Cardiac Conduction System and Electrocardiogram (ECG)

Cardiac Conduction System and Electrocardiogram (ECG)

Electrocardiogram (ECG)

• Willem Einthoven (1860-1927) was a key figure in the development of the ECG.

Cardiac Conduction System

• The heart's pacemaker is the sinoatrial (SA) node.

• It controls the timing of the cardiac cycle.

• The SA node is composed of specialized muscle tissue with characteristics of both muscle and nerves.

• It contracts and generates electrical impulses at a regular rate.

Electrocardiography (ECG)

• The beating heart generates an electrical signal that can be used as a diagnostic tool.

• The waveform produced by the heart's electrical activity is known as an electrocardiogram (ECG).

• ECG measures the voltages that arise from the contraction of cardiac tissue during the cardiac cycle.

• Voltage across the cardiac tissue can be detected by placing electrodes on the skin because the body is a good conductor of electricity.

• ECG provides information on:

  • Electrical characteristics of the heart

  • Extent of damage

  • Effects of drug intervention on cardiac function

Electrocardiography (ECG) - Cardiac Vector

• The electrical activity of the heart can be approximately modeled as a vector quantity known as the cardiac vector.

• This electrical activity can be described by the movement of an electrical dipole, which consists of a positive charge and a negative charge separated by a variable distance.

• The cardiac vector is the line joining the two charges.

• The electrical activity of the heart does not consist of two moving charges, but the electric field resulting from depolarization and repolarization of the cardiac muscle can be represented by a charged dipole model.

Typical ECG Waveform

• P wave: Signals atrial depolarization or contraction of the atria.

• QRS complex: Signals ventricular depolarization or contraction; duration is less than 0.1 s.

• T wave: Indicates ventricular repolarization, which corresponds to ventricular relaxation.

• PR interval: Time taken for the impulse to travel through the AV node, Bundle of His, bundle branches, and Purkinje fibers; normal duration is 0.12-0.2 s.

• Changes in the amplitude and duration of different parts of the ECG waveform provide diagnostic information for physicians.

P Wave

• The start of the P wave marks the beginning of depolarization at the SA node.

• The wave of depolarization takes about 30 ms to arrive at the AV node.

• There is a delay in conduction of about 90 ms to allow the ventricles to fill.

QRS Wave

• The repolarization of the atria, causing them to relax, results in a signal of opposite sign to the P wave.

  • This may be visible as a depression of the QRS complex or masked by the QRS complex.

• After the conduction delay at the AV node, the His–Purkinje cells are depolarized, giving rise to a small signal which is usually too small to be visible on the surface.

• The conduction through the His–Purkinje system takes about 40 ms, and the depolarization and contraction of the ventricles then begins, giving rise to the QRS complex.

T Wave

• Repolarization of the ventricles takes place.

• This is slower than depolarization and takes a different path, so the resulting T wave is of lower amplitude and longer duration than the QRS wave, but has the same polarity.

ECG Planes

• The heart can be considered a generator of electrical signals enclosed in a volume conductor.

• As the body and heart are 3D, the measured electrical signals will vary depending on the site of the electrodes.

• Diagnosis relies on comparisons between different people, so standardization of electrode position is important.

• To check if the heart is beating, measurements from the frontal plane are sufficient.

• For diagnosis, transverse plane measurements are also recorded.

Why Use More Than One Lead?

• Different lead positions detect voltage differently based on the direction of the electrical activity.

• A lead aligned with the direction of the cardiac vector will detect high voltage, while a lead perpendicular to it will detect no voltage.

Electrocardiogram (ECG) - Leads

• A pair of electrodes is referred to as a lead.

• Frontal plane ECG is made of 3 basic leads – electrodes placed on the left arm (LA), right arm (RA) and left leg (LL).

• With Frontal plane ECG, it is usual to describe the cardiac vector by its length in three directions at 60° to each other.

• The resulting triangle is known as Einthoven’s triangle, and the three points of the triangle represent the right arm (RA), the left arm (LA) and the left leg (LL).

