ECG examination

Content of the lecture



 * History: Willem Einthoven (1860 – 1927), ECG: 1903, Nobel prize: 1924
 * Theory: Principle of EKG
 * ECG device: Principle of operation
 * Practicum: ECG and HRV recording



The organ of the heart



 * During an EKG examination, we examine the heart as a living organ in a living organism
 * From a biophysical point of view, it is a subsystem of another system.
 * The task of the heart is to ensure blood circulation - we do not examine this function during the ECG.
 * We only monitor the secondary manifestation, which is the "electrical activity" of the heart.
 * We detect these electrical manifestations using surface electrodes → this is a non-invasive examination.



Electrical activity of the heart



 * The heart is composed of a large number of electrically active myocytes.
 * The generator of the cardiac action is the SA node, from which the irritation gradually spreads to the entire myocardium.
 * Each myocyte outwardly behaves like a variable electric dipole (it changes its position and electrical activity).
 * It is not possible to investigate the action potentials of individual myocytes in clinical conditions in a non-invasive way.
 * With the electrodes, we measure the so-called summation potential from the surface of the body, which (according to the principle of superposition) is composed of the partial contributions of all electrically active cells at a given moment in time.
 * The result is an ECG curve (or a set of several such curves) as a function of the voltage difference between two points depending on the elapsed time.



ECG waveform



 * Image = schematic recording of one period of a normal ECG (so-called sinus rhythm).
 * The horizontal axis is time [ms], running from left to right.
 * On the vertical axis is the voltage difference [mV], positive deviations go upwards in the ECG.
 * Attention, the opposite is true for neurological examinations such as EEG.
 * The isoelectric line marks the imaginary line that would appear if we did not register any cardiac activity (cardiac arrest, clinical death). It serves as a zero voltage difference level.

ECG waves



 * ECG waves are marked with letters of the alphabet P, Q, R, S, T:
 * The P wave corresponds to the depolarization (systole) of the atria.
 * The QRS waves form the so-called QRS complex, which corresponds to the depolarization (systole) of the ventricles – the most prominent manifestation on the ECG recording, overlapping the less pronounced repolarization (diastole) of the atria. The QRS complex lasts from the beginning of the Q wave to the end of the S wave.
 * The T wave corresponds to the repolarization (diastole) of the ventricles.
 * After the T wave, there is sometimes a U wave of the same polarity - the meaning is unclear.



Intervals a segments



 * In addition to individual waves, intervals and segments are important:
 * PR interval: from the beginning of the P wave to the beginning of the QRS complex;
 * PR segment: part of the isoelectric line from the end of the P wave to the beginning of the QRS complex;
 * ST segment: part of the isoelectric line from the end of the QRS complex to the beginning of the T wave;
 * QT interval: from the beginning of the QRS complex to the end of the T wave.



Heart rate



 * In the ECG recording, several periods of heart revolution are repeated one after the other.
 * The length of the cardiac period T is most accurately determined as the distance between two consecutive sharp R waves - this distance is called the RR interval.
 * Quasi-periodic course : heart periods do not repeat themselves exactly the same, there are always smaller or larger deviations.
 * HRV = Heart Rate Variability = heart rate variability : the length of the heart period changes physiologically and thus the heart rate, especially in response to breathing:
 * inhalation: increase in heart rate;
 * exhalation: decrease in heart rate.
 * Furthermore, heart rate changes depending on physical and mental stress and on a number of other factors.
 * We calculate the instantaneous heart rate f from the relationship f [Hz] = 1/T [s].
 * The secondary unit of heart rate is min-1. Derive the above relation for this unit as well!
 * In addition to the instantaneous frequency, we can also calculate the average frequency, e.g. in one minute (number of beats per minute).



Electric heart vector



 * The ECG curve does not only change over time, but we measure different waveforms between differently placed electrodes.
 * The ECG signal is therefore a spatio-temporal variable.
 * Linear model (superposition): We can view the electric field, caused by a number of spatiotemporally variable electric dipoles of individual myocytes, as a single variable electric heart vector.
 * Variability means that during the cardiac revolution this vector rotates irregularly and changes its size at the same time.
 * We are interested in the size and direction of electricity. of the cardiac vector in different phases of the cardiac revolution, especially in the moments of individual ECG waves:
 * Electric axes P, T: they are the directions of the electric cardiac vector for P, T waves;
 * QRS electrical axis : electrical direction cardiac vector for the QRS complex, it is the main electrical axis. Unless otherwise stated, this is what is meant by the electrical heart axis.
 * Electric axes are a manifestation of electrical activity and the way electricity spreads. excitement - does not depend directly on any anatomical position of the myocardium!
 * Normal direction el. axis points down to the left, but within normality it can be different for different people in a wide range of approx. 0°... +110° and it changes with age - for people over 40 years old it is -30°... +90°).
 * Large deviation from the norm:
 * It does not have to mean a pathological condition in itself, but in conjunction with other symptoms it can clarify the diagnosis.
 * It can (especially in beginners) be caused by incorrect connection of the electrodes.



