Electrical Dipoles and ECG Leads

Questions and Answers

What is formed when a positive and a negative charge are separated by a small distance?

Dipole (correct)

Electrical field

Current flow

Interval

What is the purpose of Standard Bipolar Leads in electrocardiography?

To detect the sum of multiple dipoles

To compare the electrical potentials of the left and right atria

To determine the duration and amplitude of the waveforms and intervals/segments (correct)

To measure the electrical field generated by a single dipole

What does the horizontal axis of an electrocardiogram represent?

Heart rate in beats per minute

Time in milliseconds (correct)

Amplitude in millivolts

Distance in millimeters

What happens to the width of complexes on an electrocardiogram when slower variations of electrical potentials occur?

They become wider Signup and view all the answers

What is the standard paper speed for electrocardiography in humans?

25 mm/sec Signup and view all the answers

What happens to the amplitude of complexes on an electrocardiogram when more tissue is depolarizing?

It increases Signup and view all the answers

In small animals, what is the paper speed for electrocardiography if the heart rate is 120 beats/min or lower?

25 mm/sec Signup and view all the answers

What does the P wave on an ECG represent?

Depolarization of atrial muscle Signup and view all the answers

What happens to the SA node discharge on an ECG?

It is not visible on an ECG Signup and view all the answers

Why is there no deflection for atrial repolarization on an ECG?

Because the atrial muscle is too small to produce a detectable signal Signup and view all the answers

What is the sequence of events that occurs just before the P wave on an ECG?

SA node discharge, then atrial depolarization Signup and view all the answers

What can be inferred about the SA node discharge from an ECG?

It has occurred just before the P wave Signup and view all the answers

What is the primary reason for the delay in the P-R interval?

Impulse delay through the AV node Signup and view all the answers

What is the effect of sympathetic stimulation on the P-R interval?

Decreases the P-R interval Signup and view all the answers

What is the effect of parasympathetic stimulation on the P-R interval?

Increases the P-R interval Signup and view all the answers

What is the significance of an increased P-R interval?

Associated with A-V blocks Signup and view all the answers

What is measured by the P-R interval?

Time between the start of the P wave and the first deflection of the QRS complex Signup and view all the answers

What does the QRS complex represent in an electrocardiogram?

Ventricular depolarization Signup and view all the answers

What is the characteristic of the conduction velocity in the HIS-Purkinje system?

Faster conduction velocity Signup and view all the answers

How many waves make up the QRS complex?

Three waves Signup and view all the answers

What is the duration of the QRS complex comparable to?

The duration of the P wave Signup and view all the answers

What does each wave of the QRS complex represent?

A specific part of ventricular depolarization Signup and view all the answers

What does the Q-T interval reflect?

The duration of ventricular systole and the ventricular refractory period Signup and view all the answers

What is the S-T segment correlated with?

The plateau of the ventricular action potential Signup and view all the answers

What is the Q-T interval measured from?

The beginning of the Q wave to the end of the T wave Signup and view all the answers

Why is the segment between the end of depolarization and the beginning of repolarization isoelectric?

Because all ventricular muscle is depolarized Signup and view all the answers

What period corresponds to the time when all ventricular muscle is depolarized?

The isoelectric segment Signup and view all the answers

What is the T wave in an electrocardiogram associated with?

Ventricular repolarization Signup and view all the answers

Why is the duration of the T wave longer than the QRS complex?

Because repolarization does not occur as a synchronized propagated wave Signup and view all the answers

What is a characteristic of the T wave in dogs and cats?

It has a high degree of variability Signup and view all the answers

What is possible with the polarity of the T wave?

It can be positive, negative, or biphasic Signup and view all the answers

What is a requirement for the T wave?

It should be consistent Signup and view all the answers

What is the R-R interval used to evaluate?

The regularity of the heartbeats Signup and view all the answers

What is the R-R interval equal to?

The time between one R wave and the next Signup and view all the answers

Under what condition can the R-R interval be used to calculate HR?

When the rhythm is regular Signup and view all the answers

What is the other name for the R-R interval?

Cycle length Signup and view all the answers

What is the relationship between the R-R interval and heart rate?

The R-R interval is inversely proportional to heart rate Signup and view all the answers

Why is the ECG less useful for diagnosing structural abnormalities?

Because cardiac depolarization pathways are not consistent Signup and view all the answers

What is a characteristic of the ECG waves in large animals?

They vary more among horses and cattle than dogs and cats Signup and view all the answers

What is an example of a structural abnormality that the ECG is less useful for diagnosing?

Ventricular hypertrophy Signup and view all the answers

Why do cardiac depolarization pathways affect the ECG?

Because they are irregular and inconsistent Signup and view all the answers

What is a limitation of the ECG in large animals?

It is not suitable for diagnosing structural abnormalities Signup and view all the answers

Match the ECG term to correct definition

Waveform = Movement away from the baseline in either a positive or negative deflection Segment = The time length between two specific points on an EKG that are supposed to be at the baseline amplitude Interval = The time length between two specific EKG events Complex = Group of waveforms Signup and view all the answers

An arrhythmia is an abnormal heart rhythm

True Signup and view all the answers

What does an EKG evaluate?

It evaluates rhythm and conduction Signup and view all the answers

An EKG can indicate if a patient has _ and __ disturbances

Arrhythmias; conduction Signup and view all the answers

The QRS complex represents what portion on the ventricular AP graph?

Depolarization Signup and view all the answers

The PR interval includes _ and __.

