Abstract:
Portable devices and methods for preventing deep vein thrombosis (DVT) by assuring that the ankle is flexed and extended sufficiently to promote blood flow in the lower leg are disclosed. The device includes an actuator with a free movement mode that allows a patient to move freely between activations or to initiate movement to delay a next automatic activation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application which claims priority to U.S. patent application Ser. No. 11/932,799 filed on Oct. 31, 2007; which claims priority to U.S. Provisional Patent Application No. 60/901,614 entitled “Deep Vein Thrombosis Prevention Device”, which was filed on Feb. 14, 2007, the contents of which are expressly incorporated by reference herein. 
    
    
     INCORPORATION BY REFERENCE 
     All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     Deep Vein Thrombosis (DVT) is the formation of a thrombus (clot) in a deep vein in a leg. The clot can block blood flow in the leg, or the clot may travel to the lungs causing a potentially fatal pulmonary embolism. The incidence of DVT is particularly high after hip or knee surgery, but may occur whenever patients are immobilized over a period of time. DVT occurrence is known to be high after lower extremity paralysis due to stroke or injury and is also a risk factor in pregnancy, obesity, and other conditions. 
     Current techniques for avoiding DVT have drawbacks. For example, blood thinning drugs have side effects, elastic stockings and compression devices have limited effectiveness, while compression and exercise devices have limited patient compliance. Active or passive movement of the ankle, alone or in combination with other DVT avoidance techniques, can reduce the incidence of DVT; however there has been no device to assure adequate movement that is acceptable to hospital patients and staff. 
     SUMMARY OF THE INVENTION 
     The present invention teaches a variety of methods, techniques and devices for preventing deep vein thrombosis (DVT). According to one embodiment, a DVT prevention device is attached to a patient&#39;s ankle, or any portion of any limb, to deliver active or passive movement to promote blood flow in the lower extremities. According to certain aspects, the DVT prevention device includes a battery or AC-powered actuator, an embedded computer, a software control system, sensors, and a coupling to the ankle and the foot. 
     According to another embodiment, a DVT prevention device operates in one or more modes to supply 1) passive extension and flexion of the ankle, 2) active extension and flexion of the ankle, and 3) free movement of the ankle. Patient compliance may be enhanced by allowing the patient to determine the preferred mode of operation; the device assures adequate total movement over a period of time by supplying passive movement when necessary. For example, the patient may perform enough movements in free-movement mode to delay future activations of the device, or the patient may actively resist the movement to exercise the calf muscles and promote enhanced blood flow beyond that of passive movement. 
     According to yet another aspect of the present invention, the present invention may include an output connection to allow the patient&#39;s extension and flexion of the ankle to serve as a human interface device similar to a computer mouse. If coupled to a web browser or computer game, the device can serve the dual role of preventing DVT and helping the patient to pass time more quickly. Such a device can also serve as the primary input device to those with arm or hand disabilities and may tend to avoid or mitigate carpal tunnel syndrome. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of electronics and an embedded computer that controls a deep vein THROMBOSIS (DVT) prevention device according to an embodiment of the present invention. 
         FIG. 2 a    shows a front view of a DVT prevention device attached to the leg of a patient according to an embodiment of the present invention. 
         FIG. 2 b    shows a side view of the DVT prevention device of  FIG. 2 a    near the flexion limit. 
         FIG. 2 c    shows a side view of the DVT prevention device near the extension limit. 
         FIG. 3 . shows a continuously variable actuator according to another aspect of the present invention that may be used to construct a DVT prevention device. 
         FIG. 4 . shows a single-motor actuator with a free movement mode according to another embodiment of the present invention. 
         FIG. 5 . shows a single-motor actuator as attached to an ankle according to a further embodiment of the present invention. 
         FIG. 6 . is a flowchart of a method for the prevention of DVT according to one aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a deep vein THROMBOSIS (DVT) prevention device  100  according to an embodiment of the present invention. An embedded microcontroller  102  is programmed to accept input from one or more sensors such as joint angle sensor  104  and a force (e.g., current) sensor  106 . The embedded microcontroller  102  may also be coupled to a control panel  108 . The control panel  108  may be for use by a patient, a doctor, or other health care provider. The embedded microcontroller  102  is operable to produce outputs for power drivers  112  to control the motion of one or more actuators  114 . 