Frontal plane



 * The electrical heart vector moves in 3D space.
 * Historically the oldest (Einthoven) is the examination of the projection of el. of the heart vector in the frontal plane.
 * Non-invasive examination - we place electrodes on the surface of the body, which serve as an electrolytic conductor
 * To monitor the movement of the vector in the plane, we need at least 3 places for the placement of electrodes, distributed on the body as regularly as possible in different directions from the heart (ideally: 3 angles of 120°).
 * It follows from this:
 * right shoulder,
 * left shoulder,
 * left groin (for a heart located normally on the left side).



Limb electrodes



 * The ECG signal spreads from the myocardium throughout the body, trunk and limbs.
 * It practically does not change when it passes through the limbs, the limbs act as conductors with negligible resistance (compared to the input impedance of the ECG device).
 * It is more convenient and safer to place electrodes on the limbs than on the trunk during routine examinations.
 * Limb electrodes were already used by Einthoven and are called:
 * R = Rechts – right hand (marked in red),
 * L = Links – left hand (marked in yellow),
 * F = Fuß – left foot (marked in green).
 * Limb electrodes are usually placed on the forearm near the wrist, or on the area of ​​the shin or calf near the ankle.
 * In the event of a problem or impossibility (the limb is amputated, injured, bandaged, etc.) anywhere on the limb up to the shoulder, or groin. Such placement has no practical effect on the course of the obtained ECG curve.
 * Adhesive electrodes attached directly to the body are also used for long-term ECG recordings using a portable ECG monitor (Holter).
 * The skin under the electrode should be moistened with physiological saline (solution), water or ECG gel to ensure good conductivity.



Ground electrode



 * N = neutral – grounding electrode (marked in black).
 * It is placed similarly to F, but on the opposite (i.e. usually right) leg.
 * It should not affect the actual course of the ECG curves.
 * It is not taken into account in the further interpretation of EEG leads.
 * It is therefore not counted among the limb electrodes (it is an auxiliary electrode).
 * It connects the neutral potential of the device to the patient in such a way as to minimize interference or overloading of the input amplifiers ("signal floating").



Einthoven's triangle



 * The limb electrodes form an imaginary triangle with the vertices of the RLF.
 * In Einthoven's simplifying model, we consider this triangle to be equilateral (i.e. all angles 60°).
 * Einthoven connected the measuring device between different pairs of limb electrodes and thus obtained three different leads:
 * I. = L - R,
 * II. = F - R,
 * III. = F - L.
 * The notation means that the I. lead is given by the potential difference between electrodes L and R. Etc. Pay attention to the order of the difference!
 * In the picture: the direction of the arrows and the signs + and - mean that (example for I. lead):
 * As the potential L increases (towards positive values), the curve goes upwards (and vice versa).
 * As the potential R increases (toward positive values), the curve goes down (and vice versa).
 * This designation can sometimes be confusing (+ on the side of the arrow and - on the opposite end), because in electrical engineering it is the other way around (the arrow for voltage points from + to -), but it is already a well-established habit.
 * Since the deflections in Einthoven leads are given by the potential difference of the two electrodes, they are called bipolar leads.
 * In other words, it is a bipolar connection of the electrodes.
 * Arrows indicating leads I. II. III. can be understood as vectors that have a certain direction. We can move them.



Einthoven coordinate system



 * We move all the vectors of the Einthoven triangle so that they start from one point (from the heart, located theoretically in its center).
 * This makes it clear that we get the axes of the coordinate system in the frontal plane (in contrast to the Cartesian coordinates, the axes make angles of 60°).



Heart vector in coordinate system



 * We place the electrical heart vector (e.g. QRS complex vector) into the resulting coordinate system.
 * Our goal is to find out how its action will manifest itself in individual Einthoven leads.



