Heart Contraction and ECG! Flashcards
What causes the delay in the AV node during a contraction?
- decreased number of gap junctions in successive cells in the conduction pathway
- smaller fibres
- more cell junctions per cell junctions per length distance travelled
- results in a greater resistance to conduction of excitatory ions between conducting fibres
What is a functional syncytium?
- many cells with their own cell membrane that function as one
What are the three types of cardiomyocytes
- Pacemaker cells
- Conducting cells
- Contractile cells
What is the speed of a Purkinje fibres?
- up to 5 m/s (running)
What is the speed of an AV node?
- 0.05m/s (walking)
What are gap junctions?
- they are tiny holes found within intercalated discs
- this is where two cell membranes of cardiomyocytes fuse to form permeable ‘communicating’ junctions
- the more gap junctions that are open the less resistance present therefore faster conduction times
What is the journey of electrical conduction in the heart?
SA node -(via internodal bundles)-> AV node: conducted very slowly 0.05m/s. –> Bundle of His: –> bundle branches of ventricles –> ventricular contractile myocardium: to Purkinje cells
What is the function of Internodal Bundles?
- conduct impulses from the SA node to the AV node
- ensures the atria contracts in synch
- provides faster contraction than atrial muscle 1.0m/s
Why is the atrial conduction slower?
- delay in AV node conduction
- less gap junctions, and smaller cells therefore greater resistance
- allows ventriculare refill before ventricular systole
What is the order of ventricular depolarisation?
Septum –> Apex –> atrioventricular groove
What is a Holter Monitor?
- 24/7 determination of the heart rate via the ECG
What is a Lead?
- a configuration of electrodes that is on the skin
- looks at the change of electrical potential in the direction of the Lead configuration (diagonal for Lead II)
Describe the configuration of Lead II
- the +ve on the left leg
- the -ve on the right arm
- a ground on the right leg
- a bipolar lead
What bipolar leads are there?
- Lead I, II, II
- all on the frontal plan
What is an augmented lead?
- when a positive electrode is compared to a composite reference electrode made of the two other limb electrodes connected
- there are three of these Lead II
- aVR, aVL, aVF: all on the frontal plane
What is a precordial lead?
- a positive electrode is compared to an estimate of what is happening at the centre of the heart
- V1-V6, starting in the 4th intercostal space
- a ‘chest’ lead: a configuration one electrode is placed on the chest and the ‘negative’ terminal being Wilson’s central terminal
- makes measurements of electrical activity in the transverse plane
What action potential gives the P-wave?
- depolarization of atria in response to the SA node being stimulated
What action potential gives the PR segment?
- this is the delay occuring in the AV node that allows the ventricles to fill
What does the QRS complex show?
- the depolarization of ventricles, triggers main pumping contractions
- the direction of travel of the electrical potential around the ventricles
What action potential gives the the ST segment
- the beginning of ventricle repolarization
should be flat
What action potential gives the T-wave
ventricular repolarization
What does a wide QRS complex show?
- the ventricular conduction is abnormal
- ectopic pacemaker or bundle branch block
What does a large (deep) Q wave show?
- a sign of dead tissue, usually from an old MI
What is Sinus Rhythm?
- when every P wave has a QRS complex and every QRS complex has a P wave
- when the PR interval is always 3-5 little boxes- 120-200 millisec.
What are the timings for the ECG graph paper intervals?
- little box = 40 milliseconds
- big red box = 200 milliseconds
What are the normal PR interval duration?
- 3-5 little boxes
- 20 - 200 milliseconds
What are normal QRS complex duration?
- 80-120 ms
- 2-3 boxes
What is normal QT interval Duration?
- 360-460 milliseconds
- 9-11.5 little boxes
Describe is the autonomic control of the heart
- parasympathetic input: via the vagus nerve- muscarinic stimulation, decrease contractility and heart rate
- sympathetic input- increase in contractility and HR
What are Heart blocks?
- a type of dysrhythmia
- any kind of impulse conduction black of the heart
- AV block, Bundle Branch block
What effects parasympathetic supply of the heart?
- Muscarinic antagonist: Atropine
What is sympathetic input to the heart?
- Beta agonists and Beta blocker
What causes heart blocks?
- Ischaemia of AV node or AV bundle
- Compression of AV bundle by scar or calcified tissue
What are the symptoms of heart block?
- asymptomatic
- palpitations
- dizziness, syncope, malaise
- risk of sudden death
Describe and explain first degree heart block
- When the PE interval is > 5 little boxes
- all P waves have a QRS complex
- asymptomatic
- young people
- due to delayed AV node transmission
Describe and explain Mobitz type 1, second degree heart block
- Wenckebach:
- some P-waves are blocked and are not followed by QRS ( some atrial signals fail to get to the ventricles)
- the PR interval gets longer until the QRS is missing
- usually followed and watched
Describe and explain Mobitz type II second degree heart block
- Hay: higher risk than Wenckebach
- some P waves are blocked and are not followed by QRS complex
- PR interval remains the same
- likely a problem with the Bundle of HIs
- can progress to 3rd degree heart block
Third degree heart block
- P-waves are out of sync
- PR intervals are not all the same: shows that AV node is being conducted
- symptomatic of insufficient output of the ventricles
What are escape beats?
- when the atrial beats are delayed or prevented
- ectopic beats or the AV node trigger theses escape or premature beats
What are premature beats?
- triggered by irritable heart tissue
- wide and weird looking ventricular activity
- no S wave
- wide QRS complex
What does the ECG of Atrial Fibrillation show?
- no P wave
- you see Flat line or wiggly line instead of P waves
What is the effect of Atrial fibrillation?
- often occur in the elderly
- due to slow flow of blood
- stroke risk
- require anticoagulants
What is 2nd degree heart block show?
- something wrong with the transmission from the atria to the ventricles
What is respiratory Sinus Arrhythmia?
- heart beat is slightly faster during inspiration, slightly sower during expiration
- shorter RR interval in inspiration than in expiration
What does ST Segment Elevation indicate
- Acute MI
- STEMI
- looks like a bed with a spiky headboard
What is a P-Q/P-R interval?
- beginning of the P wave and beginning of the QRS complex
What is Wilson’s central terminal?
a combination of electrodes that act together as if they were a reference electrode positioned in the centre of the heart
What are the speed of ventricular myocytes?
0.3-0.5 m/s
What is the speed of the fastest neurons?
~100m/s
What is the scale used to calculate rate from an ECG?
- the horizontal scale is 2.5cm/sec:
- 1mm per 40ms
- 5 big boxes = 1 sec
- 300/ no. big boxes
or use 1 big box = 300 bpm
