Ultrasound-Mediated Oxygen Delivery to Lower Extremity Wounds

  Abstract: Critical limb ischemia (CLI), an end result of peripheral arterial disease, remains a major clinical challenge. Wound healing in patients with CLI can be difficult due to diminished tissue oxygenation, often leading to recalcitrant ulcers and frequent limb loss. Numerous therapies, including hyperbaric oxygen therapy (HBOT), have been used to correct this regional ischemia, although often with mixed results. This case series investigates the effects of oxygen therapy delivery augmented by low-frequency ultrasound, a device that combines surface acoustic waveform (SAW) low-frequency ultrasound with hyper-oxygenated saline to deliver oxygen to wounds. Participants included 7 patients (7 men, median age 63 years, all with hypertension) with CLI and full-thickness wounds. Baseline therapy PaO₂ measurements were taken before therapy, after provision of hyper-oxygenated saline, and after sonification. The device was found to successfully oxygenate the wound beds; PaO₂ levels increased by a median of 59.7%, a maximum of 116%, and a median absolute difference peaking at 10.8 mmHg PaO₂ (P = 0.018). In conclusion, the treatment increases wound oxygen levels and may be an option in CLI therapy.

Introduction

  Critical limb ischemia (CLI), a condition that results from advanced peripheral arterial disease, retarding oxygen delivery to the affected extremity, is manifested by symptoms including constant lower extremity rest pain or nonhealing ulceration. Approximately 40% of patients with CLI require amputation of the affected limb, resulting in approximately 150,000 amputations in the United States annually.¹   Oxygen is involved with several crucial steps of wound healing and is considered the rate-limiting factor of wound improvement. Serving as a nutrient for cell metabolism, phagocytes and other cells of the immune system convert oxygen into reactive oxygen species (ROS) that are then used for wound healing. Moreover, oxygen promotes angiogenesis through inducing vascular endothelial growth factor (VEGF) transcription and myofibroblast production, and functions as a cofactor in collagen synthesis.²   Historically, low-frequency ultrasound (<100 kHz) has been used to increase the rate at which drugs and macromolecules are absorbed through cell membranes and the epidermal stratum corneum. The mechanisms associated with this process are poorly understood, although they are likely due to a combination of cavitation and acoustic streaming.^3-5 Furthermore, therapeutic ultrasound has been shown to decrease bacterial resistance to antibiotics, suggesting enhanced permeability of micro-organisms in this setting.^6,7   Normal saline solutions exhibit 6 ppm to 8 ppm of dissolved oxygen gas at sea level. By using a sparging technique, which entails the replacement of dissolved nitrogen, gas levels exceeding 24 ppm or more can be demonstrated. This gas replacement, or sparging, technique is defined by Henry’s Law of Partial Pressure.   The purpose of this study was to determine whether a surface acoustic waveform (SAW) low-frequency ultrasound device paired with hyper-oxygenated saline could synergistically deliver oxygen to wounds.

Methods

  Study population and clinical definitions. Nine patients with leg ulcers were enrolled in this single-center feasibility study conducted over a 2-month period (August 2009 to October 2009) at the Wound Healing and Hyperbaric Medicine Center, Raleigh, NC. Patients were considered for participation if the wound was full-thickness in depth and measured between 2.0 cm and 20.0 cm². Inclusion criteria also included the ability to properly position an oximeter probe in the wound bed to achieve tissue oxygenation level baselines. Exclusion criteria included active wound infection or a wound with exposed muscle tendon or bone, as well as recent therapy with hyperbaric oxygen therapy (HBOT) or skin substitutes (i.e., Apligraf [Organogensis, Inc, Canton, MA]; or Dermagraft [Advanced Biohealing, La Jolla, CA]). The Duke University Institutional Review Board approved this protocol. Informed consent was obtained from all patients before study enrollment. The primary outcome of the study was oxygen concentration in the wound bed after treatment with the treatment system.   Device. The treatment and analysis was comprised of 3 devices: a dissolved oxygen meter/probe, a hyper-oxygenated saline instillation system, and a sonophoresis ultrasound generator. The wound tissue was sonicated at low frequencies to enhance permeability and deliver hyper-oxygenated saline, at which time PaO₂ levels were measured.   The devices included: 1) The low-frequency ultrasound generator (Figure 1), a diathermy device adapted for use in enhancing permeability of tissues in the body with a transducer/applicator incorporated into a patch that adheres to the skin. This device maintains the functionality of existing ultrasonic diathermy devices cited under substantial equivalency determination k081075 issued to Nanovibronix, Inc as Pain Shield ™ and the substantial equivalency to the Duo-Son Ultrasonic Diathermy Device manufactured by Orthosonics Ltd, subject of k970131. This device was used for the sonication of wound tissue; 2) Oxylite (Oxford Optronix, Oxford, England), a dissolved oxygen meter manufactured used in conjunction with a fluro-optical fiber probe with computer interface and monitoring capabilities to accurately record interstitial PaO₂ levels pre- and post-sonophoresis (Figure 2); and 3) A SonOx instillation system (Trinity Wound Institute, LLC, Raleigh, NC) (Figure 3), used to deliver hyper-oxygenated saline to the wound bed. The system allowed for the uniform flow of fresh oxygenated sterile saline solution over the wound, while ensuring the ongoing drainage of exudate. A gas sparging technique was used to create the hyper-oxygenated saline solution, which was ultimately delivered to the wound bed.   Procedures. A lidocane injection at the wound site was used for pain management. Using a location 4 mm to 5 mm from the wound edge, a 21-gauge needle was placed longitudinally beneath the wound bed at a depth of 2 mm to 3 mm. The PaO₂ probe then was advanced to the needle tip, and a baseline PaO₂ level was recorded. A control drip solution with hyper-oxygenated saline containing approximately 30 ppm of dissolved oxygen was instituted for 20 minutes without sonication and PaO₂ levels were recorded. The wound then was sonicated with hyper-oxygenated saline using the ultrasound generator for 25 minutes. PaO₂ levels then were recorded (Figure 4). This was followed by the removal of the probe and oxygen sensor.   Statistical analysis. The Wilcoxon signed rank test was used to assess whether a statistically significant difference was noted when baseline PaO₂ levels were compared to post-sonication with hyper-oxygenated saline PaO₂ levels. Statistical significance was defined as a P < 0.05.

