ECG 2013 presentation.pptx

8/14/2019 ECG 2013 presentation.pptx

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Electrocardiography


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Electrocardiography

Electrocardiogram (ECG or EKG): a graphicrecording of electric potentials generated by theheart.

The signals are detected by means of metalelectrodes attached to the extremities and chestwall and then are amplified and recorded by the

electrocardiograph. ECG leads actually display the instantaneous

differences in potential between the electrodes.


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Electrophysiology

Initiating event for cardiac contraction- Depolarization. Source of electric currents in the heart cardiac

pacemaker cells, specialized conduction tissue, and theheart muscle itself.

ECG, records only the depolarization (stimulation) andrepolarization (recovery) potentials generated by theatrial and ventricular myocardium.

Origin of depolarization stimulus for the normalheartbeat sinoatrial (SA) node a collection of pacemaker cells . These cells exhibit automaticity .


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The first phase of cardiac activation is spread ofdepolarization wave through right and left atria,followed by atrial contraction.

Next, the impulse stimulates pacemaker andspecialized conduction tissues in theatrioventricular (AV) nodal and His-bundle areas (AV junction).

Two main branches of the bundle of His: rightand left bundles, transmit depolarizationwavefronts to the right and left ventricularmyocardium via Purkinje fibers.


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Main left bundle bifurcates into two primarysubdivisions: a left anterior fascicle and a leftposterior fascicle.

The depolarization wavefronts then spreadthrough the ventricular wall, fromendocardium to epicardium, triggeringventricular contraction.


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ECG Waveforms and Intervals

P wave = atrial depolarization The QRS complex = ventricular depolarization ST-T-U complex = ventricular repolarization. The J point = junction between end of QRS

complex and beginning of ST segment. Atrial repolarization is usually too low in

amplitude to be detected, but it may becomeapparent in conditions such as acutepericarditis and atrial infarction.


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The QRS-T waveforms correspond in a general way withthe different phases of simultaneously obtainedventricular action potentials

The rapid upstroke (phase 0) = onset of QRS. The plateau (phase 2) = ST segment Active repolarization (phase 3) = T wave. Factors that decrease the slope of phase 0 by impairing

the influx of Na + (e.g., hyperkalemia and drugs such asflecainide) tend to increase QRS duration.

Conditions that prolong phase 2 (amiodarone,hypocalcemia) increase the QT interval.

In contrast, shortening of ventricular repolarization(phase 2), such as by digitalis administration orhypercalcemia, abbreviates the ST segment.


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electrocardiogram Recorded on special graph paper divided into 1-mm 2

gridlike boxes. 1 mm horizontal divisions = 0.04 (40 ms), Heavier lines = intervals of 0.20 s (200 ms). Vertically 1 mV = 10 mm Four major ECG intervals: R-R, PR, QRS, and QT The heart rate can be computed readily from the

interbeat (R-R) number of large (0.20 s) time units between consecutive R

waves divided by 300 or the number of small (0.04 s) units divided by 1500.


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PR interval measures the time between atrial andventricular depolarization, (normally 120 200ms)

QRS interval reflects the duration of ventriculardepolarization. (normally 100 110 ms or less)

The QT interval includes both ventriculardepolarization and repolarization times andvaries inversely with the heart rate.


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A rate-related ("corrected") QT interval, QT c,can be calculated as:

QT/R-R and normally is 0.44 s. (Some references give QT c upper normal limits

as 0.43 s in men and 0.45 s in women.


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The QRS complex is subdivided intospecific deflections or waves.

Q wave = A negative initial QRS deflection in aparticular lead

R wave = The first positive deflection is termed an

S wave = A negative deflection after an R wave isan. R' and S = Subsequent positive or negative waves

respectively.

Lowercase letters (qrs) are used for waves ofrelatively small amplitude. QS wave = An entirely negative QRS complex


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ECG Leads

The 12 conventional ECG leads record thedifference in potential between electrodesplaced on the surface of the body.

