Explanation of the electrocardiogram (ECG) for non-physicians

5 minutes to understand an ECG (or almost!)

appunti del dott. Claudio Italiano

cfr anche i link correlati al tema su :  Aritmie2  cuore e diabete ecg  ecg2   atrial fibrillation  fibrillazione ventricolare   T waves of ischemia at the electrocardiogram  Ecg and heart attack, the classic signs   infarto

After writing these things, I will not let myself be seen for a while by my prof.Peppino Oreto!

Each cardiac cell is equipped with electric charges, due to the presence of ions, inside K + and outside Na +; however, the presence of proteins with negative electrical charges in the cytoplasm determines an electronegative environment inside the cell; if we want to measure this potential with two electrodes between inside and outside the cell, the value will be -100 mV (rest potential). Following a stimulation, however, this electric potential changes and describing a characteristic profile passes to positive (depolarization); this phenomenon depends on the passage of Na + inside the cell and, subsequently, with the escape of K +, the cell returns to the value of the resting potential. However, a set of cells that are invested by this phenomenon generate a wave of depolarization, which can be represented as a dipole, arranged by convention with the positive sign forward, ie in the sense of the depolarization current and with the negative tail.

At this point, if we want to detect such a depolarization activity that involves the myocardium, on the ECG trace, it is represented by a positive deflection if the electrode that explores this dipole is placed in front, that is, if it "sees" such dipole approach him, if, on the other hand, the dipole moves away, it will be negative.

The repolarization phase, in which the cells, by effect of the membrane pumps, resume the electronegative charge inside, is represented as a wave of repolarization or recovery phase or T wave. In the final analysis a cardiac cycle is given by P wave of atrial electric activity, linked to the contraction of the atria, and a QRS complex, where Q must always be very small, R is the positive deflection and S the negative deflection, which constitute the activity of the ventricles; at this point a section S and T follows, and the wave T, then a pause; the ST segment is of considerable importance for the interpretation of the ECG, because it expresses the state of the coronary and oxygenation of the myocardium, understood as blood flow; in the case of an acute infarct, this trait is generally very overlapped with respect to the isoelectric line on which it normally lies, or, in the posterior infarcts, under-segmented on V1 and V2. Finally, it is recalled that a state of ventricular hypertrophy is accompanied by signs of overload, that is ST under-stratified and sometimes T negative with asymmetric branches with R in V5 and V6 of high voltage.

Positioning of the exploring electrodes

Risultati immagini per cellula cardiaca elettricaWhen an ECG is performed, the electrodes of the so-called peripheral leads and thoracic leads must be positioned on the patient.

What is the meaning of the exploring electrodes?

They have the task of studying the heart from different angles; for example, the thoracic ones go from V1 to V6 and better explore the right ventricle, the left, the septum etc., carefully locating any anomalies or waves of injury, ischemia and necrosis depending on the location of the same. The peripheral derivations help us in the localization of the cardiac axis, they are: aVL (ie increased Left Voltage with positive electrode in the left hand), aVR (with positive electrode in the right hand), aVF (F = foot, electrode in the left foot positive); the electrodes of the derivations D1, D2, and D3., that is of the derivations of the limbs, are:

ECG: Example of derivations D1-D2-D3
D1 with pole + on the left (if the electric axis of the heart dipole is to the left, as it is in the norm, therefore, in D1 the wave R will ALWAYS be positive because the depolarization is directed to the left, with positive deflection upwards !); in D2e D3 the positive pole is at the bottom. D2 + D3 + aVF have importance in the study of lower heart infarction or ischemia.

 ECG: Example of derivations D1-D2-D3

derivazioni aVR avL, aVF : notare PR ai limiti di 5 quadrati piccoli, cioè 0,2 sec.

ECG: AVF isodifasica because the electric axis is horizontal and coincides with zero

Rapid interpretation of the electrocardiogram or ECG.

>> The chest drum
  • Let's take a look at these pieces of a patient's ecg ... Let's check the frequency and the cardiac electric axis ...

    A) Heart rate control: it is determined by calculating the point at which the R wave falls and the next one; to facilitate this task, not having the slide rule, we can observe an R wave falling on a line marked in bold on the ECG and we are going to evaluate where the other wave R falls: if it falls on the first line in bold = 300 frequency, otherwise 150, 100, 75 60, 50, as shown in the example in the diagram; for low frequencies, the notches which correspond to 3 sec are searched for on the path, the number of cycles of 6 sec is searched for in them and X 10 is multiplied.

    B) Check if there are dissociated atrial and ventricular frequencies (P waves and QRS ventricular complex waves). When reading the path, it is necessary to evaluate the distance between the wave P and the wave R, the stretch PR which is an expression of the time between the atrial and the ventricular systole and which can not go beyond 0.2 sec, on the ECG over the size of a large square:

    In these derivations, for reading:

    Is there a wave P? Yes, therefore, the rhythm is defined as a rhytm originated by sinus node. If there isn't a P wave, so it could be atrial fibrillation or a flutter that are particular arrhythmias.

    Observe in our ECG:
    - is aVL positive? Yes, so the dipole depolarization looks towards the aVL electrode, on the left that is positive.
    - is aVF positive? Is D1 positive? aVF, no, because it is the most isodifhasic, D1 yes, so the axis coincides with zero, also because the highest R is in D1 which is the derivation placed at 0 ° on the front axis.
    - Is PR regular? Yes, PR is almost, we said; The QRS is spread over 3 small squares? No, so there is no branch block or hemiblock, nor does it have enlarged configurations as for extrasystoles or branch block where the ST segment is negative. Neither there are QS waves in D3 and D2 or R-R1 appearance complexes as for BBD (right branch block) in V1 and V2; neither in V5 or V6 (see the piece of tracing with the derivations from V1 to V6 in the column, one above the other, they allow to study the heart slice by slice, starting from the right up to the left ventricle (V5 -V6).

