Session 6 & 7 Flashcards
How many limb leads are there?
6
How many electrodes/wires record the six limb leads?
4
_________ electrodes and wires that you need to connect to record from all _______ limb leads
Four
Six
Name the position/colour of the 4 limb electrodes and a mnemonic
Ride Your Green Bike
Starting with the right arm going clockwise
Red - Right Arm
Yellow - Left Arm
Green - Left Foot
Black - Right Foot
Which limb electrode acts as ‘neutral’?
Right leg
What is meant by a bipolar electrode?
Can be positive or negative depending on configuration
At which angles do leads I, II and III view the heart?
0 degrees (pointing horizontally towards the right)
60 degrees
120 degrees
How are the augmented limb leads named?
Right arm - aVR
Left arm - aVL
Left foot - aVF
In the augmented limb leads, are the limb electrodes positive or negative?
Positive
How are the augmented limb leads measured?
With reference to a negative terminal coming from the centre of the heart
What direction does aVR, aVL and aVF view the heart?
-150
-30
90
Which limb leads look at the left of the heart?
aVL
Lead I
Which limb leads look at the apex of the heart?
Lead II
Which limb leads look at the inferior heart?
Lead II
aVF
Lead III
Which lead looks at the right of the heart?
aVR
Which two limb leads view the heart almost exactly oppositely? What is the significance of this?
aVR
Lead II
Inverted images should be seen on the ECG
Describe the position of the six chest leads (C1-C6)
C1 - 4th intercostal space, right sternal border
C2 - 4th intercostal space, left sternal border
C3 - midway between C2 and C4
C4 - 5th intercostal space, midclavicular line
C5 - between C4 and C6
C6 - 5th intercostal space, midaxillary line
Before recording an ECG, ____________ must be taken from the patient
What happens to the skin before sticky electrodes are attached in an ECG?
Consent
Wiped clean with alcohol wipes
The conducting system of the heart consists of specialised cells that have lost _____________ but have the ability to generate _________ _____________
Contractility
Action potentials
In what way do the ventricles depolarise?
From endocardium to epicardium
Where is the SA node found?
Near the junction of the superior vena cava and right atrium
What is special about the depolarisation that takes place in cells of the SA node?
Depolarisation happens the fastest here
Which part of the heart sets the sinus rhythm of the heart? Why this part of the heart?
SA node
Cells in the SA node depolarise the fastest
The AV node is continuos with the…
Bundle of His
Where is the AV node located?
At the interatrial septum just above the tricuspid valve
What is the only conducting pathway from atria to ventricles?
The AV node
What structure of the heart prevents direct contact between and conduction impulses from atrial to ventricular myocytes?
So, how do impulses pass from atria to ventricles?
Fibrous ring in the heart
Through the AV node
Where are the right and left bundle branches located?
Sub-endocardially in the IV septum
Purkinje fibres are fine branches of the…
Purkinje fibres cause the rapid spread of…
Bundle of His
Depolarisation throughout the ventricular myocardium
Describe how the ventricles are depolarised and repolarised
IV septum depolarised first
Apex, RV and LF free walls depolarised next
Base of the ventricles depolarised last
Repolarisation of ventricles in the reverse order
What does an ECG record?
Changes on the extracellular surface of cardiac myocytes during depolarisation and repolarisation
A depolarisation wave from -ve to +ve will result in which complex being seen on an ECG?
What will be seen on repolarisation?
Positive complex
Negative complex
A depolarisation wave from +ve to -ve will result in which complex being seen on an ECG?
What will be seen on repolarisation?
Negative complex
Positive complex
What ECG complex will be seen if a depolarisation wave travels directly towards a positive electrode?
Obliquely towards?
Tall +ve complex
Smaller +ve complex
What ECG complex will be seen if a depolarisation wave travels at 90 degrees to a positive electrode?
No complex seen
What ECG complex will be seen if a wave of depolarisation travels obliquely away from a positive electrode?
Directly away?
Small -ve complex
Deep -ve complex
How does SA node depolarisation affect an ECG reading?
No effect, insufficient signal to register on surface ECG
How does atrial depolarisation affect an ECG reading? (Viewing from the apex)
Causes a small upward deflection of the P wave
What is the cause in the heart of a p wave on an ECG?
Atrial depolarisation
How does the delay at the AV node affect an ECG reading?
No visible effect - signal very small and isoelectric flat line segment
What is the name given to the segment of the ECG that shows delay at the AV node?
Isoelectric flat line segment
The non-conducting fibrous ring found in the heart is only crossed by the…
Bundle of His
As well as AV delay what else contributes to the isoelectric flat line segment of the ECG?
The spread of depolarisation from atrium to ventricle via the His Purkinje system
What is the cause in the heart of the Q wave of an ECG?
