Section 26. Disorders of the Cardiovascular System >
Part 4. Diagnostic Tools in Heart Disease >
Chapter 493. ElectrocardiographyGeorge F. Van Hare
Topics Discussed: cardiac conduction system; cardiology; conduction, cardiac; electrocardiogram; electrocardiographic axis; electrodes, electrocardiographic; qrs axis; scalar value; vectorcardiography; ventricular depolarization.
Sections: Approach, P Wave, Q Wave, QRS and T Waves, Ventricular Hypertrophy, ST Segment and T Waves, References.
Excerpt:"Normal electrical activation of the heart begins pacemaker activity
in the sinoatrial node, and the wave of activation spreads through
the right and left atria (Fig. 493-1). In
the right atrium the wave of depolarization passes inferiorly, and the
left atrium is activated via Bachmann bundle, which also triggers
an inferiorly directed activation front. These activation fronts
generate a potential that is detected on the body surface as the P
wave. Any force that has magnitude and direction is termed a vector and
can be represented by an arrow with direction and magnitude proportional
to the force. The impulse is delayed at the atrioventricular (AV)
node, producing the PR interval. This allows ventricular filling
to be completed before ventricular contraction begins. Beyond the
AV node, the impulse moves rapidly down the bundle of His into the
right and left bundle branches. As the impulses pass down the septum,
they activate septal muscle predominantly from the left side, so
that the initial ventricular vector passes from left to right, anteriorly
and superiorly (Fig. 493-2), and begins the
Q wave in lead V6 or the first part of the R wave in lead
V1.1 After reaching the apex, the impulse
activates the ventricular free walls from endocardium to epicardium
and from apex toward the base, thus inscribing the R and S waves;
the last part of the heart to be activated is the posterior ventricular
muscle just under the AV ring. In adults and older children, there is more left than right..."
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