WO2008055304A1

WO2008055304A1 - Improved apparatus for preventing deep vein thrombosis

Info

Publication number: WO2008055304A1
Application number: PCT/AU2007/001710
Authority: WO
Inventors: Gregory Allan Wren; Walter Meyer; Wayne Bennett
Original assignee: Vascular Enhancement Technology Pty Ltd
Priority date: 2006-11-08
Filing date: 2007-11-08
Publication date: 2008-05-15

Abstract

An apparatus (40) for enhancing blood flow through a limb (42) of a subject includes a compression sleeve (41) extendable around the subject's limb (42). Sleeve (41) includes a plurality of inflatable compressor chambers (60-64) situated next to one another along the sleeve (41). The apparatus (40) also includes a fluid delivery system (43) for delivering a fluid to each of the chambers (60-64). Fluid delivery system (43) includes a plurality of check valves (44) for preventing backflow of fluid from the chambers (60-64). In use, the chambers (60-64) compress the limb (42) in sequence to move blood within the limb (42) from one end of the sleeve (41) to the other, and as a said chamber (60-64) begins to compress the limb (42), the chamber (60-64) preceding it in sequence already compresses the limb (42), and the next preceding chamber (60-64) in sequence ceases to compress the limb (42).

Description

Improved Apparatus for Preventing Deep Vein Thrombosis

Field of the Invention

This invention relates to an apparatus for enhancing blood flow through a limb of a subject. In particular, the invention concerns an improved apparatus for preventing deep vein thrombosis.

Brief Discussion of the Prior Art

Deep vein thrombosis (DVT) is characterised by the development of a clot within a deep vein anywhere in the body but almost exclusively in the veins of the calf or thigh. DVTs are a large source of morbidity, the most common serious complication of DVT being a pulmonary embolism whereby a blood clot breaks free from a vein wall, travels to a lung and blocks an artery.

The following factors can promote blood clot formation within a vein: 1. Increased coagulation of the blood (e.g. women on hormones);

2. Increased clotting factors in the blood;

3. Damage to a vein wall (e.g. trauma to a leg), whereby coagulation factors are released and a chemical cascade causes a clot to form; and 4. Stasis of the blood, as happens in dependant limbs where gravity causes decreased blood flow through the veins. Blood clots are known to form most frequently in the legs due to stasis.

The circulatory system circulates blood around the body using various mechanisms. The heart pumps blood into the arterial system and this system distributes blood to every part of the body. Above the heart, gravity plays a role in returning blood to the heart, and below the heart, muscular contractions compress veins to move the blood towards the heart. Directional valves of the veins ensure that blood flow is directional. Small muscle groups move small amounts of blood during each contraction of the leg muscle and this is important as, since vein walls are thin and elastic, too much blood in a vein could cause the vein to distend, to suffer damage and to render the directional valves non-functional.

During long periods of muscular inactivity (e.g. when travelling on an aeroplane, car, bus or train, when confined to a wheelchair, or when bed ridden), the risk of a blood clot forming in a person increases as there may be little or no venous blood movement within the legs of the person. In addition to stasis, with blood continuing to collect within the leg veins, the directional valves may leak, the veins may distend and suffer damage and hence release clotting factors which could also initiate clot formation.

Compression sleeves for preventing DVT are known. However, although such sleeves may decrease the risk of a blood clot forming, the sleeves generally have the disadvantage that they do not decrease the risk to an acceptable level. In particular, those sleeves that compress a large area of muscle at any given time and consequently squeeze large volumes of blood through the veins may have the following disadvantages: 1. During compression of the sleeve, blood may be squeezed back into veins below the sleeve, thus increasing stasis in, and causing further distension of, the veins below the sleeve. This problem is exacerbated by long sleeve compression times, large compressed areas, and if the veins located beneath the sleeve already contain too much blood. 2. Following compression, the sleeve is relaxed, and as the empty veins in the previously compressed muscle refill with blood from below the sleeve, there is no blood to push along the blood in the veins above the sleeve and thus the blood in the veins above the sleeve lies static until the sleeve is next compressed.

