Information contained in the ECG

Disturbances of rhythm and conduction.

Ischemic damage to the myocardium.

The anatomical orientation of the heart.

Relative size of the heart chambers.

The influence of some drugs and electrolyte

disturbances (for example, hypokalemia).

Introduction to Electrocardiography

Precise interpretation of electrical activity of the heart as recorded with electrodes on the skin is difficult because:

The distribution of charge within the heart is multidirectional and constantly changing.

The configuration of cardiac action potentials varies from region to region.

The position of the heart changes continually throughout the cardiac cycle.

Conductivity through the extracellular fluid is not uniform.

The surface of the body is not simple geometrically.

Interpretation of electrical activity of the heart

Cell model to show origin of electrocardiogram record: resting cell

In the resting cell the surface is uniformly positive compared to the cell interior.

There is no potential difference between different points on the cell surface.

The ECG records a baseline (the isoelectric line).

Cardiac myocyte

positive electrode

ECG record

isoelectricNegative

electrode

Cell model to show origin of electrocardiogram record: partial depolarization creates a potential difference between two points on the cell surface

Depolarization of part of the cell membrane creates a potential difference between the resting and depolarized regions on the surface.By convention, current flows from negative to positive and from the depolarized region to the “resting” region.If the wave of depolarization is directed towards the positive electrode, the ECG records a positive (upward) deflection.

Negativeelectrode

positive electrode

ECG record

Current flows from negative to positive regions

depolarization

Cell model to show origin of electrocardiogram record: complete depolarization returns the ECG signal to baseline

When the cell is completely depolarized, once again there is no potential difference between any two points on the outside of the cell membrane.The recorded potential returns to the baseline level.

positive electrode

Negativeelectrode

ECG record

Cell model to show origin of electrocardiogram record: repolarization produces an ECG signal below baseline

As repolarization occurs, current flows in the opposite direction than during depolarization.Consequently the recorded potential is opposite in direction to the potential during depolarization.

Negativeelectrode

positive electrode

ECG record

repolarization

Cell model to show origin of electrocardiogram record: repolarization completed: ECG signal returns to baseline

Repolarization is slower than depolarization so the peak value is less than during depolarization and the duration of the wave is longer.With the return to the resting state the recorded potential returns to baseline.

Negativeelectrode

positive electrode

ECG record

Standard labeling of an ECG waveform in lead 1

Atrial depolarization & conduction through AV node

Atrial depolarization

Ventricular depolarization & atrial repolarization

Ventricular repolarization

Q

P

R

S T

QT

QRSPR

ST

Factors affecting the amplitude of the ECG signal

Obese subject with hypothyroidism

Hypertensive subject with cardiac hypertrophy (Signals are drawn to scale)

The amplitude of the ECG signal is affected by

the amount of myocardial tissue

the orientation of the heart in the chest

the thickness and type of tissue between the heart and the electrode.

For example, pericardial fluid, emphysematous lung tissue or obesity

increase electrical resistance between the heart and the electrodes,

reducing the amplitude of the ECG signal.

Correspondence between a ventricular action potential & the ECG

P TQRS

0

2

3

4

1

The QRS complex is produced by the summed upstrokes (phase 0) of the ventricular myocyte action potentials.The S-T segment corresponds to the plateaus of the action potentials.The T wave is produced by ventricular repolarization.

ECG intervals (seconds)

average range Events in heart

PR interval* 0.18** 0.12 to 0.20Atrial depolarization & conduction through AV node

QRS duration 0.08 0.06 to 0.10 Ventricular depolarization

QT interval 0.40 Varies with HRVentricular depolarization & repolarization

ST interval 0.32 ---Ventricles depolarized; ST interval normally lies on isoelectric line

* Measured from the beginning of the P wave to the beginning of the QRS complex

** Shortens as heart rate increases from average of 0.18 at a HR of 70 to 0.14 a t a HR of 130.

Normal ECG intervals: calculating the intervals in an ECG helps determine if the recording is normal or what kinds of abnormalities may exist.