These leads are divided into two groups: six limb (extremity) leads record potentials

transmitted onto the frontal plane six chest (precordial) leads record potentials

transmitted onto the horizontal plane


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The six chest leads are unipolar recordings obtained byelectrodes in the following positions:

lead V 1, fourth intercostal space, just to the rightof the sternum;

lead V 2, fourth intercostal space, just to the left of

the sternum; lead V 3, midway between V 2 and V 4; lead V 4, midclavicular line, fifth intercostal space;

lead V 5, anterior axillary line, same level as V 4; and lead V 6, midaxillary line, same level as V 4 and

V5.


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frontal and horizontal plane electrodesprovide a three-dimensional representation ofcardiac electrical activity.

The conventional 12-lead ECG can besupplemented with additional leads in specialcircumstances. For example, right precordialleads V 3R, V4R, etc., are useful in detectingevidence of acute right ventricular ischemia.


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ECG configuration

Positive (upright) deflection is recorded in alead if a wave of depolarization spreadstoward the positive pole of that lead,

A negative deflection is recorded if the wavespreads toward the negative pole.

If the mean orientation of the depolarization

vector is at right angles to a particular leadaxis, a biphasic (equally positive and negative)deflection will be recorded.


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Genesis of the Normal ECG

P Wave Normal atrial depolarization vector is oriented downward and toward the

subject's left, reflecting the spread of depolarization from the sinus nodeto the right and then the left atrial myocardium.

Since this vector points toward the positive pole of lead II and toward thenegative pole of lead aVR, the normal P wave will be positive in lead II andnegative in lead aVR .

By contrast, activation of the atria from an ectopic pacemaker in thelower part of either atrium or in the AV junction region may produceretrograde P waves (negative in lead II, positive in lead aVR).

The normal P wave in lead V1 may be biphasic with a positive componentreflecting right atrial depolarization , followed by a small (


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QRS Complex

Can be divided into two major sequential phases, and each phase can berepresented by a mean vector

First phase = depolarization of the interventricular septum from the left to theright and anteriorly (vector 1).

Second results = simultaneous depolarization of the right and left ventricles; itnormally is dominated by the more massive left ventricle, so that vector 2 points

leftward and posteriorly. Therefore, a right precordial lead (V 1) will record this biphasic depolarizationprocess with a small positive deflection (septal r wave) followed by a largernegative deflection (S wave).

A left precordial lead, e.g., V 6, will record the same sequence with a small negativedeflection (septal q wave) followed by a relatively tall positive deflection (R wave).

Intermediate leads show a relative increase in R-wave amplitude (normal R-waveprogression) and a decrease in S-wave amplitude progressing across the chestfrom right to left.

The precordial lead where the R and S waves are of approximately equal amplitudeis referred to as the transition zone (usually V 3 or V4)


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The QRS pattern in extremity leads may vary from one normal subject toanother depending on the electrical axis of the QRS, which describes themean orientation of the QRS vector with reference to the six frontal planeleads.

Normal QRS axis = 30 to +100 Axis more negative than 30 = left axis deviation , Axis more positive than +100 = right axis deviation . Left axis deviation = normal or more commonly left ventricular

hypertrophy, a block in the anterior fascicle of the left bundle system (leftanterior fascicular block or hemiblock), or inferior myocardial infarction.

Right axis deviation = normal (particularly in children and young adults), a

spurious finding due to reversal of the left and right arm electrodes, or inconditions such as right ventricular overload (acute or chronic), infarctionof the lateral wall of the left ventricle, dextrocardia, left pneumothorax,and left posterior fascicular block.


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T Wave and U Wave

Normally, the mean T-wave vector is oriented roughly concordant with themean QRS vector (within about 45 in the frontal plane).

Since depolarization and repolarization are electrically opposite processes,this normal QRS T-wave vector concordance indicates that repolarizationnormally must proceed in the reverse direction from depolarization (i.e.,from ventricular epicardium to endocardium).

The normal U wave is a small, rounded deflection (1 mm) that follows theT wave and usually has the same polarity as the T wave.

An abnormal increase in U-wave amplitude drugs (e.g., dofetilide, amiodarone, sotalol, quinidine, procainamide,

disopyramide) hypokalemia.

Very prominent U waves = a marker of increased susceptibility to thetorsades de pointes type of ventricular tachycardia

Inversion of the U wave in the precordial leads = abnormal and may be asubtle sign of ischemia.

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