Comparative Occipital Pressure Mapping in the Operating Room

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Pressure ulcers—now termed pressure injuries (PIs)—are the breakdown of skin and underlying tissues caused by prolonged pressure on bony prominences. These injuries are often seen in health care settings and can lead to patient suffering, significant morbidity, high health care costs, and even mortality. Risk factors for developing a PI include immobility, diminished perfusion, malnutrition, and sensory loss. Many of these risk factors are present in the perioperative period. Long surgical times in which the patient is immobile, hemodynamic instability during the operation, the patient’s inability to sense pain, and the patient’s positioning for surgery are all contributing factors. The incidence of PIs in surgical patients is estimated to be approximately 15%.¹ Teleten et al found a higher risk of PI for patients undergoing surgery in the lithotomy position,² and surgical patients are most at risk of developing PI in the sacrum, heels, shoulders, elbows, and occiput.³ Patients undergoing cardiac surgery were most likely to develop a PI.

            Pressure injury risk factors that are particularly common in cardiac surgery include prior PI, anemia, impaired renal function (serum creatinine > 3 mg/dL), hemodialysis, either extremes of the body mass index, total length of time in surgery and number of surgeries, ICU length of stay, post-op mechanical ventilation, vasopressor use, hypotension, and postoperative use of steroids.^1,4,5 The patient in Figure 1 had several of these risk factors for developing a PI. It is important to reexamine institutional practices for PI prevention in high-risk cardiac patients who present with more than one of these risk factors.

            Pressure injuries are believed to occur when prolonged pressure exceeding a certain threshold limits blood flow to the tissues, causing tissue ischemia and eventually necrosis. Typical capillary blood pressure is estimated at 16-33 mmHg,² and surface pressures above this threshold may occlude capillaries and put patients at risk of developing PIs, particularly when mean arterial pressures are low (ie, below 70 mmHg).⁶ The incidence of occipital PIs is low overall, making it difficult to study the effects of an intervention for preventing PIs that may start and evolve from a surgical procedure.

Few studies exist that examine the effectiveness of different products indicated for pressure redistribution in the OR. Teleten et al have studied pressure mapping of different OR surfaces with measurements of peak pressures, average pressures, and surface area in volunteers in different positions.^2,7

            One study⁸ used computational finite element modeling to look at the difference in sheer stress between the non-powered fluidized positioner and standard medical foam, and the authors saw a reduction in sheer stress on the occiput with the non-powered fluidized positioner. The same finite element modeling was used to look at the difference between the non-powered fluidized positioner and a gel donut, again showing a reduction in stress on the occiput.⁹

            Pressure mapping technology has been developed to measure peak pressures, average pressures, and surface area for pressure redistribution properties. By measuring the peak pressures and surface area, the pressure distribution over bony prominences can be measured, and favorable cushioning strategies extrapolated and designed. Ideal intraoperative cushioning over bony prominences imparts lower pressures over a large surface area, which is associated with a lower risk of developing a PI.⁷ Pressure mapping can also be used to further investigate the difference between commonly used head cushions/pillows used in the OR and guide practice.

Purpose

To find the occipital cushion/pillow with the lowest measured peak pressures and the highest measured surface area using the Pro-V3 mattress and software (XSENSOR Technology Corporation).

Three volunteer members of the OR nursing staff were recruited as participants based on their specific interest in this quality improvement (QI) project and their interest in preventing PIs. Besides these criteria for inclusion, there were no specific exclusion criteria.

This was a QI study using volunteer nurses in the OR to compare how different cushions/pillows commonly used in the OR pressure mapped with different subjects.

Cushions/Pillows. Five different pillow surfaces were tested based on what is most commonly used and available at the authors’ institution. The pillows included: standard pillow with pillowcase, non-powered fluidized positioner, medium-sized static seat cushion (17 in × 17 in × 1.5 in) placed under the shoulders and head, pediatric sized static air cushion (13 in × 13 in × 2 in) placed under the head, and foam donut (Figure 2).

Pressure sensor. Each volunteer was supine on a standard OR bed. The XSENSOR X3 PX100 (XSENSOR Technology Corporation) (24 × 24 sensors with electronic sensors 0.5 in. apart) was placed between the volunteer’s head and the selected cushioning device. Pressure mapping systems typically utilize multiple sensors that measure the pressures at numerous points on the pressure mat and output the data into a color-coded image on a handheld device for real-time assessment. These systems allow for measurement of the pressure at the interface between the body and a surface, in this case between the occiput and the pillow. The pressure mapping instrument used in this study is shown in Figure 3.

            The outcomes measured were average pressure (mmHg), peak pressure (mmHg), and surface area (sq in). Low average peak pressure, low peak pressure, and high surface area are thought to reduce the risk of PI.

A pressure map of the occiput against each cushion/pillow can be seen in Figure 2. As seen in the legend along the righthand side, areas with pressures >30 mmHg appear in red on the color map.