3. If there is any constriction of the veins above the sleeve, such as the veins located in a seated person's thighs, then the large volume of blood may distend and damage those veins.

To recapitulate, some of the known compression sleeves have disadvantages in that they can aggravate stasis below the sleeve during compression of the muscle, they cause stasis above the sleeve when the sleeve is relaxed, and may distend vein walls and render directional valves of the veins non-functional, thus increasing the risk of blood clot formation. This is also true of those sleeves which have a series of inflatable chambers and which compress areas of a leg sequentially, as the chambers, which envelop a large area of muscle, do not deflate until all of the chambers have inflated.

Examples of some known compression sleeve apparatus of the type which include a series of inflatable chambers and which are able to compress areas of a leg sequentially are disclosed in United States Patent No. 3,862,629 (Rotta) and United States Patent No. 4,624,244 (Taheri).

Rotta discloses a therapeutic device for lowering the incidence of thromboembolism and for generally improving venous and secondary blood flow. The device comprises a series of inflatable chambers connected in series by valve means operative to produce a continuous propagation of pressure pulses sequentially along the series of chambers by controlled inflation and deflation of the chambers. No distributor is required for the requisite timed inflation and deflation of successive chambers of the device. Such timed operation being achieved by a plurality of valve means which may be identical, one such valve means connecting each successive pair of connecting chambers to form a series. The supply of fluid pressure to and the exhausting of fluid pressure from successive chambers in the series are controlled by elements of each valve means which are responsive to the differential in pressure between the pair of chambers connected by that valve means. Due to the employment of such valve means, the chambers may be modular units and the valve means may be modular units whereby chambers and valve means may be plugged together to make a series of any desired length.

Taheri discloses a device for aiding cardiocepital venous flow from the foot and leg of a patient. The device includes a first flexible fabric cuff for encircling the arch and instep of a patient's foot, a first inflatable bladder/chamber in the first cuff for placement in contiguous relationship to the arch, a second cuff for encircling the leg of the patient, a plurality of sequentially ascending second inflatable bladders/chambers in the second cuff for placement in contiguous relationship to the calf of the leg of the patient, a first conduit in communication with the first chamber, and a second conduit in communication with the second chambers. A compressor supplies compressed air to a plurality of conduits which have a relief valve in communication therewith. A pulse generator is provided coupled to a counter which in turn is coupled to a program memory. The foregoing electronic components sequentially actuate various normally closed solenoid valves to inflate the chambers of the device in a sequential manner. Both the Rotta and Taheri devices operate such that as a chamber in the sequence is inflating, each preceding chamber in the sequence is either deflated or is deflating. Therefore, if either device is attached to the leg of a patient, the inflating chamber will only cause blood to move up the leg if the patient has working valves in their leg veins. However, if the valves are damaged, as is the case for those people who are most at risk from suffering from deep vein thrombosis, the inflating chamber will not only cause the blood to move up leg, it will also cause the blood to move down the leg because the other chambers are either deflated or are deflating. Moving the blood down the leg has the undesirable effect of aggravating stasis.

It would therefore be desirable to provide an improved compression sleeve apparatus which overcomes or at least ameliorates the aforementioned deficiency of prior art compression sleeve apparatus.

Summary of the Invention

It is an object of the present invention to overcome, or at least ameliorate, one or more of the deficiencies of the prior art mentioned above, or to provide the consumer with a useful or commercial choice.

Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying illustrations, wherein, by way of illustration and example, a preferred embodiment of the present invention is disclosed.

