ECG

Electrocardiography (ECG ) is a graphical representation of the electrical activity of the heart . This electrical activity is related to changes in electrical potential of specialized cells in the contraction  and specialized cells in automatism and conduction of impulses. It is collected by surface electrodes on the skin .
The electrocardiogram is paper tracing of the electrical activity in the heart . The electrocardiograph is the apparatus for an electrocardiogram . The cardiac monitor ,or scope , is a device to display the route on a screen .

This is a quick review taking only a few minutes , painless and non-invasive , devoid of danger. It can be done in doctor's office , hospital or home . However, its interpretation is complex and requires some experience of the clinician . It allows to highlight various cardiac abnormalities and has an important place in the diagnostic tests in cardiology, such as for coronary artery disease .

The Electrocardiograph

The ECG records the electrical activity of the heart . The position of the electrodes relative to the heart determines the appearance of deflections on the recording.

The detected electrical signal is of the order of millivolt . The required timing accuracy is less than 0.5 ms. ( order of magnitude of the duration of a pacemaker spike .)

The appliances were , until recently , analog . Newest are digital. The sampling rate is nearly 15 kHz3 .
A digital filter eliminates the high frequency signals to high activity other than the heart muscle and interference electric apparatus . A low-frequency filter reduces the ripple of the secondary baseline breathing.
The signal quality can be improved by averaging more complex, but this function causes artifacts in case of irregular heart rhythm or extrasystoles, ventricular above . This averaging technique is particularly suitable for devices used in exercise tests where the route is heavily artéfacté by the patient moving .
Digital layout can then be stored on a computer system . The SCP- ECG standard tends to develop . DICOM (used in medical imaging ) can also store waveform data types including ECG4 5.

The twelve leads
The 12-lead ECG has been standardized by a internationale6 convention. These give a three-dimensional picture of the electrical activity of the heart .
The 12-lead ECG has six frontal leads ( aVR aVL DI DII DIII and aVF ) and six precordial leads (V1 to V6 ) .
Six frontal derivations
DI: bipolar measurement between right arm (-) and left arm (+).
DII : bipolar measurement between right arm (-) and left leg (+).
DIII : bipolar measure between left arm (-) and left leg (+).
The letter D for derivation is not in use in Anglo-Saxon countries that simply call I, II and III.
aVR : unipolar measurement on the right arm.
aVL : unipolar measurement on the left arm.
aVF : unipolar measurement on the left leg .
The letter "a " means " increased ."

DI , DII , DIII and describe the Einthoven triangle , and we can calculate the value of all these diversions from the signal of two of them. For example, if we know the values ​​of ( DI ) and ( DII ) : Statement of the Theory Einthoven : the heart is 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 that the digital electrocardiogram not actually record that two derivations and restore the remaining 4 from them by simple calculation.

Six precordial leads
V1: 4th intercostal space right right edge of the sternum ( parasternal ) .
V2 : 4th left intercostal space left sternal border ( parasternal ) .
V3 midway between V2 and V4 .
V4 : 5th left intercostal space on the midclavicular line .
V5 : same horizontal and V4 , anterior axillary line .
V6: same horizontal and V4 , mid-axillary line .

Other derivations 
They are made in some cases to refine , for example, the topographic diagnosis of myocardial myocarde7 .
V7: same horizontal and V4 , posterior axillary line .
V8: same horizontal and V4 , under the tip of the scapula ( shoulder blade) .
V9 : same horizontal and V4 , midway between the posterior spinous and V8 .
V3R , V3 symmetrical relative to the midline.
V4R , V4 symmetrical relative to the midline.
VE, at the sternal xiphoid .

The electrical axis of the heart

Electrical axis and frontal derivations .
Polarity of the QRS complex on the first 3 frontal derivations and electrical axis of the heart .
This is the angle of the electric field generated by the heart cells at the ventricular activation. This field to a single vector is assimilated in the frontal plane . The axis is measured by comparing the amplitudes (preferably surfaces ) of the respective QRS segment ( positive - negative ) in the end lead . The largest positive QRS (R wave ) gives a good idea of the axis of the heart . As the depolarization is physiological AV node to the ventricles of the tip , the center axis of the heart is located between 30 and 60 ° but may be normal between 100 ° and -30 ° . One speaks of axial deflection left beyond -30 ° deflection and axial right beyond 100 ° . In some configurations, the electrical axis is measurable because it is situated in a plane perpendicular to the frontal plane , this is not a sign of pathological path. The electrical axis of the heart in the horizontal plane is much less used in practice. Abnormal axis may indicate disturbance in the sequence of activation of the ventricles or even cellular damage .

