Electrical Dipoles and ECG Leads
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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?

    <p>They become wider</p> Signup and view all the answers

    What is the standard paper speed for electrocardiography in humans?

    <p>25 mm/sec</p> Signup and view all the answers

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

    <p>It increases</p> 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?

    <p>25 mm/sec</p> Signup and view all the answers

    What does the P wave on an ECG represent?

    <p>Depolarization of atrial muscle</p> Signup and view all the answers

    What happens to the SA node discharge on an ECG?

    <p>It is not visible on an ECG</p> Signup and view all the answers

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

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

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

    <p>SA node discharge, then atrial depolarization</p> Signup and view all the answers

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

    <p>It has occurred just before the P wave</p> Signup and view all the answers

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

    <p>Impulse delay through the AV node</p> Signup and view all the answers

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

    <p>Decreases the P-R interval</p> Signup and view all the answers

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

    <p>Increases the P-R interval</p> Signup and view all the answers

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

    <p>Associated with A-V blocks</p> Signup and view all the answers

    What is measured by the P-R interval?

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

    What does the QRS complex represent in an electrocardiogram?

    <p>Ventricular depolarization</p> Signup and view all the answers

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

    <p>Faster conduction velocity</p> Signup and view all the answers

    How many waves make up the QRS complex?

    <p>Three waves</p> Signup and view all the answers

    What is the duration of the QRS complex comparable to?

    <p>The duration of the P wave</p> Signup and view all the answers

    What does each wave of the QRS complex represent?

    <p>A specific part of ventricular depolarization</p> Signup and view all the answers

    What does the Q-T interval reflect?

    <p>The duration of ventricular systole and the ventricular refractory period</p> Signup and view all the answers

    What is the S-T segment correlated with?

    <p>The plateau of the ventricular action potential</p> Signup and view all the answers

    What is the Q-T interval measured from?

    <p>The beginning of the Q wave to the end of the T wave</p> Signup and view all the answers

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

    <p>Because all ventricular muscle is depolarized</p> Signup and view all the answers

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

    <p>The isoelectric segment</p> Signup and view all the answers

    What is the T wave in an electrocardiogram associated with?

    <p>Ventricular repolarization</p> Signup and view all the answers

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

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

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

    <p>It has a high degree of variability</p> Signup and view all the answers

    What is possible with the polarity of the T wave?

    <p>It can be positive, negative, or biphasic</p> Signup and view all the answers

    What is a requirement for the T wave?

    <p>It should be consistent</p> Signup and view all the answers

    What is the R-R interval used to evaluate?

    <p>The regularity of the heartbeats</p> Signup and view all the answers

    What is the R-R interval equal to?

    <p>The time between one R wave and the next</p> Signup and view all the answers

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

    <p>When the rhythm is regular</p> Signup and view all the answers

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

    <p>Cycle length</p> Signup and view all the answers

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

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

    Why is the ECG less useful for diagnosing structural abnormalities?

    <p>Because cardiac depolarization pathways are not consistent</p> Signup and view all the answers

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

    <p>They vary more among horses and cattle than dogs and cats</p> Signup and view all the answers

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

    <p>Ventricular hypertrophy</p> Signup and view all the answers

    Why do cardiac depolarization pathways affect the ECG?

    <p>Because they are irregular and inconsistent</p> Signup and view all the answers

    What is a limitation of the ECG in large animals?

    <p>It is not suitable for diagnosing structural abnormalities</p> Signup and view all the answers

    Match the ECG term to correct definition

    <p>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</p> Signup and view all the answers

    An arrhythmia is an abnormal heart rhythm

    <p>True</p> Signup and view all the answers

    What does an EKG evaluate?

    <p>It evaluates rhythm and conduction</p> Signup and view all the answers

    An EKG can indicate if a patient has _________ and __________ disturbances

    <p>Arrhythmias; conduction</p> Signup and view all the answers

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

    <p>Depolarization</p> Signup and view all the answers

    The PR interval includes _________ and __________.

    <p>The PR interval includes the P wave and the PQ segment</p> 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.

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    Description

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

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