introduction to the ECG Flashcards
what is a syncytium?
one large cell having many nuclei that are not separated by cell membrane
what is a functional syncytium?
many cells functioning at once
what are the 3 types of cardiac myocytes?
- pacemaker cells= for setting the heart’s rhythm
- conducting cells= for transmitting rhythm throughout the heart
- contractile cells= for contracting to that rhythm (most numerous)
explain how the speed of propagation varies? 4
- contractile= atrial and ventricular monocytes- 0.3-0.5m/s
- conducting system (modified cardiomyocytes) purkinjie fibres up to 5m/s
- throughout the AV node 0.05m/s
how are cardiomyocytes linked? 2
- by low resistance pathways associated with gap junctions at the intercalated discs
- when one action potential depolarizes one cell–> initiating an action potential in adjacent cells
describe the spread of impulse through the atria? 7
- internodal bundles conduct the impulse from the SA node to the AV node
- bundles ensure synchronous contraction of the aorta
- conducting via atrial muscle would be slow (0.3-0.5m/s)
- conducting via bundles in the atria
- there are 4 specialised bundles in the atria
- these contain purkinhie-like cells which are cardiomyocytes modified to conduct
- bundles are in direct contact with the atrial muscle
explain the impulse at the AV node? 7
- AV node is the only point where the wave of depolarisation passes from the atria to the ventricles
- AV node delays the wave of excitation from atria to ventricles by 0.1-0.2 seconds
- the electrical delay means ventricles contract after the atria to permit longer and more effective ventricular filling
- to permit longer and more effective ventricular filling
- action potential conducted very slowly in the AV node (0.05m/s)
- AV node is composed of small, modified myocytes
- electrical connection between adjoining cells is weaker
explain ventricular propagation? 3
- AV node connects to the bundle of His followed by the purkinjie fibre system
- purkinjie fibres transmit the impulse rapidly to the main mass of the ventricles
- from there, slower conduction between contractile monocytes can occur
what are purkinjie fibres?
- very large monocytes- transmit the impulse faster- bigger diameter cells conduct faster
what order does the heart depolarise in? 3
- septum
- spex
- atrioventricular groove
what is an ECG? 5
- gross electrical measurement of the heart
- the electrical activity of the heart is measured on the skin
- the individual currents of the cardia monocytes are tiny
- these currents can be detected from the wrist and the ankle
- this is possible because the heart is a functional syncytium in which large groups of cells all make electrical charges simultaneously
what can the ECG diagnose? 4
- excellent for rate- Holter monitor (ECG) allows 24/7 rate determination- especially useful when the articular rate is different from the ventricular rate
- many subtleties- not a one stop diagnosis and patient history is essential
- diagnosis requires other techniques
- very fast and affordable
what is a lead?
2 or 3 different electrodes placed on the body in different positions
what is the set up of lead II?
- positive electrode on left leg
- negative electrode on the right arm
- ground electrode on the right leg (although it could be almost anywhere)
describe the 12 standard leads? 3
- 3 bipolar leads- I,II,III detect what happens in the frontal plane
- 3- ‘augmented’ leads
- 6 precordial leads
how is switching between leads shown on an ECG?
vertical lines
what does the P wave show?
depolarisation of the atria in response to SA node triggering
what does the PR segment show?
delay of AV node to allow filling of the ventricles
what does the QRS complex show?
depolarisation of ventricles, triggers main pumping contractions
what does the ST segment show?
beginning of ventricular repolarization, should be flat
what does the T wave show?
ventricular repolarisation
describe the problems with the QRS complex? 2
- if the ORS is wide or misshapen, then ventricular conduction is abnormal
- large (deep) Q waves are a sign of dead tissue (only MI)
what is the sinus rhythm? 4
- when the heart rhythm is generated from the sinoatrial node
- each P wave is followed by a QRS complex
- every ORS complex is preceded with a P wave
- when PR interval is always normal
what is sinus tachycardia?
