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