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Case 4: Physiology (ECG), image - Coggle Diagram
Case 4: Physiology (ECG)
Electrocardiography
- When the heart is beating there are two types of activities taking place:
- Mechanical Activity
- Electrical Activity
- Electrical activity of the heart is seen as the ECG
- Electrical activity is the trigger for Mechanical activity
Biophysical Principles of the ElectrocardiographyList the Biophysical Principles of the Electrocardiography
- ECG activity is cardiac action potentials recorded on the surface of the body
- Signals require points of references and amplification
- Signals are electrical vectors with: Polarity, Direction, magnitude and Duration
Cardiac Pacemaker and Conduction at Cellular LevelOutline the Conduction System of the Heart
- the Conduction system of the heart occurs through Gap Junction-Mediated Intercellular conduction
- Cardiac myocytes are excitable cells because they generate action potentials
- Cardiac myocytes are joined together by way of Intercalated Discs
- Intercalated discs contains Desmosomes and Gap Junctions
- Therefore, the signal can be passed on from one cell to another cell by way of Gap Junctions
- This is called the Gap Junction-mediated Intercellular Conduction
Conductions system of the HeartList the components which make up the conduction system of the Heart
- The Conduction system of the Heart is made up of:
- Pacemaker Cells found in the:
- Sinoatrial (SA) Node located in the Atrium
- Atrioventricular (AV) Node located in the junction between the Atria and the Ventricles
- Conducting cells located in the:
- Internodal pathways of the Atrium
- Bundle of His
- Left and Right Bundle Branches (located in the Interventricular Septum)
- Purkinje fibres of the Ventricles
- These conduction pathways contain cardiac myocytes that are linked/connected by Gap Junctions and Desmosomes
AutorhythmicityList the parts of the Conduction System which are able to produce Autorhythmicity
- Sinoatrial Node (Sa Node) produces 100 b/min
- AV Node and Bundle Branches produce 40-60 b/min
- Purkinje fibres produce 20-40 b/min
- Heart rate is therefore is determined by the SA Node
- This is because the Sinoatrial Node produces a Refractory period for other pacemaking sites
The Conduction System
- The manner in which Action Potentials are conducted through the conduction system depends on the specialized part of the conduction system
- Action potentials are propagated from the SA Node to the AV Node and various parts of the Atria via the different Internodal Pathways.
- The Internodal Pathways are the:
- Anterior (Internodal pathway of Bachmann)
- Middle (Internodal pathway of Wenckebach
- Posterior (Internodal pathway of Thorel)
- Action potentials can only move form the Atria to the Ventricles through a specialized system called the Bundle of His.
- Atria and Ventricles are not electrically connected, except via the Bundle His (Fibrous Trigone)
- This is because they are separated by a Atrioventricular Fibrous ring that is non-conducting
- If there is an alternative pathway conducting electrical impulses form the Atria to the ventricles such as the Bundle of Kent
- The electrical impulses can propagate from the Ventricles back to the Atria
- This represents a re-entry phenomenon
- Ventricular Repolarization starts from the Epicardium and moves into the Endocardium
Speed of ConductionList the Speeds of Conduction in the Conduction System
- Conduction pathways have a Fast Conduction Speed
- Contractile cells have a Slow Conduction Speed
- AV Node has a Very Slow Conduction Speed
- This is to allow for Atrial Contraction to take place completely before Ventricular contraction occurs
Action Potentials and ECG WavesOutline the characteristics of Action Potentials and ECG waves
- Action potential are seen in the cells and ECG waves are seen on the body surface
ECG WavesExplain the occurrence of ECG waves linking it to Cardiac Action Potentials
- Since Action potential start at the Atria, we see Atrial Action Potentials first which are represented by the positive P Wave on the ECG
- Ventricular Action Potentials follows, this is represented by the QRST wave on the ECG
ECG Waves as Polarity Vectors
- Since the ECG is measured on the body surface, it is limited in that we can not see the full components of the Action potentials
- We can only observe Action Potential Depolarization and Repolarization
- In the cells we can see Atrial Action Potential followed by Ventricular Action Potential
- On the ECG we see Atrial Depolarization, represented by the positive P Wave
- After that the electricity returns to the Isoelectric Line until we see Ventricular Depolarization which coincides with Atrial Repolarization
- This is represented by the QRS Complex
- After that electricity returns to the Isoelectric Line until we see Ventricular Repolarization, represented by the positive T Wave on the ECG
ECG Recording (Limb Leads) - DiagramExplain the formation of the Einthoven Triangle
- Electrodes are placed on the Right arm, Left arm and Left leg to measure the Limb Leads
- In order to measure Limb Lead I
- A Positive electrode is placed on the Left arm and a Negative electrode is placed on the Right arm
- Since we are able to measure vectors from the Positive electrode, Lead I would be equivalent to viewing the electrical activity of the heart from the direction of the left arm
- In order to measure Limb Lead II and Limb Lead III, an additional Positive Electrode can be placed on the Left leg.
