Electrocardiography

The electrocardiography (ECG) is the chart of the electric Potentiel which orders the muscular activity of the heart. This potential is collected by electrodes on the surface of the skin.

The electrocardiogram is the layout paper of the electric activity in the heart. The electrocardiograph is the apparatus making it possible to make an electrocardiogram. The electrocardioscope , or scope , is an apparatus posting the layout on a screen.

It is a fast examination (less than five minutes), painless and stripped of any danger. It can be made as a cabinet of Médecin, with the Hôpital, even in residence. Its interpretation remains however complex and requires a certain practice of the clinician.

History of the electrocardiography

The electric currents which circulate in the heart involve electric potentials and are the persons in charge of the cardiac muscular activity. These electric potentials are known since work of Carlo Matteucci in 1842. The first experiments are carried out in 1878 by John Burden Sanderson and Frederick Page which detects using a capillary electrometer phases QRS and T. In 1887 the first human electrocardiogram is published by Augustus D. Waller. In 1895 Willem Einthoven highlights the five deflections P, Q, R, S and T, it uses the Galvanomètre with cords in 1901 and publishes the first classifications of pathological electrocardiograms in 1906. It will obtain in 1924 a Nobel Prize for its work on the electrocardiography. Derivations précordiales are used for the medical diagnosis starting from 1932 and unipolar derivations frontal starting from 1942, which makes it possible Emanuel Goldberger to carry out the first layout on 12 ways.

It will be noted that in 1938 a transatlantic International Conference fixed the position of derivations précordiales, V1 with V6.

Today the electrocardiography is a technique relatively inexpensive, allowing using a painless examination and without danger, to monitor the cardio-circulatory apparatus, in particular for the detection of the turbid of the rate/rhythm and the prevention of the Myocardial infarction.

The electrocardiograph

The detected electrical signal is about the millivolt. The temporal precision necessary is lower than 0.5 ms (order of magnitude of the duration of a spike of Cardiac pacemaker.

The apparatuses were, up to one recent time, analogical. Most recent are numerical. The sampling rate reaches nearly 15 Khz.

A numerical filtering makes it possible to eliminate the secondary signals of high frequencies to the muscular activity other that cardiac and the interferences from the electricals appliance. A filter low frequency makes it possible to decrease the undulations of the secondary base line to breathing.

The quality of the signal can be improved by the average of several complexes, but this function involves artefacts in the event of irregularities of the cardiac rhythm or Extrasystole S, especially ventricular. This technique of average is particularly employed on the apparatuses adapted to the tests of effort where the layout is strongly artéfacté by the patient moving.

The numerical layout can be then stored on a computer support. There does not exist however, to date, of general format, as can the being the standard DICOM in Medical imagery.

Twelve derivations

The ECG with 12 derivations was standardized by an international convention. They make it possible to have a three-dimensional idea of the electric activity of the heart.

Six frontal derivations

  • DI : bipolar measurement between right-hand man and left arm.
  • DII: bipolar measurement between right-hand man and left leg.
  • DIII: bipolar measurement between left arm and left leg.
The letter D for derivation is not of use in the Anglo-Saxon countries which call quite simply them I, II and III

  • aVR : unipolar measurement on the right-hand man.

  • aVL: unipolar measurement on the left arm.
  • aVF: unipolar measurement on the left leg.
The letter has means " augmentée"

DI, DII, and DIII describe the triangle of Einthoven, and one can calculate the value of all these derivations starting from the signal of two of them. For example, if one knows the values of (DI) and (DII): Statement of the Theory of Einthoven: the heart is in the center of an equilateral triangle formed by the upper limbs and the root of the left thigh.

  • III = II - I

  • aVF = II - I/2
  • aVR = - I/2 - II/2
  • aVL = I - II/2

These equations explain why the numerical electrocardiograms more actually record only 2 derivations and restore the 4 others from those by simple calculation.

Six derivations précordiales

  • V1 : the 4th space intercostal right, flat rim of the sternum (parasternal).

  • V2: the 4th left space intercostal, left edge of the sternum (parasternal).
  • V3 halfway between V2 and V4.
  • V4: the 5th left space intercostal, on the line médioclaviculaire.
  • V5: even horizontal that V4, former line axillaire.
  • V6: even horizontal that V4, average line axillaire.

Other derivations

They are made in certain cases to refine, for example, the topographic diagnosis of a Myocardial infarction.

  • V7 : even horizontal that V4, posterior line axillaire.

  • V8: even horizontal that V4, under the point of the scapula.
  • V9: even horizontal that V4, halfway between V8 and the thorny posterior ones.
  • V 3R, symmetrical of V3 compared to the line of centers.
  • V 4R, symmetrical of V5 compared to the line of centers.
  • VE, on the level of the xiphoïde sternale.