Depolarisation of the myocardium - IV septum from left to right
In which way does the IV septum depolarise?
From left to right
What effect does depolarisation of the myocardium (IV septum) have on an ECG?
A small downward deflection - Q WAVE
What change in the heart causes the R wave of an ECG?
Depolarisation of the apex and free ventricular wall
What effect does depolarisation of the apex and free ventricular wall have on the ECG?
Produces a large upward deflection called the R wave
Why is the R wave, large and upwards on an ECG?
Depolarisation moving directly towards an electrode
How will LV hypertrophy affect the R wave of an ECG?
Will result in a taller R wave
Which change in the heart causes the S wave of an ECG?
The end of depolarisation, as depolarisation spreads upwards to the base of the ventricles
What effect does the end of depolarisation (depolarisation upwards to the base of the ventricles) have on an ECG?
Results in a small downward deflection - S WAVE
What change in the heart causes the T wave of an ECG?
Ventricular repolarisation
What effect does ventricular repolarisation have on an ECG?
Produces a medium upward deflection - T WAVE
Why is the T wave seen as an upwards deflection?
It is as a result of repolarisation and is moving AWAY from the electrode
Where does ventricular repolarisation begin? In which direction does ventricular repolarisation occur?
Epicardial surface
Opposite direction to depolarisation
P wave of an ECG is due to __________ _______________
QRS complex of and ECG is due to _____________ _______________
T wave of an ECG is due to _____________ _______________
Atrial depolarisation
Ventricular depolarisation
Ventricular repolarisation
Are all the components of the QRS complex seen in all leads when an ECG is carried out?
Not always
How many electrodes are used in the recording an ECG? How many chest? How many limb?
10
6
4
10 electrodes used in the recording of an ECG give how many leads of the heart?
Leads of the heart are effectively different _________ of the heart
12 leads
Views
The limb leads give __ views of the heart from __ electrodes in the ______________ plane
6
4
Vertical
Name three limb leads that are best for looking at the inferior surface of the heart
II
III
aVF
Name two limb leads that are useful for looking at the left side of the heart
I
aV
Chest leads give __ views of the heart from __ chest electrodes in the ____________ plane
6
6
Horizontal
What part of the heart can be described as lateral? (ECGs)
LV = Lateral Side
Which chest leads give views of the right ventricle and septum?
What is another term for these leads?
V1
V2
Septal leads
Which chest leads give views of the apex and anterior wall of the ventricles?
What is another term for these leads?
V3
V4
Anterior leads
Which chest leads give views of the left ventricle?
What is another name for these leads?
V5
V6
Lateral leads
What are the horizontal and vertical axes of an ECG?
Horizontal = Time
Vertical = Voltage
What is the normal speed that an ECG runs at?
25 mm/sec
How many small squares make up 1 second on an ECG tracing?
How many large squares make up 1 second on an ECG tracing?
25 small squares
5 large squares
What is 1 cardiac cycle with regards to the different waves seen on an ECG?
Each PQRST complex (one P wave to the next)
How many large boxes is equal to 1 minute on an ECG trace?
300 large boxes
How is the heart rate measured from an ECG?
Number of large boxes between complexes (R waves) counted
300/Number of Large Boxes between complexes = HR
How is the heart rate measured from an ECG if the heartbeat is irregular?
Count the number of QRS complexes in 6 seconds (30 large squares)
Times by 10 = HR
How long should a normal PR interval last in…
I) seconds
II) boxes
0.12-0.20 seconds
3-5 small boxes
When will a PR interval be considered prolonged?
Greater than 1 large box
A normal QRS interval should be no more than how many…
I) seconds
II) small boxes
0.12 seconds
3 small boxes
The QT interval varies with…
Heart rate
What is normal sinus rhythm? Where does it originate?
Characteristic rhythm of a healthy heart
Sinoatrial node
Name 5 factors that are considered when looking for normal sinus rhythm
Heart Rate Rhythm P wave PR interval QRS
What values for heart rate is consistent with normal sinus rhythm?
60-100 bpm
What is looked for when assessing the P wave to see if someone is in normal sinus rhythm?
Whether it is before QRS complex and identical each time
What value will PR interval and QRS take in seconds in a person with normal sinus rhythm?
PR - 0.12-0.20 seconds
QRS - less than 1.2 seconds
What is sinus bradycardia?
Sinus rhythm where there is a heart rate less than 60bpm
What is sinus tachycardia?
Sinus rhythm with a rate of 100 bpm or more
Abnormal rhythms in the heart can result from either abnormal… (2)
Impulse formation
Conduction
What two types of rhythms are seen in the heart?
