Abstract:
Apparatus for applying an adaptable pressure waveform to a patient&#39;s limb for augmenting venous blood flow in the limb comprises: an inflatable sleeve for positioning onto a limb to apply a pressure to the limb beneath the sleeve when inflated with gas; pressure transducing means for sensing the pressure of gas in the sleeve and for producing a sleeve pressure signal indicative of the sensed pressure; pressure waveform application means responsive to the sleeve pressure signal and a reference pressure waveform signal for controlling the supply of pressurized gas to the sleeve so that the sensed pressure in the sleeve is maintained near a varying pressure indicated by the reference pressure waveform signal; and waveform register means for producing the reference pressure waveform signal indicative of a reference pressure waveform during a cycle time period wherein the reference pressure waveform includes a maximum pressure at a time within the cycle time period and a rate of pressure increase in an interval leading to the maximum pressure and wherein the waveform register means includes waveform adaptation means for allowing the rate of pressure increase to be changed from a first rate to a second rate without changing the maximum pressure at the time.

Description:
FIELD OF THE INVENTION  
         [1]    1. The invention is related to an apparatus and method for periodically producing an adaptable pressure waveform in a pneumatic sleeve applied to a limb of a human patient in order to help prevent deep vein thrombosis (DVT) or to treat lymphedema in the patient.  
         BACKGROUND OF THE INVENTION  
         [2]    2. Limb compression systems of the prior art apply and release pressure on a patient&#39;s extremity to augment venous blood flow and help prevent deep vein thrombosis (DVT) or to treat lymphedema. Limb compression systems of the prior art typically include: a source of pressurized gas; one or more pneumatic sleeves for attaching to one or both of the lower limbs of a patient; and an instrument connected to the source of pressurized gas and connected to the sleeves by means of pneumatic tubing, for controlling the inflation and deflation of the sleeves and their periods of inflation and deflation. In U.S. Pat. No. 3,892,229 Taylor et al. describe an early example of one general type of limb compression system of the prior art known as an intermittent limb compression system; such systems apply pressure intermittently to each limb by inflating and deflating a single-bladder sleeve attached to the limb. In U.S. Pat. No. 4,013,069 Hasty describes an example of a second general type of limb compression system of the prior art, known as a sequential limb compression system; such systems apply pressure sequentially along the length of the limb by means of a multiple-bladder sleeve or multiple sleeves attached to the same limb which are inflated and deflated at different times. Certain intermittent and sequential limb compression systems of the prior art are designed to inflate and deflate sleeves on both limbs either simultaneously or alternately, while others are designed for use on one limb only.  
           [3]    3. The primary purpose of most of the limb compression systems of the prior art is to prevent or reduce the risk of DVT. Such limb compression systems are used to minimize venous stasis during and immediately following surgery, as well as during long periods of immobility. DVT may lead to pulmonary embolism (PE), a serious hazard for surgical and trauma patients. For example, patients over forty years of age who are undergoing hip or knee surgery, or major abdominal surgery, are at particular risk of DVT. When DVT leads to PE, this complication can result in death, with an estimated 200,000 such deaths occurring in the United States annually. To help prevent DVT and thus PE, the use of pneumatic limb compression systems of both intermittent and sequential types, used either alone or combined with anticoagulant drug therapy, have been developed in the prior art and are commonly used at present.  
           [4]    4. A purpose of other limb compression systems of the prior art is to treat chronic edema, including lymphedema. Lymphedema refers to the condition of fluid accumulation in a limb. Secondary lymphedema can be a result of trauma or surgical complications. Limb compression therapy using limb compression systems of the prior art has been demonstrated to be of significant value in treating lymphedema.  
           [5]    5. Systems of the prior art have not been capable of producing a desired pressure waveform in a pneumatic sleeve attached to a limb. This is a significant limitation, as the inventors of the present invention have inferred from the recent clinical literature that applied pressure waveforms having differing shapes produce significantly different changes to venous blood flow. In the clinical literature, the use of a wide range of devices and non-standardized techniques by clinicians to indicate changes in venous flow and venous stasis, either subjectively or quantitatively, has been reported. For example, devices employing Doppler ultrasound, photo-plethysmography, impedance plethysmography, contrast venography, oximetry and de-oximetry have all been used for such purposes in the prior art. Such changes, when detected, then may or may not have been taken into consideration in the manual adjustment of prior-art systems. For example, Tumey et al. in U.S. Pat. No. 5,443,440 describe apparatus including a sensor for determining whether patients have venous blood flow problems prior to setting parameters and use. However, a significant limitation of many prior-art limb compression systems is that such systems have not incorporated a standardized physiologic transducer and measurement algorithm which provides an indication of the change in venous blood flow produced as a result of the application of a pressure waveform to by means of the sleeve of the system. As a result, these prior-art systems cannot automatically adapt or change the pressure waveform applied to the limb, nor can they permit an operator to manually adapt or change the pressure waveform, in response to changes in venous blood flow, in order to improve the effectiveness of the therapy.  
           [6]    6. In many sequential limb compression systems of the prior art, such as the one described by Hasty in U.S. Pat. No. 4,013,069, elapsed times are pre-set to initiate the sequential pressurization of each of the multiple-chamber sleeves, or each of the multiple sleeves. This has been a significant limitation and has produced a sub-optimal augmentation of venous blood flow by such sequential limb compression systems, but has been necessary because these prior-art systems have not been capable of producing desired pressure waveforms in multiple-bladder sleeves and multiple sleeves, and have thus not been capable of using a selected parameter of the pressure waveform in one sleeve or bladder of a multiple-bladder sleeve to trigger the pressurization of another sleeve or bladder using the desired pressure waveform for that sleeve or bladder.  
           [7]    7. Many limb compression systems of the prior art are not capable of producing a desired pressure waveform in a pneumatic sleeve attached to a limb either because they do not directly measure the pneumatic pressure in the sleeve at any instant, or because they do not generate a signal indicative of the pressure suitable for permitting a feedback control system to produce the desired pressure waveform. In the prior art, for example, pressure gauges have been connected to inflatable bladders to provide visual indications of bladder pressure to operators, but such apparatus did not generate a signal suitable for controlling the production of a waveform and the apparatus was considered to be expensive, inconvenient and unnecessary.  
           [8]    8. Some limb compression systems of the prior art attempt to prevent hazardous over-pressurization by limiting the maximum pressure level produced in the sleeve without actually displaying or measuring the sleeve pressure. For example, in U.S. Pat. No. 4,841,956 Gardner et al. describe a limb compression system in which sleeve pressure is not measured, but in which the peak pressure level is limited by limiting the time period during which inflating gas flows into the sleeve. In such a system the maximum pressure actually produced in the sleeve is dependent on variables such as the flow resistance of the tubing, the design and pneumatic volume of the sleeve, and the pressure of the gas during the inflating time period. Other systems, such as that of Arkans in U.S. Pat. No. 4,396,010, use a preset pressure switch in the instrument to limit the maximum pneumatic pressure level.  
           [9]    9. In a limb compression system described by Cariapa et al. in U.S. Pat. No. 5,437,610, a pressure sensor is connected to a fluid-filled bladder within a pneumatic sleeve, but the sensor/bladder combination is adapted to measure the static pressure of the limb against the uninflated sleeve, and could not be used or adapted to produce any one of a wide range of desired pneumatic pressure waveforms in the sleeve.  