According to a first broad aspect of the present invention, there is provided an apparatus for enhancing blood flow through a limb of a subject, said apparatus including a compression sleeve extendable around the subject's limb and having a plurality of inflatable compressor chambers situated next to one another along the sleeve, and a fluid delivery system for delivering a fluid to each of the chambers and having a plurality of check valves for preventing backflow of fluid from the chambers, wherein in use the chambers compress the limb in sequence to move blood within the limb from one end of the sleeve to the other, and as a said chamber begins to compress the limb, the chamber preceding it in sequence already compresses the limb, and the next preceding chamber in sequence ceases to compress the limb. According to a second broad aspect of the present invention, there is provided a method for enhancing blood flow through a limb of a subject, said method including the steps of:

(1) extending a compression sleeve around the subject's limb, wherein the sleeve has a plurality of inflatable compressor chambers situated next to one another along the sleeve; and

(2) delivering a fluid to each of the chambers using a fluid delivery system having a plurality of check valves for preventing backflow of fluid from the chambers, wherein the chambers compress the limb in sequence to move blood within the limb from one end of the sleeve to the other, wherein as a said chamber begins to compress the limb, the chamber preceding it in sequence already compresses the limb, and the next preceding chamber in sequence ceases to compress the limb.

By preventing backflow of fluid from the chambers, the check valves are able to prevent an inflated chamber in the sequence from prematurely deflating before the next chamber in the sequence is inflated. Consequently, the present invention is less prone to aggravating stasis in the veins of the subject's limb.

The subject can be a human or other type of mammal. Preferably, the apparatus is used to move blood through an arm or leg of a person. In a particular preferred form, the apparatus is used to move blood through the lower leg of a person. The sleeve can be of any suitable size, shape and construction. Preferably, the sleeve is extendable around the calf muscle, thigh, or foot of a person. The sleeve can have any suitable number of chambers. The sleeve can have as few as three chambers but preferably the sleeve has at least five chambers and more preferably six chambers.

As mentioned, the chambers compress sequentially to move blood within the limb from one end of the sleeve to the other. Preferably, this involves the steps of:

(1) a first chamber compressing the limb such that venous blood cannot flow therepast and such that the blood beneath the first chamber is moved beneath a second chamber;

(2) the second chamber compressing the limb such that venous blood cannot flow therepast and such that blood therebeneath is moved beneath a third chamber;

(3) the third chamber compressing the limb such that venous blood cannot flow therepast and such that blood therebeneath is moved beneath a fourth chamber, and the first chamber decompressing the limb such that blood can flow therebeneath;

(4) the fourth chamber compressing the limb such that venous blood cannot flow therepast and such that blood therebeneath is moved beneath a fifth chamber, and the second chamber decompressing the limb such that blood can flow therebeneath;

(5) the fifth chamber compressing the limb such that venous blood cannot flow therepast and such that blood therebeneath is moved further up the limb, and the third chamber decompressing the limb such that blood can flow therebeneath;

(6) the first chamber compressing the limb such that venous blood cannot flow therepast and such that the blood therebeneath is moved beneath the second chamber, and the fourth chamber decompressing the limb such that blood can flow therebeneath; (7) the second chamber compressing the limb such that venous blood cannot flow therepast and such that blood therebeneath is moved beneath the third chamber, and the fifth chamber decompressing the limb such that blood can flow therebeneath;

(8) the third chamber compressing the limb such that venous blood cannot flow therepast and such that blood therebeneath is moved beneath the fourth chamber, and the first chamber decompressing such that blood can flow therebeneath; and

(9) repeating steps (3) to (8) indefinitely.

Li this way, the chambers compress and decompress the limb in a wave-like motion, allowing only short periods of stasis above and below each chamber that has compressed the limb, and allowing blood filling adjacent lower chambers that have compressed the limb whilst upper chambers move the blood toward the heart. The chambers can be of any suitable size, shape and construction. Preferably, each of the chambers extends completely or almost completely around the limb.

The sleeve can have a casing enclosing the chambers. The casing can be of any suitable size, shape and construction. Preferably, the casing consists of fabric, such as cotton. The sleeve can, in this respect, be similar to a sphygmomanometer. The casing can be secured around the limb in any suitable way. For instance, the casing can have one or more strips of hook and loop type fasteners (e.g. Velcro™), clips or press studs that mate with one another. The sleeve can further include a protective layer situated between the casing and the limb that, for hygienic purposes, can be removed and disposed of after use by the person. The protective layer can be detachably connected to the casing. For instance, the protective layer can be weakly adhered to the casing with glue. Alternatively, the protective layer may only be disposed on the casing during use when compressed between the limb and the casing.

