ECG Leads - Basic Principles and Patterns PDF

## ECG Leads ### Introduction - The heart produces electrical currents similar to a battery - These electrical currents can be measured with an electrocardiograph - The body acts as a conductor and therefore electrodes placed on the body can detect the voltage of cardiac currents - The 12 standard...

## ECG Leads ### Introduction - The heart produces electrical currents similar to a battery - These electrical currents can be measured with an electrocardiograph - The body acts as a conductor and therefore electrodes placed on the body can detect the voltage of cardiac currents - The 12 standard ECG leads demonstrate the voltage differences between electrodes on the body. ### ECG Leads as Recording Angles - ECG leads are similar to video camera angles used to record an event - Multiple ECG leads must be recorded to fully describe the heart's electrical activity ### Limb Leads #### Bipolar Leads - The leads are recorded first - Electrodes are placed on the arms and legs - The right leg electrode acts as an electrical ground - **Lead I:** The difference between the left arm (LA) and right arm (RA) - **Lead II:** The difference between the left leg (LL) and right arm (RA) - **Lead III:** the difference between the left leg (LL) and left arm (LA) - **Einthoven's Triangle**: a triangular representation of the leads. - Lead I: horizontal, positive LA, negative RA - Lead II: diagonal downward, positive LL, negative RA - Lead III: diagonal downward, positive LL, negative LA - **Lead I + Lead III = Lead II** #### Unipolar Leads - Unipolar leads record the voltage at one location relative to zero potential, not another extremity - **aVR**: represents the voltage difference between the right arm and the central terminal - **aVL**: represents the voltage difference between the left arm and the central terminal - **aVF**: represents the voltage difference between the left leg and the central terminal - **aVR + aVL + aVF = 0** ### Chest Leads - Chest leads record voltages as detected at different positions on the chest wall - **V₁:** 4th inter-costal space, right side of the sternum - **V₂:** 4th inter-costal space, left side of the sternum - **V₃:** halfway between V₂ and V₄ - **V₄:** 5th inter-costal space, midclavicular line - **V₅:** 5th inter-costal space, anterior axillary line - **V₆:** 5th inter-costal space, midaxillary line ### The 12-Lead ECG: Frontal and Horizontal Plane Leads - The 6 limb leads record electrical activity on the frontal plane of the body - Frontal plane is parallel to a large window directly in front of you - The 6 chest leads record electrical activity on the horizontal plane of the body - Horizontal plane divides the body into an upper and lower half ### 12-Lead ECG: Cardiac Monitors - 12-lead ECG is not always feasible for continuous monitoring - Bedside cardiac monitors usually place one electrode on V5, one on the right shoulder (negative), and one on the left shoulder (ground) - Changing the positive and negative electrode positions can change the ECG pattern - Ambulatory monitoring uses a portable ECG recorder to record the heartbeat over a period of time, typically 24h - Provides continuous recording of electrical activity - Printouts of any portion of the ECG can be obatined for closer examination ### Review - The electrical currents produced during atrial and ventricular depolarization and repolarization are detected by electrodes placed on the extremities and chest wall - **Extremity Leads** - 6 limb leads record voltages from the heart that are directed onto the frontal plane of the body, divided into three bipolar and three unipolar leads - Bipolar leads record the difference between voltages from the heart detected at two extremities (I, II, and III). They can be represented by Einthoven's triangle (see Fig. 3.4). They are related by the equation II = I + III - Unipolar leads records voltage at one point relative to zero potential (aVR, aVL, and aVF). They can also be represented by a triaxial diagram (see Fig. 3.6). They are related by the equation aVR + aVL + aVF = 0 - **Chest Leads**: 6 precordial lead (V₁) to (V₆). Record voltages from the heart as directed onto the horizontal plane of the body. - The 12-lead ECG is very useful for detecting abnormalities - **Monitor leads**: record voltages continuously. Used in CCUs for monitoring and used to record heart activity in ambulatory patients over a period of 24 hours. ## The Normal ECG ### Three Basic Laws of Electrocardiography 1. **A positive (upward) deflection appears in any lead if the wave of depolarization spreads toward the positive pole of that lead (Fig. 4.1)** 2. **A negative (downward) deflection appears in any lead is the wave of depolarization spreads toward the negative pole (or away from the positive pole) of that lead (Fig. 4.1).