• Because the body is an electrical volume conductor, any point on the arm, from the shoulder down to the fingers, is electrically equivalent, and recording from the left leg is electrically equivalent to recording from anywhere on the lower torso.

12 Lead ECG

• Commonly used configuration.

• Allows for a 3D look at electrical activity.

• Composed of:

  • 6 limb leads (lie in the frontal plane) and

    • 3 classical limb leads

    • 3 augmented limb leads

  • 6 chest leads (circle heart in transverse plane)

12 Lead ECG - Details

• Limb Leads: Lie in the frontal plane.

  • Bipolar Limb Leads: Measure voltage between two specific points (I, II, and III).

  • Unipolar Augmented Limb Leads: aVR, aVL, and aVF – measure the voltage difference between one limb electrode and the average of the other two electrodes.

• Chest (Precordial) Leads: V1-V6; circle the heart in the transverse plane.

  • Provide local information and are measured relative to the average of the limb electrodes.

What Each Lead Measures?

• Left Ventricle Health Leads:

  • Lead I, aVL, and -aVR: Lateral wall of the left ventricle.

  • aVF, II, and III: Inferior wall of the left ventricle.

The Four Walls of the Left Ventricle and ECG Leads Viewing These Walls

• The 12-lead ECG primarily records the electrical activity of the left ventricle. Right ventricular electrical activity is less prominent under normal circumstances.

• The left ventricle has the shape of a bullet and is divided into four walls:

  • Septal wall: V1, V2. Area involved in septal infarcts.

  • Lateral wall: I, aVL, -aVR. Area involved in lateral infarcts.

  • Anterolateral wall: V5, V6.

  • Anterior wall: V3, V4. Area involved in anterior infarcts.

  • Inferior wall: II, III, aVF. Area involved in inferior and right ventricular infarcts.

• Coronary Arteries: LAD (Left Anterior Descending artery), LCX (Left Circumflex artery), RCA (Right Coronary Artery).

Frontal Plane ECG

• Frontal plane ECG is made of 3 basic leads – electrodes placed on the left arm (LA), right arm (RA) and left leg (LL).

• These three electrodes form Einthoven’s triangle.

• The electrical signal on each lead of Einthoven’s triangle can be represented as a voltage source.

• Einthoven’s equation:

  • Lead II = Lead I + Lead III

  • I – II + III = 0

• Leads I, II, III are bipolar and measure voltage between two specific points.

Why Does the Electrical Vector Point Up or Down on ECG?

• The electrocardiograph generates an ECG lead by comparing the electrical potential difference in two points in space. In the simplest leads, these two points are two electrodes.

• One electrode serves as the exploring electrode (positive) and the other as the reference electrode.

• The electrocardiograph is constructed such that an electrical current traveling towards the exploring electrode yields a positive deflection, and vice versa.

Limb Leads

• Lead I = V{LA} – V{RA}

• Lead II = V{LL} – V{RA}

• Lead III = V{LL} - V{LA}

• Lead II = Lead I + Lead III - Einthoven’s Law

• The voltage measured in a lead represents the projection of the cardiac vector onto the lead direction

• Vector representation of limb leads - Einthoven’s triangle

Augmented Frontal Plane Leads

• Lead aVR is the voltage at RA referenced to the average of LA and LL.

• Lead aVL is the voltage at LA referenced to the average of RA and LL.

• Lead aVF is the voltage at LL referenced to the average of RA and LA.

• Give information about current flow that is right, left, superior or inferior.

• An imaginary line drawn between two electrodes (for bipolar leads) is called the axis of the lead.

Frontal Plane Augmented Limb Leads (Goldberger's Equations)

• aVR = \frac{Lead I + Lead II}{2}

• aVL = \frac{Lead I - Lead III}{2}

• aVF = \frac{Lead II + Lead III}{2}

Why We Need Augmented Leads?

• Termed unipolar leads because there is a single positive electrode that is referenced against a combination of the other limb electrodes.

