Abstract:
Apparatus for applying pressure to a patient&#39;s limb in order to augment venous blood flow in the limb and for monitoring the applied pressure, includes supplying a gas at a varying supply pressure to an inflatable sleeve that fits onto a limb to apply a varying pressure to the limb beneath the sleeve when inflated with the gas. A pressure transducer measures the pressure of gas in the inflatable sleeve and produces a sleeve pressure signal indicative of the estimated level of pressure. The apparatus measures the value of a predetermined pressure waveform parameter and produces a waveform parameter signal indicative of the measured value of the predetermined pressure waveform parameter. An interval signal is produced as indicative of an interval between a first occurrence when the measured value of the predetermined pressure waveform parameter is near a predetermined parameter level and the next occurrence when the measured value of the predetermined pressure waveform parameter is near the predetermined parameter level.

Description:
This is a continuation of U.S. patent application Ser. No. 09/105,393, filed Jun. 26, 1998 now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 08/639,782 filed Apr. 29, 1996 now U.S. Pat. No. 5,843,007. 
    
    
     FIELD OF THE INVENTION 
     The invention is related to an apparatus and method for applying varying pressure waveforms to a limb of a human patient in order to help prevent deep vein thrombosis (DVT), pulmonary embolism (PE) and death. 
     BACKGROUND OF THE INVENTION 
     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), pulmonary embolism (PE) and death. 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 thereby producing pressure waveforms to be applied to both limbs either simultaneously or alternately, while others are designed to produce pressure waveforms for application to one limb only. 
     One major concern with all pneumatic limb compression systems of the prior art is that the therapy actually delivered by these systems may vary substantially from the expected compression therapy. For example, a recent clinical study designed by one of the inventors of the present invention, and involving the most commonly used sequential pneumatic limb compression systems of the prior art, showed that the pneumatic limb compression therapy actually delivered to 49 patients following elective total hip replacement surgery varied widely from therapy expected by the operating surgeons in respect of key parameters of the therapy shown in the clinical literature to affect patient outcomes related to the incidence of deep venous thrombosis, pulmonary embolism and death. The study methodology involved continuous monitoring of the varying pressure of the compressed air in the pneumatic sleeves of these systems, permitting the values of key parameters of pneumatic compression therapy actually delivered to patients to be directly monitored throughout the prescribed period of therapy and compared to the expectations of operating surgeons. The results of this clinical study indicated that the expected therapy was not delivered to any of the 49 patients monitored: therapy was only delivered an average of 77.8 percent of the time during the expected periods of therapy; the longest interruptions of therapy in individual subjects averaged 9.3 hr; and during 99.9 percent of the expected therapy times for all 49 patients monitored in the study, values of key outcomes-related parameters of the therapy actually delivered to the patients varied by more than 10 percent from desired values. These parameters included rates of pressure rise and maximum pressures actually delivered through the sleeves. The unanticipated range of variations that was found in this clinical study between expected and delivered pneumatic compression therapy, within individual patients and across all patients, may be an important source of variations in patient outcomes in respect of the incidence of deep vein thrombosis, pulmonary embolism and death, and may be an important confounding variable in comparatively evaluating reports of those patient outcomes. The present invention addresses many of the limitations of prior-art systems that have led to such unanticipated and wide variations between the expected therapy and the therapy actually delivered to patients. 
     Due to 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, substantial variations often arise between the desired and actual pressure waveforms delivered by limb compression systems of the prior art. 
     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 under varying operational and clinical circumstances such as movement of the limb, movement of the sleeve relative to the limb and varying snugness of sleeve application, in part because they do not generate a signal indicative of the actual pressure in the sleeve suitable for permitting a feedback control system to produce the desired pressure waveform. 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. 
     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. 
