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Transcript
PHYSICS OF PACING
NASPE TRAINING
Lauren Butler
Lancashire & South Cumbria
Cardiac Network
Objectives
Pacing stimulation
Variations stimulation threshold
Sensing
Lead design
Pulse generators
Calculations !!
Strength–Duration Threshold
Curve
Stimulation threshold definition
Minimum amount of energy required to
produce depolarisation of the myocardium
Exponential relationship exists between pulse
duration and stimulus amplitude – strengthduration threshold curve
Strength-Duration Curve
Threshold (v)
5
4
3
2
1
Rheobase
0
0 0.2 0.4 0.6 0.8 1.0
chronaxie
Pulse width (ms)
1.5
Rheobase – smallest voltage amplitude that
stimulates the myocardium at an infinitely
long pulse duration
Chronaxie – threshold pulse duration at twice
the stimulus amplitude which is twice the
rheobase voltage
The chronaxie approximates the lowest
stimulation energy (microjoules) required for
myocardial deploarisation
Strength-Duration Curve
Threshold
5
Energy
(uJ)
4
3
2
1
Rheobase
0
0 0.2 0.4 0.6 0.8 1.0
chronaxie
Pulse width
1.5
Energy
Relationship between Energy, voltage,
current and pulse duration
Minimum threshold energy (chronaxie)
Charge
E = V2 x t
energy
R
volts
current
Units
uC, uJ, v, mA
0.03
0.5
5
Constant voltage vs Constant current
All pacing systems – now operate constant
voltage
Strength duration curves of constant current
are similar in shape but the current decline is
more gradual
Hence chronaxie tends to be at a higher pulse
width ∴ lowest energy requirements are
greater in a constant current device
Constant voltage – programmed to
deliver 5 volts, resultant current is
unknown
What is that resultant current
dependant upon?
Impedance (resistance) of the system
V=IxR
Impedance & Resistance
Impedance describes the impediment to
current flow of electrons within the entire
pacing system
All factors that contribute to impedance
include
„
„
„
Lead conductor resistance
Electrode resistance
Polarisation resistance
In constant voltage systems the higher the
pacing impedance the lower the current flow
Constant current stimulation !
Calculation to calculate threshold
current
I = Ir x (1 + tc/t)
I = threshold current at pulse duration t
Ir = rheobase current
tc = chronaxie pulse duration
Ideally
Lead conductor material – low
resistance (to minimize heat wastage
and therefore energy loss)
Electrode – high resistance (to minimize
current flow)
Polarisation – low
Polarisation
Layers of oppositely charged ions that
surround the electrode during the pulse
stimulus
+
+
+
+
+
+
NA+ & H3O+
IONS
Polarisation Layers
++- - ++ + ++++
++
+
++
--
HPO4- &
OHnegative
ions rush
towards
the
positive
ion layer
Polarisation impedes the movement of charge from electrode
to myocardium, thus requiring a greater voltage to stimulate
and depolarise myocardial tissue
Polarisation = increase voltage
threshold
Polarisation layers build up as the pulse
stimulus is present and reaches a peak
torwards the end of the stimulus
The longer the stimulus is present the greater
the polarisation
To reduce polarisation
Reduce pulse width and
„ Use materials which discourage polarisation
(platinum black, irridium oxide, titanium nitride,
activated carbon)
„
Ideally
Lead conductor material – low
resistance (to minimize heat wastage
and therefore energy loss)
Electrode – high resistance (to minimize
current flow)
Polarisation – low
Ideal electrode tip
High resistance & ∴ low current drain
Small radius – increases current density
and in doing so reduces voltage
threshold
Large surface area – which reduces
polarisation
Finally
Polarisation is inversely related to surface
area
To maximize surface area (to reduce
polarisation) and minimize the radius (to
increase electrode impedance) construction of
electrodes consist of a small radius with an
irregular surface made out of porous,
polarisation reducing material
Ideal electrode tip
Target Tip
Wire filament mesh (lazer bullet holes)
Coating microspheres
Microscopic pores
Uneven surface creates hot spots of
increased current density whilst keeping the
surface area high and the overall radius low
Summary – pacing impedance
Low resistance conductor coil
High resistance at electrode / myocardial
interface
„
„
„
Small radius
High surface area
High current density
Low polarisation
„
„
Short pulse width
materials
Threshold Variations
Acute changes
„
„
„
Typically rises rapidly within the first 24
hours and then gradually increases to a
peak at 1 week
over the ensuing 6-8 weeks gradually
declines - reach level – chronic threshold
Chronic threshold will be higher than the
implantation measurement but less than
the peak
Threshold Variations
Magnitude & duration of increase in
threshold may be due to
„
„
„
„
„
Lead electrode size
Shape
Design
Interface between lead & myocardium
Individual variation
Stable electrode-myocardial
interface
Passive Vs Active
„
„
Active leads have higher threshold at
implant but frequently reduces between 15
– 30 minutes post placement
Due to hyperacute injury due to
advancement of screw
Cortico-Steroid eluting electrodes
reduce acute & chronic thresholds
(available passive and active leads)
Cellular changes
Acute injury to cellular membranes
Development myocardial oedema
Electrode surface coated with platelets & fibrin
Subsequent release chemotactic factors
