Early Experience and Recommendations for the Use of an Automated Estimator of Solution Volume for Negative Pressure Wound Therapy With Instillation

NPWTi is a device that combines NPWT with intermittent dwelling of a solution (VAC Veraflo; 3M Inc). NPWTi is used for a wide variety of wound indications and anatomical locations.¹ The latest derivation of this device was introduced in 2013 and has steadily gained acceptance over the past 10 years. This rise in acceptance has coincided with a series of peer-reviewed publications that include a multicenter, randomized, controlled trial; a meta-analysis; and an economic model. NPWTi has been demonstrated to be superior to other wound therapies, including traditional NPWT, in such important clinical and economic considerations as: (1) bacterial counts, (2) time for wound conversion to a healthier state, (3) number of operative visits, (4) length of hospitalization, (5) likelihood of 30-day readmissions, and (6) overall health care costs.^2-4

NPWTi is largely automated, with the flexibility to change the settings for duration of dwell and negative pressure based on clinician preference. The volume of solution instilled per cycle requires an estimation by the clinician. The estimate of solution amount is determined by the wound dimensions, anatomical location, and whether a bridge is utilized. This solution volume estimation is often difficult and can vary widely. There is a balance between providing enough solution to saturate the entirety of the wound bed while preventing the overestimation of solution volume needed. Overestimation can lead to loss of seal of the drape and periwound maceration. Clinicians who have experience with this device are able to identify this balance; however, new or infrequent users have anecdotally reported difficulty determining this estimation. Further, some clinicians have not attempted to use the device due to this perceived difficulty.

A new software upgrade on NPWTi provides for automatic calculation of the volume of solution needed (Smart Instill Feature; 3M Inc). This AESV is based on benchtop studies examining the saturation curves of the foam dressing as well as other variables (eg, fluid dynamics across an irregular surface)(internal data; 3M Inc). This software upgrade deliberately provides a conservative estimation of the volume of solution needed for a given wound. To the authors’ knowledge, this is the first manuscript describing early observations for the use of the AESV through a case series across 3 different institutions of 23 patients for whom this NPWTi software upgrade was used.

This is a case series of 23 patients who received NPWTi with AESV. Because this was a nonexperimental, small case series that utilized an established device (US Food and Drug Administration approved), IRB approval was not required. Three institutions contributed to this cases series: 2 academic institutions and 1 community hospital. The data were compiled into a single data set and evaluated together. Patients selected for AESV were already scheduled to receive NPWTi. There was no attempt to preselect these patients, and there were no eligibility criteria utilized. Data were obtained from the electronic medical records from each institution. Deidentified patient and wound demographic information, including age, sex, wound etiology, wound size, NPWTi therapy settings, duration of therapy, and volume of solution was collected.

The authors examined the quality of the wounds to subjectively determine whether there was “adequate” or “inadequate” volume of solution determined by the AESV immediately after the termination of therapy. This subjective determination of whether or not the AESV was “adequate” or “inadequate” was based on the appearance of the quality of the wound. The quality of the wound included the amount of granulation tissue versus nonviable tissue present. No formal or standardized guidance was provided to authors as to the assessment of quality of tissue or the length of NPWTi use before examination of the wound. The volume of solution was determined “adequate” if the authors believed that the wound appearance was similar to the results of the authors’ prior experience with NPWTi in which the authors calculated their own volume estimations. The authors each have approximately 10 years of experience with NPWTi. No formal comparative or descriptive statistical comparisons were performed. Means and ranges are presented.

A total of 23 patients (15 male, 8 female) with a mean age of 59 years (range, 21-85 years) were included in this case series (Table). Data in the Table are grouped by anatomical location of each patient’s wound, beginning with the upper extremity and descending to the foot and comprising 13 distinct anatomical regions: upper extremity (n = 1), torso (n = 1), spine (n = 1), abdomen (n = 1), flank (n = 1), sacrum (n = 5), gluteal (n = 1), groin (n = 2), trochanter (n = 1), thigh (n = 3), below-knee amputation stump (n = 1), lower leg (n = 2), and foot (n = 3). Wound etiologies included infection (n = 5), surgical dehiscence (n = 6), pressure injury (n = 7), hematoma (n = 2), burn (n = 2), and ischemia (n = 1). The mean wound area was 155 cm² (range, 5-1700 cm²), and the mean wound volume was 223 cm³ (range, 7.5-1700 cm³).

The negative pressure setting was -125 mm Hg, but other device settings and length of therapy varied widely among the 23 patients. The duration of negative pressure ranged from 1 to 3 hours; the dwell period also varied with a range of 1 to 15 minutes. The total length of therapy ranged from 2 to 51 days.

The mean volume of instillate recommended by the AESV was 27 mL (range, 6-100 mL). The authors determined that the AESV provided “sufficient” volume 65% of the time (15/23 cases).

NPWTi has tremendous potential in the adjunctive care of chronic and acute wounds. Its ability to convert wounds efficiently and effectively to a healthier state can have a positive impact on the delivery of care. However, its utility has been hampered by the perceived difficulty of estimating adequate volume of solution. New and intermittent users have anecdotally reported a high leak/loss of seal and periwound maceration due to overestimation of volume. Thus, both groups have significant reservations about this therapy and may be quick to abandon this option. Further, individuals who have never used this therapy have been reluctant to adopt NPWTi due to this perception.

