Threshold, sensing and lead impedance

Question 1

Threshold

Answer 1

The minimum amount of energy required to depolarise the heart outside of its refractory period

Question 2

Wedensky effect

Answer 2

The tendency of myocardial cells to have a lower stimulation threshold when threshold determination is completed from higher to lower than if the threshold determination is completed from lower to higher

Question 3

When does the Wedensky effect occur

Answer 3

As a result of hyperpolarised state create across the myocardial cell membranes from the delivery of electrical impulse of high output

Question 4

Sensing

Answer 4

Allows the pacemaker to detect appropriate intrinsic atrial or ventricular electrograms

Question 5

Sensitivity can be programmed

Answer 5

The sensitivity value represents the minimum local EGM amplitude that is registered as a sensed event. It is quantitative and defines the voltage level at which events start to be sensed. Any EGM component smaller than the programmed sensitivity is ignored.

Question 6

The ideal sensitivity

Answer 6

• The ideal sensitivity will result in the appropriate detection of intrinsic events (atrial or ventricular depolarization) with a large safety margin in case the amplitude of the local EGM decreases

Question 7

Oversensing

Answer 7

Oversensing can result in the pacemaker detecting unwanted electrical signals as ventricular or atrial depolarisations and inhibit pacing and causes unnecessary shocks in ICDs

Question 8

Undersensing

Answer 8

When atrial or ventricular depolarisations are not detected properly by the pacemaker causing extra pacing at the wrong time in the cardiac cycle

Question 9

Hysteresis

Answer 9

the number of pulses per minute below the programmed LRL that the heart rate must drop to, in order to cause initiation of pacing

Question 10

AV hysteresis

Answer 10

Alterations in paced AV interval relative to the patients intrinsic AV conduction

Question 11

How does AV hysteresis work

Answer 11

A longer paced AVI is set to allow the patients own intrinsic rhythm to kick in thus maintaining intrinsic AV conduction.