     Some limb compression systems of the prior art attempt to record and display the total cumulative time during which pneumatic compression therapy was delivered to a patient&#39;s limb, but do not differentiate between times when values of parameters of the delivered therapy were near the desired values for the therapy and when they were not. For example, commercially available systems such as system the Plexipulse intermittent pneumatic compression device (NuTech, San Antonio Tex.) and AirCast intermittent pneumatic compression device (Aircast Inc., Summit, N.J.) record the cumulative time that compressed air was delivered to each compression sleeve. These are typical of prior-art systems which include simple timers that record merely the cumulative time that the systems were in operation. 
     In U.S. Pat. No. 5,443,440 Tumey et al. describe a pneumatic limb compression system capable of recording compliance data by creating and storing the time, date and duration of each use of the system for subsequent transmission to a physician&#39;s computer. The compliance information recorded by this system contains only information relating to times when the system was operating and the cumulative duration of operation. Tumey et al. cannot and does not determine occurrences when pressure-related values of parameters of the delivered therapy matched the desired values of the parameters and occurrences when they did not. 
     A major limitation of Tumey et al. and other limb compression systems of the prior art is that values of key parameters of pneumatic compression therapy that are known to affect patient outcomes are not monitored and recorded. This is a serious limitation because evidence in the clinical literature shows that variations in applied pressure waveforms produce substantial variations in venous blood flow, and that delays and interruptions in the delivery of pneumatic compression therapy affect the incidence of DVT. One key parameter identified by the inventors of the present invention is the interval between successive occurrences of delivered pressure waveforms having desired values of certain waveform parameters known to affect patient outcomes, such as rate of pressure rise and maximum pressure. Because this key parameter is not monitored as therapy is delivered by prior-art systems, variations between delivered and expected therapy cannot be detected as they occur, and clinical staff and patients cannot be alerted to take corrective measures for improving therapy and patient outcomes. 
     Because prior-art systems do not monitor the interval between successive occurrences of delivered pressure waveforms having desired values of certain waveform parameters known to affect patient outcomes, and because such prior-art systems do not therefore have alarms to alert clinicians and patients that a maximum time interval has elapsed during which the expected therapy was not delivered to the patient, then the operator and the patient cannot adapt such systems during therapy, including for example sleeve re-application and changing certain parameters of therapy, to help assure that the prescribed and expected therapy is actually delivered to the patient throughout as much as possible of the prescribed duration of therapy. 
     In addition to the monitoring limitations of prior-art systems described above, prior art systems do not measure and record parameters related to the 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 times during which a waveform matching a desired waveform in respect of key parameters was periodically applied, the interval between applications of waveforms matching a desired waveform and the number of cycles of the waveform which were applied. 
     Additionally, limb compression systems do not subsequently produce the recorded values of key outcomes-related parameters for use by physicians and others in determining the extent to which the prescribed and desired pressure waveforms were actually applied to the patient for use by third-party payors in reimbursing for therapy actually provided, and for use in improving patient outcomes by reducing variations in parameters of therapy known to produce variations in patient outcomes. 
     SUMMARY OF THE INVENTION 
     The present invention provides apparatus and a method for applying pressure to a patient&#39;s limb through a pneumatic sleeve in order to augment venous blood flow in the limb and for monitoring the applied pressure, to help prevent deep vein thrombosis, pulmonary embolism and death. More specifically, the present invention includes means for supplying a gas at a varying supply pressure, an inflatable sleeve adapted for positioning onto a limb to apply a varying pressure to the limb beneath the sleeve when inflated with the gas, pressure transducing means for measuring the pressure of gas in the inflatable sleeve, waveform parameter measurement means for measuring the value of a predetermined pressure waveform parameter, and interval determination means for producing an indication of the interval between two occurrences when the measured value of the predetermined pressure waveform parameter is near a predetermined parameter level. 
     In the present invention, the pressure waveform parameter can be a predetermined variation in the measured level of pressure of gas in the sleeve that augments the flow of venous blood into the limb proximal to the sleeve from the limb beneath the sleeve. Also, the sleeve of the present invention can include two ports and separate tubing connecting it to the gas supply means and the pressure transducing means so that the pressure transducing means only communicates pneumatically with the gas supply means through the sleeve. 
