Changes in temperature of heel skin under pressure in hip surgery patients

Objectives: To examine the effect of external pressure of the bed surface on heel skin temperature in adults in the first three days after hip surgery. Design: A quasi-experimental study in a prospective within subjects repeated-measures design. Setting: Two acute care hospitals. Participants: Eighteen subjects (9 men and 9 women) with a mean age of 58.3 (± 16.1) years were recruited after hip surgery at two hospitals. Methods: Temperature sensors were placed on the plantar surface of each foot, close to the heels. Measures were taken when the heels were 1) suspended above the bed surface for 20 minutes (preload), 2) on the bed surface for 15 minutes (loading), and 3) suspended again above the bed surface for 15 minutes (unloading). Main Outcome Measures: Heel skin temperature and demographic data. Results: Heel temperature increased during loading and unloading in both legs on postop days 1 (p = 0.003) and 3 (p = 0.04) but not on post-op day 2. Heel temperature in the non-operative leg decreased in the first 3 minutes of unloading on post-op days 2 (p = 0.02) and 3 (p < 0.01). Conclusion: Heel temperature increased with loading and unloading on postoperative days 1 and 3. Upon immediate unloading, hyperemic response was present only in the non-operative leg. Keeping the heels off the bed surface at all times may avoid heel skin temperature changes and prevent tissue damage. Further research is needed to identify the mechanisms that explain the effect of external pressure on heel temperature. Introduction Pressure ulcers are a frequent complication of hip fracture and its surgical repair. 3 The sacrum and heels are the most common sites of pressure ulcer and the incidence of heel ulcers is increasing. Factors that contribute to heel pressure ulcers in the operative leg include decreased perfusion 5 and reduction in femoral blood flow during hip surgery. Likely explanations for decreased perfusion in the non-operative leg are increased pressure at the groin intraoperatively, lateral positioning on the operating table, use of hypotensive anesthesia, and pre-existing vascular diseases. In addition, the nonoperative leg is often used as a pivot point for turning and repositioning in bed, making it susceptible to intermittent friction and pressure. Consequently, perfusion in both the operative and non-operative legs may be compromised and the heels are at risk of pressure ulcer development. In addition, surgical pain may deter the patient from moving the operative leg, with the heel resting on the bed surface for extended periods of time. Skin temperature has been used as a measure of perfusion, yet the relationship between temperature and perfusion is not linear. Heat loss in the subcutaneous tissue and the skin is regulated by vasoconstriction and vasodilatation. Skin temperature is influenced by the difference between ambient temperature and blood temperature and perfusion is correlated with blood temperature. Depending on clinical circumstances, external pressure can increase or reduce skin blood flow. Release of pressure may lead to reactive hyperemia; that is, a transient increase in blood flow. At the other extreme, external pressure can reduce blood flow and produce ischemia, the sine qua non of pressure ulcer development. Few studies have explored whether a change in skin temperature is related to external pressure and pressure ulcer development. If a change in skin temperature can reflect skin blood flow, it can also represent the change in external pressure. This study was conducted to examine the relationship between external pressure and heel skin temperature in the non-operative and operative leg over three days after surgery in adults. The study questions were: 1. Is there a difference in heel skin temperature in the non-operative leg compared to the operative leg during repeated loading and unloading conditions in hip surgery patients on postoperative days 1, 2, and 3? 2. Is there a difference in heel skin temperature during the first 3 minutes of unloading compared to the last minute of loading on postoperative days 1, 2, and 3 after hip surgery? Background Blood Flow and External Pressure Vascular flow is affected by the application and removal of external pressure. Increased blood flow in the human hand was noted when external pressure of various non-extreme magnitudes was applied. Pressure-induced vasodilatation is thought to be the body’s protective mechanism against external pressure. In another study, arterial blood flow doubled in the forearm of healthy men when compression stockings were applied to the forearms for 10 minutes. The increased blood flow and local vessel dilation was attributed to a decrease in transmural pressure in the blood vessel. Effect of Pressure on Skin Temperature Blood flow and external pressure affect skin temperature. A gradual rise in skin temperature at the pressure site was recorded in rats when increasing external pressure