The protective layer can consist of any suitable material or materials. Preferably, the protective layer consists of a plastic-backed absorbent sheet wherein an absorbent surface of the sheet contacts the limb and can absorb sweat from the limb, whereas the plastic backing prevents the sweat from reaching the casing. The sleeve can further include a firm backing sleeve (i.e. layer) for ensuring that the compressive force of the chambers is largely exerted on the limb. The backing sleeve can extend adjacent an outer surface of the chambers around the limb. Preferably, the backing sleeve extends within the casing along the outer surface of the chambers. The backing sleeve can be of any suitable size, shape and construction. The backing sleeve can be made of any suitable material or materials. The backing sleeve can be of unitary construction or can comprise two or more attachable pieces. Preferably, the backing sleeve is made of hard rubber or plastics material. Such a backing sleeve can have some degree of flexibility yet ensure that the compressive force of the chambers is largely exerted on the limb.

The chambers may be of any suitable construction. For instance, a rubber bladder or a plastic bag can be heat sealed to form the chambers. The rubber bladder or plastic bag can be loosely enclosed by the casing. Alternatively, the chambers can consist of discrete bladders connected to the casing. Preferably, the chambers at the ends of the sleeve are narrower and hold less fluid (i.e. have a smaller volume) than the intervening chambers.

Each chamber may have a pressure relief valve for preventing over inflation of the chamber. The pressure relief valve preferably opens when the chamber pressure reaches a level which exerts in excess of about 35 mm Hg on the limb.

The chambers can be inflated by the fluid delivery system at a predetermined rate, to a predetermined pressure. Any suitable inflation rate and pressure can be used. The rate of inflation of a chamber could be, for example, within the range of about 1 to 30 seconds, but is preferably about five seconds. The pressure of an inflated chamber could be, for example, within the range of about venous pressure to 40 mm Hg, but is preferably below about 35 mm Hg. The pressure that the chambers are inflated to may be adjusted. An inflated chamber is preferably below arterial pressure to decrease the risk of ischaemia which is a reduction of the blood supply to part of the body. Each chamber may be inflated for any suitable period of time. For example, each chamber may be inflated for 1 to 30 seconds. The period of time that each chamber is inflated for may be adjusted. The chambers are preferably relatively small so that the apparatus is relatively compact, and so that the chambers can be inflated and deflated relatively quickly.

The check valves may be of any suitable type or combination of types. Preferably, the check valves are ball check valves, however other types of check valves may be used instead. For example, the check valves may be diaphragm check valves, swing check valves, clapper check valves, stop-check valves, lift-check valves, or any combination thereof.

Preferably, the fluid delivery system includes a pump, and a manifold extending between the pump and the compressor chambers, for delivering fluid to each of the chambers. Although any suitable type of fluid (e.g. air, water, oil) can be used, preferably the fluid is air. The pump can be a pulsating pump or a non-pulsating pump, to produce either a pulsating fluid stream or a non-pulsating fluid stream. The fluid may be pumped into each chamber in a pulsatile manner to increase agitation of the blood beneath the chambers and to decrease stasis.

Preferably, the fluid delivery system includes a valve assembly for inflating and deflating each chamber in sequence. Any suitable type of valve assembly may be used. Such an assembly may have pressure activated, time activated and/or electrically activated valves for inflating and deflating each chamber. The assembly may further include pressure relief valves and valves for producing a pulsatile fluid stream. The valve assembly preferably includes a restrictor for restricting the rate at which each chamber is able to be deflated. In a preferred embodiment of the invention the valve assembly includes a plurality of three-way valves for inflating and deflating the chambers, wherein at a first valve setting air is communicated from the pump to a specific chamber, at a second valve setting the chamber is sealed such that it remains inflated, and at a third valve setting air within the chamber is bled to the atmosphere and the chamber deflates. A timer and a drive are preferably operatively coupled to each three-way valve.