     The present invention includes means to allow an operator to select the predetermined pressure waveform parameter and the predetermined parameter level from a plurality of predefined parameters and parameter levels. Also, alarm means are included for producing an indication perceptible to the operator and the patient when the determined interval exceeds a predetermined maximum interval. 
     The interval determination means of the present invention can include means for measuring a number of intervals during therapy, each corresponding to the time between an occurrence when the measured value of the parameter is near the predetermined parameter level and the next occurrence when the measured value of the parameter is near the predetermined parameter level. The interval determination means can further include a clock for determining the clock times when occurrences are measured. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a pictorial representation of the preferred embodiment in a typical clinical application. 
     FIG. 2 is a block diagram of the preferred embodiment. 
     FIG. 3 are graphical representations of pressures applied to a region of a patient by the preferred embodiment 
     FIGS. 4,  5 ,  6  and  7  are software flow charts depicting sequences of operations carried out in the preferred embodiment. 
     FIGS. 8 and 9 are pictorial representations of a sleeve for applying pressures to a patient&#39;s foot. 
     FIGS. 10 and 11 are pictorial representations of sleeve for applying pressures to a patient&#39;s calf. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     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. 
     The preferred embodiment of the invention is described in three sections below: instrumentation, software and sleeves. 
     I. Instrumentation 
     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. 
     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. 
     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.    
     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.    
     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 and text representations of the time intervals between the production of pressure waveforms having desired predetermined parameters in inflatable sleeves connected to channels “A” and “B” of instrument  2 ; (e) messages which provide operating information to the operator. 
     Controls  22  shown in FIGS. 1 and 2 provide a means for an operator to control the operation of instrument  2 . 
     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 . 
     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 . 
     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 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 , irrespective of 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 . 
     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 . 
     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 . 
     Valve  38  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 . 
     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 , irrespective of 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 . 
     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 . 
     As shown in FIG. 2, pneumatic pump  48  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. 
     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 . 
     For each reference waveform stored in waveform register  56  a corresponding set of reference values for predetermined waveform parameters is also stored in waveform register  56 . The predetermined waveform parameters are representative of desired characteristics of an applied pressure waveform used to augment the flow of venous blood. For example for an individual reference waveform these waveform parameters may include: (a) the maximum pressure applied during the cycle time period; (b) the rate of rise of pressure during a portion of the reference waveform cycle time period; (c) pressure thresholds which must be exceeded for predetermined time periods. Example reference values of these parameters are: (a) 45 mmHg for maximum pressure applied during the cycle time period; (b) 10 mmHg per second rate of pressure rise maintained for a period of 3 seconds; (c) a pressure threshold of 30 mmHg exceeded for a period of 7 seconds. As described further below, microprocessor  32  uses the reference values of these waveform parameters to verify that pressure waveforms having desired characteristics have been applied to the patient. 
     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. Similarly the corresponding reference values of the predetermined waveform parameters could be mathematically derived from the reference pressure 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 and the corresponding reference values of predetermined waveform parameters will be communicated to microprocessor  32 . 
     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 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. 
     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 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 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. 
     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. 
     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 retrieving from waveform register  56  a reference pressure waveform, as determined by the level of a channel “A” waveform selection signal produced by microprocessor  32  in response to an operator manipulating controls  22 . 
     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 . 
     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. 
     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”. 
     Microprocessor  32  also analyzes the channel “A” sleeve pressure signal generated by pressure transducer  26  representative of the pressure waveform being produced in foot sleeve  4 , in order to measure predetermined waveform parameters. The specific waveform parameters measured by microprocessor  32  are determined by the reference values of the waveform parameters corresponding to the channel “A” reference pressure waveform signal. If for example, microprocessor  32  has retrieved from waveform register  56  a reference value for the maximum pressure applied during the cycle time period microprocessor  32  will analyze the sleeve pressure signal and measure the value of the maximum applied pressure during the cycle time period. 