The electric axis of the heart

It is the angle of the electric field generated by the cardiac cells during ventricular activation. One compares this field to a single vector in the frontal plan. The axis is measured by the comparison of the amplitudes (ideally of surfaces) respective of segment QRS (positivity - negativity) in frontal derivations. The greatest positivity of the QRS (wave R) gives an good idea of the axis of the heart. Like physiological depolarization is made node AV towards the point of the ventricles, the average axis of the heart is located between 30 and 60° but it can be normal between -30° and +100°. One speaks about left axial deviation beyond -30° and right axial deviation beyond +100°. In certain configurations, the electric axis is not measurable because located in a plan perpendicular to the frontal plan, this is not a pathological sign of layout. The electric axis of the heart in the horizontal plane is used definitely less in practice. An abnormal axis can be the sign of disturbances in the sequence of activation of the ventricles or even of cellular damage.

right Axis . Center heart between +90 and +120° (surface of the QRS in D3 > D2, VF comparable with D3, negative in VR). This angulation is physiological (subject longiligne) or evokes a right ventricular overload.

left Axis . Center heart between +30 and - 30° (surface of the QRS in D1 > D2, VL comparable with D2, almost isoelectric in VF). This angulation is physiological (subject bréviligne) or evokes a left ventricular overload.

Axis hyperdroit . Center heart > 120° (surface of the QRS in D3 > D2, negative in D1 and positive in VR). This angulation evokes a right ventricular overload or a posterior hémibloc left beyond 100°.

Axis hypergauche . Center heart < -30° (surface of the positive in D1 and negative QRS in D2-D3). This angulation evokes a left ventricular overload or a former hémibloc left beyond -45°. Indifferent axis. Center heart between +60 and +90° (surface of the QRS in D2 > D1 > D3, positive in VL, D1 comparable with VF)

Axis in No man' S Land . Center located in No man' S Land (180-270°). If there is not error in the position of the electrodes, such an axis evokes a ventricular origin of the QRS in favor of a ventricular tachycardia. It represents an activation of the point of the heart into the base and thus the opposite of what occurs in the event of activation via the beam of His.

perpendicular Axis . Center incalculable heart because perpendicular to the frontal plan (all the QRS appreciably have the same amplitude and same morphology). This aspect is secondary with a rocker of the heart towards the sagittal plan.

vertical Axis . Center heart between 60 and 90° (surface of the QRS in D2 > D3 > D1, negative in VL, and D2 comparable with VF).

Medical use of the ECG

What is what a good ECG?

It must comprise:
  • 12 derivations comprising some complexes, as well as a longer layout of at least a derivation, allowing to visualize well the Cardiac rhythm,
  • identity of the patient,
  • the date and the hour of the layout, and possibly, the circumstances of this last (systematic, pain, palpitations…),
  • a correct calibration: 25 m/s and 1 cm/microvolt, this last having to be proven by a visible signal of calibration on the layout.

The layout must be in addition unscathed as much as possible of electric parasites on the whole of derivations and with a rectilinear base line (and not undulating).

Bases of the interpretation of a ECG

The reading and the interpretation of a ECG require a great practice which can be acquired by the Médecin only by one practical regular. There exist Logiciel S delivered with certain electrocardiographs which can help with the diagnosis, but their approximate reliability does not make it possible to in no case to replace the doctor.

A normal ECG does not eliminate to in no case a disease of the Cœur. An abnormal ECG can be also completely pain-killer. The doctor makes use of this examination only like one tool among others, allowing to bring arguments to support his diagnosis.

The electric layout comprises several repetitive accidents called “waves”.

  • the wave P corresponds to the depolarization (and the contraction) of the auricles. One analyzes his form, his duration, his height, his axis (cf above) and his synchronization with:
  • the wave QRS (called also complex QRS) which corresponds to the depolarization (and the contraction) of the ventricles. The wave Q is the first negative wave of the complex. The wave R is the first positive component of the complex. The wave S is the second negative component. According to derivation and his form, one speaks thus about aspect “QS”, “RS”, even “RSR'” (for a form in M with two positivities). The form and the size of the QRS depend on the disease of the cardiac muscle under unclaimed but with a very important variability.
  • the wave T corresponds to the essence of the repolarization (relieving) of the ventricles, this one starting as of the QRS for some cells.
  • the wave T atriale is masked by wave QRS and corresponds to the repolarization (relieving) of the auricles. This one is negative.

The interval PR is time between the beginning of P and the beginning of the QRS. He is the witness of the time necessary to the transmission of the electric impulse of the auricles to the ventricles.

The interval " QT" measured beginning of the QRS at the end of the wave T corresponds to the whole of depolarization and the cardiac repolarization. Its lengthening even its shortening is related in certain circumstances to the appearance of a disorder of the ventricular rate/rhythm complexes named " Twists of pointes" potentially mortal. Unfortunately its measurement is burdened with many uncertainties making its study difficult.

One can also calculate the heart rate (many QRS per unit of time) and see whether the rate/rhythm is regular or not. If the heart rate is regular, the heart rate is equal to: 60/durée in seconds of interval R-R.

In the event of anomaly, the layout must be ideally compared with an old ECG at the same patient: an abnormal ventricular repolarization does not have the same significance at all if it has existed for several years that if it is recent.

Complementary techniques

Cardiac Holter

It is about a portable device allowing the recording of one or more derivations of the ECG during several hours.

See cardiac article Holter

ECG during the test of effort

See article test of effort.

Scope of monitoring

ECG with high amplification

This type denregistrement is especially employed to detect the appearance of arrhythmia and the modification of segment STT over one 24 hours duration. the electrodes used are as for all the ECG of the electrodes in Ag/AgCl. recommendations related to the choices of the ways recorded at the time of lacquisition of ECG HOLTER were the subject of several work. The recordings are either analogical or numerical.

Invasive electrophysiology

See too

Random links:De ciclo simple | Kristian Birkeland | -852 | Jean Thuillier | Godsend, prohibited experiment | Isaac Ganon | 64