![]() This is tested in a similar way to voltage threshold, except that in this instance, the voltage remains fixed, and the pulse width is incrementally decreased until capture fails. The above shows the paced beats capturing from their commencing at 1.75V, through until 0.5V, where no beat is initialised, ergo the pacing threshold is 0.75V The patient may feel their heart racing at this point, but it is not permanent the amount of energy generated by the IPG is steadily decreased until it fails to cause a depolarisation, and thereby gives the voltage threshold. Testing this is a relatively simple process: the demotion of the heart that was mentioned above must be made (temporarily, at least) constant, so the heart is paced to such an extent so as to stop any natural beats from being generated. This must be programmed so as to enable stimulation of the heart, and minimise the drain on the battery. If the thresholds are too low, depolarisation will not be initialised, and if they are too high, the pacemaker battery will become depleted much quicker. This is known as the pulse width threshold. In addition this electrical energy needs to be delivered over a certain period of time to facilitate the paced beat. Essentially, the pacemaker needs the energy to “demote” the heart, and put itself in charge of the beat. Pacing voltage thresholds denote the minimum amount of energy required for the pacemaker to cause the cardiac muscle to depolarise. a failure at a nodal level, an implanted pacemaker will send it’s own signal to the heart so as to initialise a depolarisation. If the SA node fails to ‘fire’ as a result of an underlying pathology, the result will be a pause in the successive cardiac cycle and rhythm.
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