Supraventricular
Ventricular
Supraventricular rhythms consist of rhythms from which parts of the heart? (3)
SA node
Atrium
AV node
Ventricular rhythms consist of rhythms from which part of the heart?
Ventricles
How will abnormal supraventricular rhythms affect…
I) Ventricular depolarisation
II) Conduction
Normal ventricular depolarisation will take place
Conduction will be normal via the His-Purkinje system
How will abnormal supraventricular rhythms affect the QRS complex of an ECG?
Will be normal (narrow)
How do abnormal ventricular rhythms affect…
I) Depolarisation in the ventricles
II) Conduction
Depolarisation takes longer
Conduction not via normal His-Purkinje system
How do abnormal ventricular rhythms affect the ECG seen?
Wide and bizarre QRS complexes
A wide QRS complex is typically longer than ___ small boxes
3
Will abnormal ventricular rhythms always produce the same bizarre QRS complexes?
No, depends on the foci of the ventricular rhythm
Give two example of conditions that can result from abnormal ventricular rhythms
Ventricular tachycardia
Ventricular fibrillation
Ventricular premature beats
How is the rhythm of the heart interpreted from an ECG?
By looking at the rhythm strip at the bottom of the 12 lead ECG
What is the rhythm strip of an ECG?
A long (approx 10 seconds) recording of limb lead II
Describe the impulses/depolarisation seen in atrial fibrillation
There are multiple foci throughout the atria
Depolarisation and impulses chaotic
How does atrial fibrillation affect the p waves seen on an ECG?
No p waves will be seen instead just a wavy baseline
How does atrial fibrillation affect the contraction of the atria?
They quiver rather than contract
What happens to the rapid, irregular impulses that arrive at the AV node in atrial fibrillation?
Only some are conducted to ventricles when the AV node is not refractory
Why are only some of the impulses from the atria conducted to the ventricles at the AV node in atrial fibrillation?
Impulses arrive rapidly/irregularly
AV node is in refractory period
How does atrial fibrillation affect…
I) Ventricular depolarisation/contraction
II) QRS complex of the ECG
III) Pulse
IIII) Heart rate
Normal ventricular depolarisation and contraction
Normal shape (narrow) and irregular
Irregularly irregular
Irregularly irregular
What are AV conduction blocks?
Where there is delay/failure of conduction of impulses from atrium to ventricles via AV node and the bundle of His
What are two causes of AV conduction blocks? Which is more common?
Acute Myocardial Infarction (more common)
Degenerative Changes
What are the three types of AV conduction blocks?
First degree heart block
Second degree heart block
Third degree heart block (complete heart block)
What are the two types of second degree heart block?
Mobitz type 1
Mobitz type 2
What mechanism works in the heart to avoid pacemakers ‘competing’ with each other?
In the case of failure of the SA node the NEXT FASTEST to depolarise will take over (AV node)
Which part of the heart takes over from the SA node in case of failure?
In this case, ___________ rhythm is replaced by _______________ ___________ rhythm
AV node
Sinus
Ventricular escape rhythm
What happens in first degree heart block?
There is SLOW conduction in the AV node and bundle of His
How does first degree heart block affect the ECG?
I) P wave
II) PR interval
II) QRS
P wave normal
PR prolonged (>5 small squares)
QRS normal
If the PR interval is prolonged in an ECG, it is typically >__ small squares
5
As well as lead II being used in a rhythm strip, what other two leads are commonly used?
V1, V5
What happens in Mobitz type 1 second degree heart block?
There is progressive lengthening of the PR interval until one P is not CONDUCTED
This allows the AV node time to recover
Cycle begins again
How is the PR interval of an ECG affected in Mobitz type 1 second degree heart block?
Progressively lengthened
What happens in mobitz type 2 second degree heart block?
There is sudden non-conduction of a beat resulting in dropped QRS complex
Which type of heart block has a high risk of progression to complete heart block?
Mobitz type 2 second degree heart block
Describe the PR interval and QRS complex seen in mobitz type 2 second degree heart block
PR interval normal
QRS complex dropped
What happens in third degree heart block?
Atrial depolarisation is normal but impulses are not conducted to ventricle
Ventricular pacemaker takes over (ventricular escape rhythm)
How does third degree heart block affect…
I) atrial depolarisation
II) conduction to ventricles
III) pacemaker of the heart
Normal atrial depolarisation
Not conducted
Ventricular pacemaker takes over (ventricular escape rhythm)
What sort of heart rate is seen in third degree heart block?
Very slow - ~30-40 bpm
What are the implications of the very slow heart rate seen in third degree heart block? What is therefore required for treatment?
Heart rate is too slow to maintain BP and perfusion
Urgent pacemaker
How does third degree heart block affect the QRS complexes seen on an ECG?