- These 3 Limb Leads form an Isosceles Triangle called the Einthoven Triangle
- Which places the Heart at it's Centre
ECG Waves as Polarity VectorsDescribe the concept of Polarity Vectors
- The polarity of a vector depends on whether it is a Depolarization or a Repolarization as well as the Position of the Recording Electrode
- For example: For Lead II
- The Positive Electrode is on the Left Leg and the Negative Electrode is on the Right arm
- Therefore, the viewing angle (Lead II) is from The Apex of the Heart looking upwards from below.
- If there is no Electrical Signal the Vector is at Zero, also known as the Isoelectric Line
- If there is an Electrical Signal (Vector) coming towards the Electrode, there is a Positive Wave on the ECG
- If there is an Electrical Signal (Vector) moving away from the Electrode, there is a Negative Wave on the ECG
- If there is a Depolarization moving Perpendicular to the Viewing angle, the signals cancel out
- And the Vector remains at Zero, Isoelectric Line
ECG Waves and the Conduction SystemDescribe the ECG waves produced when we use Lead II to construct an ECG
- If we use Lead II to construct a typical ECG wave then the following will appear on the ECG:
- When the electrical impulse is propagated from the SA Node to the AV Node via the Internodal Pathways of the Atrium.
- The Electrical impulse is moving towards the Lead II (Viewing angle) and causes Atrial Depolarization
- This is represented by a Positive T Wave (Atrial Depolarization)
- From the AV Node the electrical impulse is then propagated to the Bundle of His.
- The electrical impulse moves posteriorly away from the Lead II (Viewing angle)
- This is represented by a Negative Q Wave (Q wave is often Absent)
- From the Bundle of His the electrical impulse is then propagated to the Left and Right Bundle Branches of the Interventricular Septum
- The electrical impulse moves down the IV Septum, towards Lead II
- This is represented by a Positive R Wave
- From the Left and Right Bundle Branches the electrical impulse is propagated along the Purkinje Fibres of the Ventricles.
- The electrical impulse moves up the Ventricular wall, away from the Lead II (Viewing angle)
- This is represented by a Negative S Wave.
- Therefore, the QRS Complex signals Ventricular Depolarization (& Atrial Repolarization)
- Ventricular Repolarization starts from the Epicardium (Outside) to the Endocardium (Inside)
- This is represented by a Positive T Wave
ECG Recording: ECG as Waves of Depolarization or RepolarizationOutline the formation of the Intervals and Segments of the ECG wave
- ECG waves are either Upstrokes or Downstrokes depending on whether there is Depolarization or Repolarization
- With Atrial Depolarization there is a Positive P Wave
- After that the electricity returns to the Isoelectric Line
- This is followed by Ventricular Depolarization which gives the QRS Complex
- Q Wave is Negative, and then the electricity returns to the Isoelectric Line
- R Wave is Positive, and then electricity returns to the Isoelectric Line
- S Wave is Negative, and then electricity returns to the isoelectric Line
- The Width of the QRS complex often indicates the location of the originating electrical impulse
- Ventricular Repolarization is represented by the T Wave is Positive
- After that the electricity returns to the isoelectric Line
- T Wave is normally asymmetrical and is usually larger than the P Wave
- As the heart rate increases the P Wave and the T Waver can share the same space on an ECG
- Abnormally shaped T waves can show acute episodes of Cardiac Ischaemia, Electrolyte Imbalances and the use of Cardiac Medications such as Digoxin
ECG Recording: Intervals and Segments of the ECG WaveList the Intervals and Segments of the ECG Wave
- PR Interval
- PR Segment (Within the Interval)
- QT Interval
- ST Segment
- TP Segment
Describe the PR Interval of the ECG waves
- PR Interval is measured from the start of P Wave to the start of the QRS Complex
- PR Interval measures the duration between Atrial and Ventricular Contraction
- If the PR Interval is longer than normal (0.2 seconds) this suggests that conduction is abnormally slow through the AV Junction
- This is known as the First Degree Block
- If the PR Interval is shorter than normal this indicates a Junctional Rhythm
Describe the PR Segment of the ECG waves
- PR Segment is the between the P wave and the beginning of the QRS Complex
- PR Segments signifies the time take to conduct the electrical impulse through the slow Atrioventricular (AV) Node
Describe the QT Interval of the ECG waves
- QT Interval is measured from the Start of the QRS Complex too the End of the T Waves
- QT Interval represents a complete Ventricular cycle of Depolarization and Repolarization
- A prolonged QT is associated with a high incidence of sudden death
Describe the ST Segment of the ECG Waves
- ST Segment is measured from the end of the S Wave to the start of the T wave
- ST segments shows Early Repolarization of the Ventricles, when the Ventricles have fully Depolarized
- Therefore, Calcium is coming in and there is maximum contraction
- ST segments usually lies along the ECG Baseline/ Isoelectric Line
- The Heart does not have any electrical activity during this time
- When the ST Segment is not on the Isoelectric Line it is said top be Elevated or Depressed
