Pulsed Shortwave Therapy
Pulsed Shortwave Therapy (PSWT) is a widely used modality in the UK (Al Mandil and Watson 2006), though it is often called Pulsed Electromagnetic Energy (PEME) which is less than fully appropriate in that many modalities come under the heading of PEME, PSWT being only one of them and the use of the term should be avoided. The older term Pulsed Shortwave Diathermy is not really appropriate either in that the modality is not primarily employed as a diathermy (literally ‘through heating’).
PSWT employs the same operating frequency as traditional SWD ie. 27.12MHz. The output from the machine is pulsed such that the ‘on’ time is considerably shorter than the ‘off’ time, thus the mean power delivered to the patient is relatively low even though the peak power (ie during the on pulses) can be quite high (typically around 150 – 200 Watts peak power with modern machines).
The control offered by the machine will enable the user to vary (a) the mean power delivered to the patient and (b) the pulsing parameters governing the mode of delivery of the energy. It would seem from current research that the mean power is probably the most important parameter. pulse shortwave parameters
Read about the main machine parameters here ...
There are two basic types of output from these machines, the ELECTRIC field, comparable to the condenser (capacitor) field in traditional SWD & secondly, the MAGNETIC field, comparable to inductothermy. Some machines offer the facility to pulse either output, with the magnetic field being delivered via a drum containing a coiled conductor housed in some form of ‘monode’ or ‘drum’ applicator (which goes by several different names). Manufacturers include a special screen in the face of the drum to eliminate any of the electric field. All identified research in which pulsed shortwave has been evaluated has been conducted with the monode type applicator. No evidence has been identified that demonstrates a measurable clinical benefit when PSWT is applied using the plate (electrostatic) applicators.
The output of the monode applicator can be thought of as a form of pulsed inductothermy. The pulsed electromagnetic field which is emitted from the applicator will be transmitted through the tissues, and will be absorbed in those of low impedance i.e. the conductors which are tissues like muscle, nerve, those which are highly vascular, tissues in which there is oedema, effusion or recent haematoma.
With respect to the effects of pulsed SWD, there is an element of tissue heating which occurs during the `on' pulse, but this is dissipated during the prolonged ‘off' phase & therefore, it is possible to give treatment with no NET increase in tissue temperature. In the figures below, (A) demonstrates no accumulation of either thermal or non thermal effects. In (B), the pulses are sufficiently close to generate an accumulative non-thermal effect and in (C) there is an accumulation of both thermal and non thermal effects. The settings applied on the machine will determine which of these is achieved in a particular treatment, with the mean power appearing to be the most critical parameter. The `non thermal' effects of the modality are generally thought to be of greater significance. They appear to accumulate during the treatment time & have a significant effect after a latent period, possibly in the order to 6-8 hours. It is suggested (Hayne 1984) that the energy levels required to produce such an effect in humans is low.
An active research programme has been conducted for several years now relating to the thermal nature of PSWD. It was unclear just what power levels were required to bring about a real tissue heating, and in fact, there has been some opinion that PSWD was a non thermal modality per se. Research has demonstrated that PSWD does have a thermal component, and real tissue heating can occur under different treatment settings. This is important in that if the modality is to be applied in circumstances where the heating would be inappropriate or contraindicated, it is essential to know the power/energy levels where the thermal effects begins. From the work we have done, it has been shown that a measurable heating effect can be demonstrated at power levels over 5 watts, though on average, it will become apparent at some 11 watts mean power. More recent work by Seiger and Draper (2006) suggests that it may still be safe to apply higher mean power levels than previously thought, even with metal in the tissues.
If a ‘non thermal’ treatment is the intended outcome of the treatment, it is essential that the mean power applied remains below a level that is likely to induce significant heating effects, and at present, this is taken as being at 5 Watts mean power. If a thermal effect is an intentional outcome of the intervention, then it may be perfectly appropriate to deliver power levels in excess of 5 watts, but if doing so, the therapist must ensure that the precautions are taken as for any other thermal intervention.
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Effects of Pulsed Shortwave Therapy
These can be basically divided into two types those of the electric field & those of the magnetic field. There appears to be almost no literature/research concerning the effects of pulsing the electric field, & almost all the research reviewed is concerned with the therapeutic effects of the magnetic field. This is not to say that pulsing the electric field has no effect, but that the research evidence for such an effect is lacking. The information which follows relates therefore to the effect of pulsing the magnetic field (i.e. via the drum or monode applicator).
The primary effects of the pulsed magnetic field appears to be at the cell membrane level & is concerned with the transport of ions across the membrane. Some interesting publications have strongly supported the ‘non thermal’ effects at cell membrane level (Luben 1997, Cleary 1997).
Normal cell membranes exhibit a potential difference due to the relative concentration differences of various ions on either side of the membrane (reviewed in Charman 1990). Of these ions, sodium (Na+), potassium (K+), calcium (Ca++), chloride (Cl-), & bicarbonate (HCO3-) are probably the most important. Cell membrane potentials vary according to the cell type, but a typical membrane potential is 70mV, internally negative. It is actively maintained by a series of pumps & gated channels, & cellular energy (ATP) must therefore be utilised to maintain the potential.
A cell involved in the inflammatory process demonstrates a reduced cell membrane potential & consequently, the cell function is disturbed. The altered potential affects ion transport across the membrane, & the resulting ionic imbalance alters cellular osmotic pressures. The application of PSWT to cells affected in this way is claimed to restore the cell membrane potential to their ‘normal’ values & also restores normal membrane transport & ionic balance. The mechanism by which this effect is brought about is not yet established, but the two theories suggest that this is either a direct ionic transport mechanism or an activation of various pumps (sodium/potassium) by the pulsed energy (Sanseverion 1980). Evidence (Luben & Cleary 1996) supports the contention that the energy is absorbed in the membrane and that via a mechanism of signal transduction, stimulates or enhances intracellular effects.
There appears to be a strong similarity in the mechanism of effect of ultrasound, laser and pulsed shortwave – all three modalities appear to have their primary effect at cell membrane level, with the resulting ‘up regulation’ of cellular behaviour being the key to the therapeutic effects.
It is claimed that the applied energy has little or no effect on normal cells as `sick' cells respond to lower energy levels than normal cells.
The clinical effects of PSWT are primarily related to the inflammatory and repair phases in muscuoskeletal / soft tissues. The effects list is remarkably similar to that of ultrasound and laser therapy – which is not surprising given their probable common mode of action.
Goldin et al (1981) list the following as the primary effects of pulsed SWD:
- Increased number of white cells, histocytes & fibroblasts in a wound.
- Improved rate of oedema dispersion.
- Encourages absorption of heamatoma.
- Reduction (resolution) of the inflammatory process.
- Prompts a more rapid rate of fibrin fibre orientation & deposition of collagen.
- Encourages collagen layering at an early stage.
- Stimulation of osteogenesis.
- Improved healing of the peripheral & central nervous systems. (the claim for CNS healing has not been substantiated)