- tachycardia driven by the SA node beating too quickly- it has normal PR intervals
what is the normal PR interval and its duration?
start of the P wave to the start of the QRS complex
3-5 little boxes (120-200ms)
what is the normal QT interval and its duration?
from the start of the QRS complex to the end of the T wave
9-11.5 boxes (360-460ms)
what is the ST segment on the graph?
from the end of the QRS complex to the start of the T wave
what is the normal QRS complex duration?
2-3 boxes (80-120ms)
how long do these boxes count for on an ECG:
- little box
- big box
- 5 big boxes
- 40ms
- 200ms
- 1 second
how do you calculate ventricular rate on an ECG?
count between the R waves
how to calculate heart rate on an ECG?
count between P waves
how do you work out rate on an ECG?
300/big boxes
what does the parasympathetic input of the heart lead to? 5
- vagus nerve leads to
- muscarinic stimulation
- decreased heart rate
- decreased contractility
- decreased conduction velocity
what does the sympathetic input of the heart lead to? 3
- increase in heart rate
- increase in contractility
- increase in conduction velocity
what happens during parasympathetic withdrawal? 3
what can cause it?
- increased heart rate
- increased contractility
- increased conduction velocity
- atropine–> muscarinic antagonist
what is a heart block? 2
- a type of dysrhythmia (bad rhythm)
- any kind of impulse conduction that blocks the heart
what is an AV heart block?
- a delay or failure of atrial signal stimulating ventricle
what can be the cause of a AV heart block? 3
- ischaemia of AV node or AV bundle
- compression of AV bundle by scar of calcified tissue
- inflammation of the AV node or bundle
what are the symptoms of a heart block? 4
- can be asymptomatic
- palpitations
- hypotension like symptoms: dizziness, syncope
- risk of sudden death
what is a first degree heart block? 6
- when the PR interval is bigger than 5 little boxes
- all P waves are followed by QRS
- almost always asymptomatic
- often young people
- delayed AV node transmission
- rarely treated
what is mobitz type 1? 6
-second degree heart block
-some P waves are blocked are blocked and are not followed by QRS (some ORS are missing)
-wenckebach
-PR interval gets longer until QRS wave fails to follow the P wave
-likely cause is AV node damage
usually no treatment given
what is mobitz type 2? 8
- second degree heart block
- hay
- some P waves are blocked and are not followed by QRS
- PR interval remains the same
- likely a problem in the bundle of His
- high risk
- can progress to a 3rd degree heart block
- implant pacemaker
what is a third degree heart block? 6
- atrial signals consistantly fall to arrive at ventricles
- ventricular rate is consistent(30-40 bpm)
- time between atrial beats and ventricular beats is variable
- PR interval varies radically- sometimes more then 12 boxes
- intrinsic ventricular rate is low
- atrial beats are consistent
what are premature beats?
early and triggered by irritable tissue
what are escape beats? 2
- are late and triggered by natural rhythmicity of non-atrial tissue
- occur when the atrial signal is very delayed or prevented (in third degree heart block)
what is premature ventricular control? 4
- unusually wide and weird looking ventricular activity
- no S wave, instead a wide negative dip where the T wave should be
- often beat triggered in middle of myocardium- the two ventricles are electrically unsynchronised
- width is determined by slowed conduction velocity
what is atrial fibrillation? 6
- disorganised electrical activity in atria
- no P wave, instead a flat and wiggly line
- ventricular rate is fat and irregular: many signals reach the AV node
- AF is very common in the elderly
- can lead to thrombus formation in atrium due to slow flow of blood, this leads to a stroke risk
- anticoagulants can try and prevent thrombus
what is respiratory sinus arrhythmia? 5
- heartbeat is slightly faster during inspiration, slightly slower during expiration
- normal
- usually only present in children and athletes
- caused by respiratory centres in the brains medulla
- observe ventricular rate: inverse of RR interval
what does ST segment elevation show? 2
- acute sign of MI
- isoelectric baseline: from end of T to the next P
