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Electrocardiography
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Electrocardiography (ECG or EKG*) is the process of recording the electrical
activity of the heart over a period of time using electrodes placed on a patient's
body. These electrodes detect the tiny electrical changes on the skin that arise
from the heart muscle depolarizing during each heartbeat.
In a conventional 12 lead ECG, ten electrodes are placed on the patient's limbs
and on the surface of the chest. The overall magnitude of the heart's electrical
potential is then measured from twelve different angles ("leads") and is recorded
over a period of time (usually 10 seconds). In this way, the overall magnitude
and direction of the heart's electrical depolarization is captured at each moment
throughout the cardiac cycle. The graph of voltage versus time produced by this
noninvasive medical procedure is referred to as an electrocardiogram
(abbreviated ECG or EKG).
During each heartbeat, a healthy heart will have an orderly progression of
depolarization that starts with pacemaker cells in the sinoatrial node, spreads out
through the atrium, passes through the atrioventricular node down into the
bundle of His and into the Purkinje fibers spreading down and to the left
throughout the ventricles. This orderly pattern of depolarization gives rise to the
characteristic ECG tracing. To the trained clinician, an ECG conveys a large
amount of information about the structure of the heart and the function of its
electrical conduction system. Among other things, an ECG can be used to
measure the rate and rhythm of heartbeats, the size and position of the heart
chambers, the presence of any damage to the heart's muscle cells or conduction
system, the effects of cardiac drugs, and the function of implanted pacemakers.
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Medical uses
Reasons for performing electrocardiography include:
 Suspected heart attack
 Suspected pulmonary embolism
 A third heart sound, fourth heart sound, a cardiac murmur or other
findings to suggest structural heart disease
 Perceived cardiac dysrhythmias
 Fainting or collapse
 Seizures
 Monitoring the effects of a heart medication
 Assessing severity of electrolyte abnormalities, such as hyperkalemia
The United States Preventive Services Task Force does not recommend
electrocardiography for routine screening procedure in patients without
symptoms and those at low risk for coronary heart disease. This is because an
ECG may falsely indicate the existence of a problem, leading to misdiagnosis,
the recommendation of invasive procedures, or overtreatment. However,
persons employed in certain critical occupations, such as aircraft pilots,[8] may
be required to have an ECG as part of their routine health evaluations.
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Continuous ECG monitoring is used to monitor critically ill patients, patients
undergoing general anesthesia, and patients who have an infrequently occurring
cardiac dysrhythmia that would be unlikely be seen on a conventional ten
second ECG.
Interpretation
A typical ECG tracing is a repeating cycle of three electrical entities: a P wave
(atrial depolarization), a QRS complex (ventricular depolarization) and a T
wave (ventricular repolarization). The EKG is traditionally interpreted
methodically in order to not miss any important findings.
Rate and rhythm
A heart rate between 60 and 100 beats per minute is considered normal. A heart
rate slower than 60 beats per minute is said to be bradycardic and a rate faster
than 100 beats per minute is said to be tachycardic. The physiologic rhythm of
the heart is normal sinus rhythm, wherein the sinoatrial node initiates the
cardiac cycle. In normal sinus rhythm a p-wave precedes every QRS complex
and the rhythm is generally regular. If this is not the case, the patient may have
a cardiac arrhythmia.
There are different types of rhythms that can cause the heart rate to be too fast
or too slow. Many athletes can have a normal resting heart rate of less than 60
beats a minute. The key indicator of whether a slow heart rate is a problem is
whether the person is having any kind of symptoms. One of the primary
rhythms that can cause the heart rate to be slow and symptomatic is known as a
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heart block. There are many types of heart block, but the most common is an
AV block.
There are also many rhythms that can cause the heart rate to be fast, the most
common of which is sinus tachycardia. In sinus tachycardia, the depolarization
is still starting in the normal pacemaker of the heart, called the Sino-Atrial or
SA node. When the heart rhythm is no longer initiated in the SA node but is
also initiating in various atrial foci then the heart rate can become irregular and
can develop into atrial fibrillation or atrial flutter. Atrial fibrillation can become
unstable when the heart rate is above 100. The risk of rapid atrial fibrillation is
that the heart is not beating efficiently and the blood that is pooling in the atria
of the heart can begin to clot, putting the person at a high risk of stroke or heart
attack. This is the reason many people with atrial fibrillation or A-fib have to
take blood thinners for the rest of their life. Other fast rhythms include
supraventricular
tachycardia,
ventricular
fibrillation,
and
ventricular
tachycardia. The heart rate can be approximated quickly by dividing 300 by the
number of large boxes between two consecutive QRS complexes on the EKG
paper.
Axis
The heart's electrical axis is the general direction of the ventricular
depolarization wavefront (or mean electrical vector) in the sagittal plane (the
plane of the limb leads and augmented limb leads). The QRS axis can be
determined by looking for the limb lead or augmented limb lead with the
greatest positive amplitude of its R wave. A lead can only detect changes in
voltage that are aligned with that lead; therefore the lead that is best aligned
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with the axis of ventricular depolarization will have the tallest positive QRS
complex.
The normal QRS axis is generally down and to the left, following the
anatomical orientation of the heart within the chest. An abnormal axis suggests
a change in the physical shape and orientation of the heart, or a defect in its
conduction system that causes the ventricles to depolarize in an abnormal way.
A normal axis can be quickly identified if the QRS complexes in leads I and
aVF are both upright. Lead I is positioned at 0° and lead aVF is positioned at
90°. If the QRS is upright in both, its vector of depolarization must be
somewhere between these two angles, and is therefore normal axis.
To make it easy to remember, if lead I is negative and lead II is positive so they
are facing each other and if we imagine they are hands (we shake hands with the
right hand so this is right axis deviation), Alansari Sign.
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