Electrocardiogram (ECG) Notes

Electrocardiogram (ECG)

• Electro: Electrical activity
• Cardio: Heart
• Gram: Output "on paper"
• Graph: Instrument used
• Graphy: Technique
• Life healthcare ≠ sick care
• Clinical Laboratory Testing
• ECG Test
• ECG - Electrocardiogram
• Electrocardiograph
• Electrocardiography

Introduction

• The electrocardiograph (ECG) is an instrument that records the electrical activity of the heart.
• The ECG records the electrical signal of the heart as the muscle cells depolarize (contract) and repolarize.
• Normally, the SA Node generates the initial electrical impulse and begins the cascade of events that results in a heartbeat.
• Cells at rest have a negative charge with respect to the exterior; depolarization consists of positive ions rushing into the cell.
• ECG provides valuable information about a wide range of cardiac disorders, such as an inactive part (infarction) or an enlargement (cardiac hypertrophy) of the heart muscle.
• Electrocardiographs are used in catheterization laboratories, coronary care units, and for routine diagnostic applications in cardiology.

Conduction System of the Heart

• Also called the pacemaker of the heart - "SA node".

  1. Sino Atrial Node (SA Node): Initiates the impulse; the start and beginning of cardiac activity.
  2. Internodal Atrial Pathway: The impulse goes through it to the AV node.
  3. Atrioventricular Node (AV Node)
  4. Bundle of His
  5. Purkinje System

• Electrical conduction through the heart involves studying the electrical activity.

• When the heart beats, an impulse starts, and within one minute, we study how many times the heart is beating.

• Within one impulse, we study where the impulse enters the heart, what it does, and what comes out.

ECG Waveform

• For a complete ECG waveform, we have P, Q, R, S, T, U, V, but some are very small and negligible.

• One ECG waveform consists of P, Q, R, S, which is called the QRS complex.

• Conduction Pathway:

  1. SA node (Pacemaker)
  2. AV node (delay)
  3. AV bundle & branches (Insulated)
  4. Purkinje fibers (activation)

• Fibro-fatty atrioventricular groove separates atrial and ventricular tissue.

• Contractile, Conductive, and Nonconductive tissues are present.

ECG Machine

• Block Diagram of an Electrocardiograph Machine:
  • Electrodes are placed where we start obtaining signals.
  • Includes:
    • Electrodes
    • Lead selector
    • Bridge
    • Power
    • Preamp amplifier
    • Output circuit
    • Auxiliary circuits
    • Chart transport motor
    • Frequency selective feedback network
    • Pen motor

ECG Electrodes

• Limb electrodes: Placed on the hands and legs.

• Chest electrodes: Placed on the chest.

• Classification of electrodes depending on how they are placed on the patient:

  1. Valve electrode
  2. Disposable electrodes
  3. Capels
  4. Vacuum electrodes
  5. Clamp electrodes

Types of ECG Electrodes

• Limb electrodes
• Floating electrodes (vacuum or bulb)
• Pregelled disposable electrodes: Pregelled because they have gel; only used one time.
• Vacuum electrodes have changed into disposable ones.

Limb Electrodes

• The most common type of electrodes routinely used for recording ECG are rectangular or circular surface electrodes.
• The material used is German silver, nickel silver, or nickel-plated steel.
• They are applied to the surface of the body with electrode jelly.
• The typical value of the contact impedance of these electrodes, which are of normal size, is nearly $2$ to $5 kΩ$ when measured at $10 Hz$.
• The electrodes are held in position by elastic straps.
• They are also called limb electrodes as they are most suitable for application on the four limbs of the body.
• They are reusable and last several years.

Advantages and Disadvantages of Limb Electrodes

• Advantages:
  • Generally preferred for use during surgery because the patient’s limbs are relatively immobile.
  • Chest electrodes cannot be used as they would interfere with the surgery.
• Disadvantages:
  • Not suitable for use in long-term patient monitoring because the long flowing leads are inconvenient to the patient.
  • Electromyographic voltages generated by the activity of the limb muscles makes them unsuitable for use when monitoring conscious and semi-conscious patients.

Floating Electrodes

• Limb electrodes generally suffer from motion artifacts caused due to the relative motion at the interface between the metal electrode and the adjacent layer of electrode jelly.
• The interface can be stabilized by the use of floating electrodes in which the metal electrode does not make direct contact with the skin.
• The electrode consists of a lightweight metalled screen or plate held away from the subject by a flat washer which is connected to the skin.
• Floating electrodes can be recharged, i.e., the jelly in the electrodes can be replenished if desired.

