If you have any questions about the EP Lab, Let us know. Cardiac Physiology
1. Cardiac Cell Properties
a. Automaticity is the mechanism that generates new depolarizations and thus causes the heart to beat. Cells in the sinus node will usually have the fastest automaticity and will thus, control the heart rate. If the SA node automaticity fails to produce a depolarization, alternate cells in the heart will produce depolarizations, but at a slower rate.
b. Excitability refers to the ability of a cell to respond to an outside stimulus.
c. Refractoriness is the property that prevents a cell from depolarizing, or responding to a new stimulus. These three properties help regulate how cardiac cells function.
2. Action Potential:
The action potential is a representation of the changes in voltage of a single cardiac cell. While there are some differences in the action potentials of various types of cardiac tissue (discussed below), the model below is most commonly used for education purposes. This action potential is based on the purkinjie fibers. (Information in this section was collected from the book, “Electrophysiologic Testing”, by Richard N. Fogoros, M.D.)
a. Transmembrane Potential – difference in voltage inside a cell when compared to the voltage outside the cell. The inside of a cell will generally be -80mv to -90mv more negative than the regions outside the cell.
b. Phase 0 – Depolarization – Rapid Na+ channels are stimulated to open, flooding the cell with positive sodium ions. This causes a positively directed change in the transmembrane potential. This shift in voltage is reflected by the initial spike of the action potential.
c. Depolarization of one cell triggers the Na+ channels in surrounding cells to open as well, causing the depolarization wave front to propagate cell by cell throughout the heart.
d. The speed of depolarization of a given cell (the slope of phase 0), determines how soon the next cell will depolarize. The interaction between the slope of the initial waveform and the time interval before the next cells depolarization is referred to as conduction velocity. By changing the rate of depolarization (slope of phase 0), you can change the conduction velocity.
e. After completion of depolarization, the cell begins to repolarize, or return to its original resting state. The cell can not depolarize again until this happens. Phases 1-3 are the repolarization phases and coincide with the time that the cell is refractory and can not respond to a new stimulus.
f. Phase 1 is the initial stage of repolarization.
g. Phase 2 is the plateau stage where the rate of repolarization is slowed by the influx of Ca+ ions into the cell. The Ca ions enter the cell slower than the Na ions and help prevent the cell from repolarizing too quickly, thus extending the refractory period (f). This mechanism helps regulate the rate at which cardiac tissue can depolarize.
h. Phase 3 is the later stages of repolarization. Once repolarization is complete, the cell will be able to respond to a new stimulus.
i. Phase 4 occurs after repolarization is complete. During this phase, known as the quiet or quiescent phase, there is no ion exchange across the cellular membrane in most cardiac cells.
j. In some cells, there is a leakage of ions across the cell membrane during phase 4. This causes a gradual increase in the transmembrane potential. When the transmembrane potential reaches the threshold voltage, it triggers depolarization to occur and a new action potential occurs.
k. This change in transmembrane potential during phase four which leads to a new depolarization of the given cell is referred to as automaticity.
3. Differences in Action Potentials:
The cells in different regions of the heart do not all have the same action potential, and thus have varying conduction velocities.
a. Sinus Node and AV Node have slower action potentials due to a lack of the rapid Na+ channels. Conduction velocity in these regions is controlled by the Ca+ channels.
b. Atrial myocardial tissue has a faster conduction velocity and a shorter refractory period than the SA or AV node.
c. Purkinjie fibers and ventricular myocardium have faster conduction velocities and a somewhat longer refractory period than atrial myocardial tissue.
4. Autonomic Innervation – Sympathetic and Parasympathetic fibers contribute to heart rate control via input which primarily comes through the Vagus nerve.
a. Sympathetic tone – Increase in sympathetic tone causes enhanced automaticity, increased conduction velocity and decreased action potential duration. This causes cardiac cells to fire more rapidly, respond to signals more rapidly and recover more quickly.
b. Parasympathetic tone – Increase in parasympathetic tone causes decreased automaticity and conduction velocity and increased refractory periods.
c. Large numbers of sympathetic and parasympathetic fibers are found in both the Sinus node and the AV node making these regions more responsive to changes in sympathetic or parasympathetic tone.
As blood leaves the lungs, it returns to the heart through the pulmonary veins. Blood flow from the veins is deposited into the left atrium where it passes through the mitral valve into the left ventricle.
The left atrium is one of the most structurally complex chambers in the heart. There are usually four pulmonary veins that connect to the posterior wall of this chamber. The area where the veins connect to the atrium is a region of intertwined venous and heart muscle tissues. Many physicians feel that stretching of the tissues where the veins connect to the atrium that happens over time may be a contributing factor to the rhythm atrial fibrillation.
Beyond the veins, the left atrium itself has many similarities to the right atrium. Below is a list of additional structures that can be found in the left atrium.
-Left Atrial Appendage:
-Atrial Septum:
- Fossa Ovalis:
- Mitral Valve:
The left ventricle is the largest and strongest chamber of the heart. This big muscle is responsible for most of the pumping action that delivers the blood throughout the body. As the left ventricle contracts, blood passes through the left ventricular outflow track, across the aortic valve and into the aorta where it is routed to smaller arteries throughout the body.
A normal heart beat originates in an area of the right atrium called the sinus node. The sinus node is located on the upper portion of the lateral wall of the right atrium, close to where the SVC, or Superior Vena Cava connects to the right atrial chamber. Specialized cells in the sinus node generate an electical signal that spreads outward throughout the heart. As this wavefront of electrical activity passes through the muscle of the heart, it causes the fibers of the muscle to first, contract and then to relax. This cycle of contraction and relaxation is the "beating" of your heart.
For most people, the heart beats between 60-100 times per minute. When your heart contracts at this rate and the singals comes from the sinus node, it is called a normal sinus rhythm. When the sinus rate occurs faster than 100 beats per minute, it is called a sinus tachycardia. A sinus rhythm that fires slower than 60 beats per minute is called a sinus bradycardia. If the rhythm originates elsewhere in the heart, it is considered an abnormal rhythm. To learn more about abnormal rhythms, select the link above, or click abnormal rhythms.
