Flashcards in Electrocardiogram Deck (41):
What do surface electrodes see for different changes in polarisation?
Surface electrodes will see a positive deflection for a depolarisation in their direction and a negative deflection for a repolarisation in their direction.
When does repolarisation take place and in what direction?
After depolarisation (about 280ms) cells begin to repolarise and obviously takes place in the opposite direction to that of depolarisation. The Epicardial surface repolarises first and the endocardium last.
How does the heart achieve a very controlled precise order of depolarisation?
Cardiac myocytes are connected by gap junctions and a system to oversee this so that the correct part of the heart depolarises at the correct time and in the right order. It also allows a pause between the atria and ventricles contracting and allows more rapid spread of depolarisation from the initial point. Sa node, internodal tracts, AV nodes, bundle of his, left and right bundle branch and then purkinje fibres.
Describe the 4 limb leads in an ECG
There are 4 other electrodes other than the C1-C6 electrodes called your limb leads: AVR, AVL and AVF plus an earthing lead. These are coloured red, yellow, green and black and can be remembered in the order Ride Your Green Bike. Lead one is red and is placed on the right upper limb and the yellow is placed in the same area but on the left hand of the body, the green lead is placed on the left ankle and finally the black lead is placed on the right ankle as an earther.
Describe the 6 thoracic electrodes and where they are placed.
V1 - 4th intercostal space (R)
V2 - 4th intercostal space (L)
V3 - in between V2 and V3
V4 - 5th intercostal space mid clavicular line
V5 - parallel to V4 anterior axillary line
V6 - parallel to V4 mid axillary line
What planes do the different leads measure in?
The limb leads measure in the vertical plane and the thoracic leads in the horizontal plane.
How do 10 electrodes create 12 leads
Although only 10 electrodes this creates 12 leads because the combination of the 3 limb leads combine to create the bipolar leads called I, II and III. Then the limb leads use other electrodes as their negative electrode in a unipolar lead called the AVR (augmented vector right), AVL (augmented vector left) and AVF (augmented vector foot) leads.
What is the difference between unipolar and bipolar leads?
Unipolar leads - Read from the labelled positive electrode and utilise other electrodes as the negative
Bipolar - uses on positive and one negative electrode from the limb leads
Describe the orientation that the augmented and bipolar leads read in Where the head = -90degrees, the feet 90 degrees left = 0 degrees and right = 180 degrees.
I = 0 degrees
II = 60 degrees
III = 120 degrees
aVF = 90 degrees
aVL = -30 degrees
aVR = -150 degrees
If the change in polarisation is parallel to the electrode what will the lead read?
If a change in polarisation is exactly parallel to a lead then this will cause no deflection of the baseline of the ECG.
If the change in polarisation is at an angle to the electrode what will the lead read?
The deflection will be slightly less than if it were straight on.
Other than direction what else changes the height of the deflection of the ECG trace?
The amplitude also depends upon how much muscles is contracting.
Describe what each of the letters in the PQRST wave mean
P represents atrial polarisation which is small due to there being little atrial muscle, (there is then an isoelectric segment as the depolarisation is conducted from AV node to bundle of his), Q represents septal depolarisation spreading to ventricle this happens faster on the left side as it has more muscle and so the depolarisation travels towards the right side of the heart. It is only a small negative deflection because it isn’t moving directly away, R is the main ventricular depolarisation which heads straight towards lead 2, S is the end ventricular depolarisation which heads away from lead 2, T is ventricular repolarisation heading from apex upwards and so causes positive deflection.
What might interfere with an ECG trace?
Lead misplacement, muscle contraction i.e. movement, shivering, talking and coughing, Interference such as alternating currents and poor electrode contact due to sweat, cable pull and hair.
How would us assess the regularity of a rhythm using and ECG trace?
Assess whether the rhythm is regular, irregularly irregular or regularly irregular.
How do you measure the heart rate if it is regular
All ECG machines run at 25mm per second and so one big box length represents 0.2 seconds. If the rhythm is regular you can calculate the heart rate by dividing 300 by the number of small boxes between each R peak.
How do you measure the heart rate if it is irregular
All ECG machines run at 25mm per second and so one big box length represents 0.2 seconds.If the rhythm is irregular, you can count the number of R peaks in a 30 big box length i.e. 6 seconds worth and then multiply by 10.
