Week 113: Syncope Flashcards
What happens during diastole?
The heart fills
What happens during systole?
The heart contracts
Where does the cardiac cycle start?
The electrical activation of the sino-atrial node
Where is the sino-atrial node located?
High in the right atrium
What does the activation of the sino-atrial node do?
Initiates a wave of depolarisation that spreads over the right atrium and into the left atrium via the Bundle of Bachmann
How does the wave of depolarisation initiated by the sino-atrial node go into the left atrium?
Via the bundle of Bachmann
What does the electrical activation of the atria lead to?
Coordinated atrial pumping
Where else does the electrical impulse from the sino-atrial node travel?
To the atrioventricular node
What is special about the atrioventricular node?
It is the only electrical connection between the atria and the ventricles
What does the AV node do once the signal has reached it?
Delays the electrical impulse
Why does the AV node delay the electrical impulse?
To allow the atrial systole to finish before ventricular systole starts
How long is the delay at the AV node?
150-200ms
What happens after the delay at the AV node?
The electrical impulse crosses the AV node and enters the specialised conducting tissue of the ventricles
What are the specialised conducting tissues of the ventricles?
The bundle of His, bundle branches, and their divisions
How long does it take the specialised conducting tissue to distribute the electrical impulse throughout the ventricle?
50-60ms
What does the electrical signal spreading through the ventricles do?
Go into the myocytes and initiate contraction
Myocyte-myocyte spread of signal is much slower than through the specialised conducting tissue
What are myocytes?
Muscle cells or fibre found in muscle tissue
Develop from myoblasts by myogenesis
What accounts for most leads observing the left ventricle having a positive deflection?
The specialised conducting tissue is sub-endocardial so the wave of excitation spreads endocardially to epicardially and then onto an observing electrode
What happens after depolarisation?
The action potential of the myocytes has a prolonged plateau phase, during which the ventricular myocytes are contracted and a little current flows
What happens after the plateau phase?
Repolarisation: the intracellular level of calcium falls rapidly and the myocytes relax, starting diastole
Where does repolarisation start and end?
Starts: sub-epicardially
Ends: sub-endocardially
Where are the four limb leads in an ECG positioned?
Red: right arm
Yellow: left arm
Green: left leg
Black: right leg
Where is VI placed?
Red: fourth intercostal space, right sternal border
Where is V2 placed?
Yellow: fourth intercostal space, left sternal border
Where is V3 placed?
Green: midway between V2 and V4
Where is V4 placed?
Brown: fifth intercostal space, left mid-clavicular line
Where is V5 placed?
Black: level with V4, left anterior axillary line
Where is V6 placed?
Violet: level with V4, left mid-axillary line
What is lead I?
The voltage between the positive left arm electrode and right arm electrode
I = LA - RA
What is lead II?
The voltage between the positive left leg electrode and right arm electrode
II = LL - RA
What is lead III?
The voltage between the positive left leg electrode and left arm electrode
III = LL - LA
What is the aVR lead?
aVR = - (I + II)/2
What is the aVL lead?
aVL = I - II/2
What is the aVF lead?
aVF = II - I/2
How long is each large square recording?
200ms
How long is each small square recording?
40ms
How do you work out the heart rate from an ECG?
300/number of squares = rate
How do you work out the axis deviation?
- aVF +ve, lead I +ve: normal
- aVF -ve, lead I +ve: left axis deviation
- aVF +ve, lead I -ve: right axis deviation
What does it mean when there is a bifid P-wave?
M shaped P-wave implies mitral valve (lead II)
- Left atrial enlargement/hypertrophy
What does an increased QRS amplitude imply?
Right ventricular hypertrophy
What does left axis deviation imply?
- Left ventricular hypertrophy: more ventricular muscle to depolarise
- Damage to conducting tissue: delaying depolarisation
What does right axis deviation imply?
- Right ventricular hypertrophy
- Disease in left ventricular conducting tissue
What can cause PR interval lengthening?
Sleep
What can cause PR interval shortening?
Exercise (high heart rate)
What causes a prolonged (>200ms) but regular PR interval?
First degree heart block
> 200ms
What happens in third degree heart block?
A complete dissociation between the P-wave and QRS complex
AV dissociation
What are the two types of second degree heart block?
Mobitz I - Wenckebach’s Phenomenon
Mobitz II
What happens in Wenckebach’s Phenomenon/Mobitz I?
The PR interval becomes gradually longer each beat until a QRS is dropped then the pattern restarts
What happens in Mobitz II?
- Prolonged PR interval
- Absent QRS at regular intervals
How does atrial fibrillation appear on an ECG?
Like a blunt saw
What causes a shortened PR interval?
Wolff-Parkinson-White Syndrome
Lown-Ganong-Lavine Syndrome
How does ventricular tachycardia appear?
Very fast high waves
What produces a prolonged QRS of around 160ms?
Right bundle branch block
- Often a RSR pattern too
What produces a long QRS of around 200ms?
Left bundle branch block
Give four causes of bradycardia
Athlete
Cold
Beta blockers
Hyperthyroidism
Give two examples of tachycardia
Beta agonists
Fear
Which X-ray is best for the heart?
PA chest xray
What is the RR interval and what is its length?
The interval between an R wave and the next R wave
- Usually 60-100 bpm
- 600-1200ms
What is the P wave and what is its length?
During normal atrial depolarisation the main electrical vector is directed from the SA node to the AV node and spreads between the atria
80ms
What is the PR interval and what is its length?
- Beginning of the P wave to the beginning of the QRS complex
- Reflects the time the electrical impulse teak to travel from the SA node through the AV node and entering the ventricles
- PR interval is a good estimate of AV node function
120-200ms
What is the PR segment and what is its length?
