Mastering the Technique of 12 Lead ECG Recording: A Comprehensive Review
This course aims to review the correct technique of recording a 12 lead ECG, understand related concepts to ECG electrode position and functionality, and differentiate normal from abnormal ECG morphology and rhythm. It also covers important rules in interpreting ECGs, including features of depolarization and repolarization waves.
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1. 12 Lead ECG. 1. Recording. 2. Normal Features.
2. Aims and Objectives. Review and perform correct technique to record a 12 lead ECG. To understand related concepts to ECG electrode position and functionality. To understand the features of a normal ECG. To create a checklist to differentiate normal from abnormal ECG morphology and rhythm.
3. Early ECG recording.
4. Important Rules. A wave of depolarisation travelling toward a positive electrode results in a positive deflection in the ECG trace. A wave of depolarisation travelling away from a positive electrode results in a negative deflection. A wave of repolarisation travelling toward a positive electrode results in a negative deflection. A wave of repolarisation travelling away from a positive electrode results in a positive deflection. A wave of depolarisation or repolarisation travelling perpendicular to an electrode axis results in a biphasic deflection of equal positive and negative voltages (i.e no net deflection). The instantaneous amplitude of the measured potentials depends upon the orientation of the positive electrode relative to the mean electrical vector. The voltage amplitude is directly related to the mass of tissue undergoing depolarization or repolarisation.
5. Recording a 12 lead ECG. Standard limb leads (bipolar leads). Record potential difference between two electrodes. Lead I (+ive electrode left arm and -ive on right arm). Lead II (+ive electrode left leg and -ive on right arm) Lead III (+ive electrode on left leg and -ive on left arm). Einthoven Triangle - equilateral triangle with heart at centre.
6. Limb Lead Axis. Positive electrode for lead I is at 0 degrees (horizontal plane). Positive electrode for lead II is at +60 degrees. Positive electrode for lead III is at +120 degrees. Known as Axial Reference System .
7. ECG Appearance Limb Leads. Wave travelling at +60 degrees greatest deflection in lead II. Wave travelling at +120 degrees greatest in lead III. If travelling at +90 degrees similar deflections seen in leads II and III.
8. Augmented Limb Leads (Unipolar). Single positive electrodes referenced against a combination of other limb electrodes. Positive electrodes located at aVR (right arm), aVL (left arm) and aVF (left leg). Same electrodes as for leads I, II and III (ECG machine automatically converts signals for these leads). Coupled with bipolar leads make 6 limb leads of the ECG. Record activity along a single 'frontal plane' relative to the heart - used to determine the cardiac axis.
9. Augmented limb lead axis. aVL at -30 degrees to lead I. aVR at -150 degrees to lead I. aVF at +90 degrees relative to lead I. e.g. wave deflected towards +90 degrees aVF shows greatest positive deflection. OR e.g. wave deflected towards +60 degrees then aVR will show greatest NEGATIVE deflection.
10. ECG Appearance Augmented Leads. For a normal axis ECG at +60 degrees. Greatest deflection positive aVF (+90 degrees). Greatest negative aVR (-150 degrees).
11. Chest Leads (Unipolar). 6 positive electrodes placed across the chest to record electrical activity of different regions of the heart. Records perpendicular to the frontal plane. Labelled V1 - V6. Record using same rules as previous - +ive deflection if impulse travelling toward that lead, -ive deflection if away.
12. Chest Lead Position Related to Heart. Leads V1 and V2 view antero-septal region. V3 and V4 view antero-apical region. V5 and V6 view antero-lateral region.
13. ECG appearance Chest Leads. Depolarisation normally occurs from left to right (IV septum). Largest muscle mass (LV) produces largest action potential. Leads overlying this region show greatest deflection.
14. QRS appearances V1-V6.
15. Position of Limb ECG Electrodes.
16. Chest Lead Electrode Position. V1 fourth intercostal space at the right sternal edge V2 Fourth intercostal space at the left sternal edge V3 Midway between V2 and V4. V4 Fifth intercostal space in the mid- clavicular line V5 Left anterior axillary line at the same horizontal level as V4 V6 Left mid-axillary line at the same horizontal level as V4 and V5
17. ECG Paper. Standardised paper. Speed 25mm/sec (1 small square 0.04sec large square 0.2 sec). Amplitude 1 mV (10mm to 1 mV).
18. Recording. Relaxed. Comfortable supine. 12 lead ECG. Rhythm strip (which leads?). Special cases (wheelchair, amputations, emergency).
19. Somatic Tremor. Check the patient is relaxed and not moving or twitching (check fingers and toes). If the patient is cold, cover them with a blanket.
20. Interference. Baseline wander. Check electrode (sticker) contacts are secure and correct. Check the patient is relaxed and still. Patient perspiration can cause bad connections, dry and clean the skin and replace the sticker. It may be necessary to shave hair off male patients.
21. Key References. Society for Cardiological Science and Technology. (2006). Clinical Guidelines by Consensus: Recording a Standard 12 lead ECG. AHA / ACC / HRS Scientific Statements. Recommendations for the Standardisation and Interpretation of the ECG. Journal of the American College of Cardiology, 49 (10).
22. ECG Recording Practical. ECG Recording Workbook.