           [10]    10. Some limb compression systems known in the prior art attempt to estimate sleeve pressure in an inexpensive and convenient manner, based on a variety of apparatus and methods. These systems do not measure pressure directly in the pneumatic sleeve applied to the limb but instead estimate sleeve pressure indirectly and remotely from the sleeve. For example, in U.S. Pat. No. 5,031,604 Dye describes a system in which sleeve pressure is estimated by measuring pneumatic pressure near the instrument end of the tubing connecting the instrument to the sleeve. As another example, Arkans in U.S. Pat. No. 4,375,217 describes a system in which the static pressure in the sleeve is estimated at a location on the tubing between the instrument and the sleeve. All such apparatus and methods which estimate sleeve pressure by measuring a pneumatic pressure remotely from the sleeve suffer from a significant disadvantage, which makes them unsuitable for incorporation into an instrument for producing a desired pressure waveform in the sleeve: the accuracy of the estimates of pressure made by such systems is significantly affected by variations in the length and flow resistance of the tubing attached to the sleeve, and by variations in sleeve design, sleeve inflation volume and sleeve application technique. For example, the inventors of the present invention have determined that variables related to the design and size of the sleeve, as well as the snugness of application of the sleeve, can result in discrepancies at any instant of well over 50 percent between the remotely estimated sleeve pressure and the actual pressure in the sleeve. As a separate consideration regarding the flow resistance of the tubing employed in prior-art systems which measure pressure in this manner, it has been necessary to locate such systems close to the patient to minimize flow resistance in the tubing, resulting in unnecessary noise and clutter around the patient.  
           [11]    11. Other systems known in the prior art interrupt the flow of gas in the tubing in an effort to estimate sleeve pressure by measuring pneumatic pressure at the instrument end of the tubing under zero-flow conditions. One such system is the Jobst Athrombic Pump System 2500 (Jobst Institute Inc., Charlotte N.C.). However, estimates of sleeve pressure made in this manner cannot practically be incorporated into limb compression systems for producing pressure waveforms having large amplitudes and short cycle periods. Also, more generally, such systems suffer from the disadvantage that pressure estimates are available discontinuously and are not suitable for real-time control of the pressure in the sleeve to produce a desired pressure waveform.  
           [12]    12. In the prior art, incorporation of a force sensor to measure the force applied by a sleeve to a limb has been described by Tumey et al. in U.S. Pat. No. 5,443,440. Also, the use of separate measurement apparatus for measuring the pressure applied by a sleeve to a limb has been described by Arkans in U.S. Pat. No. 4,331,133, wherein a separate measurement cuff is placed between the sleeve and the limb and the pressure applied by the sleeve is estimated indirectly. Both the above-referenced force sensor of Tumey et al. and separate measurement apparatus of Arkans have several disadvantages which make them unsuitable for incorporation into a system for periodically applying a desired pressure waveform to a limb: calibration of the force sensor/measurement cuff is difficult, time-consuming and error-prone; significant errors can arise during use due to use-related changes in the interface between force sensor/measurement cuff and the sleeve, or between the force sensor/measurement cuff and the limb; and minor anomalies such as wrinkling or folding of the sleeve or cuff surface when inflated can produce significant anomalies in measured force/pressure.  
           [13]    13. Because of errors and limitations associated with estimation of the pressure applied by a sleeve to a limb, prior-art systems have not had the capability of accurately producing a desired pressure waveform in combination with sleeves having differing designs and varying pneumatic volumes, or when sleeve application techniques vary and the resulting sleeve snugness varies, or when sleeves are applied to limbs of differing sizes, shapes and tissue characteristics. As a result, clinical staff using such prior-art systems have very inaccurate and limited knowledge of what pressure waveforms have actually being applied to the patient, relative to what was prescribed.  
           [14]    14. In addition to their limitations related to the delivery of desired pressure waveforms for pneumatic compression therapy, an important limitation of all systems known in the prior art is that they do not allow clinically important characteristics of pressure waveforms to be changed independently of each other. For example, characteristics of pressure waveforms that have been shown in the clinical literature to significantly affect the augmentation of venous blood flow and thus the incidence of DVT, PE and patient outcomes include: the maximum pressure, the rate of pressure increase to the maximum pressure, the time at which the maximum pressure occurs and the duration after the maximum pressure that the pressure remains above a threshold. No systems known in the prior art allow each of these characteristics to be changed independently of the other. For example, the apparatus described by Tumey et al. in U.S. Pat. No. 5,443,440 allows a user to change one characteristic, the preset maximum pressure value, but changing the maximum pressure value also necessarily changes the rate of pressure increase leading up to the maximum pressure and other clinically important characteristics of the pressure waveform inherently produced by the apparatus of Tumey at al. Tumey et al. is thus representative of prior art systems wherein an inherent pressure waveform is produced by a combination of pneumatic, electronic, mechanical and other components of the apparatus as well as user settings, and wherein any attempt to change one clinically significant aspect of the inherent pressure waveform changes other clinically important characteristics of the delivered pressure waveform. This is an important limitation of all such prior-art systems because the ability to change such characteristics of pressure waveforms independently is desirable in order to optimize pneumatic compression therapy and thus improve patient outcomes.  
           [15]    15. In U.S. Pat. No. 5,443,440 Tumey et al. further describe a limb compression system capable of creating and storing the time, date and duration of each use of the system for subsequent transmission to a physician&#39;s computer. However, sequential and intermittent limb compression systems known in the prior art do not record parameters related to the periodic application of a desired pressure waveform, such as any differences between the actual shape of the pressure waveform produced in the pneumatic sleeve and the shape of a desired reference pressure waveform, the time and duration during which the waveform was periodically applied, and the number of cycles of the waveform which were applied. Additionally, limb compression systems known in the prior art do not subsequently produce the recorded values of these parameters for use by physicians in determining the extent to which the desired pressure waveform was actually applied, for use by third-party payors in reimbursing for therapy actually provided, and for use in patient outcome studies where variations in these parameters of therapy are thought to be related to variations in patient outcomes, leading to optimization of waveform-related parameters and thus improved therapy.  
         SUMMARY OF THE INVENTION  
         [16]    16. The invention is directed to apparatus for applying an adaptable pressure waveform to a patient&#39;s limb for augmenting venous blood flow in the limb, comprising: an inflatable sleeve for positioning onto a limb to apply a pressure to the limb beneath the sleeve when inflated with gas; pressure transducing means for sensing the pressure of gas in the sleeve; pressure waveform application means for controlling the supply of pressurized gas to the sleeve so that the sensed pressure in the sleeve is maintained near a varying pressure indicated by a reference pressure waveform; and waveform register means for producing the reference pressure waveform during a cycle time period wherein the reference pressure waveform includes a maximum pressure at a time within the cycle time period and a rate of pressure increase in an interval leading to the maximum pressure and wherein the waveform register means includes waveform adaptation means for allowing the rate of pressure increase to be changed from a first rate to a second rate without changing the maximum pressure at the time. The waveform adaptation means of the invention also allows the maximum pressure of the reference pressure waveform to be changed from a first level to a second level at the time within the cycle time period without changing the rate of pressure increase to the maximum pressure.  
           [17]    17. The invention may include provision for automatic adaptation of the reference pressure waveform and may also allow for adaptation of the reference pressure waveform by an operator.  
           [18]    18. Alarm means may be included for producing an alarm signal near an alarm time when the difference between the pressure indicated by the level of the sleeve pressure signal and the pressure indicated by the reference pressure waveform signal is greater than a predetermined pressure difference. The apparatus may also include therapy register means for enabling an operator to determine at a time subsequent to the alarm time the sleeve pressure and the reference waveform pressure indicated by the levels of the sleeve pressure signal and the reference pressure waveform signal recorded near the alarm time.  
           [19]    19. Advantageously, the inflatable sleeve of the invention includes a first sleeve connector means communicating pneumatically with the inflatable sleeve and a second sleeve connector means communicating pneumatically with the inflatable sleeve, and the first sleeve connector means does not communicate pneumatically with the second sleeve connector means except through the sleeve.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [20]    20.FIG. 1 is a pictorial representation of the preferred embodiment in a typical clinical application.  
         [21]    21.FIG. 2 is a block diagram of the preferred embodiment.  
         [22]    22.FIG. 3 are graphical representations of pressures applied to a region of a patient by the preferred embodiment  
         [23]    23.FIGS. 4, 5,  6  and  7  are software flow charts depicting sequences of operations carried out in the preferred embodiment.  
         [24]    24.FIGS. 8 and 9 are pictorial representations of a sleeve for applying pressures to a patient&#39;s foot.  
