Neuromuscular electrical stimulation of the foot muscles for...

Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems

Reexamination Certificate

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Reexamination Certificate

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06615080

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the use of single channel Neuromuscular Electrical Stimulation (NMESS ) of the lower extremity for the prevention of Deep Vein Thrombosis (DVT) and Pulmonary Embolism (PE) by reducing the pooling of blood in the soleal veins of the calf.
BACKGROUND OF THE INVENTION
Venous thromboembolic disease (VTED) continues to be a cause of significant morbidity and mortality for individuals immobilized during prolonged travel, after orthopedic surgery, neurologic disorders, and a variety of other conditions.
Virchow in 1845, postulated that changes in blood flow, vessel wall, and blood constituents were responsible for venous thrombosis.
1
Studies have shown that at least two of the three factors are nee de d to trigger thrombosis. Reduction of blood flow, especially in the venous sinuses of the calf muscles has long been recognized as an important risk factor.
2,3,4
The venous pooling triggers coagulation and at the same time consumes local anticoagulants. This explains the high risk of DVT and PE in spinal cord injury, stroke, and post-surgery where immobility of the lower limbs occurs.
Homans
5
observed in 1954 that “prolonged dependency stasis, a state imposed by airplane flights, automobiles trips and even attendance at the theater, is able unpredictably, to bring on thrombosis . . . ” Until recently it was not known if only travel, without other risk factors, was sufficient to cause venous thrombosis. In 1977, Symington
6
showed that trips as short as three to four hours can induce deep vein thrombosis (DVT) and pulmonary embolism (PE), although longer trips are more usual. In 1999, Ferrari
7
demonstrated that, in people over 50 years of age, a history of recent travel is a risk factor by itself for thromboembolic disease. He further confirmed that travel longer than four hours increased the risk by four times, even in healthy individuals over 50 years of age. With the decreased legroom in airliners and the escalating time spent in the cramped situation of economy class, Cruickshank coined the term Economy Class Syndrome for the increasing travel-associated venous thromboembolic disease (VTED).
8
In travel associated VTED additional risk factors include pressure on the calves from the back of a seat exacerbating venous stasis,
9
dehydration,
10
hemocentration,
11
and a decrease in fibrinolysis.
12
Current approaches to prophylaxis include mechanical compression using pneumatic compression devices, and anticoagulation therapy. While both have demonstrated effectiveness, they have problems as well. Pneumatic compression equipment is clearly too cumbersome for mobile patients, or during prolonged travel. In addition, AC current is required for these devices and is not practical for these conditions. No manufacturer has designed a battery-operated device. Even if this were done, the battery would be too large and heavy to provide the needed electric current for more the six hours.
Anticoagulation therapy carries the risk of bleeding complications and must be started several days in advance to be effective.
Electric stimulation has theoretical advantages in that it can be started at the time prophylaxis is needed, and can be portable using DC current sources. Previous studies have shown that electrical stimulation is an effective means of increasing venous blood flow and preventing DVT. Nicolaides et al
13
studied 116 patients undergoing different operations under general anesthesia. An AC-operated Thrombophylactor supplied single impulse in square waveform. The intensity was strong enough to produce “brisk plantar flexion of the foot without violent movement of the leg”. This stimulation was discontinued at the end of the operation just before the patient woke up. It was found that a resting period less than 4 seconds did not allow the soleal veins to fill completely before the next stimulus and resulted in a progressive reduction in stroke volume from the calf muscles. On the other hand, a resting period longer than 5 seconds allowed the soleal veins to fill in the interval between stimuli, resulting in the maximum stroke volume. The optimum rate of stimulation was found to be 12-15 per minute and optimal duration of 50 milliseconds. This stimulus parameter produced the greatest improvement in femoral venous Doppler blood flow. It was shown by a 92% relative reduction in DVT incidence as determined by
125
I-fibrinogen testing.
Lindstrom et al
14
used groups of stimuli to produce a short-lasting tetanus vs. single stimulus on ten patients being operated on for various abdominal diseases. The aim of the study was to investigate whether a summation of contractions by grouped impulses was more efficient than a single impulse in reducing venous stasis. All patients were anaesthetized during the study. A Whitney strain gauge plethysmograph was used to record changes in calf volume during stimulation. Two surface electrodes were attached just below the knee joint and above the ankle on the back of the leg. A stimulator was used to deliver a square waveform, which could be varied in duration, amplitude and frequency. It was found that this short tetanus reduced calf venous volume three times more effectively than single stimulus. The strength of individual impulses was of the order of 40-50 mA. Using a stimulus duration of 50 milliseconds, they found that grouped stimuli at 8/min, 6 impulses/group and 8 impulses/second were most efficient at reducing venous volume as recorded by plethysmograph. In this study, a lower frequency of 8 grouped impulses per minute was used instead of 12-15 single impulses per minute as advocated in the Nicolaides study.
Merli et al
15
performed a prospective study of DVT prophylaxis in acute spinal cord injury (SCI) patients. In the study 48 patients were randomly assigned to saline placebo, subcutaneous low dose heparin or heparin with ES for 23 hours per day over 28 days. Stimulus parameters included a frequency of 10 Hz, duration of 50 microseconds, cycle of 4 seconds “on” and 8 seconds “off”. Both tibialis anterior and gastrocnemius muscles were stimulated to produce a moderately strong contraction. Surveillance for DVT was evaluated by daily 125-I fibrinogen scanning. A significant decrease in the incidence of DVT was noted in the subcutaneous heparin with ES group, but not in the group with placebo or heparin alone.
Jaweed et al
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studied the effects of ES in enhancing femoral venous flow in six normal subjects. A square wave with stimulus duration of 200 microseconds was given at frequencies of 2, 5 and 10 Hz in 2 to 3 sessions over four weeks. Maximum peak velocity was measured bilaterally in the supine posture. It was found that electrical stimulation (ES) at 10 Hz significantly increased femoral venous flow but not at lower frequencies of 2 and 5 Hz.
There a number of U.S. patents that teach methods of applying electrical stimulation for the prevention of DVT. These include the following patents: the Powell, III patent, U.S. Pat. No. 5,358,513; the Tumey patent, U.S. Pat. No. 5,674,262; the Dennis, III patent, U.S. Pat. No. 5,782,893; the Katz patent, U.S. Pat. No. 5,643,331; and the more recent Katz patent, U.S. Pat. No. 6,002,965. What these patents have in common is that they all describe methods of electrically stimulating the calf muscle.
These methods do enhance blood flow by causing the calf muscle to contract. But they also have some drawbacks that the present invention overcomes.
Positioning the electrodes on the calf muscle can be problematic. The differences in size and shape of peoples calf muscles requires fitting cuffs to ensure accurate placement of the electrodes. Often these electrode presenting cuffs or bands shift or slide down the leg with use. One irritating problem is the removal of the electrode from the calf when the hair on the leg becomes bound to the sticky electrode
Another difficulty with these calf stimulators is that the fatty tissue layer has a relatively high resistance to electric current. This fatty surface layer, between the electrode and the muscle benea

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