Figure 2-4. Standard graph paper measurements. A Graph paper. B Enlargemen
of one square of graph paper.
c. Electrical Impulses of Electrocardiogram Waves. Each part of the cardiac
cycle produces a different electrical impulse. The electrical flow of the heart starts with
the SA node (right atrium) and continues to the Purkinje fibers (ventricles). Electrodes
transmit impulses to a recording pen which graphs the impulses in a series of up and
down waves called deflection waves. The cardiac cycle includes all of the wave
patterns produced by electrical activity beginning with the pacemaker impulses and
including ventricular repolarization. An isoelectric line occurs when there is no current
strong enough to produce either a positive or negative deflection. The positive and
negative forces are equal with the result that a flat line is shown (usually following the
"T" wave). An electrical force (from the heart) toward the positive electrode will draw
the stylus in an upright wave while an electrical force toward the negative electrode will
draw the stylus in a downward wave. A single cardiac cycle is expected to produce one
heartbeat. Deflections above or below the isoelectric line are called waves. Each wave
is labeled with a letter. The waves are called the P wave, QRS complex, and the T
wave. The letters were arbitrarily selected and do not stand for any words. Here are
the waves:
(1) P wave. A small upward (positive) wave that indicates atrial polarization
(the spread of an impulse from the SA node through the muscle of the two atria). The
atria contract a fraction of a second after the P wave begins.
MD0571
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