Path of excitation in the heart

Sino-atrial node originates action potentials

Atrial myocytes

Atrioventricular node

Bundle of His

Right & left bundle branches

Ventricular myocytes

Purkinje fibers

Conduction velocity for action potentials is greater in the Purkinje fibers than in myocytes so excitation spreads rapidly through the ventricles allowing coordinated contraction of the ventricular muscle.

APs are propagated between myocytes via gap junctions.

Ventricular endocardium is depolarized before epicardium.

The duration of action potentials in the endocardium is greater than the duration of APs in the epicardium.

Myocytes in the epicardium begin to repolarize before myocytes in the endocardium.

Therefore repolarization spreads through the ventricle in the opposite direction to depolarization, so in the ECG depolarization (QRS) and repolarization (T wave) are both positive deflections.

Path of repolarization

depolarization

repolarization

Right & left bundle branches

endocardium

Purkinje fibers

epicardium

QRS

P T

Einthoven’s triangle: placement of electrodes

Einthoven’s triangle is a system of placing electrodes to measure the electrical activity of the heart in the frontal plane.Electrodes (red circles) are placed on

the right arm (RA), left arm (LA) and

left leg (LL).

The electrodes are grounded to the

right leg (RL).

LA

LL

-

+ +

+--RA

RL (ground)

ECG Leads measure potential differences (voltages) between electrodes.

Definition of leads in Einthoven’s triangle (frontal plane):Lead I: between RA and LALead II: between RA and LLLead III: between LA and LL

RA LA

LL

-

+ +

+-

-

Lead I

Lead II Lead III

Leads I, II & III are bipolar: they measure the potential difference (PD) between 2 points.

Lead I (zero)

- 30

+ 90

Normal range for the mean electrical axis is – 30 to + 90 degrees.

Size & polarity of the potentials are related to the path of depolarization or repolarization

RA LA

LL

-

+ +

+--

Lead I

Lead IILead III

The shaded arrow is a vector (has magnitude & direction).This vector represents depolarization during the QRS complex & is called the mean electrical axis of the heart in the frontal plane.The mean electrical axis is normally is 60 degrees to the horizontal.

Example of a shift in the mean electrical axis

RA LA

LL

-

+ +

+--

Lead I

Lead II

Lead III

A shift in the electrical axis changes the magnitude & direction of the ECG

signals. When evaluating an ECG the axis may be determined from the magnitude and direction of the signals in the six frontal plane leads.

Example: a shift to the right to 120 degrees:Lead I polarity is reversed because projection of the vector on lead I is from + to pole.Lead II magnitude is decreased, lead III magnitude is increased.

Einthoven’s triangle and the axial reference system for the ECG

LA

Lead III

RA

LL

-

+ +

+- -Lead I

Lead II

Leads I, II & III of Einthoven’s triangle

I

IIIII

+ 90 °

0 °

- 90°

+ 120 °

Leads I, II & III in axial reference system

An axial reference system is used to compare tracings from all 6 leads in the frontal plane.The 6 leads include Leads I, II and III and the unipolar limb leads AVR, AVL, & AVF (next slide)

The unipolar limb leads AVR, AVL, & AVF are measured as the PD between one limb and the electrical average of the other two limbs.

aVF (augmented left foot)

+aVR

+ aVF

+aVL

aVL (augmented left arm)aVR (augmented right arm)

Axial reference system: six leads measuring the ECG in the frontal plane

The mean electrical axis is determined from the magnitude and direction of the signals in the six frontal plane leads.

For each lead, the arrow points toward the positive pole.

- 150 °aVR

aVF

aVL

I

IIIII

- 30 °

+ 90 °

0 °

- 90°

+ 120 °

Normal range for the mean electrical axis is – 30 to + 90 degrees

An axis beyond + 90 ° is called right axis deviation.

An axis beyond - 30 ° is called left axis deviation.

V1

V2V3

V4

V5

V6

Six standard precordial leads

The 6 precordial leads (V1 to V6) measure potentials in a transverse plane

around the apex of the heart at right angles to the frontal plane.

They measure the PD between the electrode and ground so they are unipolar leads.