Results

  Patient characteristics. Baseline age, gender, ethnicity, and comorbidities of the 9 patients enrolled are summarized in Table 1. The majority of patients were middle-aged Caucasian men with diabetes mellitus and hypertension. No adverse events were noted. Two patients withdrew prior to readings. Only 7 patients’ results were included.   Primary endpoint. Figure 5 illustrates the oxygen tension (PaO₂ increase versus baseline oxygen level.)Baseline dissolved oxygen levels were considered to be hypoxic ( > 40 mm Hg) and did not increase with the control drip of hyper-oxygenated saline. However, sonication increased PaO₂ levels by 24% to 116% compared to baseline, with a median percent change of 59.7% and median absolute difference peaking of 10.8 units mmHg PaO₂ (P = 0.018).

Discussion

  This safety and feasibility study measuring the efficacy of the treatment by the combination of using SAW ultrasound technology with hyper-oxygenated saline showed a statistically significant increase in PaO₂ levels in the wounds. It was the first study of its kind to evaluate whether hyper-oxygenated saline could be delivered to the wound bed via SAW ultrasound technology, resulting in elevated PaO₂.   The use of oxygen therapy to reduce hypoxemia was reported in the 1960s with systemic HBOT administration², which is currently approved by Medicare and Medicaid services for use on selected lower extremity wounds.^8-10   Because oxygen is required for wound healing^2,11 and ultrasound technology is portable, cost-effective, and easily applicable, the combination of both may provide an effective alternative for treating CLI wounds. If proven effective, it could be a paradigm shift in the way oxygen is delivered in a clinical setting for CLI treatment. In the future, this therapy may be evaluated for the delivery of other treatment modalities, such as antibiotics and growth factors.   Many wound patients cannot tolerate, afford, or gain access to HBOT. A diseased micro-vascular system or edematous tissue promotes inadequate transportation of oxygenated blood to the wounded tissue. Ultrasound-assisted dissolved oxygen therapy provides a potential means by which oxygen can reach the wound via the external surface of the body through the use of an active energy source rather than relying on capillaries. Oxygen is delivered directly to the wound, and the systemic side effects or logistical problems are mitigated. This approach also may aid in promoting better outcomes in patients that receive HBOT but suffer from the noted physiologic conditions that could limit full benefits of the established therapy.   Dissolved topical oxygen systems have evolved over the past 50 years. Methods that deliver topical dissolved oxygen include: catalytically produced dissolved oxygen at the wound surface; diffusible dissolved oxygen bound to a carrier such as a fluorocarbon; or reservoir of gaseous oxygen sparged through the vehicle. There may be a significant advantage to delivering oxygen topically in its dissolved form, because it is biologically available immediately upon administration. The fundamental challenge to topical oxygenation methods is to create a large oxygen gradient to allow oxygen delivery into zones of tissue hypoxia. Devices that deliver unbound dissolved oxygen with an active ultrasound energy source have demonstrated significant oxygen penetration through viable porcine tissue.¹²   There were several limitations to this study. The small patient sample size was the main limitation. Without knowing the clinical benefits, patients were reluctant to enroll in this study; however, even in lieu of a small sample size, the findings were robust. Additionally, the authors treated all wounds as though they were the same. The authors did not assess clinical response to improved oxygenation, such as reduction in wound size or improvement in granulation tissue.

Conclusion

  Surface acoustic waveform technology that aids in the delivery of the dissolved gas at a depth in the wound bed has proved to be physiologically effective in promoting angiogenesis, promoting collagen formation, and promoting fibroblast production. If this can be achieved clinically, dissolved oxygenation procedures will become complementary to systemic methods of oxygenation (hyperbaric) and will allow the treating physician greater therapeutic versatility in treating wounds, possibly in a homecare environment adjunctively or as an alternative to HBOT for a certain populace of patients.   The ability to control duration and depth of topical oxygen delivery into human tissue will allow new strategies in individualizing patient therapy. A deeper understanding into the pharmacokinetics of dissolved oxygen administration will allow manipulation of specific healing processes. Larger studies that explore controlled oxygen therapies using ultrasound-based delivery systems based on pharmacokinetics are warranted.

References

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