Lecture 12 Flashcards
What is deflection?
deviation from a straight line (in ECG upward/downward peak/wave from baseline/isoelectric line
+ve deflection is upright
How does an ECG work?
Electrical changes (depolarisation/repolarisation) recorded via electrodes placed on limbs and chest wall and transcribed onto graph paper
How many views does an ECG provide electrical activity from?
12 separate views
Therefore if there is an abnormality in activity we can localise it to a region of the heart
What is meant by the heart being an intrinsic rhythm?
If supported by ions and energy, can beat outside our body.
What are pacemaker cells?
Specialised cardiac myocytes which spontaneously generate AP’s which initiate the cardiac cycle.
(primary cardiac pacemaker- in SAN in right atrium)
How do action potentials spread?
-Depolarisation wave
-AP spreads via gap junctions
Causing a coordinated contraction of atria and ventricles
EXCITATION-CONTRACTION COUPLING
How is propagation of an action potential generated?
Ions move into the cell, allowing it to reach TV, this then causes more positive ion channels to open which are shared with the cell next door which then helps to generate the TV there.
How does electrical activity spread throughout the heart?
- initiated at SAN (top of RA)
- causing depolarisation of right/left atria
- hits AVN located in interatrial septum near tricupsid valve
- THE AP SLOWS DOWN WHEN IT HITS THE AVN
- AVN to Bundle of His through annulus fibrosis
- Bundle of His then enters the interventricular septum where it divides into the left/right bundle branches
- these branches then terminate in extensive network of fibres called purkinje fibres
What is the annulus fibrosis & its function?
4 Fibrous rings that separates the atria from the ventricles
- anchors myocardium & cardiac valves
- electrical insulator between the atria and ventricles
Role of the purkinje fibres?
Conduct depolarising wave to ventricles
What do the left/right bundle branches do?
Right: travels along right side of the inter-ventricular septum to excite the right ventricle
Left: travels along left side of the inter-ventricular septum to excite the left ventricle
What is the function of the SAN?
- sets HR and sinus rhythm
- fastest rate of depolaristaion in heart
- 60-100 times per min
What is the function of AVN?
- slows conduction
- giving time for stria to contract before ventricles
- firing rate 40-60 times per min without stimulation from the SAN
Can the ventricles cause AP firing?
Yes, they have an electrical conducting system but it is not usually manifested
-firing rate 20-40 times per min
What does an ECG measure?
Changes in electrical potential mVolts produced in successive areas of the myocardium
What does an ECG lead mean?
- cable
- electrical view of the heart obtained from the electrodes
- records electrical activity transmitted to chest wall/limb
How many electrodes are used to record an ECG & how many leads are there?
10 electrodes (6 on chest, 4 on limbs)
(connected by 10 cables to the machine)
-12 leads
What is the function of the right leg electrode?
Grounding electrode (not used for any leads)
Where are the limb leads attached and how many are there?
3 limb leads which are BIPOLAR 1: voltage difference between RA and LA (LA is positive electrode) 2: voltage difference between RA and LL (LL is positive electrode) 3: voltage difference between LA and LL (LL being the positive electrode)
What is a bipolar lead?
They have negative and positive electrodes
Which direction do leads go?
Positive to negative to look at
but the current goes from negative to positive
What are the augmented limb leads?
aVR
aVL
aVF
These are all unipolar- only have a positive electrode
The other electrode is the average of the 2 remaining ones, acting as neutral/reference
What is the aVR?
Positive electrode on the right arm
other two electrodes on the LA and LL
What is the aVL?
Positive electrode on the left arm
other two electrode on the RA and LL
What is the aVF? (f for foot)
Positive electrode on left leg
other two electrodes on the RA and LA
How are augmented unipolar leads different from standard limb leads?
The connections between the electrodes
the electrodes used are the same
Why is the positive electrode important?
The cardiac view from the lead is from the perspective from the +ve electrode
When does the ECG trace have a positive deflection?
If electrical current is travelling to the +ve electrode of the lead.
OR
If repolarisation is occuring away from the positive electrode.
What effects the amplitude of deflection?