         [25]    25.FIGS. 10 and 11 are pictorial representations of sleeve for applying pressures to a patient&#39;s calf.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [26]    26. The embodiment illustrated is not intended to be exhaustive or limit the invention to the precise form disclosed. It is chosen and described in order to explain the principles of the invention and its application and practical use, and thereby enable others skilled in the art to utilize the invention.  
         [27]    27. In the context of the preferred embodiment, a pressure waveform is generally considered to be a curve that represents the desired or actual amplitude of pressure in a pneumatic sleeve applied to a patient over time, and is described by a graph in rectangular coordinates whose abscissas represent times and whose ordinates represent the values of the pressure amplitude at the corresponding times. A cycle time period of the pressure waveform is generally considered to be the period of time during which one desired pressure waveform is completed. A phase of the pressure waveform is generally considered to be a portion of the pressure waveform occurring during an interval of time within the cycle time period of the pressure waveform. In the context of the preferred embodiment, periodic generation of a pressure waveform is generally considered to be the repetitive production of the pressure waveform in a pneumatic sleeve applied to a patient.  
         [28]    28. The preferred embodiment of the invention is described in three sections below: instrumentation, software and sleeves.  
       I. Instrumentation  
       [29]    29.FIG. 1 depicts instrument  2  connected to two inflatable sleeves, foot sleeve  4  and calf sleeve  6 . Foot sleeve  4  is suitable for applying a compressive pressure waveform to the plantar region of the foot, and is depicted applied to the right foot of a patient  8 . Foot sleeve  4  is shown in detail in FIGS. 8 and 9 and described further below. Calf sleeve  6  is suitable for applying a compressive pressure waveform to the calf and is depicted applied to the left calf of patient  8 . Calf sleeve  6  is shown in detail in FIGS. 10 and 11 and is also described below. Alternatively, other designs of sleeves, applied to other regions of the lower or upper limb, may be employed. Instrument  2  has two channels, channel “A” and channel “B”. Inflatable sleeves  4  and  6  applied to patient  8  are connected to channels “A” and “B” of instrument  2 . instrument  2  repetitively produces a desired pressure waveform in foot sleeve  4  connected to channel “A” of instrument  2 , and repetitively produces another desired pressure waveform in calf sleeve  6  connected to channel “B” of instrument  2 , in order to augment the flow of venous blood from the portions of the limbs beneath sleeves  4  and  6  into portions of the limbs proximal to sleeves  4  and  6 . Channel “A” and channel “B” of instrument  2  operate independently, and may generate different or similar pressure waveforms, as determined by an operator.  
         [30]    30. To enable a better appreciation of the versatility of the invention, instrument  2  is depicted in FIGS. 1 and 2 with channel “A” connected to foot sleeve  4  and channel “B” connected to calf sleeve  6 , to apply pressures to the foot of the right leg and to the calf of the left leg of patient  8 , as may be desirable during a surgical procedure. In other clinical applications, channels “A” and “B” of instrument  2  may be connected to two foot sleeves for applying pressure waveforms to each foot of a patient, or to two calf sleeves for applying pressure waveforms to each calf of a patient. Alternatively, instrument  2  may be connected to only one sleeve, or two sleeves of different design applied to the same limb for applying pressure waveforms sequentially in time.  
         [31]    31. As can be seen in FIG. 1, an inflatable portion of foot sleeve  4  communicates pneumatically with channel “A” of instrument  2  by means of pneumatic connector  9  and pneumatic tubing  10 , and by means of pneumatic connector  11  and pneumatic tubing  12 . Connector  9  comprises sleeve connector  9   a  non-releasably attached to foot sleeve  4  and mating tubing connector  9   b  non-releasably attached to tubing  10 . Connector  11  comprises sleeve connector  11   a  non-releasably attached to foot sleeve  4  and mating tubing connector  11   b  non-releasably attached to tubing  12 . In the preferred embodiment connector  9   a  is physically incompatible with connector  11   b  and does not mate with connector  11   b . Connector  11   a  is physically incompatible with connector  9   b  and does not mate with connector  9   b.    
         [32]    32. An inflatable portion of calf sleeve  6  communicates pneumatically with channel “B” of instrument  2  by means of pneumatic connector  13  and pneumatic tubing  14 , and by means of pneumatic connector  15  and pneumatic tubing  16 . Connector  13  comprises sleeve connector  13   a  non-releasably attached to calf sleeve  6  and mating tubing connector  13   b  non-releasably attached to tubing  14 . Connector  15  comprises sleeve connector  15   a  non-releasably attached to calf sleeve  6  and mating tubing connector  15   b  non-releasably attached to tubing  16 . In the preferred embodiment connector  13   a  is physically incompatible with connector  15   b  and does not mate with connector  15   b . Connector  15   a  is physically incompatible with connector  13   b  and does not mate with connector  13   b.    
         [33]    33. As shown in FIG. 1, venous blood flow sensor  18  is applied to the right popliteal region located behind the knee of patient  8  and located proximally to calf sleeve  6 , and venous blood flow sensor  18  is electrically connected to instrument  2 . Sensor  18  estimates venous blood flow in the limb proximal to calf sleeve  6  using an ultrasonic Doppler technique and is further described below.  
         [34]    34. Liquid crystal graphic display  20  shown in FIGS. 1 and 2 forms part of instrument  2  and is used to display information to the operator of instrument  2 . Display  20  is employed for the selective presentation of any of the following information as described below: (a) menus of commands for controlling instrument  2 , from which an operator may make selections; (b) parameters having values which characterize the sleeve pressure waveforms to be produced in inflatable sleeves connected to channels “A” and “B” of instrument  2 ; (c) text messages describing current alarm conditions, when alarm conditions are determined by instrument  2 ; (d) graphical representations of venous blood flow signals produced by sensor  18 ; and (e) messages which provide operating information to the operator.  
         [35]    35. Controls  22  shown in FIGS. 1 and 2 provide a means for an operator to control the operation of instrument  2 .  
         [36]    36. Referring the block diagram of instrument  2  depicted in FIG. 2, foot sleeve  4  communicates pneumatically with valve manifold  24  through pneumatic connector  9  and pneumatic tubing  10 . Foot sleeve  4  also communicates pneumatically with pressure transducer  26  through pneumatic connector  11  and pneumatic tubing  12 . Valve  28  and valve  30  communicate pneumatically with manifold  24 . Valve  28 , valve  30 , manifold  24  and pressure transducer  26  comprise the principal pneumatic elements of channel “A” of instrument  2 .  
         [37]    37. In the preferred embodiment valve  28  is an electrically actuated, normally closed, proportional valve and valve  30  is an electrically actuated, normally open, proportional valve. Valves  28  and  30  respond to certain valve control signals generated by microprocessor  32 . The level of the valve control signals presented to each of valves  28  and  30  by microprocessor  32  determines the degree to which valve  28  opens and the degree to which valve  30  closes. The level of the valve control signals thereby affects the pressure of gas in foot sleeve  4  by changing the rate of gas flow into and out of manifold  24 .  
         [38]    38. Pressure transducer  26  communicates pneumatically with the inflatable portion of foot sleeve  4  by means of tubing  12  and connector  11 . As shown in FIGS. 1 and 2 pressure transducer  26  does not communicate pneumatically with valve manifold  24  except through foot sleeve  4 . In this way, pressure transducer  26  directly and continuously measures the pressure of gas in the inflatable portion of foot sleeve  4 , and is unaffected by variables including the flow resistance of tubing  10 , the flow resistance of connector  9 , the design of foot sleeve  4 , the pneumatic volume of the inflatable portion of foot sleeve  4 , and the snugness of application of foot sleeve  4  to the limb of patient  8 . Pressure transducer  26  is electrically connected to an analog to digital converter (ADC) input of microprocessor  32  and generates a channel “A” sleeve pressure signal, the level of which is representative of the pressure of gas in foot sleeve  4 .  
