Limb leads

The electrocardiogram can be obtaining using standard limb leads and/or precordial leads. A lead should be selected that is stable over time and that has a sharp demarcation at the end of the T wave to facilitate the measurement of the QT interval duration. One can also position a monophasic action potential electrode catheter through the femoral or carotid artery to obtain endocardial monophasic action potentials (see below Modification of the Method). 

Rabbits are anesthetized with methohexital and alpha-chloralose and ventilated mechanically. After establishing an intravenous port, the animals receive the alphai-agonist methoxamine at 15 fig/kg/min, a dose that should have only a slight effect on systemic arterial blood pressure. The ECG is measured using standard limb leads and an monophasic action potential... 

Q5 How does the ECG recorded from a standard limb lead correlate with the excitation processes in the heart ... 

The conventional limb leads record potentials between two apices of the triangle. Lead 1 records potentials between the right arm and left arm, lead 2 records potentials between the right arm and left leg and lead 3 records potentials between the left arm and left leg. These are known as bipolar leads. In addition there are three unipolar leads attached to the limbs, which record potentials between the limb and a reference zero. 

A routine ECG is composed of 12 leads. Sis are called limb leads (I, II, III, aVR, aVL, and aVF), because they are recorded between arm and leg electrodes, and six are called precordial or chest leads (Vj, V2, V3, V4, V5, and Vg) and are recorded across the sternum and left precordium. Each lead records the same electrical impulse but in a different position relative to the heart. Areas of pathology shown on the ECG can be localized by analyzing differences between the tracing in question and what is known to be normal in the 12 different leads. 

QRS interval prolongation of greater than 0.12 seconds in the limb leads (Figure 1-4) strongly indicates serious poisoning by tricyclic antidepressants (see p 90) or other membrane-depressant drugs (eg, quinidine [p 324], fleoainide [p 78], ohioroquine [p 165], and propranolol [p 131]). 

FIGURE 24.1 The 12-lead ECG is formed by the three bipolar surface leads I, II, and III the augmented Wilson terminal referenced limb leads aVR, aVL, aVF and the Wilson terminal referenced chest leads Vi, V2, V3, V4, V5,... 

The most basic ECG examination is with three skin surface electrodes on the limbs one at the left arm (LA), one at the right arm (RA), and one at the left leg (LL). Figure 10.1 illustrates how six limb leads are derived from these three electrodes. 

Figure 10.1 The six limb leads derived from three limb electrodes (both arms and left leg). Here,...

Figure 10.4 shows the six limb leads arranged in a so called Cabrera sequence. The text box on Figure 10.4 contains a set of data extracted from the six ECG lead waveforms. 

Figure 10.4 ECG waveforms of the six standard extremity leads shown with the augmented leads interlaced between the bipolar limb leads (Cabrera sequence).

The 12-lead ECG clinical diagnostic test is composed of the six limb leads and the six unipolar precordial leads an example is shown in Figure 10.4. 

The limb leads presuppose that the patient is lying down so that EMG signals from the limb and body muscles do not intervene. Slow baseline respiration waves are seen if the characteristic frequency of the low-pass filter is 0.05 Hz or lower. 

The input voltages and are referenced to circuit ground. In the difference mode, this amplifier can be used to sense potential changes at one point in a circuit relative to another point in a circuit. A typical application is in EKG preamplifiers where two limb leads are connected to the input of a difference amplifier. They may also be used in null-indicator circuits to indicate equality... 

The axes of the three bipolar limb leads (I, II, and III) form a shape known as Einthoven s triangle. Because the electrodes for these leads are about equidistant from the heart, the triangle is equilateral. 

To record a 12-lead ECG, place electrodes on the patient s arms and legs (with the ground lead on the patient s right leg). The three standard limb leads (I, II, and III) and the three augmented leads (aVp, aV, and aVp) are recorded using these electrodes. Then, to record the precordial chest leads, place electrodes as follows ... 

Identify the limb lead with the smallest QRS complex or the equiphasic QRS complex. In this example, it s lead III. 

Stage 1—Diffuse ST-segment eievations of 1 to 2 mm in most limb leads and most precordial leads reflect the inflammatory process. Upright T waves appear in most leads. The ST-segment and T-wave changes are typically seen in leads I, II, III, aVp, aVp, and V2 through Vg. 

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