Electrocardiography is an instrumental diagnostic method that allows one to study the electrical fields arising during heart contractions. The advantage of the method is its relative cheapness and the value of the data obtained during the procedure. With its help, it is possible to determine the heart rate, disturbances in the functioning of the myocardium and cardiac conduction, and assess the physical condition of the heart muscle.
During an ECG, a concept such as electrocardiographic leads (potential difference in electrocardiography) is used. When diagnosing heart disease, ECG leads are used in the arms, legs and sternum.
Indications for electrocardiography
The use of ECG is indicated in the following cases:
- during routine examinations, preventive examinations;
- to assess the condition of the heart muscle in patients before upcoming surgery;
- during examination of patients with diseases such as diabetes, pathologies of the lungs, thyroid gland, diseases of the endocrine system;
- for diagnosing arterial hypertension;
- during diagnosis of cardiac ischemia, atrial fibrillation, to determine which organ wall is affected;
- to identify heart defects in newborns and adult patients;
- when detecting disturbances in heart rhythm and conduction of cardiac impulses;
- to monitor the condition of the heart muscle during medical treatment.
Important! The norm or deviation of the obtained data is determined exclusively by a specialist with the necessary knowledge in this field.
Electric potential in ECG
Many patients are interested in why, when examining the heart muscle, the electrodes of the device are placed not only on the chest, but also in the limbs? To understand this, you should find out some features of the functioning of the organ. During contractions, the heart synthesizes certain electrical signals, creating a kind of electric field that spreads throughout the body, including the right and left limbs. These waves diverge throughout the body in concentric circles. When measuring the potential in any area, the electrocardiograph will show equal potential values. The same electrical potential at any point is called equipotential in medical practice. The above measurements are carried out in the area of the hands and feet.
During electrocardiography, special sensors are used that are fixed on the patient’s chest and limbs
Another such circle is the human chest. Electrocardiography data is often recorded from the surface of the heart muscle (during open surgery in the heart), from other parts of the organ’s conduction system, for example, from the His bundle and others. That is, recording the ECG line curve is performed by recording the electrical signals of the chest and limbs. In this case, doctors receive a cardiogram recorded in all leads, since the electrical potentials of the heart muscle are, as it were, diverted from certain parts of the body.
Electrocardiogram. Part 1 of 3: Theoretical Basics of ECG
I’m starting the long-promised ECG cycle, which consists of 3 parts:
- theoretical foundations of ECG,
- ECG interpretation plan,
- some common pathological conditions on the ECG.
Required basic knowledge:
- cardiac conduction system (required),
- how the heart works (preferably).
The cycle was prepared based on the textbook “Electrocardiography” by V. V. Murashko and A. V. Strutynsky
, which is used in teaching medical students from the third year onwards.
This is an entry level guide. Practical work with ECG requires more in-depth knowledge, for example, the level of V. N. Orlov’s “Guide to Electrocardiography”
.
If you are not involved in medicine, but really want to understand a little about ECG, I recommend buying and mastering the book by Murashko and Strutynsky. The most important information is highlighted there separately, and questions and tasks for self-testing have answers
, which allows you to study on your own.
Electrocardiography
is a whole science that studies electrocardiograms (ECG), about which thick works and monographs are written. However, you can learn to distinguish a normal ECG from a pathological one. Mastery comes only with experience, when the number of deciphered ECGs reaches hundreds and thousands. At first, looking at each ECG will take up to 10-15 minutes, but experienced doctors and functional diagnostic specialists need no more than half a minute. The physical foundations of the ECG are studied in the first year of physics, and they begin to really decipher the ECG only in the third year of propaedeutics of internal diseases.
To understand the topic, you must know the conduction system of the heart, otherwise it will be extremely difficult to understand what processes are reflected on the ECG.
What exactly does the ECG machine record?
An electrocardiograph records the total electrical activity of the heart
, or more precisely, the difference in electrical potential (voltage) between 2 points.
Where does the potential difference arise
?
It's simple. At rest, myocardial cells are charged negatively from the inside and positively charged from the outside, while a straight line (= isoline) is recorded on the ECG tape. When an electrical impulse (excitation) arises and propagates in the conduction system of the heart, cell membranes move from a resting state to an excited state, changing polarity to the opposite (the process is called depolarization
).
In this case, the membrane becomes positive from the inside, and negative from the outside due to the opening of a number of ion channels and the mutual movement of K+ and Na+ ions (potassium and sodium) from and into the cell. After depolarization, after a certain time, the cells enter a resting state, restoring their original polarity (inside minus, outside plus), this process is called repolarization
.
The electrical impulse sequentially spreads throughout the parts of the heart, causing depolarization of myocardial cells. During depolarization, part of the cell becomes positively charged from the inside, and part is negatively charged. A potential difference arises
.
When the entire cell is depolarized or repolarized, there is no potential difference. The stage of depolarization corresponds to contraction of
the cell (myocardium), and the stage
of repolarization corresponds to relaxation
.