The fluid delivery system preferably includes a controller for controlling the operation of the valve assembly. The controller preferably includes at least one sensor for monitoring the fluid delivery system. The pump can be separate from the sleeve or mounted to the sleeve. The pump can be mounted to the sleeve in any suitable way. If mounted to the sleeve, the vibrations of the pump can vibrate both the muscles and veins to decrease the risk of clot formation. A single pump can deliver air to one or more sleeves.

The apparatus is preferably relatively small and portable so that a user is able to ambulate while wearing the apparatus.

The apparatus may include a plurality of compression sleeves for compressing different limbs, or for compressing different parts of the same limb. For example, the apparatus may include one sleeve for compressing the calf muscle or thigh of a leg, and another sleeve for compressing a foot of the leg.

Brief Description of the Drawings In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings in which:

Figure 1 depicts an apparatus for moving blood through a limb of a person according to a first preferred embodiment of the present invention;

Figure 2 depicts the apparatus of figure 1 secured to the lower leg of a person;

Figure 3 depicts the compression sleeve of the apparatus illustrated in figure l; Figure 4 is a transverse cross-section of the compression sleeve depicted in figure 3;

Figure 5 depicts the fluid delivery system of the apparatus illustrated in figure l;

Figure 6 depicts a ball check valve of the fluid delivery system illustrated in figure 5;

Figure 7 is a schematic electronic circuit diagram of a fluid delivery system controller of the apparatus illustrated in figure 1; and

Figure 8 is a timing diagram which depicts the operation of the solenoids illustrated in figure 7.

Detailed Description of the Drawings In the figures, like reference numerals refer to like features. Referring to figures 1 to 7, an apparatus 40 for enhancing blood flow through a lower leg of a person includes an inflatable compression sleeve 41 (see figures 1 to 4) extendable around the person's lower leg 42 (see figure 2), a fluid delivery system 43 (see figure 5) for delivering air to the sleeve 41 and having a plurality of non-return/ball check valves 44 (see figures 5 and 6) for preventing backflow of the fluid from the sleeve 41. The sleeve 41 is similar to a sphygmomanometer.

With particular reference to figure 1, the air delivery system 43 is contained within a housing 50 which is secured to the sleeve 41 such that the sleeve 41 extends across the back of the housing 50. A first end 51 of the sleeve 41 is secured to an elongate eyelet 52. A second end 53 of the sleeve 41 is shown inserted through the eyelet 52. Strips 54 of Velcro™ hook and loop fastener are secured to the sleeve 41 adjacent to its second end 53. The strips 54 are used to secured the sleeve 41 relative to the person's lower leg 42. The sleeve 41 can be tightened and loosened around leg 42 by varying the amount of the sleeve 41 which is pulled through the eyelet 52. As shown in figure 2, the apparatus 40 is strapped to the leg 42 by the sleeve 41 such that the housing 50 is located over the tibia of the leg 42 and such that the sleeve 41 extends around the calf muscle of the leg 42. The sleeve 41 is tightened around the leg

42 so that the apparatus 40 is held firmly in place.

With particular reference to figures 3 and 4, sleeve 41 includes five inflatable compressor chambers 60-64 situated next to one another along the sleeve 41. The chambers 60-64 are elongate and extend around the leg 42, as seen in figure 2.

Sleeve 41 has a casing 65 enclosing the chambers 60-64. Sleeve 41 also includes a firm rubber or plastic backing sleeve 66 for ensuring that the compressive force of the chambers 60-64 is largely exerted on the leg 42. The backing sleeve 66 extends within the casing 65 alongside the chambers 60-64 around the leg 42. The backing sleeve 66 is somewhat flexible yet ensures that the compressive force of the chambers 60-64 is largely exerted on the leg 42.

A protective sheet (not depicted) may be situated between the casing 65 and the leg 42 that, for hygienic purposes, can be removed and disposed of after use by the person. The protective sheet may have an absorbent surface for absorbing sweat from the leg, and may be plastic-backed to prevent the sweat from reaching the casing 65.

The protective sheet would decrease the need for the casing 65 to be cleaned regularly. Different size sleeves 41 can, if necessary, be used for different size people.

