1
Q
Conduction system of the heart
A
- SAN originates generates action potential (automatic pacemaker)
- SAN to AVN with delay via internodal pathways to allow atrial contraction
AVN via Bundle of His to bundle branches and Purkinje fibres at the end (fast)
2
Q
Where is the SAN?
A
Top of right atrium
3
Q
How do ventrices polarise?
A
From bottom up (purkinje fibres)
4
Q
SAN
A
From SVC to IVC
- central SAN tissue where rhythm originates
- peripheral SAN tissue where it exits
5
Q
Origins of pacing in SAN
A
- unlike normal action potentials
- no true resting potential
- depolarised managed by sppecialised slow Na+ and Ca2+ channels making action potential different
6
Q
Action potential of SAN
A
- unlike normal
- funny current (pacing controlled by slow Na+ channels)
- threshold reached = action potential (voltage gated Ca2+ channels)
- slower flow of calcium than in other cells so slower rate of depolarisation
7
Q
3 phases of SAN action potentials
A
4 = funny current (constant changing as slow open Na+ channels), reaches -50 so transient calcium channels open so more flow in helping reach threshold 0 = depolarisation, increased calcium flow as long lasting channels open and transient close 3 = repolarisation (potassium channels open allowing depolarisation and calcium close)
8
Q
What happens if there is failure with SAN?
A
- AVN will take over as have pacemaker cells here, similar action potentials, at slower rate of rhythmicity
- will be much slower rate than required
- = escape rhythm
9
Q
Ventricular myocyte depolarisation
A
- much faster like nerve cells
- still significant difference with nerve cells however (heart cells have much greater time period)
- cause by fast sodium channels
- large plateau before repolarisation (caused by Calcium channels)
- have sudden rapid repolarisation which don’t get at nerve or pacemaker cells (transient potassium channels open and move out but calcium channels flooding in slows this repolarisation), increases absolute refractory period preventing building of action potentials (tetanus) so blood pumps out of ventricles
10
Q
How is bradycardia treated?
A
Typically with pacemakers
11
Q
How does bradycardia occur?
A
SAN = lack of sufficient electrical impulses AVN = heart block (3 degrees)
12
Q
3 degrees of AVN heart blocks
A
1 - signal is travelling from SAN to AVN but is just slower
2 - some beats dropped so irregular
3 - no signal receieved at all = escape rhythm as AVN takes over
13
Q
How is tachycardia treated?
A
Drugs or defibrillators
14
Q
What do you get in tachycardia?
A
- AF
- atrial flutter
- SVT
- ventricular tachycardia
- VF
15
Q
Reasons for pacing
A
- need something to sense slow/irregular beats and reinitiate standard rhythm for successful blood flow
16
Q
SA block
A
- sudden stop of the P wave
17
Q
SA pause or delay
A
- missing PQRST component
18
Q
External pacemakers methods
A
- trans-thoracic
- transcutaneous
- external pacing
- emergency use only until transvenous pacing or other therapies applied
- after defibb
- for immediate bradycardia Tx
- should not be relied upon for extended time period
- use patch electrodes fixed to chest
19
Q
Transvenous pacing
A
- temporary external pacing
- alternative to transcutaneous
- pacemaker wire is threaded through vein to enter RA or RV via subclavian or groin via x-ray guidance
- sedation is usually used
- issues = infection so guidance say avoid and put in permanent pacemaker instead
20
Q
Implanted pacemakers
A
- local anaesthesia
- under skin
- pectoral muscle region
- battery life old issue but now lifetime guarantee mostly
- 1-2 hours surgery
- single or biventricular (bi = cardiac synchronisation therapy with you have HF with abnormal signals so have to stimulate both ventricles as not synchronised)
21
Q
2 types of pacemaker implantation
A
- endocardial (inner)
- epicardial (outer, need open heart surgery to remove)
22
Q
Why is endocardial implanted pacemaker used?
A
- more common
- adults
- leads last longer
- easier surgery (transvenously, local anaesthetic)
- greater risk of dislodgement
- risk of venous occlusion
23
Q
Why is epicardial implanted pacemaker used?
A
- children as still growing
- no interference with venous flow
- longer recovery times
- more pulmonary complications
24
Q
How are pacemakers implanted?
A
- non dominant side
- reduces risk of lead damage and device movement
- less uncomfortable
- increases lifetime of leads
25
Other considerations of pacemaker implantation
- vascular problems
- dialysis shunts
- mastectomy
- sporting needs (shooting)
26
Types of electrode
Unipolar or bipolar
27
Unipolar electrode
- cathode inside heart
- anode (positive) in pulse generator in device
- current flow between pacing electrode to electrode on device itself through body tissue
28
Bipolar electrode
- cathode and anode placed close together
- current flows between
- proximally have anode electrode ring, distal electrode at tip (cathode)
29
Why is bipolar used?
