Cardiac Conduction System and Electrocardiogram (ECG)

Flashcards based on lecture notes about the cardiac conduction system and electrocardiograms.

What is the heart's pacemaker?

The sinoatrial (SA) node

What does the sinoatrial node control?

The timing of the cardiac cycle

What is the waveform produced by the heart's electrical activity known as?

An electrocardiogram (ECG)

What does the ECG measure?

The voltages that arise from contraction of the cardiac tissue during the cardiac cycle.

Why can voltage across the cardiac tissue be detected by placing electrodes on the skin?

Because the body is a good conductor of electricity.

What information does the ECG provide?

Information on electrical characteristics of the heart, extent of damage, effects of drug intervention on cardiac function

What is the electrical activity of the heart approximately modeled as?

A vector quantity known as the cardiac vector.

How can the electrical activity of the heart be described?

By the movement of an electrical dipole which consists of a positive and a negative charge separated by a variable distance.

What does the P wave signal?

Atrial depolarization or contraction of atria.

What does the QRS complex signal?

Ventricular depolarization or contraction.

What does the T wave indicate?

Ventricular repolarization, which corresponds to ventricular relaxation.

What does the PR interval represent?

Time taken for the impulse to travel through the AV node, bundle of His, and bundle branches to the Purkinje fibers.

Where does the P wave begin?

At the beginning of depolarization at the SA node.

What does the repolarization of the atria result in?

A signal of opposite sign to the P wave, which may be visible as a depression of the QRS complex or masked by it.

What happens after the conduction delay at the AV node?

The His-Purkinje cells are depolarized, giving rise to a small signal (usually too small to be visible on the surface).

What is the polarity of the T wave?

The same polarity as the QRS wave.

Why is standardization of electrode position important in ECG?

Diagnosis relies on comparisons between different people.

What plane measurements are sufficient to check if the heart is beating?

Frontal plane.

What is the frontal plane ECG made of?

Electrodes placed on the left arm (LA), right arm (RA), and left leg (LL).

What is Einthoven's triangle?

The resulting triangle when the cardiac vector is described by its length in three directions at 60 degrees to each other, with the points representing the right arm (RA), left arm (LA), and left leg (LL).

What does the 12-lead ECG consist of?

6 limb leads (frontal plane) and 6 chest leads (transverse plane).

What is Einthoven's equation?

Lead II = Lead I + Lead III

What do leads I, II, and III measure?

Voltage between two specific points (bipolar).

What is the axis of the lead?

An imaginary line drawn between two electrodes for bipolar leads.

What is aVL referenced to?

The average of RA and LL

What does lead aVR measure?

Voltage at RA referenced to the average of LA and LL.

What does lead aVF measure?

Voltage at LL referenced to the average of RA and LA.

What information do augmented limb leads provide?

Information about current flow that is right, left, superior, or inferior.

Why are augmented leads termed unipolar leads?

Because there is a single positive electrode that is referenced against a combination of the other limb electrodes.

What is Wilson's central terminal (WCT)?

A virtual reference terminal defined as the average of the potential at the three limb electrodes: (VLA + VRA + VLL)/3

What is the speed of ECG paper?

25 or 50 mm/s

What is Fibrillation?

Involuntary recurrent contraction of the cardiac muscle which disrupts normal sinus rhythm of the heart and results in deficiency in the propulsion of blood from the heart chamber.

What are the characteristics of Atrial fibrillation?

Atrial fibrillation is a Random contraction of the atria, causing an irregular and rapid heart rate.

What are the characteristics of Ventricular fibrillation?

Randomised contraction of the ventricles due to random repetitive excitation of ventricular muscle fibres without coordination

What is used to treat ventricular fibrillation?

Defibrillators are commonly used.

What type of electrodes are typically used in ECG?

Silver/Silver chloride electrodes

What is the function of Electrodes?

Electrodes are transducers that convert ionic currents to electrical currents

What is applied between the electrode and skin?

Electrode paste or jelly

What is High common mode rejection ratio?

Greater than 80 dB before the first amplification stage.

What is Amplifier protection circuit?

First component receiving the signals from the electrodes.

What does Diodes limit?

The effect of high voltage/current, such as in the cases of electrostatic discharge or defibrillation shock.

What are the benefits of the Driven right leg circuit?

Provides a reference point on the patient that normally is a ground potential

What does the Driver amplifier do?

Performs the final amplification as well as band-pass filtering before analog-to- digital conversion is performed.

What are the Sources of Noise

Mains Interference, Electrode contact noise, Motion artefacts, EMG from chest wall