Wide QRS complexes
Apart from causing a wide QRS complex, how are the P-P intervals and R-R intervals affected on an ECG as a result of third degree heart block?
Constant
Constant but much slower
Describe the relationship between the P waves and QRS complexes in third degree heart block?
There is no relationship between the P waves and QRS complexes
What causes ventricular ectopic beats?
An ectopic focus in ventricle muscle
How does an impulse that originates in the ventricle muscle spread (e.g. In ventricular ectopic beats)?
It is not spread rapidly via the His-Purkinje system follows abnormal conducting pathways
How do ventricular ectopic beats affect…
I) depolarisation of the ventricles
II) QRS complex
Slower depolarisation of the ventricles
Wider QRS complex with different shape
What is ventricular tachycardia?
A run of 3 or more consecutive ventricular ectopics
Is ventricular tachycardia dangerous? What can it lead to?
Persistent ventricular tachycardia is dangerous and requires urgent treatment
Ventricular fibrillation
How will ventricular tachycardia appear on an ECG?
With rapid, regular and broad beats
What is ventricular fibrillation?
Abnormal, chaotic and fast ventricular depolarisation
Describe the impulses seen in ventricular fibrillation
There are numerous impulses from numerous ectopic sites in ventricular muscle
Describe the contraction seen in ventricular fibrillation
No coordinated contraction of the ventricles - ventricles quiver
How can ventricular fibrillation affect cardiac output? What can this cause?
No cardiac output
Cardiac arrest
In case of cardiac arrest as result of ventricular fibrillation, what must be done to restore rhythm in the heart?
CPR and immediate defibrillation
Describe the pulse and heart rate seen in ventricular fibrillation
No pulse or heart beat
What causes cardiac arrest, atrial fibrillation or ventricular fibrillation?
Ventricular fibrillation
Ventricular fibrillation causes cardiac arrest. Name 4 other differences between atrial and ventricular fibrillation.
AF - ventricular depolarisation normal (chaotic in VF)
AF - normal ventricular contraction (no coordinated contraction in VF)
AF - irregularly irregular HB and pulse (no HB/pulse in VF)
AF - cardiac output present (no cardiac output in VF)
Ischaemia and therefore myocardial infarction is due to…
Reduced perfusion of the myocardium
Why can myocardial infarction cause changes in different ECG leads in different cases?
MI doesn’t affect all parts of the heart, changes will be seen in the leads facing the affected area
Why is it important to look at the PQRST complexes in all 12 leads when investigating an MI?
MI doesn’t affect all parts of the heart - changes will only be seen in the leads facing the affected area
What parts of the heart do the six chest leads look at?
V1, V2 - Septal Leads
V3, V4 - Anterior Leads
V5, V6 - Lateral Leads
What parts of the heart of do the limb leads (excluding aVR) look at?
I, aVL - Lateral Leads
II, III and AVF - Inferior Leads
Where do major coronary arteries lie in the heart?
On the epicardial surface of the heart
Which muscle in the heart is furthest away from major coronary arteries on the epicardial surface? What is the implication of this?
Subendocardial muscle
Most vulnerable to MI
What ECG changes can be seen in leads facing the affected area of an MI? (2)
ST segment depression
T wave inversion (-ve instead of +ve)
What causes the ECG changes (ST segment depression and T wave inversion) seen in MI?
Abnormal current during repolarisation
If ischaemia/MI is less severe, ischaemic ECG changes may only be seen as a result of…
Exertion - e.g. Exercise
When will ischaemic ECG changes be seen at rest in an MI?
When there is a severe reduction in the size of the lumen of the coronary artery
What causes STEMI to occur?
Complete occlusion of the lumen of a coronary artery
Describe the extent of the muscle injury seen in a STEMI
Muscle injury extends full thickness - endocardium to epicardium
Injury to the epicardium in a STEMI causes which change in the ECG?
ST elevation in leads facing the area
If perfusion is not re-established in a STEMI what will happen?
Muscle necrosis
What feature of an ECG can indicate that muscle necrosis has taken place as a result of a STEMI?
Developed Q waves that persist after the acute STEMI
Q waves are usually narrow and caused by…
Depolarisation of the septum
What appearance do pathological q waves take?
Wider/deeper than normal
> 1 small square wide
2 small squares deep
How does hyperkalaemia affect the resting membrane potential?
Less negative RMP
How does hypokalaemia affect the resting membrane potential?
More negative RMP
How does the excitability of the heart change as a result of hyperkalaemia?
Becomes less excitable
What effect does the less negative RMP seen in hyperkalaemia have on the cell during an action potential?
Inactivates voltage gated Na+ channels, heart becomes less excitable causing conduction problems
