# Basic Pacing Concepts Part II

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Basic Pacing Concepts Part II
Welcome to Basic Pacing Concepts, a course module in CorePace. The Basic Pacing module addresses concepts such as pacing system components, stimulation, sensing, EMI, and rate response.

Electrical Concepts

Every Electrical Pacing Circuit Has the Following Characteristics:
Voltage Current Impedance The terms resistance and impedance are used interchangeably in pacing (unless engineers are talking!).

Voltage Voltage is the force or “push” that causes electrons to move through a circuit In a pacing system, voltage is: Measured in volts Represented by the letter “V” Provided by the pacemaker battery Often referred to as amplitude The terms “amplitude” and “voltage” are often used interchangeably, undoubtedly to express “Voltage Amplitude” which refers to the voltage output.

Current The flow of electrons in a completed circuit
In a pacing system, current is: Measured in mA (milliamps) Represented by the letter “I” Determined by the amount of electrons that move through a circuit Current: Measured in amperes (I) 1 Ampere =1000 milliamps Movement of electricity or free electrons through a circuit One ampere is a unit of electrical current produced by 1 volt acting through a resistance of 1 ohm

Impedance The opposition to current flow
In a pacing system, impedance is: Measured in ohms Represented by the letter “R” (W for numerical values) The measurement of the sum of all resistance to the flow of current Impedance is the sum of all resistance to the flow of current. The resistive factors to a pacing system include: Lead conductor resistance The resistance to current flow from the electrode to the myocardium Polarization impedance, which is the accumulation of charges of opposite polarity in the myocardium at the electrode-tissue interface. Resistance is a term used to refer to simple electric circuits without capacitors and with constant voltage and current. Impedance is a term used to describe more complex circuits with capacitors and with varying voltage and current. Therefore, the use of the term impedance is more appropriate than resistance when discussing pacing circuits.

Voltage, Current, and Impedance Are Interdependent
The interrelationship of the three components can be likened to the flow of water through a hose Voltage represents the force with which . . . Current (water) is delivered through . . . A hose, or lead, where each component represents the total impedance: The nozzle, representing the electrode The tubing, representing the lead wire

Voltage and Current Flow
Spigot (voltage) turned up (high current drain) Using the garden hose as an analogy, the higher the voltage, the greater the push, or “flow” of electrons (and the greater the current drain). Spigot (voltage) turned low (low current drain)

Resistance and Current Flow
“Normal” resistance “Low” resistance Resistance affects current flow. Leads with an insulation breach, such as the garden hose pictured in the middle, will measure a low resistance reading with a resultant high current flow, and possible premature battery depletion. Conversely, if there is a high resistance, such as a lead conductor break, the current flow will be low or non-existent. High current flow “High” resistance Low current flow

Ohm’s Law is a Fundamental Principle of Pacing That:
Describes the relationship between voltage, current, and resistance V I R V = I X R I = V / R R = V / I Can be expressed in three ways: V = I x R R = V ÷ I I = V÷ R If any two values are known, the third may be calculated (cover the value you are seeking and the others appear in the appropriate format to calculate the unknown value). x

When Using Ohm’s Law You Will Find That:
If you reduce the voltage by half, the current is also cut in half If you reduce the impedance by half, the current doubles If the impedance increases, the current decreases

Ohm’s Law Can Be Used to Find Amounts of Current Passing Through Pacemaker Circuitry
If: Voltage = 5 V Impedance = 500 W What will the current be? I = V/R I = 5 V ÷ 500 W = Amperes 0.010 x 1000 = 10 mA If: Voltage = 5 V Impedance = 500 ohms 1 Ampere = 1000 milliamps (mA) Then use the formula I = V ÷ R to find I (the current). The amount of current that flows through a pacemaker system is very small: Use milliamps as a unit of measurement rather than Amps. Convert amps to milliamps by multiplying amps by 1000 or move the decimal three places to the right.

