraction. Anterior, middle, and posterior internodal pathways of very rapid conduction are described (Figure 13-1).

The AV node is a somewhat smaller mass of modified cardiac muscle cells located in the interatrial septum, immediately superior to the opening of the coronary sinus. The AV bundle (of His) arises from this node and lies within the membranous portion of the interventricular septum. It courses toward the apex of the heart, and at the upper portion of the muscular portion of this septum, it divides into right and left bundle branches. The bundle branches lie on their respective sides of the septum just beneath the endocardium. The bundles then divide to form a subendocardial plexus of Purkinje fibers. The right bundle is described as supplying the interventricular septum, the anterior papillary muscle (reached by the septomarginal or moderator band), and the wall of the right ventricle. The left bundle supplies the interventricular septum, anterior and posterior papillary muscles, and the wall of the left ventricle.

The SA node or pacemaker typically will depolarize at a rate of approximately 70 times per minute. This rate is referred to as a sinus rhythm. The SA node is innervated by fibers of the sympathetic and parasympathetic divisions of the autonomic nervous system. Stimulation of the SA node by sympathetic nerve impulses increases the rate of depolarization of the SA node, and stimulation by parasympathetic fibers decreases this rate.

Figure 13-1. Cardiac conduction system: 1 = sinoatrial node, 2 = anterior internodal pathway, 3 = middle internodal pathway (Wenckebach bundle), 4 = posterior internodal pathway, 5 = atrioventricular node, 6 = atrioventricular bundle of His, 7 = moderator band, 8 = right bundle branch, 9 = terminal conducting fibers of Purkinje, 10 = left bundle branch. (Reproduced, with permission, from the University of Texas Health Science Center Houston Medical School.)

Cardiac Valves

The outflow from the two atria and the two ventricles is guarded by the AV and the semilunar valves, respectively. The leaflets of these cardiac valves and the myocardial muscle fibers are attached to the fibrous cardiac skeleton. This structure consists of four fibrous rings to which the leaflets attach, the right and left fibrous trigone, and the membranous portion of the interventricular septum.

The right AV or tricuspid valve between the right atrium and right ventricle consists of anterior, posterior, and septal leaflets or cusps. Tendinous cords attach to the margins of adjacent valve cusps and prevent separation and inversion (prolapse) of the leaflets into the atrium during ventricular contraction. The proximal attachment of the tendinous cords is to conical projections of cardiac muscle called papillary muscles; there are three papillary muscles, named anterior, posterior, and septal, like the cusps. The tendinous cords of the anterior papillary muscle attach to the anterior and posterior cusps. Those of the posterior papillary muscle attach to the posterior and septal cusps, and the cords of the septal papillary muscle attach to the septal and anterior cusps.

The left AV or bicuspid (mitral) valve between the left atrium and ventricle consists of anterior and posterior cusps. The tendinous cords of the anterior and posterior papillary muscles (which are larger because of the increased pressure demands) are attached to adjacent cusps and function in a manner similar to that described for the tricuspid valve.

The outflow from the right and left ventricles is guarded by the pulmonary and aortic semilunar valves, respectively. Both semilunar valves are similar in structure; both are circular in shape and consist of three cuplike cusps, with the opening to these cups directed superiorly. The space formed is called the pulmonary or aortic sinus, so named for the cusp that creates it. As blood is ejected from the ventricles, the cusps lie close to the pulmonary or aortic wall. At the end of contraction, the elasticity of the vessel walls results in backflow of blood that fills the sinuses, resulting in apposition of the three cusps and closure of the valves. The cusps of the pulmonary semilunar valve are the anterior, right, and left cusps, and the aortic valve has right, left, and posterior cusps. The right and left coronary arteries arise from the aorta at the right and left aortic sinuses, respectively.

COMPREHENSION QUESTIONS

13.1 As a pathologist, you are examining the heart of a victim of fatal trauma and note a tear at the junction of the SVC and the right atrium. This tear would likely damage which of the following?

A. SA node

B. AV node

C. AV bundle

D. Right bundle branch

E. Left bundle branch

13.2 As a pathologist, you must examine the AV bundle histologically. In which of the following tissue samples will you find the AV bundle?

A. Right atrium

B. Left atrium

C. Interatrial septum

D. Membranous interventricular septum

E. Muscular interventricular septum

13.3 A 57-year-old man develops a myocardial infarction and is noted to have a heart rate of 40 beats/min. The cardiologist diagnoses an occlusion of the right coronary artery. Which of the following structures is most likely to be affected?

A. AV node

B. Bundle of His

C. Purkinje fibers

D. Mitral valve

ANSWERS

13.1 A. The SA node or pacemaker lies within the right atrial wall, where it is joined by the SVC.

13.2 D. The AV bundle is located in the membranous portion of the interventricular septum.

13.3 A. An inferior wall myocardial infarction involving the right coronary artery may affect the AV node, leading to bradycardia.

ANATOMY PEARLS

⯈ The cardiac conduction system is composed of specially modified cardiac muscle cells (not nervous tissue).

⯈ The SA node is the pacemaker that spontaneously produces a sinus rhythm of 70 beats/min. It lies at the junction of the SVC and the right atrium.

⯈ The AV node lies in the interatrial septum, and the AV bundle and the right and left bundle branches lie in the membranous and muscular portions of the interventricular septum, respectively.

⯈ Stimulation of the SA node by sympathetic nerve impulses increases its rate of depolarization, whereas parasympathetic impulses decrease its depolarization rate.

References

Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy, 2nd ed. New York, NY: Thieme Medical Publishers; 2012:89−93. 

Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy, 7th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014:135−150, 159. 

Netter FH. Atlas of Human Anatomy, 6th ed. Philadelphia, PA: Saunders; 2014: plates 215−223.

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