Segment: The length between two specific points on an ECG that are supposed to be at the baseline amplitude not negative or positive. Complex: The combination of multiple waves grouped together. Each will be explained individually in this tutorial, as will each segment and interval. The P wave indicates atrial depolarization. The T wave comes after the QRS complex and indicates ventricular repolarization.
Back to Healio. ECG Basics Save. A lead composed of a single positive electrode and a reference point is a unipolar lead. Chest leads: , , , , , and. It is a normal tracing shows normal sinus rhythm. This course is meant to give the nurse a practical education concerning the lead EKG. We will present the basic clinical aspects of the lead.
We will demonstrate how and where to attach the leads in certain places. We will also demonstrate the basics of interpreting the results. Then all you need is practice. Each lead EKG machine will have its own instructions for use.
Be sure you are familiar with the machines at your facility. The instructions for the machine will show you how to attach the leads to the patient. In most instances, the patient electrodes will be attached with the use of either flat elastic straps or by cloth Velcro straps.
In most cases, the metal electrodes will also need to be coated with conductive gel prior to attachment to the patient. Be sure to clean the electrodes before and after each use, as gel will tend to build up. Be sure to read the entire instruction manual for the machine including: how to start the machine, how to load the paper, how to calibrate the machine if needed , and any other pertinent information needed to safely and accurately run the machine. Once you know how to operate the machine at your facility the next concept to understand is the placement of the leads.
The limb leads are usually first. As the name implies, the limb leads are attached to the four limbs. This is usually accomplished by attaching the leads, according to instructions, on each wrist and each ankle.
As mentioned earlier, the electrodes will be sensing the electrical impulses from the heart muscle at ht various locations and with various voltages, either positive or negative. The lead EKG tracing below was obtained with universal lead placement. These marks are for the purpose of showing the leads as they are changed. Every time you see a blip mark, the next lead is being recorded on the tracing. Location of the Frontal Plane Axis: In order to accurately interpret the lead EKG, you must have an understanding of the electrical activity of the heart.
The direction in which the impulses flow in the heart is important. It is also important to understand that 12 different leads pick up those impulses as they travel in many different directions through the heart. Remember that the normal conduction of the heart begins in the SA node. The wave of depolarization moves across the atria, through the AV node, into the Bundle of HIS, down the Bundle Branches, and finally through the Purkinje fibers which conduct the electrical impulses throughout the ventricles.
The activity of the heart produces electrical potentials that can be measured on the surface of the skin. Using the galvanometer EKG machine , differences between electrical potentials at different sites of the body can be recorded. WCT is computed by connecting all three limb electrodes via electrical resistance to one terminal.
This terminal will represent the average of the electrical potentials recorded in the limb electrodes. WCT serves as the reference point for each of the six electrodes which are placed anteriorly on the chest wall. The chest leads are derived by comparing the electrical potentials in WCT to the potentials recorded by each of the electrodes placed on the chest wall.
There are six electrodes on the chest wall and thus six chest leads Figure Each chest lead offers unique information that cannot be derived mathematically from other leads. Since the exploring electrode and the reference is placed in the horizontal plane, these leads primarily observe vectors moving in that plane.
Hair on the chest wall should be shaved before placement of electrodes. This improves quality of the registration. The ECG leads may be presented chronologically i. Chronological order does not respect that leads aVL, I and -aVR all view the heart from a similar angle and placing them next to each other can improve diagnostics. The Cabrera system should be preferred. In the Cabrera system, the leads are placed in their anatomical order.
As mentioned earlier, inverting lead aVR into —aVR improves diagnostics additionally. All modern ECG machines can display the leads according to the Cabrera system, which should always be preferred. Note the clear transition between the waveforms in neighbouring leads. There are conditions that may be missed when utilizing the lead ECG. Fortunately, researchers have validated the use of additional leads to improve diagnostics of such conditions.
