12 & 15-Lead ECG's
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12 & 15-Lead ECG's

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Module 4-Electrocardiography

Module 4-Electrocardiography

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12 & 15-Lead ECG's 12 & 15-Lead ECG's Document Transcript

  • ELECTROCARDIOGRAPHY: A standard ECG is composed of 12 separate electrodes. Lead wires connect the electrodes placed on the body to the ECG machine. Four leads are attached to the four extremities and these leads are either ―bipolar‖ or ―unipolar‖. There are 6 limb leads, 3 of which are bipolar and 3 of which are unipolar, and 6 unipolar chest leads. Limb Leads: The limb leads are recordings of electrical impulses flowing in a coronal plane received from skin sensors (or electrodes) placed on the right and left arm and the left leg. The electrode that is placed on the right leg is used only to stabilize the tracing and is called the common ground. The placement of the limb leads and assignment of electrical charge is done according to the convention of Von Einthoven’s triangle, Figure 4.1. Page 2 of 9
  • FIGURE 4.1: VON EINTHOVEN’S TRIANGLE Page 3 of 9
  • The deflection of the stylus in an upward (positive) or downward (negative) motion depends on whether the point at which the electricity is measured is more positive or negative than the reference point, Figure 4.2. FIGURE 4.2: DEFLECTION OF STYLUS a) Bipolar Limb Leads: Compare the electrical potentials between one positive and one negative electrode that are placed on two extremities. The only bipolar leads are the limb leads designated LI, LII and LIII, or L1, L2 and L3. LII or L2 is also known as the rhythm strip and is most often used for cardiac monitoring and basic arrhythmia interpretation because the QRS is the most upright and the ―p‖ wave most visible and upright in this lead. TABLE 1: BIPOLAR LIMB LEADS Lead Reference Measured at I Rt arm –ve Lt arm +ve II Rt arm –ve Lt leg +ve III Lt arm –ve Lt leg +ve Direction of impulse Towards positive lead Towards positive lead Towards positive lead Deflection b) Unipolar Limb Leads: The negative electrode is eliminated and the electrical potential is determined at single lead sites only. Unipolar leads are always designated with an ―a‖ (aVR augmented vector to the right; aVL - augmented vector to the left; aVF augmented vector foot). Augmented view leads require amplification of voltage in the ECG machine to get tracings of the same amplitude as leads I, II, III, because the signal is weaker. Page 4 of 9
  • TABLE 2: Unipolar Limb Leads Lead Reference Measured at midway between aVR Lt arm & Lt leg Rt arm aVL aVF Rt arm & Lt leg Rt arm & Lt arm Lt arm Lt leg Direction of impulse Deflection Towards negative lead Towards positive lead Towards positive lead Precordial Leads: With the precordial or chest leads, there are six unipolar electrodes placed in six different, defined positions as shown Figure 4.3. The leads are numbered from V1–V6. Multichannel ECG machines require the placement of six chest electrodes, one at each designated position. The recording obtained is a simultaneous record of electrical events. The ECG tracings will show progressive wave changes from V1 to V6 (negative to positive) due to the different views of the heart each lead provides. Figure 4.3 and Table 3 describe the surface anatomy for each lead placement and the direction of the QRS tracing. Page 5 of 9
  • FIGURE 4.3: PLACEMENT OF CHEST LEADS TABLE 3: PRECORDIAL LEADS Precordial Anatomical Position Lead V1 4th intercostal space on the right side of the sternum V2 4th intercostal space on the left side of the sternum V3 Midway between V2 and V4 V4 slightly slightly 5th intercostal space, midclavicular line V5 QRS Deflection Intercostal space in anterior axillary line; midway between V4 and V6 5th intercostal space, midaxillary line (curves up) V6 Page 6 of 9
  • NOTE: The QRS in leads V1 and V2 represent the right ventricles and the right atrium. The QRS in V3–V6 represent the larger left ventricle (therefore the reason for stronger deflections on the tracing). It is important to know which way the current flows for each lead, because the tracing produced is characteristic for that lead. Since the ―view‖ of the heart given by each lead is characteristic for normal patients, a deviation from the normal may be a clue to an abnormality. We must recognize a normal tracing for each lead so that an error in hookup will be discovered as soon as the tracing is produced. Page 7 of 9
  • Figure 4.4: ECG WAVES, INTERVALS AND SEGMENTS DESCRIPTION OF ECG WAVES, INTERVALS AND SEGMENTS P WAVE The P wave is produced by the depolarization of the atria. It indicates SA node function and is best seen on leads LII and V1, where it is normally upright. P-R INTERVAL It is measured from the onset of the P wave to the beginning of the QRS complex. Normal duration is 0.12–0.20 sec PR SEGMENT Measured from end of P wave to beginning of QRS complex; normally isoelectric Q WAVE The Q wave is the first negative (downward) deflection following the P wave and the P-R interval. The Q wave signifies septal depolarization Page 8 of 9
  • R WAVE The R wave is the first positive (upward) deflection. It follows the Q wave, if there is one. It represents the contraction of the left ventricle, thus the height of the R wave S WAVE The S wave is a negative deflection following the R wave and is usually the last portion of the QRS complex QRS INTERVAL From the beginning of the Q wave to the end of the S wave, it represents the time in which ventricular depolarization occurs. Normal duration is 0.04–0.10 sec. (<0.10) ST INTERVAL Measured from the end of the S wave to the end of the T wave; represents repolarization of the ventricles T WAVE T wave follows S wave and ST segment—shows repolarization of the ventricles U WAVE U wave follows T wave. The source is unknown most common theory is the U wave represents after depolarizations from mechanical forces in the ventricular wall. Will be same direction as T wave and approx. 25% of the size. Only visible with heart rates < 65 and inversely proportional in that the lower the heart rate the larger the U wave ST SEGMENT Measured from end of S wave to beginning of T wave; normally isoelectric QT INTERVAL Measured from beginning of QRS complex to end of T wave RR INTERVAL Distance between the R waves is referred to as the RR interval. It is used to calculate heart rate as left ventricular contraction (R wave) is one heartbeat. Page 9 of 9