Before thinking about pacing thresholds, consider the heart’s normal conduction pathway, and cardiac action potentials. A specific amount of energy is required in order to initialise cellular depolarisation, and in a normal cycle, it begins in the sinoatrial (SA) node before spreading throughout the tissue and causing the rest of the beat.
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. In an event such as this, i.e. a failure at a nodal level, an implanted pacemaker will send it’s own signal to the heart so as to initialise a depolarisation.
Pacing voltage thresholds denote the minimum amount of energy required for the pacemaker to cause the cardiac muscle to depolarise. Essentially, the pacemaker needs the energy to “demote” the heart, and put itself in charge of the beat. In addition this electrical energy needs to be delivered over a certain period of time to facilitate the paced beat. This is known as the pulse width threshold.
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 must be programmed so as to enable stimulation of the heart, and minimise the drain on the battery. 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.
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.
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
Pulse Width Threshold
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.
In the example of voltage threshold, the final value was 0.75V, so for this example, assume that the voltage is fixed at tat same number. The pulse width in this image produces capture at 0.8ms down to failure at 0.3ms, giving a threshold of 0.4ms.
Combining the two examples would give a total threshold of 0.75V @ 0.4ms.
Why Test Thresholds?
There are a number of reasons to test and potentially alter thresholds. Primarily, it is done to find a safety margin. In a standard scenario, once the values have been confirmed using the above testing methods, the pacemaker is programmed to respond at about double the thresholds. This is done to help ensure that the pacemaker causes a contraction EVERY time the heart fails to do so on its own.
Threshold changes can indicate developing issues with either the device or the patient. They can also be altered by taken medications.
It is necessary to ensure maximum conservation of battery life. If there was no cost regarding patient safety or finances, it would be reasonable to assume we should simply max out the pacing functions and take all of the strain from the heart completely. Due to the expensive nature of IPGs and leads, as well as the trauma and infection risk associated with invasive procedures, minimising the drain on the device power supply is desirable, as it also minimises the number of devices a patient will have.