Sensing

Question 1

What 3 components determine polarisation effect?

Answer 1

1. Pulse width
2. Stimulation output
3. Electrode radius

Question 2

Amplitude and slew rate can abruptly decline 1 week post implant - when do values typically return?

Answer 2

6-8 weeks post implant.

Question 3

True / False

Unlike pacing, sensing circuits should have low/no resistance.

Answer 3

True.

Sensing circuits that follow pacing circuit design may result in diminished sensing.

Question 4

Define the difference between source impedance and input impedance.

Answer 4

Source impedance = Voltage drop from origin of IEGM to proximal portion of lead.

Input impedance = Impedance of sensing amplifier itself.

Question 5

The IEGM seen by the pacemaker is the ratio between which two impedances?

Answer 5

Source impedance vs Input impedance.

Large difference = less signal attenuation.

Question 6

What 5 factors affect sensing accuracy?

Answer 6

1. Lead integrity
2. Electrode characteristics and placement
3. Lead polarity
4. EMI
5. Properties of the myocardium

Question 7

Oversensing / Undersensing

A fractured wire with intermittent contact will lead to which outcome?

Answer 7

Oversensing.

Question 8

Oversensing / Undersensing

A fractured wire with no contact will lead to which?

Answer 8

Undersensing.

Question 9

What are historically acceptable sensing values for P and R waves in pacemakers?

Answer 9

P wave >2mV

R-wave >5mV

Question 10

Where are myopotentials most likely to arise from when ICD is programmed integrated bipolar?

Answer 10

Diaphragm or Pectorals.

These muscles are in the devices ‘Field of View’. More likely to occur in integrated bipolar leads vs true bipolar.

Question 11

List 4 ways to reduce ICD over sensing (x4 programmable / x1 interventional).

Answer 11

1. Reduce max sensitivity
2. Decrease sensitivity decay
3. Decrease LRL or Increase AV delay (Minimise RV pacing)
4. Prolong detection interval - Transient over sensing will terminate
5. Implant separate RVOT sensing lead (Further from diaphragm)

Question 12

List 3 ways to stop T-wave over sensing.

Answer 12

1. Reduce max sensitivity
2. Increase refractory periods
3. Reduce auto sensitivity aggressiveness

Question 13

The IEGM shows two non-physiologic sensed intervals. List two potential sources.

Answer 13

1. Loose set screw in header
2. Lead defect

Question 14

Yes / No

Is it necessary to have both near field and far field sensing channels in ICD systems?

Answer 14

Yes.

Far field helps to show morphology changes and also can detect atrial signals - thus showing VT vs SVT.

Question 15

If ICD sensing can’t be sorted with programming, what is the next course of action?

Answer 15

Re-intervention should strongly be considered E.g. Small R-wave // Big T-wave.

Question 16

Generally speaking, how does the ICD noise detection algorithm work in ICDs?

Answer 16

Identifies marked differences between near field and far field IEGMs.

Tip - Ring = Near Field

Coil - Can = Far Field

Question 17

Noise secondary to a conductor failure will typically be seen on which IEGM?

Answer 17

Near field (Tip - Ring).

A conductor fracture is less likely to interfere with far field EGM.

Question 18

Noise detection algorithm is ineffective against EMI - Why?

Answer 18

Noise will appear on both near and far field channels - thus therapy will likely be delivered.

Question 19

Is ICD sensitivity more or less sensitive following a paced beat?

Answer 19

More sensitive.

Question 20

With ICD sensing are post pacing blanking intervals longer or shorter than post sensing intervals?

Answer 20

Post pacing blanking periods are longer.

Question 21

List 3 programmable ways to decrease TWOS

Answer 21

1. Decrease max sensitivity
2. Prolong refractory periods
3. Alter auto sensitivity (Amplitude and Delay Decay)

Question 22

Near Field / Far Field

Which is more likely to detect morphology changes during tachycardia?

Answer 22

Far Field.

A greater field of view gives a larger observed degree of vector change.

Question 23

Define the difference between consecutive and probabilistic counters.

Answer 23

Consecutive = one following other.

Probabilistic = certain quantity in a certain time period.

Question 24

What do VF counters generally accept % wise.

Answer 24

75%.

Question 25

How long should an ICD take to charge.

Answer 25

5-12 seconds.

Question 26

The following statement best describes which form of sensing circuit? 'Both anode (+) and cathode (-) are located in the heart'.

Answer 26

Bipolar sensing.

Question 27

The following statement best describes which form of sensing circuit? 'Anode (+) is in the generator pocket and Cathode (-) is in the heart'.

Answer 27

Unipolar sensing.

Question 28

Atrial and Ventricular Slew rates should exceed what respective values at implant?

Answer 28

P-wave = 0.5V/s. R-wave = 0.75V/s.

Question 29

The ability of a device to adequately sense is determined by which two values?

Answer 29

1. Impulse Amplitude 2. Slew rate

Question 30

The following formula describes what? ΔV/ΔT

Answer 30

Slew rate. ## Footnote *Delta change in voltage / delta change in time.*

Question 31

Sensing amplifiers are typically most sensitive between what Hz bandwidth?

Answer 31

30-40Hz.

Question 32

What is the approximate sensing field of view for both Unipolar and Bipolar sensing circuits? (in cm)

Answer 32

Unipolar = 30-50cm - FOV is lead tip to Generator can. Bipolar = 3-5cm - FOV is lead tip to lead ring.

Question 33

# Unipolar / Bipolar Which pacemaker sensing circuit is most susceptible to inappropriate pacing inhibition due to oversensing?

