Physics of pacing - csnlc.nhs.uk

NASPE TRAININGLauren ButlerLancashire & South Cumbria Cardiac Network

PHYSICS OF PACING

Objectives

Pacing stimulationVariations stimulation thresholdSensingLead designPulse generatorsCalculations !!

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 – strength-duration threshold curve

Strength-Duration Curve

Pulse width (ms)

Threshold (v)

0 0.2 0.4 0.6 0.8 1.0 1.5

5

4

3

2

1

0

Rheobase

chronaxie

Rheobase – smallest voltage amplitude that stimulates the myocardium at an infinitely long pulse durationChronaxie – threshold pulse duration at twice the stimulus amplitude which is twice the rheobase voltageThe chronaxie approximates the lowest stimulation energy (microjoules) required for myocardial deploarisation

Strength-Duration Curve

Pulse width

Threshold

0 0.2 0.4 0.6 0.8 1.0 1.5

5

4

3

2

1

0

Rheobase

chronaxie

Energy (uJ)

Energy

Relationship between Energy, voltage, current and pulse duration Minimum threshold energy (chronaxie)

E = V2 x tR

0.03 0.5 5

voltscurrent

energy

Charge

Units

uC, uJ, v, mA

Constant voltage vs Constant current

All pacing systems – now operate constant voltageStrength duration curves of constant current are similar in shape but the current decline is more gradualHence 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 unknownWhat is that resultant current dependant upon?Impedance (resistance) of the system V = I x R

Impedance & Resistance

Impedance describes the impediment to current flow of electrons within the entire pacing systemAll factors that contribute to impedance include

Lead conductor resistanceElectrode resistancePolarisation 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 tIr = rheobase currenttc = 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

++++++

+

+++

+

+++

- - ---

--

-

----

-negative ions rush towards the positive ion layer

HPO4- &

OH-

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 stimulusThe longer the stimulus is present the greater the polarisationTo reduce polarisation

Reduce pulse width andUse 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 thresholdLarge surface area – which reduces polarisation

Finally

Polarisation is inversely related to surface areaTo 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 TipWire filament mesh (lazer bullet holes)Coating microspheresMicroscopic 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 coilHigh resistance at electrode / myocardial interface

Small radiusHigh surface areaHigh current density

Low polarisationShort pulse widthmaterials

Threshold Variations

Acute changesTypically rises rapidly within the first 24 hours and then gradually increases to a peak at 1 weekover the ensuing 6-8 weeks gradually

declines - reach level – chronic thresholdChronic 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 sizeShapeDesignInterface between lead & myocardiumIndividual variation

Stable electrode-myocardial interface

Passive Vs ActiveActive leads have higher threshold at implant but frequently reduces between 15 – 30 minutes post placementDue to hyperacute injury due to advancement of screw

Cortico-Steroid eluting electrodes reduce acute & chronic thresholds (available passive and active leads)

Cellular changesAcute injury to cellular membranesDevelopment myocardial oedemaElectrode surface coated with platelets & fibrinSubsequent 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 macrophagesFinally – 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 dayIncreases 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 timeDespite initial satisfactory placementOften occurs in parallel within atria and ventriclesOften recurs with further placement of new leadsSteroid-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 depolarisationAs 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 massFourier transformation determines frequency

densityAtrial 80 – 100 HzVentricular 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

Voltage

(ΔV)

IEGM – Slew rate

Slew rate = ΔV/ Δt(Volts/second)

Time(Δt)

R wave amplitude chronic = 85% R wave amplitude acute

Slew rate (V) chronic = 50%–60% slew rate (V) acute

IEGM – slew ratePeak slope of developing EGM Represents maximal rate of change of the electrical potential between the two sensing electrodesWhat should the slew rate be?Slew rate > 0.5 v/sec in both chambersHigh slew = high frequency content = increased chance sensingSlow 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 – AfterpotentialsAfterpotentials if sensed – inhibition or delay subsequent pacing stimulusAmplitude 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)

Leading

edgeTrailing

edge

Afterpotential – opposite polarity to stimulus

Constant voltage pulse with leading & trailing 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 implantAfter 6 – 8 weeks, approach implant levelsActive fixation leads – marked decrease immediately after implant which increases within 20 – 30 minutesCortico-steroid eluting leads have little effect on measurements