Drawbacks of Electrodes

• Electrodes which are employed in stress testing or long-term monitoring present additional problems because of the severe stresses, perspiration, and major body movement encountered in such studies.
• Both design considerations and application techniques of electrodes used in electrocardiography are necessary to prevent random noise on the baseline, baseline wandering, and skin contact over extended periods causing a loss of signal.

Pregelled Disposable Electrodes

• To overcome problems due to prolonged application, special disposable electrodes have been developed.
• The main design feature of these electrodes which helps in reducing the possibility of artifacts, drift, and baseline wandering is the provision of a high-absorbency buffer layer with isotonic electrolyte.
• This layer absorbs the effects of movement of the electrode in relationship to the skin, and attempts to maintain the polarization associated with the half-cell potential constant.
• Since perspiration is the most common cause of electrode displacement, the use of an additional porous overlay disc resists perspiration and ensures secure placement of the electrode on the skin even under stress conditions.

Construction of Pregelled Disposable Electrodes

• The lead wire’s female connector “snaps” on, allowing a convenient snap-on pull-off connection with a 360-rotation providing mechanical and electrical connection.

• The plastic eyelet or sensor has a diameter of $0.5 – 1.5 cm$ and is electroplated with silver up to a thickness of $10 mm$.

• The surface of the $Ag$ layer is partially converted to $AgCl$.

• The tape is made from one of the adhesive coated occlusive foams made from a plastic, such as polyethylene, or a porous backing, such as non-woven cloth.

• The electrode diameters range from $4 – 6 cm$.

• Advantages of these electrodes:

  • There is no risk of infection which is possible with reusable electrodes.
  • Their smaller size makes them less prone to detachment.
  • Less time is required per ECG procedure.

Lead Placement

• 4 limb leads + 6 chest leads
• Limb leads: RA, RL, LL, LA
• Chest leads: V1, V2, V3, V4, V5, V6
• Connecting these together gives a proper ECG waveform.

Electrocardiogram (ECG) Characteristics

• Generated in the heart
• Amplitude range: $0.5 - 4 mV$
• Frequency range: $0.01-250 Hz$
• Measurement: surface electrodes

ECG Leads: Einthoven's System

• In 1908, Willem Einthoven developed a system capable of recording small signals and recorded the first ECG.
• The leads were based on the Einthoven triangle associated with the limb leads.
• Leads put the heart in the middle of a triangle.
• In defining the bipolar leads, Einthoven postulated that at any given instant of the cardiac cycle, the electrical axis of the heart can be represented as a two-dimensional vector.
• The ECG measured from any of the three basic limb leads is a time-variant single-dimensional component of the vector.
• He proposed that the electric field of the heart could be represented diagrammatically as a triangle, with the heart ideally located at the center.
• The triangle is known as the “Einthoven triangle”.
• The sides of the triangle represent the lines along which the three projections of the ECG vector are measured.

Limb Lead Placement

• Bipolar leads: LEAD-I, LEAD-II, LEAD-III
• Augmented leads: AVR, AVL, AVF
• Einthoven's triangle shows the relationship between the leads.

Chest Leads and Extra Leads Placements

• 6 chest leads: V1, V2, V3, V4, V5, V6
• Standard chest lead placement:
  • V1
  • V2
  • V3
  • V4
  • V5
  • V6
• V3R & V4R, V7 & V8 are used to assess immediate heart events.

Electrode Placement

• 10 electrodes in total are placed on the patient.
• Firstly, self-adhesive ‘dots’ are attached to the patient with single electrical contacts.
• The 10 leads on the ECG machine are then clipped onto the contacts of the ‘dots’.

Electrode Placement in 12 Lead ECG

• 6 are chest electrodes called V1-6 or C1-6

• 4 are limb electrodes

  • Right arm: Red
  • Left arm: Yellow
  • Left leg: Green
  • Right leg: Black (Right leg electrode is a neutral or “dummy”!

Electrode Placement for Chest Electrodes

• V1 - 4th intercostal space right sternal edge
• V2 - 4th intercostal space left sternal edge
• To find the 4th space, palpate the manubriosternal angle (of Louis).
• Directly adjacent is the 2nd rib, with the 2nd intercostal space directly below.
• Palpate inferiorly to find the 3rd and then 4th space
• V4 - over the apex (5th ICS mid-clavicular line)
• V3 - halfway between V2 and V4
• V5 - at the same level as V4 but on the anterior axillary line
• V6 - at the same level as V4 and V5 but on the mid-axillary line

Right Side Leads & Posterior Lead ECG

• RIGHT SIDE LEADS
  • The usual 12-lead ECG evaluates only the left ventricle.
  • If the RV needs to be assessed for damage or dysfunction. Eg. Inferior wall MI - RV lead to rule out the RV involvement
• POSTERIOR LEAD ECG
  • These ECGs used to assess the posterior side of the heart, standard 12 lead ECGs can`t assess(posterior surface of myocardium).
  • V7, V8, V9, These leads are placed opposite of the anterior leads V4,V5, AND V6 on the left side of the patient back following the same horizontal line.

