6. ECG Flashcards
Which factors are used to describe the electric field of charges?
Electric field lines (also called lines of force) and equipotential surfaces
How to describe electric field of charges in case of the field line plot?
the tangent of field lines shows the direction of the electric field vector at any point, while the density of lines indicates field strength
How to describe electric field of charges in case of the potential map?
By using equipotential surfaces (i.e., surfaces containing points of equal potential), which are perpendicular to the electric field lines.
What is an an electric dipole?
a pair of equal positive and negative point charges at a given distance from each other
The electric field of the charges of the heart can be approximated by the ___
dipole-moment vector
The role of dipole-moment vector
It characterizes its magnitude
→ This vector (the cardiac dipole) changes its direction and length quasi-periodically (corresponding to the cardiac cycle) and, consequently, the electric field of the dipole changes as well (potential map of the chest during the R-wave, see Fig. 1c)
The activity of the neural, skeletal and cardiac muscle cells is caused by an electrical phenomenon.
What is that phenomenon?
action potential
2 methods that measure action potential
- In between an extracellular electrode and a capillary microelectrode inserted into the intracellular space → the transmembrane potential can be measured directly
- in between two extracellular macroscopic electrodes (surface electrodes), the potential changes outside the cell bundles can be measured.
Describe Origin of the depolarization and repolarization signal shapes in the skeletal muscle cells
Skeletal muscle cells have short action potentials of uniform duration (1 -2 ms)
→ the positive depolarization front of the electromyogram (EMG) measured by surface electrodes is followed by a negative repolarization front of the same direction (
Describe Origin of the depolarization and repolarization signal shapes in the ventricular cardiac muscle cells
Ventricular cardiac muscle cells have action potentials of decreasing duration from the endocardium towards the epicardium
→ the epicardial cells that were activated later become repolarized earlier than the endocardial cells.
→ a positive depolarization front propagating from the endocardium towards the epicardium outwards in the heart wall is followed by a positive repolarization front propagating inwards
Describe Origin of the depolarization and repolarization signal shapes in the atrial cardiac muscle cells
Although the action potential curve of atrial cardiac muscle cells resembles that of the vetricular muscle cells
the direction and the polarity of the depolarization and repolarization front is similar to that of skeletal muscle cells (de- and repolarization waves with opposite polarity).
Describe the steps of the excitation and conduction of the heart during the cardiac cycle
- The sinoatrial (SA) node generates action potentials with a certain frequency.
- The depolarization front from the sinoatrial node spreads towards the atrial muscle (P-wave) → causing the contraction of the atria.
- Action potentials from SA node arrive to AV node → where an action potential is generated again after a certain while (AV pacemaker cells)
- The stimulus reaches every part of the ventricular muscle through the bundle of His, the Tawara-branches, and the Purkinje-fibers.
- The ventricular muscles contract almost simultaneously (QRS complex).
- The depolarization front propagates from the endocardium to the epicardium, which is followed by the repolarization front (T-wave) in the opposite direction.
=> the heart stays in rest until the next action potential is generated in the sinoatrial node.
the steps of the excitation and conduction of the heart during the cardiac cycle
The sinoatrial (SA) node generates ___ with a certain frequency.
- action potentials
the steps of the excitation and conduction of the heart during the cardiac cycle
The (1)___ from the sinoatrial node spreads towards the (2)____, which causes the (3)____ of the atria.
- depolarization front
- atrial muscle (P-wave)
- contraction
the steps of the excitation and conduction of the heart during the cardiac cycle
The depolarization front from the sinoatrial node spreads towards the atrial muscle (P-wave), which causes the contraction of the atria.
→ Why is this stimulus limited only to atrial muscle?
because there is an electrically insulating tissue layer between the atria and ventricles.
the steps of the excitation and conduction of the heart during the cardiac cycle
In the meantime, the action potential of the sinoatrial node arrives via the internodal tracts to the atrioventricular (AV) node, where an ___ is generated again after a certain while (AV pacemaker cells).
action potential
the steps of the excitation and conduction of the heart during the cardiac cycle
In the meantime, the action potential of the sinoatrial node arrives via the internodal tracts to the atrioventricular (AV) node, where an action potential is generated again after a certain while (AV pacemaker cells).
