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Electrocardiographic Lead Systems

Recording electrodes are configured to form various types of leads

A lead records the fluctuation in extracellular voltage generated between its electrodes

The standard clinical ECG includes recordings from 12 leads

6 in the frontal plane

3 bipolar leads or standard limb leads

3 modified (augmented) unipolar limb leads

6 in the transverse plane

the unipolar precordial leads

Limb leads

Lead I

-negative connection to right arm, positive connection to left arm

-defines an axis in the frontal plane at 0

Lead II

-negative to right arm, positive to left leg

-defines an axis in the frontal plane at 60

Lead III

-negative connection to left arm, positive connection to left leg

-defines an axis in the frontal plane at 120

-The output – the potential difference between two limbs

ECG Leads - Overview

Each lead is an axis in one of the planes (frontal, transverse, sagittal) onto which the heart projects its electrical activity

Each lead looks at the heart from a unique angle and plane it has its own unique point of view

The recording from a single lead shows how that lead views the time-dependent changes in voltage of the heart

P Wave - Characteristics

-Shape

-dome; sometimes biphasic (V1, V2), or discretely notched (V5, V6, aVL), due to the partial asynchrony of atrial activation

-Polarity

-positive in leads I,II, aVL, aVF, V4 – V6

-negative in aVR

-Axis: 0 – 75

-Duration: < 0.12 s

-Amplitude:

-< 0.25 mV in limb leads

-the terminal negative deflection in V1 < 0.1 mV in depth

T Wave

= Repolarisation of ventricles

Generally T wave = positive

This is due to the repolarization of the membrane.

(QRS wave), the heart depolarizes.

Repolarization of the ventricle happens in the opposite direction of depolarization and is negative current.

This double negative (direction and charge) is why the T wave is positive; although the cell becomes more negatively charged, the net effect is in the positive direction, and the ECG reports this as a positive spike.

T-wave inversion (negative T waves) can be a sign of coronary ischemia, Wellens' syndrome, left ventricular hypertrophy, or CNS disorder.

Tall and narrow ("peaked" or "tented") symmetrical T waves may indicate hyperkalemia.

Flat T waves (less than 1 mV in the limb leads and less than 2 mV in the precordial leads) may indicate coronary ischemia or hypokalemia

ST Segment

Augmented Unipolar Limb Leads

aVR (augmented voltage right)

-positive connection to the right arm

-the axis defined in the frontal plane is -150

aVL (augmented voltage left)

-positive connection to the left arm

-the axis defined in the frontal plane is -30

aVF (augmented voltage foot)

-positive connection to the left leg

-the axis defined in the frontal plane is +90

Represents the period when the ventricles are depolarized.

It is isoelectric.

=Period between the end of the QRS complex and the beginning of the T wave where there is no conduction and the line is flat

Normal ST segment has/may have a slight upward concavity

Flat, downsloping, or depressed ST segments may indicate coronary ischemia.

ST elevation may indicate myocardial infarction. An elevation of >1mm and longer than 80 milliseconds following the J-point

ST depression may be associated with hypokalemia

Define

A non-invasive test of the electrical activity of heart's conduction system, which is transformed into recordings on graph paper – an electrocardiograph

Why is it used

  • Arrhythmias
  • Conduction disturbances
  • Myocardial ischemia
  • Cardiac enlargement and hypertrophy
  • Electrolyte disturbances
  • Increased susceptibility to sudden death (QT-prolongation syndromes)

P Wave

Represents Depolarisation of the atria.

Absence of P wave = Atrial fibrillation; Sinoatrial block; junctional rhythm

Peaks above (>) 2.5 mm indicate Right Atria inlargement or Hypokalemia (low potassium).

A decreased P wave = hyperkalemia.

Bifid P waves (split into two parts) = left atrial hypertrophy = increase in size of organ’s cells

Characteristics of the Normal ECG

Normal ECG consist of a number of waves, positive and negative, connected through segments of isoelectric line

The waves correspond to depolarization and repolarization of the heart surface during the cardiac cycle

  • P wave – atrial depolarization
  • PQ segment – delayed conduction in the AV node
  • QRS complex – ventricular depolarization
  • T wave – ventricular repolarization

S wave:

This is a small negative wave after the large R wave which represents depolarisation in the Purkinje fibres.

The S wave travels in the opposite direction to the large R wave because, as can be seen on the earlier picture, the Purkinje fibres spread throughout the ventricles from top to bottom and then back up through the walls of the ventricles.

QRS WAVE:

Definition:

Represents ventricular depolarisation

Description:

At this point the electrical stimulus passes from the bundle of His into the bundle branches and Purkinje fibres.

The amount of electrical energy generated is recorded as the QRS complex.

More voltage is required to cause ventricular contraction and therefore the wave is much bigger.

How?

The extracellular fluids contain salts and therefore conduct electricity. As these fluids are distributed throughout the body, the body acts like a volume conductor. When cardiac muscle depolarizes, extracellular currents between depolarized and resting cells cause potentials that can be measured at the body surface.

Sinus Rhythm

= normal rhythm of the heart

You will not always see a Q wave or an S wave on an ECG.

This is why only 3 waves are emphasised when you are learning from scratch.

Unipolar Precordial Leads

Usually six standard chest leads are recorded:

V1: fourth intercostal space to the right of the sternum

V2: fourth intercostal space to the left of the sternum

V4: fifth intercostal space at the midclavicular line

V3: halfway between V2 and V4

V6: fifth intercostal space at the midaxillary line

V5: halfway between V4 and V6.

Q wave:

- Represents depolarisation in the septum.

- The picture below shows this small negative wave immediately before the large QRS complex.

Whilst the electrical stimulus passes through the bundle of His, and before it separates down the two bundle branches, it starts to depolarise the septum from left to right (shown by arrows).

This is only a small amount of conduction (hence the Q wave is less than 2 small squares),

It travels in the opposite direction to the main conduction (which is right to left) so the Q wave points in the opposite direction to the large QRS complex.

From Heart Vectors to ECG Waves

The amplitude and polarity of the cardiac potentials sensed in a lead are proportional to the size of the projection of the heart vector on the lead vector

a.The projection of the heart vector points toward the positive pole of the lead, the lead records a positive potential positive wave on ECG

b.The projection is directed away from the positive pole of the lead the lead records a negative potential negative wave on ECG

QRS Complex

Nomenclature

Q wave

-Initial downward deflection

-Duration: < 0.03 – 0.04 s; exception: in lead V1, V2 any Q is abnormal

-Amplitude: < ¼ R wave, < 0.2 – 0.3 mV

R wave

-First upward deflection

-Criteria for shape or size are not absolute; high amplitude in V5, V6

-A second upward deflection is designated R’

S wave

-The second negative deflection if there is a Q wave, or the first downward deflection if not

-Duration: < 0.04 s

-High amplitude in V1, V2

R wave:

As shown in the diagram; the R wave represents the electrical stimulus as it passes through the main portion of the ventricular walls.

- The wall of the ventricles are very thick due to the amount of work they have to do and, consequently, more voltage is required.

This is why the R wave is by far the biggest wave generated during normal conduction.

More muscle means more cells. More cells mean more electricity . More electricity leads to a bigger wave.

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