Precordial Leads vs Augmented Leads

• Precordial Leads:
  • Measure potentials close to the heart, V1- V6
  • Unipolar leads
• Augmented Leads:
  • Three additional limb leads are also used: aVR, aVL, and aVF
  • These are unipolar leads
  • Each lead uses the average of the other two leads as reference
  • $VR = ΦR – (ΦL + ΦF)/2$

How Does the ECG Work?

• Electrical impulse (wave of depolarization) picked up by placing electrodes on patient

• The voltage change is sensed by measuring the current change across 2 electrodes – a positive electrode and a negative electrode

• If the electrical impulse travels towards the positive electrode this results in a positive deflection

• If the impulse travels away from the positive electrode this results in a negative deflection

  • Towards the electrode = positive deflection
  • Away from the electrode = negative deflection

• Direction of Impulse Axis

Lead Formation

• Lead I is formed using the right arm electrode (red) as the negative electrode and the left arm (yellow) electrode as the positive.
• Lead II is formed using the right arm electrode (red) as the negative electrode and the left leg electrode as the positive.
• Lead III is formed using the left arm electrode as the negative electrode and the left leg electrode as the positive.
• aVL, aVF, and aVR are composite leads, computed using the information from the other leads.

Leads and What They Tell You

• Limb leads look at the heart in the coronal plane
  • aVL, I and II = lateral
  • II, III and aVF = inferior
  • aVR = right side of the heart
• Each lead can be thought of as ‘looking at’ an area of myocardium
• Chest leads V1 to V6 ‘look’ at the heart on the transverse plane
  • V1 and V2 look at the anterior of the heart and R ventricle
  • V3 and V4 = anterior and septal
  • V5 and V6 = lateral and left ventricle
• P wave = atrial depolarization
• QRS = ventricular depolarization
• T = repolarization of the ventricles

Bipolar Leads

• Lead I: + _ Vo
• Lead II: + _ Vo
• Lead III: + _ Vo

Augmented Leads

• aVR: Vo
• aVF: Vo
• aVL: Vo

Unipolar Chest Leads

Instrumentation Setup

ECG Block Diagram

• Assign different electrodes for different leads
• Ensure proper base and alignment of the circuit
• Ensure that the signal obtained from the power amplifier does not go out of the circuit that is maintained.
• Includes:
  • Electrodes
  • Lead selector
  • Bridge
  • Power
  • Preamp amplifier
  • Output circuit
  • Auxiliary circuits
  • Chart transport motor
  • Frequency selective feedback network
  • Pen motor
• D printer - How do you print ECG

Working of ECG

• The potentials picked up by the patient electrodes are taken to the lead selector switch.
• In the lead selector, the electrodes are selected two by two according to the lead program.
• By means of capacitive coupling, the signal is connected symmetrically to the long-tail pair differential preamplifier.
• The preamplifier is usually a three or four stage differential amplifier, which gives a stabilizing effect.
• The amplified output signal is picked up and is given to the power amplifier (push-pull differential type).
• The base of the other transistor is driven by the feedback signal resulting from the pen position and connected via frequency selective network.
• The output of the power amplifier is single-ended and is fed to the pen motor, which deflects the writing arm on the paper.

Working of ECG (Contd.)

• A direct writing recorder is used as the ECG signal has limited bandwidth.
• Frequency selective network is an R–C network, which provides necessary damping of the pen motor and is preset by the manufacturer.
• The auxiliary circuits provide a $1 mV$ calibration signal and automatic blocking of the amplifier during a change in the position of the lead switch. It may include a speed control circuit for the chart drive motor.
• A ‘stand by’ mode of operation is generally provided on the electrocardiograph.
• In this mode, the stylus moves in response to input signals, but the paper is stationary. This allows the operator to adjust the gain and baseline position controls without wasting paper.

Direct Writing Recorders

• A coil of thin wire, wound on a rectangular aluminum frame is mounted in the air space between the poles of a permanent magnet
• Hardened-steel pivots attached to the coil frame fit into jeweled bearings so that the coil rotates with a minimum of friction. A lightweight pen is attached to the coil.
• Springs attached to the frame return the pen and coil always to a fixed reference point.
• When current flows through the coil, a magnetic field is developed which interacts with the magnetic field of the permanent magnet.
• It causes the coil to change its angular position as in an electric motor.
• The direction of rotation depends upon the direction of flow of current in the coil.
• The magnitude of pen deflection is proportional to the current flowing through the coil.

