ECG is a electrocardiogram.
ELECTROCARDIOGRAPHY:
Electrocardiography (ECG or EKG from Greek: kardia, meaning heart) is a transthoracic (across the thoraxor chest) interpretation of the electricalactivity of the heartover a period of time, as detected by electrodesattached to the surface of the skin and recorded by a device external to the body.
An ECG is used to measure the heart’s electrical conduction system. It picks up electrical impulses generated by the polarization and depolarization of cardiac tissue and translates into a waveform.
The waveform is then used to measure the rate and regularity of heartbeats, as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart, such as a pacemaker.
MEDICAL USES
General symptoms indicating use of electrocardiography include:
Cardiac murmurs
Syncope or collapse
Seizures
Perceived cardiac dysrhythmias
It is also used to assess patients with systemic disease, as well as monitoring during anesthesia and critically ill patients
FUNCTION:
· ECG produces a pattern reflecting the electrical activity of the heart and usually requires a trained clinician to interpret it in the context of the signs and symptoms the patient presents with.
· It can give information regarding the rhythm of the heart(whether or not the electrical impulse consistently arises from the part of the heart where it should and at what rate), whether that impulse is conducted normally throughout the heart, or whether any part of the heart is contributing more or less than expected to the electrical activity of the heart.
· It can also give information regarding the balance of salts (electrolytes in the blood (e.g. hyperkalaemia or even reveal problems with sodium channels within the heart muscle cells (Brugada syndrome).
· Modern ECG machines often include analysis software that attempts to interpret the pattern but the diagnoses this produces may not always be accurate.
· It is one of the key tests performed when a heart attack (myocardial infarction or MI) is suspected; the ECG can identify whether the heart muscle has been damaged in specific areas, though not all areas of the heart are covere
·
PRINCIPLES:
· The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat
· heart muscle cell has a negative charge, called the membrane potential, across its cell membrane . Decreasing this negative charge toward zero, via the influx of the positive cations, Na+ and Ca++, is called depolarization, which activates the mechanisms in the cell that cause it to contract.
· During each heartbeat, a healthy heart will have an orderly progression of a wave of depolarisation that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventricular node and then spreads all over the ventricles .
· This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart, which is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
· ,more than two electrodes are used, and they can be combined into a number of pairs (For example: left arm (LA), right arm (RA), and left leg (LL) electrodes form the three pairs LA+RA, LA+LL, and RA+LL). The output from each pair is known as a lead. Each lead looks at the heart from a different angle.
· Different types of ECGs can be referred to by the number of leads that are recorded, for example 3-lead, 5-lead, or 12-lead ECGs (sometimes simply "a 12-lead"). A 12-lead ECG is one in which 12 different electrical signals are recorded at approximately
· the same time and will often be used as a one-off recording of an ECG, traditionally printed out as a paper copy.
· Three- and 5-lead ECGs tend to be monitored continuously and viewed only on the screen of an appropriate monitoring device, for example during an operation or whilst being transported in an ambulance. There may or may not be any permanent record of a 3- or 5-lead ECG, depending on the equipment used
ECG GRAPH PAPER:
· The output of an ECG recorder is graph (or sometimes several graphs, representing each of the lead with time represented on the x-axis and voltage represented on the y-axis.
· A dedicated ECG machine would usually print onto grapher that has a background pattern of 1-millimeter squares (often in red or green), with bold divisions every 5 mm in both vertical and horizontal directions.
· It is possible to change the output of most ECG devices but it is standard to represent each mV on the y axis as 1 cm and each second as 25 mm on the x-axis (that is a paper speed of 25 mm/s). Faster paper speeds can be used, for example, to resolve finer detail in the ECG. At a paper speed of 25 mm/s, one small block of ECG paper translates into 40 ms. Five small blocks make up one large block, which translates into 200 ms. Hence,
· there are five large blocks per second. A signal may be included with a record. A standard signal of 1 mV must move the stylus vertically 1 cm, that is, two large squares on ECG paper
LEADS :
Illustration depicting lead placement during electrocardiography
Ø The term "lead" in electrocardiography causes much confusion because it is used to refer to two different things. In accordance with common parlance,
Ø the word lead may be used to refer to the electrical cable attaching the electrode to the ECG recorder. As such, it may be acceptable to refer to the "left arm lead" as the electrode (and its cable) that should be attached at or near the left arm. Usually, 10 of these electrodes are standard in a "12-lead" ECG.
Ø Alternatively (and some would say properly, in the context of electrocardiography), the word lead may refer to the tracing of the voltage difference between two of the electrodes and is what is actually produced by the ECG recorder.
Ø Each will have a specific name. For example "lead I" is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" is the voltage between the right arm and the left leg. (This rapidly becomes more complex as one of the "electrodes" may in fact be a composite of the electrical signal from a combination of the other electrodes). Twelve of this type of lead form a "12-lead" ECG
PLACEMENT OF ELECTRODES:
Ø Ten electrodes are used for a 12-lead ECG. The electrodes usually consist of a conducting gel, embedded in the middle of a self-adhesive pad onto which cables clip. Sometimes the gel also forms the adhesive.
Ø They are labeled and placed on the patient's body as follows:
Ø Proper placement of the limb electrodes, color-coded as recommended by the American Heart Association (a different colour scheme is used in Europe):
Ø The limb electrodes can be far down on the limbs or close to the hips/shoulders, but they must be even (left vs right).
Ø When exercise stress tests are performed, limb leads may be placed on the trunk to avoid artifacts while ambulatory (arm leads moved subclavicularly and leg leads medial to and above the iliac crest)
12 LEADS:
Electrode label | Electrode placement |
RA | On the right arm, avoiding thick muscle. |
LA | In the same location where RA was placed, but on the left arm. |
RL | On the right leg, lateral calf muscle. |
LL | In the same location where RL was placed, but on the left leg. |
V1 | In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone). |
V2 | |
V3 | Between leads V2 and V4. |
V4 | |
V5 | |
V6 |
WAVES AND INTERVALS:
A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex, a T wave, and a U wave, which is normally invisible in 50 to 75% of ECGs because it is hidden by the T wave and upcoming new P wave.[
The baseline of the electrocardiogram (the flat horizontal segments) is measured as the portion of the tracing following the T wave and preceding the next P wave and the segment between the P wave and the following QRS complex (PR segment).
In a normal healthy heart, the baseline is equivalent to the isoelectric line (0 mV) and represents the periods in the cardiac cycle when there are no currents towards either the positive or negative ends of the ECG leads.
However, in a diseased heart, the baseline may be depressed (e.g., cardiac ischaemia) or elevated (e.g., myocardial infarction) relative to the isoelectric line due to injury currents during the TP and PR intervals when the ventricles are at rest.
The ST segment typically remains close to the isoelectric line as this is the period when the ventricles are fully depolarised and thus no currents can be in the ECG leads.
Since most ECG recordings do not indicate where the 0 mV line is, baseline depression often gives the appearance of an elevation of the ST segment and conversely baseline elevation gives the appearance of depression of the ST segmen
RHYTHM STRIP:
Although multiple leads, and thus multiple electrical vectors, are commonly used in unison to gain diagnostic and therapeutic insight into cardiac status, monitoring one lead, referred to as a rhythm strip, can be useful to trend cardiac function in terms of heart rate, regularity, pauses, and basic rhythm .
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