- Therefore is is important to note that the ST Segment begins at the Junction J Point and stops at the beginning of the T wave
- ST deviation is a sign of Myocardial Ischaemia, Myocardial Infarction and/or Cardiac Disease
ECG recording: ECG and the Cardiac Cycle (Diagram)Provide a brief explanation of how the ECG Wave forms a Cardiac Cycle with a Drawing
- ECG is part of the Cardiac Cycle
- The events between the QRS Complex and the T Wave represent Systolic Events
- And the events between the T wave and the next P Wave represent the Diastolic events
- This creates a cyclic pattern where each event is followed by following event in a predictable manner
ECG RecordingOther Leads: Augmented Vector Leads - DiagramOutline the other Leads of ECG Waves
- We can also measure 3 Other Augmented Vector (AV) Leads
- Apart from Lead I, Lead II, and Lead III there is:
- aVL (augmented Voltage L)
- aVR (augmented Voltage R)
- aVF (augmented Voltage F)
- These 3 Leads provide 3 different views
ECG RecordingOther Leads: Chest (Precordial) Leads
- There are also Precordial (Chest) Leads on the Chest Surface
- In total there are 6
- V1, V2, V3, V4, V5, and V6
- V1 is located on the Right Sternal Border in the 4th Intercoastal Spaced
- V2 is located on the Left Sternal Border in the 4th Intercoastal Space
- V3, V4, V5, and V6 are located in the Left 5th Intercoastal Space
12-Lead ECGDescribe what is the 12-Lead ECG
- In Total there are 6 Vertical Plane Leads and 6 Horizontal leads:
- 3 Limb Leads and 3 augmented Voltage Leads
- V1, V2, V3, V4, V5 and V6
- In total these gives the 12-Lead ECG
ECG Interpretation of the 12-Lead ECG
- It is important to note that each Lead produces a different ECG Wave, BUT they are all recording the SAME Electrical Phenomenon but from different reference points
- Example: Look at Lead II and Lead aVR
- Lead II has upright waves and Lead aVR has downright waves
- For Lead II the reference angle is from below the Heart and the most of the Waves are Depolarization waves moving towards Lead II
- Therefore, there are predominantly Positive Waves
- In Lead II, BOTH Atrial and Ventricular Depolarization are moving in the direction of the eye
- For Lead aVR the positive Electrode is on the Right Arm
- Viewing angle is from the Top of the Heart looking Downwards
- The Depolarization of BOTH the Atria and The Ventricles is moving away from Lead aVR
- Resulting in predominantly Negative Waves
12-Lead ECG for Various Views
- Different ECG Leads look at different parts of the Heart
- Example: In The Coronary Circulation, different parts of the Heart are supplied by different Coronary Vessels
- If the Coronary Vessels are damaged, they will show differently on the different ECG Lead Waves
List the different parts of the Heart Wall with their respective ECG Leads
- Right Inferior Wall: Supplied by the Right Coronary Artery-Posterior Descending
- Lead II, Lead III and Lead aVF
- Anterior wall: Supplied by the Left Anterior Descending artery
- Left Lateral Wall: Supplied by the Left Circumflex Artery
- Lead I, Lead V5, Lead V6 and aVL
- Right Septal Wall: Supplied by the Left Anterior Descending artery Septal
- Therefore, depending on which wall we need to assess, we will assess the ECG waves of the Lead that properly views the respective Heart Wall
ECG InterpretationMagnitude of the ECG Voltage
- Typically on a recorded ECG in order to get an idea of whether the ECG Waves are normal sizes or not:
- At the beginning of the Tracing is a Calibration Mark
- The Calibration Mark is = 1 mV Amplitude
- This tells us whether the subsequently recorded Waves are normal Voltage ECG, Low-Voltage ECG or High-Voltage ECG
- Low-Voltage ECG can occur when there is something separating the Heart from the recording Surface such as Pericardial Effusion
- As a result the signals are so small that they can not easily reach the surface
Heart Rate and RhythmExplain how to use the ECG to calculate Heart Rate and Rhythm
- ECG can also be used to measure the Heart Rate using the Sweep Speed
- Sweep Speed is the how fast the trace can sweep across the oscilloscope screen, it calculates how long it takes from one beat to the following beat
- The Standard Sweep Speed is 25 mm/s
Sweep Speed Calculation
- Each Dark square is 0.2 s = 1/300 minutes
- Hence, the Heart Rate is = 300/ No. of Squares
Specific ECG WavesDescribe the Specific ECG waves in order
- P Wave
- P Waves represents Atrial Depolarization
- The movement of electrical impulses from the SA Node to the AV Node via the Internodal pathways
- Abnormalities of the P Wave:
- When the P Wave is absent this is known as the Non-Sinus Rhythm
- When the P Wave is peaked or larger than normal in amplitude this is known as a Peaked P Wave. Example in Right Atrial Enlargement due to Pulmonary Hypertension
- When the P Wave is m-shaped this is known as a Double P Wave. Example in Left Atrial Enlargement due to Mitral Stenosis
- The reason why the P Wave would be m-Shaped is because Right Atrial and Left Atrial representation Superimpose
- PR Interval
- PR Interval is the duration between the start of the P Wave to the start of the QRS Complex
- PR Interval depicts Atrioventricular conduction mainly the AV Node Transmission, since it is the slowest
- Abnormalities of the PR interval:
- If the PR Interval is longer than normal (0.2 seconds) this suggests that conduction is abnormally slow through the Atrioventricular (AV) Junction.
