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Disturbances Of The Heart In General
Source: Disturbances Of The Heart
Of prime importance in the treatment of diseases of the heart is a
determination of the exact, or at least approximately exact,
condition of its structures and a determination of its ability to
This is not the place to describe its anatomy or its nervous
mechanism or the newer instruments of precision in estimating the
heart function, but they may be briefly itemized. It has now been
known for some time that the primary stimulus of cardiac contraction
generally occurs at the upper part of the right auricle, near its
junction with the superior vena cava, and that this region may be
the "timer" of the heart.
This is called the sinus node, or the sino-auricular node, and
consists of a small bundle of fibers resembling muscle tissue. Lewis
[Footnote: Lewis: Lecture in the Harvey Society, New York Academy of
Medicine, Oct. 31, 1914.] describes this bundle as from 2 to 3 cm.
in length, its upper end being continuous with the muscle fibers of
the wall of the superior vena cava. Its lower end is continuous with
the muscle fibers of the right auricle. From this node "the
excitation wave is conducted radially along the muscular strands at
a uniform rate of about a thousand millimeters per second to all
portions of the auricular musculature."
Though a wonderfully tireless mechanism, this region may fall out of
adjustment, and the stimuli proceeding from it may not be normal or
act normally. It has been shown recently not only that there must be
perfection of muscle, nerve and heart circulation but also that the
various elements in solution in the blood must be in perfect amounts
and relationship to each other for the heart stimulation to be
normal. It has also been shown that if for any reason this region of
the right auricle is disturbed, a stimulus or impulse might come
from some other part of the auricle, or even from the ventricle, or
from some point between them. Such stimulations may constitute
auricular, ventricular or auriculoventricular extra contractions or
extrasystoles, as they are termed. In the last few years it has been
discovered that the auriculoventricular handle, or "bundle of His,"
has a necessary function of conductivity of auricular impulse to
ventricular contraction. A temporary disturbance of this
conductivity will cause a heart block, an intermittent disturbance
will cause intermittent heart block (Stokes-Adams disease), and a
prolonged disturbance, death. It has also been shown that
extrasystoles, meaning irregular heart action, may be caused by
impulses originating at the apex, at the base or at some point in
the right ventricle.
In the ventricles, Lewis states, the Purkinje fibers act as the
conducting agent, stimuli being conducted to all portions of the
endocardium simultaneously at a rate of from 2,000 to 1,000 mm. per
second. The ventricular muscle also aids in the conduction of the
stimuli, but at a slower rate, 300 mm. per minute. The rate of
conduction, Lewis believes, depends on the glycogen content of the
structures, the Purkinje fibers, where conduction is most rapid,
containing the largest amount of glycogen, the auricular musculature
containing the next largest amount of glycogen, and the ventricular
muscle fibers the least amount of glycogen.
Anatomists and histologists have more perfectly demonstrated the
muscle fibers of the heart and the structure at and around the
valves; the physiologic chemists have shown more clearly the action
of drugs, metals and organic solutions on the heart; and the
physiologists and clinicians with laboratory facilities have
demonstrated by various new apparatus the action of the heart and
the circulatory power under various conditions. It is not now
sufficient to state that the heart is acting irregularly, or that
the pulse is irregular; the endeavor should be to determine whit
causes the irregularity, and what kind of irregularity is present.
Next: Clinical Interpretation Of Pulse Tracings