Right axis :Axis of the heart between +90 and +120 ° ( surface QRS D3 > D2, D3 in VF comparable , negative VR ) . This angulation is physiological in children and in the lanky subject, it is abnormal in case of right ventricular overload ( as in acute or chronic pulmonary heart or mitral stenosis ) .
Left axis: Axis of the heart between +30 and -30 ° ( surface QRS D1 > D2, D2 VL comparable to almost isoelectric VF ) . This angulation is physiological in adults over 50 years and in obese , it is pathological if left ventricular overload (as in hypertension , aortic valve disease , mitral regurgitation ) .
Hyperdroit axis : Axis of the heart > 120 ° ( surface QRS D3 > D2, D1 negative and positive VR ) . This angle is always pathological and can evoke a congenital heart disease, a left posterior hemiblock beyond 100 ° or right ventricular overload.
Hypergauche axis :Axis of the heart < -30 ° ( surface positive and negative QRS D1 D2- D3). This angulation evokes a left ventricular overload or left anterior hemiblock beyond 45 °.
Axis indifferent: Axis of average heart , between 30 and 60 ° ( surface QRS D2 > D1 > D3 , positive in VL , VF comparable to D1 ) , which is physiological .

Axis in the no man's land :Axis in the no man's land ( 180-270 °). If there is no error in the position of the electrodes , such an axis refers QRS ventricular origin for a ventricular tachycardia . It reflects activation of the tip to the base of the heart and therefore the opposite of what happens in the case of activation via the bundle of His .

Axis perpendicular : Heart since incalculable axis perpendicular to the frontal plane ( QRS all have substantially the same amplitude and the same morphology ) . This is secondary to toggle the heart to the sagittal plane.

Vertical axis: Axis of the heart between 60 and 90 ° ( surface QRS D2 > D3 > D1, negative in VL , and comparable to D2 VF ) , physiological adolescent or slender subject. In older or obese patient , it can evoke a right heart overload.

Medical use of ECG
A good ECG  must have
The analysis starts with an ECG monitoring the interpretability of the path. The calibration of the unwinding speed of the paper must be 25 mm / s , and the amplitude of 1 cm to 1 mV . In this case , 0.1 mV = 0.04 s = a 1 mm square on the line side .

 It must include:
12 leads some having complex , and a longer path at least one bypass , allowing to visualize the heart rate
the identity of the patient ,the date and time of the track, and possibly the circumstances of the latter (systematic , pain, palpitations ... )

correct calibration : calibration of the unwinding speed of the paper of 25 mm / s and amplitude calibration of 1 cm / mV . These two pieces of information are systematically reported on the trace and calibration amplitude is shown by a calibration signal visible on the plot. A good calibration is essential for analyzing the trace . The Ashmann unit is defined as 0.1 mV equal 0.04 s , which corresponds to a square of 1 mm side. Any change in calibration changes the amplitude deflections and makes the ECG uninterpretable under references conventionally used .

The route must be also free as possible from electrical noise on all leads and a baseline straight (not wavy ) .

Search a malposition of electrodes must be made. The P wave should be negative in aVR and positive in D1 , D2 and V6. Furthermore, the QRS complex must have a morphology and amplitude progressing harmoniously in the precordial leads .

Basics of ECG interpretation
Reading and ECG interpretation requires a great habit that can be acquired by the physician as a regular practice. There are software programs come with some electrocardiographs can assist in the diagnosis , but they can not substitute the doctor.

A normal ECG does not eliminate any cases of heart disease . An abnormal ECG can also be quite trivial. The doctor uses this test as a tool among others to provide arguments to support his diagnosis.

After the checks mentioned above on the interpretability of the plot , the ECG analysis continues with the study of rhythm and heart rate (number of QRS per unit time ) :

A normal heart rhythm is called a sinus rhythm,  cardiac activity under the control of the sinus node is characterized by:

a. a steady pace with a constant R -R space
b. the presence of a P-wave and before each QRS QRS after each of a P wave ,
c. P wave axis and normal morphology ,
d. a constant PR interval .

If the rate is regular , we can determine a heart rate which is equal to the inverse of the RR interval 
( multiplied by 60 , to be expressed as a number of beats per minute ) . In practice, it may be determined by dividing 300 by the number of small squares of 5 mm between two QRS complexes ; storing the sequence "300 , 150, 100 , 75, 60, 50 " and allows rapid estimation of the frequency, for example if there are 2 squares between two QRS frequency is 150 beats per minute, s' 4 there is 75 square , where currently it is of square 6 508.9 .


Representation of a normal ECG
The plot has several repetitive electrical accidents called " waves ", and intervals between waves. The main measures to be carried out during the analysis of ECG are those of the P wave , PR interval , QRS complex , the registration of intrinsécoïde deflection time , the J point , space QT , ST segment and T wave finally.

P wave corresponds to the depolarization ( and contraction ) of the atria , right and left . Its morphology ( in positive or biphasic and monophasic V1 or V2 in all other derivations ) , duration ( which is 0.08 to 0.1 seconds ) is analyzed , its amplitude ( less than 2.5 mm 2 and D2 the V and V2 mm ) , its axis ( determined in the same manner as the axis of the QRS , normally between 0 and 90 ° , typically about 60 ° ) and synchronization with the QRS wave .

PR interval ( or PQ ) is the time between the beginning of the beginning of the P and QRS . He is the witness of the time required for the transmission of electrical impulses from the sinoatrial node to the atria myocardial tissue of the ventricles ( atrioventricular conduction) . The normal duration , measured from the beginning of the P wave to the beginning of the QRS complex is 0.12 to 0.20 seconds . The duration of the PR interval decreases when the heart rate increases. It normally is isoelectric .