- This is known as the 1st Degree Block
- If the PR Interval is prolonged leading to the Lack of Ventricular Beats this is known as the 2nd Degree Heart Block
- If the Ventricular and Atrial activities are disconnected where by there is complete Atrioventricular Dissociation.
- This is known as the 3rd Degree Heart Block
- QRS Complex which represents Ventricular Depolarization
- This is when the electrical impulses moves from the Bundle of His, to the Left and Right Bundle Branches to the Purkinje Fires
- Q represents Initial Depolarization and the Bundle of His
- R is the Bundle Branches
- S Represents Late Depolarization at the Purkinje Fibres
- Abnormalities of the QRS Complex:
- If there is any abnormalities in any of these sections they can then be seen as abnormalities of the negative Q Wave
- Normally the Q Wave is small or absent because it moves in a small space (From the AV Node to the Bundle of His)
- Therefore, when the Q Wave becomes larger than normal, This is known as the Significant Q Wave
- Due to an Old Anterior Infarct
- A Wide QRS Complex is called the RsR Phenomenon due to the Bundle Branch Block
- QT Interval and ST Segment
- QT Interval measures from the start of the QRS Complex to the end of the T Wave
- QT Interval represents Ventricular Depolarization and repolarization
- ST Segment is located between the end of the S Wave and the start of the T wave
-ST Segment is the Isoelectric line when the Electricity is at Zero
- Abnormalities of the QT interval and the ST Segment:
- If there is QT Elongation where the QT interval is prolonged
- Then various forms of Long/Short QT Syndrome will occur
- If the level of the ST segment is above or below the Isoelectric Line this is called ST Elevation or ST Depression
- Due to a Recent Infarct seen on Lead II of ECG
- T Wave
- T Wave represents Ventricular Repolarization
- This phenomenon starts from the Epicardium going into the Endocardium
- Abnormalities of the T Wave:
- Peaked T Wave such as in Hyperkalemia (High Potassium)
- T Wave inversion such as in Ischaemia or Digonxin Toxicity
Magnitude of the ECG Voltage: Cardiac Electrical Axis (Diagram)
- Cardiac Electrical Axis is used to indicate where the majority of the Cardiac Impulses/ Waves are moving towards
- In the Heart the Axis is moving from Top Down
- So that the Net Axis is in the Normal Axis of - 30 Degree to +90 Degree
- If the Net Axis falls out, then there is an Axis Deviation
- Left Axis Deviation or Right Axis Deviation
- This can be caused by left Ventricular Hypertrophy or Right Ventricular Hypertrophy
Calculation of the Cardiac Axis (Diagram)Outline how to Calculate the Cardiac Axis
- Plot the Size of the Net Deflection of the asked Leads
- Drop perpendicular lines between the 2 Points and Find the Net Direction
- Net Direction is the Axis Angle
ECG Waves: QRS Complex - Ventricular Size/ HypertrophyOutline how ECG Waves can be used to calculate Ventricular Size/ Hypertrophy
- Right Ventricular Hypertrophy
- There is Right Ventricular Hypertrophy when the R Wave is greater in amplitude than the S Wave in the V1 Lead
- Left Ventricular Hypertrophy
- There is Left Ventricular Hypertrophy if the sum of the S Wave Amplitude size in V1 and R Wave Amplitude size V5 is greater than or equal to 35 mm
- S in V1 + R in V5 >/= 35 mm
Arrhythmogenesis
- The Types of abnormalities in the ECG can tell us which Arrhythmia is present:
- Pacemaker dysfunction
- Conduction defects (Heart Block)
- Re-entry Pathway (Pre-excitation)
- Bundle Branch blocks
- Ectopic excitation
- Mechanisms: Arrhythmias will occur if there is:
- Focal activity (automaticity/triggered)
- Conduction deficits
-