In This Example, the Voltage is Halved
If: Voltage = 2.5 V Impedance = 500 W Current = ? I = V/R V = 2.5 V ÷ 500 W = Amperes 0.005 x 1000 = 5 mA If: Voltage = 2.5 V Impedance = 500 ohms 1 Ampere = 1000 milliamps Then use the formula I = V ÷ R to find I (the current). The amount of current that flows through a pacemaker system is very small: Use milliamps as a unit of measurement rather than amps. Convert Amps to milliamps by multiplying Amps by 1000 or move the decimal three places to the right.

In This Example, the Impedance is Reduced By Half
If: Voltage = 5 V Impedance = 250 W Current = ? I = V/R I = 5 V ÷ 250 W = Amperes 0.020 x 1000 = 20 mA If: Voltage = 5 V Impedance = 250 ohms 1 Ampere = 1000 milliamps Then use the formula I = V ÷ R to find I (the current). The amount of current that flows through a pacemaker system is very small: Use milliamps as a unit of measurement rather than Amps. Convert Amps to milliamps by multiplying Amps by 1000 or move the decimal three places to the right.

Impedance Changes Affect Pacemaker Function and Battery Longevity
High impedance reading reduces battery current drain and increases longevity Low impedance reading increases battery current drain and decreases longevity Impedance reading values range from 300 to 1,000 W High impedance leads will show impedance reading values greater than 1,000 ohms

Lead Impedance Values Will Change Due to:
Insulation breaks Wire fractures

An Insulation Break Around the Lead Wire Can Cause Impedance Values to Fall
Insulation breaks expose the wire to body fluids which have a low resistance and cause impedance values to fall Current drains through the insulation break into the body which depletes the battery An insulation break can cause impedance values to fall below 300 W Insulation break Decreased resistance Insulation around the lead wire prevents current loss from the lead wire. Electrical current seeks the path of least resistance. An insulation break that exposes wire to body fluids which have low resistance causes: Lead impedance to fall Current to drain into the body Battery depletion Impedance values below 300 W. Insulation breaks are often marked by a trend of falling impedance values. An impedance reading that changes suddenly or one that is >30% is considered significant and should be watched closely.

Impedance values across a break in the wire will increase
A Wire Fracture Within the Insulating Sheath May Cause Impedance Values to Rise Impedance values across a break in the wire will increase Current flow may be too low to be effective Impedance values may exceed 3,000 W Lead wire fracture Increased resistance Insulation may remain intact but the wire may break within the insulating sheath. Impedance may exceed 3,000 W. Current flow may be too low to be effective. If a complete fracture of the wire occurs: No current will flow Impedance number will be “infinite” When suspecting a wire break, look for a trend in an increase in impedance values rather than a single lead impedance value.

Stimulation

Transmembrane Potential
Stimulation Process -50 50 -100 Phase 1 Phase 2 Phase 0 Transmembrane Potential (Millivolts) Phase 3 Threshold The stimulation process can be described in “phases:” The output voltage produces an electrical field at the electrode-tissue interface. The electrical field permeates cardiac cells via ionic movement and changes voltage on the cell membrane, which brings the cell membrane “above threshold” and alters its permeability. Phase 0 is the result of this part of the process. During Phase 0, sodium rushes in, which results in depolarization followed by cellular repolarization via sodium/potassium infusion. NOTE: The electrical field generated by the stimulation pulse must last long enough to excite the tissue. To effectively raise the membrane potential, the intensity of the stimulation must be balanced with the length of time it is applied. Phase 4 Time (Milliseconds) 100 200 300 400 500

Stimulation Threshold
The minimum electrical stimulus needed to consistently capture the heart outside of the heart’s refractory period Capture Non-Capture VVI / 60

Two Settings Are Used to Ensure Capture:
Amplitude Pulse width Remind audience that amplitude had been discussed when voltage was described.

