Ultrasound is a mechanical vibration delivered at a frequency above the range of human hearing. Clinical ultrasound units currently being manufactured typically deliver ultrasound at frequencies of 1 and 3 MHz with duty cycles ranging from 20 to 100 percent. Duty cycles less than 100% are usually termed pulsed ultrasound while a 100% duty cycle is referred to as continuous ultrasound.
- Inflammatory Phase - ultrasound causes a degranulation of mast cells resulting in the release of histamine. Histamine and other chemical mediators released from the mast cell are felt to play a role in attracting neutrophils and monocytes to the injured site. These and other events appear to accelerate the acute inflammatory phase and promote healing.
- Proliferative Phase - ultrasound has been noted to effect fibroblasts and stimulate them to secrete collagen 1. This can accelerate the process of wound contraction and increase tensile strength of the healing tissue 2. Connective tissue will elongate better if both heat and stretch are combined. Continuous ultrasound at higher therapeutic intensities provides and effective means of heating deeper tissue prior to stretch.
As the frequency of ultrasound is increased, the penetration of the signal decreases. For most dermal wounds, it is preferable therefore, to utilize a frequency of 3 MHz. 1 MHz wound be more effective on deeper structures or periwound skin.
As with other medical devices, inappropriate use can result in serious complications. The basic precautions should be taken:
- Treat at the lowest intensity that will produce the desired result.
- Assure that the applicator is kept in constant motion throughout treatment and that the proper acoustic coupling medium is used.
- Reduce the intensity or terminate treatment if the patient complains of any increase in pain.
- Prior to ultrasound treatment, remove dressings and clean wound of foreign debris or dressing residue.
When it is desired to provide mild heating to the periwound tissue in order to stimulate circulation, higher intensity ultrasound can be given. Extreme are should be used, however, to assure the tissue is capable of handling the thermal levels delivered.
To treat periwound tissue, 1 MHz, continuous ultrasound is usually employed. Again, an ultrasound applicator 1.5 to 2 times the size of the treatment area should be used. With an aqueous coupling medium in place, the sound head is placed lightly against the skin surface and moved in a slow and deliberate manner. The intensity is typically set to between 1 and 1.5 watts per square centimeter. This parameter is extremely variable and depends on the patient's circulatory, sensory and mental status. Thermal ultrasound should never be used in situations where impaired perception prevents the patient from sensing heat or pain. If at anytime during the treatment the patient should experience an increase in pain, the intensity should be turned down or the treatment discontinued.
Treatment duration is slightly longer than that of pulsed ultrasound since a mild thermal effect is desired. Initial treatment is about 2-3 minutes per zone and can be increased by 30 second increments to a maximum of 5 minutes per zone and delivered 3 times per week.