Perpendiculars



 * We will distribute electricity heart vector to individual coordinate axes.
 * As in the Cartesian coordinate system, we run perpendiculars from the end of the vector towards the coordinate axes.



Projections of electric heart vector



 * The intersections of the perpendiculars with the axes define the projections of the el vector. heart axes to coordinate axes.
 * The height of the curve on the ECG recording (at the corresponding moment in time) in individual Einthoven leads I. II. III. will now match those folders.
 * Determination of the cardiac axis vector:
 * In practice, we will encounter the opposite task: to determine the heart axis vector (e.g. QRS complex) from the measured curves.
 * We do exactly the opposite:
 * We plot the components of the vector, determined as the height of the QRS complex in individual leads, on the axes.
 * We run perpendicular to the axes.
 * The intersection of the perpendiculars determines the vector to be searched for.



Projections in a triangle



 * We know that we can move vectors arbitrarily (keeping the direction and magnitude).
 * We can arrange the coordinate axes again in a triangle.
 * Decomposing a vector into its components and reassembling it can also be well imagined in a triangular arrangement.



Projections in a centered triangle



 * In the same way, we can move the triangle back to its original position (with the beginning of the el. heart vector in the middle).
 * We can see that decomposing the vector into components and recomposing it still works.
 * The + signs in the projections of the heart vector mean that in the given case the QRS complexes in all Einthoven leads point upwards (the R wave is higher, i.e. its potential is more positive than the average of the potentials of the Q and S waves).

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Electrical heart axis inclination



 * If the investigated vector is the vector of the heart axis (i.e. it is the time point of the peaks of P, T or R waves), we can determine the slope of the corresponding heart axis in the above way.
 * The slope angle is measured by:
 * The angle 0° lies on the horizontal axis (ie, the axis of the I. lead) in the direction of the electrode L (similarly as in geometry).
 * The angles rise to positive values ​​as the vector rotates down, i.e. towards the electrode F (opposite to the geometry).
 * In normal cases, therefore, the slope of the cardiac axis QRS takes on positive values ​​(in the range of approx. 0°... 110°).

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Wilson unipolar leads



 * In contrast to bipolar leads (Einthoven), in unipolar leads we observe potential changes on one electrode, related to some reference point.
 * For Wilson leads, this point is the Wilson clamp, at which the average value of the potential of all limb electrodes is created.
 * If we denote R, L, F the potentials on the limb electrodes and W the potential of the Wilson clamp, then the following applies: W = (R+L+F)/3.
 * For the potential difference on the unipolar Wilson leads VR, VL, VF, then:
 * VR = R - W = (2R-LF)/3,
 * VL = L - W = (2L-RF)/3,
 * VF = F - W = (2F-LR)/3.
 * In the graphic representation, Wilson's leads are represented by three vectors, starting from the center of the triangle to its vertices (electrodes).
 * Wilson leads thus create a three-axis system similar to Einthoven leads, but rotated by 30°.
 * Wilson's and Einthoven's leads thus complement each other and 6 leads (3 bipolar and 3 unipolar) together form a six-axis system.

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Wilson clamp



 * There remains the question of how to realize the Wilson clamp so that at every moment there is an average value of the potentials of all three electrodes.
 * Simply: By connecting all three electrodes via three resistors of the same size into one node.

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Augmented leads


Lengthening = augmentation, that's why Goldberg's leads are called augmented ( and at the beginning): aVR, aVL, aVF.
 * Compared to Einthoven's, Wilson's leads have a much smaller amplitude (the vectors are shorter).
 * Therefore, Goldberger in 1942 increased the voltage of unipolar leads by placing the reference points on opposite sides of the triangle.
 * As a result, the length of the vectors was extended by 1/2, i.e. to 3/2 of the original length, and the voltage of Goldberg's leads increased as much as compared to Wilson's.
 * The reference point potential of the Goldberg leads is the average of the potentials of the two remaining electrodes. Therefore:
 * aVR = (2R - L - F)/2,
 * aVL = (2L - R - F)/2,
 * aVF = (2F - R - L)/2.
 * Comparing the relations for the Wilson lead voltages, we see that they are indeed 3/2 times larger.

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Reference points of augmented leads



 * Similar to the Wilson clamp, the average potentials of adjacent electrodes are achieved by resistance dividers, composed of resistors of the same size.
 * We need 3 dividers, each with 2 resistors, for a total of 6 resistors of the same size.