Answer 33

Unipolar. ## Footnote *Due to larger sensed field of view. Impulses of non cardiac origin are 'seen' by the device and confused for genuine cardiac signals - hence inhibition of output.*

Question 34

# Overpace / Underpace If a device is oversensing it is most likely to do which of the following?

Answer 34

Underpace (Inhibit inappropriately). ## Footnote *The device is fooled into thinking there are intrinsic cardiac impulses, thus will inhibit pacing output.*

Question 35

What Hz bandwidth do device bandpass filters typically allow?

Answer 35

20-40Hz.

Question 36

The following statement best describes what generator component? 'Permits frequencies in a defined range yet attenuats frequencies outside that range'

Answer 36

Bandpass Filter.

Question 37

# True / False Review this statment with respect to a single lead system 'Oversensing will lead to under-pacing, while under sensing will lead to over-pacing'

Answer 37

True. ## Footnote *Extraneous impulses of non cardiac origin will fool the pacemaker to inhibit necessary output. An inability to sense intrinsic events will fool the pacemaker to think the myocardium is inactive, thus delivering inappropriate output.*

Question 38

# True / False Review the following statement respect to a dual lead system programmed DDD. 'Atrial oversensing will lead to ventricular under-pacing'.

Answer 38

False. ## Footnote *DDD is a tracking mode. Thus it is likely the pacemaker will track the oversensed impulses on the atrial channel and over-pace the ventricle in response.*

Question 39

What is the Atrial impulse density as an expression of Hz bandwidth?

Answer 39

Atrial signals are typically 80-100Hz.

Question 40

What is the Ventricular impulse density as an expression of Hz bandwidth?

Answer 40

Ventricular signals are typically 10-30Hz.

Question 41

# Increase / Decrease What effect will an increase in stimulation pulse width have on the corresponding polarisation effect?

Answer 41

Increase. ## Footnote *Increasing pulse width or 'time the current is switched on' will heighten the polarisation effect.*

Question 42

# Increase / Decrease What effect will an increase in electrode radius have on the corresponding polarisation effect?

Answer 42

Increase. ## Footnote *Fractal coatings on electrode tips help to increase surface area without increasing physical radius, thus keeping polarisation effect low.*

Question 43

# Increase / Decrease What effect will a decrease in stimulation voltage have on the corresponding polarisation effect?

Answer 43

Decrease. ## Footnote *Stimulation voltage is directly related to polarisation effect. Lower voltages correspond to lower polarisation.*

Question 44

Amplitude and slew rate can abruptly decline 1 week post implant - discounting dislodgement, why does this occur?

Answer 44

Localised Oedema / Inflammation ## Footnote *Anti-inflammatories are typically affixed to the lead tip to largely negate this in modern systems.*

Question 45

What are acceptable sensing values for P and R waves in ICDs?

Answer 45

P wave \>2mV R-wave \>10mV

Question 46

Why is ICD sensitivity is typically more sensitive following a paced beat?

Answer 46

To prevent the phenomenon of pacing into VF.

Question 47

# True / False 'Voltage difference between two electrodes creates the sensed signal sent to the device'.

Answer 47

True.

Question 48

# True / False 'A generator with sensitivity fixed to 3mV will sense only cardiac signals 3mV and greater'.

Answer 48

True. ## Footnote *Anything smaller than 3mV will not be sensed.*

Question 49

The following statement best describes what? 'The change in signal amplitude vs. time'.

Answer 49

Slew Rate.

Question 50

List 3 examples of extra-cardiac signals that present with very high slew rates.

Answer 50

1. Skeletal Muscle Myopotentials 2. Electrical Interference 3. Electromagnetic Interference ## Footnote *The signals are normally attenuated via the bandpass filter.*

Question 51

List an example of a cardiac signal that presents with a very low slew rate.

Answer 51

The T-wave. ## Footnote *Normally attenuated via the bandpass filter.*

Question 52

# True / False 'A bandpass filter discriminates signals by their amplitude'.

Answer 52

False. ## Footnote *A bandpass filter discriminates via frequency (Hz) not amplitude (mV).*

Question 53

# True / False 'A level detector discriminates signals by their amplitude'.

Answer 53

True.

Question 54

# True / False 'A V-sensed event with no preceding A-event (as detected by the pacemaker) will be labelled a PVC'

Answer 54

True. ## Footnote *This also holds true if the atrial event falls within the PVAB.*

Question 55

Define the difference between true and functional undersensing.

Answer 55

True = Amplitude of signal is too small to be detected at current programmed sensitivity levels. Functional = Signal falls within a programmed blanking or refractory period and is thus not detected.

Question 56

List one possible explanation for undersensed atrial events post PPM defined PVC, despite them occurring after the normal programmed PVARP interval.

Answer 56

Most systems extend the PVARP interval following a PPM defined PVC. ## Footnote *This interval is significantly longer than the normal PVARP interval, upwards of 400msec.*

Question 57

Which of the following is the correct description of each of the four recordings?

Answer 57

E.

Question 58

Which is important for sensing evoked potentials necessarry for capture confirmation algorythms? 1. Low impedance lead 2. Higher french lead 3. High impedance lead 4. Low polarization lead 5. High polarization lead

Answer 58

4 - Low polarization lead.

Question 59

A patients device is undersensing at 5mV. What would be the next appropriate step. 1. Decrease sensitivity to 7mV 2. Increase sensitivity to 4mV 3. Increase the output 4. Increase sensitivity to 10mV

Answer 59

2 - Increase sensitivity (by decreasing the number) to an acceptable threshold.