More control of the stimulus
| Also less susceptible to artefact as close together so not going through lots of body tissue
30
Capture
- depolarisation of heart due to artificial stimulus
| - shows up on ECG as a spike just before P wave or R wave, depending on what is being stimulated
31
How to generate capture?
- consider threshold = minimum amount of energy needed to depolarise myocardium
- this is because want to use low energy as high can cause burns/damage but also don't want to drain battery
- lower voltage/pulse amplitude = more time to trigger (shown on duration amplitude curves)
32
Why does capture threshold change over time?
Resistance increases as you get build up of scar tissues around electrode so high voltage needed for sufficient current density
- other factors such as hypoxia and acidosis increase threshold
33
How to reduce stimulation thresholds
- reduce diameter and increase surface area of lead tip = high current density
- adjust electrical material with rough surface = large SA as more contact points without increasing diameter size
- steroid collar also works around the electrode
34
Steroid use with electrode
- tips which release steroids decrease pacing thresholds and suppress inflammation of surrounding cardiac tissue
- anti-inflammatory drugs such as glucocorticosteroids are used, containing in silicone core and surrounded by electrical material, slowly leaks out over years, reduces fibrous formations around device, reduces energy requirements of pacemaker
35
Electrode Fixation Techniques
- need to produce minimal electrochemical reaction & withstand flexing/corrosion
- normally helical coils of platinum-irridium alloys encapsulated in silicon rubber = minimise stresses within system + provide good tensile strength, multiple wounds so if one fails there is a backup, helical coil = reversible flexibility, silicone = flexibility + electrical insulation + biological compatibility with human body
36
2 types of electrode fixation
- cork screw fixation = active, extendable screws into cardiac tissue
- tine fixation = passive fixator, anchors itself
37
Leadless pacemaker problems
- loss of pacing output due to battery failure
- battery malfunction
- reduction in electrolytes = increases internal resistance = reduces current = failure of output
38
3D printed pacemakers
- high resolution images and modelling to print a heart model and moulded elastic membrane on top
- tested on rabbits heart
- electrode and sensory array for different information (pH, temp etc.)
39
Implantable power sources
- Lithium cells
40
Lithium cells
- high reliability
- high energy density
- low self-discharge
41
Requirements for any pacemaker battery
- understanding min and max voltage and discharge of current
- provide continuous operation
- long service life
- robust to impacts
- good performance across variety of conditions
- biocompatible with body
- doesn't corrode
- high reliability
- high energy density
- low self-discharge
42
Which implantable power source may be used in future?
Lithium carbon monofluoride
- high energy density
- compatible with titanium casing which reduces weight by over 50%
43
Lithium Iodine Batteries
- low rate discharge
- hermetically sealed glass-to-metal stainless steel case
- current in microamps
- optimum temperature 37 degrees C
- relatively high energy density
44
Lithium Silver vanadium oxide batteries
- high rate of cell discharge
- low & consistent current drain over lifetime (0.5 to 2 ampere range)
- high energy density and long shelf life
45
Generic pacing codes
- standard 5 letter code
- describes pacing functions of the devices
- 3 letter code for temporary pacing
46
VOO
- pace the ventricles
- no sensing
- no response to sensing
- not generally used as a single function but as a default function as reverts to this if interferences obscures sensing
47
AOO
- atrial pacing
- no sensing
- no response to sensing
- similar to VOO but for atria
48
VVI
- pacing of ventricles
- sensing of ventricle
- response inhibited
- if natural beat is sensed (natural R wave) => pacemaker is inhibited (ventricular inhibited)
- advantages = conserves battery power and avoids competitive rhythms
- disadvantages = potentially no AV synchrony so output may be low, no response to increased need
49
AAI
- atrial pacing
- atrial sensing
- inhibited
- same as VVI but for atria
50
VVT
- ventricular pacing & sensing
- rarely used now as battery continually triggered wasting it
- ventricle triggered if there is a QRS
51
DDD
- atrium and ventricles both paced and sensed
- pacing inhibited in atrium if sense ventricular or atrial activity
- pacing inhibited in ventricle by sensed ventricular activity
- ventricular pacing triggered by sensing atrial activity
52
Rate adapting pacemakers
Use activity sensors to determine rate of pacing
- activity and accelerometer sensors
- minute ventilation sensor
- stimulus (T of QT interval sensor)
- peak endocardial acceleration sensor
- right ventricular impedance based sensor
Clinical Measurements
flashcards
Decks in class (11)
# Cards
-
Week 1 - Intro, Taking Measurements, Signal & Noise79
-
Cardiovascular and Cardiac Output Measurements92
-
Respiratory Measurements65
-
Biopotential, Motor & Sensory Function41
-
Neurophysiological Disorders30
-
Electroencephalography26
-
Electroneurography16
-
ECG39
-
Pacing52
-
Defibrillators51
-
Gait Analysis34