The stimulus quickly reaches every part of the ____ through the bundle of His, the Tawara-branches, and the Purkinje-fibers.
ventricular muscle
the steps of the excitation and conduction of the heart during the cardiac cycle
Following the depolarization front, the ventricular muscles (1)___ almost simultaneously (QRS complex).
The (2)___ propagates from the endocardium to the epicardium, which is followed by the (3)___ in the (4)___ direction.
- contract
- depolarization front
- repolarization front (T-wave)
- opposite
the steps of the excitation and conduction of the heart during the cardiac cycle
Subsequently, the heart stays in ___ until the next action potential is generated in the sinoatrial node.
rest
Why isn’t the electric signal of the conductive elements of the heart (sinoatrial node, atrioventricular node, bundle of His) recognizable in electrocardiograms of usual resolution?
The electric signal of the conductive elements of the heart (sinoatrial node, atrioventricular node, bundle of His) is so small → smaller than the noise level
A given deflection in the electrocardiogram depends on ___
which part of the heart muscle is depolarized
At any particular time, the depolarization and repolarization fronts can be considered as __
double (+ and –)-charge layers composed of a number of elementary electric dipoles
What is the integral vector?
by a strict definition, it is the spatial dipole vector representing the electric field of the heart.
Often, however, its frontal-plane projection is called the integral vector, which is constructed by using the Einthoven’s triangle.
As a result of the electrical activity of the heart the potential of every point of the body surface may change in time, therefore, _____ may be used to characterize the electrical function of the heart.
the voltage between any two points of the body surface
What is electrocardiogram (ECG)?
A recording of the voltage associated with the electrical activity of the heart measured between well defined points of the body (location of electrodes) as a function of time (U(t))
Which information does the electrocardiogram (ECG) provide?
It provides information about the position of the heart, the heart rate, the rhythm and origin of excitation and conduction, repolarization and malfunctions regardless of their anatomical, mechanical, metabolic, or circulatory origin.
What is the aim of electrocardiography?
The measurement of the action potential of the heart and the spatial and temporal construction of the cardic electric dipole
What is active (different) electrode?
an electrode, the electric potential of which changes continuously during the cardiac cycle.
What is inactive (indifferent) electrode?
an electrode (or a system of electrodes), the electric potential of which stays essentially constant as a function of time.
Is there any point on body surface that has a constant potential?
No
Since no point on the body surface has a constant potential, inactive electrodes are constructed by connecting____
multiple electrodes through resistors.
What is the the Wilson Central Terminal?
A common points that connect electrodes attached to the right arm, the left arm, and the left foot
The Wilson Central Terminal – has a (1)___ potential providing the initial point or (2)___ potential of the (3)___, because the signed potential variations of the L, R, and F leads even out.
- nearly constant
- zero
- integral vector
What is unipolar lead?
It records the potential difference between an active and an inactive electrode.
What is bipolar lead? (Fig.5)
It records the potential difference between two active electrodes.
What type of leads are Einthoven’s standard limb leads?
bipolar leads
→ the electrodes on the arms represent the shoulders and the electrode on the left foot represents the hip; limbs function only as electric conductors
In Einthoven’s standard limb leads (which are bipolar leads) the electrodes on the arms represent the shoulders and the electrode on the left foot represents the hip
→ limbs function only as ____
electric conductors.
What is Einthoven’s triangle?
an imaginary equilateral triangle connecting the two shoulders and the hip. Its corners correspond to the limb electrodes, and the sides to the standard limb leads.
What is main electrical axis (mea) of the heart?
the integral vector constructed from the R-waves measured by the limb leads.
The angle enclosed by the MEA and the horizontal is called the ___
angle of the mean electrical axis.
What occurs in WILSON’S CHEST LEADS?
In Wilson’s chest (or precordial) leads six unipolar recordings are measured with the six precordial active electrodes (C1, C2 … C6) using the central terminal (CT) as reference.
