Waves and Measurements

We have learned that arrhythmias are manifestations of the electrical activity of the heart. You have learned that electrocardiography is the study of the heart’s electrical activity. We must transform the electrical activity into a form that can be seen in order to study it.

Electrodes Can be attached to the skin Then connected to a machine that will display the electrical activity on graph paper.

Electrical activity is displayed best if you assure good contact between the electrode and the skin by: Abrading the skin slightly. Removing any obstacles like dirt or hair Using a contact medium, like saline or commercial gel.

When the EKG machine is turned off: The writing point (stylus) will produce a straight line. This line is called the isoelectric line. No current is flowing, and the electrical forces are equal.

Once the EKG machine is connected to the patient and turned on: The current is flowing, and the needle will move above or below the isoelectric line.

Rule of Electrical Flow States that “if the electricity flows toward the positive electrode, the patterns produced on the graph paper will be upright. If the electricity flows away from the positive electrode (or toward the negative electrode), the pattern will be a downward deflection”.

Review The electrical current is flowing toward the positive electrode. This will produce a positive deflection. True or False?

TRUE

Monitoring Leads We must be sure that the electrodes are always in same place on patients. They are always positioned the same, or standardized.

The positioning of electrodes for monitoring will allow you to see the electrical pattern of the heart for a single view. By rearranging the electrodes, you can get several views.

Lead Each view of the heart is called a lead. Leads can be changed by a knob on the EKG monitor.

True or False? Leads can be changed by simply using a knob on the EKG machine. One lead shows only one view of the heart.

Leads… A combination of electrodes that reflects the flow of electricity between two points on opposing sides of the heart. Bipolar- one positive and one negative Unipolar-single positive, and a neutral opposing location situated to reference the center of the heart.

Single leads that give good pictures of the basic waves are called monitoring leads, because they are used to monitor patterns such as arrhythmias. The most common one is Lead II. MCL1 is also common.

Positioning of Lead II Negative (Below right clavicle) Positive (Apex) Ground

Positioning of MCL1 Negative (below left clavicle) Positive Ground (4th intercostal, r. sternal border) Ground

The primary deflection in Lead II is positive or upright. Why?

Because… The electrical current is flowing from the SA node in the right atrium to toward the ventricles. The current is going in the direction toward the positive electrode.

Graph Paper It is standardized and uniform. The size of the graph and the speed at which the paper moves through the EKG machine are always constant. This allows us to have a “norm” and a comparison.

Graph Paper The lines on the graph paper can help determine the direction and the magnitude of the deflections. Voltage-compare to horizontal lines Time-compare to vertical lines

What is “voltage”? It is the strength of the current. The height of the deflection will indicate the voltage(amplitude). Measured by the horizontal lines of the graph paper.

Cardiac Cycle Heart has 4 chambers. 2 atria at the top work together 2 ventricles at the bottom work together In the normal heart, blood will enter both atria simultaneously and it is then forced into both ventricles simultaneously while the atria contract.

How do we measure “time” on the graph paper? The vertical lines tell you how much time it took for the electrical current within the heart to travel from one area to another. Standard rate is 25 millimeters per second. It takes .20 seconds to get from one heavy vertical line to the next. The distance (in time) between two light vertical lines is .04 seconds.

Chambers of the Heart

Remember… Before the atria can contract, an electrical current must first stimulate the muscle cell. For a myocardial cell to contract it must first receive an electrical stimulus. During each phase of the cardiac cycle, a distinct pattern is produced on the EKG graph paper.

What makes up a cardiac cycle? It begins with the initiating impulse from the pacemaker and includes all phases until the ventricles are repolarized. You will see on the graph paper all of the wave patterns that this causes.

Waves… The deflections above or below the isoelectric line. In one cardiac cycle, there are 5 prominent waves. P, Q, R, S, T The area between waves is called an interval. A straight line (inactivity) between waves is called a segment.

P Wave The first wave. It starts with the first deflection from the isoelectric line. This wave is indicative of atrial depolarization(current is discharging or flowing).

True or False? If you see a P wave, this means that the atria contracted.

FALSE It does mean that an electrical current stimulated it, but the muscle cell might not have contracted in response to it. You cannot tell by simply looking at the EKG. You have to use also the mechanical function assessments. How do you do that?

When the impulse leaves the atria and goes to the AV node, there is a slight delay. This is because the tissues of the node conduct impulses slower than other myocardial tissue. This is the PR segment (inactivity).

The SA node, the atria, and the ventricles all conduct impulses faster than the AV node. This allows time for the atrial contraction and complete filling of the ventricles.

PR Interval All of the electrical activity before it goes to the ventricles.

QRS Complex Remember…the PR interval begins at the first sign of the P wave and ends at the first deflection of the next wave. This wave is called the QRS complex.

True or False? The PR interval includes all atrial activity and all nodal activity, but does not include ventricular activity.

TRUE

What does QRS wave indicate? Ventricular depolarization Larger than the P wave because ventricular depolarization involves greater muscle mass than atrial depolarization.

Components of QRS Q wave is negative deflection, follows the P wave, but before R wave R wave is first positive deflection following the P wave S wave is the second negative deflection after the R wave.

Review What does the QRS complex indicate?

It indicates… Ventricular depolarization.

Another wave? Yes… T wave, ventricular repolarization. NOTE: The atria repolarize as well, but it is usually hidden in the QRS complex. Why? Because their repolarization usually occurs at the same time as ventricular depolarization.

Normal Measurements PRI, between .12 and .20 seconds, otherwise it is “short” or “prolonged” A long PRI would suggest a delay in the AV node, not a P wave or atrial problem

What about the ST segment and T wave? The section between the S wave and the T wave. Most segments are straight lines, but the ST is usually not because of the transition between the QRS complex and the T wave.

How long is a QRS complex? Anything less than .12 seconds. This would mean that the ventricles took a normal amount of time to depolarize if they did it in less than .12 seconds.

Now that you know the waves… PRACTICE, PRACTICE, PRACTICE!!!