    In the case that a QRS complex is of size over 0, 2 sec, it means that there is a block between the conduction of the impulses between the atria and the ventricles; we know, in fact, that there is a specific myocardial tissue called a node of the SENO where the pulses are automatically made, or pacemakers: from here through specific ways the signal passes to the atrioventricular node or AV, to be sent to the right and left bundles of His. Well, the PR trait is an expression of the atrio-> ventricular conduction of the depolarization wave, which if it goes beyond the established time means that the tissue has problems in conduction: ATRIOVENTRICULAR BLOCK (see the arrhythmias on this site).

    Another thing to be observed in the ECG is the presence of the wave P and if it is positive or negative, that is, if the signal starts from the atriums, or if the electrical signal leads to the other halls by another path marker . Another thing to observe are the QRS complexes, because if there is a block of the downstream conduction, ie in the right branch (BBD) or in the left branch (BBS) of the beam of His the QRS will be spread out and of odd shape, to M , with cusps R and R1, ie as 2 peaks, because the depolarization of the ventricles will be asynchronously:

    Nel caso del blocco di branca sinistra, l’aspetto sarà di un QRS slargato come con due cuspidi unite da un tratto a concavità verso l'alto. Tale aspetto occorrerà valutarlo in V1 e V2 per il BBD,  e nelle derivazioni toraciche V5 e V6 per un BBS.

    Rhythm (Arrhythmias )

    Sinus, if it starts from the sinus node and if there are waves P on the ECG, expression of atrial electromechanical contraction; if such waves are lacking being replaced by many small waves "f", we will have atrial fibrillation, if the waves will be bigger serrations with saw teeth, we will have bigger "F" waves from atrial flutter.

    Ischemia, injury, heart attack

    The waves of injury, ischemia and infarction, are represented by the ONDA T INVERTITA (ISCHEMIA), which is sometimes also a recent infarct, or worse when in a second the ST segment is leveled and the negative T wave appears of symmetrical bran lesions INVERTED !! with respect to the R wave; the ischemic picture is more severe if other aspects appear, the ST segment being overlapped or under-Risultati immagini per Q WAVES stemi nstemiexposed together with other myocardiosmecific enzymes, signs of necrosis (troponin and transaminase, CKMB, CPK).

    ST ST segment overleaf

    It is the expression of a recent and acute episode of heart attack (LESION); if the ST tract is under-stratified we can think of a positive Master's test, or subendocardial ischemia.

    Waves Q of  necrosis

    Pay attention to the presence of high T-shaped curtains in a patient with pain in the stomach or chest! The ECG is sometimes characterized by overdeveloped ST and T which tend to be negative: it is a sign of acute infarction due to fibrinolisare; the Q waves of necrosis, ie Q waves of dimensions 1/3 with respect to the R wave, the result of an already happened and stabilized infarct; at this point it is necessary to evaluate in which derivations the waves of lesion, ischemia or infarction appear: for example if in V1 V2 V3 V4 we think of a front infarction; if V1-V2.V3 at the septum;

    see >> auscultation of the heart with real tones recorded!

    Risultati immagini per Q WAVES stemi nstemiIf the lesion waves occur in aVL and in D1 we think of the derivations in which the lateral part of the heart is projected, then to a lateral infarct; the posterior infarcts will be read in D2, D3 and aVF; finally, in the posterior infarcts V1 and V2 they will have ST sub-leveling downwards, but in reality, with the mirror test, the R wave is nothing but a sort of inverted Q wave! And that's why grading in V1 and V2 must be attenuated. In other cases, the sub-divided V5 may express a left ventricular engagement from the savraccaric and the same V2, for the right ventricle. Cardiac enzymes, troponin, CPKMB, GOT LDH isoenzyme etc. must always be requested. If they raise double = heart attack! Immediately as soon as possible, fibrinolytic therapy with a fibrinogen tissue activator must be implemented; however, in the absence of this, the rescue doctor can also perform calciparin subcutaneous and intravenous aspirin pending final treatment at the UTIC.Altri segni da saper riconoscere al tracciato

    (vedi per approfondire cfr > ecg 2  ecg altri segni)

    Hypertrophy of the ventricles

    HYPERTROPHY Left Ventricle = when measuring the S wave in V1 and the R wave in V5, the measurement will be greater than 35 mm + T inverted

    HYPERTROPHY Right Ventricle = high voltage R waves in V1-V2-V3-V4.

    HYPERTROPHY LEFT Atrium = P-diffusive wave as a horizontal S;

    HYPERTROPHY  right Atrium = high voltage P wave.Risultati immagini per hypertrophy ventricle

    Hyperkalaemia: T-pointed wave or QRS slarged
    Hypopotassiemia: flat T wave
    Hypercalcemia: short QT
    hypocalcemia: QT: long;

    In the course of pericardial effusion, ST is flattened and suprasliveled with a concave tract and ST goes over the isoelectric line.


    Long and under-lined QT, P uncinata; wave U after T, as bending to U. Traced to "roller coaster" in case of overdose.




    Per le aritmie ipocinetiche ed ipercinetiche si rimanda alla pagina che le tratta su questo sito e lo stesso per quanto concerne l’infarto e l’ischemia del miocardio.

    indice argomenti di cardiologia