         [39]    39. Valve  28  communicates pneumatically with manifold  24  and through tubing  34  to gas pressure reservoir  36 , a sealed pneumatic chamber having a fixed volume of 750 ml. When activated valve  28  permits the flow of gas from reservoir  36  to manifold  24  and therefrom supplies pressurized gas through tubing  10  and connector  9  to the inflatable portion of foot sleeve  4 . Valve  30  pneumatically connects manifold  24  to atmosphere, allowing a controlled reduction of pressure from foot sleeve  4 .  
         [40]    40. Valve  38 , valve  40 , manifold  42  and pressure transducer  44  comprise the principal pneumatic elements of channel “B” of instrument  2 , and are configured as shown in FIG. 2 and described below. Calf sleeve  6  communicates pneumatically with valve manifold  42  through pneumatic connector  13  and pneumatic tubing  14 . Calf sleeve  6  also communicates pneumatically with pressure transducer  44  through pneumatic connector  15  and pneumatic tubing  16 .  
         [41]    41. Valve  28  and valve  40  communicate pneumatically with manifold  42 . In the preferred embodiment valve  38  is an electrically actuated, normally closed, proportional valve and valve  40  is an electrically actuated, normally open, proportional valve. Valves  38  and  40  respond to valve control signals generated by microprocessor  32 . The level of the valve control signals influence the pressure of gas in calf sleeve  6  by determining the gas flow into and out of manifold  42 .  
         [42]    42. Pressure transducer  44  communicates pneumatically with the inflatable portion of calf sleeve  6  by means of tubing  16  and connector  15 . As shown in FIGS. 1 and 2 pressure transducer  44  does not communicate pneumatically with valve manifold  42  except through calf sleeve  6 . In this way, pressure transducer  44  directly and continuously measures the pressure of gas in the inflatable portion of calf sleeve  6 , and is unaffected by variables including the flow resistance of tubing  14 , the flow resistance of connector  13 , the design of calf sleeve  6 , the pneumatic volume of the inflatable portion of calf sleeve  6 , and the snugness of application of calf sleeve  6  to the limb of patient  8 . Pressure transducer  44  is electrically connected to an analog to digital converter (ADC) input of microprocessor  32  and generates a channel “B” sleeve pressure signal, the level of which is representative of the pressure of gas in calf sleeve  6 .  
         [43]    43. Valve  38  communicates pneumatically with manifold  42  through tubing  46  to gas pressure reservoir  36 . When activated valve  38  permits the flow of gas from reservoir  36  to manifold  42  and therefrom supplies pressurized gas through tubing  14  and connector  13  to the inflatable portion of calf sleeve  6 . Valve  40  pneumatically connects manifold  42  to atmosphere, allowing a controlled reduction of pressure from calf sleeve  6 .  
         [44]    44. As shown in FIG. 2, pneumatic pump  4  communicates pneumatically with reservoir  36  through tubing  50 . Pump  48  acts to pressurize reservoir  36  in response to control signals from microprocessor  32 . Reservoir pressure transducer  52  communicates pneumatically with reservoir  36  through tubing  54  and generates a reservoir pressure signal indicative of the pressure in reservoir  36 . Pressure transducer  52  is electrically connected to an ADC input of microprocessor  32 . In response to the reservoir pressure signal and a reservoir pressure reference signal, microprocessor  32  generates control signals for pump  48  and controls the pressure in reservoir  36  to maintain a pressure near the reference pressure represented by the reservoir reference pressure signal.  
         [45]    45. Multiple predetermined reference pressure waveforms suitable for application by foot sleeve  4 , and multiple predetermined pressure waveforms suitable for application by calf sleeve  6 , are stored within waveform register  56 . In the preferred embodiment pressure waveforms are stored in waveform register  56  as a set of values describing the amplitude of pressure at all times within one complete waveform cycle time period. It will be apparent to those skilled in the art that certain reference pressure waveforms could alternatively be stored as series of coefficients for a mathematical equation describing the waveforms, or a scaling factor and a set of values representing a normalized waveform. Waveform register  56  responds to a waveform selection signal produced as described below. The level of the waveform selection signal determines which one of the stored predetermined reference pressure waveforms will be communicated to microprocessor  32 .  
         [46]    46.FIG. 3 illustrates three examples of reference pressure waveforms, reference pressure waveforms A, B and C, which are maintained in waveform register  56 . The waveforms over the complete cycle time period are shown. Each reference pressure waveform cycle has one or more discrete phases. In the context of the preferred embodiment, a phase of a reference pressure waveform is considered to be a variation in the amplitude of pressure during a time interval within the cycle time period having a shape and maximum pressure and rate of pressure increase to the maximum pressure adapted to produce a desired augmentation of the flow of venous blood proximally from a selected sleeve which is positioned on a limb near a desired location. Reference pressure waveforms A and C illustrate waveforms having two phases. Reference pressure waveform B illustrates a reference pressure waveform having a single phase. In the preferred embodiment the cycle time periods of reference pressure waveforms range between 50 and 200 seconds. The time intervals corresponding to phases of the reference pressure waveforms range between 2 and 20 seconds.  
         [47]    47. Reference pressure waveforms A and B shown in FIG. 3 are typical waveforms for application by calf sleeve  6 . Reference pressure waveform C is a typical waveform for application by foot sleeve  4 . Reference pressure waveforms A and C depicted in FIG. 3 have two different phases, indicated as phase  1  and phase  2  in FIG. 3. The variation in pressure amplitude, maximum pressure and rate of pressure increase to the maximum pressure of phase  1  of each reference pressure waveform A and C shown in FIG. 3 is adapted to augment the flow of venous blood into the limb proximal to the sleeve from the limb beneath the sleeve by increasing the maximum blood velocity during the phase  1  time interval of the reference pressure waveform. The variation in pressure amplitude, maximum pressure and rate of pressure increase to the maximum pressure of phase  2  of waveforms A and C is adapted to augment the flow of venous blood into the limb proximal to the sleeve from the limb beneath the sleeve by increasing the mean blood velocity during phase  2  time interval of the waveform. Pressure waveform cycle B is shown with a single phase that is adapted to augment both mean and maximum venous blood flow proximally into the limb from the region underlying the pressurizing sleeve.  
         [48]    48. In addition to the examples described above other reference pressure waveforms adapted to augment the flow of venous blood are also maintained in waveform register  56 . For example, multiple reference pressure waveforms with the same maximum pressure but differing rates of increase to the maximum pressure may be maintained in waveform register  56 . Similarly, multiple reference pressure waveforms with differing maximum pressures and the same rate of pressure increase to the maximum pressure may be maintained in waveform register  56 .  
         [49]    49. Referring again to FIG. 2, microprocessor  32  operates, when directed by an operator of instrument  2  through manipulation of controls  22 , to repetitively generate a selected reference pressure waveform in foot sleeve  4  connected to channel “A” of instrument  2 . Microprocessor  32  continues to repetitively produce the desired pressure waveforms in foot sleeve  4  until an operator through manipulation of controls  22  directs microprocessor  32  to suspend the generation of pressure waveforms, or alternatively until microprocessor  32  suspends the generation of pressure waveforms in response to an alarm signal as described below.  
         [50]    50. To generate pressure waveforms in foot sleeve  4  connected to channel “A”, microprocessor  32  first generates a channel “A” sleeve reference pressure waveform signal by: (a) retrieving from waveform register  56  a reference pressure waveform, as determined by the level of a channel “A” waveform selection signal; and (b) scaling the amplitude of the retrieved reference pressure waveform uniformly so that the amplitude of the scaled reference pressure waveform is equivalent to the desired amplitude of the channel “A” reference pressure waveform.  
         [51]    51. If subsequently desired by an operator, the level of the channel “A” waveform selection signal may be adjusted. Also, the amplitude of the channel “A” reference pressure waveform may be adapted by the operator of instrument  2  through manipulating controls  22 . Alternatively, the level of the sleeve waveform selection signal and amplitude of the channel “A” reference pressure waveform may be automatically set by microprocessor  32  as a result of microprocessor  32  retrieving the values of previously stored parameters from configuration register  58  as described below or as a result of microprocessor  32  when instructed by an operator, automatically determining a new level for the channel “A” waveform selection signal and a new amplitude for the channel “A” reference pressure waveform as further described below.  
