Monroe Community College
Rochester, New York
EMERGENCY MEDICAL SERVICES TRAINING COURSES
PQRST Wave Genesis in the Normal Heart.
by Peter Bonadonna, EMT-P
Paramedics today, routinely perform and interpret 12 Lead ECGs with or without physician consultation.
To be competent at reading 12 Lead ECGs and to have a better assessments of
arrhythmia strips, it is necessary to learn the anatomy, physiology, and intricate movements
of electrical flow in the normal heart. One can only recognize abnormal
when they thoroughly know normal and all of its variations. Below is a diagram
that describes the different types of cells at work in the heart. ALWAYS REMEMBER THAT
WHAT WE SEE ON THE ECG IS PRODUCED ONLY BY THE BLUE ELECTRICAL CELLS!
- Pacer (purple) Cells that have the ability to self excite and produce a small electrical wave
- Conductive Cells that have the ability to transmit an electrical signal
a. Nerve (black)
b. Myocardial electrical (blue)
c. Calcium neuromuscular transmitter (orange)
- Motor (contractile heart muscle) (red/green) Cells that contract and eject blood out of a chamber
The Sino-Atrial Node or SA Node.
Term described: Sinus = cavity and cyclical timing, Atrial = top
chamber of the heart
The SA Node is located at the connection of the superior
venacava and right atrium at the anterior most portion of this juncture.
The SA Node is a specialized group of pacer cells.
It is a small structure considering the important nature of its function.
The membranes of these cells leaks sodium back into the cell faster than
any other cell (see article on cellular electrical physiology).
As a result the threshold or trip point is sooner
than all other cells of the heart. Having the fastest rate is what grants
the SA Node the privilege of ruling the heart. When the SA Node depolarizes
nothing is recorded on the surface electrocardiogram. The Impulse formed
by the SA Node propagates out of the node and moves in an inferior, anterior,
leftward direction. in essence, it moves in the direction of the left
leg. Because of the direct connection to the Right Atrium, and due to the
poor atrial conduction system, the right atrium depolarizes slightly before
the left. If you divide the sinus P wave in any lead, the first half will
be right atrium and the second half will be left atrium. (see diagram)
The AV Node is stimulated when the wave of depolarization reaches the
bottom of the right atrium. Note on the diagram that AV stimulation occurs
in the middle of the P wave, not at the end of the P. The left atrial depolarization
heads more laterally and posteriorly than the right atrium. This explains
why the P wave in V1 or MCL1 is biphasic (first positive and then negative).
see illustration. Between the SA node and AV node are "tracts"
that electrically favor the wave of depolarization in atrial tissue. These
pathways called internodal or intranodal pathways have never been proven
by histologic or electron microscope. We think they exist because of direct
myocardial electrical mapping. (see illustration for locations and names
of these pathways).
The Atrio-Ventricular Node or AV Node.
Term described: Anatomic location between the atrium and ventricles.
The AV Node is a large collection of cells located in the inferior right
atrium. It use to be mistakenly believed that the AV Node had pacemaker
ability. It does not! The AV Node is merely a receiving station to sense
the atrial depolarization and delay the wave propagation before shipping
it down the HIS-purkinje network. This delay is very important to the proper
timing between atrial contraction and ventricular contraction. This proper
timing can account for 20% of the hearts cardiac output. The AV node and
the bundle of HIS can be viewed like a long head of hair. See illustration.
When the AV Node is stimulated, the electrical activities contained within,
are not recorded on the surface electrocardiogram. The impulse is delayed
and then sent silently down the HIS, bundle branch, purkinje network. When
the electrical impulse emerges at the myofibril level, a wave of depolarization
begins in myocardial electrical cells. This wave front can be measured
on the EKG and produces a narrow QRS. The QRS is narrow because when depolarization
travels the His-purkinje network, all of the electrical cells are activated at
once. This shortens ventricular depolarization time to about 0.08 seconds in
If ventricular depolarization occurs cell by cell using the gap junctions (intercalated disks)
then the time it takes to completely activate the entire myocardial mass will increase. This
is evidenced by a wide QRS.
This image illustrates primary ventricular activation (like a PVC or Pacer)
More to come..................
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Updated: February 12, 2001