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
ventricular muscle, so the major cardiac vectors point to the left
and posteriorly and produce a tall R wave in V6 and a deep
S wave in V1. In a normal newborn infant with a thick right
ventricle, the major cardiac vectors pass to the right and anteriorly
and produce a dominant R wave in V1 and a large S wave
in V6. After depolarization has occurred, there is slower
repolarization that produces the T wave.
Diagram of conduction system. AV, atrioventricular node;
His, bundle of His, dividing into right and left bundles; LA, left
atrium; RV and LV, right and left ventricles; SA, sinoatrial node;
SVC and IVC, superior and inferior venae cavae.
Normal ventricular depolarization, starting with 1 and
ending with 5. A: Section through ventricles
with thicker-walled left ventricle and thinner-walled right ventricle.
Arrows are vectors, indicating direction and magnitude of electrical forces
at each time. B: Vectors are superimposed
on common center, and their tips are joined by a dashed line. C: The
dashed line remains to give a vector loop. The vector at any moment
would be a line joining the central point to the corresponding part
of the loop. The arrows...
Pop-up div Successfully Displayed
This div only appears when the trigger link is hovered over.
Otherwise it is hidden from view.