The ECG records the total potential difference from all myocardial cells, or, as it is called, the electromotive force of the heart
(cardiac emf). EMF of the heart is a tricky but important thing, so let’s return to it a little lower.
Schematic location of the cardiac EMF vector
(center) at one point in time.
ECG leads
As stated above, the electrocardiograph records the voltage (electrical potential difference) between 2 points
, that is, in some
lead
. In other words, the ECG device records on paper (screen) the magnitude of the projection of the electromotive force of the heart (cardiac emf) onto any lead.
Standard ECG is recorded in 12 leads
:
- 3 standard
(I, II, III), - 3 reinforced
from limbs (aVR, aVL, aVF), - and 6 chest
(V1, V2, V3, V4, V5, V6).
1) Standard leads
(suggested by Einthoven in 1913). I - between the left hand and the right hand, II - between the left foot and the right hand, III - between the left foot and the left hand.
simplest
(single-channel, i.e. recording no more than 1 lead at any time) the cardiograph has 5 electrodes:
red
(applied to the right hand),
yellow
(left hand),
green
(left leg),
black
(right leg) and chest ( sucker). If you start with the right hand and move in a circle, you can say that it is a traffic light. The black electrode represents “ground” and is needed only for safety purposes for grounding, so that a person does not get an electric shock in the event of a possible breakdown of the electrocardiograph.
Multichannel portable electrocardiograph
. All electrodes and suction cups differ in color and location.
2) Reinforced limb leads
(proposed by Goldberger in 1942). The same electrodes are used as for recording standard leads, but each of the electrodes in turn connects 2 limbs at once, and a combined Goldberger electrode is obtained. In practice, recording of these leads is done by simply switching the handle on a single-channel cardiograph (i.e., there is no need to rearrange the electrodes).
aVR
- enhanced abduction from the right hand (short for augmented voltage right - enhanced potential on the right).
aVL
- enhanced abduction from the left arm (left - left)
aVF
- enhanced abduction from the left leg (foot - leg)
3) Chest leads
(proposed by Wilson in 1934) are recorded between the chest electrode and the combined electrode from all 3 limbs. The chest electrode placement points are located sequentially along the anterolateral surface of the chest from the midline of the body to the left arm.
I don’t indicate too much detail, because it is not necessary for non-specialists. The principle itself is important (see figure). V1 - in the IV intercostal space along the right edge of the sternum. V2 V3 V4 - at the level of the apex of the heart. V5 V6 - along the left mid-axillary line at the level of the apex of the heart.
Location of 6 chest electrodes when recording an ECG
.
The 12 leads indicated are standard
.
additional
are “written” :
- according to Neb
(between points on the surface of the chest), - V7 - V9
(continuation of chest leads to the left half of the back), - V3R - V6R
(mirror reflection of chest leads V3 - V6 on the right [right] half of the chest).
Lead meaning
For reference: quantities can be scalar and vector. Scalar quantities only have magnitude
(numerical value), for example: mass, temperature, volume.
Vector quantities, or vectors, have both magnitude and direction
; for example: speed, force, electric field strength, etc. Vectors are indicated by an arrow above the Latin letter.
Why were so many leads
?
The EMF of the heart is the vector of the EMF of the heart in the three-dimensional world
(length, width, height) taking into account time. On a flat ECG film we can see only 2-dimensional values, so the cardiograph records the projection of the EMF of the heart on one of the planes in time.
Body planes used in anatomy
.
Each lead records its own projection of the cardiac EMF. First 6 leads
(3 standard and 3 enhanced from the limbs) reflect the EMF of the heart in the so-called
frontal plane
(see figure) and allow you to calculate the electrical axis of the heart with an accuracy of 30° (180° / 6 leads = 30°). The missing 6 leads to form a circle (360°) are obtained by continuing the existing lead axes through the center to the second half of the circle.
The relative position of standard and enhanced leads in the frontal plane
. But there is an error in the figure: aVL and lead III are NOT on the same line. Below are the correct drawings.
6 chest leads
reflect the EMF of the heart
in the horizontal (transverse) plane
(it divides the human body into upper and lower halves). This makes it possible to clarify the localization of the pathological focus (for example, myocardial infarction): interventricular septum, apex of the heart, lateral parts of the left ventricle, etc.
When analyzing an ECG, projections of the EMF vector of the heart are used, therefore such an ECG analysis is called vector
.
Note
. The material below may seem very complex. This is fine. When you study the second part of the series, you will return to it, and it will become much clearer.
Electrical axis of the heart (EOS)
If you draw a circle
and draw lines through its center corresponding to the directions of three standard and three enhanced limb leads, then we obtain a
6-axis coordinate system
. When recording an ECG in these 6 leads, 6 projections of the total EMF of the heart are recorded, from which the location of the pathological focus and the electrical axis of the heart can be assessed.
Formation of a 6-axis coordinate system
. Missing leads are replaced by a continuation of existing ones.
Electrical axis of the heart
- this is a projection of the total electrical vector of the ECG QRS complex (it reflects the excitation of the ventricles of the heart) onto the frontal plane.