V1 V2 V3

V4

V5

V6

LV

There are 12 leads in a standard ECG

Six leads meaure potentials in the frontal plane

3 Leads (I, II & III) for Einthoven’s Triangle

3 Calculated leads:

aVR (augmented right arm)

aVL (augmented left arm)

aVF (augmented left foot)

Six leads measure potentials in a transverse plane across

the heart.

Precordial leads V1 to V6

12 Lead ECG: Normal sinus rhythm at a rate of 71 beats/min, a P wave axis of 45°, and a PR interval of 0.15 sec.

©2007 UpToDate

Cardinal features of sinus rhythm:The P wave is upright in leads I and IIEach P wave is usually followed by a QRS complexThe normal adult resting heart rate is 60 99 beats/min

Example: Ventricular depolarization recorded by precordial leads V1 & V6

(2) Depolarization begins in the septum(1) At rest ECG leads show baseline PD

(3) Depolarization spreads to apex(4) Depolarization spreads to LV & RV

V1

-

LV

- -

-

---- - -

-

-

--

++ +

+

+

+

+++

V6 V6

-

- -

+++

V1

V6

V1

-

- -

+++

++++

- -

++

--

V1

V6

-

- -

+++

++++

- -

+ ++

+

+ + +-

-

-

-

-

-

Depolarization is complete

(4) Depolarization spreads to LV & RV; signal from LV dominates

(5) Depolarization is complete; trace returns to baseline.

Note that step 4 is repeated from previous slide

V6

V1

-

- -

+++

++++

- -

+ ++

+

+ + +

V1

V6

-

- -

+++

++++

- -

+ ++

+

+ + +-

-

-

-

-

-

Calibration of the ECG

Paper speed = 25 mm/sec1 mm = 1/25 sec or 0.04 sec

0.2 sec Calibration Signal (1 mV)

0.1 mV

0.04 sec 1 mm

paper speed = 25 mm/sec

1 sec

Small box = 1 mm2

Paper speed = 25 mm/sec1 mm = 0.04 sec = 1 small boxHeart rate = beats/min = [mm per minute]/[number of mm between beats],

Substituting numbers:

Since 1 small box = 0.04 sec, intervals can be calculated from trace also

beatmmminmm

min

beatsrateHeart

min

beats94

beatmm16

minsec60

secmm25

HR

Calculation of heart rate from the ECG

16 mm

Heart Block

Heart block is due to delay or blockade of excitation in the conducting system.First degree heart block refers to a prolonged PR interval (> 0.2 sec) and is due to a defect at the AV node & may be functional or structural.

Functional block occurs when the conducting impulse reaches a region that is still refractory from the preceding depolarization. Causes include some anti arrhythmic drugs, or AV nodal ischemia.Structural block is due to irreversible damage in the conducting system, for example due to infarction or degeneration with aging.First degree block is usually benign.Second degree heart block refers to intermittent complete blockade of the conducting signal so that some P waves are not followed by a QRS complex. There are two types and several causes.

Third degree heart block is complete dissociation between the atrial rhythm and the QRS complex. The ventricular rhythm is due to an ectopic pacemaker distal to the AV node. Third degree block requires treatment with a pacemaker.

PP

QRS

2nd degree block

P P PP

QRSQRS

3rd degree block

Reentry and arrythmias

Errors in conduction can cause abnormal rhythms or arrhythmias

One common cause of potentially serious arrythmias is re-entry

Reentry occurs when an anatomical or pathological fault causes action potentials to continuously circulate around an abnormal path in the myocardium

Reentry may occur as multiple, irregular continuously moving circuits leading to fibrillation or disorganized contraction of the myocardium

Reentry can occur where there is a branching path of myocardial tissue

Normally APs in the connecting branch extinguish each other.

Unilateral block with slow conduction

Reentry: If conduction is slow, as APs travel in a retrograde direction, they reach tissue which has passed the refractory period and is excitable, so the APs continues to cycle.

Unilateral block with normal conduction velocity

Cells in blocked area are in refractory period so AP cannot propagate in a retrograde direction

Conditions for reentry: A branching pathway of myocardium with unilateral block plus decreased conduction velocity.Conduction blocks may be caused by ischemia, inflammation, fibrosis or some drugs.Reentry circuits may cause tachycardia, atrial or ventricular fibrillation.