Writing Stylus

• The writing stylus can have an ink tip or it can have a tip that is the contact for an electro-sensitive, pressure sensitive or heat sensitive paper.
• If a writing arm of fixed length is used, the ordinate will be curved.
• In order to convert the curvilinear motion of the writing tip into a rectilinear motion, various correcting mechanisms have been devised to change the effective length of the writing arm as it moves across the recording chart.

Isolation Preamplifier - Need

• All ECG circuits have to have the right leg (RL) electrode connected to the chassis, and from there to the ground to provide a ready path for any ground seeking current through the patient and presented an electrical hazard.
• The American Heart Association guidelines state that the leakage current should not be greater than $10 microamperes$ when measured from the patient’s leads to the ground or through the main instrument grounding wire with the ground open or intact.
• For this, patient leads would have to be isolated from the ground for all line operated units.

Block Diagram of Isolation Preamplifier

Isolation Amplifier - Working

• Difference signals obtained from the right arm (RA), left arm (LA) and right leg (RL) is given to a low-pass filter.
• Filtering is required on the input leads to reduce interference caused by electrosurgery and radio frequency emissions and sometimes from the $50 kHz$ current used for respiration detection.
• The filter has a cut off frequency higher than $10 kHz$. A multistage filter is needed to achieve a suitable reduction in high frequency signal.
• The filter circuit is followed by high voltage and over voltage protection circuits so that the amplifier can withstand large voltages during defibrillation.
• The lead selector switch is used to derive the required lead configurations and give it to a dc-coupled amplifier.
• A dc level of $1 mV$ is obtained by dividing down the power supply, which can be given to this amplifier through a push button for calibration of the amplifier.

Isolation Amplifier - Working (Contd.)

• Isolation of the patient circuit is obtained using a low capacitance transformer whose primary winding is driven from a $100 kHz$ oscillator.
• The transformer secondary is used to obtain an isolated power supply of $±6 V$ for operating the devices in the isolated portion of the circuit and to drive the synchronous modulator at $100 kHz$, which linearly modulates an ECG signal given to it.
• The oscillator frequency of $100 kHz$ is chosen (higher the frequency the smaller the transformer) so that the switching time is not too fast
• A square wave is utilized to minimize the power requirements of the driven transistors.
• A synchronous demodulator is chosen to give low noise performance utilizing switching FET’s.

Effects of Artifacts on ECG

• Abnormal patterns of ECG may be due to pathological states or on occasion they may be due to artifacts

  • Interference from the Power Line
  • Shifting of the Baseline
  • Muscle Tremor

Power Line Interference

• Power line interference may be due to the stray effect of the alternating current on the patient or because of alternating current fields due to loops in the patient cable.
• Other causes of interference are loose contacts on the patient cable as well as dirty electrodes.
• When the machine or the patient is not properly grounded, power line interference may even completely obscure the ECG waveform.
• The most common cause of $50 Hz$ interference is the disconnected electrode resulting in a very strong disturbing signal.
• It is strong enough to damage the stylus of an unprotected direct writing recorder, and therefore needs quick action
• Static charges on the synthetic uniform of the operator may result in a random noise on the trace. This noise is very difficult to remove except in those machines which have very high CMRR.
• The noise can be reduced by partially shielding the patient by means of the bed springs.

Shifting of the Baseline

• A wandering baseline is usually due to the movement of the patient or electrodes.
• The baseline shift can be eliminated by ensuring that the patient lies relaxed and the electrodes are properly attached.
• Baseline wander is usually observed immediately after application of the electrodes.
• It is due to a relatively slow establishment of electrochemical equilibrium at the electrode-skin interface.
• This can be minimized by selecting the proper electrode material, which will reach equilibrium quickly with a good electrode jelly.

Muscle Tremor

• Irregular trembling of the ECG trace without wandering of the baseline occurs when the patient is not relaxed or is cold.
• It is generally found in the case of older patients.
• Muscle tremor signals are especially bothersome on limb leads when a patient moves or the muscles are stretched.
• Therefore, for long-term monitoring, the electrodes are applied on the chest and not on the limbs.
• For normal routine ECG recordings, the patient must be advised to get warm and to relax so that muscle tremor from shivering or tension is eliminated.

Patient Cable

• The most critical component of the ECG recorder is the patient cable.
• The conventional PVC insulation gets degraded and becomes rigid and breakable because of the arification of the softener.
• Some manufacturers supply a patient-cable made of silicon-rubber, which provides better elasticity over long periods

Push-Pull Amplifier

• A push–pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load

Capacitive Coupling

• A coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked.
• This technique helps to isolate the DC bias settings of the two coupled circuits.
• Capacitive coupling is also known as AC coupling and the capacitor used for the purpose is also known as a DC-blocking capacitor.