ECG Flashcards
Syncytium
One large “cell” having many nuclei that are not separated by cell membrane
Functional syncytium
Many cells functioning as one
Propagation speed
Contractile - atrial and ventricular myocytes 0.3-0.5 m/s
Conducting system (modified cardiomyocytes)
Purkinje fibres up to 5 m/s
cf fastest neurons ~100 m/s
AV node 0.05 m/s
Impulse spread in 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
0.3-0.5 m/s
Conducting via bundles is much faster
1.0 m/s
Delay at AVN
30 ms from SA to AV node
90 ms delay before enters penetrating portion of AV bundle
40 ms delay in penetrating bundle
Increased RESISTANCE
smaller fibres which lead to:
More resistance along length of fibre (smaller diameter)
More intercellular junctions (shorter length)
With diminished numbers of gap junctions per surface area
ventricular propagation
AV node connects to the bundle of His followed by Purkinje fibre system
Purkinje fibres are very large myocytes - transmit the impulse faster
Bigger diameter cells conduct faster - 5m/s
After slower conduction between contractile myocytes can occur
0.3 - 0.5 m/s
First part of ventricular wall to be depolarised is septum, then apex
Last part is atrioventricular groove
Ventricular propagation order
First part of ventricular wall to be depolarised is septum, then apex
Last part is atrioventricular groove
ECG definition
The ECG is a gross electrical measurement of the heart
The electrical activity of the heart is measured on the skin
ECG use in diagnosis
Excellent for rate
Holter monitor (ECG) allows 24/7 rate determination
Esp. useful when Atrial rate ≠ Ventricular rate
Many Subtleties
Not a one-stop-diagnosis
Patient Hx essential for interpretation
Diagnosis requires other techniques
ECG is very fast and affordable
Lead
A configuration of electrodes (usually consisting of a positive electrode, a negative electrode, and sometimes a ground).
A standard “12-lead ECG” looks at the heart from 12 different angles, creating measurements for 12 leads, using 10 separate electrodes.
Lead II has the positive electrode on left leg, negative electrode on right arm, and the ground electrode on the right leg (although the ground could be almost anywhere).
12 standard leads
3 bipolar leads
I, II, III
Frontal plane
3 “augmented” leads,
aVR, aVL, and aVF
Frontal plane
6 “precordial”
on the thorax near the heart
called V1, V2, V3, V4, V5, V6
transverse plane (spine to sternum)
Augmented Lead
Precordial chest
Bipolar Leads
Augmented Lead – a positive electrode is compared to a composite reference electrode made of the two other limb electrodes connected
Precordial chest (Unipolar) Leads – a positive electrode is compared to an estimate of what is happening at the centre of the heart
Bipolar Leads: a positive electrode is compared to a negative electrode (e.g. lead II)
Coronal plane
QRS complex
QRS = Transmission of depolarisation through the ventricular myocardium
If QRS is wide or misshapen, then ventricular conduction is abnormal, e.g. ectopic pacemaker or bundle branch block
More cells -> bigger contribution
Almost no contribution from His and conduction system
Large (deep) Q waves are a sign of dead tissue (old MI)
Sinus Rhythm
When the heart rhythm is generated from the Sino-Atrial Node
Each P wave is followed by a QRS complex
Each QRS complex is preceded by a P wave
When PR interval is always normal (3-5 little boxes)
Sinus Tachycardia is a tachycardia driven by the SA node beating too quickly
It has normal PR intervals, and each P matched with a QRS
ECG timing
PR interval - from start of P wave to start of QRS complex (name is misleading)
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 – 200 ms
QRS complex Duration = 2-3 boxes
80 – 120 ms
QT interval Duration = 9-11.5 boxes
360 – 460 ms
Calculating rate
Horizontal scale is 2.5 cm/sec.
One little box = 1 mm = 40 ms (milliseconds)
A big box = 5 little boxes = 200 ms
To calculate rate, count how many big boxes occur between two P waves
For ventricular rate, count between R waves
Autonomic Control of CVS
Rate & Contractility
The Heart: parasympathetic input
Vagus nerve: Muscaranic stim -> Decrease HR + contractility + conduction velocity
Parasympathetic withdrawal -> Increase HR + contractility + conduction velocity
The Heart: sympathetic input
Sympathetic stimulation -> Increase HR + contractility + conduction velocity
Stellate nerves
Beta agonists increased rate
Beta blockers decreased rate
Activation of
Beta(1)-adrenoceptors
Beta(2)-adrenoceptors
Alpha-adrenoceptors
Activating Beta(1)-adrenoceptors cause inotropism and chronotropism
Activating Alpha-adrenoceptors cause vasoconstriction
Activating Beta(2)-adrenoceptors cause vasodilatation in skeletal muscle (decreases peripheral resistance during exercise)
Atropine
Anticholinergic drug that reduces parasympathetic activity. Until recently it was medically used to treat asystole and extreme bradycardia
Heart Blocks
A type of dysrhythmia
any kind of impulse conduction block of the heart
Includes AV block, Bundle Branch Block, etc
Atrio-ventricular Heart Blocks
Delay or failure of atrial signal stimulating ventricle
CAUSES
Ischaemia of AV node or AV bundle
Compression of AV bundle by scar or calcified tissue
Inflammation of the AV node or bundle
SYMPTOMS (esp. for serious 3rd degree heart block)
Can be asymptomatic
Palpitations
Hypotension-like: Dizziness, Malaise, Syncope
Risk of Sudden Death
First degree heart block
When PR interval > 5 little boxes (200 ms)
Normal PR < 5 little boxes
But 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 Heart Block = Some P waves are blocked and are not followed by QRS – Some QRS complexes are “”missing”
Mobitz Type I (Wenckebach)
PR interval gets longer until QRS wave fails to follow P wave
Likely cause is AV node damage
Usually no treatment given *E
Mobitz Type II
Second degree heart block
Mobitz Type II (Hay) Some P waves are blocked and are not followed by QRS PR interval remains the same Likely Problem in Bundle of His High risk Can progress to 3rd degree heart block Treatment: Implant Pacemaker
