VOL: 97, ISSUE: 24, PAGE NO: 58
Kate Ballard, BSc, RGN, is clinical nurse specialist, tissue viability, and clinical trial coordinator, Tissue Viability Unit, Guy’s Hospital, London
Hildegard Charles, PhD, MSc, DN, RGN, RSCN, is clinical nurse specialist in leg ulcers, St Charles Hospital Leg Ulcer Clinic, London
Ultrasound has been used in the treatment of soft-tissue injuries for 50 years (Dyson, 1990). It is used routinely by physiotherapists and it has been estimated that each year over a million NHS treatments involve its use (ter Haar et al, 1985).
What is ultrasound?
Ultrasound is a mechanical vibration resonating at a high frequency beyond the upper limit of human hearing (Hart, 1998). Ultrasound cannot travel through air, so for the soundwaves to travel from the hand-held applicator into the wound tissue a hydrogel sheet or film dressing and ultrasound coupling gel must be used.
When ultrasound enters the body it has a thermal and non-thermal effect on the cells and tissues (Young, 1996).
As ultrasound travels through body tissue a percentage is absorbed, resulting in the generation of heat. The degree of absorption depends on the nature of the tissue, the extent of vascularisation and the frequency of the soundwaves (Young, 1996). Thermal diffusion and local blood flow then dissipate the heat created by the ultrasound. Complications can occur if the ultrasound beam hits bone or a metal prosthesis. These structures will reflect the ultrasound waves, and further heat is created during the beam’s return journey (Lehmann and Guy, 1972). It is therefore important to know the local anatomy and the patient’s medical and surgical history when using ultrasound.
This describes a number of physical mechanisms known as cavitation, acoustic streaming and standing waves.
Cavitation is the formation by ultrasound of small bubbles or cavities in gas-containing fluids. These bubbles vibrate to cause changes to the permeability of cell membranes. Acoustic streaming is the unidirectional movement of a fluid in an ultrasound field. It can stimulate cell activity if it occurs at the boundary of the cell membrane and the surrounding fluid. Standing waves occur when an ultrasound wave hits the interface between two different tissues, such as muscle and bone, resulting in reflection of a percentage of the wave. The increased pressure produced in standing wave fields can cause transient cavitation and consequently free radical formation (Dyson et al, 1974). To prevent this it is important that therapists move the applicator continuously throughout treatment and use the lowest intensity required to have a therapeutic effect.
Non-thermal effects are thought to be primarily responsible for ultrasound-accelerated wound-healing (Young, 1996).
Ultrasound and wound-healing
Ultrasound can stimulate wound-healing (Young and Ballard, 2000). However, evidence to support its use in the treatment of chronic wounds is sparse.
The degree of effectiveness of ultrasound treatment depends on several parameters: power and intensity of the ultrasound; ultrasonic frequency; pulsing regime of the ultrasound; phase of wound-healing when treatment is first applied, and duration and number of treatments.
Power and intensity
Ultrasound intensity is measured in watts (w) or w/cm2. Therapeutic ultrasound machines can supply intensities of up to 3.0 w/cm2, but there is no evidence to support the use of intensities above 1.0 w/cm2 in wound-healing. The majority of studies reporting beneficial effects have used average intensities of 0.1-0.5 w/cm2 (Hart, 1998). When used incorrectly there is a risk of excessive heating of the tissues.
The depth of penetration by the ultrasound wave depends on the frequency selected. Higher frequencies are absorbed more readily by body tissues and are used to treat more superficial wounds. Lower frequencies are less readily absorbed and thus penetrate further and are selected for the treatment or investigation of deeper tissues.
Frequencies of 1-3 MHz have proved effective in the treatment of venous leg ulcers (Dyson et al, 1976; Callam et al, 1987).
Ultrasound can be applied continuously or in pulses. Continuous ultrasound results in a thermal effect whereas pulsed ultrasound, which allows heat dissipation, is used to achieve the non-thermal effects. Pulsed ultrasound is therefore the regime usually used in wound-healing (Young, 1996).
