Device Concepts

Question 1

Capacitance

Answer 1

Storage of charge

Question 2

Inductance

Answer 2

Storage of current flow

Question 3

Ampere

Answer 3

Shortened to “amp”

-Unit of electric current (amount of electrical flow)
-Measures how much electricity is moving through the circuit

A (ampere) —> mA (milliamp) —> uA (microamp)

1 A = 1,000 mA
1 mA = 1,000 uA
1 A = 1,000,000 uA

Question 4

Charge

Answer 4

Electrical particles

Question 5

Ohm

Answer 5

Unit that measures electrical resistance

Ohm —> kOhm (kilo-ohm) —> MOhm (mega-ohm)

1 Ohm = 1,000 kOhms
1 kOhm = 1,000 MOhms
1 MOhm = 1,000,000 Ohms

Question 6

Volt

Answer 6

-Electrical pressure
-The force that pushes electricity through a wire
-The electrical pressure used to stimulate the heart muscle

Question 7

Hertz

Answer 7

Measures how often something repeats in 1 second

1 Hz = 1 cycle per second

1 Hz = 60 beats per minute

Question 8

Capacitor reforming

Answer 8

-Electrolytic capacitors develop relatively large leakage currents over time and this can be reduced by recharging/reforming the capacitor
-Failure to reform with sufficient frequency can cause significant delays in the first shock during therapy delivery (subsequent therapies are fine)
-ICDs perform automatic capacitors reformations periodically
-It’s a charge-discharge cycle

Question 9

Resistor

Answer 9

-Component that slows down the flow of electricity
-Limits current
-Creates resistance
-Helps control current delivered to the heart and energy used from the battery

Question 10

Diode (Zener diode)

Answer 10

-Designed to protect the circuitry from high external voltages (like what occurs with defibrillation)
-When the input voltage presented to the pacemaker exceeds the Zener voltage, the excess voltage is shunted back through the leads to the myocardium

Question 11

Impedance

Answer 11

-Term applied to the resistance to current flow in the pacing system
-Implies inclusion of all factors that contribute to current flow impediment
-Note that resistance is technically not interchangeable as resistance doesn’t include the effects of storage of charge or storage of current flow

Measured in ohms

Normal: 300-1200
Lead fracture (conductor failure): >2000
Loose set screw: abnormally high impedances
Insulation break: <200

Normal shock impedance: 25-100 ohms

Question 12

Ohm’s law

Answer 12

V = I x R

Voltage = Current (amps or mA) x Resistance/Impedance (ohms)

-The lower the pacing impedance, the greater the current flow
-The higher the pacing impedance, the lower the current flow

Question 13

Voltage

Answer 13

Pacing output strength

Measured in volts

V (voltage in volts)= I (current in amps or mA) x R (resistance/impedance in ohms)

Question 14

Current

Answer 14

Flow of electricity delivered to the myocardium

Measured in amps or mA

I (current in amps or mA) = V (voltage in volts) / R (resistance or impedance measured in ohms)

Question 15

Resistance

Answer 15

The opposition in the lead system

Measured in ohms

R (resistance or impedance in ohms) = V (voltage measured in volts) / I (current measured in amps or mA)

Question 16

Watt

Answer 16

-Total energy being used
-Measurement of power

Watts = Volts x Amps

Question 17

Farad

Answer 17

Unit used to measure capacitance

Most medical device capacitors are in microfarads (uF)

How much electricity a capacitor can hold

(If a capacitor is a bucket, the Farad is the size of the bucket)

Question 18

Slew rate

Answer 18

-How fast voltage can change
-Represents the maximal rate of change of an electrical potential between the sensing electrodes

-Measured in volts per second
-Slew rate (V/s) = Voltage (in mV) / time (in ms)

-Should be > 0.5 V/s

-The higher the frequency content the higher the slew rate and the more likely the signal will be sensed
-The slow broad signals (T wave) have a lower slew rate and lower frequency density and are less likely to be sensed

Question 19

Amplitude

Answer 19

-The difference in voltage recorded between 2 electrodes
-Measured in mV

Normal ranges:
Atrial: 1.5-5 mV
Ventricular: 5-25 mV

Question 20

Sensing amplifier

Answer 20

-ICDs must amplify ventricular EGMs 10 times more than brady devices that use fixed gain sensing (because of amplitude of VF signals)
-Each sensing cycle beginning after the end of the blanking periods starts at high sensing threshold (least sensitive) and increases until new depolarization is reached or predetermined minimum threshold has been reached
-Sensitivity is minimal immediately after a depolarization and maximal late in diastole

Question 21

Afterpotentials

Answer 21

-The excess of positive charge surrounding the cathode after termination of the pulse stimulus
-Most likely to be sensed when programmed to high voltage and long pulse duration in combination with maximal sensitivity
-Refractory and blanking periods have helped to prevent the pacemaker from reacting to afterpotentials
-Can cause crosstalk

Question 22

Far field

Answer 22

2 definitions:
-EGMs recorded between large, widely spaced electrodes, one of which is not in contact with the heart (example: shock electrodes)
-signals from a remote source recorded on an intracardiac bipole, such as a far field R wave recorded from an atrial bipole (Signals not generated by the tissue the lead electrode is in contact with)

