Hello everyone and welcome back! A big thanks to everyone who viewed and commented on our blog. As promised, we are back and ready to talk more high voltage pulsed current (HVPC). Last week we discussed a little bit of the history and physical presentation of the HVPC device. Today, we will introduce you to some of the parameters specific to HVPC.
As its name suggests, high voltage stimulation (HVS) uses high voltage current, which is the main difference between HVS and other estim devices. Due to the lower impedance associated with high voltage, HVS is able to penetrate the skin more easily. Because of this, the depth of tissue penetration is proportional to the current pulse amplitude (Belanger, 2002). Up to 500V may be used to facilitate wound and tissue healing, but to also produce analgesia and muscle contraction.
HVS can be used to produce both a mechanical contraction and a chemical change within the body. This is achieved via delivering a short duration (microsecond) micramperage current, being driven by a high voltage current, to the symptomatic region of the body (Belanger, 2002). Again, higher peak currents combined with short pulse duration allows for a deeper tissue penetration.
The dosage associated with HVS requires specific parameters to be set according to the desired therapeutic effects. The amplitude may range between 1-500V (Belanger, 2002); however what the patient perceives as most comfortable is what should be implemented. The patient should be comfortable and pain free at all times throughout treatment.
The most widely accepted parameters for HVS use include (Belanger, 2002):
· Waveform: Monophasic
A twin peak pulse is used and may accumulate a weak charge in the tissues. However, the second peak is unlikely to be effective, as the second peak occurs within the refractory period – a period where no physiological responses can occur. The monophasic twin peak waveform is perceived to provide more comfort for the patient. Here's a comparison of the waveforms:
 |
| High voltage monophasic twin peak pulsed current |
 |
| Biphasic pulsed current produced by AC devices such as TENS |
· Amplitude: Volts
HVS utilises low amperage (microamps) which allows for deeper penetration of the tissues, with less risk of irritating or burning the skin and other tissues.
· Pulse duration: 20-200 µs
HVS machines are called micropulse stimulators, hence the most correct name for these devices is high voltage (pulsed) stimulators (HVS). The pulse duration is fixed on most HVS devices by the manufacturer and cannot be altered by the operator. The output of HVS devices is dependent on the pulse interval – the longer the pulse interval the smaller the output. As the pulse duration decreases, higher voltage or current is needed to produce a response. Therefore, output may be as high as 300-400mA, but due to the longer pulse interval may be as low as 1.5-2mA.
· Frequency:
o Low frequency stimulators: 0-150Hz.
o High frequency stimulators: greater than 150Hz
120-150Hz is the most common used range with clinical effects seen in frequencies up to 400Hz (Belanger, 2002). There is little evidence to indicate that frequencies greater than 800Hz provide additional therapeutic benefits (Belanger, 2002). Clinical effects may even be lost as the pulses are delivered during the refractory period where no response can occur.
The combination of this very short pulse duration and high peak current, but overall low total current per second (microcurrent) allows a relatively comfortable stimulation sensation for the patient. Stay tuned for more, when we next discuss what therapeutic effects HVS is capable of achieving.
Until next time.... A & M.
References:
Bèlanger, A. (2002). Evidence-based guide to therapeutic physical agents. Lippincott Williams & Wilkins. Pg 109-122
