Stimulation

Question 1

True or False:

‘Rheobase is described as the lowest stimulation voltage amplitude at an infinitely long pulse duration’.

Answer 1

True

Question 2

True or False:

‘Rheobase = Threshold Voltage at Pulse Width of 2.0ms’.

Answer 2

True

Strength Duration Curve is essentially flat at a pulse width of 2.0ms.

Rheobase = Measured Threshold @ 2.0ms.

Question 3

True / False

‘Chronaxie is described as the lowest stimulation voltage at an infinitely long pulse duration’.

Answer 3

False (That’s the Rheobase definition).

Chronaxie is pulse width at double rheobase voltage Threshold pulse duration at twice the rheobase voltage amplitude (i.e. pulse duration at double the voltage threshold).

Question 4

The following statement best describes what?

‘Approximates the point of minimum threshold stimulation energy (micro Joules/μJ)’

Answer 4

The Chronaxie.

Question 5

Are modern pulse generators constant voltage or constant current generators?

Answer 5

Constant voltage.

Internal resistance is very low compared with the load resistance it is giving power to.

Question 6

Define strength duration curve differences between constant voltage and constant current generators.

Answer 6

Constant current = current decline is more gradual.

Constant voltage = current decline is more abrupt.

Question 7

The following statement best describes what?

‘Stimulation thresholds measured by decrementing the stimulus voltage until loss of capture are usually 0.1-0.2V lower than when the stimulus voltage is gradually increased from sub-threshold until capture is achieved’.

Answer 7

The Wedensky effect

Question 8

Which equation calculates Stimulation Energy Consumption of a device circuit?

Answer 8

E = V2 / R x t

E = Energy / Joules / μJ
V = Voltage / Volts / V
R = Resistance / Ohms
T = Time / Pulse Width / Msec

Question 9

Doubling the pulse duration does what to the respondent stimulation energy?

Answer 9

Results in a two-fold increase in stimulation energy.

Question 10

Doubling the stimulation voltage does what to the respondent stimulation energy?

Answer 10

Results in a four-fold increase in stimulation energy.

Question 11

True / False

‘Longer pulse durations than necessary don’t result in a wastage of stimulation energy without providing a substantial increase in output safety margin’.

Answer 11

False.

Longer pulse durations than necessary will waste energy.

Question 12

In CRT systems, is the RV electrode considered the anode or cathode?

Answer 12

RV Electrode is the Anode (Positive).

Question 13

In CRT systems, is the LV the anode or cathode?

Answer 13

LV Electrode is the Cathode (Negative).

Question 14

With respect to CRT, list 3 reasons why Anodal capture is undesirable.

Answer 14

1. Earlier activation of the RV
2. Not ideal in patients who require a desirable offset between the LV and RV
3. May prove deleterious to biventricular

Question 15

True / False

‘Auto threshold testing can use evoked response to determine capture/non-capture’.

Answer 15

True.

Commonly evoked response is used solely or in conjunction with other variables.

Question 16

The following statement best describes what?

‘Electrical event that results from myocardial capture after an output pulse’.

Answer 16

Evoked Response.

Question 17

Evoked response is commonly mistaken for what type of Artifact?

Answer 17

Polarization artifact.

High stimulation energy = Greater Polarization artifact. If auto capture keeps failing, try lowering starting voltage to reduce polarization artifact.

Question 18

How does autocapture measure differences between evoked response and polarization artifact?

Answer 18

Delivers Paired impulses.

First pulse captures myocardium and gives evoked response.

Second pulse occurs in the absolute refractory period, thus causing only polarization artifact.

Question 19

What is retry mode? (Boston scientific)

Answer 19

Automatic Capture is active, the device is operating in beat-to-beat mode and is unable to confirm capture due to reasons such as loss of capture or due to signal/noise issues. Retry pacing amplitudes will be at double previous measured threshold.

Question 20

Automatic capture algorithms use beat-to-beat capture apart from which manufacturer?

Answer 20

Medtronic

Utilizes algorithm called capture management.

Question 21

What are the two benefits of automated capture management?

Answer 21

1. Improve safety related to pacing by avoiding situations where loss of capture could occur due to a rise in the stimulation threshold
2. Improve battery longevity by reducing the programmed safety margin closer to the stimulation threshold

Question 22

Voltage drop across lead body (distal to proximal) results in what?

Answer 22

Heat.

A form of wasted energy that unnecessarily depletes battery.

Question 23

Increases / Decreases

A small electrode tip size does what to the impedance?

Answer 23

Increases impedance.

Question 24

Why can’t we have even smaller electrode tip sizes than that found presently?

Answer 24

A smaller fixation helix will increase likelihood of micro-dislodgement.

Question 25

If high impedances are good for battery longevity - why dont we install higher impedance leads?