     With further reference to  FIG. 1 , power is supplied to the DVT prevention device  100  through an actuator power supply  116 . Power may come through a battery  118  or from an AC adapter  120 . In one embodiment, the battery  118  is wirelessly recharged by inductive coupling to a pad conveniently placed, such as at the foot of a hospital bed. Such a wireless recharge device has been announced by Wildcharge at the 2007 Consumer Electronics show. 
     In certain embodiments, such as cases where the patient can supply significant force to exercise the ankle, the battery charging requirements may be reduced or eliminated by recharging the battery from energy captured from running the actuator  114  as backdriven motor generator. This may provide an extra incentive to the patient to exercise, especially if the amount of exercise is recorded and presented to the patient, the patient&#39;s family and the hospital staff. 
     The control panel  108  may be as simple as an on/off switch, or may include switches and displays to allow adjustments for the range of motion, minimum repetition frequency, movement statistics, battery charge, and the like. 
     One embodiment includes a USB or wireless connection  122  to allow the DVT prevention device  100 , or a pair of devices (e.g., one device each on the left and right ankles), to act as a human interface device (HID) that may be connected, for instance, to a PC. For example, the right ankle position may determine the left/right location of a computer curser and the left ankle position may determine the up/down location of the curser. When a patient uses the computer, for instance to surf the internet or play a game, the ankles must be flexed and extended, and in the process the blood flow to the leg is enhanced. The computer connection may significantly enhance patient compliance, which is a major problem with existing compression devices. 
       FIG. 2  shows three views of a DVT prevention device  200 , according to another embodiment of the present invention, attached to an ankle  202 . An actuator  204  is attached to upper and lower ankle attachment points such that activation of the actuator  204  may extend or flex the ankle  202 .  FIG. 2 a    shows a front view of the DVT prevention device  200 ,  FIG. 2 b    shows a side view of the DVT prevention device  200  near a flexion limit, and  FIG. 2 c    shows a side view of the DVT prevention device  200  near an extension limit. The limits may be programmatically or physically limited within the patient&#39;s range of motion. As will be appreciated, a typical extension limit (also known as Planar Flexion) is about 45 degrees from the standing position of the ankle, and a typical flexion limit (also known as Doral Flexion) is about −20 degrees from the standing position. 
     With further reference to  FIG. 2 , a rigid foot support structure  206  is placed under the foot and a rigid ankle support  208  structure is placed behind the calf. The two support structures  206  and  208  are connected to each other with a hinge  210 . The actuator  204  is mounted to the upper rigid structure  208 . Straps or padded supports  212  hold the ankle support structure  208  and actuator  204  to the lower leg. An output shaft  214  of the actuator  204  is connected to a linkage  216  attached to the foot support structure  206 . One or more straps  212  hold the foot support structure  206  to the foot. 
       FIG. 3  shows a continuously variable actuator  300  suitable for use as an actuator according to certain embodiments of the present invention. One suitable example of the continuously variable actuator is described in more detail in the Horst et al.&#39;s U.S. patent application Ser. No. 11/649,493, filed Jan. 3, 2007, the contents of which are incorporated herein by reference. The actuator  300  uses a flexible belt  302  connected by belt supports  304  and  306 , two motor-driven lead screws  308  and  310  driven by motors  312  and  314 , respectively, and a motor driven cam  316  driven by motor  318  to provide variable drive ratio forces in either direction or to allow the output shaft  320  to move in a free-movement mode. Also shown are two driven carriages  322  and  324 , and two passive carriages  326  and  328 . 