Pulsing regimes such as two milliseconds on and eight milliseconds off have been reported as effective in the treatment of a variety of wounds (Dyson et al, 1976).
Duration of treatment
Treatment duration depends on the size of the wound. It is common practice to divide the wound area into zones of approximately 1.5 times the area of the applicator head and then apply ultrasound therapy for one to two minutes per zone (Oakley, 1978).
Frequency of treatments
Acute wounds should be treated as soon as possible, ideally within hours of injury. They are often treated once or twice a day to reduce pain and swelling (Young, 1996). Ultrasound accelerates the inflammatory phase, moving the wound into the proliferative phase of repair sooner (Young and Dyson, 1990a). Ultrasound has been shown to affect many of the processes that occur during the proliferative phase, for example angiogenesis (Young and Dyson, 1990b), fibroblast activity (Dyson, 1987) and wound contraction (Hart, 1993). These effects have been achieved using low intensity (maximum of 0.5w/cm2) which utilises primarily non-thermal mechanisms.
In the studies that have been conducted on chronic wounds, such as venous leg ulcers, treatment is generally given one to three times a week (Callam et al, 1987; Dyson et al, 1976).
Care must be taken when using electrotherapies. Dyson (1988) lists the following basic precautions to be taken to ensure that ultrasound is used effectively and safely:
- Only use ultrasound as a wound-healing therapy if adequately trained to do so;
- Only treat patients with conditions known to respond favourably to this treatment;
- Use the lowest intensity that produces the required effect, as higher intensities may be damaging;
- Move the applicator constantly throughout treatment, to avoid the damaging effect of standing waves;
- If the patient feels additional pain during treatment, either reduce the intensity of ultrasound to a painfree level or abandon the treatment;
- Use properly calibrated and maintained equipment;
- If there is any suggestion that there may be a problem with the wound - for example, suspected infection - do not use ultrasound.
Eighty-two-year-old Annie Cox has been attending the community leg ulcer clinic for four years for treatment of bilateral venous disease and leg ulcers.
On her right leg Mrs Cox has a chronic ulcer which had been treated at the clinic for the previous three years. The ulcer on her left leg developed more recently following trauma to the leg where she had caught it on the bottom of a cupboard door. Initially the new ulcer measured 7cm2 and was covered with sloughy tissue. A hydrocolloid was chosen as the wound contact material, used in conjunction with short stretch compression bandaging. This treatment continued for three months with minimal improvement. To enhance the healing process the specialist nurses at the clinic decided to use ultrasound to supplement the treatment already being used.
The treatment regimen
On arrival at the clinic the compression bandaging and hydrocolloid dressing were taken down, Mrs Cox’s leg was washed in a bucket of tap water and the ulcer irrigated with normal saline. The wound was then covered with a hydrogel sheet dressing and the ultrasound applied in combination with an ultrasound-conducting gel. The ultrasound regime was pulsed - two milliseconds on, eight milliseconds off - with an intensity of 0.5w/cm2 with a 3MHz probe.
The duration of treatment started at one minute for the first week, increasing to three minutes for the second week and five minutes for the third week. Five minutes was the maximum treatment time, due to the small size of the ulcer. The ultrasound was applied twice weekly.
After the ultrasound therapy was completed the ulcer was redressed as before. The ulcer completely healed after six weeks of ultrasound plus conventional treatment.
Evidence to support the use of ultrasound in this instance is anecdotal and the case study aims only to illustrate the use of ultrasound in the treatment of a recalcitrant ulcer. However, such ulcers cause much anxiety to patients (Franks et al, 1994) and, although electrotherapy regimes are currently out of vogue, ultrasound, administered by a properly trained practitioner is worth considering as a viable option for use on a long-standing leg ulcer or patient with recurrent disease. Physiotherapists use ultrasound routinely, and it may be possible to liaise with the physiotherapy department to borrow a machine for use on a consenting patient.