Amplitude is the Amount of Voltage Delivered to the Heart By the Pacemaker
Amplitude reflects the strength or height of the impulse: The amplitude of the impulse must be large enough to cause depolarization ( i.e., to “capture” the heart) The amplitude of the impulse must be sufficient to provide an appropriate pacing safety margin

Pulse Width Is the Time (Duration) of the Pacing Pulse
Pulse width is expressed in milliseconds (ms) The pulse width must be long enough for depolarization to disperse to the surrounding tissue 5 V The greater the pulse width, the shorter the battery longevity. An increase in pulse width leads to an increase in total energy delivered. The longer the duration of the stimulus, the lower the amplitude required to capture the heart 0.5 ms 0.25 ms 1.0 ms

The Strength-Duration Curve
The strength-duration curve illustrates the relationship of amplitude and pulse width Values on or above the curve will result in capture .50 1.0 1.5 2.0 .25 Stimulation Threshold (Volts) Capture The strength-duration curve illustrates the tradeoff of amplitude (intensity of stimulation, measured in volts) and duration (the length of time the stimulation is applied, measured in milliseconds). Capture occurs when the stimulus causes the tissue to react. On the graph, capture occurs on or above the curve. Anything below the curve will not capture. The lowest voltage on a stimulation curve which still results in capture at an infinitely long pulse duration is called rheobase. A voltage programmed below is inefficient and will lead to non-capture. The pulse duration at twice the rheobase value is defined as the chronaxie. Chronaxie time is typically considered most efficient in terms of battery consumption. 0.5 1.0 1.5 Duration Pulse Width (ms)

Clinical Usefulness of the Strength-Duration Curve
Adequate safety margins must be achieved due to: Acute or chronic pacing system Daily fluctuations in threshold 2.0 1.5 Stimulation Threshold (Volts) 1.0 Capture .50 Safety margins have traditionally been selected as follows, when a threshold is determined by decrementing the pulse width at a fixed voltage: At a given voltage where the pulse width value is < .30 ms: Tripling the pulse width will provide a two-time voltage safety margin. Pulse widths at a given voltage value which are >.30 ms are not typically selected, because they are less efficient (expend more energy), while not providing further safety. In this case, the voltage should be doubled to provide a two-time safety margin. Adequate safety margins must be selected due to daily fluctuations in threshold that can occur due to eating, sleeping, exercise, or other factors that affect thresholds. Also, a patient with an acute pacing system will typically be programmed to a setting allowing a higher safety margin, due to the lead maturation process, which can occur within the first 6-8 weeks following implant. .25 0.5 1.0 1.5 Duration Pulse Width (ms)

After Patient Safety, the Second Most Important Goal in Programming is to Extend Battery Life
The best way to extend the service life of a battery is to lower voltage settings while maintaining adequate safety margins Amplitude values greater than the cell capacity of the pacemaker battery require a voltage multiplier, resulting in decreased battery longevity Most lithium-iodine batteries (those used most often in pacemakers today), the battery voltage is 2.8 V at the beginning of battery life. If an output is programmed above this setting (for example, to 5 V), a voltage multiplier must be used to achieve the higher amplitude. The impact on battery longevity is significant. Using two capacitors can reduce longevity by as much as half.

Factors That Affect Battery Longevity Include:
Lead impedance Amplitude and pulse width setting Percentage paced vs. intrinsic events Rate responsive modes programmed “ON”

Electrode Design May Also Impact Stimulation Thresholds

Fibrotic “capsule” develops around the electrode following lead implantation There are three phases that make up the lead maturation process: The acute phase, where thresholds immediately following implant are low The peaking phase, where thresholds rise and reach their highest point, usually around one week post-implant; followed by a decline in the threshold over the next 6 to 8 weeks1 as the tissue reaction subsides. The chronic phase, where thresholds assume a stable reading to a level somewhat higher than that at implantation but less than the peak threshold.1 The lead maturation process occurs due to the trauma to cells surrounding the electrode, which causes edema and subsequent development of a fibrotic capsule. The inexcitable capsule reduces the current at the electrode interface, requiring more energy to capture the heart. 1Hayes DH et al. Cardiac Pacing and Defibrillation: A Clinical Approach. Armonk, NY: Futura Publishing Company; Page 7.