     Microprocessor  32  computes the differences between the measured values of the waveform parameters and the corresponding reference values of the waveform parameters. If the absolute differences between the measured and reference values are less than predetermined maximum variation levels microprocessor  32  retrieves a channel ‘A’ interval time from interval timer  58  and stores this channel ‘A’ interval time along with other information as described below in a location in therapy register  60 . Microprocessor  32  then generates a channel ‘A’ interval timer reset signal which is communicated to interval timer  58 . 
     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 and corresponding reference values of waveform parameters, interval times, alarm signals and valve control signals are produced independently of those produced for channel “A”. 
     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 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; or d) the initiation of a pressure waveform cycle by channel “B” upon the channel ‘A’ interval time exceeding a predetermined threshold. 
     When instrument  2  is operating to generate pressure waveforms sequentially in foot sleeve  4  and calf sleeve  6  connected to channels “A” and “B”, the channel “B” interval time is computed and stored in therapy register  60  when the absolute values of the differences between the measured and reference values of both the channel “A” and channel “B” pressure waveform parameters are less than predetermined maximum variation levels. Microprocessor  32  then generates a channel ‘B’ interval timer reset signal which is communicated to interval timer  58 . 
     Interval timer  58  shown in FIG. 2 maintains independent timers for channel ‘A’ and channel ‘B’. In the preferred embodiment the timers are implemented as counters that are incremented every 100 ms. The rate at which the counters are incremented determines the minimum interval time that can be resolved. Microprocessor  32  communicates with interval timer  58  to read the current values of the counters and also to reset the counters. Interval timer  58  includes a battery as an alternate power source and continues to increment the counters during any interruption in the supply of electrical power from power supply  62  required for the normal operation of instrument  2 . 
     Microprocessor  32  generates alarm signals to alert the operator of instrument  2 , and patient receiving therapy from instrument  2 , if an excessive interval has elapsed between the application of pressure waveforms having desired reference values of waveform parameters. Microprocessor  32  periodically retrieves from interval timer  58  the current values of the channel ‘A’ and channel ‘B’ interval timers, if an interval time value exceeds a predetermined maximum of 5 minutes microprocessor  32  will generate an alarm signal associated with either channel ‘A’ interval time or channel ‘B’ interval time. 
     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. Therapy register  60  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, or to select a reference pressure waveform for generation in a sleeve (b) alarm events resulting from microprocessor  32  generating alarm signals as described above; and (c) interval time events resulting from microprocessor  32  determining the interval between the application of pressure waveforms having predetermined desired parameters. 
     Microprocessor  32  communicates with therapy register  60  to record events as they occur. Microprocessor  32  records an event by communicating to therapy register  60 : 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 and which channel of instrument  2  the event is associated with as determined by microprocessor  32 . Also, if the event relates to channel “A” of instrument  2 , therapy register  60  records the values at the time of the event of the following parameters: the channel “A” waveform selection signal, the channel “A” sleeve pressure signal, the channel “A” reference pressure waveform signal and the channel “A” interval time. Alternatively, if the event relates to channel “B” of instrument  2 , therapy register  60  records the values at the time of the event of the following parameters: the channel “B” waveform selection signal, the channel “B” sleeve pressure signal, the channel “B” reference pressure waveform signal and the channel “B” interval time. 
     Therapy register  60  retains information indefinitely in the absence or interruption of electrical power from power supply  62  required for the normal operation of therapy register  60 . 