            The Table shows the outcomes of the average pressure, peak pressure, and surface area measurements for each type of cushion/pillow.

            The medium static air seat cushion had the lowest peak pressure for 2 of 3 volunteers (Volunteer #1: 44.40 mmHg; Volunteer #2: 33.10 mmHg). The non-powered fluidized positioner had the highest peak pressure for 3 of 3 volunteers (Volunteer #1: 71.80 mmHg; Volunteer #2: 58.6 mmHg; Volunteer #3: 55.7 mmHg). The difference in peak pressure between the medium static air seat cushion and the non-powered fluidized positioner was not statistically significant (P = .2) using a paired t-test.

            The medium static air seat cushion had the lowest average pressure for 2 out of 3 volunteers (Volunteer #1: 18.8 mmHg; Volunteer #2: 18.7 mmHg), while the non-powered fluidized positioner had the highest average pressure for 3 out of 3 volunteers (Volunteer #1: 29.50 mmHg; Volunteer #2: 21.3 mmHg; Volunteer #3: 30.6 mmHg). The difference in average pressure between the medium static air seat cushion and the non-powered fluidized positioner was not statistically significant (P = .064) using a paired t-test.

            The foam donut had the lowest peak pressure and average pressure for 1 out of 3 volunteers (Volunteer #3: peak pressure, 38.5 mmHg, average pressure, 21.9 mmHg).

            The surface area was greatest for the standard pillow with pillowcase for volunteer #1 (49.00 sq in), medium static air seat cushion for 1 volunteer (24.7 5 sq in), and donut for 1 volunteer (47.25 sq in).

When comparing the data between the different head cushions, the least favorable pressure profile was the non-powered fluidized positioner, which had the highest average pressure and peak pressure for all 3 volunteers. The most favorable pressure profile was the medium static air seat cushion, with the lowest average and peak pressures for 2 out of 3 of the volunteers. A large surface area to distribute pressure is considered favorable; none of the cushions consistently demonstrated a larger surface area of pressure distribution. Extrapolating these pressure mapping results to the risk of developing an occipital pressure injury, the data would suggest the use of a medium static air seat cushion over the non-powered fluidized positioner.

            At the authors’ institution, the non-powered fluidized positioner is routinely used in the cardiac OR, where patients have several risk factors for developing PIs: long surgical times, prolonged ICU stay, high risk of hypotension, frequent vasopressor use, and postoperative mechanical ventilation.^1,4,5 There is a belief among providers that using the non-powered fluidized positioner helps reduce the risk of developing occipital PIs and redistributing pressure on the occiput. Despite the common practice at the authors’ institution of using the non-powered fluidized positioner as a pillow/head cushion throughout surgery, the results of the current study do not support this as the best option. The non-powered fluidized positioner is marketed as an easily customizable fluidized pressure redistribution device to assist in positioning and repositioning patients.¹⁰ When using the non-powered fluidized positioner as a head cushion in the current study, however, it had an unfavorable pressure mapping profile, with high peak pressures and high average pressures. The pressure mapping data of the current study favors using the medium-sized static air cushion (17 in × 17 in × 1.5 in), placed under the shoulders and head instead.

Implications. Future studies should include pressure mapping with recording of patients’ measured pressures and surface area while under anesthesia over the entire duration of the surgical procedure. These studies need to include measurements of perfusion and pressure mapping with postoperative follow-up so that early occipital pressure injuries that occur in the OR can be identified.

            Further tools for early identification, such as dressings that would measure pressure and oxygen saturation or temperature at the bony prominence in real-time, should be developed and validated.

Limitations of this project include the small sample size and the fact that the study was conducted a single site. This study used a convenience sample of staff volunteers with an interest in QI and preventing PIs; as such, there may be differences between this population and the surgical patient population. The small sample size makes it impossible to evaluate other potential contributors to occipital pressure injuries, such as sex, body habitus, ethnicity, or even hair style. It is also possible that the occipital pressures differ between a conscious patient and one who is receiving general anesthesia. Duration of evaluation was another limitation of this study. Presumably, given a longer measurement time, head cushions could change shape or distribute pressure differently. Using different head positions could have implications as well. There are other features of head cushions that may be favorable for positioning—such as elasticity, mouldability, and ability to adjust to head contours—that were not directly addressed in this study.

Pressure mapping can be used to compare cushioning strategies for patients to determine which strategies have a more favorable pressure re-distribution, with a goal of ultimately reducing the incidence of pressure injuries.

Authors: Lauren Walden, MD; Oleg Teleten, MS, RN, CWCN; Lisa Peterson, MS, RN; Aubrey Yao, MD; and Holly Kirkland-Kyhn, PhD, FNP, GNP, CWCN, FAANP 

Affiliations: University of California Davis Health System, Sacramento, California

Disclosure: The authors disclose no financial or other conflicts of interest.

Correspondence: Lauren Walden, MD; University of California Davis Health System, 4301 X St, Sacramento, CA 95817; lmwalden@ucdavis.edu

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