Electrical Dipoles and ECG Leads

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.