Lead technology

• What is the difference between unipolar and bipolar stimulation?

  Location of the anode

  • Unipolar = the anode is the case of the pulse generator
  • Bipolar = the anode is located on the pacing lead (2cm proximal)

  The cathode in both systems is at the lead tip in contact with the myocardium
• Why is the impedance and threshold slightly higher in bipolar systems?

  There are 2 conductors in one lead, compared to the single conductor unipolar lead
• How can you identify the difference between bipolar and unipolar pacing on an ECG?

  Pacing spikes!

  • Unipolar = BIG pacing spikes, useful for identifying pacing (mal)function
  • Bipolar = small pacing spikes, often difficult to see on ECG
• How does a unipolar circuit work?

  1. Impulse begins at generator (anode)
  2. Flows through the lead tip (cathode)
  3. Stimulates heart
  4. Returns through tissue and body fluid to anode
• What are the characteristics of a unipolar pacing lead?

  1. One electrode at tip
  2. One conductor coil
  3. Smaller in diameter than bipolar
  4. Lots of artefact!
• How does a bipolar circuit work?

  1. Impulse begins at generator
  2. Flows through lead tip (cathode)
  3. Stimulates heart
  4. Returns to ring electrode above lead tip (anode)
• What are the characteristics of a bipolar pacing lead?

  1. Creates a circuit between ring and tip electrode
  2. Two conductor coils
  3. Inner layer of insulation
  4. Less susceptible to oversensing of non-cardiac signals
• What type of lead is used as standard practice?

  Bipolar

  • Can still be reprogrammed in unipolar configuration when required but often has fewer issues than a genuine unipolar circuit
• What is an intracardiac electrogram (iECG)?

  Electrical signal from within the heart, produced by movement of electrical current through the myocardium

  • Pacing inhibition and shock delivery rely on iECG!
  • Able to distinguish between SVT and VT via presence or absence of relationship between atrium and ventricles
  • Also identifies FFRWS and skeletal muscle signals
• How does depolarisation relate to iECGs?

  • In depolarisation, the outside of the cell is electrically neutral with respect to inside
  • As wavefront of depolarisation travels TOWARDS endocardial electrode, it becomes positively charged
  • This manifests as a POSITIVE deflection
  • As wavefront of depolarisation travels UNDER electrode, cell becomes negatively charged
  • This manifests as a NEGATIVE deflection
  • The intrinsic deflection indicates the moment of activation under recording electrode
• Why is the ventricular electrogram bigger than the atrial?

  Greater mass of myocardium

  • Hence more prone to FFRW sensing
• What is Fourier transformation?


  Frequency spectrum expressed as a series of sine waves of varying frequency and amplitude

  • Maximum density for R waves = 10-30Hz
  • Filtering below 10Hz (low frequency) reduces force of T wave/FFRWS/myopotentials
  • For a signal to be sensed, it must be of sufficient amplitude and the intrinsic deflection must have sufficient slope
• What is the typical frequency for iECGs?

  30Hz (up to 40Hz)
• How does bipolar and unipolar sensing work?

  Measures the difference between 2 electrodes

  • Bipolar = interelectrode difference of 2-3cm - both electrode in heart
  • Unipolar = interelectrode difference of 30-50cm - one electrode in heart, one in pulse generator

  Both electrodes contribute to the sensed signal... lots of influence from non-cardiac signals
• What are the benefits of bipolar sensing?

  • Mostly immune to myopotentials... unless break in circuit
  • Less influenced by EMI (e.g. diathermy or cautery)
  • More sensitive to direction of depolarisation waveform, both electrodes are activated at the same time when wave is perpendicular to iECG (and so is often biphasic)
• Why can you not use unipolar leads in ICDs?

  ICDs are more sensitive and have short refractory periods to interrupt tachyarrhythmias and reduce risk of TWOS

  • Oversensing can lead to inappropriate shocks

  Must use bipolar leads!
• What is an afterdepolarisation?

  Following a paced beat, an after potential of opposite charge is induced in myocardium at the electrode tip

  • Positive ions surround electrode after a (negative) cathodal stimulation
  • The after potential can be inappropriately sensed = inhibition of next paced beat
  • Amplitude of after potential directly proportional to amplitude and duration of paced beat
  • Much more likely to occur at high output pacing and/or a long pulse width!
• What refractory period prevents the inappropriate sensing of afterpotentials?

  PVAB - prevents inhibition and/or triggering. Signal is blanked
• What is a dangerous complication of inappropriately sensing afterpotentials?

  Crosstalk

  • Inhibition of pacing
  • Signal may be sensed in the opposite chamber in DDD systems
• What is an injury current?

  ST segment elevation on iECG due to pressure exerted by distal electrode on myocardium

  • Occurs in both atrial and ventricular leads, in both active and passive fixation
  • The absence of an injury current usually indicates poor electrode contact with myocardium
  • Area of fibrotic or scarred myocardium?
  • Returns to isoelectric line shortly after (hours-days post implant)
  • Often much more intense in active fixation due to screw-in mechanism
• What are the 5 components of lead design?