** 3. **If the mean depolarization path is directed at right angles (perpendicular) to any lead, a small biphasic (RS or QR) deflection is seen (Fig. 4.1)** ### Normal P Wave - The P wave represents atrial depolarization - Initiated by the sinus node in the right atrium - Spreads from right to left and downward toward the AV junction - Represented by an arrow that points downward and to the left ### Normal QRS Complex - The QRS complex represents ventricular depolarization - The left side of the septum is stimulated first by a branch of the left bundle of His - **The first phase of ventricular depolarization**: - Depolarization spreads from the left ventricle to the right across the septum (septal stimulation) - Represented by a small arrow pointing from the left septal wall to the right side of the heart (Fig. 4.6, A). - **The second phase of ventricular depolarization**: - Simultaneous stimulation of the main mass of both the left and right ventricles - Represented by a larger arrow pointing through the left ventricle toward the left chest (Fig. 4.6, B). ### Normal R Wave Progression - The R wave tends to become gradually larger as you move toward the left chest leads - The transition zone is where the R waves and S waves are about equal - May take place in V₂ (early transition), V₄(normal transition), or V₅ and V₆ (delayed transition) ### Normal Chest Leads | Lead | Description of Complex| |---|---| |V₁| rS-type complex. The small initial r wave represents the left-to-right spread of septal stimulation. The large S wave reflects septal stimulation .| |V₆| qR complex| |V₂ to V₅| R wave increases in size and S wave decreases in size| ### Normal Extremity Leads - They are affected by the electrical position of the heart, which can be either horizontally or vertically - **Horizontal QRS axis:** -Leads I and av, show qR complexes - Leads II, III, and aV, show rS complexes - **Vertical QRS axis:** -Leads II, III, and aV, show qR complexes - Leads I and aV, show rS complexes (this is the exact opposite of the horizontal axis) ### Review - **The three basic laws of electrocardiography are:** - **A positive (upward) deflection is seen in any lead if depolarization spreads toward the positive pole of that lead.** - **A negative (downward) deflection is seen if depolarization spreads toward the negative pole (or away from the positive pole) of any lead.** - **If the mean depolarization path is directed at right angles (perpendicular) to any lead, a small biphasic (RS or QR) deflection is seen.** - **Normal Sinus Rhythm:** - Atrial depolarization starts in sinus node and spreads downward and to the patient's left, toward the postitive pole of lead II and away from the postitive pole of lead aV.. - Therefore the normal P wave is positive in lead II and negative in lead aV.. - **Normal Ventricular Depolarization**: - **First Phase:** stimulation of the ventricular septum from left to right - produces a small (septal) r wave in the right chest leads and a small (septal) q wave in the left chest leads - **Second Phase:** simultaneous outward through the right and left ventricles, the stimulus spreads simultaneously outward through the right and left ventricles. Because the mass of the normal left ventricle overbalances the mass of the right ventricle, the spread of depolarization through the left ventricle predominates on the normal ECG. This spread of a stimulus through the left ventricle produces a tall R wave in the left chest leads (e.g., V₅ and V₆). The stimulus produces a deep S wave in the right chest leads (V₁ and V₂). - **Chest leads** in between these extreme positions show a relative increase in R wave amplitude and a decrease in S wave amplitude. - **Extremity Leads**: The shape of the QRS complex varies with the electrical position (axis) of the heart: - **Horizontal Heart**: Leads I and aV, show a qR pattern - **Vertical Heart**: Leads II, III, and aV, show a qR pattern - **Normal T Wave**: generally follows the direction of the main deflection of the QRS complex in any lead. - **Chest Leads**: the T wave may normally be negative in leads V₁ and V₂. In most adults, the T wave becomes positive by lead V₂ and remains positive in the left chest leads. - **Extremity Leads**: It is always positive in lead II and negative in lead aV.. - **Horizontal Heart**: The QRS complex and T wave are positive in leads I and aV.. - **Vertical Heart**: The QRS complex and T wave are positive in leads II, III, and aV..