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. ST-T wave abnormalities associated with LVH most commonly occur in the anterolateral leads, and are typically seen as a horizontal or downsloping ST segment and inverted T wave.
This pattern is often referred to as "strain" and thought to be related to conduction delays through the thickened diameter of the muscle wall. Pericarditis, an inflammation of the pericardium, typically produces diffuse ST segment elevation in most leads. Again, as with any ECG evaluation, it is critical to evaluate the clinical context in which the ECG abnormalities occur.
Pericarditis typically produces a friction rub that is heard on auscultation; the pain is generally sharp and stabbing in nature, and is often relieved by anti-inflammatory medications see Acute Pericarditis. Early repolarization is a normal variant and is not indicative of coronary disease. It often occurs in young, healthy individuals, but in the setting of chest pain, it may be confused with myocardial injury. Most of the ST changes that occur in early repolarization involve the precordial leads with J point elevation and a pattern of concave upward ST segments.
Electrical impulses reach the ventricles by way of AV junction. Depolarization then occurs in a wave-like fashion in the ventricles by way of the right and left bundle branches. The left bundle branch bifurcates into the anterior and posterior branches, whereas the right bundle branch is undivided. Any condition that affects the normal electrical conduction in the ventricles will cause a delay, resulting in a widening of the QRS complex.
The next step is to determine in which branch or branches the conduction is delayed or blocked. Once widened, QRS complexes are identified.
After an electrical impulse leaves the AV node, it normally travels downward and activates the intraventricular septum in a left to right direction. Since V1 is to the right of the septum and the impulse is coming toward it, a small R wave normally appears in V1. In RBBB, the impulse travels normally down the left bundle branch, activating the septum and then the left ventricle. A normal R wave is therefore produced in V1, since the septum is activated normally.
Once the left ventricular depolarization has occurred, the impulse then spreads across to the right ventricle moving toward V1 producing a second positive deflection. The second R wave is also sometimes referred to as R prime. These incomplete blocks are sometimes referred to as intraventricular conduction defects.
Hemi-blocks, or fascicular blocks, may occur in the anterior or posterior branches of the left bundle branch. They do not produce a widening of the QRS complex. They are recognized by the axis changes that are produced. In general, a left anterior hemi-block causes a left axis deviation, and a left posterior hemi-block produces right axis deviation. Because the depolarization sequence in BBBs is abnormal, repolarization may also be affected, producing ST segment and T wave abnormalities.
Wagner GS. Marriott's Practical Electrocardiography. Grauer K. Baltimore, Md: Mosby Year Book; Article Content Of the many diagnostic tools used to screen for and evaluate cardiac abnormalities, the lead electrocardiogram ECG is among the most basic.
No caption available. Definition The lead ECG is a graphic representation of the electrical activity of the heart on two planes. There are two ways to learn ECG interpretation — pattern recognition the most common and understanding the exact electrical vectors recorded by an ECG as they relate to cardiac electrophysiology — and most people learn a combination of both.
This tutorial pairs the approaches, as basing ECG interpretation on pattern recognition alone is often not sufficient. The standard ECG has 12 leads. Below is a normal lead ECG tracing. The different parts of the ECG will be described in the following sections. A comprehensive collection of medical revision notes that cover a broad range of clinical topics. A collection of anatomy notes covering the key anatomy concepts that medical students need to learn.
A collection of interactive medical and surgical clinical case scenarios to put your diagnostic and management skills to the test. Each clinical case scenario allows you to work through history taking, investigations, diagnosis and management. A collection of free medical student quizzes to put your medical and surgical knowledge to the test! Table of Contents. ECG is the abbreviated term for an electrocardiogram.
It is used to record the electrical activity of the heart from different angles to both identify and locate pathology. The PR interval begins at the star t of the P wave and ends at the beginning of the Q wave. It represents the time taken for electrical activity to move between the atria and the ventricles.
The ST segment starts at the end of the S wave and ends at the beginning of the T wave. The ST segment is an isoelectric line that represents the time between depolarisation and repolarisation of the ventricles i. The RR interval begins at the peak of one R wave and ends at the peak of the next R wave. It represents the time taken for the ventricles to depolarise and then repolarise. The paper used to record ECGs is standardised across most hospitals and has the following characteristics:.
An ECG electrode is a conductive pad that is attached to the skin to record electrical activity. Only 10 physical electrodes are attached to the patient, to generate the 12 leads.
The data gathered from these electrodes allows the 12 leads of the ECG to be calculated e.
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