13 14 Heart and Circulation I and II (1)

Depolarization begins 1 mv complete depolarization

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Depolarization Begins + 1 mV - - - - - - - - - - - - - - - - - - - - Complete Depolarization + + + + + + + + + + + + + + + ECG + - + 1 mV 0 mV Complete Depolarization Apex Base
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Repolarization Wave of Electrocardiogram      Repolarization + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - + + + + + ECG + - Complete Depolarization 0 mV Complete Repolarization + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - ECG + - Depolarized + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - ECG + - 0 mV + 1 mV Repolarization Begins + 1 mV 0 mV Complete Repolarization Apex Base
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Base Apex (+) (-) Major Electrical Axis of the Heart During Ventricular Depolarization and Repolarization Major Electrical Axis Note : ECG electrodes placed along the direction of the electrical axis will record positive voltages during ventricular depolarization, and vice versa.
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Why 12 ECG Leads? The heart is a three-dimensional object. Visualizing the shape of a three-dimensional object requires observation from multiple angles.
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Placement of 12 ECG Leads
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Right Arm Left Leg Right Leg (Ground) Left Arm Standard Leads (Connection to 2 limbs) Lead I + - + - Major Electrical Axis Lead II + - Lead III + - VII = VI + VIII
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ECG Waves P wave (Atrial Depolarization) QRS Complex (Ventricular Depolarization) T wave (Ventricular Repolarization) P wave One Cardiac Cycle
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ECG Time Intervals P wave QRS Complex T wave P wave P-Q Interval Q-T Interval P-Q Interval = A-V Conduction Time Q-T Interval = Ventricular Contraction Time R-R Interval = Cardiac Cycle Time Heart Rate = 1/R-R Interval
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Sequence of Cardiac Activation
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Augmented Leads (Connection to 3 Limbs) 1. Positive Terminal – Connected to One limb 2. Negative Terminal – Connected to Remaining two limbs
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Augmented Lead aVR Left Arm Right Arm Left Leg aVR + - Note : The aVR ECG lead is positioned almost directly opposite to the direction of the major electrical axis of the heart. Therefore, the QRS complex in an aVR ECG is inverted. Augmented Leads : One limb (positive terminal) versus the remaining two limbs together (negative terminal)
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Augmented Lead aVL Left Arm Right Arm Left Leg aVL + -
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Left Arm Right Arm Left Leg aVF + - Augmented Lead aVF
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Directionality of Standard and Augmented ECG Leads Standard and augmented ECG leads are designed to measure the electrical activity of the heart from 6 different angles on the two-dimensional frontal surface of the body.
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Precordial ECG Leads (Connection to 3 Limbs + 1 Point) Positive Terminal – Connected to one point on the surface of chest. Negative Terminal – Connected to all 3 limbs (left arm, right arm, and left leg) - represents center of heart. Attachment sites for the Positive Terminals of the 6 Precordial Leads
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Directionality of Precordial Leads Heart Body Cross-section at Heart Level V1 V2 V3 V4 V5 V6 Front Back Left Side Right Side
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A Normal 12-Lead ECG
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Healthy Partial Heart Block (2:1) Complete Heart Block ECG Patterns for Atrioventricular Block
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Regulation of Cardiac Muscle Contraction Sarcoplasmic Reticulum Ca2+ Ca2+ Ca2+ Influx via Ca2+ Channels Ca2+ (+) Ca2+ Ca induced Ca Release via Ca2+ Channels Ca2+ Uptake via Ca2+-ATPase Na+ Ca2+ Extrusion via Na+-Ca2+ Exchange Cardiac Muscle Cell Note : Both Ca2+ influx and Ca2+ release from SR are important for activating cardiac muscle contraction.
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Excitation-Contraction Coupling in Cardiac Muscle Cells (From: Physiology 22: 167-173, 2007)
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