The document summarizes the key components of a normal electrocardiogram (ECG). It describes that a normal ECG consists of P, QRS, and T waves. The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. It also outlines the placement of electrodes for the 12-lead ECG and which leads monitor different walls of the heart.
2. Characteristics of the Normal Electrocardiogram
• The normal electrocardiogram is composed of a P
wave, a QRS complex, and a T wave.
• The QRS complex is often, but not always, three
separate waves: the Q wave, the R wave, and the S
wave.
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4. • Q wave is first downward deflection after P wave
• R wave is positive deflection after Q wave
• S wave is negative deflection after R wave
• T wave follows QRS
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7. • The P wave is caused by electrical potentials
generated when the atria depolarize before atrial
contraction begins.
• The QRS complex is caused by potentials generated
when the ventricles depolarize before contraction,
that is, as the depolarization wave spreads through
the ventricles.
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8. • Both the P wave and the components of the QRS
complex are depolarization waves.
• The T wave is caused by potentials generated as the
ventricles recover from the state of depolarization.
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9. • This process normally occurs in ventricular muscle
0.25 to 0.35 second after depolarization, and the T
wave is known as a repolarization wave.
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10. The Electrocardiogram (ECG)
■ An ECG is a series of waves and deflections
recording the heart’s electrical activity from a
certain “view.”
■ Many views, each called a lead, monitor voltage
changes between electrodes placed in different
positions on the body.
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11. The Electrocardiogram (ECG)
■ Leads I, II, and III are bipolar leads, which
consist of two electrodes of opposite polarity
(positive and negative).
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12. The Electrocardiogram (ECG)
■ Leads aVR, aVL, and aVF are unipolar leads and
consist of a single positive electrode and a
reference point (with zero electrical potential) that
lies in the center of the heart’s electrical field.
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13. The Electrocardiogram (ECG)
■ Leads V1–V6 are unipolar leads and consist of a
single positive electrode with a negative reference
point found at the electrical center of the heart.
■ Voltage changes are amplified and visually
displayed on an oscilloscope and graph paper.
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14. The Electrocardiogram (ECG)
■ An ECG tracing looks different in each lead because
the recorded angle of electrical activity changes with
each lead.
■ Several different angles allow a more accurate
perspective than a single one would.
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15. The Electrocardiogram (ECG)
■ The ECG machine can be adjusted to make any skin
electrode positive or negative.
■ The polarity depends on which lead the machine is
recording.
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16. The Electrocardiogram (ECG)
■ A cable attached to the patient is divided into
several different-colored wires: three, four, or
five for monitoring purposes, or ten for a 12-lead
ECG.
■ Incorrect placement of electrodes may turn a
normal ECG tracing into an abnormal one.
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17. ♥ Clinical Tip
• Patients should be treated according to their
symptoms and signs, not merely their ECG.
• To obtain a 12-lead ECG, four wires are attached to
each limb and six wires are attached at different
locations on the chest.
• The total of ten wires provides twelve views (12
leads).
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19. Limb Leads
• Electrodes are placed on the right arm (RA), left
arm (LA), right leg (RL), and left leg (LL).
• With only four electrodes, six leads are viewed.
■ Standard leads: I, II, III
■ Augmented leads: aVR, aVL, aVF
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20. Standard Limb Lead Electrode Placement
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