These are now discussed. Infarction of the right ventricle is unusual but may occur if the right coronary artery is occluded proximally. None of the standard leads in the lead ECG is adequate for diagnosing right ventricular infarction.
However, V1 and V2 may occasionally display ECG changes indicative of ischemia located in the right ventricle. In such scenarios, it is recommended that additional leads be placed on the right side of the chest. Considering myocardial ischemia and infarction, elevation of the ST-segment discussed later is an alarming finding as it implies that there is extensive ischemia.
Ischemic ST-segment elevations are often accompanied by ST-segment depressions in ECG leads which view the ischemic vector from the opposite angle. Such ST-segment depressions are therefore termed reciprocal ST-segment depressions, because they are mirror reflections of the ST-segment elevations.
Electrical activity emanating from this part of the left ventricle marked with an arrow in Figure 23 cannot be readily detected with the standard leads, but the reciprocal changes ST-segment depressions are commonly seen in V1—V3. In order to reveal the ST-segment elevations located posteriorly, one must attach the leads V7, V8 and V9 on the back of the patient.
Please note that right ventricular infarction and posterolateral infarction will be discussed in detail later on. The conventional placement of electrodes can be suboptimal in some situations. Electrodes placed distally on the limbs will record too much muscle disturbance during exercise stress testing; electrodes on the chest wall may be inappropriate in case of resuscitation and echocardiographic examination etc.
Efforts have been made to find alternative electrode placements, as well as reducing the number of electrodes without loosing information. In general, lead systems with less than 10 electrodes can still be used to compute the all standard leads in the lead ECG. Such calculated ECG waveforms are very similar to the original lead ECG waveforms, with some minor differences that may affect amplitudes and intervals.
As a rule of thumb, modified lead systems are fully capable of diagnosing arrhythmias but one should be cautious when using these systems to diagnose morphological conditions e. Indeed, in the setting of myocardial ischemia one millimeter may make a life-threatening difference. Lead systems with reduced electrodes are still used daily to detect episodes of ischemia in hospitalized patients. This is explained by the fact that when monitoring continuously — i.
Instead the interest lies in the dynamics of the ECG and in that scenario the initial recording is of little interest. This is used in all types of ECG monitoring arrhythmias, ischemia etc. It is also used for exercise stress testing as it avoids muscle disturbances from the limbs. As stated above, the initial recording may differ slightly in amplitudes so that it is not valid to diagnose ischemia on the initial tracing. For monitoring ischemia over time, however, Mason-Likar is an effective system.
Refer to Figure 24 A. The left and right arm electrodes are moved to the trunk, 2 cm beneath the clavicle, in the infraclavicular fossa Figure 24 A. The left leg electrode is placed in the anterior axillary line between the iliac crest and the last rib. The right leg electrode can be placed above the iliac crest on the right side.
Placement of the chest leads is not changed. As mentioned above, it is possible to construct mathematically a lead system with fewer than 10 electrodes. In general, mathematically derived lead systems generate ECG waveforms that are almost identical to the conventional lead ECG, but only almost. It is generated by means of 7 electrodes Figure 22 B. Using these leads, 3 orthogonal leads X, Y and Z are derived.
These leads are used in vectorcardiography VCG. Orthogonal means that the leads are perpendicular to each other. These leads offer a three-dimensional view of the cardiac vector during the cardiac cycle. However, the VCG has lost much ground in recent decades as it has become evident that the VCG has very low specificity for most conditions. VCG will not be discussed further here. Lead X is derived from A, C and I. Lead Y is derived from F, M and H.
EASI also provides orthogonal information. The Cabrera format of the lead ECG. Cardiac electrophysiology: action potentials, automaticity, electrical vectors. Video lecture on ECG interpretation. No products in the cart. Sign in Sign up. Search for:. Introduction to ECG Interpretation. Clinical electrocardiography and ECG interpretation.
Arrhythmias and arrhythmology. Mechanisms of cardiac arrhythmias: from automaticity to re-entry reentry. Conduction Defects. Overview of atrioventricular AV blocks.
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