         [52]    52. The channel “A” sleeve reference pressure waveform signal is used by microprocessor  32 , in combination with a channel “A” sleeve pressure signal generated by pressure transducer  26  and the reservoir pressure signal as described below, to maintain the pressure in the sleeve connected to channel “A” of instrument  2  near the pressure represented by the channel “A” sleeve reference pressure waveform signal by generating control signals for valves  28  and valve  30 .  
         [53]    53. Microprocessor  32  subtracts the pressures represented by the levels of the channel “A” reference pressure waveform signal and the channel “A” sleeve pressure signal. The difference in pressure between the sleeve pressure and the reference waveform pressure is used by microprocessor  32  along with the pressure represented by the level of the reservoir pressure signal to calculate levels of control signals for valves  28  and  30 . Valves  28  and  30  respond to the control signals to increase, decrease or maintain the pressure in foot sleeve  4  connected to channel “A” such that the pressure within foot sleeve  4  at the time is maintained near the pressure represented by the level of the channel “A” reference pressure waveform signal.  
         [54]    54. To alert the operator when the pressures being generated in foot sleeve  4  are not within a desired limit of the pressures indicated by the channel “A” reference pressure waveform signal, microprocessor  32  generates alarm signals. Microprocessor  32  first compares the pressure in foot sleeve  4  to the pressure indicated by the level of the channel “A” reference pressure waveform signal. If the pressure in foot sleeve  4  exceeds the reference pressure by a pre-set limit of 10 mmHg, microprocessor  32  generates an alarm signal indicating over-pressurization of the sleeve connected to channel “A”. If the pressure in foot sleeve  4  is less than the reference pressure signal by a pre-set limit of 10 mmHg, microprocessor  32  generates an alarm signal indicating under-pressurization of the sleeve connected to channel “A”.  
         [55]    55. Microprocessor  32  also maintains a therapy duration counter to track the actual number of pressure waveforms that have been generated in foot sleeve  4  by channel “A” and the length of time that these pressure waveforms have been produced. Microprocessor  32  compares this actual channel “A” sleeve therapy duration to a channel “A” sleeve therapy duration time limit, and if the actual therapy duration time exceeds the therapy duration time limit, microprocessor  32  generates an alarm signal indicating that the therapy duration time limit for the channel “A” sleeve has been exceeded.  
         [56]    56. To generate pressure waveforms in calf sleeve  6  connected to channel “B” of instrument  2 , microprocessor  32  operates in an equivalent manner to the operation of channel “A” as described above. Reference pressure waveforms, alarm signals and valve control signals are produced independently of those produced for channel “A”.  
         [57]    57. When instructed by an operator of instrument  2  through manipulation of controls  22 , microprocessor  32  will initiate the sequential generation of pressure waveforms in foot sleeve  4  and calf sleeve  6  connected to channels “A” and “B”. The timing of the sequential generation of pressure waveforms in sleeves  4  and  6  may be selected by the operator to be: a) the initiation of a pressure waveform cycle by channel “B” at a predetermined time interval following the initiation of a pressure waveform cycle by channel “A”; or b) the initiation of a pressure waveform cycle by channel “B” upon the pressure within foot sleeve  4  connected to channel “A” exceeding a predetermined pressure level; or c) the initiation of a pressure waveform cycle by channel “B” upon slope of the pressure waveform within foot sleeve  4  connected to channel “A” exceeding a predetermined slope threshold.  
         [58]    58. Venous blood flow sensor  18  is located on a portion of either the right or the left limb, proximal to either foot sleeve  4  or calf sleeve  6  to sense the velocity of venous blood flowing in a vein located beneath flow sensor  18 . The velocity of blood flow in the vein proximal to the sleeve is augmented as determined by the shape of pressure waveforms generated in the sleeve and applied to the limb beneath the sleeve. FIG. 1 illustrates a typical location for the application of sensor  18  to the lower limb. Sensor  18  operates using Doppler ultrasound to generate a venous blood flow signal indicative of the velocity of blood flow in a vein beneath sensor  18 , which is processed by sensor interface  60  and communicated to microprocessor  32 , as depicted in FIG. 2.  
         [59]    59. During the generation of a pressure waveform in a sleeve connected to either channel “A” or “B” of instrument  2 , microprocessor  32  analyzes the venous blood flow signal from sensor  18  to determine, for each phase of the pressure waveform, the peak venous blood flow velocity and the mean time-averaged venous blood velocity resulting from the application of the pressure waveform. The magnitude of these velocities are indicative of the effectiveness of the therapy that is delivered to a patient by the preferred embodiment. Although a Doppler ultrasound sensor has been incorporated into the preferred embodiment, other sensors may alternately be employed using photo- plethysmography, oximetry, de-oximetry or impedance plethysmography to provide an indication of augmentation of venous blood flow in one or both limbs simultaneously.  
         [60]    60. To assist the operator of instrument  2  in adapting the amplitude of a reference pressure waveform, microprocessor  32 , as instructed by an operator of instrument  2  through manipulation of controls  22 , may automatically adapt amplitude of the selected reference pressure waveform for channels “A” and “B” to increase the peak and time averaged venous blood flow velocities, as selected. This is further described in the software description given below. Alternatively, microprocessor  32 , when instructed by an operator of instrument  2  through manipulation of controls  22 , may automatically select a reference pressure waveform for channels “A” and “B” to increase the peak and time averaged venous blood flow velocities.  
         [61]    61. Configuration register  58  shown in FIG. 2 is comprised of non-volatile memory and operates in conjunction with microprocessor  32  as described below. Configuration register  58  contains the values of previously recorded parameters representing reference pressure waveform selections, amplitudes of reference pressure waveforms and therapy time duration alarm limits for use by microprocessor  32  as described below, and retains the recorded values of these parameters indefinitely in the absence of electrical power supplied to configuration register  58  and in the absence or interruption of electrical power from power supply  62  required for the normal operation of instrument  2 . The values of the parameters representing waveform selections, amplitudes of reference pressure waveforms and therapy time duration limits initially recorded in configuration register  58  are given in the table below:  
                                                                                     Therapy           Reference       Duration           Waveform       Time           Selection   Amplitude   Limit                                    Channel “A”   Foot   3   180 mmHg   8 Hrs           Calf   1    50 mmHg   8 Hrs       Channel “B”   Foot   180 mmHg   8 Hrs           Calf   1    50 mmHg   8 Hrs                  
 
         [62]    62. Microprocessor  32  communicates with configuration register  58  to record and retrieve levels of the configuration parameters recorded in configuration register  58  as also described below.  
         [63]    63. Real time clock  64  shown in FIG. 2 maintains the current time and date, and includes a battery as an alternate power source such that clock operation continues during any interruption in the supply of electrical power from power supply  62  required for the normal operation of instrument  2 . Microprocessor  32  communicates with real time clock  64  for both reading and setting the current time and date.  
         [64]    64. Therapy register  66  shown in FIG. 2, records “events” related to the pressure waveforms generated in sleeves connected to channels “A” and “B” of instrument  2 , and thereby related to the therapy delivered to a patient by the preferred embodiment. “Events” are defined in the preferred embodiment to include: (a) actions by the operator to initiate the generation of pressure waveforms in a sleeve, to suspend the generation of pressure waveforms in a sleeve, to select a reference pressure waveform for generation in a sleeve, to adapt the amplitude of a pressure of a waveform, or to adjust the therapy time duration alarm limits; (b) alarm events resulting from microprocessor  32  generating alarm signals as described above; and (c) events associated with determining an amplitude for a reference pressure waveform automatically as described below.  