  1. Electrode(s)
  2. Conductors (s)
  3. Insulation
  4. Connector pin
  5. Fixation mechanism
• Electrodes


  The smaller the radius of the electrode, the greater the current density

  • Resistance is higher with smaller electrodes
  • Enables constant voltage pacing with improved battery longevity
  • Sensing impedance and afterdepolarisation are decreased with larger surface area
  • The ideal pacing lead would have a SMALL radius (increased current density) and a LARGE surface area (decreased polarisation)
  • Solution = shape!
• What are the features of modern pacing leads?


  Small electrode tip with a complex surface structure

  • Screw shaped, helical, hooks and barbs used
  • Complex shapes produce an irregular pattern of current density at edges of lead tip, whilst covering a large surface area
• What is the surface structure of an early vs. modern pacing lead?


  Early = polished metal surface

  Modern = textured surface - allowing an increase in surface area without an increase in radius
• How can lead design reduce polarisation?

  • Pores on lead tip
  • Large (130 microns)
  • Microscopic (20-100 microns)
  • Coating
  • Elgiroy / platinum / iridium oxide
  • Carbon electrodes
  • Roughening of surface of lead tip, known as activation
• Sensing improved by porous electrode design, but threshold generally improved by active fixation and steroid eluting tips
• What is the chemical composition used in new pacing leads?

  
  

  Platinum-iridium / elgiroy / titanium coated with platinum or carbon (improved with activation)

  • Electrodes must be biologically inert and resistant to degradation, to minimise inflammation and fibrosis
  • Metals such as zinc, copper, mercury, lead, nickel and silver = toxic
  • Stainless steel = corrosion
  • Titanium requires a surface coating of oxides, which may impede charge transfer
  • However, coated with carbon or platinum = excellent long term!
• What electrodes do each manufacturer use?

  Medtronic = platinum iridium with polyurethane outer jacket

  Boston = iridium oxide

  Abbott = titanium nitrade
• What steroid is used in steroid eluting electrodes?

  Dexamethasone sodium phosphate

  
  

  Silicone core impregnated with dexamethasone, surrounded by a porous titanium electrode and coated with platinum
• What are the benefits of using steroid eluting electrodes?

  Reduces inflammation at myocardial surface

  • Controls chronic evolution of thresholds over time
  • Reduces risk of exit block
  • Reduces risk of calcification or fibrosis
  • Duration of drug elution not defined, but evidence of reduced myocardial inflammation for 10+ years!
• What are the characteristics of passive leads?

  • Tines
  • Trapped in trabeculae immediately upon correct positioning
  • Rapidly covered in fibrous tissue, very difficult to remove after 6 months - more difficult than active!
  • Minimises trauma to myocardium... smaller fibrous capsule = lower chronic threshold
  • Larger introducer (7F+) required due to increased diameter of lead
• What are the characteristics of active leads?

  • Screw helix
  • Most common = extendable-retractable design
  • Allows screw helix to extend from tip once through the vascular system and over tricuspid valve = less trauma
  • Especially useful in atrium, provides stable fixation even in AF or post CABG when atrial appendage is injured
  • Test in deployed and non-deployed at implant
• Why was the J-retention wire discontinued?

  Very easy to implant, but...

  • Caused metal fatigue from flexing duration cardiac motion
  • Frequent fractures
  • Increased risk of tamponade and perforation
• What is a lead conductor (DBS/drawn brazed strand)?


  Wire between electrode tip and connector pin

  • At 70bpm, heart contracts 36 million times per year = significant stress on lead!
  • Leads must flex with heartbeat and have longitudinal/transverse/rotational components
  • Fracture and metal fatigue can occur
  • Bipolar leads = COAXIAL design
• What is a coaxial design?


  Occurs in bipolar leads

  • Conductor coil to distal electrode is INSIDE the outer conductor coil
  • Inner coil connects to tip electrode
  • Outer coil connects to ring electrode
  • Sleeve of insulation separates the two coils
  • Can make lead less flexible than unipolar leads
  • Remember, unipolar leads only have a single conductor coil!
• What are the characteristics of silicone insulated leads?

  • Very flexible
  • Repairable
  • Easy to mould into shape
  • Tears and cuts easily
  • High friction in blood (this was an old exam question!!)
  • Absorbs lipids - prone to calcification and fibrosis
  • Larger diameter, requires bigger sheath
  • 30+ year history of success
• What are the characteristics of polyurethane insulated leads?

  • Stiff
  • Not repairable
  • Low friction in blood
  • High resistance to tears and cuts
  • Small diameter, smaller sheath required
  • Sensitive manufacturing process - cooling
  • Prone to cracking from environmental stress - suturing, stylet insertion, oxidisation
  • 10+ year history of success
• What insulation do the manufacturers use?


  Medtronic = polyurethane

  Boston = silicone

  Abbott = 'optim' design - hybrid of silicone and polyurethane
• What is the diameter of a standard IS-1 lead?

  3.2mm with no sealing rings in header
• Why were connector pins incompatible?


  No clinical standard

  • Mix of lead connectors and pulse generator headers
  • Evolved from 5.6mm to 3.2mm (IS-1)
  • Some old leads may not fit in all headers
  • E.g. old Medtronic leads require IS-1B with a long receptacle for lead terminal.