Ecg recording setup

In lead aVF the exploring electrode is placed on the left leg, so this lead observes the heart directly from south. The equations follow:. 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.

Start learning ECG. No products in the cart. Sign in Sign up. Search for:. Introduction to ECG Interpretation. Clinical electrocardiography and ECG interpretation. Arrhythmias and arrhythmology. A lead is a glimpse of the electrical activity of the heart from a particular angle. Put simply, a lead is like a perspective. In lead ECG, there are 10 electrodes providing 12 perspectives of the heart's activity using different angles through two electrical planes - vertical and horizontal planes.

By using 4 limb electrodes, you get 6 frontal leads that provide information about the heart's vertical plane:. On the other hand, the augmented leads-aVR, aVL, and aVF-are unipolar and requires only a positive electrode for monitoring. The Einthoven's triangle explains why there are 6 frontal leads when there are just 4 limb electrodes.

As a result, they form an equilateral triangle. Keep in mind that RL is neutral also known as point zero where the electrical current is measured. By using 6 chest electrodes, you get 6 transverse leads that provide information about the heart's horizontal plane: V1, V2, V3, V4, V5, and V6. Like the augmented leads, the transverse leads are unipolar and requires only a positive electrode.

The negative pole of all 6 leads is found at the center of the heart. This is calculated with the ECG. A slight ECG artifact is not uncommon. However, you can reduce further interference through the following steps:. Exact placement of each electrode on the patient is important. Incorrect placement can lead to false or misleading diagnosis. A 3-Lead ECG uses 3 electrodes that are labeled white, black, and red. These colors are not universal as two coloring standards exist for the ECG discussed below.

These 3 leads monitor rhythm monitoring but doesn't reveal sufficient information on ST elevation activity. The connections for all three standard limb leads are shown to the left below. The figure to the right shows a diagrammatic representation of the Einthoven Triangle Hypothesis. Willem Einthoven attempted to explain the principles of the ECG in scientific terms. In Einthoven's triangle, the heart may be considered to lie at the centre of an equilateral triangle and the corners of the triangles are the effective sensing points - the right arm, left arm and left leg electrodes.

Einthoven's Triangle.