Sleeves for small people are typically 100 mm wide, and each chamber 60-64 of the sleeve 41 is 20 mm wide. Sleeves for medium-sized people are typically 140 mm wide, and each chamber 60-64 is 28 mm wide. Sleeves for large people are typically between 180-220 mm wide, and each chamber 60-64 is 36 mm - 44 mm wide.

The fluid delivery system 43 shown in figure 5 includes an air pump 70, hoses

71-75 extending from the ball check valves 44, and a hose 76 extending between the pump 70 and the hoses 71-75. Each ball check valve 44 is connected to a respective solenoid-operated three-way valve 77, and each valve 77 is connected to a respective chamber 60-64 by a respective hose 78. Hoses 71-75, 76 and 78 form part of a manifold

79 of the fluid delivery system 43.

Each valve 77 is able to be moved by its solenoid between three different positions. At a first one of the positions the valve 77 is closed so that air cannot be pumped through it and into its associated chamber 60-64 by the pump 70. At a second one of the positions the valve 77 is open so that air can be pumped through it and into its associated chamber 60-64 by the pump 70. At a third one of the positions the valve 77 vents air contained in its associated chamber to atmosphere. The check valves 44 prevent the air in the chambers 60-64 from flowing back through the hoses 71-75 when the valves 77 are in the open position so that each chamber 60-64 can only be deflated when its associated valve 77 is in the venting position.

Referring to figure 6, each check valve 44 includes a hollow housing 80, and an input passage 81 which allows air from the hoses 71-75 to enter the housing 80. The input passage 81 communicates with a hollow valve seat 82 on which rests a moveable ball 83. An output passage 84 is located on the other side of the housing 80 so that air which passes through the housing 80 can enter the three-way valve 77 which the valve

44 is connected to.

When the pressure of the air in the input passage 81 exceeds the pressure of the air in the output passage 84, the ball 83 is lifted off the seat 82 so that air can flow through the housing 80 to the three-way valve 77 and the chamber 60-64 which is associated with the valve 44. When the pressure in the input passage 81 is less than the pressure of the air in the output passage 84, the ball 83 is pressed against the seat 82 so that air in the chamber 60-64 which is associated with the valve 44 is prevented from flowing backwards through the valve 44. Each valve 77 has an associated restrictor 85 for restricting the rate at which the chamber 60-64 associated with each valve 77 deflates when the valve 77 is operated to deflate the chamber 60-64.

Each chamber 60-64 of the apparatus 43 may also have an associated pressure relief valve which prevents the chambers 60-64 from over-inflating. Each pressure relief valve may open when the pressure inside its associated chamber 60-64 exceeds a certain pressure. For example, each pressure relief valve may open when the pressure inside its associated chamber 60-64 exceeds 35 mm Hg.

Apparatus 40 may be modified to include a pulsing valve so that air which is output by the pump 70 is delivered to the inflatable compression chambers 60-64 in a pulsatile manner. The pulsing valve may be situated between the air pump and the hoses 71-75.

Figure 7 depicts a controller 90 for controlling the five three-way valves 77 and the pump 70. Each three-way valve 77 has a respective solenoid 91 for controlling the operation of the valve 77. Controller 90 includes a 5VDC voltage regulator 92 which is powered from a 12VDC unregulated power supply. A pressure sensor 93 is connected to the output of the regulator 92. The output of the pressure sensor 93 is connected to the input of a differential amplifier 94 which is also connected to the regulator 92. The output of the differential amplifier 94 and the output of the regulator 92 are connected to inputs of a comparator 95. An output of a set point reference circuit 96 is connected to an input of the comparator 95. An output of the comparator 95 and the output of the regulator 92 are connected to inputs of a microcontroller 97. A crystal time base circuit 98 is connected to an input of the microcontroller 97. Outputs of the microcontroller 97 are connected to inputs of a Darlington transistor array driver 99. A series connected switch 100 and diode 101 are connected in parallel with the solenoids 91 which are connected to a 12VDC unregulated power supply. Switch 100 is connected to a 12VDC battery.

