University:
Michigan State University
Course:
MED 611 | Cardiology Clerkship
Academic year:

2019
Views:

160

Pages:

43
Author:

Campbell Wagner

INTRODUCTION TO ECG What we will learn today • Definition of ECG • The components of an ECG strip • The characteristics of the ECG paper • The ECG machine and it’s electrodes • 12- lead ECG • How to properly perform an ECG The electrocardiogram (ECG) • The ECG is a recording of the electrical activity of the heart. • When the cardiac impulse passes through the heart, electrical current also spreads into the adjacent tissues all the way to the surface of the body. • If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded • A simple & non-invasive diagnostic test The Normal ECG • ECG is a plot of voltage on the vertical axis against time on the horizontal axis • The ECG waves are recorded on a special graph paper that is divided into standard-sized squares. Each large square is 5 mm long and each small square is 1 mm long. ➢ECG is recorded at a speed of 25mm/sec, So: ➢ Each large square on the horizontal axis represents =0.2 sec (200ms) ➢ Each small square on the horizontal axis represents =0.04 sec (40ms) ➢Vertically, the ECG graph measures the height (amplitude) of a given wave or deflection,10 mm (10 small squares) equals 1 mV with standard calibration. ➢Paper speed and voltage are usually printed at the bottom of the ECG paper strip. Main Components of ECG • The P wave is caused by electrical potentials generated when the atria depolarize. • The QRS complex is caused by potentials generated when the ventricles depolarize. The first downward deflection is called a Q wave. An upward deflection is called an R wave. Any downward deflection following an R wave is called an S wave • The T wave is caused by potentials generated as the ventricles repolarize. • The repolarization wave of the atria occurs at the same time as the QRS complex that’s why it’s not seen on the ECG record normally. ➢ PR interval • From the beginning of the P wave till the beginning of the QRS complex. It is about 0.16 second. ➢ QT interval • From the beginning of the QRS complex to the end of the T wave (ventricular depolarization & repolarization). This interval is about 0.35 second. ➢ R-R interval • Represents one cardiac cycle & is essential in calculating the heart rate. ➢ PR segment • Extends from the end of P wave to the beginning of QRS complex. It should be isoelectric. ➢ ST segment • Extends from the end of QRS complex to the beginning of T wave. It should be isoelectric. Electrocardiograph Machine The ECG machine • This machine compares, amplifies and filters the electrical potential differences recorded by the ELECTRODES and presents the results as LEADS • Electrode is a conductive pad which is attached to the skin and allows recording of electrical currents • An ECG lead is a graphical description of the electrical activity of the heart from a particular angle across the body. It is created by analysing the data obtained from two or more electrodes 12-lead ECG • 12-lead ECG is generated from 10 electrodes. • These 10 electrodes allow the electrical activity of the heart to be looked at from 12 different positions. There are 4 limb electrodes and 6 chest electrodes. • Limb electrodes: ➢LA – Left arm ➢RA – Right arm ➢LL – Left leg ➢RL – Right leg ✓ RL is a neutral lead and is solely present to complete the electrical circuit. It plays no role in the formation of the ECG itself. Limb electrodes Clamp Electrodes Disposable, adhesive Limb leads (Frontal Plane) ➢ Bipolar Limb Leads • Lead I: RA (-) to LA (+) • Lead II: RA (-) to LL (+) • Lead III: LA (-) to LL (+) ➢ Augmented Unipolar Limb Leads ➢ Connect two limbs to the negative (reference) electrode through very high resistance (the recorded voltage will be almost zero). The third limb is connected to the positive(exploring) electrode and its voltage is thus recorded. • Lead aVR: RA (+) to [LA & LL] (-) • Lead aVL: LA (+) to [RA & LL] (-) • Lead aVF: LL (+) to [RA & LA] (-) Recording a wave of depolarization Einthoven’s Triangle & Law • Einthoven’s triangle, the two arms and the left leg form apices of a triangle surrounding the heart. • Einthoven’s law states that if the electrical potentials of any two of the three bipolar limb leads are known at any given instant, the third one can be determined mathematically. Because: • The sum of the voltages in leads I and III equals the voltage in lead II Chest electrodes Position of the chest electrodes: ➢V1: Right sternal edge, 4th intercostal space ➢V2: Left sternal edge, 4th intercostal space ➢V3: Midway between V2 and V4 ➢V4: Left mid-clavicular line, 5th intercostal space ➢V5: Left anterior axillary line, 5th intercostal space ➢V6: Left mid-axillary line, 5th intercostal space Disposable, adhesive Suction bulb electrodes Chest Leads (Horizontal plane) ➢These are unipolar leads. • The positive (exploring) electrode is placed on the anterior surface of the heart and the negative (reference) electrode is attached to the three limbs through very high resistance • Leads V1 & V2 look at the right ventricle and the septum. • Lead V3 look at the anterior wall of the left ventricle • Leads V4, V5 & V6 look at the lateral wall of the left ventricle. I, aVL, V4, V5, V6 V1, V2 V3 What do we need to perform a 12 lead ECG • ECG machine • 4 limb electrodes • 6 chest electrodes • Razors and skin cleansing wipes (alcohol swabs) • Conducting gel MAKING A RECORDING – PRACTICAL POINTS • Fully explain the procedure to the patient • Make sure the patient is relaxed, warm and lying in a Semi-recumbent • • • • • • • • position comfortably Ask the patient to take off any jewellery, belts and clothes with metallic parts and to turn off the cell phone. Shaving might be necessary since hair is a bad conductor of electrical activity. Make sure the skin is clean and dry The electrodes should be accurately positioned. Apply the gel in sufficient quantities. Ensure good contact between the electrodes and the skin. Make sure the patient isn’t moving or talking while making the record Check the calibration & speed settings on the machine. • You can watch this video for further clarification • //www.youtube.com/watch?v=1k4B_fIX_t0 Wandering baseline : Patient movement or abnormal breathing, loose electrodes

Introduction to ECG

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