Task Leader(s): David M. Brienza, PhD

Co-Investigator: Yi-Ting Tzen, BS, PT (SHRS Doctoral student)

Specific Aims

• Aim 1: To investigate the blood flow response to local skin cooling at various cooling rates in weight-bearing sacral tissues
  
  
• Aim 2: To develop methods for selecting temperature settings to enhance tissue tolerance to loading pressure in people with various degrees of neurological impairments

Hypotheses and Rationale

The general objective of this study is to investigate the effectiveness of local cooling on enhancing tissue tolerance to loading pressure based on laser Doppler skin blood flow measurements. Two hypotheses will be tested to assess the effectiveness of cooling on enhancing tissue tolerance to loading pressure (Aim 1). The hypotheses are as follows:

Hypothesis 1a:
The skin under local cooling (at fast and slow rates) has a shorter recovery time of reactive hyperemia as compared with the skin without local cooling after 60 mmHg of pressure for 20 minutes.

Hypothesis 1b:
The skin under fast cooling (-4°C/min) has a shorter recovery time of reactive hyperemia as compared with the skin under slow cooling (-0.33°C /min) after 60 mmHg of pressure for 20 minutes.

Rationale for Hypotheses 1a & 1b: Within the same subject, the intensity of reactive hyperemia could be used to assess the imposed tissue stress. The recovery time of reactive hyperemia time is highly correlated with oxygen debt and metabolic wastes. This measurement will be used to assess the influences of local cooling on tissue metabolic rates. Other parameters used to characterize reactive hyperemia (i.e. total skin blood flow, half life, time to peak flow, and peak flow) will also be used to investigate benefits associated with local skin cooling. Several research groups have adopted the use of reactive hyperemia to assess the effects of pressure on skin blood flow response.

The application of local cooling to lower skin temperature to 25°C with different rates will elicit varying skin blood flow responses. With a fast cooling rate of -4°C per minute, vasoconstriction is less obvious as compared with a slower cooling rate of -0.33°C per minute. A smaller vasoconstriction indicates more blood flow supply under skin cooling. Given the metabolic rate of local cells is dependent on temperature surrounded them, if we reduce metabolic rates with cooling and, at the same time, allow more blood flow supply to the compressed skin and muscles, the ischemic tolerance of tissues would be increased. Such benefits could be applied to cushion technology by adding the cooling elements and active feedback (such as in Project D2 - Development of low-shear, cool cushion). In order to guide the development, we will examine the effects of cooling rates on skin blood flow response to loading pressure. Our finding will direct future research regarding the application of local cooling on preserving tissue viability.

To develop methods for selecting temperature settings to maximize tissue protection by using local cooling, we will test the following hypotheses:

Hypothesis 2a:
Persons with SCI at T6 and above have a longer recovery time from reactive hyperemia as compared with persons with SCI at T6 and below.

Hypothesis 2b:
Persons with SCI at T6 and below have a longer recovery time from reactive hyperemia as compared with healthy controls.

Rationale for Hypotheses 2a & 2b: In order to develop methods for selecting a cooling setting to accommodate an individual's impairment, a customized temperature setting and rate might be needed to maximize the protection provided by local cooling. In order to achieve Specific Aim 2, we propose to compare cooling responses in three groups. Persons with SCI at T6 and above are well known for impaired cardiovascular regulation including skin blood flow response to pressure or heating. The impairment in this population might cause a different skin blood flow response to cooling. Thus temperature and rate settings for persons with SCI should at least account for this factor.

Although persons with SCI below T6 have intact sympathetic control of the cardiovascular system, the skin blood flow response to cooling in this population might still be impaired as compared with healthy controls. In order to further examine this factor, only people with complete injury (ASIA grade A) will be recruited into the study. Based on results from this research design, we will be able to predict microvascular response to local cooling in individuals with incomplete SCI.

Expected Findings and Deliverables

Local cooling has a great potential to complement current technology used to prevent pressure ulcers in persons with SCI. We anticipate that the proposed research will provide objective evidence supporting that the application of local cooling preserves tissue viability of the weight-bearing soft tissues. Our approach will used to guide the selection of temperature and rate settings to accommodate people with various degrees of neurological impairment. Such findings will facilitate the development of commercial cushions with cooling capacity for the prevention of pressure ulcers in persons with SCI.

---

Project Update

Configuration of the test instrumentation, IRB approval and a pilot study on ten young healthy adults have been completed. The developed integrated system worked well to control pressure and temperature and no adverse events occurred. Results of the pilot study have been reported in three extended abstracts, a Masters thesis, and a draft peer-reviewed manuscript. The pilot study and analysis revealed the need for modification to our instrumentation. We have completed instrument modification and started screening subjects with SCI. The system successfully controls skin temperature at the two different cooling rates that will be evaluated in the second phase of testing. Based on the analysis of the pilot data, an attenuated reactive hyperemic response was found after local cooling application. This indicates that local cooling provides a protective effect to the skin under localized pressure. Further experiments using slow and fast cooling rates will determine the cooling rate that is most beneficial for enhancing tissue tolerance under localized pressure in individuals with SCI.

---

Publications

Tzen Y, Jan YK, Brienza DM. Development of a system to study the effect of local cooling on skin blood flow response to interface pressure. The Rehabilitation Engineering and Assistive Technology Society of North America (RESNA) Annual Conference, Washington, DC, 2008.

Tzen Y. Effects of local cooling on skin perfusion response to pressure: implications to pressure ulcer prevention. MS Thesis, published in University of Pittsburgh Electronic Dissertation Database, 2008.

Tzen Y, Jan YK, Porach EA, Karg PE, Brienza DM. Effects of local cooling on sacral skin perfusion response to pressure: implications for pressure ulcer prevention. National Pressure Ulcer Advisory Panel (NPUAP) Biannual Conference, Washington, DC, 2009.

Tzen Y, Loughlin PJ, Brienza DM. Analyzing the mechanisms of local cooling on pressure induced reactive hyperemia by applying short-time Fourier transform (STFT). The Rehabilitation Engineering and Assistive Technology Society of North America (RESNA) Annual Conference, New Orleans, LA, 2009.