The controller 90 is adapted to control the three-way valves 77 via the solenoids 91 so as to obtain a variable and sequential pneumatic output from the three- way valves 77, which in turn control the inflation of the inflatable chambers 60-64 of the apparatus 40. The pressure sensor 93 continuously senses the pressure inside the inflatable chambers 60-64 of the apparatus 40. The set point reference for the chamber pressure can be adjusted using the set point reference circuit 96 from 0-300mmHg. The chamber pressure signal output by the sensor 93 and the set point signal output by the set point reference circuit 96 are compared in an electronic bridge arrangement (not depicted), and the output of the bridge arrangement is used to control the pump motor 70.

The controller 90 also has a sixteen position binary rotary switch 102 which is connected to an input of the microcontroller 97. The period of time that each inflatable chamber 60-64 of the apparatus 40 is inflated during one cycle of the apparatus 40 can be selected from amongst sixteen different time periods by turning the rotary switch 102.

The total period of time which elapses from when the first inflatable chamber 60 is inflated to when the fifth inflatable chamber 64 is deflated during a single cycle of the apparatus can be varied from 25 seconds to approximately 75 seconds.

The microcontroller 97 is programmed to control the sequence and timing of operation of the three-way valves 77, and the operation of the pump motor 70. The microcontroller 97 drives the motor 70 and the solenoids 91 via the Darlington transistor array driver 99.

Figure 8 depicts the timing of the control signals which are output by the Darlington transistor array driver 99 to the solenoids 91 during one cycle of the apparatus 40. The cycle can be repeated indefinitely. It can be seen that each solenoid 91 is turned on after the preceding solenoid 91 has been turned on for 5 seconds, and that each solenoid 91 remains on for 10 seconds. While a solenoid 91 is turned on, the valve 77 which includes that solenoid 91 is opened so that air from the pump 70 is able to flow through the valve 77 and into the chamber 60-64 which is connected to the valve 77. When the solenoid 91 is turned off, the valve 77 which includes that solenoid 91 closes so that air from the pump 70 is no longer able to inflate the chamber 60-64 which is connected to the valve 77, and the solenoid 91 vents the chamber 60-64 through the restrictor 85 associated with the valve 77 so that the chamber 60-64 deflates.

It can be seen from figure 8 that shortly after a first chamber in the sequence commences inflating, a second chamber in the sequence commences inflating so that the valves 77 associated with both of those chambers will be open to the pump 70 at the same time. When the second chamber starts to inflate, the pressure inside the first chamber is greater than that inside the second chamber so that there is a pressure differential between the first and second chambers. The check valves 44 prevent air from the first chamber from flowing back out of that chamber and into the second chamber while the valves 77 of those two chambers are open at the same time. If the check valves 44 were not present, air from the first chamber would flow out of that chamber into the second chamber so as to equalise the pressure in the two chambers. This would result in premature deflation of the first chamber which would result in a premature reduction in the pressure it applies to the leg 42.

Air is pumped from the pump 70 at a pressure of at least 35 mm Hg. The three-way valves 77 deliver the air to specific chambers 60-64 and bleed air from specific chambers 60-64. The inflation/deflation sequence is as follows:

(1) chamber 60 inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 60 moves beneath chamber 61;

(2) chamber 61 inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 61 moves beneath chamber 62; (3) chamber 62 inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 62 moves beneath chamber 63, and chamber 60 deflates such that blood can flow beneath chamber 60;

(4) chamber 63 inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 63 moves beneath chamber 64, and chamber 61 deflates such that blood can flow beneath chamber 61 ;

(5) chamber 64 inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 64 moves further up the limb, and chamber 62 deflates such that blood can flow beneath chamber 62;

(6) chamber 60 again inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 60 moves beneath chamber 61, and chamber 63 deflates such that blood can flow beneath chamber 63;

(7) chamber 61 again inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 61 moves beneath chamber 62, and chamber 64 deflates such that blood can flow beneath chamber 64; (8) chamber 62 again inflates such that venous blood cannot flow past that chamber and such that the blood beneath chamber 62 moves beneath chamber 63, and chamber 60 deflates such that blood can flow beneath chamber 60; and

(9) steps (3) to (8) are repeated indefinitely. In this way, blood is moved up the leg. The apparatus 40 can be used for DVT prevention by persons traveling on planes, trains, buses or cars, or by persons seated at desks for long periods of time, or persons unable to use their legs, such as paraplegics or the elderly, or by persons having leg disorders, including swollen ankles, varicose veins or non-functional directional valves in the veins.