Acute inflammatory reaction – mononuclear cells &
polymorphonuclear leukocytes
Following acute response – accelerated cellular injury
due to release of proteolytic enzymes & oxygen free
radicals by invading macrophages
Finally – fibroblasts within myocardium produce
collagen creating fibrous capsule around electrode tip
Fibrous capsule – increase in electrode
radius with a possible decrease in
electrode surface area
Other factors
„
„
Increase threshold during sleep, reduces
during day
Increases with hyperglycemia, hypoxia,
acute viral illness, after eating, electrolyte
fluctuations, drugs
Remember the threshold may also
increase at fast pacing rates (short
cycle lengths) as the pacing stimulus
encroaches into the refractory period of
the preceeding beat
Exit Block
Progressive rise in threshold over time
Despite initial satisfactory placement
Often occurs in parallel within atria and
ventricles
Often recurs with further placement of new
leads
Steroid-eluting electrodes prevent exit block
in most but not all patients
(type steroid – usually 1mg dexamethasone
sodium)
Sensing
Wavefront electrical activity approaches
electrode – which creates a positive
deflection on IEGM as electrode tip
becomes positive in relation to negative
region of depolarisation
As wavefront passes tip - large negative
deflection (called intrinsic deflection)
Smaller waveforms pre & post intrinsic
deflection due to depolarisation of
surrounding myocardium
Ventricular IEGM’s are larger than Atrial
IEGM’s – muscle mass
Fourier transformation determines frequency
density
„
„
Atrial 80 – 100 Hz
Ventricular 10 – 30 Hz
This allows filter systems to be incorporated
into sensing circuits of pacemakers to
enhance sensing – myopotentials overlap 10
– 200Hz (Unipolar sensing!)
Blanking & refractory periods have helped
IEGM – Slew rate
Time
(Δt)
Voltage
(ΔV)
Slew rate = ΔV/ Δt
(Volts/second)
R wave amplitude
chronic = 85% R
wave amplitude acute
Slew rate (V) chronic =
50%–60% slew rate (V)
acute
IEGM – slew rate
Peak slope of developing EGM
Represents maximal rate of change of the
electrical potential between the two sensing
electrodes
What should the slew rate be?
Slew rate > 0.5 v/sec in both chambers
High slew = high frequency content =
increased chance sensing
Slow broad signals (T waves) low slew rate,
low frequency content = less likely to be
sensed
Polarisation again!
After termination of pacing stimulus – an
excess of positive charge surrounds the
cathode which gradually decays until neutral
– Afterpotentials
Afterpotentials if sensed – inhibition or delay
subsequent pacing stimulus
Amplitude afterpotentials – related to size
and pulse duration of pulse
„
High PW, High Output, Maximum sensitivivity
= not good
Blanking periods reduce this – if not cross
talk (highlighted in unipolar sensing systems)
Constant voltage pulse with leading & trailing edge
Afterpotential – opposite
polarity to stimulus
Leading
Trailing
edge
edge
Remember – devices with autocapture may be at risk of
inappropriate sensing
Acute Vs Chronic sensing
Amplitude & slew rate may abruptly decline
within the 1st week post implant
After 6 – 8 weeks, approach implant levels
Active fixation leads – marked decrease
immediately after implant which increases
within 20 – 30 minutes
Cortico-steroid eluting leads have little effect
on measurements
Source and Input impedances
Source impedance?
The sensing circuit of the system also
has impedance
Source impedance – voltage drop that
occurs from origin of IEGM to proximal
portion of lead
„
„
„
Electrode myocardial resistance
Lead conductor resistance
Effects of polarisation
Source & Input impedances
Input Impedance?
Impedance of sensing amplifier itself
EGM actually seen by pacer – determined by
ratio between sensing amplifier (input
impedance) and the lead (source impedance)
The bigger the difference/ratio the less
attenuation of signal occurs
Input impedances are large
Source impedances – typically 400 - 1500Ω
Impedance mismatch – clinically due to
insulation or conductor failure – under or
oversensing
Lead Design
Components
„
Electrode, conductor, insulator, connector pin
Mechanically stable & flexible in vivo
Satisfactory electrical conductive and resistive
properties
Durable insulation with low friction coefficient but high tensile strength
Good mechanical contact/grip between
electrode & myocardium
Sensing circuitry
Incorporates noise reversion circuits
Reverts to fixed rate pacing when rate of
noise exceeds the noise reversion rate
Incorporate Zener diode – protects circuitry
from high voltage sources e.g. defibrillation
If voltage exceeds zener voltage, the excess
is shuntedback through the leads to the
myocardium and is dissipated
Electrode
Small radius, large surface area
Electrode shape, surface composition
Also – biologically inert, resist
degradation, do not elicit marked tissue
reaction at myocardial interface
Materials – reduce polarisation whilst
achieving the above
Metals for electrode use (or not)
Toxic reactions
„
Zinc, copper, mercury, nickel, lead, silver
Susceptible to corrosion
„
Stainless steel alloys
Surface coating oxides impedes current
transfer
„
Titanium & tantalium
Metals for electrode use
In use today
„
„
„
„
„
„
„
Platinum-iridium
Elgiloy (alloy cobalt, nickel, chromium,
molybdenum, iron & manganese) !