         [65]    65. Microprocessor  32  communicates with therapy register  66  to record events as they occur. Microprocessor  32  records an event by communicating to therapy register  66 : the time of the event as read from real time clock  64 , and a value identifying which one of a specified set of events occurred as determined by microprocessor  32 . Also, if the event relates to channel “A” of instrument  2 , therapy register  66  records the values at the time of the event of the following parameters: the channel “A” waveform selection signal, the channel “A” reference pressure waveform amplitude, the channel “A” sleeve pressure signal, and the channel “A” sleeve therapy duration. Alternatively, if the event relates to channel “B” of instrument  2 , therapy register  66  records the values at the time of the event of the following parameters: the channel “B” waveform selection signal, the channel “B” reference pressure waveform amplitude, the channel “B” sleeve pressure signal, and the channel “B” sleeve therapy duration.  
         [66]    66. Microprocessor  32 , when directed by an operator of instrument  2  through manipulation of controls  22 , subsequently displays, prints or transfers to an external computer the values associated with events stored in therapy register  66 . Therapy register  66  retains information indefinitely in the absence or interruption of electrical power from power supply  62  required for the normal operation of therapy register  66 .  
         [67]    67. Safety circuit  68  acts to prevent abnormal valve actuations resulting from: failure of the electronic circuitry associated with controlling valves  28 ,  30 ,  38  and  40 ; failure in microprocessor  32 ; or software error. Safety circuit  68  operates independently of microprocessor  32  such that safety circuit  68  continues to operate normally during a malfunction or complete failure of microprocessor  32 . An abnormal valve actuation may cause abnormal pressure waveforms to be applied to a patient, resulting in injury or unintended therapy. Upon detecting an abnormal valve actuation safety circuit  68 , will cause: a) the supply of electrical power to valves  28 ,  30 ,  38  and  40  to be disconnected; b) an audio tone to be emitted by loud speaker  70 ; c) a message to be displayed upon display panel  20 ; and d) the operation of microprocessor  32  to be suspended. When valves  30  and  40  are disconnected from electrical power sleeves connected to channel “A” and “B” will be allowed to vent to atmosphere as valves  30  and  40  are normally open valves. Similarly, when valves  28  and  38  are disconnected from electrical power, pressurized gas in reservoir  36  is prevented from flowing to sleeves connected to channels “A” and “B” as valves  28  and  38  are normally closed valves.  
         [68]    68. To detect abnormal valve actuations safety circuit  68  monitors the electrical current supplied to each of valves  28 ,  30 ,  38 , and  40 . The amount of current supplied to a valve is indicative of the state of the valve, actuated or de-actuated. Safety circuit  68  receives from microprocessor  32  mode signals indicative of: the mode of operation of each channel, defined to be either an “active” mode during which pressure waveforms are being generated, or an “inactive” mode during which pressure waveforms are not being generated. Also, safety circuit  68  receives from microprocessor  32  the channel “A” and “B” reference pressure waveform signals indicative of the current sleeve pressure levels.  
         [69]    69. The table below summarizes the abnormal combinations of valve actuations which are detected by safety circuit  68  for channel “A”, equivalent abnormal actuations are also detected by safety circuit  68  for channel “B”.  
                                                   Valve 28           Channel “A”   Channel “A” reference   (normally   Valve 30       mode   pressure waveform level   closed)   (normally open)                   Inactive   don&#39;t care   Actuated   De-actuated       Inactive   don&#39;t care   Actuated   Actuated       Inactive   don&#39;t care   De-actuated   Actuated       Active   &lt;2 mmHg   Actuated   De-actuated       Active   &lt;2 mmHg   Actuated   Actuated       Active   &lt;2 mmHg   De-actuated   Actuated       Active   &gt;=2 mmHg    Actuated   Actuated                  
 
         [70]    70. Referring to FIG. 2, and as described above operator input is by means of controls  22 . Signals from controls  22 , arising from contact closures of the switches that comprise controls  22  are communicated to microprocessor  32 .  
         [71]    71. Microprocessor  32  will, in response to generated alarm signals, alert the operator by text and graphic messages shown on display panel  20  and by audio tones. Electrical signals having different frequencies to specify different alarm signals and conditions are produced by microprocessor  32  and converted to audible sound by loud speaker  70  shown in FIG. 2.  
         [72]    72. Power supply  62  provides regulated DC power for the normal operation of all electronic and electrical components within instrument  2 .  
       II. Software  
       [73]    73.FIGS. 4, 5,  6  and  7 , are software flow charts depicting sequences of operations which microprocessor  32  is programmed to carry out in the preferred embodiment of the invention. In order to simplify the discussion of the software, a detailed description of each software subroutine and of the control signals which the software produces to actuate the hardware described above is not provided. The flow charts shown and described below have been selected to enable those skilled in the art to appreciate the invention. Functions or steps carried out by the software are described below and related to the flow charts via parenthetical reference numerals in the text.  
         [74]    74.FIG. 4 shows the initialization operations carried out by the main program. FIG. 5 shows a software task associated with processing input from an operator and updating therapy register  66 . FIG. 6 shows a software task for controlling the channel “A”. FIG. 7 shows a software task associated with the automatic determination of the amplitude of a reference pressure waveform.  
         [75]    75.FIG. 4 shows the initialization operations carried out by the system software. The program commences ( 400 ) when power is supplied to microprocessor  32  by initializing microprocessor  32  for operation with the memory system and circuitry and hardware of the preferred embodiment. Control is then passed to a self-test subroutine ( 402 ). The self-test subroutine displays a “SELF TEST” message on display panel  20  and performs a series of diagnostic tests to ensure proper operation of microprocessor  32 , its associated hardware and safety circuit  68 . Should any diagnostic test fail ( 404 ), an error code is displayed on display panel  20  ( 406 ) and further operation of the system is halted ( 408 ); if no errors are detected, control is returned to the main program.  
         [76]    76. As can be seen in FIG. 4, after the “self-test” has been completed successfully, control is next passed to a subroutine ( 410 ) which retrieves from configuration register  58  the values of previously recorded configuration parameters. The configuration parameters for each channel are: a reference pressure waveform selection, reference pressure waveform amplitude and therapy time duration alarm limit for both calf and foot sleeves, as described above.  
         [77]    77. Upon completion, this subroutine returns control to the main program. Control is next passed to a subroutine ( 412 ) which tests the values of the retrieved configuration parameters for validity by: (1) calculating a checksum for the retrieved values of the parameters and comparing it to a checksum previously calculated and recorded in configuration register  58 ; (2) testing each retrieved parameter value to ensure it is within pre-defined allowable limits. If any of the values of the retrieved parameters are found to be invalid an error message is displayed on display panel  20  ( 414 ), and configuration parameters are set to default values defined in software ( 416 ). If the retrieved parameters are valid, the reference pressure waveform selections, reference pressures waveform amplitudes and therapy time duration alarm limits for both calf and foot sleeves are set to the previously recorded values of the configuration parameters ( 418 ).  
         [78]    78. Next, a software task manager is initialized ( 420 ). The software task manager executes at predetermined intervals software subroutines which control the operation of instrument  2 . Software tasks may be scheduled to execute at regularly occurring intervals. For example the subroutine shown in FIG. 6 and described below executes every 2 milliseconds. Other software tasks execute only once each time they are scheduled. The task manager ( 422 ) continues to execute scheduled subroutines until one of the following occurrences: a) power is no longer supplied to microprocessor  32 ; b) the operation of microprocessor  32  has been interrupted by safety circuit  68  in response to a detected fault condition; or c) the operation of microprocessor  32  has been halted by software in response to the software detecting an error condition.  
         [79]    79.FIG. 5 shows a flowchart of the software task associated with updating display  20  and processing input from an operator. This task is executed at regular predetermined intervals. Control is first passed to a subroutine that updates the menus of commands and values of displayed parameters shown on display  20  ( 500 ). The menus of commands and parameters shown on display  20  are appropriate to the current operating state of instrument  2  as determined and set by other software subroutines.  