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Hexaxial system



 * Unipolar and bipolar leads thus complement each other and 6 leads (3 bipolar and 3 unipolar) create a common six-axis system into which the electrical heart vector can be projected.

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Precordial leads


We mark the chest leads V1, V2, V3, V4, V5, V6.
 * So far we have looked at the projection of the electrical heart vector in the frontal plane.
 * Projection in the transverse plane is created by 6 thoracic (precordial) leads.
 * These are unipolar leads, i.e. we monitor the potential of each electrode relative to a common reference.

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12-lead ECG

 * A standard 12-lead ECG therefore consists of:
 * 3 bipolar limb leads of Einthoven I., II., III;
 * 3 unipolar limb augmented leads aVR, aVL, aVF;
 * 6 unipolar precordial leads.

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History



 * Einthoven did not have electronic amplifiers at his disposal, so he monitored the ECG curve using string galvanometers.
 * Containers with water or electrolyte are used as limb electrodes.

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Differential amplifier



 * At the input of the ECG leads, differential amplifiers with a high input impedance are used, which does not affect the measurement.
 * A differential amplifier has two inputs, direct (indicated by the + symbol) and inverting (indicated by the - symbol).
 * At its output, it amplifies the difference (differential voltage) between the two inputs:
 * An increasing potential at the direct input causes an increase in the voltage at the output of the amplifier.
 * A rising potential at the inverting input causes a voltage drop at the output of the amplifier.
 * The two inputs of the differential amplifier connect to the same places as the string galvanometers used to connect in prehistoric EKG devices.
 * The principle of unipolar and bipolar leads remains the same.
 * Differential amplifiers will make it possible to reduce interfering voltages (interfering voltages of the same polarity applied to the differential inputs cancel each other out).
 * Sometimes the name "differential amplifier" is used, which can be misleading, since the difference is amplified and not the differential.

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Electrodes at the inputs



 * The electrode connected to the direct input is sometimes called active.
 * The electrode connected to the inverting input is sometimes called the reference.
 * There may or may not be qualitative differences between the two electrodes. It's often just a matter of convention.

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Bipolar wiring



 * In bipolar connection, the inputs of differential amplifiers are usually connected to two electrodes.
 * One and the same electrode can be connected to the inputs of different amplifiers.
 * Chains are often created in this way, when the amplifiers amplify the differences between adjacent electrodes.

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Einthoven's bipolar connection



 * An Einthoven connection is a bipolar connection where the end of the chain is connected to the beginning.
 * The differential amplifiers are thus connected in a circle (or in a triangle).
 * It is important to connect direct and inverted inputs at different leads.

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Unipolar connection



 * The direct inputs are each connected to one active electrode.
 * The inverted (reference) inputs are connected to a common reference electrode.
 * The common reference electrode is usually replaced by an artificial reference, created by connecting active electrodes through resistances of the same size to one point (Wilson clamp).
 * A Wilson circuit can be implemented using three amplifiers (as shown).
 * Unipolar connection of six precordial electrodes can be implemented in a similar way using six differential amplifiers.

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Preparation of electrodes and patient



 * Untangle the cables with the electrodes and put them aside.
 * We notice that the leads of the limb electrodes are longer than those of the chest.
 * We place the patient relaxed on his back.
 * Places for attachment of electrodes are cleaned with alcohol and moistened with physiological solution.

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Attachment of limb electrodes



 * First, we attach the grounding electrode N to the right leg.
 * We fasten the clamped limb electrodes F, R, L.
 * The use of ECG gel is usually not necessary, but we can use it if necessary. Usually, proper moisturizing of the skin is sufficient.

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Placement of thoracal electrodes



 * The first ribs are hidden behind the collarbone, the fossa just below it is the first intercostal.
 * We count up to the fourth intercostal space (usually at the level of the nipples in men).
 * Using balloons, we place the suction chest electrodes (instead of V1...V6, the cables are marked as C1...C6):
 * C1 and C2 on the fourth intercostal space on both sides of the sternum,
 * C6, C5 to the fifth intercostal space, from the left side of the chest (under the pit in the armpit),
 * C3, C4 evenly between.
 * We will check the even distribution of the electrodes on a smooth line and, if necessary, correct the position.

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Related articles

 * Electrocardiography
 * Heart Autorhythmicity
 * Electrocardiogram
 * Bradyarrhythmia
 * Tachycardia

Used literatre

 * VOKROUHLICKÝ, L and J KVASNIČKA. Základy elektrografie. 1. edition. Praha. 1984