Electrical Foot Stimulation: A Potential New Method of Deep Venous Thrombosis Prophylaxis

James J. Czyrny; Robert E. Kaplan; Gregory E. Wilding; Christopher H. Purdy; Jack Hirsh

Disclosures

Vascular. 2010;18(1):20-27. 

In This Article

Methods

Institutional Review Board approval and informed consent were obtained from all subjects. Forty healthy subjects between the ages of 50 and 80 years participated in the study. Half of the subjects were nonobese with a body mass index (BMI) < 30. The other half of the subjects were obese with a BMI > 30. Exclusion criterion included a prior history of DVT or pulmonary embolism, the presence of a cardiac pacemaker, or any trauma or surgery involving any part of the lower extremities.

Subjects were studied over two sessions. At each session, subjects received either electrical foot stimulation or IPC of the calf of one leg. The other lower extremity served as a control. Subjects received one therapy at one session and the other therapy at a second session at least 48 hours later. The subjects were randomly assigned using a computer-generated protocol as to which leg would be treated and the order in which the type of therapy was to be given.

The study otherwise followed the protocol used in our initial study of electrical foot stimulation. Subjects were seated for 4 hours in chairs placed at a fixed distance apart. They were constantly monitored throughout the study to ensure that they remained seated. Subjects were allowed to use a bathroom located several feet away only twice during the 4-hour period. During the 4-hour study period, subjects were offered a maximum of 16 ounces of fluid and a normal lunch.

Electrical foot stimulation (see Figure 1) was produced by surface electrodes placed on the sole of the foot over the plantar muscle group. Electrical stimulation was delivered by the Focus Neuromuscular Stimulation System (Empi, Inc., St. Paul, MN). The crucial stimulus parameters were biphasic symmetric square wave at 50 pulses per second, phase duration of 300 microseconds, a starting ramp uptime of 2 seconds and a finishing ramp downtime of 2 seconds per stimulation cycle, and a stimulation cycle of 12 seconds "on" and 48 seconds "off" per minute. Stimulation was increased to an intensity just sufficient to create a slight visible muscle twitch. This level of intensity caused no evident discomfort in any of the subjects in our first study. Subjects were continually monitored throughout this study for any indication of discomfort.

IPC of the leg consisted of external IPC with a kneehigh device. A Tyco Healthcare Kendall Novamedix A-V impulse system Model 6060 (Tyco Healthcare, Mansfield, MA) was used. Operating parameters were 130 mm Hg impulse pressure with 3-second impulse duration and "Program Preset 1" for DVT prophylaxis. The compression followed the approved standard patient protocol used at our institution detailed in Utilization of Intermittent Pneumatic Compression (IPC) Stockings for DVT Prophylaxis.[12–14]

Popliteal and femoral venous peak blood flow velocities were measured bilaterally using a Doppler ultrasound device at 0, 15, 120, and 240 minutes. Measurements were taken at the midpoint of the "on cycle" for electrical foot stimulation and pneumatic compression. The same ultrasound technician obtained all Doppler studies on all subjects throughout the study. All Doppler tracings were read by the same independent reader in a blinded fashion. Immediately following completion of each 4-hour session of electrical foot stimulation or IPC, subjects were asked to complete a brief questionnaire (Appendix) regarding their acceptance and tolerance of electrical stimulation or IPC.

This study was designed to compare venous blood flow velocity in the seated position with our device with IPC. The seated position was chosen because most patients who are discharged from the hospital will be sitting for a significant period of time during the day as they recover. Additionally, long-distance travelers, who are at risk for VTE, are also in the seated position.

Statistical Analysis

The primary objective of this clinical trial was to demonstrate the noninferiority of the experimental treatment (electrical foot stimulation) relative to standard accepted treatment (IPC). The noninferiority test was chosen because it matched the clinical goal to assess if the new device was similarly effective to compression. Noninferiority is a widely accepted standard when assessing new treatment modalities. This is particularly true of new treatment modalities that are safer, have fewer side effects, or offer greater compliance because of enhanced ease of use. IPC is presently used in the acute hospital setting as a means of mechanically increasing venous blood flow velocity. It has been demonstrated to reduce the risk of DVT for a number of conditions. In this way, the equivalence of electrical foot stimulation to IPC could be assessed. Owing to this, the statistical methods used in this study followed those recommended for reporting noninferiority trials.[15]

The study used a repeated measures design with two within-group factors: time from baseline and stimulation (stimulation, control). That is, all treatment comparisons were based on paired differences at each time point. Blood flow velocity measurements (at time = 120 and 240 minutes) were analyzed using two different approaches. A noninferiority index of 5.0 cm/s was chosen at the start of the study. The primary analysis was based on the use of paired t-tests after accounting for the noninferiority index. The secondary analysis accounted for baseline differences and made use of a mixed model. Specifically, change from baseline was modeled as a function of the fixed effects treatment group and baseline and a random subject effect. Tests corresponding to this model also were based on the same noninferiority index (5.0 cm/s). The questionnaire results were analyzed using the Fisher exact test, and p values < .05 were considered to be statistically significant. All analyses were done with SAS 9.1 (SAS Institute, Cary, NC).

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