Section Two: Short Answers (30 marks) Flashcards
The part of the ECG tracing that reflects ventricular repolarisation is?
T wave
Name four major electrolytes that affect cardiac function.
- Calcium
- Magnesium
- Sodium
- Potassium
Explain the electro physiological role of the sodium potassium exchange pump in the cardiac conduction system.
- Pumps sodium irons out of the cell and potassium ions into the cell.
- ATP is used as the energy source to complete this action.
- Pump can transport up to three sodium ions for every two potassium ions.
- More positive ions are transferred outward which repolarises the cell and returns to resting state.
What are risk factors to cardiovascular disease?
Advanced age
Gender (men)
Smoking
Poor diet
Insufficient exercise
Obesity
Hypertension
Components of cardiac conduction system
SA node
AV node
Bundle of his
Right and left bundle branches
Purkinje fibres
What do the electrolytes do within cardiac conduction
Calcium: mediates muscles contractions
Potassium: determine the resting membrane potential and govern repolarisation in cardiac myocytes
Sodium: needed for action potential and triggering contraction
Magnesium: coordinates activity of the heart muscle.
What is the normal duration for the P wave?
<0.10 seconds
What does it mean if the P wave is abnormal
Atrial Arrhythmias such as Atrial Fib, Atrial Flutter.
Atrial Enlargment.
Hypokalemia.
What does the P-R interval represent?
The time that it takes for an impulse to travel from the SA node, depolarise the atria and arrive at the AV node.
What is the regular time of the P-R interval?
0.12-0.2 seconds - 3-5 small boxes
How do you distinguish the Q wave?
The first negative deflection after the P wave.
How do you distinguish the R wave?
The first positive deflection after the P wave.
How do you distinguish the S wave?
The subsequent negative deflection. It must come below the isoelectric line.
What is the normal duration of the QRS complex?
Less than or equal to 0.12. Narrow means its generated high up, wider means its been delayed in conduction.
What does the T wave represent?
Ventricular repolarisation.
What does a U wave indicate?
Repolarisation of purkinje fibres.
When would you see a U wave on an ECG?
During bradycardia.
What does a prominent U wave indicate?
Hypokalemia
What is the QT interval?
From the beginning of the QRS complex to the end of the T wave which is complete polarisation and repolarisation.
What’s the danger with a prolonged QT interval?
Torsardes - R on T a ventricle in its relative refractory period and another depolarisation lands on top
What is a significant ST elevation?
1mm in the limb leads - AVR AVL AVF
2mm in chest leads
What ST depression is significant?
Any depression in the ST segment is significant.
What does ST depression indicate?
Ischemia
What is a fusion beat?
A fusion beat occurs when a supraventricular and a ventricular impulse coincide to produce a hybrid complex. It indicates that there are two foci of pacemaker cells firing simultaneously: a supraventricular pacemaker (e.g. the sinus node) and a competing ventricular pacemaker.
Wolff-Parkinson-White Syndrome
Short P-R interval
Delta wave
QRS widening
Bundle of Kent - accessory pathway faster, path of least resistance resulting in early depolarisation of the ventricle causing the slurred upstroke (delta wave)
The heart is supplied by two branches of the autonomic nervous system, what are they and what do they do?
- Sympathetic Nervous System (SNS) - Adrenergic: Chronotropic effect.
- Parasympathetic Nervous System (PNS) - Cholinergic: Slows down rate
What are the two basic groups of cells within the myocardium?
- Pacemaker Cells - Specialised cells of the electrical conduction system - have automaticity to generate action potentials.
- Working myocardial cells - possess the property of contractibility and therefore physically do the pumping of the blood.
When in a solution ions of opposite charges will typically pair up, what will this do to the solution?
Neutralise it
If ions of opposite charges were to be pulled apart, or ions of the same charges pushed together, the energy required to do so is referred to as what?
Potential energy
What are the chemical symbols for the four elements below?
