Intro to ECG

Question 1

What is an electrocardiogram?

Answer 1

• The electro/chemical changes that cause myocardial contraction, shown by the cardiac action potential

Question 2

How do cardiac cells contract?

Answer 2

• The potential difference across the cellular membrane must change from negative to positive in relation to the inside of the cell (this initiates an action potential)
• Changes in potential difference occur through the flow of ions through specialised ion channels in the cellular membrane and also relatively freely through gap junctions
• Depolarisation initiates at SAN due to nature of SAN not having a stable resting potential, and it also depolarises very quickly
• The myocardium (cardiac muscle) has electrical currents that sequentially depolarise each individual cell, resulting in a change in cellular morphology that allows muscle contraction

Question 3

Describe the cardiac electrical field

Answer 3

The electric dipole (lead) consists of 2 equal and opposite charges, +q and -q, separated by a distance d. The dipole is a vector (has magnitude and direction)

Question 4

Where are the leads placed on the frontal plane?

Answer 4

6 limb leads

• Right arm
• Left arm
• Right leg
• Left leg

Question 5

What are the 6 limb leads in the frontal plane and state whether they are bipolar or unipolar

Answer 5

• Lead I - Bipolar
• Lead II - Bipolar
• Lead III - Bipolar
• aVR - Unipolar
• aVL - Unipolar
• aVF - Unipolar

Question 6

What does it mean if a lead is bipolar or unipolar?

Answer 6

• Unipolar leads- register activity in the heart which is directed towards, or located below the electrode
• Bipolar leads- register the voltage between 2 electrodes

Question 7

How are the aVR, aVL and aVF leads placed?

Answer 7

Placed to create an equilateral triangle with the heart in the middle - This is the Einthoven triangle

Question 8

Describe the aVR lead

Answer 8

An augmented and unipolar limb lead, constructed to obtain specific information from the right upper portion of the heart, including the outflow tract of the right ventricle and the basal portion of the inter ventricular septum

Lead II travels from aVR towards aVF to become 2nd inferior lead

Placed on right wrist or shoulder

Question 9

Describe the aVL lead

Answer 9

• Unipolar lead placed on left wrist or shoulder, looks at upper left side of heart
• Lead I travels towards aVL creating a second high lateral lead

Question 10

Describe the aVF lead

Answer 10

• Placed on left ankle or left lower abdomen, looks at bottom, or inferior wall of heart
• Lead II travels from aVR to aVF to become a 2nd inferior lead
• Lead III travels from the aVL towards aVF to come a 3rd inferior lead

Question 11

What are the 6 chest leads and where are they placed?

Answer 11

• V1 - 4th intercostal space, right sternal edge
• V2 - 4th intercostal space, left sternal edge
• V3 - equidistant from V2 - V4
• V4 - 5th intercostal space, midclavicular line
• V5 - left anterior axillary line in horizontal line with V4
• V6 - mid axillary line, horizontal with V4 and V5

Question 12

What is R wave progression?

Answer 12

From V1 to V6, the pattern is that of a change from the S wave being prominent to the R wave being prominent

Question 13

What leads convey activity of the inferior surface of the heart?

Answer 13

• II
• III
• aVF

Question 14

What leads convey activity of the septal area of the heart?

Answer 14

• V1
• V2

Question 15

What leads convey activity of the anterior surface of the heart?

Answer 15

• V3
• V4

Question 16

What leads convey activity of the lateral surface of the heart?

Answer 16

• V5
• V6
• I
• aVL

Question 17

Describe sinus rhythm

Answer 17

• Initiated by SAN, atria contract
• Depolarisation spreads to AVN
• AVN slows depolarisation to allow enough time for ventricles to fill and prevent damage to ventricles
• Depolarisation spreads to bundle of His, which splits into purkinje fibres, which causes ventricular contraction beginning a the apex up towards the base of the heart

Question 18

Describe what sinus rhythm would look like on an ECG

Answer 18

• 1 P wave before each QRS
• Normal PR interval (less than 0.2 seconds)
• Normal QRS duration

Question 19

What is the Bachmann’s bundle?

Answer 19

• Often referred to as the intertribal bundle
• Muscular bundle composed of parallel muscle strands connecting left and right area
• Serves as the primary electrical connection between the right and left atria, allowing the waves of depolarisation to spread through both atria

Question 20

What does a normal ECG look like?

