12 Lead ECG 1 Recording 2 Normal Features

12 Lead ECG. 1. Recording. 2. Normal Features.

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.

Early ECG recording.

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.

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.

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.

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.

Augmented Limb Leads (Unipolar). • Single positive electrodes referenced against a combination of other limb electrodes. • Positive electrodes located at a. VR (right arm), a. VL (left arm) and a. VF (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.

Augmented limb lead axis. • a. VL at -30 degrees to lead I. • a. VR at -150 degrees to lead I. • a. VF at +90 degrees relative to lead I. • e. g. wave deflected towards +90 degrees a. VF shows greatest positive deflection. • OR e. g. wave deflected towards +60 degrees then a. VR will show greatest NEGATIVE deflection.

ECG Appearance Augmented Leads. • For a normal axis ECG at +60 degrees. • Greatest deflection positive a. VF (+90 degrees). • Greatest negative a. VR (-150 degrees).

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 V 1 - V 6. • Record using same rules as previous +ive deflection if impulse travelling toward that lead, -ive deflection if away.

Chest Lead Position Related to Heart. • Leads V 1 and V 2 view antero-septal region. • V 3 and V 4 view antero-apical region. • V 5 and V 6 view antero-lateral region.

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.

QRS appearances V 1 -V 6.

Position of Limb ECG Electrodes.

Chest Lead Electrode Position. V 1 – fourth intercostal space at the right sternal edge V 2 – Fourth intercostal space at the left sternal edge V 3 – Midway between V 2 and V 4 – Fifth intercostal space in the midclavicular line V 5 – Left anterior axillary line at the same horizontal level as V 4 V 6 – Left mid-axillary line at the same horizontal level as V 4 and V 5

ECG Paper. • Standardised paper. • Speed 25 mm/sec (1 small square 0. 04 sec – large square 0. 2 sec). • Amplitude 1 m. V (10 mm to 1 m. V).

Recording. • • • Relaxed. Comfortable – supine. 12 lead ECG. Rhythm strip (which leads? ). Special cases (wheelchair, amputations, emergency).

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.

Interference. • Baseline wander. • Check electrode (sticker) contacts are secure and correct. • Check the patient is relaxed and still. • Patient perspiration cause bad connections, dry and clean the skin and replace the sticker. It may be necessary to shave hair off male patients.

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).

ECG Recording Practical. ECG Recording Workbook.