- Calcium
- Potassium
- Sodium
- Magnesium
Calcium = Ca++
Potassium = K+
Sodium = Na+
Magnesium = Mg++
What is the major intracellular cation?
Potassium K+, 98% stored inside cells
What is the difference between a cation and anion?
Cations are positively charged
Anions are negatively charged
What is the major extracellular cation?
Sodium Na+
Difference in charges between the inside and outside of the cell creates what?
A membrane potential
Measured in millivolts
What is it called when ions move against the concentration gradient and how does this occur?
Active transport - requiring energy in the form of ATP
When the cell membrane separating ions becomes permeable, what happens to the potential energy?
Potential energy is used and released
Why is the inside of the cardiac cells negatively charged?
Because there is relatively more positively charged sodium on the outside than there is potassium on the inside creating an electrical gradient keeping the inside of the sell negatively charged in comparison.
This is due to the continuous operation of the sodium-potassium ion pump across the living cell membrane, moving 3 sodium out for every 2 potassium in.
what is resting membrane potential?
A state where there is more charged anions and chlorides intercellular than extracelluar, inside of the cell is recorded negative.
The potential is the resting membrane voltage - average neg membrane potential is -70 to -90 mV.
What prevents potassium K+ diffusing through the semipermeable membrane across to the extracellular fluid where the concentration gradient is lower?
The electrical gradient from negatively charged anions and chlorides retain the potassium within the cell.
How does sodium Na+ diffuse into the intracellular fluid and when does this occur?
By passing through sodium channels that only open when the cell is depolarise with an action potential.
What prevents sodium Na+ moving intrecellularly with the concentration gradient?
The cell membrane is not very permeable to sodium.
What are the two types of protein-lined channels?
- Potassium K+ ion channels
- Sodium Na+ ion channels
What is the catalyst for the opening and close of the protein lined channels?
As the voltage inside the cell changes with the movement of ions the channels open and close.
What effects the diffusion of ions through the ion/protein lined channels?
The electrical charge within the cell - as the cell depolarises and repolarises the voltage is changing within the cell and prompting the opening and closing of the channels.
True or False: The potassium K+ channels open as the Sodium Na+ channels close?
True
A stimulus opens the sodium Na+ channels initiating depolarisation. Sodium Na+ ions diffuse down the concentration gradient into the intracellular fluid resulting in excess sodium within the cell … what occurs next?
Rapid depolarisation
What is the process called when a cell is returning to its resting membrane potential?
Repolarisation
When observing the resting membrane potential become depolarised and repolarised on an ECG what segment are you looking at?
The QRS segment and T wave
If the QRS segment isn’t tall on an ECG what might this indicate?
Lack of voltage possibly due to a problem with the cell membrane
What is the purpose of the Sodium-Potassium pump and what is its energy source?
The purpose is to work against the concentration gradient and uses ATP as the energy source
Which of the processes of electrolyte transfer requires energy to occur?
Active transport by way of the sodium-potassium pump.
How can imbalances in sodium, potassium or calcium affect the electrical activity of the heart?
Sodium Na+: + or - can alter fluid volume and cause tachycardia
Potassium K+: + or - causes charge imbalances effects the ability of the cell membrane to depolarise and repolarise effectively.
Calcium Ca++: + or - can cause ventricular dysthymias due to affect of contractility, resulting in poor ventricle contraction and therefore poor cardiac output.
How does hypokalemia present on a 12 lead ECG?
Flattened or inverted T waves
U waves
ST depression
Widen PR interval
Potential to cause dysrhythmias
How does hyperkalemia present on a 12 lead ECG?
Severe dysrhythmias - atrial fibrillation and asystole
Why do sodium Na+ ions go through a fast channel?
To generate rapid depolarisation
As cells drift towards threshold level (the point at which a cell depolarises) what occurs?
Fast sodium channels open, resulting in a rush of sodium ions into the cell and therefore rapid depolarisation.
Slow channels have selective permeability to calcium Ca++, what roles does calcium Ca++ play in the cardiac cells?