Quantitatively, the electrical axis of the heart is expressed by the angle?
between the axis itself and the positive (right) half of the axis of standard lead I, located horizontally.
It is clearly seen that the same EMF of the heart
in projections onto different leads gives different shapes of curves.
Determination rules
The positions of the EOS in the frontal plane are as follows: the electrical axis of the heart
coincides
with the one of the first 6 leads in which
the highest positive teeth
, and
is perpendicular
to the lead in which the size of the positive teeth
is equal to
the size of the negative teeth. Two examples of determining the electrical axis of the heart are given at the end of the article.
Variants of the position of the electrical axis of the heart:
- normal
: 30° > ? - vertical
: 70° > ? - horizontal
: 0° > ? - sharp axis deviation to the right
: 91° > ? - sharp axis deviation to the left
: 0° > ?
Options for the location of the electrical axis of the heart
in the frontal plane.
Normal electrical axis of the heart
approximately corresponds to its
anatomical axis
(in thin people it is directed more vertically from the average values, and in obese people it is directed more horizontally).
For example, with hypertrophy
(growth) of the right ventricle, the heart axis deviates to the right.
In case of conduction disturbances,
the electrical axis of the heart may deviate sharply to the left or right, which in itself is a diagnostic sign. For example, with a complete block of the anterior branch of the left bundle branch, a sharp deviation of the electrical axis of the heart to the left (? ? 30°) is observed, and the posterior branch to the right (? ? +120°).
Complete block of the anterior branch of the left bundle branch
.
The EOS is sharply deviated to the left
(? ?? 30°), because the highest positive waves are visible in aVL, and the equality of the waves is noted in lead II, which is perpendicular to aVL.
Complete block of the posterior branch of the left bundle branch
.
The EOS is sharply deviated to the right
(? ? +120°), because The tallest positive waves are seen in lead III, and the equality of the waves is noted in lead aVR, which is perpendicular to III.
Read further:
- Electrocardiogram. Part 2 of 3: ECG interpretation plan
- ECG part 3a. Atrial fibrillation and supraventricular paroxysmal tachycardia
Types of leads
The most commonly used is 12-lead ECG. These include:
- three standard leads;
- three reinforced;
- six leads from the chest.
A 12-lead ECG has diagnostic value and is used during electrocardiography to assess the condition of the heart and identify its various diseases.
Standard type leads
Each of the specific points of the electric field has its own potential. Electrocardiography allows you to record the potential difference at several measured points.
Standard leads are recorded as follows:
- Lead 1 – in this case, the positive electrode is fixed on the left hand, the negative electrode on the right hand;
- Lead 2 – sensor with a plus value on the left leg, negative electrode on the right hand;
- Lead 3 – a positive electrode is attached to the left leg, a negative electrode is attached to the left arm.
The indicators of the first, second and third leads are responsible for the work of one or another part of the heart muscle.
During an ECG, standard leads are considered the main type
Enhanced leads
The data is recorded by obtaining the difference between the electrical potential of one of the limbs, to the area of which the positive electrode is attached, and the average potentials of the other limbs.
Such leads in the diagram are designated by a combination of the letters aVF, aVL and aVR.
The connection of the electrical center of the heart muscle with the area of electrode attachment determines the axis of reinforced unipolar leads. This axis is divided into two equal parts. One of them is positive, directed towards the active electrode. The second is negative, directed towards the Goldberg electrode with a negative charge.
Thoracic leads
Electrocardiography leads in the chest area are designated by the letter V, proposed by Wilson. During electrocardiography, 6 chest leads are used. To do this, the electrode is placed on one or another point of the chest. ECG chest leads are schematically designated by a combination of Latin letters and numbers.
Electrode attachment area:
- area of the fourth intercostal section to the right of the chest – V1;
- area of the fourth intercostal section to the left of the chest - V2;
- the area between V2 and V4 – V3;
- midline of the clavicle and fifth intercostal space – V4;
- anterior axillary line and area of the fifth intercostal space – V5;
- the middle part of the axillary region and the space of the sixth intercostal space - V6.
Chest leads are located in the patient's sternum area
The use of a 12-lead ECG is the most common option. Electrocardiographic disturbances in each of them determine the overall electromotive force of the heart, that is, they are a consequence of the simultaneous influence on the discharge of the changing electrical potential in the walls of the heart, parts of the ventricles, the upper part of the organ and at its base.
Additional leads
To obtain more accurate information about the state of the heart muscle during electrocardiography, additional Neb leads are used. To carry out this type of diagnosis, sensors are used that are usually used for standard leads.
Neb lead data helps to identify pathological conditions associated with myocardial disorders in the posterior part of the organ, the anterior wall and the upper parts of the heart.
Important! Often additional leads are necessary to make a diagnosis for a particular disease.