Steroid Eluting Leads Steroid eluting leads reduce the inflammatory process and thus exhibit little to no acute stimulation threshold peaking and low chronic thresholds Porous, platinized tip for steroid elution Steroid eluting leads reduce inflammation by employing a capsule of dexamethasone sodium phosphate, which gradually emits steroid over time, nearly eliminating the peaking phenomenon of the lead maturation process. Silicone rubber plug containing steroid Tines for stable fixation

Effect of Steroid on Stimulation Thresholds Implant Time (Weeks) Textured Metal Electrode Smooth Metal Electrode 1 2 3 4 5 Steroid-Eluting Electrode 7 8 9 10 11 12 Volts This graph compares the stimulation thresholds of contemporary pacing leads. Older electrodes exhibited higher threshold peaking than that of steroid leads shown on the slide. The different types of electrodes exhibit a wide range of threshold peaking. Steroid-eluting electrodes continue to show lower chronic stimulation thresholds and no significant peaking. Threshold changes are shown here over a 12-week period post-implant, where a comparison is made between: • smooth metal electrode • textured metal electrode • steroid-eluting electrode Traditionally, implant stimulation thresholds are relatively low. Non-steroid-eluting electrodes exhibit a peaking phase from week 1 to approximately week 6, due to the maturation process at the electrode-tissue interface. Steroid-eluting electrodes exhibit virtually no peaking. The chronic phase of stimulation threshold occurs 8-12 weeks post-implant which is characterized by a plateau. This plateau is higher than the acute phase, due to fibrotic encapsulation of the electrode. Steroid-eluting lead chronic thresholds remain close to implant values. 3 6 Pulse Width = 0.5 msec

General Medtronic Pacemaker Disclaimer
INDICATIONS Medtronic pacemakers are indicated for rate adaptive pacing in patients who may benefit from increased pacing rates concurrent with increases in activity (Thera, Thera-i, Prodigy, Preva and Medtronic.Kappa 700 Series) or increases in activity and/or minute ventilation (Medtronic.Kappa 400 Series). Medtronic pacemakers are also indicated for dual chamber and atrial tracking modes in patients who may benefit from maintenance of AV synchrony. Dual chamber modes are specifically indicated for treatment of conduction disorders that require restoration of both rate and AV synchrony, which include various degrees of AV block to maintain the atrial contribution to cardiac output and VVI intolerance (e.g., pacemaker syndrome) in the presence of persistent sinus rhythm. 9790 Programmer The Medtronic 9790 Programmers are portable, microprocessor based instruments used to program Medtronic implantable devices. 9462 The Model 9462 Remote Assistant™ is intended for use in combination with a Medtronic implantable pacemaker with Remote Assistant diagnostic capabilities. CONTRAINDICATIONS Medtronic pacemakers are contraindicated for the following applications: ·       Dual chamber atrial pacing in patients with chronic refractory atrial tachyarrhythmias. ·       Asynchronous pacing in the presence (or likelihood) of competitive paced and intrinsic rhythms. ·       Unipolar pacing for patients with an implanted cardioverter-defibrillator because it may cause unwanted delivery or inhibition of ICD therapy. ·       Medtronic.Kappa 400 Series pacemakers are contraindicated for use with epicardial leads and with abdominal implantation. WARNINGS/PRECAUTIONS Pacemaker patients should avoid sources of magnetic resonance imaging, diathermy, high sources of radiation, electrosurgical cautery, external defibrillation, lithotripsy, and radiofrequency ablation to avoid electrical reset of the device, inappropriate sensing and/or therapy. Operation of the Model 9462 Remote Assistant™ Cardiac Monitor near sources of electromagnetic interference, such as cellular phones, computer monitors, etc. may adversely affect the performance of this device. See the appropriate technical manual for detailed information regarding indications, contraindications, warnings, and precautions.  Caution: Federal law (U.S.A.) restricts this device to sale by or on the order of a physician.

Medtronic Leads For Indications, Contraindications, Warnings, and Precautions for Medtronic Leads, please refer to the appropriate Leads Technical Manual or call your local Medtronic Representative. Caution: Federal law restricts this device to sale by or on the order of a Physician. Note: This presentation is provided for general educational purposes only and should not be considered the exclusive source for this type of information. At all times, it is the professional responsibility of the practitioner to exercise independent clinical judgment in a particular situation.

Continued in Basic Pacing Concepts Part III