CVS3-ECG

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ELECTROCARDIOGRAM (ECG)| Tutorial D-1 CVS

130110110177|Gabriella Chafrina| 18/09/1

The heart consist of 3 types of cells:

Pace maker cells

Smalls cells approximately 5 to 20 um long

Able to depolarize spontaneously

1 electrical cycle of depolarization & repolarization = action potential

Dominant pace maker cells : sinoatrial (SA) node, which rate can vary

(: symphatetic stimulation, : vagal stimulation)

Electrical conducting cells

Long, thin cells

The electrical conducting cells of the ventricles join to form distinct electrica

pathways

Myocardial cells

Responsible for heavy labor of repeatedly contracting and relaxing When a wave of depolarization reaches a myocardial cell, calcium

is released within the cell, causing the cell to contract. This process

is called excitationcontraction coupling

Time and voltage

The waves reflect the electrical activity of the myocardial cells

Three chief characteristics of the waves: duration, amplitude, and configuration

EKG paper

Is a long, continuous roll of graph paper

The light lines = small squares of 1 x 1 mm

The dark lines = large squares of 5 x 5 mm

The horizontal axis measures time

1 small square = 0.04 s

1 large square = 0.04 s x 5 =0.2 s

The vertical axis measures voltage

1 small square = 0.1 mV

1 large small = 0.5 mV

Atrial DepolarizationP Waves

The sinus node fires spontaneously and wave of depolarization begins to

spread outward into the atrial

P wave is a recording of the spread of depolarization through the atrial

myocardium from start to finish 1stpart of the P waveright atrial depolarization

2ndpart of the P waveleft atrial depolarization

A Pause

A conduction pause separates the atria from the ventricles.

Atrioventricular (AV) mode slows conduction to allow the atria to finish contracting

before the ventricles begin to contract. So, its permit the atria to empty their volume of

blood completely into the ventricles before the ventricles contract.

Influence by ANS like SA node.

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Ventricular DepolarizationVentricular Conducting Systsem + QRS complex

Ventricular conducting system

1/10s after depolarizing waves escapes AV nodeenter electrical conducting cell

consist of 3 part:

Bundle of His

This emerges from the AV node and divides into right and left bundle branches

Bundle branches

Right bundle branch carries the current down the right side of the

interventricular septum to the apex of the right ventricle

Left bundle branch divides into 3 major fascicles :

o Septal fascicle: depolarizes the interventricular septum in a left-to-right-

direction

o Anterior fascicle: runs along the anterior surface of the left ventricle

o Posterior fascicle: sweeps over the posterior

Terminal Purkinje fibers

Termination of the right and left ventricular branches and its fascicles. These

fibers deliver the electrical current into the ventricular myocardium

QRS complex

The beginning of ventricular myocardial depolarization and ventricular

contraction is marked by QRS complex

The amplitude of the QRS complex is greater that the P wave because

ventricles are larger than the atria

Parts of QRS complex:

First deflection downwardQ wave.

First upward deflectionR wave.

Second upward deflectionR' (R-prime).

First downward deflection following an upward deflectionS wave

If entire configuration consists of one downward deflectionQS wave.

The earliest part of the QRS complex represents depolarization of the

interventricular septum by the septal fascicle of the left bundle branch

Myocardial cells depolarizepass through a brief refractory period

(resistant to the further stimulation)repolarize

RepolarizationT waves

Myocardial cells depolarizepass through a brief refractory period (resistant to

the further stimulation)repolarize

Ventricular repolarization inscribes the T wave, third wave on ECG

Segments and Intervals

Segment : a starlight line connecting 2 waves

Interval : at last 1 wave + the connecting straight line

PR Interval : measures the time from the start of atrial depolarization to the start of ventricular depolarization

PR segment: measures the time from the end of atrial depolarization to the start of ventricular depolarization

ST segment : measures the time from the end of ventricular depolarization to the start of ventricular repolarizati

QT interval : measures the time from the start of ventricular depolarization to the end of ventricular repolarizatio

QRS interval : measures the time of ventricular depolarization

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Making Waves

A wave of depolarization moving toward a positive electrode causes a positive deflection on the EKG

A wave of depolarization moving away from a positive electrode causes a negative deflection on the EKG

A wave of repolarization moving toward from a positive electrode cause a negative deflection on the EKG

A wave of repolarization moving away from a positive electrode causes a positive deflection on the EKG

A perpendicular wave produces a biphasic wave

12lead EKG

6 limb leads: view the heart in the vertical plane called the frontal plane 3 standard leads

Lead I

The left arm positive and the right arm negative

Angle of orientation: 0o

Lead II

The legs positive and the right arm negative


Lead III

The legs positive and the right arm negative


3 augmented leads Lead AVL

The left arm positive and the other limbs negative

Angle of orientation: -30o

Lead AVR

The right arm positive and the other limbs negative

Angle of orientation: -150o

Lead AVF

The legs positive and the other limbs negative

Angle of orientation: +900

6 precordial leadsin a horizontal plane

V1 : 4thintercostals space to the right of the sternum V2 : 4thintercostals spave to the left sternum

V3 : between V2 an V4

V4 : 5thintercostals space in the midclavicular line

V5 : between V4 and V5

V6 : 5thintercostals space in the misaxillary line

Anterior groups : Leads V1, V2, V3, V4

Inferior groups : Leads II, III, AVF

Left lateral goups : leads I, AVL, V5, V6

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Definition: partial intraventricular block every other heartbeat that make regular alternating amplitude, direction, or

configuration of the QRS complexes in any or all leads

- The RR intervals remain unchanged (regular)

- Total electrical alternans refers to involvement of the P, QRS, and T waves and occasionally the U wave

Etiology:

Alternans of the QRS complex is rare in patients with cardiac tamponade and occurs in some patients with a lrge

pericardial effusion, particularly with malignancy

Total electrical alternans is almost diagnostic of cardiac tampinade, although it occurs in fewer than 10% of patients

with tamponade and may be associated with a swinging heart on echocardiography

Severe coronary artery and hypertrophic heart disease is a reare cause of electrical alternans

Supraventricular tachycardia with a very rapid ventricular rate, mainly occurring in patients with Wolff-Parkinson-Wh

syndrome (orthodromic) reentrant tachycardia, is another cause

conditions that depress the heart (ex: ischemia, myocarditis, or digitalis toxixity) can cause incomplete intraventricular

heart block

Pathophysiology:

The pathophysiologic mechanisms that cause electrical alternans can be divided into 3 categories

o Repolarization alternans (ST, T, U alternans)o Conduction and refractoriness alternans (P, PR, QRS alternans)

o Alternans due to cardiac motion

True electrical alternans is a repolarization or conduction abnormality of the Purkinje fibers or myocardium

Electrical alternans due to cardiac motion is effectively artifact, as the heart swings in relation to the chest wall and

electrodes, with a period twice that of the heart rate.

- In pericarditis, if effusion sufficiently largeheart may actually rotate freely within the fluid filled sac/heart swing

electric axis keep changes when ECGElectrical alternans

- Rapid heart rateimpossible for some Purkinje system to recover from previous refractory period quickly enough to

respond during every succeeding heart beat

Reference:

M. Gabriel Khan; Rapid ECG Interpretation

Malcolm S. Thaler; The Only EKG Book Youll Ever Need

Scott W. Sharkey;A Guide to Interpretation of Hemodynamic Data in The Coronary Unit

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