What is an ECG, features of its registration, pros and cons of portable devices
We have all heard the abbreviation ECG more than once. Many of us even know how it stands for. Since childhood, we have all been familiar with the not very pleasant feeling of cold and wet electrodes on our chest. And the first thing we are told to do when questions about the heart arise is an electrocardiogram. But questions arise about how accurate this study is, how informative, what it can track, what affects the final result, how to interpret, how often to do it, what are the pros and cons of an ECG.
Optical heart rate sensors are no longer surprising. You can even buy a bracelet for the price of several cups of coffee that will be able to measure your pulse. But recently, more and more devices with ECG measurement function have appeared on the market. What is this, a marketing ploy or a really useful tool?
I will try to answer all these questions today. But let's take things in order.
What is an ECG? Electrocardiography is an accessible and informative technique for studying the functional state of the heart by graphically recording electrical impulses arising during cardiac activity. Electrocardiography is a relatively inexpensive but valuable method of electrophysiological instrumental diagnostics in cardiology. The direct result of electrocardiography is an electrocardiogram (ECG). It is the occurrence of electrical impulses in the heart that causes the rhythmic alternation of contraction (systole) and relaxation (diastole) of the heart muscle over a certain time range.
An electrocardiograph is a special medical device that records impulses coming from sensors mounted on the body and converts them into a graph, i.e. an electrocardiogram, which is subject to further analysis by a cardiologist. Since ECGs are performed in hospitals and at home, there are stationary and portable cardiographs.
In short, the electrocardiograph records the total electrical activity of the heart, or more precisely, the difference in electrical potentials (voltage) between 2 points
The main components of the device are:
- electrodes placed on a person’s arms, legs and torso;
- switch-regulator;
- signal amplifier;
- filter against network interference.
Modern cardiographs have high sensitivity to the bioelectrical activity of the heart muscle and accuracy in transmitting impulse oscillations.
Purpose and objectives of the ECG An electrocardiogram is taken for the correct diagnosis of heart disease. Using this procedure, the following parameters are assessed:
- Determination of frequency (see also pulse) and regularity of heart contractions (for example, extrasystoles (extraordinary contractions), or loss of individual contractions - arrhythmias).
- Indicates acute or chronic myocardial damage (myocardial infarction, myocardial ischemia).
- Can be used to identify metabolic disorders of potassium, calcium, magnesium and other electrolytes.
- Detection of intracardiac conduction disorders (various blockades).
- Screening method for coronary heart disease, including stress tests.
- Gives an idea of the physical condition of the heart (left ventricular hypertrophy).
- May provide information about non-cardiac diseases such as pulmonary embolism.
- Allows you to remotely diagnose acute cardiac pathology (myocardial infarction, myocardial ischemia) using a cardiophone.
- Must be used when undergoing medical examination.
Reasons to undergo an ECG examination is done in the following main cases:
- chronically high blood pressure;
- making a diagnosis for chest pain;
- obesity;
- jumping heart rate;
- monitoring in athletes
Among other things, an ECG can be prescribed in many other cases. An ECG is mandatory before operations, as it is one of the main tests on the basis of which permission to perform surgery is given.
Type of electrocardiogram Graphic recording of an ECG is a broken line, the sharp corners (teeth) of which are located above and below the horizontal line on which time cycles are recorded. The teeth show the depth and frequency of rhythmic changes. An electrocardiogram reflects only electrical processes in the myocardium: depolarization (excitation) and repolarization (restoration) of myocardial cells.
Graphically, the ECG is a sequence of the QPRST complex. Each tooth of which is designated by a corresponding letter, and the intervals between the letters are nothing more than a display of the phases of the heart.
Correlation of ECG intervals with the phases of the cardiac cycle (ventricular systole and diastole)
- *Systole is one of the states of the heart muscle during heartbeat, namely the contraction of the left and right ventricles and the ejection of blood into the aorta from the left ventricle and into the pulmonary trunk from the right ventricle. Diastole (from the Greek diastole - expansion) - expansion of the cavities of the heart (associated with relaxation of the muscles of the atria and ventricles), during which it fills with blood; together with systole (contraction) constitutes the cycle of cardiac activity.
Waves and intervals on the ECG.
- P (atrial contraction),
- Q, R, S (all 3 teeth characterize ventricular contraction),
- T (ventricular relaxation),
- U (non-permanent wave, rarely recorded).
A segment on an ECG is a straight line segment between two adjacent waves. The PQ and ST segments are of greatest importance. For example, the PQ segment is formed due to a delay in the conduction of excitation in the atrioventricular (AV) node.
The interval consists of a tooth (a complex of teeth) and a segment. Thus, interval = tooth + segment. The most important are the PQ and QT intervals.
Waves, segments and intervals on the ECG.
There is no point in going into the topic of waves, intervals and segments, as well as into the analysis of the ECG itself. Cardiologists study for years to correctly interpret this complex complex. But you now have a general understanding of what an ECG is and what it looks like. Let's move on to how the electrical impulses themselves are recorded from the human body.