QRS wave ( also called QRS complex ) corresponding to depolarization ( and contraction ) of the ventricles , left and right . The Q wave is the first negative wave complex. The R wave is the first positive component of the complex . The S wave is the second negative component . Following the derivation and shape, hence the term aspect " QS ", " RS" or "SERP " '( for M-shape with two positives ) . The shape and amplitude of the QRS vary according to diversions and possible pathology of the heart muscle underneath. The QRS complex has a normal duration of less than 0.1 seconds , often less than 0.08 s . The normal QRS axis is between 0 and 90 ° . The transition zone corresponding to the precordial lead wherein the isoelectric QRS are normally located in V3 or V4 .

Point D is the point of transition between the QRS complex and the ST segment . It normally is isoelectric .
Segment ST corresponds to the time between the beginning of the ventricular depolarization represented by the QRS complex and the beginning of the T wave . The normal ST segment J is isoelectric point at the beginning of the T wave

QT interval measured from QRS onset to the end of the T wave is the set of ventricular depolarization and repolarization ( time electrical systole ) . Its length varies depending on the heart rate , it decreases the heart rate increases and increases when the heart rate decreases. Elongation or shortening its is bound in certain circumstances the occurrence of complex ventricular rhythm disorder called " torsades de pointes " potentially fatal. And the QTc ( corrected QT ) is the measure of the QT interval corrected by the frequency with the formula QTc = QT / square root of RR space does one use . Cardiac hypoxia and disorders of the blood calcium concentration affect this intervalle10 .

T wave corresponds to most of the repolarization ( relaxation ) of the ventricles , the latter starting from the QRS for a few cells. Its length is 0.20 to 0.25 seconds , the duration of the analysis is in the analysis of the duration of the QT interval . The normal line of the T-wave , calculated in the same way that the axis of the QRS , is between - 10 and 70 ° , often around 40 ° . The T wave is normally pointed, asymmetrical and ample in most derivations . It can be negative in V1 or in D3 and aVF . Its amplitude generally depends of the R-wave that preceded , it is between 1/8 and 2/3 of that of the R-wave and does not usually exceed 10 mm .
T-wave is masked by the atrial wave in the QRS and repolarization ( relaxation ) of the atria . This is negative .

U wave is a small deflection sometimes seen after the T wave in the precordial leads V to V4. It is positive in all leads except aVR , its origin is discussed.

In case of failure, the route should ideally be compared with an old ECG in the same patient : an abnormal ventricular repolarization has not the same meaning if it has existed for several years if it is recent.

Normal ECG

Features ECG normal:
Rhythm: sinus ( the majority of QRS complexes are controlled by a sinus P wave after the sinoatrial )
P wave : time < 0.12 s ; Amplitude < 0.25 mV ; Positive and monophasic in all leads except aVR (where it is negative) and V1 (where it is biphasic ) ; Axis between 0 and 90 °
PR interval : isoelectric ; between 0.12 and 0.20 s
QRS complexes : Time < 0.11 s ; Time to onset of deflection intrinsécoïde <0.04 s and 0.06 s in V1 V6 ; Axis between 0 and 90 ° ; Transition zone in V3 or V4
Repolarization : Point J and isoelectric ST segment; positive T waves , asymmetric , close to that of the QRS axis. U waves absent or below the waves under T. QT interval heart rate.

NB . Many variations exist to the normal , and make the interpretation difficile12 .



Complementary techniques
Holter heart:  This is a portable device for recording one or more ECG leads during several hours.
ECG during the exercise test
Patient monitor : Surveillance monitor , to monitor the patient's rhythm . The number of electrodes is variable , ranging from three ( two electrodes for a bipolar lead and a neutral electrode ) , 5 (for recording standard derivations : four electrodes plus neutral ) or higher . The monitoring console can be located close to the patient or to offset distance ( in a nursing station , for example) connected thereto via a wired or wireless system . This is called " telemetry ". The device can also monitor other physiological parameters such as respiratory rate , blood pressure ... and includes a computer to process the data and present it in the form of reports. Several transmission frequencies without son were reserved for strictly medical purposes, to minimize the risk of interference with other appareils13 . Among the transmission frequencies without dedicated to medical son , we find particularly WMTS and ISM bands .

High amplification ECG : This record is used mainly for detecting the occurrence of arrhythmia and ST -T change over a period of segment 24 hours . The electrodes are used as for all ECG electrodes Ag / AgCl . Recommendations related to the choice of channels recorded on the acquisition of ECG Holter were the subject of several studies . Records are either analogue or digital .

Electrophysiology percutaneous: Examination of percutaneous cardiac electrophysiology is often called " Electrophysiological Study ." This is an examination performed under local anesthesia . Catheters are inserted , usually , in a femoral and guided under fluoroscopy to the heart vein . These catheters are provided with one or more electrode (s ) connected (s ) to an amplifier, to record the potential differences . The signal is interpreted as a function of the location of the catheter and the surface electrocardiogram .

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