Answer 25

Implantation of high pacing impedance leads for ventricular stimulation does not result in a benefit with respect to pacemaker longevity as compared with standard impedance leads.

Question 26

Define polarisation with respect to the cathode.

Answer 26

Movement of positive IONs to cathode, cathode repels negative ions.

Question 27

What has a greater effect on polarisation, Voltage or Time?

Answer 27

Time. ## Footnote *Effect magnitude directly relates to the duration of stimulus.*

Question 28

List two ways to minimize polarization (1x Programmable, 1x Structural).

Answer 28

1. Shorter pulse durations 2. Larger surface area

Question 29

Smaller electrode surface area = better impedance yet Larger surface area = reduced polarisation. What's the solution?

Answer 29

Electrodes with small geometric radii but a complex, porous surface that increases microscopic surface area may be used (aka Fractal Coating). In addition, various coatings (e.g. platinum or iridium oxide) may help minimize polarisation.

Question 30

# Lead tip / Device can When programmed unipolar, which structure is the cathode and which is the anode?

Answer 30

Lead tip = cathode (negative) Can = Anode (positive)

Question 31

# Lead tip / Lead ring When programmed bipolar, which structure is the cathode and which is the anode?

Answer 31

Ring electrode = Anode (Positive) Tip = Cathode (Negative)

Question 32

In Quadripolar ICD leads, is the tip the anode or cathode?

Answer 32

Tip = Cathode = Negative

Question 33

In Quadripolar ICD leads, is the ring the anode or cathode?

Answer 33

Ring = Anode = Positive

Question 34

# True / False 'Chronaxie is the threshold pulse duration at twice the rheobase voltage amplitude'.

Answer 34

True. ## Footnote *Chronaxie = Pulse width at double the voltage threshold.*

Question 35

# True / False 'Constant current systems use less energy than constant voltage systems'.

Answer 35

False. ## Footnote *Energy requirements in a constant current system are higher than in a constant voltage system. All implantable systems are constant voltage for this reason.*

Question 36

# True / False 'Typically all constant current pacemakers are implantable pacemakers'.

Answer 36

False. ## Footnote *All implanted pacemakers are constant voltage. Most external / temporary pacemakers are constant current as energy is plentiful and easily accessible from the mains power socket.*

Question 37

# True / False 'Larger pulse widths result in a greater polarisation effect'.

Answer 37

True. ## Footnote *Polarisation is resistance to flow. A longer pulse width will increase the respondent polarisation effect.*

Question 38

# Yes / No Is it possible to program Bipolar with the following configuration? 'Lead Tip = Cathode, Device Can = Anode'.

Answer 38

No. ## Footnote *The device cannot be included in a bipolar circuit. Device can only form part of the circuit in unipolar configuration.*

Question 39

Capture hysteresis is also known as '\_\_\_\_\_ effect'

Answer 39

Wedensky effect.

Question 40

List 3 potential causes of stimulation exit block

Answer 40

1. Antiarrhythmic drugs 2. Electrolyte imbalance 3. Myocardial Infarction at pacing site

Question 41

Energy requirements for pacing and defibrillation differ by a factor of: A. 10 B. 100 C. 100,000 D. 1,000,000

Answer 41

D. 1,000,000

Question 42

# True or False: Exercise increases myocardial stimulation threshold.

Answer 42

False ## Footnote *It decreases stimulation threshold.*

Question 43

At a pulse duration of 2.0ms the threshold voltage is 1.5V. At a programmed amplitude of 3.0V the pulse width threshold is 0.4ms. What is most likely? 1. Rheobase 3.0V 2. Rheobase 0.4ms 3. Chronaxie 0.4ms 4. Chronaxie 2.0ms 5. Chronaxie 0.8ms

Answer 43

3 - Chronaxie is pulse width capture at x2 rheobase voltage. ## Footnote *Rheobase is 1.5V in this example.*

Question 44

When connecting a unipolar lead to the generator - what is the next step? 1. Ensure both screws are fully torqued onto lead pin 2. Verify pacing before placing device into pocket 3. Place device into pocket, then verify pacing

Answer 44

3 - Place device into pocket, then verify pacing. ## Footnote *Unipolar leads only have one set screw and will not pace until generator is in contact with the skin.*

Question 45

For bipolar pacing systems, which is true? 1. Neither the anode or cathode are in the heart 2. Both the anode and cathode are in the heart 3. The anode is in the heart, cathode is in the device pocket 4. The cathode is in the heart, anode is in the device pocket

Answer 45

2 - Both anode and cathode are in the heart.

Question 46

Calculate the energy delivered below. '5V, 565ohm, 0.4ms'

Answer 46

E = (V²/R)\*T E = (5²/565)\*0.4 E = 0.018 E = 18 microjoules