Calcium Ca++ plays an electrical role by contributing to the number of positive charges in the cell. Calcium Ca++ is also required for contractility of the cardiac muscles.
What occurs when the threshold potential of a cell membrane is met?
The permeability of the surrounding cells changes, resulting in the action potential being spread right across the entire cell membrane.
Define propagation of action potential.
When action potential at any point on a cell membrane acts as stimulus to the surrounding cells.
Cardiac action potential has 5 phases - what are they?
Phase 0: Rapid depolarisation
- Fast sodium channels open, sodium Na+ floods into the cell.
Phase 1: Initial repolarisation
- Sodium Na+ Channels close
- Potassium K+ channels begin to open
Phase 2: Plateau
- Calcium Ca++ channels begin to open & calcium Ca++ enters,
- Potassium K+ continues to leave,
- Myocytes contract
Phase 3: Rapid repolarisation
- Calcium Ca++ channels close
- Slow potassium K+ channels are open
- Return back to -70 to -90 mV
Phase 4: Resting potential
- The cell still have an excess of sodium inside and potassium outside.
- Sodium-Potassium pump is activated restoring the sodium/potassium balance.
During phase 2 of the cardiac action potential, there is a plateau. What causes this and why is it important?
The plateau is a prolonged phase of depolarisation and prolonging of the action potential through the influx of calcium Ca++ and efflux of potassium K+ creating an electrically balanced membrane potential.
This allows time for a complete contraction - the ventricles have time to fill and eject the complete volume of blood.
What occurs during the refractory period?
Cells are incapable of repeating a particular action.
What is the difference between absolute and relative refractory periods?
Absolute: Cardiac muscle cell cannot respond to any stimulation, regardless of how long the stimulus is applied.
Relative: Cardiac muscle cell is more difficult than normal to excite but can still be stimulated.
What is the purpose of the refractory period?
To ensure that the cardiac muscle is fully relaxed before another contraction starts to allow time for complete emptying of the atria and ventricle before then refill and contract again.
True or False: The atria has a shorter refractory period than the ventricles? Justify your answer.
True - the atria have a faster refractory periods as they are needed to react to action potential first to generate a coordinated beat.
What nodes and branches is the cardiac electrical conduction system composed of?
Nodes:
1. SA Node - right atrium
2. AV Node - right atrium
Branches:
1. Left bundle branch
2. Right bundle branch
What are the SA and AV nodes?
A group of specialised cardiac cells that contain automaticity.
What is the passage of the cardiac conduction system.
- SA node depolarises
- Impulses travel across internodal bundles to ensure both atria contract
- down to the AV node
- AV node extends into the AV bundle (bundle of His)
- AV bundle (bundle of His) extends down into the interventricular septum.
- AV bundle (bundle of His) divides into left and right bundle branches.
- The LBB and RBB descend into the apex of the ventricle and continue to branch off repeatedly.
- The branches are known as Purkinje fibres which reach into the ventricle wall.
What is the atrioventricular junction formed by? And what is its purpose?
- AV node
- Bundle of His (AV Bundle)
Serves as the only electrical link between the atria and ventricles.
What ECG leads look at the AV Junction?
Septal leads - V1 & V2
The left bundle branch subdivides into what?
- Anterior-superior fascicles
- Posterior-inferior fascicles
What is the role of the Purkinje fibres?
To spread electrical impulses from cell to cell throughout the myocardial fibres causing propagation of action potential.
What are the conduction rates for the: SA node, AV node, & the Ventricles
SA Node: 60 - 100 bpm
AV Node: 40 - 60 bpm
Ventricles: 20 - 40 bpm
What are the four conduction tracts that make up the atrial conduction system?
- atrioventricular node
- Bachman’s bundle in the left atrium
- weckebach’s tract in the middle internordal tract
- thorel’s tract in posterior intermodal tract
How long does it take for the impulse to travel from the SA node to the AV node?
0.04 seconds
