Introduction to the ECG Flashcards
What is a functional syncytium?
One large cell having many nuclei that are not separated by cell membranes. Many cells acting as one. (Cardiomyocytes)
List and describe the 3 types of cardiac myocyte
Pacemaker cells- For setting hearts rhythm
Conducting cells- For transmitting rhythm throughout the heart.
Contractile cells- For contracting to that rhythm throughout the heart
How does speed of transmission vary throughout the heart?
Contractile- atrial and ventricular monocytes (0.3-0.5 m/s)
Conducting system- purkinje fibres (5m/s) and AV node (0.05m/s)
Cardiomyocytes are linked by low resistance pathways in close association with what?
Gap junctions
Intercalated discs
Explain how a signal is moved between cells
Action potential arrives at a cell and causes depolarisation which then initiates an action potential in the adjacent cell.
What is the purpose of internodal bundles?
Conduction between SAN and AVN. Ensure synchronous contraction of both atria.
Why are bundles used to conduct the impulses instead of through the atria?
Atria conduction is 0.3m/s-0.5m/s
Bundles conduction is 1.0m/s
Bundles conduct much faster leading to quicker transmission
How many specialised bundles are in the atria?
4
What happens to the impulse at the AVN?
Waves of depolarisation travel at 0.05m/s across the small modified cardiomyocytes to the AVN. The AVN delays the wave of excitation to allow the ventricles to be sufficiently filled by the atria.
How many ms delay is there in the transmission of SAN to AVN?
160ms
What causes the delay between transmission of signal from SAN to AVN?
Mostly due to 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
What does the AV node connect to?
Bundle of his followed by the purkinje fibres
Why do the purkinje cells conduct impulses faster than other cells?
Larger diameter
Which parts of the ventricle are depolarised in order?
Septum, apex, atrioventricular groove
How does an ECG manage to measure the electrical activity of the heart?
The heart is a functional syncytium and although the electrical charge of one individual cardiac myocyte is too small, a group of them together is large and changes occur simultaneously.
What is the purpose of leads in an ECG?
A lead is a configuration of the electrodes. Looks at the heart at different angles depending on where the leads have been placed
What is the most common lead analysed?
Lead 2
How is lead 2 organised on the body?
Positive electrode on left leg, negative electrode on right arm and ground electrode on the right leg
List the different types of standard leads
3 Bipolar leads (1,2,3)
3 Augmented leads
6 Precordial leads
In which plane do the bipolar leads look?
Frontal plane
What is the lower lead called on the bottom of the trace?
Rhythm strip- shows the beats/heart rate from lead 2
Give the axes for an ECG
Voltage (y) and time (x)
List the different sections of the ECG wave in order
P wave PR segment QRS complex ST segment T wave
What does the P wave represent?
The depolarization of the atria in response to the SSN triggering
What does the PR segment represent?
Delay in the AV node to allow filling of the ventricles
What does the QRS complex represent?
Transmission of depolarisation through the ventricular myocardium
What does the ST segment represent?
Beginning of the ventricular repolarization- should be flat
What does the T wave represent?
Ventricular repolarization
When is the QRS complex abnormal?
Wide/misshapen complex is abnormal
Large Q waves are a sign of an old MI
What are the conditions for sinus rhythm?
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)
What is sinus tachycardia?
A tachycardia driven by the SA node beating too quickly. It has normal PR intervals, and each P matched with a QRS
How many seconds is 1 small box on an ECG trace?
0.04 seconds
How many seconds is 1 big box on an ECG trace?
0.2 seconds (200 ms)
Define PR interval and state its normal length
From start of P wave to start of QRS complex
3-5 boxes (120-200ms)
Define QT interval and state its normal length
From start of QRS complex to end of T wave
9-11.5 boxes (360-460ms)
Define ST segment
From end of QRS complex to start of T wave
How long is a normal QRS complex?
2-3 boxes (80-120 ms)
What is the box equivalent of 1 second?
5 Big boxes
How is rate calculated?
The number of big boxes between two P waves
How is ventricular rate calculated?
The number of boxes between two R waves
Give the formula for calculating rate
300/no. big boxes
Give the rate of 1 box
300 bpm
Give the rate of 2 boxes
150 bpm
Give the rate of 3 boxes
100 bpm
Give the rate of 4 boxes
75 bpm
Give the rate of 5 boxes
60 bpm
Give the rate of 6 boxes
50 bpm
Give the rate of 10 boxes
30 bpm
What is heart block?
A type of dysrhythmia
Any type of impulse conduction block of the heart
What is AV heart block?
A delay of failure of atrial signal stimulating the ventricle
What are the causes of AV heart block?
Ischemia of AVN or AV bundle
Calcified or scar causing compression on AVN
Inflammation of the AVN or AV bundle
List the symptoms of AV heart block
Can be asymptomatic Palpitations Syncope Malaise Sudden death (3rd degree) Hypotension
Describe first degree heart block
When the PR interval is greater than 5 little boxes (200 ms) Almost always asymptomatic Usually in adolescents Delayed AVN transmission Rarely treated
What is second degree heart block?
Some QRS complexes are missing since some P waves are blocked and therefore are not followed by QRS complex
Name the two types of second degree heart block
Mobitz type 1 (wenckebach)
Mobitz type 2 (Hay)
Describe mobitz type 1 (wenckebach)
When the PR interval gets longer until the QRS wave fails to follow the P wave
Likely caused by AVN damage
Usually no treatment is given
Describe mobitz type 2 (Hay)
Some P waves are blocked out and are not followed by QRS. The PR interval remains the same.
Likely problem in the bundle of his
High risk- can progress to 2nd degree
Treatment: Implant a pacemaker
Describe third degree heart block
Atrial signals consistently fail to arrive at ventricles. Ventricular rate is consistent. Time between atrial beats and ventricular beats is variable. PR interval varies. Sometimes > 12 boxes. Intrinsic ventricular rate is quite slow ( < 60 bpm). Atrial beats are consistent
Describe atrial fibrillation
Disorganised electrical activity in atria
No P wave. Instead:
Flat line OR Wiggly line instead of P
Ventricular rate is fast & irregular
Many signals reach AV node
AF is very common in the elderly
Can lead to thrombus formation in atrium
Due to slow flow of blood which increases stroke risk
Patient given anticoagulants as prophylaxis
Describe respiratory sinus arrythmia
Heart beat is slightly faster during inspiration, slightly slower during expiration
Normal: sign of a healthy heart
Usually only present in children & athletes
Caused by respiratory centres in brain’s medulla
Observe ventricular rate: inverse of RR interval
What is an RR interval
Duration in time between two QRS complexes
What does an ST segment elevation suggest?
Myocardial infarction