- directly towards/away = large signal
- obliquely towards/away = smaller signal
- at right angles = no signal
What are the precordial chest leads?
UNIPOLAR positive eletrodes placed around the chest
V1/2/3/4/5/6
(unipolar so the other electrode is the average of the 3 limb electrodes which is averaged to the centre of the chest)
Where is the ground lead of the chest leads?
In the centre of the chest.
Measures electrical activity in the front/back direction or the right/left direction
What gives a downward/negative displacement?
Depolarisation going directly away from the positive electrode.
What do we see on an ECG during SAN depolarisation?
Nothing, insufficient signal to register on ECG
What does the P wave symbolise? (lead 2)
Small upward deflection
(upward because it is towards the positive electrode)
-atrial depolarisation
What is the isoelectric segment after the p wave?
-Delay at the AVN.
(conduction slowed down here to allow the ventricles to contract separately from the atria)
-fibrous ring only crossed by bundle of his, so depolarisation can only reach ventricle via this: it contributes to part of the isoelectric segment
What is the Q wave?
Downward deflection (due to oblique movement) Depolarisation of interventricular septum -muscle depolarises from left to right in the interventricular septum
What is the R wave?
Large upward deflection due to depolarisation
- upwards as the depolarisation is moving directly towards electrode
- large due to large muscle mass (if left ventricle is hypertrophied then the R wave will be correspondingly taller)
What is the S wave?
Depolarisation spreads upwards from apex of ventricles producing a downward deflection.
- down because moving away
- small because not moving directly away
What is the T wave?
Ventricular repolarisation (opposite way to depolarisation) -upwards as repolarisation is away from electrode
What do the waves not indicate?
Contraction/relaxation of ventricles/atria
This immediately follows the waves
What are leads 1 and aVL good for viewing?
Left side of heart (lateral wall of left ventricle) (muscle necrosis due to occlusion of left coronary artery: lateral wall MI)
Which limb leads are good for viewing the inferior surfaec of the heart?
Leads 2 & 3, AVF
muscle necrosis due to occlusion of right coronary artery:inferior myocardial wall infarction
What leads look at the front/septum region of the heart?
V1 & V2
Face right ventricle and septum
Which precordial leads look at the apex and anterior walls of RV/LV?
V3 & V4
Which leads are the antero-septal leads?
V1-4
Which precordial leads face the LV?
V5 & V6
What leads detect anterior cardiac wall necrosis?
V3 & V4
Due to occlusion of left anterior descending artery (branch of the left coronary artery-widow maker, as carries around 50% of blood in coronary ciculation)
At what rate does ECG paper move?
25 mm/sec (1mm is one tiny square)
1mm therefore represents 0.04 seconds
What are the axis in and ECG tracing?
Verticle axis: voltage in mV
Horizantal axis: time in secs
How many large squares is one second?
5
How many large squares is 1 min?
300
How do you calculate the HR when rhythm is regular?
Start of R wave to end of R wave is 1 beat/cycle
(each beat requires a full cycle)
Then scale up from time interval to 1 min
How do you calculate heart rate if rhythm is irregular?
Count number of QRS complexes in 6 seconds
- multiply by 10 to get total number of heart beats in 60 seconds
When do you not count a cycle?
When there is no QRS complex
What is the normal PR interval?
0.12-0.2 (3-5 tiny squares, if more than 1 box it is prolonged)
What does a prolonged PR interval mean?
Delayed conduction through AVN and Bundle of His
What is the normal length of a QRS interval?
<0.12 secs (< 3 tiny boxes)
What happens if there is a widened QRS complex?
Depolarisation not spreading via rapid conducting His-Purkinje system. Depolarisation arising in ventricle.
How long should the QT interval be?
= 0.44-0.45 secs (11 tiny boxes)
What is the QT interval?
Time taken for depolarisation and repolarisation of ventricle
(varies with HR)
What is wrong with a prolonged QT interval?
Indicates prolonged ventricular repolarisation
-associated risk for dangerous arrythmias
Which way do precordial leads measure electrical activity?
Front to back
Right-Left
(unlike limb leads they don’t measure up-down plane)