Source and Input impedancesSource impedance?The sensing circuit of the system also has impedanceSource impedance – voltage drop that occurs from origin of IEGM to proximal portion of lead

Electrode myocardial resistanceLead conductor resistanceEffects of polarisation

Source & Input impedancesInput Impedance?Impedance of sensing amplifier itselfEGM 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 occursInput impedances are large Source impedances – typically 400 - 1500ΩImpedance mismatch – clinically due to insulation or conductor failure – under or oversensing

Lead Design

ComponentsElectrode, conductor, insulator, connector pin

Mechanically stable & flexible in vivoSatisfactory electrical conductive and resistive propertiesDurable insulation with low friction co-efficient but high tensile strengthGood mechanical contact/grip between electrode & myocardium

Sensing circuitry

Incorporates noise reversion circuitsReverts to fixed rate pacing when rate of noise exceeds the noise reversion rateIncorporate 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 areaElectrode shape, surface compositionAlso – biologically inert, resist degradation, do not elicit marked tissue reaction at myocardial interfaceMaterials – reduce polarisation whilst achieving the above

Metals for electrode use (or not)

Toxic reactionsZinc, copper, mercury, nickel, lead, silver

Susceptible to corrosionStainless steel alloys

Surface coating oxides impedes current transfer

Titanium & tantalium

Metals for electrode useIn use today

Platinum-iridiumElgiloy (alloy cobalt, nickel, chromium, molybdenum, iron & manganese) !Platinised titanium coated platinumPyrolytic carbon coated titaniumPyrolytic carbon coated graphiteIridium oxideplatinum

Activated carbon – least susceptible to erosion, the activation process increases surface area and allows for tissue ingrowth *

Lead fixation

Passive typesFins, fines, tines

Most active fixation screws are now electrically active forming the cathode itself, although some are inactiveSteroid 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 failureFixed active screws – difficult to pass down veins, apply torque as the lead passes down vein and through tricuspid valveRemember different types

Screws, hooks, barbs

Lead conductor

Multifilament design – to facilitate high tensile strength and reduce metal fatigueOther types include – unifilar and cable designsBipolar 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 materialsAlloy 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 InsulationPolyurethane Vs Silicone

Polyurethane polymers with widest usePellathane 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 failureIn contact with silver chloride conductors –oxidative stress may occur causing failure of insulation from insidePolyurethane – easier to move

Advantages polyurethane

High tear strengthHigh cut resistanceLow friction in bloodHigh abrasion resistanceLess thrombogenicSuperior compressive properties

Advantages Silicone

FlexibleGood performance record > 30 yrsEasy fabrication & moulding

Disadvantages Polyurethane

Relatively stiffSensitive to manufacturing processesPotential – environmental stress cracking (Pellathane 80A)Potential MIO (Pellathane 80A & 55D)

Disadvantages Silicone

Tears easilyCuts EasilyHigher friction in bloodMore 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 IS13.2 mm diameter In-line bipolar configuration (not bifurcated)

Pulse Generator

Power sourceOutput circuitry (pacing)Input circuitry (sensing) Timing circuitHeader blockRR sensorTelemetry Microprocessor (storage diagnostics)

Power source

PreviouslyNuclear, photoelectric cell, rechargeable nickel-cadmium cell & biogalvanic energy

NowLithium-IodineLithium – anode & provides electronsIodine – cathode & recieves the electronsAnode & 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 compositionLithium Iodine

BOL = 2.8vERI = 2.4v (90 % has been used)Exponentially decreases until EOL is reachedEOL = 1.8v

Longevity – contributing factors

Chemical composition of batterySize of batteryExternal load

Pulse amplitude, duration, stimulation frequency, total pacing lead impedance, amount current required to operate circuitry & store diagnostics

Amount – internal dischargeVoltage decay characteristics

Longevity

Longevity (years) = Ampere-hours x 114current drain

Finally - calculations

Ohms Law V= I x REnergy E = V2/R x tConstant current stimulation (Rheobase/chronaxie)Slew Rate = Δv/ΔtLongevity = ampere-hrs/current drain x 114TO DO

Other energy equationsNoise sampling equation

And Finally !

E = V2/R x tE = VITE = I2RTNoise sampling Period (NSP) – Convert to Hz (frequency per second) to enable trigger responseHz = 1/t (ms) eg.NSP = 125ms, Hz = 1000/125 = 8Hz