Question 23

Sensing filters

Answer 23

Maximum frequency densities in sinus rhythm:
-Atrium: 80-100 Hz
-Ventricle: 10-30 Hz

Myopotential frequencies:
-10-200 Hz

High pass filter:
-Removes very low frequency signals (respiration artifact, motion artifact)
-Allows fast depolarization signals

Low pass filter:
-Removes very high frequency signals (electromagnetic interference, muscle noise)

Band pass filter:
-Allows only the frequency band typical for cardiac signals
-Filters out high and low frequency noise

Question 24

Cross talk

Answer 24

-Atrial afterpotentials of sufficient strength and duration to be sensed by the ventricular channel
-Atrial output pulse is detected by the ventricular sensing channel
-Can result in inappropriate ventricular inhibition
-Particularly seen in unipolar systems

Ways to mitigate this:
-Adjust ventricular blanking periods
-Ventricular safety pacing algorithms
-Reduce atrial output amplitude
-Adjust ventricular sensitivity settings

Question 25

Sensing

Answer 25

Measured amplitude of intrinsic cardiac electrograms as detected by the implanted device sensing circuitry Normal ranges: -Atrial 1.5-5 mV -Ventricular 5-25 mV

Question 26

Threshold

Answer 26

-Definition: minimal amount of energy required to cause successful depolarization of the myocardium in contact with the electrode (and propagation of that depolarization to surrounding myocardium) -At very small pulse widths = small changes result in significant changes on the pulse amplitude -At longer pulse widths = small changes have little effect on the pulse amplitude -May vary slightly with circadian pattern, increasing during sleep and decreasing during the day (reflection of autonomic tone changes?) -May rise after eating, hyperglycemia, hypoxemia, acute viral illness, or electrolyte fluctuations -Fluctuations are typically minimal except severe hypoxemia or electrolyte abnormalities can lead to loss of capture

Question 27

Myopotentials

Answer 27

Electrical signals produced by skeletal muscle activity -Examples: pectoral muscles, diaphragm, arm/shoulder muscles

Question 28

Electromagnetic interference (EMI)

Answer 28

-When external electrical or magnetic signals interfere with the sensing function of a pacemaker/ICD -Can cause oversensing, pacing inhibition, inappropriate ICD shocks, noise on intracardiac electrograms -Common sources: MRI, electocautery during surgery, arc welding, high voltage power lines, radiation therapy (can cause device reset, loss of telemetry, device failure), TENS unit

Question 29

Charge time

Answer 29

-Measure of both battery and capacitor function -An ICD charges it capacitor from the battery until it reaches the programmed shock voltage -Generally <10 seconds -Charge time increases when: battery ages, high shock energy is programmed, or internal resistance rises

Question 30

ERI

Answer 30

Measured in the unloaded state

Question 31

EOL

Answer 31

End of life -Voltage is the minimum loaded voltage -Recorded while the battery is maximally stressed Most devices lose telemetry and programming capabilities Devices frequently revert to a fixed high-output pacing mode to maintain patient safety (benefit of lithium based power cells in current devices)

Question 32

Exit block

Answer 32

-When the capture threshold exceeds the programmed output of the pacemaker and loss of capture occurs -Consequence of lead maturation, results from progressive rise in thresholds over time

Question 33

BOL

Answer 33

Beginning of life Cell generates approximately 2.8 V

Question 34

Chronaxie

Answer 34

The threshold pulse width duration at twice the rheobase voltage (Example: If rheobase is 1.1 V then chronaxie would be whatever the pulse width duration is at 2.2 V) Typical values for modern steroid eluting leads range from 0.24-0.5 ms -It approximates the point of minimum threshold energy (microjoules) required for myocardial depolarization -Can be helpful in determine optimal pulse width -Programming the pulse width at or near chronaxie: -Achieves the most energy efficient pacing configuration -Maximizes battery longevity while maintaining reliable myocardial capture

Question 35

Rheobase

Answer 35

Minimum voltage (V) amplitude required to achieve myocardial capture at an infinitely long pulse duration (ms) on the strength duration curve Represents the theoretical lowest possible capture threshold regardless of pulse width

Question 36

NBG code

Answer 36

5 position standardized system to describe pacemaker programming modes Developed by the (N)orth american society of pacing and electrophysiology and (B)ritish pacing and electrophysiology (G)roup 1 - Chamber paced O (none). A (atrium). V (ventricle). D (dual). 2 - Chamber sensed O (none). A (atrium). V (ventricle). D (dual, A+V) 3 - Response to SENSED events O (none). T (triggered). I (inhibited). D (dual, T+I) 4 - Rate modulation capability O (none). P (simple programming). M (multi programmable). C (communicating) 5 - Multisite pacing capability O (none). A (atrium). V (ventricle). D (dual, A+V)

Question 37

Signal related undersensing

Answer 37

Failure to sense a depolarization signal due to insufficient amplitude or frequency

Question 38

Functional undersensing

Answer 38

-Failure to sense signals that have sufficient amplitude and frequency -Most commonly occurs when sensed events time in a device blanking or refractory period

Question 39

Near field

Answer 39

Recorded from closely spaced bipoles in contact with the source myocardium