Taking an ECG and leads Where does the potential difference arise in the heart? It's simple. At rest, myocardial cells are charged negatively from the inside and positively from the outside, while a straight line (isoline) is recorded on the ECG tape. When an electrical impulse (excitation) arises and propagates in the conduction system of the heart, cell membranes move from a resting state to an excited state, changing polarity to the opposite (the process is called depolarization). In this case, the membrane becomes positive from the inside, and negative from the outside due to the opening of a number of ion channels and the mutual movement of K+ and Na+ ions (potassium and sodium) from and into the cell. After depolarization, after a certain time, the cells enter a resting state, restoring their original polarity (minus from the inside, plus from the outside), this process is called repolarization.
The electrical impulse sequentially spreads throughout the parts of the heart, causing depolarization of myocardial cells. During depolarization, part of the cell becomes positively charged from the inside, and part is negatively charged. A potential difference arises. When the entire cell is depolarized or repolarized, there is no potential difference. The stage of depolarization corresponds to contraction of the cell (myocardium), and the stage of repolarization corresponds to relaxation. The ECG records the total potential difference from all myocardial cells, or, as it is called, the electromotive force of the heart (cardiac emf).
Schematic location of the cardiac EMF vector (in the center)
Now there will be a very important point regarding the leads and understanding of the accuracy of the signal.
A standard ECG is recorded in 12 leads:
- 3 standard (I, II, III),
- 3 reinforced from limbs (aVR, aVL, aVF),
- and 6 chest (V1, V2, V3, V4, V5, V6).
Standard leads (proposed by Einthoven in 1913). I - between the left hand and the right hand, II - between the left foot and the right hand, III - between the left foot and the left hand.
The simplest (single-channel, i.e. recording no more than 1 lead at any time, as in the photo above) cardiograph has 5 electrodes: red (applied to the right hand), yellow (left hand), green (left leg), black ( right leg) and pectoral (suction cup). If you start with the right hand and move in a circle, you can say that it is a traffic light. The black electrode represents “ground” and is needed only for safety purposes for grounding, so that a person does not get an electric shock in the event of a possible breakdown of the electrocardiograph.
Reinforced limb leads (proposed by Goldberger in 1942). The same electrodes are used as for recording standard leads, but each of the electrodes in turn connects 2 limbs at once, and a combined Goldberger electrode is obtained. In practice, recording of these leads is done by simply switching the handle on a single-channel cardiograph (i.e., there is no need to rearrange the electrodes).
aVR - enhanced abduction from the right hand (short for augmented voltage right - enhanced potential on the right). aVL - enhanced abduction from the left arm (left - left) aVF - enhanced abduction from the left leg (foot - leg)
Chest leads (proposed by Wilson in 1934) are recorded between the chest electrode and the combined electrode from all 3 limbs. The chest electrode placement points are located sequentially along the anterolateral surface of the chest from the midline of the body to the left arm.
Location of 6 chest electrodes when recording an ECG
The 12 leads indicated are standard. If necessary, additional leads are “written”:
- according to Neb (between points on the surface of the chest),
- V7 - V9 (continuation of chest leads to the left half of the back),
- V3R - V6R (mirror reflection of chest leads V3 - V6 on the right [right] half of the chest)
Lead values
For reference: quantities can be scalar and vector. Scalar quantities have only magnitude (numerical value), for example: mass, temperature, volume. Vector quantities, or vectors, have both magnitude and direction; for example: speed, force, electric field strength, etc. Vectors are indicated by an arrow above the Latin letter.
Why were so many leads invented? The EMF of the heart is the vector of the EMF of the heart in the three-dimensional world (length, width, height) taking into account time. On a flat ECG film we can see only 2-dimensional values, so the cardiograph records the projection of the EMF of the heart on one of the planes in time.
Body planes used in anatomy.
Each lead records its own projection of the cardiac EMF. The first 6 leads (3 standard and 3 reinforced from the limbs) reflect the EMF of the heart in the so-called frontal plane (see figure) and allow you to calculate the electrical axis of the heart with an accuracy of 30° (180° / 6 leads = 30°). The missing 6 leads to form a circle (360°) are obtained by continuing the existing lead axes through the center to the second half of the circle.
6 chest leads reflect the EMF of the heart in the horizontal (transverse) plane (it divides the human body into upper and lower halves). This makes it possible to clarify the localization of the pathological focus (for example, myocardial infarction): interventricular septum, apex of the heart, lateral parts of the left ventricle, etc.
When analyzing an ECG, projections of the EMF vector of the heart are used, therefore this ECG analysis is called vector. If we draw a circle and draw lines through its center corresponding to the directions of the three standard and three enhanced limb leads, we obtain a 6-axis coordinate system. When recording an ECG in these 6 leads, 6 projections of the total EMF of the heart are recorded, from which the location of the pathological focus and the electrical axis of the heart can be assessed.
Formation of a 6-axis coordinate system.
The electrical axis of the heart is the projection of the total electrical vector of the ECG QRS complex (it reflects the excitation of the ventricles of the heart) onto the frontal plane. Quantitatively, the electrical axis of the heart is expressed by the angle between the axis itself and the positive (right) half of the axis of standard lead I, located horizontally.