Answer 20

• Wave - A positive or negative deflection from baseline that indicates a specific electrical event. The waves on an ECG are P, QRS, T
• Interval - Time between two specific ECG events. Intervals commonly measured on an ECG include the PR interval, QRS interval, QT interval and RS interval
• Segment - The length between 2 specific points on an ECG that are supposed to be at the baseline amplitude (neither negative nor positive). The segments on an ECG include the PR segment, ST segment and TP segment
• Complex - The combinations of multiple waves grouped together. This is the QRS complex.
• Point - There’s only one point on an ECG and this is the J point, which is where the QRS complex ends and the ST segment begins

Question 21

How is rate calculated from a 12-lead ECG?

Answer 21

• When cardiac rhythm is regular, heart rate can be determined by the interval between 2 successive QRS complexes
• On standard paper with the most common tracing settings, the heart rate is calculated by dividing the number of large boxes (5mm or 0.2 seconds) between two successive QRS complexes into 300

Question 22

What is tachycardia?

Answer 22

• Increased heart rate
• Heart rate >100bpm

Question 23

What is bradycardia?

Answer 23

• Decreased heart rate
• Heart rate <50bpm

Question 24

How is rhythm calculated from a 12-lead ECG?

Answer 24

• Rhythm refers to the pattern of electrical impulses that causes the heartbeat
• Normal cardiac rhythm is a sinus rhythm that follows a predictable patter
• When cardiac rhythm is regular, heart rate can be determined by the interval between 2 successive QRS complexes
• On standard paper with the most common tracing settings, the heart rate is calculated by dividing the number of large boxes (5mm or 0.2 seconds) between two successive QRS complexes into 300

Question 25

What does axis mean?

Answer 25

Axis is the average direction of electrical movement through the heart during a depolarisation

Question 26

How is the axis calculated from a 12-lead ECG?

Answer 26

- The most efficient way to estimate axis is to look at LEAD I and LEAD aVF. - A positive QRS in lead I puts these in roughly the same direction as lead I - A positive QRS in lead aVF similarly aligns the axis with lead aVF. Combining both coloured area- the quadrant of overlap determines the axis

Question 27

What does the P wave show?

Answer 27

- Represents atrial depolarisation - In healthy individuals, a P wave precedes each QRS complex - The height of the P wave should be no more than 3mm (ideally 2.5mm)

Question 28

What does the PR interval show?

Answer 28

- PR interval begins at the start of the P wave and ends at the beginning of the Q wave - It represents the time taken for electrical activity to move between the atria and the ventricles, and includes the time the electrical impulse is being held at the AVN

Question 29

What does the Q wave show?

Answer 29

- The first negative deflection (current flowing away from the lead) - Septal depolarisation - this is when depolarisation is being distributed at the bundle of His, it's distributed between the left and right branches - It's is distributed to the left ventricle first, and the movement moves from left to right - The positive charge moves away from the positive electrode

Question 30

What does the R wave show?

Answer 30

- The first positive deflection (current flowing towards lead) - Action potentials spread through the purkinje fibres so movement is through to the apex, and then moves up to the ventricles - This is the R wave - The left ventricle has thicker muscles and a greater number of myocytes, so generates a greater number of action potentials and therefore has a larger positive deflection - The right ventricle has a smaller electrical charge due to fewer action potentials being generated - The left side is bigger, therefore mean movement is towards the left -> towards the electrode = positive wave

Question 31

What does the S wave show?

Answer 31

- Second negative deflection - Ventricular depolarisation - Action potentials begin moving superiorly towards the base of the heart - It moves away from the lead = negative deflection

Question 32

What does the QRS complex show?

Answer 32

- Depolarisation of the ventricles - Appears as 3 closely related waves on the ECG (Q,R and S waves)

Question 33

What does the QT interval show?

Answer 33

- Total time from ventricular depolarisation to complete repolarisation - This begins at the start of the Q wave and extends to the ends of the T wave - If there's no Q wave, then the starting point would be the beginning of the R wave

Question 34

What does the ST interval show?

Answer 34

- Isoelectric (flat) segment between the end of QRS and start of T wave - Ventricular repolarisation - Depression in flat line can indicate ischaemia - Elevation in flat line can indicate myocardial infarction

Question 35

What does the T wave show?

Answer 35

- Ventricular repolarisation - Negative charge travels upwards due to repolarisation - The flow of charge is negative - negative charge flow towards negative electrode = positive deflection

Question 36

How can an ECG show evidence of hypertrophy?

Answer 36

- More tissue = higher voltage - Will cause a taller R wave or deeper S wave

Question 37

Why would a P wave not be present on an ECG?

Answer 37

- SA node fails to initiate impulse resulting in no P waves/QRS complex