PqRST Wave Genesis in the Normal Heart.
by Peter Bonadonna, EMT-P
Paramedics, Physician Assistants and Nurse Practitioners 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!
a. Nerve like (black) Really specialized muscle that acts like nerves i.e. bundle of his and bundle branches
b. Myocardial electrical cells (blue)
c. Calcium neuromuscular transmitter (orange)
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 the adult.
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, VT, IVR, or a paced ventricle)