→ The active electrodes are placed roughly in the horizontal plane. The leads are designated as V1, V2, V3, V4, V5, and V6
Describe GOLDBERGER’S AUGMENTED LEADS
By omitting the the limb electrode to be measured from Wilson’s central terminal, an ECG signal of sufficient amplitude (see Fig. 7) may be obtained.
In this case the active limb electrode (e.g. R) is connected to a “truncated” Wilson’s central terminal (e.g. ( L + F)/2) as reference.
Goldberger’s leads are also positioned in the frontal plane.
Describe vectorcardiography
It records the three-dimensional variation of the integral vector
The role of 12-lead ECG system
provides information about the temporal change of the cardiac dipole in the respective spatial projection.
→ The chest leads give planar projections in the horizontal plane crossing the heart (Fig.6)
→ the limb leads supply information about projections in the frontal plane (Fig. 8).
Structure of 12-channel ECG-recorder
the electrode wires are connected to the input of the program selector.
→ This is a set of switches used to select from the electrodes those required for the actual recording and to connect them to the input of the amplifier.
→ The central terminal (for Wilson’s leads) or the truncated central terminal (for Goldberger’s leads) is also formed by the program selector.
Why we need to use differential amplifier in electrocardiography?
a special type of the amplifier
→ For the amplification of the weak ECG signals and suppression of the high amplitude noise
The differential amplifier has two sensitive inputs and amplifies ___
the difference of the two input signals
The differential amplifier has two sensitive inputs and amplifies the difference of the two input signals (Fig. 11). As the power-line noise appears at the (1)___ on the inputs of the amplifier, only the (2)___ ECG signal will be amplified.
- “common mode”
- “differential-mode”
The most common lead system in clinical vectorcardiography is the __
Frank lead system
→ The signals of the electrodes are converted into x-y-z voltage compnents via a resistor matrix by using appropriate weighting and addition.
Assign the name of waves
Construction of integral vector
- Let us map the voltages UI, UII, and UIII onto the corresponding sides of the triangle, so that the arrows point towards the positive electrodes
- The resultant integral vector can be constructed from any two of the three components by perpendicular projection.
- The direction of the integral vector approximates the direction, in the frontal plane of the cardiac dipole, and its magnitude is also proportional to that of the cardiac dipole in the corresponding moment of time.
- This construction can be carried out for any (P, T) wave.
- The largest integral vector, which is constructed from the R-wave, is called the mean electrical axis of the heart (MEA).
Construction of integral vector
What can the resultant integral vector be constructed from?
any two of the three components by perpendicular projection.
Construction of integral vector
What does the direction of the integral vector approximate?
The direction of the integral vector approximates the direction, in the frontal plane of the cardiac dipole
Construction of integral vector
The direction of the integral vector approximates the direction, in the frontal plane of the cardiac dipole, and its magnitude is also proportional to ___
Magnitude of the cardiac dipole in the corresponding moment of time.
The integral vector (reading)
Origin of ECG signals
The electrical signal begins in the sinoatrial node (1) which is located in the right atrium and travels to the right and left atria, causing them to contract and pump blood into the ventricles. This electrical signal is recorded as the P wave on the ECG.
Multipole series expansion
Any charge distribution, whether discrete or continuous, can be represented as the superposition of a monopole, dipole, quadrupole, octupole, and so on.
Multipole series expansion
Any charge distribution, whether discrete or continuous, can be represented as the superposition of a monopole (1 charge), dipole (1 positive and 1 negative charge), quadrupole (2 positive and 2 negative charge), octupole, and so on.
Characteristics of ECG signals
ECG signals are typically characterized by the PR, QRS, QT interval, ST-segment, and heart rate (HR) parameters.
- P: Depolarization of the atria
- PQ: Delay due to the fibrous septa and the lower conduction of the AV node
- Q: Depolarization of the septa (from left to right)
- R: Depolorization of the ventricles
- S: Depolorization of the ventricles spreading upwards (towards negative electrode)
- T: Depolarization of the ventricles