         [80]    80. Control is next passed to a subroutine ( 502 ) which processes the input from controls  22 . In response to operator input by means of controls  22  other software tasks may be scheduled and initiated ( 504 ). For example, if the operator has selected a menu command to adapt the amplitude of the channel “A” reference pressure waveform so that the amplitude of the channel “A” reference pressure waveform is equivalent to a desired amplitude, an appropriate software task is scheduled to effect the scaling of the channel “A” reference pressure waveform.  
         [81]    81. Control is then passes to a subroutine ( 506 ) which determines if the operating parameters (reference pressure waveform selections, amplitudes of reference pressure waveforms, therapy duration limits, initiation or suspension of the application of pressure waveforms) of instrument  2  which affect the therapy delivered to a patient have been adjusted by an operator of instrument  2 . Current values of operating parameters are compared to previous values of operating parameters. If the current value of any one or more parameters differs from its previously set value control is passed to a subroutine ( 508 ) for recording events in therapy register  66 . This subroutine ( 508 ) records an event by storing the following in therapy register  66 : the time of the event as read from real time clock  64 ; and a value identifying which one or more of a specified set of events occurred as determined by subroutine ( 506 ). Also, if the event relates to channel “A” of instrument  2 , the values of the following parameters at the time of the event are also stored in therapy register  66 : channel “A” waveform selection signal, amplitude of the channel “A” reference pressure waveform, channel “A” sleeve pressure signal and the channel “A” sleeve therapy duration. Alternatively if the event relates to channel “B” of instrument  2 , the values of the following parameters at the time of the event are stored in therapy register  66 : channel “B” waveform selection signal, amplitude of the channel “B” reference pressure waveform, channel “B” sleeve pressure signal and the channel “B” sleeve therapy duration.  
         [82]    82. As shown in FIG. 5 control is next passed to a subroutine ( 510 ) which compares the current alarm conditions to previous alarm conditions. If any one or more alarm conditions exist which did not previously exist, control is passed to a subroutine ( 512 ) for recording the alarm event in therapy register  66 . Subroutine ( 512 ) records an alarm event by storing in therapy register  66  the time of the event as read from real time clock  64 ; a value identifying which one or more of a specified set of alarm events occurred as determined by subroutine ( 510 ). Also, if the alarm event relates to channel “A” of instrument  2 , the values of the following parameters at the time of the event are also stored in therapy register  66 : channel “A” waveform selection signal, amplitude of the channel “A” reference pressure waveform, channel “A” sleeve pressure signal and the channel “A” sleeve therapy duration. Alternatively, if the event relates to channel “B” of instrument  2 , the values of the following parameters at the time of the event are stored in therapy register  66 : channel “B” waveform selection signal, amplitude of the channel “B” reference pressure waveform, channel “B” sleeve pressure signal and the channel “B” sleeve therapy duration. The software task shown in FIG. 5 then terminates ( 514 ).  
         [83]    83.FIG. 6 depicts a software task associated with controlling channel “A” of instrument  2 . A similar software task exists for controlling channel “B”, but for simplicity only the task associated with channel “A” will be described. The software task shown in FIG. 6 is scheduled to execute continuously once every two milliseconds. As shown in FIG. 6, if channel “A” is not currently generating pressure waveforms ( 600 ) in foot sleeve  4  the valve control signal for valve  28  is set to a level that ensures valve  28  remains closed ( 602 ). The valve control signal for valve  30  is set to a level that ensures valve  30  remains open ( 604 ). Opening valve  30  vents any gas in foot sleeve  4  connected to channel “A” to atmosphere, and closing valve  28  prevents gas from flowing from reservoir  36  to foot sleeve  4  connected to channel “A”.  
         [84]    84. The channel “A” sleeve pressure signal is then sampled ( 606 ). If the pressure in foot sleeve  4  connected to channel “A” is above a predetermined threshold of 10 mmHg ( 608 ), an alarm flag is set ( 610 ) to indicate that the sleeve connected to channel “A” is pressurized at a time when it should not be pressurized. The software task associated with controlling channel “A” then terminates ( 612 ).  
         [85]    85. As shown in FIG. 6, if channel “A” is currently generating pressure waveforms ( 600 ) in foot sleeve  4 , control is passed to a subroutine which updates the therapy duration timer ( 614 ) for channel “A”. Control next passes to a subroutine ( 616 ) which compares the current therapy duration time to the therapy time duration limit for channel “A”. If the therapy time duration limit has been exceeded, an alarm flag is set ( 618 ) to indicate that the therapy time duration limit for channel “A” has been exceeded.  
         [86]    86. The software task continues by sampling the value of the channel “A” sleeve pressure signal ( 620 ). Control is then passed to a subroutine ( 622 ) which samples the channel “A” reference pressure waveform signal. The value of the sample obtained from the reference pressure waveform signal is representative of the desired sleeve pressure at the instant of time when the subroutine executes. An error signal is computed ( 624 ) by calculating the difference between the pressure indicated by the value of the channel “A” sleeve pressure signal and the value of the sample of the channel “A” reference pressure waveform signal. Control is passed to a subroutine ( 626 ) that compares the error signal to predetermined limits and sets an alarm flag ( 628 ) if the limits have been exceeded. Next, the signal from reservoir pressure transducer  52  is sampled ( 630 ). Control then passes to a subroutine ( 632 ) which calculates levels for the control signals for valve  28  and valve  30 . The subroutine ( 632 ) uses the current levels of the error signal and reservoir pressure signal, as well as previously stored levels of these signals, to compute new levels for the valve  28  and  30  control signals. When the calculation subroutine ( 632 ) completes, the software task shown in FIG. 6 terminates ( 612 ).  
         [87]    87. As described above an operator of instrument  2  may elect to adapt the amplitude of a reference pressure waveform generated in a sleeve connected to either channel “A” or “B” automatically. The software task depicted in FIG. 7 is associated with the automatic adaptation of the amplitude of a reference pressure waveform. The task begins by sampling the venous blood flow signal from sensor  18  ( 700 ). Next control is passed to a subroutine ( 702 ) which processes the venous blood flow signal from venous blood flow sensor  18 . This subroutine ( 702 ) calculates the mean time-averaged venous blood flow velocity and the peak venous blood flow velocity for each phase of the currently generated pressure waveform in the sleeve. The sampling and calculation continues until the reference pressure waveform cycle time period has elapsed ( 704 ). At completion of the generation of a pressure waveform, control is passed to a subroutine ( 706 ) which compares the velocities of mean time-averaged venous blood flow and peak venous blood flow for each phase of the recently generated pressure waveform to predefined target velocities. If the target velocities for mean time-average venous blood flow and peak venous blood flow have not been achieved control is passed to a subroutine ( 708 ) which calculates a new amplitude for the next pressure waveform to be generated in the sleeve. Control next passes to a subroutine ( 710 ) that re-schedules the amplitude adaptation task shown in FIG. 7 to execute again during the generation of the next pressure waveform. The amplitude adaptation task then terminates ( 716 ).  
         [88]    88. If the comparison of venous blood flow velocities performed in subroutine ( 706 ) indicates that the predetermined target velocities have been met, control is passed to a subroutine ( 712 ) which causes the amplitude of the reference pressure waveform to be maintained at its current level.  
         [89]    89. Control is next passed to a subroutine ( 714 ) which records in therapy register  66  an amplitude adaptation event by storing in therapy register  66  the time of the event as read from real time clock  64  and a value identifying that an amplitude adaptation event occurred. Also, if the event relates to channel “A” of instrument  2 , the values of the following parameters at the time of the event are also stored in therapy register  66 : channel “A” waveform selection signal, amplitude of the channel “A” reference pressure waveform, channel “A” sleeve pressure signal and the channel “A” sleeve therapy duration. Alternatively, if the event relates to channel “B” of instrument  2 , the values of the following parameters at the time of the event are stored in therapy register  66 : channel “B” waveform selection signal, amplitude of the channel “B” reference pressure waveform, channel “B” sleeve pressure signal and the channel “B” sleeve therapy duration. The software task shown in FIG. 7 then terminates ( 716 ).  
