EKG Rhythms – Reading the Graph
Welcome to this video tutorial on EKG interpretation. An electrocardiogram, also known as an ECG or EKG, is a graphic representation of the electrical activity going on within the heart. Electrodes placed on the patient’s body detect tiny electrical changes on the skin that arise from the heart muscle depolarizing with each heartbeat. A standard 12-lead EKG machine uses electrodes that consist of a positive and negative terminal to make up an EKG lead.
A common type of continuous monitoring is known as telemetry, which requires no cables. Electrical impulses are transmitted through an antenna to a monitor usually at the nurses’ station. EKG tracing is recorded on graph paper divided into millimeter squares. Each horizontal millimeter square represents 0.04 second, and each thick line, or large square, represents 0.2 second. 5 large squares equals 1 second, and 300 large squares equals 1 minute. Vertically, each small square is 1 millimeter in height, and represents 0.1mV of voltage, or amplitude.
Each large square is 0.5mV, or 5 millimeters. Graph paper is normally marked in 3 second intervals, making it easier to look at a 6 second interval, and multiply by 10 to figure the heart rate. Let’s look at a couple of definitions. The initiation of a cardiac impulse begins with the process of depolarization. The cell is electrically activated, or excited, and excites adjacent cells, thus conducting cardiac impulses from one cell to another. Repolarization is the process by which the cell is returned to the resting state.
It does not actively trigger any mechanical activity, such as muscular contraction or relaxation. It is just an electrical process in which the resting potential is restored. Now we’re going to look at a breakdown of the phases on an EKG tracing. The P wave indicates atrial depolarization—the atria are contracting, pumping blood into the ventricles.
The PR interval is from the beginning of the P wave to the beginning of QRS complex. It is the time between atrial depolarization and the start of ventricular depolarization, or conduction—it indicates the transit time for that electrical signal to travel from the sinus node to the ventricles. The QRS complex is measured from the beginning of the Q wave to the end of the S wave. It indicates the conduction of impulse through the Bundle of His to Purkinje fibers, causing the contraction of ventricles—which is ventricle depolarization, or contraction.
Atrial repolarization, or relaxation, is also occurring, it just doesn’t show up on our EKG as it’s covered up by the QRS complex. The ST segment is the end of the QRS to the beginning of the T wave—it indicates the heart’s resting period. The T wave indicates ventricular repolarization—the ventricles return to their resting state. In the QT interval, this is from the beginning of the Q wave to the end of the T wave.
It indicates the total time for ventricular depolarization and repolarization. U wave follows the T wave and represents the final phase of ventricular repolarization, and it may or may not be present. There are 8 steps to interpreting an EKG. First, look at the rhythm. Examine the R to R intervals for ventricular rhythms, or the P to P intervals for atrial rhythms.
Look at the rate. Count the QRS complexes over a 6 second interval and multiply by 10, or over 3 seconds and multiply by 20, and your heart rate will be in beats per minute. The P wave represents atrial depolarization, and is normally 0.06 to 0.11 seconds long, or 1.5 to 2.75 boxes. It’s smooth, rounded, and upright.
Ask yourself, “Are they present? Do they occur regularly? Is there 1 P wave for every QRS complex? Are they smooth, rounded, and upright? Do they all have similar shapes?” The PR interval indicates AV conduction time, normally 0.12 to 0.20 seconds, or 3 to 5 small boxes in adults and longer in the elderly. The interval shortens with an increased heart rate.
“Does it fall within that normal range of 0.12 to .2 seconds? Is it constant? If they vary, is there a steady lengthening to the point where an expected QRS does not appear?” Next, look at the QRS complex. This is ventricular depolarization, or the contraction of ventricles. It is normally 0.06 to 0.12 seconds, or 1.5 to 3 boxes. “Does it fall within the normal time range? Are they similar in appearance across the EKG tracing?” Next, look at the ST segment. This represents the early part of ventricular repolarization. It’s normally flat relative to baseline. The T wave indicates repolarization of the ventricles.
Take note if they have a negative, or downward deflection, or tall pointed peaks. The last step is to look at the QT interval. This represents the time of all ventricular activity, including depolarization and repolarization. It is normally 9 to 11 small boxes in duration, but varies with patient gender, age, and heart rate.
Let’s review the main features of the EKG interpretation. The P wave is the electrical signal that causes both atria to contract—atrial depolarization. The QRS complex is the electrical signal that causes both the ventricles to contract—ventricle depolarization. The T wave is the electrical signal that causes the ventricles to relax—or ventricle repolarization. Thank you for watching this video tutorial on EKG interpretation.