Introduction to ECG Flashcards
Syncytium
One large cell having many nuclei that are not separated by cell membrane
Functional syncytium
Many cells functioning as one
Pacemaker cells
For setting heart’s rhythm
Conducting cells
For transmitting rhythm throughout the heart
Contractile cells
For contracting to that rhythm (most numerous)
Gap junction
Specialised intercellular connection between two cells with adjacent membranes
Intercalated discs
Undulating double membrane separating adjacent cells in cardiac muscle fibres
Support synchronised contraction of cardiac tissue
Spread of impulse through the atria
Internodal bundles conduct impulse from SA node to AV node
- bundles ensure synchronous contraction of the atria
- conducting via atrial muscle would be slow
- conducting via bundles is much faster
4 internodal bundles
Anterior, middle and posterior go to AV nodes
Bachmann’s goes to left atrium
Impulse at AV node
Wave of depolarisation passes to AV node
AV node delays wave of excitation to allow atria to contract and empty so ventricles can fill prior to contraction
Delay at AV node
Small diameter of AV nodal cells
Reduced number of gap junctions between any two cells
Smaller cells so more gap junctions must be traversed to travel the same distance longitudinally
Ventricular propagation
AV node connects to bundle of His followed by purkinje fibre system
Purkinje fibres transmit the impulse rapidly to the main mass of the ventricles
First part of ventricular wall to be depolarised is septum then apex then atrioventricular groove
Purkinje fibres
Very large myocytes
Transmit the impulse faster
Up to 5m/s
Bundle of His
Transmits impulses from AV node to the ventricles
ECG
Gross electrical measurement of the heart
Electrical activity of heart measured on the skin
Individual currents of cardiac myocytes are tiny
Currents detected from wrist and ankle
Lead
Configuration of electrodes (usually consisting of a positive electrode, negative electrode and sometimes a ground)
Standard 12 lead ECG looks at heart from 12 different angles
Lead II
Positive electrode on left leg
Negative electrode on right arm
Ground electrode on right leg
12 standard leads
3 bipolar leads
- I, II, III
- frontal plane
3 augmented leads
- aVR, aVL, aVF
- frontal plane
6 precordial
- on the thorax near the heart
- V1, V2, V3, V4, V5, V6
- transverse plane (spine to sternum)
Bipolar leads
Positive electrode is compared to a negative electrode
Precordial leads
Positive electrode is compared to an estimate of what is happening at the centre of the heart
Augmented lead
Positive electrode is compared to a composite reference electrode made of the two other limb electrodes connected
P-wave
Depolarisation of atria in response to SA node triggering
PR segment
Delay of AV node to allow filling of ventricles
QRS complex
Depolarisation of ventricles, triggers main pumping contractions
If wide or misshapen, ventricular conduction is abnormal
Large Q waves are sign of dead tissue
T-wave
Ventricular repolarisation
ST segment
Beginning of ventricle repolarisation, should be flat
Ectopic beat
Heart contraction that electrically begins somewhere other than the SA node
Sinus rhythm
Heart rhythm is generated from the SA node
Each P wave is followed by QRS complex
Each QRS complex preceded by P wave
PR interval is always normal (3-5 little boxes)
Sinus tachycardia
Tachycardia driven by SA node beating too quickly
Timing of ECG
PR interval- from start of P wave to start of QRS complex
QT interval- from start of QRS complex to end of T wave
ST segment- from end of QRS complex to start of T wave
Normal intervals
PR interval duration: 3-5 boxes, 120-200ms
QRS complex duration: 2-3 boxes, 80-120ms
QT interval duration: 9-11.5 boxes, 360-460ms
Calculating rate
1 little box is 40ms
1 big box is 200ms
4 big boxes is 1 second
Rate=300/big boxes
The heart: parasympathetic input
Vagus nerve
Muscarinic stim ->
Decrease HR, contractility and conduction velocity
Atropine
Vasculature not innervated by parasympathetic system
The heart: sympathetic input
Sympathetic stimulation ->
Increase HR, contractility, conduction velocity
Stellate nerves
Beta agonists
Beta blockers
Beta (1) adrenoreceptors
Causes inotrposim and chronotropism
Alpha adrenoreceptors
Cause vasoconstriction
Beta (2) adrenoreceptors
Cause vasolidation in skeletal muscle
Adrenaline
Leads to net decrease in total peripheral resistance via beta (2) receptors that vasodilate vessels of the muscle and liver
Noradrenaline
Leads to strong increase in total peripheral resistance via alpha (1) receptors that vasoconstrict most vessels of the body
Atropine
Anticholinergic drug that reduces parasympathetic activity
Heart block
Type of dysrhythmia
Any kind of impulse conduction block of the heart
Includes AV block, bundle branch block
AV heart block
A delay or failure of atrial signal stimulating ventricle
Causes: - ischeamia of AV node/ bundle - compression of AV bundle by scar or calcified tissue - inflammation of the AV node/ bundle
Heart block symptoms
Can be asymptomatic
Palpitations
Hypotension like: dizziness, malaise, syncope
Risk of sudden death
First degree heart block
When PR interval > 5 little boxes (200ms)
All P’s followed by QRS
Almost always asymptomatic
Often young people (adolescents)
Delayed AV node transmission
Rarely treated
Mobitz type I second degree heart block
Second degree- some P waves blocked and are not followed by QRS
Mobitz type I- PR interval gets longer until QRS wave fails to follow P wave
Mobitz type II second degree heart block
Second degree- some P waves blocked and are not followed QRS
Mobitz type II- PR interval remains the same, likely problem in bundle of his
Can progress to 3rd degree heart block
Third degree heart block
Atrial signals constantly fail to arrive at ventricles
Ventricular rate is consistent: 30-40bpm
Time between atrial and ventricular beats is variable
PR interval varies radically
Intrinsic ventricular rate is quite slow
Atrial beats are consistent
Atrial fibrillation
Disorganised electrical activity in atria
- no P wave
- flat line or wiggly line instead
Ventricular rate is fast and irregular
Very common in the elderly
Can lead to thrombus formation in the atrium
Respiratory sinus arrhythmia
Heart beat is slightly faster during inspiration, slightly slower during expiration
Usually only present in children and athletes
Caused by respiratory centres in brain’s medulla