was applied. Similar skin temperature response has been shown in human when trochanter skin was subjected to pressures of 100 mmHg, 200 mmHg, and 300 mmHg for periods of 11 to 67 minutes. With loading, skin temperature increased initially and then decreased but with the removal of pressure, the temperature increased and peaked after 3 to 5 minutes. The increase in skin temperature during unloading in another study was affected by both the magnitude and the duration of applied pressure. Greater pressure and longer duration resulted in higher post-unloading temperature. A study on the sacrum and gluteus maximus muscle in both older and younger adults also demonstrated that skin temperature increased after removal of external pressure. Increased perfusion (i.e. reactive hyperemia) was observed when pressure was removed. Skin Temperature & Pressure Ulcer Formation Skin temperature was examined in undamaged but erythematous skin using ultrasonography. Data showed that capillaries and venules were engorged with red blood cells, suggesting that the engorged blood vessels caused reduced blood flow, persistent erythema, and decreased skin temperature. Similarly, another study showed that skin perfusion was higher in the center of the nonblanchable erythema (stage 1 pressure ulcer) and decreased toward the edge in patients in the acute care and geriatric units. It is possible that the increased central temperature reflected inflammation while the decreased distal flow in the small vessels was due to obstruction of vessels with red blood cells. In another study, increased skin perfusion was also found in the damaged area as compared to the undamaged skin on the sacrum in older adults. In this situation, the temperature in the damaged skin did not change. These finding are not consistent with those of others; however, the authors suggest that increased perfusion in the damaged skin might have dissipated enough heat so that skin temperature was not increased. Pain Pain induces sympathetic nervous activities with subsequent arteriolar vasoconstriction leading to reduced peripheral blood flow that may impact skin temperature. Pressure Ulcers & Hip Surgery Patients Pressure ulcers are a frequent complication of hip fracture and surgery with a common site of ulcer development being the heels. Perfusion in both legs may be impaired by anesthesia, operative position, and pre-existing vascular disease. These unresolved issues led to the present study to explore the relationships between external pressure and temperature of the heels during loading and unloading in postoperative hip fracture patients. Methods In this quasi-experimental study nested within a larger longitudinal study, a prospective within-subjects repeated-measures design was used to study the effects of loading and unloading on skin temperature. An actual clinical condition was used and the weight of each subject’s foot served as the loading pressure, with the recognition that loading pressure varies by foot weight and heel architecture. All subjects in this study were positioned on a pressure-reduction mattress (Hill-Rom, Model Advanta) as part of routine nursing care. The independent variable was the loading condition with two levels, loading and unloading. The baseline measure (preload), intervention (loading), and post-test measure (unloading) were sequenced as follows: Preload was defined as the time period right before loading and after the 20-minute stabilization when heels were off the bed surface. Studies examining skin temperature on the plantar aspect of the foot found that stabilization of temperature is achieved after 15 minutes. 21 Procedures in this study were consistent with these recommendations, ensuring a stable baseline. Loading was the 15-minute time period when the heels touched the bed (generated by the subject’s full weight of the foot); and Unloading was defined as the 15-minute time period when the heels were kept off the bed surface after the heels were placed on the bed surface for 15 minutes. The dependent variable was the heel skin temperature. Subjects were encouraged to maintain maximal pain control during data collection. The non-operative leg served as a comparison to the operative leg. A convenience sample of 18 subjects was recruited from two San Francisco area hospitals after approval by the Institutional Review Board for each facility as well as that of the University of California, San Francisco. A preliminary power analysis on 9 subjects showed significant differences with 80% power. Included were persons > 21 years undergoing hip surgery who had both lower extremities intact, a score ≤ 5 on the Mental Status Questionnaire (MSQ), an ankle-brachial index (ABI) > 0.9 in both legs, intact plantar surface foot sensory perception measured with a 5.07 Semmes-Weinstein (S-W) monofilament, and a fasting glucose for diabetics of < 140 mg/dL or a preoperative random plasma glucose of 120-180 mg/dL. Persons with existing chronic foot ulcers or medical conditions asso