EKG LI Icha
EKG LI Icha
EKG LI Icha
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NORMAL ELECTROCARDIOGRAM
Cardiac contraction relies on the organized flow of electrical impulses through the heart. Electrocardiogram is an easily obtained
recording of that activity and provides a wealth of information about cardiac structure and function.
It is important to note that in the intact heart the sequence by which regions repolarize is actually opposite to that of their
depolarization, it occurs because the myocardial action potential durations are more prolonged in cells near the inner endocardium
than in myocytes near the outer epicardium. Thus, the cells closer to the endocardium are the first to depolarized but are the last to
repolarize direction of repolarization recorded by the ECG machine is usually inverse of what presented in the single cell, in EKG
repolarixation are usually oriented in the same direction.
Electrocardiographic Leads
Wire electrodes are placed directly on the skin, held in place by adhesive tabs, on each of the four limbs and on the chest in the
standard arrangement. The right-leg electrode is not used for the measurement but serves as an electrical ground.
- Complete ECG (12-lead ECG) is produced by recording electrical activity between the electrodes in specific patterns, results in 6
references axes in the bodys frontal plane (limb leads) plus 6 in the transverse plane (chest leads).
- ECG machine records lead aVR by selecting the right-arm electrode as the (+) pole with respect to the other electrodes
Unipolar (no single (-) pole, rather , the other limb electrodes are averaged to create a composite (-) reference).
- When the instantaneous electrical activity of the heart points in the direction of the right arm, upward deflection is recorded in
lead aVR.
- Lead aVF is recorded by setting the left leg as (+) pole positive deflection recorded when the forces are directed to the feet.
- Lead aVL selected when the left-arm electrode is made the (+) pole and it records an upward deflection when electrical activity
is aimed in that direction.
a. Bipolar limb leads
- Bipolar limb leads are part of the standard ECG recording. Bipolar means that the electrocardiogram is recorded from two
electrodes located on different sided of the heart.
- It indicates that one limb electrode is the (+) pole and another single electrode provides the (-) reference.
- ECG machine inscribe an upward deflection if electrical forces are heading toward (+) electrode and records downward
deflection if the forces are heading (-) electrode.
- Lead I connects the left arm (positive terminal) to the right arm (negative terminal).
- Lead II connects right arm (negative terminal) to the left arm (positive terminal), electrocardiograph records positively.
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- Lead III connects left arm (negative terminal) to the left leg (positive terminal). It means the electrocardiograph records
positively when the left arm is negative with respect to the left leg.
Magnitude and direction of electrical activity are represented by the ECG recording in each lead. There are 4 points that should be
studied :
a. Electrical force directed toward the (+) pole of a lead results in an upward deflection on the ECG recording of that lead.
b. Forces that head away from the (+) electrode result in a downward deflection in that lead.
c. Magnitude of the deflection (upward or downward) reflects how parallel the electrical force is to the axis of the lead being
examined, more parallel force to the lead, the greater magnitude of the deflection.
d. Electrical force directed perpendicular to electrocardiographic lead does not register any activity (flat).
Six standard limb leads examine the electrical forces in the frontal plane of the body. Because the electrical activity travels in three
dimensions, recording from a perpendicular plane are also essential by use of the six electrodes placed on the anterior and left
lateral aspect of the chest, creating chest (precordial) leads.
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Conduction of electrical impulses through the heart is an orderly process. Normal beat begins at the sinoatrial (SA) node.
- Wave of the depolarization spreads rapidly through the right and left atria then reaches the atrioventricular (AV) node
encounter expected delay.
- Impulse then travels rapidly through the bundle of His and into the right and left bundle branches, then divide into purkinje
fibers which radiate toward the myocardial fibers (stimulate depolarize and contract).
QRS complex may take one of several shapes but can always be subdivided into individual components.
- If first deflection is downward Q wave
- First upward deflection R wave
- Downward deflection following the R wave S wave
Course of normal ventricular depolarization is recorded by 2 of the ECG leads aVF and aVL. aVF represent electrical activity from
the perspective of the inferior aspect of the heart, aVL records from the perspective of the left lateral side.
Initial portion of ventricular myocardium that is stimulated to depolarize with each cardiac cycle is the midportion of the
interventricular septum, on the left side. Depolarization reverses the cellular charge, surface of that region become negative with
respect to the inside.
- Initial current is directed toward the right ventricle and inferiorly.
- The force is directed away from the (+) pole region of lead aVL recorded initial downward deflection.
- At the same time, electrical force is directed toward the (+) pole region of lead aVF recorded initial upward deflection.
- As the lateral wall of the ventricles are depolarized, forces of the thicker left side outweigh those of the right arrows
orientation increasingly directed toward the left ventricle.
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- At the completion of depolarization, myocytes are again homogenously charged ECG return to baseline in both leads.
- Lead aVL inscribes initial small Q wave followed by the tall R wave. In lead aVF, initial upward (R wave) deflection followed by
downward (S wave).
First region to depolarize is the midportion of the interventricular septum on the left side, then it proceeds toward the right
ventricle, then toward cardiac apex, and finally around the lateral wall of both ventricles.
REFERENCES