Page 33 - Cardiac Nursing

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CHAPTER 1 / Cardiac Anatomy and Physiology 9

working atrial myocardial cells, the penetrating AV bundle, and goes inferiorly and posteriorly across the left ventricular inflow
the branching AV bundle. 19,20 tract to the base of the posterior papillary muscle; it then spreads
Fibers from the AV node converge into a shaft termed the bun- diffusely through the posterior inferior left ventricular free wall. It
dle of His (also called the penetrating AV bundle or common bun- is approximately 20 mm long and 6 mm thick. This fascicle is of-
dle). It is approximately 10 mm long and 2 mm in diameter. 18 ten the least vulnerable segment of the ventricular conducting sys-
The bundle of His passes from the lower right atrial wall anteri- tem because of its diffuseness, its location in a relatively protected
orly and laterally through the central fibrous body, which is part nonturbulent portion of the ventricle, and its dual blood supply
of the fibrous skeleton. (Table 1-1).
As first noted by His in 1893, 21 the His bundle provides the Three, rather than two, major divisions of the left bundle
only cellular connection between the atria and ventricles and is of branch are sometimes found, with a group of fibers ramifying
pivotal functional importance. Cardiac impulse transmission is from the left posterior fascicle and terminating in the lower sep-
20
slowed at this site, providing time for atrial contraction to dispel tum and apical ventricular wall. This trifascicular configuration
blood from the atria into the ventricles. This slowing boosts ven- of the bundles explains some conduction defects involving partial
tricular volume and increases the cardiac output during subse- bundle-branch block. Sometimes instead of three discrete bundles
quent ventricular contraction. At the membranous septal region the common left bundle fans out diffusely along the septum and
of the heart, the right atrium and left ventricle are opposite each the free ventricular wall. 28
other across the septum, with the right ventricle in close proxim- Purkinje fibers, first described in 1845, form a complex net-
22
ity. Three of the four cardiac valves are nearby. Thus, pathology work of conducting tissue ramifications that provide a continua-
of the fibrous skeleton, tricuspid, mitral, or aortic valves can af- tion of the bundle branches in each ventricle. 29 The Purkinje
fect functioning of one or more of the other valves or may affect fibers course down toward the ventricular apex and then up to-
cardiac impulse conduction. Dysfunction of the AV conducting ward the fibrous rings at the ventricular bases. They spread over
tissue may affect the coordinated functioning of the atria and the subendocardial ventricular surfaces and then spread from the
ventricles. endocardium through the myocardium; thus, spreading from in-
Abnormal accessory pathways, termed Kent bundles, occasion- side outward, providing extensive contacts with working myocar-
ally join the atria and ventricles through connections outside the dial cells, and coupling myocardial excitation with muscular con-
main AV node and His bundle. 23,24 Tracts from the His bundle to traction.
upper interventricular septum (termed paraspecific fibers of Ma-
haim) sometimes occur and are also abnormal. 25,26 AV conduc-
tion is accelerated when impulses bypass the delay-producing AV CORONARY CIRCULATION
junction and travel instead through these abnormal connections.
When accelerated AV conduction occurs, cardiac output often de- The heart is continuously active. Like all tissues, it must receive
creases because there is inadequate time for atrial contraction to oxygen and metabolic substrates; carbon dioxide and other
boost ventricular filling. 27 wastes must be removed to maintain aerobic metabolism and
The His bundle begins branching in the region of the crest of contractile activity. However, unlike other tissues, it must gener-
the muscular septum (Fig. 1-11). The right bundle branch typi- ate the force to power its own perfusion. The heart requires con-
cally continues as a direct extension of the His bundle. The right tinuous perfusion.
bundle branch is a well-defined, single, slender group of fibers
approximately 45 to 50 mm long and 1 mm thick. It initially Coronary Arteries
courses downward along the right side of the interventricular
septum, continues through the moderator band of muscular tis- The major coronary arteries in humans are the right coronary
sue near the right ventricular apex, and then continues to the artery and the left coronary artery, sometimes called the left
base of the anterior papillary muscle. If a small segment of the main coronary artery. These arteries branch from the aorta in the
bundle is damaged, the entire distal distribution is affected be- region of the sinus of Valsalva (Figs. 1-12 and 1-13). They ex-
cause of the right bundle’s thinness, length, and relative lack of tend over the epicardial surface of the heart and branch several
arborization. times. The branches usually emerge at right angles from the par-
The left bundle branch arises almost perpendicularly from the ent artery. 30 The arteries plunge inward through the myocardial
His bundle as the common left bundle branch. This common left wall and undergo further branching. The epicardial branches
bundle, approximately 10 mm long and 4 to 10 mm wide, then exit first. The more distal branches supply the endocardial (in-
divides into two discrete divisions, the left anterior bundle branch ternal) myocardium. The arteries continue branching and even-
and the left posterior bundle branch. The left anterior bundle tually become arterioles, then capillaries. Partially because the
branch, or left anterior fascicle, is approximately 25 mm long and blood supply originates more distally, the endocardium is more
3 mm thick. It usually arises directly from the common left bun- vulnerable to compromised blood supply than is the epicardial
dle after the origin of the posterior fascicle and close to the origin surface.
of the right bundle. It branches to the anterior septum and courses There is much individual variation in the pattern of coronary
over the left ventricular anterior (superior) wall to the anterior artery branching. In general, the right coronary artery supplies the
papillary muscle, crossing the aortic outflow tract. Anterior and right atrium and ventricle. The left coronary artery supplies much
septal myocardial infarctions and aortic valve dysfunction often of the left atrium and ventricle. The following discussion describes
affect the left anterior bundle branch. the most common arterial pattern. Table 1-1 lists the major car-
The large, thick, left posterior bundle branch, or left posterior diac structures, their usual arterial supply, and some common
fascicle, arises either from the first portion of the common left variations (e.g., either the right or the left coronary artery may
bundle or from the His bundle directly. The left posterior fascicle supply the AV node).