       FIG. 4  shows a single-motor actuator  400  suitable for use as an actuator according to another embodiment the present invention. In the single-motor actuator  400 , a motor  402 , which may have an internal gear head, drives a lead screw  404  to move a nut  406  linearly. The lead screw  404  may be an acme screw, a ball screw with a ball nut for lower friction and higher motor efficiency, or any other suitable screw. The ball nut  406  is always between a flexion stop  408  and an extension stop  410  connected to an output shaft  412 . When the ball nut  406  is in a center of travel, the output shaft  412  is free to move linearly in either direction without having movement impeded by interaction with the ball nut  406 . This position provides free movement of the output shaft  412 , and likewise free movement of the ankle or other relevant body part, even with no power applied to the actuator  400 . When it is time to extend or flex the ankle, the ball screw  404  is turned to move the ball nut  406  to the left or the right where the ball nut  406  eventually pushes against the flexion or extension stop. Further movement of the ball nut  406  in the same direction moves the flexion stop  408  or the extension stop  410 , and hence moves the output shaft  412 , thus causing the ankle to flex or extend, respectively. The output shaft  412  is supported by one or more linear bearings  414  allowing the output shaft  412  to move freely in one dimension while preventing substantial movement or twisting in other dimensions 
     To further elaborate, lead screws include types of screws such as acme screws and ball screws. Ball screws have nuts with recirculating ball bearings allowing them to be backdriven more easily than acme screws. When using a ball screw, motion of the nut causes the lead screw and hence the motor to rotate. Therefore, when the ball nut is engaged by one of the stops, the patient may exercise the leg muscles by extending or flexing the foot to cause motion of the output shaft and hence cause motion of the motor. Exercise may be accomplished either by resisting the passive motions imparted by the actuator, or through a separate exercise mode where all motion is caused by the patient. In either case, software running in the embedded processor controls the amount of current delivered to/from the motor and therefore the amount of exercise resistance 
       FIG. 5  shows the single motor actuator  400  of  FIG. 4  attached to an ankle support  212  and coupled to a foot support  206  through a linkage  216 . The ball screw  404  in the actuator  400  is shown in a position about to extend the ankle by pushing to the right. Near the extension and flexion limits, some compliance may be built in to provide more comfort to the patient and to assure that there is no possibility of injuring the patent. This may be accomplished by springs in the actuator  400  or springs in the linkage  216 , or both (not shown), that expand or compress before damaging forces are applied 
     To further elaborate, a free-movement mode of the actuator  400  allows the patient to move the ankle with little resistance. The free movement mode obviates the need to remove the DVT prevention device when walking (for instance, to the restroom); this improves patient compliance because there is no need for the patient or hospital staff to remove and reattach the DVT protection device frequently. 
       FIG. 6  is a flowchart of a method for operating a device in the prevention of DVT according to one embodiment of the present invention. In step  602 , a person such as a medical professional sets up the device with appropriate limits for range of motion and minimum time between ankle movements. This step  602  may also be performed automatically. Then, in step  604 , a DVT prevention device is attached to one or both ankles of the patient, and if necessary the device is turned on. In step  606 , a test is made to determine if too much time has elapsed since the last flexion of the ankle. If the predefined time limit between flexion has been exceeded, step  608  runs a device actuator through one flexion/extension cycle or other suitable sequence. This cycle may be purely passive motion, or the patient may actively resist tending to cause more blood flow. If the time limit has not been exceeded or if the cycle is at the end of the passive or active movement cycle, the actuator is put into free movement mode in step  610 . Finally, in step  612 , the movements of the ankle are monitored to help determine the appropriate time for the next movement. Step  612  is followed by step  606 , repeating the sequence until the prevention method stops, the device is removed, or the device is turned off. 
     In the flowchart of  FIG. 6 , step  606  determines if the specified time has elapsed in order to initiate movement of the ankle. The “specified time” can be determined by any suitable manner including one or more of any of the following ways:
         1. A fixed elapsed time since the last ankle movement   2. A moving average over time of the frequency of ankle movements.   3. A dynamic algorithm that approximates blood flow in the leg by taking into account the frequency of movement, the intensity of active movement, and the patients age and condition.       

     A fixed time algorithm is simplest to implement, but may move the ankle more than necessary. Using a frequency of movement algorithm, the patient can have more control and has more positive feedback for initiating movements beyond the minimum. A dynamic algorithm rewards patient-initiated exercise (resisting the passive movement) and also customizes the frequency of movement based on the patient&#39;s condition. The algorithm can be determined through clinical studies of different patients using the device while monitoring blood flow. 
     The invention is not limited to the specific embodiments described. For example, actuators need only have a way to move and allow free movement of the ankle and need not have strictly linear movement. The actuator may be driven from a brushed or brushless motor or may be activated through pneumatics, hydraulics, piezoelectric activation, electro-active polymers or other artificial muscle technology. The usage of the device is not confined to hospitals but also may be beneficial to those bedridden in nursing homes or at home. The device may also be beneficial to avoid DVT for those traveling long distances by airplane, automobile or train.