• The ECG recorder does the actual switching and rearranging of the electrode designations

• aVL lead is at -30° relative to the lead I axis; aVR is at -150° and aVF is at +90°

• These six leads record electrical activity along a single plane, termed the frontal plane relative to the heart

Transverse Plane Leads

• 6 precordial leads form a curve from the right heart across the septum to the left ventricle

Transverse Plane Precordial Leads

• A virtual reference terminal called Wilson’s central terminal is defined as the average of the potential at the three limb electrodes.

• WCT = \frac{V{LA} + V{RA} + V_{LL}}{3}

• 6 precordial leads (V1 to V6) are defined with respect to WCT.

• Unipolar leads.

12 Lead Placement: Transverse Plane Precordial Leads

• Electrode placement following either AHA (American Heart Association) or IEC (International Electrotechnical Commission) standards.

• Specific color codes and inscriptions are used to identify the location of each electrode (RA, LA, RL, LL, V1-V6).

Identifying Disease

• ECG paper speed and voltage (amplitude).

• At 50 mm/s:

  • 0.4 seconds = 200 milliseconds

  • 0.2 seconds = 100 milliseconds

  • 0.04 seconds = 20 milliseconds

• At 25 mm/s:

  • 0.4 seconds = 400 milliseconds

  • 0.2 seconds = 200 milliseconds

  • 0.04 seconds = 40 milliseconds

• Amplitude:

  • 1 mV = 10 mm

• Time:

  • At 50 mm/s 10 mm = 0.2 seconds = 200 milliseconds

  • At 25 mm/s 10 mm = 0.4 seconds = 400 milliseconds

Arrhythmias

• 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.

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

• Ventricular fibrillation – randomized contraction of the ventricles due to random repetitive excitation of ventricular muscle fibres without coordination

• Leads to rapid ischemia if normal cardiac rhythm is not re-established –Defibrillators.

Example on Identifying Cardiac Infarction

• STEMI (ST-Elevation Myocardial Infarction)

Electrodes

• Typically, Silver/Silver chloride electrodes are used.

• Electrodes are transducers that convert ionic currents to electrical currents.

• Electrode paste or jelly is applied between the electrode and skin.

• The combination of the ionic electrode paste and the silver metal of the electrode forms a local solution of the metal in the paste at the electrode-skin interface.

• Some of the silver dissolves into solution producing Ag^+ ions.

Block Diagram of a Basic ECG Amplifier Circuit

• The potentials recorded on the body surface have amplitudes on the order of 1 mV.

• Therefore, a combination of amplification and filtering is needed to obtain diagnostic quality tracings.

ECG Circuit

• Amplifier protection circuit:

  • First component receiving the signals from the electrodes.

  • Diodes are often used to limit the effect of high voltage/current, such as in the cases of electrostatic discharge or defibrillation shock.

• Lead selector circuit:

  • Each electrode connected to the patient is attached to the lead selector of the ECG.

  • Functions to determine which electrodes are necessary for a particular lead and to connect them to the remainder of the circuit

  • This circuit is usually controlled by the microprocessor to select the combination of electrodes required to be amplified.

• Preamplifier

  • Initial amplification is performed.

  • The preamplifier should have high common-mode rejection ratio (CMMR) and input impedance.

• Isolation circuit

  • Barrier to 50/60 Hz current

  • The isolation circuit protects both the patient and the ECG machine from possible high voltages.

• Driven right leg circuit

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

  • Sets the right leg as the reference signal.

• Driver amplifier

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

  • Band-pass filtering with cutoff frequencies of 0.05–150 Hz and zero-phase distortion.

  • Amplification is performed to achieve an amplitude resolution on the order of 10 μV or better prior to analog-to-digital conversion.

ECG System Requirements

• Ability to measure signals in the range of 0.05 – 10 mV

• Input impedance of an electrode should be greater than 5 M Ohm (at a frequency of 10 Hz)

• Bandwidth 0.1 to 150 Hz

• High common mode rejection ratio > 80 dB before the first amplification stage.

• The instrument should not allow leakage currents greater than 10 uA to flow through the patient

• Isolation methods should be used to keep the patient from becoming part of the AC electrical circuit in case of patient to powerline fault

• Committee on ECG of the American Heart Association

Sources of Noise

• Mains Interference

• Electrode contact noise

• Motion artifacts

• EMG from chest wall