RESNA 28th Annual Conference - Atlanta, Georgia
Yih-Kuen Jan, PhD, PT1; David M. Brienza, PhD1, and Michael L. Boninger, MD2
1Department of Rehabilitation Science and Technology,
2Department of Physical Medicine and Rehabilitation, University of Pittsburgh, Pittsburgh, PA
Prevention of pressure ulcers may be possible using alternating pressure support surfaces. Alternating pressure appears to stimulate a protective increase in skin blood flow, but the physiologic mechanism by which this occurs is not well understood. Ten healthy adults were subjected to both constant loading for 20 min at 30 mmHg and alternating pressure for 20 min (5-min cycle ´4) at either 60 mmHg or 3 mmHg on their sacrum. A wavelet-based spectrum analysis was used to decompose laser Doppler blood flow signal. The results indicate that alternating pressure stimulates an increase in skin blood flow compared to constant loading (p<0.01). Power increased in the 0.05-0.15 Hz frequency band during the low-pressure phase of alternating pressure and decreased during the high-pressure phase. Our study suggests that optimization of alternating pressure parameters to compensate for impaired control mechanisms in patients may be possible using wavelet analysis of blood flow oscillations.
Alternating pressure, laser Doppler flowmetry, pressure ulcers, skin blood flow, wavelet transform.
Pressure ulcers exact a devastating loss of function, increase the risk of death, and increase healthcare costs. Conservative estimates of the costs associated with the management of pressure ulcers of all etiologies exceed $6.4 billion annually (1). Prevention of pressure ulcers may be possible using alternating pressure support surfaces. Alternating pressure appears to stimulate a protective increase in skin blood flow, but the mechanism by which this occurs is not well understood (2; 3).
Using wavelet-based spectrum analysis we have isolated five characteristic frequency bands embedded in the laser Doppler blood flow signal and correlated to physiologic responses (4; 5). The frequency bands 0.008-0.02, 0.02-0.05, and 0.05-0.15 Hz may be linked to endothelial nitric oxide, neurogenic control, and myogenic responses, respectively. This method allows studying interactions of different blood flow control mechanisms, and may greatly advance the understanding of the blood flow responses to pressure loading. We utilized this method to study skin blood flow responses to incremental pressure (0 to 60 mmHg, 5 mmHg step/3 min) (6; 7). The results show that mean blood flow decreased as pressure increased from 0 to 15 mmHg; mean blood flow increased as pressure increased from 15 to 60 mmHg. We extended our method and findings to study blood flow responses to alternating pressure. This study will provide insight into mechanisms by which skin blood flow is affected by alternating pressure operating parameters.
The application of alternating pressure on the sacrum stimulates myogenic responses as measured from wavelet analysis of blood flow oscillations, thereby enhancing skin blood flow.
Ten unimpaired subjects (5 male and 5 female) were recruited into the study. The demographic data were as follows: age 30.0 ± 3.1 years, height 162.9 ± 6.8 cm, and weight 58.3 ± 8.6 kg. Laserflo Blood Perfusion Monitor 2 (Vasamedics, Eden Praire, MN) and Softip pencil probe (P-435, Vasamedics) were used to measure capillary blood perfusion (mL LD/min per 100g tissue). A five-channel scanning thermistor thermometer (Cole-Parmer Instrument Company, Vernon Hills, IL) was used to measure skin temperature changes under the indenter. A computer-controlled indenter system was developed for use in this study and is described elsewhere (8).
A crossover design was used in which the order of treatment, constant or alternating pressure on the sacrum, was randomly assigned. Subjects were kept relaxed for at least 30 min prior to testing in order to achieve a stable skin blood flow and to allow them to acclimate to the room temperature. Subjects underwent the following loading regimens in randomized order: 1) an alternating pressure regimen consisting of 20 min (5 min cycles ´4) with the loading pressure applied alternately at either 60 mmHg or 3 mmHg and 2) a constant pressure regimen consisting of a 20 min loading period at 30 mmHg. A washout period of 40 min between regimens was used.
A wavelet-based time frequency analysis was used to decompose the laser Doppler blood flow signal (4). Three characteristic frequency bands were identified (0.008-0.02 Hz, 0.02-0.05 Hz, and 0.05-0.15 Hz) that are associated with endothelial nitric oxide, neurogenic, and myogenic origins, respectively. Paired-t tests were used to compare mean blood flow and power for constant loading and AP in each characteristic frequency band.
The results indicate alternating pressure stimulates an increase in skin blood flow compared to constant loading (p<0.01) (Figure 1). Skin blood flow during the high-pressure phase of four AP cycles shows an increasing trend (Figure 2).
Skin temperature was not shown to be significantly different during pre-loading, constant loading, alternating pressure, and post-loading periods (p>0.05). Mean skin temperature for all subjects increased by 1.04 °C from constant loading to post-loading and 1.35 °C from alternating pressure to post-loading.
An increase in power in the 0.008-0.02 Hz frequency band (i.e. endothelial related control) and a decrease in power in the 0.05-0.15 Hz frequency band (i.e. myogenic responses) during alternating pressure was observed compared to skin blood flow prior to loading (Figures 3 & 4). Power increased in the 0.05-0.15 Hz frequency band (i.e. myogenic responses) during the low-pressure phase of alternating pressure and decreased during the high-pressure phase of alternating pressure.
A significant finding of this study was that skin blood flow during alternating pressure was higher compared to constant pressure over 20 minutes. Since increased skin blood flow may enhance tissue viability, our study suggests that alternating pressure may enhance tissue viability. Our analysis of the skin blood flow signal power in individual frequency bands thought to be associated with corresponding endothelial related, neurogenic, and myogenic control mechanisms showed that these control mechanisms respond differently to constant and alternating pressure. Our study clearly shows skin blood flow of tissue under loading is dependent on the loading patterns of the externally applied pressure. Using this analysis may allow for the optimization of alternating pressure loading patterns for enhanced tissue integrity in individuals with impairments to specific control mechanisms.
Several researchers have shown that at-risk subjects have lower occlusion pressure thresholds, impaired reactive hyperemia, and diminished maximal vasodilatory function (i.e. smokers, elderly, SCI, cardiovascular dieses). These impaired microcirculatory functions result in diminished responses to different stimuli. Although different pathology exists in different patients, the final common pathway is insufficient blood flow supply (9). Myogenic responses and metabolic control are inherent properties of cutaneous microcirculation and are spared in people with SCI population. Thus, using alternating pressure to stimulate an increase in skin blood flow in SCI to enhance tissue viability and prevent pressure ulcers may be possible. For individuals with endothelial dysfunction (i.e. smokers, elderly population) the configuration of alternating pressure should focus on enhancing myogenic responses which could be monitored from wavelet analysis of blood flow oscillations.
Increased power in the metabolic frequency band and decreased power in the myogenic frequency band for alternating pressure compared to the pre-loading period were observed. Our result showing increased power in the metabolic frequency band during alternating pressure does not support our hypothesis. This unexpected result may be due to the high-pressure phase (60 mmHg) of the alternating pressure cycle completely inhibiting the myogenic responses of the sacral cutaneous microcirculation. This finding is supported by several research studies showing decreased myogenic response when perfusion pressure was decreased (e.g. increased externally applied pressure) (10).
This project is partly supported by the Department of Education, National Institute on Disability and Rehabilitation Research (H133G040222) and the Department of Veterans Affairs, Rehabilitation Research and Development Service (F2181C).
Yih-Kuen Jan, PhD, PT
Forbes Tower, Suite 5044
Pittsburgh, PA, 412-624-7732