The authors each have at least 10 years of experience with NPWTi and are therefore able to estimate solution volume more accurately, which has led to infrequent instances of leaks and periwound maceration while maximizing the clinical results. The AESV can potentially increase the utility for novice or infrequent users as well as increase the adoption of NPWTi by nonusers.

The data from this case series suggest that for larger wounds (>120 cm³), the AESV underestimates the volume of solution needed (Figure 1). These “insufficient” wounds did not display the robust change in wound quality that the authors previously experienced when estimating their own volumes. However, it must be noted that even in these cases, the results were still better than compared with past observations of the impact of traditional NPWT on the wound bed. It is important to remember that AESV deliberately provides very conservative estimates of volume of solution needed, calculating a maximum amount of 200 mL. This inbuilt constraint is a purposeful attempt to reduce the potential for leaks and periwound maceration. Thus, the current results are not surprising for larger wounds that require larger volumes of solution. In these cases, the suboptimal clinical outcomes observed were most likely the result of the AESV providing inadequate volume of solution. Further, there does not appear to be any observable linear correlation between the wound volume and solution volume estimate for the wounds deemed “insufficient.” There also does not appear to be any correlation between solution volume estimate with any specific anatomical location or bridging technique used.

The negative results presented in this case series may reflect an inadequate amount of volume of solution required to bathe the entirety of the wound. It is important to remember that the AESV uses a calculation that predominantly relies on the benchtop model of a saturation curve of the foam dressing. It also provides a conservative volume of solution recommendation. If the wound is large and/or irregular, the foam may not be in full contact with the entire wound surface. Thus, the AESV will underestimate the volume of solution needed. The amount and degree of the foam packed into the wound may also influence the volume of solution applied by the AESV.

The authors believe that NPWTi requires “super-saturation” of the foam for the solution to bathe the entire wound surface. In other words, the excess fluid not contained in the foam dressing during the dwell cycle may be of significant benefit. Since the AESV does not take this into account, there is an underestimation of amount of solution needed. There is no consensus of what the “optimal” volume of solution is for a particular wound. There have been 5 multidisciplinary expert panel publications over the past 10 years, but none of these publications make any definitive volume recommendations due to this uncertainty.^1,5-8 There are generally accepted practices to evaluate sufficient solution saturation, such as observing for color changes to the foam, retrograde flow of solution to the track pad when pushing on the foam, and monitoring for the onset of tenting of the drape. Others have developed equations that consider wound size multiplied by a constant (eg, 0.2x) to estimate the solution volume. No definitive or reproducible method, beyond individual user experience, has been able to determine sufficient volume of solution.

For novice or intermittent users of NPWTi, the authors of the current study concur that the AESV is sufficient to obtain positive results and is a good starting point. The clinical results for the use of NPWTi should be better than what is observed with traditional NPWT. It is undesirable to use NPWTi if the clinical result is no better than what is observed with traditional NPWT, but prior studies have clearly demonstrated that NPWTi provides superior results to standard NPWT.^9-14

Experienced users, including the authors, have anecdotally reported the overestimation of the volume of solution when they initially began using NPWTi. Despite the sensitivity to leaks and periwound maceration, the robust impact on wounds justified close monitoring and continued use. With growing proficiency, experienced users began to suggest a reduction of solution volume by approximately 20% of the initial estimate. This 20% reduction in volume did not result in poorer outcomes. The authors postulate that there is an “ideal” volume somewhere between the automated volume estimator and that of the experienced user (Figure 2). The novice or intermittent user should begin with the AESV and then, after gaining experience and confidence, titrate the volume of instillation upwards until the desired effect is achieved in terms of reducing leaks and periwound maceration, while maintaining positive clinical effects. AESV also can provide a degree of confidence for individuals who have not used NPWTi due to the perceived difficulty of solution volume estimation.

There are multiple confounding variables that make interpretation of the current observations difficult. These data reflect a wide variability for the settings for NPWTi as well as the total duration of use. This variability reflects the authors’ subjective determination of the what the wounds needed as well as the individual circumstances of the patient. Further, there was no standardization of the type of solution used or, most importantly, the assessment of wound quality. The results of this case series may reflect different practice patterns among the authors and their respective institutions rather than the clinical outcomes of the device. This is an inherent limitation in interpreting the results of any case series. The focus of this case series was to subjectively assess whether the AESV provided a reasonable amount of solution as compared with the authors’ prior experience with NPWTi. It is not the goal of this case series to determine the clinical efficacy or effectiveness of NPWTi; however, these initial observations may still be helpful to clinicians.

Most clinicians agree that a good clinical outcome for NPWTi is defined as a wound that is free of nonviable tissue (ie, slough, eschar, fibrotic tissue, necrotic tissue) and contains a majority of granulation tissue (Figure 3). There is no widely accepted measure to quantify this subjective assessment. Thus, it is difficult to determine whether a given volume of solution is adequate to produce a positive clinical outcome because it is difficult to objectively measure it. The authors’ individual subjective assessment of whether there was sufficient volume may be a critical flaw of this case series. However, the authors share a common definition (as previously described), but there was no standardization or operational definition provided before the assessments were performed.

The authors of the current study believe that NPWTi provides a superior result over other wound therapies, including traditional NPWT, for the inpatient management of acute or chronic wounds. The AESV provides a good therapeutic baseline for the novice or intermittent user. However, decisions on settings, including solution volume, should be customized to the needs of the wound and patient. There is no doubt there will be variability from patient to patient. It is important to acknowledge that this paper reflects only a small case representation of early experiences with this software upgrade. Thus, the interpretation of the authors’ impressions should be carefully considered.