The PR interval includes the P wave and the PQ segment Signup and view all the answers

Study Notes

Electric Dipole and its Effects

A dipole is a pair of positive and negative charges separated by a small distance, which can generate local current flow and a small electrical field.
Cardiac muscle cells, when depolarizing or repolarizing, exhibit different charges along their membranes, behaving like a dipole.

Electrocardiogram (ECG) Leads

Standard bipolar leads are the most commonly used leads.
Lead I: compares the left arm (LA+) with the right arm (RA-).
Lead II: compares the left leg (LL+) with the right arm (RA-).
Lead III: compares the left leg (LL+) with the left arm (LA-).

ECG Waveforms and Intervals

The ECG allows for the determination of duration and amplitude of waveforms and intervals/segments.
Standard calibration paper speed is 25 mm/sec in humans.
In small animals, paper speed is 50 mm/sec, but can be 25 mm/sec if the heart rate is 120 beats/min or lower.

ECG Axis Interpretation

The horizontal axis represents time in milliseconds.
Slower variations of electrical potentials result in wider complexes (e.g., due to fibrosis, more time is required for depolarization/repolarization).
The vertical axis represents amplitude in millivolts.
Increased tissue depolarization results in increased amplitude (height) of the complexes (e.g., more muscle mass results in higher waves).

ECG Deflections

The first ECG deflection is the P wave, which represents depolarization of the atrial muscle.
The discharge of the SA node is assumed to have occurred just before the P wave.
The discharge of the SA node does not produce a visible deflection on the ECG, likely due to the small number of cells involved.
Atrial repolarization also does not produce a visible deflection on the ECG.

P-R Interval

Represents the time taken for the electrical impulse to conduct through the atria, AV node, and Bundle of His.
The impulse is mostly delayed in the AV node, which affects the overall P-R interval length.

Measurement of P-R Interval

Measured from the start of the P wave to the first deflection of the QRS complex.

Effects of Stimulation on P-R Interval

Sympathetic Stimulation: • Decreases the P-R interval. • Increases conduction velocity.
Parasympathetic Stimulation: • Increases the P-R interval. • Decreases conduction velocity.

Clinical Significance of P-R Interval

An increased P-R interval is associated with A-V blocks.

Ventricular Depolarization

The activation of the HIS-Purkinje system and ventricular muscle leads to the generation of a QRS complex.
The QRS complex is a representation of ventricular depolarization, comprising three waves.
The total duration of the QRS complex is comparable to that of the P wave.
The conduction velocity of the QRS complex is faster.
Each wave of the QRS complex corresponds to a specific region of the ventricle undergoing depolarization.

Q-T Interval

Represents the approximate duration of ventricular systole and ventricular refractory period
Marks the ending of depolarization and beginning of repolarization, making it isoelectric since all ventricular muscle is depolarized
Measured from the beginning of the Q wave to the end of the T wave

S-T Segment

Correlates with the plateau of the ventricular action potential (AP)

Ventricular Repolarization (T Wave)

Represents ventricular repolarization
Has a longer duration than the QRS wave because repolarization does not occur as a synchronized propagated wave
Exhibits high degree of variability in dogs and cats
Can be:
- Positive
- Negative
- Biphasic
- Very low amplitude, but should be consistent

ECG Variability in Large Animals

In large animals, there is considerable variability in the polarity and size of the ECG waves.
ECG waves exhibit more variation in horses and cattle compared to dogs and cats.
Significant individual variations exist among large animals.
Cardiac depolarization pathways are not consistent in large animals.
ECG is primarily useful for detecting arrhythmias in large animals.

Limitations of ECG

ECG is less useful for diagnosing structural abnormalities in large animals.
Example of a structural abnormality that ECG is not suitable for diagnosing: Ventricular hypertrophy.

Electrophysiology

A dipole is a positive and negative charge separated by a small distance, generating local current flow and a small electrical field.
Cardiac muscle cells, during depolarization and repolarization, show different charges along their membranes and act like a dipole.

Electrocardiography (ECG)

Standard bipolar leads are used to measure the electrical activity of the heart.
Lead I compares LA (+) and RA (-), Lead II compares LL (+) and RA (-), and Lead III compares LL (+) and LA (-).
ECGs allow for the determination of duration and amplitude of waveforms and intervals/segments.
Standard calibration paper speed is 25 mm/sec in humans, and 50 mm/sec in small animals (or 25 mm/sec if HR is 120 beats/min or lower).

ECG Components

The P wave represents depolarization of atrial muscle and occurs just after the discharge of the SA node.
The P-R interval represents the time it takes for the electrical impulse to conduct through the atria, AV node, and Bundle of His (mostly through the AV node).
The QRS complex represents ventricular depolarization and is generated as the impulse activates the His-Purkinje system and ventricular muscle.
The Q-T interval reflects the approximate duration of ventricular systole and the ventricular refractory period.
The S-T segment correlates with the plateau of the ventricular action potential.
The T wave represents ventricular repolarization and has a longer duration than the QRS complex.
The R-R interval represents the time between one R wave and the next, and is used to evaluate the regularity of heartbeats and calculate heart rate.

Variations in ECGs

ECG waves can vary in polarity and size among individuals, particularly in large animals such as horses and cattle.
ECGs are more useful for detecting arrhythmias than diagnosing structural abnormalities, such as ventricular hypertrophy.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Understanding electrical dipoles and their role in generating local current flow and electrical fields, particularly in cardiac muscle cells and ECG leads.