Platinised titanium coated platinum
Pyrolytic carbon coated titanium
Pyrolytic carbon coated graphite
Iridium oxide
platinum
Activated carbon – least susceptible to
erosion, the activation process increases
surface area and allows for tissue ingrowth *
Lead fixation
Passive types
„
Fins, fines, tines
Most active fixation screws are now
electrically active forming the cathode itself,
although some are inactive
Steroid eluting – both passive & active
Active fixation
„
Distorted anatomy, congenital defects, post
surgical (A lead), high right sided pressures, septal
placement
Active fixation mechanisms
Retractable – easier passage down
vasculature but mechanism has higher rate of
failure
Fixed active screws – difficult to pass down
veins, apply torque as the lead passes down
vein and through tricuspid valve
Remember different types
„
Screws, hooks, barbs
Lead conductor
Multifilament design – to facilitate high tensile
strength and reduce metal fatigue
Other types include – unifilar and cable
designs
Bipolar leads – parallel, co-axial (most
common), individually coated wires wrapped
in a single multifilar arrangement, mixture
Bipolar and Unipolar leads
Unipolar – smaller
diameter lead body
Bipolar - Co-axial
design
Conductor materials
Alloy MP35N
„
Cobalt, nickel, chronium, molybdenum
Elgiloy
„
Susceptible to corrosion as a lead, OK as an
electrode material
Nickel-silver – Drawn Brazed Strand (DBS)
has 6 nickel alloy wires drawn together with
heated silver
„
breaks down polyurethane due to MIO (Metal Ion
Oxidation) process – seen in Medtronic 6972
leads
Lead Insulation
Polyurethane Vs Silicone
Polyurethane polymers with widest use
„
Pellathane 80A and Pellathane 55d
BUT ** Pellathane 80A had a high failure rate
due to small insulation cracks appearing after
heating and cooling processes during
manufacture. With environmental stresses
the cracks deepened – insulation failure
In contact with silver chloride conductors –
oxidative stress may occur causing failure of
insulation from inside
Polyurethane – easier to move
Advantages polyurethane
High tear strength
High cut resistance
Low friction in blood
High abrasion resistance
Less thrombogenic
Superior compressive properties
Advantages Silicone
Flexible
Good performance record > 30 yrs
Easy fabrication & moulding
Disadvantages Polyurethane
Relatively stiff
Sensitive to manufacturing processes
Potential – environmental stress
cracking (Pellathane 80A)
Potential MIO (Pellathane 80A & 55D)
Disadvantages Silicone
Tears easily
Cuts Easily
Higher friction in blood
More thrombogenic
Some leads have silicone body with poyurethane
coating (Fast pass coating)
When implanting 2 leads – same vein – use same
material for easier use
Lead connector
Standardized IS1
3.2 mm diameter
In-line bipolar configuration (not
bifurcated)
Pulse Generator
Power source
Output circuitry (pacing)
Input circuitry (sensing)
Timing circuit
Header block
RR sensor
Telemetry
Microprocessor (storage diagnostics)
Power source
Previously
„
Nuclear, photoelectric cell, rechargeable nickelcadmium cell & biogalvanic energy
Now
„
„
„
„
„
Lithium-Iodine
Lithium – anode & provides electrons
Iodine – cathode & recieves the electrons
Anode & cathode separated by electrolyte which
serves as a conductor of ionic movement but as a
barrier to electron transfer
Circuit completed by external load (leads &
myocardium)
Battery Voltage
Battery voltage depends on chemical
composition
Lithium Iodine
„
„
„
„
BOL = 2.8v
ERI = 2.4v (90 % has been used)
Exponentially decreases until EOL is
reached
EOL = 1.8v
Longevity – contributing factors
Chemical composition of battery
Size of battery
External load
„
Pulse amplitude, duration, stimulation
frequency, total pacing lead impedance,
amount current required to operate
circuitry & store diagnostics
Amount – internal discharge
Voltage decay characteristics
Longevity
Longevity (years) = Ampere-hours x 114
current drain
Finally - calculations
Ohms Law V= I x R
Energy E = V2/R x t
Constant current stimulation (Rheobase/chronaxie)
Slew Rate = Δv/Δt
Longevity = ampere-hrs/current drain x 114
TO DO
„
„
Other energy equations
Noise sampling equation
And Finally !
E = V2/R x t
E = VIT
E = I2RT
Noise sampling Period (NSP) – Convert
to Hz (frequency per second) to enable
trigger response
Hz = 1/t (ms)
eg.NSP = 125ms, Hz = 1000/125 = 8Hz