A correct understanding of the normal and pathological vectors of depolarization and repolarization of myocardial cells allows us to obtain a large amount of important clinical information. The right ventricle has a small mass, leaving only minor changes on the ECG, which leads to difficulties in diagnosing its pathology, compared to the left ventricle.
That is, in simple terms, the more leads we can register, the more complete the clinical picture will be. By registering only one lead, we deprive ourselves of important information. But more on this below.
Regarding accuracy Electrocardiographs are measuring instruments. And all measuring instruments must undergo verification, that is, some kind of research, on the basis of which it can be concluded that a particular device is truly accurate in the results that it produces. I don’t know how it is abroad, but in Russia ECGs are verified by a device of the Diatest-4 type, which is a signal generator that meets the requirements as a means of verification of myographs, rheographs, electrocardiographs, encephalographs, ensuring the setting of signal parameters with the required accuracy.
What else affects the reliability of the resulting ECG, besides the accuracy class of the device itself?
- interference in the electrical network;
- excitement of the subject;
- poor-quality contact of the sensor (a special gel is used to improve the passage of the electrical signal through the electrode);
- human factor (negligent attitude of medical personnel, who may incorrectly apply electrodes).
There are still a lot of nuances that can affect the result obtained as a result of ECG registration.
In addition, it should be separately noted that an ECG is not a panacea. An ECG is not capable of detecting absolutely all types of pathologies and abnormalities in the heart. Moreover, ECG is a real-time display of the heart, and many abnormalities are intermittent and intermittent. That is, having some kind of deviation in the functioning of the heart is far from a fact that it will manifest itself at the time of recording the ECG. And in this case, the ECG will be absolutely useless, even if you use the most extensive analysis using 12 leads.
And now it’s time to talk about portable devices for recording ECGs.
Appla Watch and analogues The presentation of the latest Apple Watch model caused a lot of noise. Mainly due to the fact that the gentlemen introduced a new watch feature that allows you to take an ECG directly from the owner’s hand. Almost every week another device with a similar function appears on the market, and journalists from specialized and not so specialized publications call such gizmos a breakthrough in engineering and a new word in the field of self-diagnosis. But let's figure out how it works and what we get as a result.
This watch has a special electrode on the back of the case. To take an ECG, it is necessary to “close” the circuit between the two hands and the heart, i.e. press the button on the side of the watch with your second hand, which will launch the ECG analysis function. What can be determined based on one lead, measuring the potential between the right and left arms? Only heart rate, which can be very useful for screening for atrial fibrillation. After measuring the ECG in this way, the only answer you will receive is that you have normal sinus rhythm or atrial fibrillation. Atrial fibrillation (atrial fibrillation, obsolete) is the most common cardiac arrhythmia. Atrial fibrillation is associated with various cardiovascular diseases that contribute to the development and maintenance of arrhythmia. There are several forms of AF, but the bottom line is that this type of rhythm disorder can be intermittent and not monitored by periodic ECG measurements using just 1 lead. The presence of AF in itself does not necessarily indicate a critical situation. Many people with AF have no symptoms at all, but others may periodically experience a fast or irregular heart rate. AFib becomes dangerous when it causes abnormalities in a person's vital signs, such that a person's blood pressure drops so much that they may lose consciousness, experience shortness of breath, or have a dangerously high pulse. If your heart rate exceeds 100-110 beats per minute, you are experiencing AF with tachysystole. At this point, the emergency room doctors could monitor your pulse with intravenous medications. And while short periods of asymptomatic AF may not be dangerous, persistent AF increases the risk of heart attack, blood clots in the lungs, and cardiac arrest. Depending on the patient's risk factors, some may require blood thinning. The longer your heart remains in uncontrolled AF, the more difficult it is to reverse cardiac changes.
What Apple Watch and analogues cannot do
- Currently, Apple Watch with ECG functionality is not designed to detect heart problems other than AFib.
- They are also not suitable for people who have already been diagnosed with AF - they need to see a doctor regularly.
- They cannot accurately detect the risk of a heart attack. Even a full 12-lead ECG may miss certain signs of a heart attack.
- They are not considered a US Food and Drug Administration (FDA) approved device. The FDA simply issues approvals, “pre-market 510k approval forms,” that explicitly state that the device is not intended for use by people under 22 years of age. The device is considered a Class II home use device, a class that includes condoms and pregnancy tests.
- They are not a device for continuously monitoring the electrical activity of the heart. They can only track the ECG when you touch the wheel with your other hand.
- It is physically impossible to construct an ECG with one electrode. To measure electrical activity, it is necessary to organize a closed circuit passing through the heart. Even a wireless device worn on the other hand will not cope with this, since it will not be part of the same circuit.
All of the above applies to absolutely all portable devices (watches, bracelets, phone cases, etc.) whose operating principle is based on recording potential changes only between two electrodes.
Overall, the new Apple Watch (and its analogues) look like a great tool, but they are not medical-grade devices, and are not a substitute for a professional medical assessment if symptoms arise. And even if the ECG on your Apple Watch looks normal, it doesn't mean you don't have AFib or other cardiac abnormalities.
