CVS: Introduction to ECG Flashcards
1
Q
What is an electrocardiogram?
A
The electro/chemical changes that cause myocardial contraction are shown by the cardiac action potential
2
Q
How do cardiac cells contract?
A
- 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
3
Q
Describe the cardiac electrical field
A
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)
4
Q
Where are the leads placed on the frontal plane?
A
6 limb leads
- Right arm - Left arm - Right leg - Left leg
5
Q
What are the 6 limb leads in the frontal plane and state whether they are bipolar or unipolar
A
- Lead I - Bipolar
- Lead II - Bipolar
- Lead III - Bipolar
- aVR - Unipolar
- aVL - Unipolar
- aVF - Unipolar
6
Q
What does it mean if a lead is bipolar or unipolar?
A
- Unipolar leads - register activity in the heart which is directed towards, or located below the electrode
- Bipolar leads- register the voltage between 2 electrodes
7
Q
How are the aVR, aVL and aVF leads placed?
A
Placed to create an equilateral triangle with the heart in the middle (Einthoven triangle)
8
Q
What do the six frontal plane leads or limb leads consist of?
A
- Three bipolar leads (I,II and III) - they are designated as such as each records the difference in electrical potential between two limbs (as leads are bipolar this is why there’s 6 leads due to 3 x 2 being 6)
- Lead I = right arm → left arm
- Lead II = right arm → left leg
- Lead III = left arm → left leg
- Three unipolar leads aVR, aVL, aVF (derived leads: I, II, III)
9
Q
What is the lead aVR?
A
- 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 interventricular septum
- Lead II travels from aVR towards aVF to become a 2nd inferior lead
- Placed on right wrist or right shoulder
10
Q
What is the lead aVL?
A
Unipolar lead placed on left wrist or shoulder, looks at the upper left side of the heart
Lead I travels towards aVL creating a second high lateral lead
11
Q
What is the lead aVF?
A
- Placed on the left ankle or left lower abdomen, and looks at the bottom, or inferior wall of the heart
- Lead ll travels from aVR towards aVF to become a 2nd inferior lead
- Lead III travels from the aVL towards aVF to become a 3rd inferior lead
12
Q
What are the 6 chest leads and where are they placed?
A
- 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
13
Q
What is R wave progression?
A
From V1 to V6, the pattern is that of a change from the S wave being prominent to the R wave being prominent
14
Q
What leads convey activity of the inferior surface of the heart?
A
- II
- III
- aVF
15
Q
What leads convey activity of the septal area of the heart?
A
- V1
- V2
16
Q
What leads convey activity of the anterior surface of the heart?
A
- V3
- V4
17
Q
What leads convey activity of the lateral surface of the heart?
A
- V5
- V6
- I
- aVL
18
Q
Describe Sinus rhythm
A
- Initiated by SAN, atria contract
- Depolarisation spreads to the 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 at the apex up towards the base of the heart
19
Q
Describe how sinus rhythm is seen on an ECG
A
- One P wave before each QRS
- Normal PR interval (less than 0.2 seconds)
- Normal QRS duration
20
Q
What is the Bachmann’s bundle?
A
- Often referred to as the interatrial bundle
- It’s a muscular bundle composed of parallel muscle strands connecting the left and right atria
- Serves as the primary electrical connection between the right and left atria
- This is what allows the wave of depolarisation to spread through both atria
21
Q
What does a normal ECG look like?
A
- Normal ECG contains waves, intervals segments and one complex:
- 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 RR 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
22
Q
How is rate calculated from a 12-lead ECG?
A
- When cardiac rhythm is regular, heart rate can be determined by the interval between two 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
23
Q
What is tachycardia?
A
- Increased heart rate
- Heart rate >100bpm
24
Q
What is bradycardia?
A
- Decreased heart rate
- Heart rate <50bpm
25
How is rhythm calculated from a 12-lead ECG?
- Rhythm refers to the pattern of electrical impulses that causes the heartbeat
- Normal cardiac rhythm is a sinus rhythm that follows a predictable pattern
- When cardiac rhythm is regular, heart rate can be determined by the interval between two 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
26
How is the axis calculated from a 12-lead ECG?
Axis is the average direction of electrical movement through the heart during a depolarisation
- 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
27
What does the P wave show?
- Represent 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)
28
What does the PR interval show?
- 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.
29
What does the Q wave show?
The first negative deflection (current flowing away from the lead)
- Septal depolarisation:
- When depolarisation is being distributed at the bundle of His, it’s distributed between the left and right branches
- It is distributed to the left ventricle first, and the movement moves from left to right
- The positive charge moves away from the positive electrode
30
What does the R wave show?
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 muscle 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
31
What does the S wave show?
Second negative deflection
- Action potentials begin moving superiorly towards the base of the heart
- This is ventricular depolarisation
- It moves away from the lead = negative deflection
- This is the S wave
32
What does the QRS complex show?
- Represents depolarisation of the ventricles
- Appears as three closely related waves on the ECG (Q, R and S waves)
33
What does the QT interval show?
- The total time from ventricular depolarisation to complete repolarisation
- This begins at the start of the Q wave and extends to the end of the T wave
- If there's no Q wave, then the starting point would be the beginning of the R wave.
34
What does the ST interval show?
- 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
35
What does the T wave show?
Ventricular repolarisation
- Negative charge travels upwards due to repolarisation
- The flow of charge is negative - negative charge flow towards negative electrode = positive deflection
36
How can an ECG show evidence of hypertrophy?
- More tissue = higher voltage
- Will cause a taller R wave or deeper S wave
37
Why would a P wave not be present on an ECG?
SA node fails to initiate impulse resulting in no P wave/ QRS complex