- Connects the P wave and QRS complex
- The impulse vector is from the AV node to the bundle of His to the bundle branches then Purkinje fibres
50-120ms
What is the QRS complex and what is its length?
- Reflects the rapid depolarisation of the right and left ventricles
- More muscle in ventricles than atria therefore QRS complex has higher amplitude than P wave
80-120ms
What is the ST segment and what is its length?
- Connects the QRS complex and the T wave
- Represents the period when the ventricles are repolarised
80-120ms
What is the T wave and what is its length?
- Represents depolarisation of the ventricles
- Beginning or QRS complex to the apex of the T wave is the refractory period
- Final half of T wave is the relative refractory period
160ms
What is the ST interval and what is its length?
- End of the S triangle to the T wave
320ms
What is the QT interval and what is its length?
- Beginning of QRS complex to the end of the T wave
Up to 420ms in a heart rate of 60 bpm
What is TLOC?
Transient loss of consciousness due to not getting blood to the brain
What is syncope?
Unusual heart rhythm
What are the main causes of TLOC?
Reflexes
Orthostatic (postural hypertension)
Cardiac arrhythmia (syncope)
Structural cardiac diseases
What is vasovagal syncope?
- Reflex TLOC
- To do with the vagus nerve
- Getting too warm
What is situational syncope?
- Reflex TLOC
- Post cough
- Post micturition/defecation
What is orthostatic syncope?
Nervous system failures
Give three structure disease examples of causes of TLOC?
- Aortic stenosis: not good blood flow
- Myocardial ischaemia: can be longer term
- Aortic dissection: blood stops going in the right direction
What options for monitoring TLOC are there?
- Lying/standing BP: check immediately on standing and then 3 minutes later
- Halter monitor
How does blood flow through the heart?
- The atria contract and the tricuspid and mitral valves open
- Blood flows into the ventricles
- Ventricles start to contract and the tricuspid and mitral valves close
- Aortic and pulmonary valves open and blood shoots into the aorta and pulmonary artery
How does the sympathetic system regulate the heart?
It speeds the heart up
How does the parasympathetic system regulate the heart?
It slows down the heart
What is the mean arterial pressure?
Diastolic (low BP) + (systolic-diastolic)/3
What is the central venous pressure?
Usually quite low as the vein are expanded and act as a blood reservoir
What happens to the pressure in veins and arteries when the heart beats?
- Reduction in venous pressure
- Increase in arterial pressure
Increase is higher than the decrease
Why does the increase in arterial pressure occur when the heart beats?
Because the arterial system cannot expand as well as veins to cope with the volume
What is the compliance of arteries?
20% that as of the veins
What is the mean arterial pressure - what is trying to be measured?
cardiac output x peripheral resistance
Where does the main drop in blood pressure occur?
Arterioles
What are the vasa vasorum?
- A network of small blood vessels that supply blood to larger blood vessels
- They supply oxygen to the blood vessel
- Supply the tunica interna/externa
What are conduit arteries?
Arteries that don’t kink easily
What is a major vessel that gives the Windkessel effect?
The aorta and its immediate branches (are elastic)
Where does the major control of arterial flow occur?
Arterioles
What passes through cell membranes and what has to go through cell junctions?
- Oxygen and carbon dioxide can diffuse through cell membranes
- Glucose has to pass through cell junctions
What are veins?
- Capacitance vessels that hole ⅔ of blood volume
- Become collapsed when blood is lost so as to get the arterial system to deal with blood loss
How does venous return occur?
- Mainly through muscle pumps: as exhale the lung collapses which sucks blood up towards the heart through creation of negative pressure in the thoracic cage
What is a function syncytium?
A group of joined up cells that can communicate directly with one another
What is an example of cells that are a function syncytium?
Cardiac myocytes
What two things control the firing of the SA node?
- Background sodium current and funny current: both currents result in gradual depolarisation of the heart
- Central nervous system
How does the cardiac action potential work?
- Resting membrane potential -90mV
- At -60mV, Na+ enters
- Na+ channel closes at +20
- Brief depolarisation
- Plateau phase where Ca2+ released
- K+ leaves
- Ca2+ channels close and K= brings about depolarisation
- Absolute refractory period
What is a β1/β2 blocker?
Propranolol
Give 3 β1-adrenoceptor antagonists
Atenolol
Metoprolol
Bisoprolol
How do β-adrenoceptor antagonists work?
- Produce a fall in blood pressure by decreasing cardiac output (initially)
- After continuous treatment cardiac output returns to normal but blood pressure stays low
What are common side effects of beta blockers?
- Cold hands
- Fatigue
- Provocation of asthma
What is propranolol used for?
Hypertension
Angina
Arrhythmias
What are bisoprolol and metoprolol used for?
Hypertension
Heart failure
What are diuretics used for?
Hypertension
Give 3 examples of thiazide diuretics
Bendroflumethiazide
Chlortalidone
Spironolactone
What is doxazosin and what is it used for?
α1-adrenoceptor agonist
Used in hypertension
Give 2 examples of calcium channel blockers and how they work and what they are used for
Nifedipine
Amlodipine
Vascular smooth muscle tone is determined by the cytosolic calcium concentration
Used in hypertension and angina
How do ACE inhibitors work?
- Decrease circulating angiotensin II (a vasoconstrictor)
- Use results in a fall in peripheral resistance and a lowering of blood pressure
What are ACE inhibitors used for?
Hypertension
Give two examples of ACE inhibitors
Lisinopril
Enalapril
What are common side effects with ACE inhibitors?
- Dry cough
What does adenosine do?
- Stimulate A1-adenosine receptors
- Antiarrhythmic
What does digoxin do?
- Stimulate vagal activity causing release of Acetylcholine
- Antiarrhythmic