A couple of thoughts to consider. Devices of this kind are not measuring instruments, they are not verified, and accordingly there is no control over the accuracy of the data obtained with this method of ECG registration. In addition, no special gels are used to improve the conductivity of electrical impulses through the skin, which has a fairly high resistance.
So the decision to purchase this type of device or not is up to you.
What to do? In addition to a regular ECG, I recommend performing an ECG under stress, i.e. on a treadmill, elliptical, bicycle ergometer. The picture under load will be even more complete. But this type of analysis is also not a panacea. A more informative type of research for pathologies that are periodic in nature is the use of a 24-hour monitor or Holter. There is also such a thing as echocardiography - an ultrasound method aimed at studying the morphological and functional changes of the heart and its valve apparatus. It is based on capturing ultrasound signals reflected from the structures of the heart. This method allows you to establish the condition of soft tissues, determine the thickness of the walls of the heart, the condition of the valve apparatus, the volume of the heart cavities, the contractile activity of the myocardium, see the work of the heart in real time, and trace the speed and characteristics of blood movement in the atria and ventricles of the heart.
Once again, I want to emphasize that when doing an ECG, you need to understand that various pathologies may not be detected. Even myocardial infarction sometimes cannot be tracked on an ECG or EchoCG. In this case, you need to do a blood test for markers of cardiomyocyte damage. But this is only in extreme cases. Currently, the most convincing marker is troponin. Troponin is a protein that is a component of thin muscle filaments and part of the troponin complex located in striated muscle.
How often should an ECG be done? I would recommend at least once a year to monitor the health of your heart muscle. If you are actively involved in sports, especially cyclic sports, then it is best to do this kind of analysis before the start of a new training season and possibly at its peak in the summer. Well, try to treat your heart as carefully as possible and track your heart rate and heart rate variability, which is very useful and informative during the preparation period.
How does an electrocardiograph work?
An electrocardiograph is a device designed to detect various pathologies and diseases of the heart muscle. The diagnostic method is based on obtaining the difference in electrical potentials. During normal heart function, this difference is weak or absent.
Most standard devices are equipped with 12 lead cables and 10 electrodes. During the procedure, 6 electrodes are attached to the patient’s chest, the remaining 4 are attached to the lower and upper extremities. Electrical impulses pass through the electrodes into the leads. In this case, the device records the data, recording it in the form of a graph. The resulting cardiogram is used to make a diagnosis.
An electrocardiograph is a device that allows you to record the electrical impulses of the heart on paper in the form of a graph.
Data decoding is carried out by a doctor, with their help the following indicators are determined:
- heart rate;
- cardiac conduction defects;
- which wall of the heart is affected;
- regularity of contractions;
- metabolic disturbances in the electrolyte balance of the organ;
- normal or pathological state of the myocardium;
- physical assessment of the condition of the heart muscle.
Electrocardiography allows you to identify both serious pathologies and heart defects, as well as minor disorders that do not require serious treatment.
More often, a standard scheme is used for diagnosis, but several types of electrocardiography can be used in medical practice:
- intraesophageal - in this case, the patient is inserted into the esophagus with an active electrode. This type of study is used for the differential diagnosis of supraventricular disorders with ventricular ones;
- Holter electrocardiography – the procedure is repeated over a long period of time, recording and comparing the data obtained;
- bicycle ergometry – carrying out the procedure during physical activity on the body (using an exercise bike);
- high-resolution electrocardiography and other methods.
Each type of laboratory test is prescribed by a doctor in accordance with the characteristics of the disease and the patient’s indications.
What are standard ECG leads and how are they formed.
Since our site is dedicated to cardiography, it does not hurt to describe in more detail the process of recording a cardiogram in six standard limb leads with an ECG Light USB cardiograph. This material is technical in nature and will be useful to radio amateurs and professional developers. I note that the medical aspects of the formation of an electrocardiogram are not described here! To study the medical side of the issue, I advise you to read “The ABC of ECG” by Yu. Zudbinov (I don’t publish a link to the book - Google will help, it won’t be difficult to find it).
When recording a cardiogram, clothespin electrodes are placed on the limbs of the subject to remove the potential.