What is a normal PR interval
PR interval – 0.12-0.2 seconds (3-5 small boxes)
What is a normal QRS interval
QRS interval – less than 0.12 seconds (less than 3 small boxes)
What is a normal QT interval
Must be corrected with heart rate and charts should be available for this but the correct interval should be no more than 0.45 seconds in adult males or 0.47 seconds in adult females.
What is Sinus Rhythm
This is the normal rhythm a healthy heart should show on an ECG. The rate should be between 60-100bpm, the P wave should be upright in leads 1 and 2, normal PR and QRS interval and every P wave is followed by a QRS and every QRS is preceded by a P wave.
Describe Atrial Fibrillation
Atrial Fibrillation – there are multiple P waves i.e. multiple atrial depolarisations which are all very chaotic. This is randomly conducted to the ventricles causing an irregularly irregular pulse. As a result of this atrial contraction is abnormal and the atria quivers instead. P wave may appear completely absent in this as well.
Describe first egree heart Blocks
First Degree Heart Block – P wave is normal but there is a slow conduction to the AV node due to ischaemia or degenerative change. QRS is normal and pulse is regular.
Describe type 1 second degree Heart Blocks
Second degree heart block Type 1 (Mobitz type 1, Wenkebach Phenomenon) – This is where there is a progressive lengthening of the PR interval which until one P wave is not conducted at all and there is a missing QRS complex, this happens to allow AVN to recover.
Describe type 2 second degree Heart Blocks
Second degree heart block Type 2 – PR interval normal but there is a sudden lack of conduction of a beat and so the QRS complex is missing randomly. This poses a high risk of progression towards complete heart block and so a pace maker must be inserted.
Describe a complete heart block
Complete Heart Block – P waves appear at regular interval but are not associated with a QRS complex as the AV node is completely blocked. Instead the QRS is stimulated by the ventricular tissue itself which is very slow (30-40 bpm). This usually results in a very wide QRS complex but the heart rate is too slow to maintain BP and perfusion. A pacemaker is required immediately.
What do you call the process by which the SAN normally overrides all other electrical pacemakers
What are Ectopic Beats
Ectopic beats (premature contractions) occur due to ectopic foci which are abnormal pacemaker sites within the heart tissue causing extra random stimulations during the heart contraction. Normally these are suppressed by the higher rate of the SA node
What is special about ventricular ectopic beats?
Ventricular ectopic beats – These appear as very large QRS waves amongst the normal QRS waves due to the depolarisation being conducted through muscle tissue rather than the fast purkinje fibres. It will also be different in shape.
What is Ventricular Tachycardia
If there are more than 3 consecutive Ectopic ventricular beats.
What is ventricular fibrillation
Ventricular Fibrillation occurs due to multiple ectopic arising from many different areas of the ventricles as a result there is no co-ordinated contraction and so your heart goes into cardiac arrest.
What are pathological Q waves
Pathalogial Q waves last more than 0.04 seconds i.e. one small box and are greater than 2mm deep. They tend to be present in full thickness MI’s and the Q wave remains after other changes resolve.
What is the axis and where does the axis normally lie
The axis is the general direction in which depolarisation spreads across the heart. The axis is usually from -30 to +90 degrees.
What is left and right axis deviation?
Right axial deviation is greater than 90 degrees and left axial deviation is therefore less than -30.
Describe what you would see in left axis deviation and why you see it.
Left axial deviation is generally asociated with left ventricular hypertrphy as conduction blocks are present in the anterior part of the left bundle of his. This can be seen in an ECG as the QRS in lead 2 will be inverted and in lead 1 will be upright.
Leaving = left ventricular hypertrophy
Describe what you would see in left axis deviation and why you see it.
right axis deviation is associated with right ventricular hypertrophy. In an ECG this presents with the QRS in lead 1 being predominantly facing downwards and the QRS in lead 2 being upright.
Reaching = right ventricular hypertrophy
What is Long QT syndrome
This is when the QT interval is longer than it should be. Note this value changes with gender and age.
What causes long QT sysdrome
Problem with repolarisation of the ventricles caused by: drugs, excessive vomiting/diarrhoea, anorexia, thyroid dysfunction and hypomagnesia. Can also be genetic such as mutated ion channels or HCM
What is hypertrophic cardiomyopathy
A genetic disease resulting in thickened and therefore stiff heart muscle
What are the symptoms of long QT syndrome
Syncope, palpitations, seizures, torsades de pointes and T wave variation between beats.