Physical Therapy in the Treatment of Venous Leg Ulcers: Biophysical Mechanisms
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The clinical results strongly correspond with results obtained regarding hemodynamic events inside the wound.
The authors maintain that the biophysical mechanisms of wound healing after HVS are: enhanced local blood circulation, increased proliferation of capillaries, and lymphatic vessels, as well as increased production of DNA, and collagen and fibroblast proliferation.12
The present results did not confirm that improved blood flow in the wound is a significant mechanism after application of HVS. Theoretical interpretation of the effects of HVS is a complicated issue. One phenomenon contributing to the explanation of wound healing is the so-called “skin battery.” The sodium pump drives the “skin battery.” The surface between the positively charged wound surface and the negatively charged undamaged skin surrounding the wound generates an electrical current that travels through the tissues in the moist wound environment. This is a precondition for proper tissue reconstruction. Absence or reduction of the potential difference may delay the regenerative process. By stimulating the wound with the positive pole, the potential difference may be increased or restored, thus restimulating the healing process.4 Further research is needed, in the authors’ opinion.
The biophysical effects of ultrasound are traditionally separated into thermal and nonthermal effects (cavitation, acoustic streaming). Changes in blood flow due to heating at clinically acceptable doses are probably confined to the skin.
In a study using duplex ultrasound scans (with the option of gray-scale or Doppler mode) to measure saphenous vein cross-sectional area, heat stress (via a thermal suit perfused with water at 49˚C) resulted in doubling of the cross-sectional area, and therefore blood volume, in this vein. An increase in blood flow facilitated rapid turnover of warm blood, which assisted cooling.13
In muscle, the use of radioactive tracers in human subjects showed that heating agents, including ultrasound, do not cause an increase in blood flow that is comparable to that caused by even moderate exercise.13 This finding was confirmed recently using venous occlusion plethysmography and laser Doppler flowmetry before and after the administration of continuous ultrasound (1.5 W/cm2 for 5 minutes). A reasonable explanation for the discrepancy between these studies and studies demonstrating that muscle blood flow increased with heating is that the latter studies used only plethysmography to measure blood flow. This technique, however, does not measure tissue-specific changes in blood flow in tissues, such as muscle.13
Robinson and Buono14 noted that researchers using the Xenon-33 washout technique to measure muscle blood flow concluded that continuous ultrasound at an intensity of 1.5 W/cm2 applied for 5 minutes to the forearm did not increase blood flow. It is possible, however, that ultrasound at higher intensities may increase muscle blood flow. For example, although no increase in muscle blood flow was found at tolerable ultrasound intensities, increased muscle blood flow did occur at intolerable ultrasound intensities (high intensity continuous ultrasound is intolerable due to pain caused by excessive heating). The contention that high temperatures are necessary to increase muscle blood flow is supported by a study that used microwave heating to achieve temperatures in excess of 44.5˚C. Muscle blood flow increased from a pretreatment value of 10 mL/min/100 g to 44 mL/min/100 g. However, this increase was far less than the increase from 2 to 4 mL/min/100 g at rest to 80 mL/min/100 g of muscle achieved with extreme exercise.