The sleeve 41 squeezes blood out of the calf muscle and moves small amounts of blood directionally through the veins in an almost continuous way to decrease stasis above and below the sleeve 41. Since at most only two chambers 60-

64 are inflated at any given point in time, there is only a 10 second period of stasis and blood filling can occur beneath all non-inflated chambers 60-64. Moreover, air can be pumped into the chambers 60-64 in a pulsatile manner to agitate the blood beneath the inflating chamber and to further decrease stasis and potential blood clot formation. As the flow of blood is always in one direction, there will be negligible or no directional valve damage and vein distension.

The apparatus 40 does not rely on intact, healthy directional valves of the veins to work as the wave-like motion of the sleeve 41 pushes blood continuously upwards. The apparatus 40 could conceivably work almost as well in persons lacking functional directional valves.

Throughout the specification and the claims, unless the context requires otherwise, the term "comprise", or variations such as "comprises" or "comprising", will be understood to apply the inclusion of the stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout the specification and claims, unless the context requires otherwise, the term "substantially" or "about" will be understood to not be limited to the value for the range qualified by the terms.

It will be appreciated by those skilled in the art that variations and modifications to the invention described herein will be apparent without departing from the spirit and scope thereof. The variations and modifications as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth.

It will be clearly understood that, if a prior art publication is referred to herein, that reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

Claims

CLAIMS:

1. An apparatus for enhancing blood, flow through a limb of a subject, said apparatus including a compression sleeve extendable around the subject's limb and having a plurality of inflatable compressor chambers situated next to one another along the sleeve, and a fluid delivery system for delivering a fluid to each of the chambers and having a plurality of check valves for preventing backflow of fluid from the chambers, wherein in use the chambers compress the limb in sequence to move blood within the limb from one end of the sleeve to the other, and as a said chamber begins to compress the limb, the chamber preceding it in sequence already compresses the limb, and the next preceding chamber in sequence ceases to compress the limb.

2. The apparatus of claim 1, wherein the sleeve further includes a firm backing sleeve for ensuring that the compressive force of the compressors is largely exerted on the limb.

3. The apparatus of claim 2, wherein the backing sleeve extends adjacent to an outer surface of the chambers and around the limb.

4. The apparatus of claim 2 or 3, wherein the backing sleeve is made from a flexible material.

5. The apparatus of any one of the preceding claims, wherein the pressure that the chambers are inflated to by the fluid delivery system is adjustable.

6. The apparatus of any one of the preceding claims, wherein the period of time that each chamber is inflated is adjustable.

7. The apparatus of any one of the preceding claims, wherein the check valves are selected from the group comprising ball check valves, diaphragm check valves, swing check valves, clapper check valves, stop-check valves, and lift-check valves.

8. The apparatus of any one of the preceding claims, wherein the fluid delivery system includes a pump, and a manifold extending between the pump and the compressor chambers, for delivering fluid to each of the chambers.

9. The apparatus of claim 8, wherein the pump is selected from the group comprising a pulsating pump and a non-pulsating pump.

10. The apparatus of any one of the preceding claims, wherein the fluid delivery system includes a valve assembly for inflating and deflating each chamber in sequence.

11. The apparatus of any one of the preceding claims, wherein the valve assembly includes a restrictor for restricting the rate at which each chamber is able to be deflated.

12. The apparatus of claim 10 or 11, wherein the valve assembly includes a plurality of three-way valves for inflating and deflating the chambers.

13. The apparatus of any one of claims 10 to 12, wherein the fluid delivery system includes a controller for controlling the operation of the valve assembly.

14. The apparatus of claim 13, wherein the controller includes at least one sensor for monitoring the fluid delivery system.

15. According to a second broad aspect of the present invention, there is provided a method for enhancing blood flow through a limb of a subject, said method including the steps of: