ECG's Flashcards
What is a functional syncythm and how does it enable the use of ECG’s to detect the hearts electrical activity?
A functional syncythm is a system in which many cells function as one, as in the heart.
ECGs detect the hearts activity due to summation of each cells activity which can occur because the heart is a functional syncythm.
Describe the three types of cardiomyocyte
How does the speed of propagation differ between atrial/ventricular cardiomyocytes, AV node and purkinje fibres?
Pacemaker- set heart rhythm
Conducting- transmit the rhythm
Contractile- contract to the rhythm
AV node: 0.05m/s
Atrial and ventricular contractile cardiomyocytes have an intermediate speed of 0.3-0.5m/s
Conducting purkinje fibres up to 5m/s
Describe the stricture of myocardium.
Cardiomyocytes linked by low resistance pathways associated with gap junctions and intercalated discs.
Intercalated discs are clusters of gap junctions and structural components which hold the myocardium intact.
Cardiomyocytes have a double membrane
Which structure travels between the atria and ventricles?
Fibrous skeleton
Outline transmission through atria
Impulse initiated at SAN
Internodal bundles allow transmission 3/4x faster
Describe what happens to the impulse at the AV node
Consider the timeframe of one ventricular contraction.
From SAN to AVN it takes 30ms
There is a delay in signal transmission of 0.1/0.2s due to slow AP conduction (0.05m/s). This allows 20/25% more ventricular filling.
From AVN to penetrating portion of AV bundle it takes 90ms
From here the delay to the penetrating bundle is 40ms
Describe what purkinje cells are and their function in ventricular propagation
Via four bundles containing purkinje cells in ventricular contraction
They are large myocytes that transmit the impulse rapidly to the main mass of the ventricles vis the route septum->apex-> AV groove
Outline 3 advantages of an ECG
Excellent for measuring rate (pulse) e.g. the Holder monitor allows 24/7 surveillance which is useful when atrial rate differs from ventricular rate.
Fast
Affordable
What is meant by a lead?
Describe the standard ECG?
A configuration of electrodes (positive and negative). Each lead provides 3-4 seconds of information. Rhythm signals provide 12 seconds.
12- lead ECG including :
3 bipolar leads (I,II,III) which occur in the frontal plane
3 “augmented” leads also in frontal plane
6 ‘‘precordial’’ (on thorax) (V1-V6) in transverse plane (spine to sternum)
Describe the structure of a normal electrical wave
What is the Y axis of an ECG?
P wave- depolarisation of atria in response to SAN triggering
PR segment- delay of AV node to allow ventricular filling
QRS complex- depolarisation of ventricles, triggers main pumping contraction
ST segment- beginning of ventricle repolarisation, should be flat
T wave- Ventricular repolarisation
Y AXIS= VOLTAGE
Describe what it means to have a
a) wide QRS complex
b) large Q wave
a) ventricular conduction is abnormal e.g. ectopic pacemaker or bundle branch block
B) sign of dead tissue
What is sinus rhythm?
What are its requirements?
When the heart rhythm is generated from the SAN
Requirement:
Each P wave is succeeded by a QRS complex
Each QRS complex is preceded by a P wave
PR interval normal 120-200ms (3-5 boxes)
What is sinus tachycardia?
Tachycardia driven by the SAN beating too quickly
Normal PR interval
P matched with QRS
Outline the normal intervals as per
a) PR interval
b) QRS complex
c) QT interval
a) 120-200ms (3-5 boxes)
b) 80-120ms (2-3 boxes)
c) 360-460ms (9-11.5 boxes)
How do you calculate overall rate?
Ventricular rate?
How is the RR interval significant?
count boxes between two successive P waves. (1 small box = 0.04s or 40ms; 1 large box= 200ms)
Count between R waves
Heart rate is equivalent to the inverse of the RR interval
Outline autonomic control of the CVS
Describe the action of Atropine
Is the hearts own vasculature innervated by parasympathetic system?
Parasympathetic control via the vagus nerve decreases heart rate, contractility and conduction velocity
Sympathetic control via sympathetic nerve increases heart rate, contractility and conduction velocity.
Withdrawal of the parasympathetic effect has the same result as increasing sympathetic effect. Atropine works in this way as it is a muscarinic antagonist.
NO!
What is the effect on rate of beta agonist and beta antagonists (blockers)?
What other nerves is employed by the sympathetic system?
Beta agonists increase rate, antagonists decrease the rate.
Stellate nerves
Briefly outline what heart block is, its causes and symptoms.
Its a type of dysrhythmia.
AV heart block is a delay/failure of atrial signal stimulation to ventricle
Causes include :
- Ischaemia of AV node/bundle
- Compression of AV node/bundle by scar tissue/calcification
- Inflammation of AV node/bundle
Symptoms include palpitations, hypotension and death.
It can also be asymptomatic
Describe 1st degree heart blood
PR Interval >200ms
Asymptomatic, young people
Cause: Delayed AV node transmission
Rarely treated
Describe 2nd degree heart blood
Mobitz type 1
Elongated PR interval. PR interval increases with QRS complex until QRS fails to follow P. wave
Caused by AV node damage
No treatment
Mobits type 2 Some P waves not followed by QRS complex Caused by block in Bundle of His Increases risk of 3rd degree HB Treatment: implanted pacemake
Describe 3rd degree heart blood
Atrial signals consistently fail to arrive at ventricle
Ventricular rate consistently 30-40bpm
Variable time between atrial and ventricular beats
Intrinsic ventricular rate is very slow <60bpm
Inverted QRS
Describe atrial fibrillation
Disorganisation of electrical activity in atria
No P wave, instead flat/wriggly line
Ventricular rate fast and irregular
Elderly people
May cause thromboformation due to slow flow of blood which increases risk of stroke. GIVE ANTI-COAGS
Describe respiratory sinus arrhythmia
Heart beat is slightly elevated in inspiration and decreases during expiration
Occurs in children and athletes
Cause: respiratory segment in medulla
Observe inverse RR interval (HR)
What might ST elevation indicate?
Acute MI
What is the isoelectric baseline?
End of T to next P.