       III. Sleeves  
       [90]    90.FIG. 8 is a plan view to illustrate details of foot sleeve  4 . Foot sleeve  4  is manufactured in a single size designed to accommodate 95% of normal adult feet. Foot sleeve  4  includes exterior layer  900  which forms a non-inflating portion, and bladder assembly  902  which forms an inflating portion. Exterior layer  900  is fabricated from a synthetic cloth material and has an outer and inner surface which allows engagement with a Velcro™ hook material.  
         [91]    91. As shown in plan view FIG. 8 and cross sectional view FIG. 9, bladder assembly  902  contains layer  904  and layer  906 . Layers  904  and  906  are fabricated from a flexible gas-impermeable thermoplastic polyvinylchloride sheet material permanently bonded together to form inflatable bladder  908 . The flexibility of this gas-impermeable polyvinylchloride sheet material is predetermined and substantially inextensible when bladder  908  is pressurized up to 300 mmHg.  
         [92]    92. Ports  910  and  912  are thermoplastic right-angle flanges. Port  910 , in combination with tubing  10  and connector  9 , provides a pneumatic passageway suitable for increasing or decreasing the gas pressure within bladder  908  of foot sleeve  4 . Port  912 , in combination with pressure transducer  26 , tubing  12  and connector  11 , is used in the preferred embodiment to enable direct, accurate and continuous measurement of gas pressure in foot sleeve  4  by transducer  26  in a manner unaffected by variables such as the flow resistance of tubing  10 , the flow resistance of connector  9 , the design of foot sleeve  4 , the pneumatic volume of the inflatable portion of foot sleeve  4  and the snugness of application of foot sleeve  4 . Alternatively, it will be appreciated that direct, accurate and continuous measurement of pneumatic pressure within bladder  908  of foot sleeve  4  could be accomplished by embedding an electronic pressure transducer within bladder  908 .  
         [93]    93. Referring to FIG. 8 and FIG. 9, stiffener  914  located between exterior layer  900  and bladder assembly  902 , is permanently attached to layer  900 . The shape of stiffener  914  is pre-determined being of sufficient width and length to cover the medial planter vein of the foot. Stiffener  914  fabricated from a thermoplastic sheet material has a predetermined thickness and rigidity to direct the inflated portion of bladder  908  above stiffener  914  toward the limb producing the desired applied pressure waveform when bladder  908  is inflated.  
         [94]    94. As shown in FIG. 8, fasteners  916  attached to layer  900  consist of rectangular sections of Velcro™ hook material which removably engage with the cloth surface of layer  900  ensuring that foot sleeve  4  remains secured to a limb when bladder  908  is inflated.  
         [95]    95. Foot sleeve  4  is manufactured by die cutting layer  900  from the desired synthetic cloth material. Two holes are cut into layer  908  providing access for ports  910  and  912  allowing them to protrude through layer  900  when bladder assembly  902  is secured in place. Stiffener  914  die cut from a thermoplastic sheet material into a predetermined shape is then permanently heat sealed to layer  900  using Radio Frequency (RF) sealing equipment. Fasteners  916  are sewn to layer  900  such that the hooks of fasteners  916  face away from layer  900 .  
         [96]    96. Fabrication of bladder assembly  902  begins by die cutting layers  904  and  906  from a flexible polyvinylchloride sheet material. Two holes are die cut into layer  904  allowing ports  910  and  912  to be inserted into position and bonded in place using RF sealing equipment. With ports  910  and  912  facing away from layer  906 , layers  904  and  906  are heat sealed together forming bladder  908 . With fasteners  916  facing ports  910  and  912  of bladder assembly  902 , ports  910  and  912  are inserted into the holes in layer  900  such that ports  910  and  912  protrude through layer  900 . Manufacturing of foot sleeve  4  is completed by permanently fastening bladder assembly  902  to layer  900  using RF sealing equipment and by inserting pneumatic connectors  9 A and  11 A into the opening of ports  910  and  912  respectively.  
         [97]    97.FIG. 1 illustrates foot sleeve  4  communicating pneumatically with instrument  2  by means of pneumatic connectors  9  and  11 . As described above connector  9 A is physically incompatible with connector  11 B and does not mate with connector  11 B. Connector  11 A is physically incompatible with connector  9 B and does not mate with connector  9 B.  
         [98]    98.FIG. 10 is a plan view to illustrate details of calf sleeve  6 . Calf sleeve  6  is manufactured in a single size designed to conform to a variety of calf shapes and sizes accommodating 95% of the normal adult population. As illustrated in plan view FIG. 10 and cross sectional view FIG. 11, calf sleeve  6  includes bladder  1100 , which forms an inflatable portion surrounded by and an non-inflatable portion. Bladder  1100  of calf sleeve  6  is formed by permanently bonded together layers  1102  and  1104  using Radio Frequency (RF) sealing equipment.  
         [99]    99. Layers  1102  and  1104  are fabricated from a flexible gas-impermeable thermoplastic polyvinylchloride sheet material. The rigidity and thickness of this gas-impermeable sheet material is predetermined allowing layers  1102  and  1104  to be substantially inextensible when bladder  1100  is pressurized up to 60 mmHg.  
         [100]    100. Ports  1106  and  1108  are thermoplastic right-angle flanges. Port  1106 , in combination with tubing  14  and connector  13 , provides a pneumatic passageway suitable for increasing or decreasing the gas pressure within bladder  1100  of calf sleeve  6 . Port  1108 , in combination with pressure transducer  44 , tubing  16  and connector  15 , is used in the preferred embodiment to enable direct, accurate and continuous measurement of gas pressure in calf sleeve  6  by transducer  44  in a manner unaffected by variables such as the flow resistance of tubing  14 , the flow resistance of connector  13 , the design of calf sleeve  6 , the pneumatic volume of the inflatable portion of calf sleeve  6  and the snugness of application of calf sleeve  6 . Alternatively, it will be appreciated that direct, accurate and continuous measurement of pneumatic pressure within bladder  1100  of calf sleeve  6  could be accomplished by embedding an electronic pressure transducer within bladder  1100 .  
         [101]    101. Shown in FIG. 10, Velcro™ loop fasteners  1110  and Velcro™ hook fasteners  1112  removably engage each other allowing application and removal of calf sleeve  6 . Fasteners  1110  and  1112  ensure that calf sleeve  6  remains secured a limb when bladder  1100  is inflated. Velcro™ loop fasteners  1110  and Velcro™ hook fasteners  1112  have a thermoplastic coating on one side allowing loop fasteners  1110  to be bonded to the outer surface of thermoplastic layer  1104  and hook fasteners  1112  to be bonded to the outer surface of thermoplastic layer  1102 .  
         [102]    102. Calf Sleeve  6  is manufactured by die cutting layers  1102  and  1104  from a polyvinylchloride thermoplastic sheet material. Two holes are die cut into layer  1104  providing access for ports  1106  and  1108 . Ports  1106  and  1108  are inserted through the holes in layer  1104  and bonded to layer  1104  using RF sealing equipment. Velcro™ loop fasteners  1110  are permanently RF sealed to the outer surface of layer  1104  by positioning the thermoplastic coating on fasteners  1110  in contact with thermoplastic layer  1104 .  
         [103]    103. With ports  1106  and  1108  facing away from layer  1102 , layer  1104  and layer  1102  are RF sealed together forming bladder  1100 . Hook fasteners  1112  are then RF sealed to the outer surface of layer  1102  as illustrated in FIG. 10. Manufacturing of calf sleeve  6  is completed by inserting pneumatic connectors  13 A and  15 A into the opening of ports  1106  and  1108  respectively.  
         [104]    104.FIG. 1 illustrates calf sleeve  6  communicating pneumatically with instrument  2  by means of pneumatic connectors  13  and  15 . As described above connector  13 A is physically incompatible with connector  15 B and does not mate with connector  15 B. Connector  15 A is physically incompatible with connector  13 B and does not mate with connector  13 B.