Usually in cardiography the signal from the left hand is called L , from the right hand - R , from the left leg - F , the signal that is sent to the right leg -
N. In the technical documentation for cardiographs you can read that they record an electrocardiogram in one/two/three/six/twelve standard leads. What does it mean? A cardiac lead is simply the location of two points on the body (for bipolar leads) between which the ECG signal is recorded. For example, if we say that single-channel cardiographs record a cardiogram in the first standard lead, this means that the ECG is taken between the left and right hands. Three-channel electrocardiographs record an electrocardiogram in three standard leads: in the first lead - an ECG between the arms; in the second lead - ECG between the left leg and right arm; in the third lead - ECG between the left leg and left arm. Usually, to the three standard leads (designated by Roman numerals I, II, III), three more enhanced leads from the limbs (aVR, aVL, aVF) are added, which are recorded relative to the “virtual zero” and formed by the analog part of the cardiograph or calculated by software. Enhanced limb leads are the potential difference between the active positive electrode located on one limb and the average potential of the other two limbs. It is easier to understand the essence of the amplified leads using the registration scheme (here is a picture of my own execution :-)): aVR (amplified from the right hand) = signal from the right hand - (sum of signals from the left hand and left leg) /2;
aVL (amplified from the left hand) = signal from the left hand - (sum of signals from the right hand and left leg) / 2;
aVF (amplified from the left leg) = signal from the left leg - (sum of signals from the left and right hands) /2;
Enhanced leads can and should be calculated programmatically if the cardiograph has a software part. If the device is portable with a built-in thermal printer, then the enhanced leads are formed by the analog part of the cardiograph exactly as shown in the diagram. For computer devices there are practically no restrictions on calculations, so there will be no multiplying entities, complicating the circuitry and occupying ADC channels with unnecessary data. And in general, in the modern era of computer technology, when spaceships have been roaming the open spaces for decades, it would be a sin not to take advantage of these very technologies! Simply put, through simple mathematical transformations we obtain expressions for calculating reinforced leads (if you are interested in the full derivation of the formulas, write by email):
aVR (amplified from the right hand) = - (sum of signals in the first and second leads) /2;
aVL (amplified from the left hand) = signal in the first lead - (signal in the second lead) / 2;
aVF (amplified from the left leg) = signal in the second lead - (signal in the first lead) / 2;
We look at the scheme for recording cardiographic leads, remember school geometry, namely the addition of vectors, and get a simple expression for the ECG in the first lead through the second and third:
ECG in the first lead = difference between the ECG in the third and second leads.
Thus, the cardiogram signals are calculated in all standard limb leads from two ECG signals of the second and third leads. As you can see, simple arithmetic and nothing more.
Now the circuit of a household USB cardiograph, or rather the circuit of its biopotential amplifier (BPA), becomes more understandable. The signal from the right hand is fed to the non-inverting input of the op-amp DA4:B, the signal from the left foot is fed to its inverting input. Those. amplifier DA4:B generates an ECG in the second standard lead, then the ECG signal is amplified by DA4:C and transmitted through capacitor C23 to the input of the ADC (port C0 of the ATMega48 microcontroller). Similarly, the signal from the left hand goes to the non-inverting input of the operational amplifier DA4:A, the signal from the left leg goes to the inverting input, and at the output of DA4:A we get an ECG in the third standard lead. We amplify in a similar way and transmit it through capacitor C27 to the second channel of the ADC (port C1). ECG signals in the second and third leads are transmitted to the PC, ECG in the first and enhanced leads is obtained in the ECG Control software using the simple expressions we received.
Careful readers will have noticed that the amplified signal from the left leg is also fed to the inverting input of op-amp DA2:B, and then to the right leg. This is done to suppress common-mode interference, i.e. DA2:B is essentially a neutralization amplifier for the cardiograph UPS block.
That's all! Thank you all for your attention, if you have any difficulties, ideas or suggestions while reading, write in the comments!
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Do you need preparation for an ECG?
No specific preparation is required for an ECG, but in order to obtain the most correct results of the study, several aspects should be taken into account. The day before the diagnosis, experts recommend:
- sleep well;
- try to eliminate excessive emotional experiences;
- intra-nutritive electrocardiography is performed exclusively on an empty stomach;
- a few hours before the test, it is recommended to reduce fluid and food intake;
- During the diagnosis, you need to take off your clothes, relax, and not be nervous.
The day before the procedure, you should stop smoking and drinking alcohol.
During the ECG procedure, the patient is advised to relax and breathe evenly.
You should not engage in sports or heavy physical work. If you need to take certain medications, this must be discussed with your doctor. In addition, it is not recommended to visit a sauna, steam bath, or perform other procedures associated with the effects of heat on the body.
Important! No special preparation is required before performing an electrocardiogram, but following the above tips will help you obtain the most accurate data from any type of ECG.
How is an ECG interpreted?
Cardiogram analysis is deciphered exclusively by a specialist. Indicators include P, Q, R, S, T waves and ST and PQ segments. In turn, teeth directed upward are called positive, and teeth directed downwards are called negative.
Key ECG indicators:
- the source of excitation under normal conditions is accompanied by sinus rhythm;
- rhythm frequency – the gap between the R waves is no more than 10%;
- normal heart rate is 60-80 beats/min;
- rotation of the electrical axis of the heart muscle - from semi-horizontal to semi-vertical;
- The R wave is accompanied by a positive character;
- T wave – must be positive;
- PQ section – from 0.02 to 0.09 sec;
- section ST – runs along the isoline; normally there may be deviations of no more than 0.5 mm.
Electrocardiography is a method often used in medical practice that allows one to obtain detailed information about the condition of the heart and some other organs in a short period of time. The data obtained during diagnosis is used to identify many diseases, help to start treatment in a timely manner, and prevent serious complications.