     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  60 . For example, microprocessor  32  in response to an operator of instrument  2  manipulating controls  22  will retrieve from therapy register  60  all events associated with determining interval times and the corresponding information associated with those events. Microprocessor  32  will then tabulate the retrieved information and will present on graphic display  20  a display detailing the history of interval times between the application of pressure waveforms having desired reference parameters for channels ‘A’ and ‘B’ of instrument  2 . Also for example, microprocessor  32  in response to controls  22  will calculate and present on graphic display  20  the elapsed time between a first event recorded in therapy register  60  and a second event recorded in therapy register  60  by computing the difference between the time at which the first event occurred and the time when the second event occurred. 
     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 . 
     Microprocessor  32  will, in response to generated alarm signals, alert the operator and patient 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  66  shown in FIG.  2 . 
     Power supply  62  provides regulated DC power for the normal operation of all electronic and electrical components within instrument  2 . 
     II. Software 
     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. 
     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  60 . FIG. 6 shows a software task for controlling channel “A” of instrument  2 . FIG. 7 shows a software task associated with the determination of time intervals between the application of pressure waveforms having predetermined desired parameters. 
     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 . Should any diagnostic test fail ( 404 ), an error code is displayed on display  20  ( 406 ) and further operation of the system is halted ( 408 ); if no errors are detected, control is returned to the main program. 
     Next, a software task scheduler is initialized ( 410 ). The software task scheduler 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 ( 412 ) continues to execute scheduled subroutines until one of the following occurrences: a) power is no longer supplied to microprocessor  32 ; or b) the operation of microprocessor  32  has been halted by software in response to the software detecting an error condition. 
     FIG. 5 shows a flowchart of the software task associated with updating display  20 , processing input from an operator and testing for interval time alarm conditions. This task is executed at regular predetermined intervals of 50 milliseconds. 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. 
     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 display the history of interval times between the application of pressure waveforms having desired reference parameters for channel ‘A’ software tasks will be scheduled to retrieve from therapy register  60  events associated with determining interval times and compute and display the history. The history of interval times may include the longest interval, and the cumulative total of all interval times between the application of pressure waveforms. 
     Control then passes to a subroutine ( 506 ) which determines if the operating parameters (reference pressure waveform selections, 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  60 . This subroutine ( 508 ) records an event by storing the following in therapy register  60 : 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 and which channel of instrument  2  the event is associated with 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  60 : channel “A” waveform selection signal, channel “A” sleeve pressure signal, channel “A” reference pressure waveform signal and channel “A” interval time. 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  60 : channel “B” waveform selection signal, channel “B” sleeve pressure signal, channel “B” reference pressure waveform signal and the channel “B” interval time. 
     As shown in FIG. 5 control is next passed to a subroutine ( 510 ) which retrieves from interval timer  58  the values of the interval times for channel “A” and channel “B” of instrument  2 . If the channel “A” interval time is a above a predetermined threshold of 5 minutes ( 512 ) an alarm flag is set ( 514 ) to indicate that the channel “A” interval time has been exceeded. If the channel “B” interval time is above a predetermined threshold of 5 minutes ( 516 ) an alarm flag is set ( 518 ) to indicate that the channel “B” interval time has been exceeded. 
     Control is next passed to a subroutine ( 520 ) 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 ( 522 ) for recording the alarm event in therapy register  60 . Subroutine ( 522 ) records an alarm event by storing in therapy register  60  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 ( 520 ). 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  60 : channel “A” waveform selection signal, channel “A” sleeve pressure signal, channel “A” reference pressure waveform signal and the channel “A” interval time. 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  60 : channel “B” waveform selection signal, channel “B” sleeve pressure signal, channel “B” reference pressure waveform signal and the channel “B” interval time. The software task shown in FIG. 5 then terminates ( 524 ). 
     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”. 
     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 ). 
     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 samples the value of the channel “A” sleeve pressure signal ( 614 ). This subroutine ( 614 ) also stores the value in the memory of microprocessor  32  to permit microprocessor  32  to perform measurements of pressure waveform parameters as described further below. Control is then passed to a subroutine ( 616 ) 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 ( 618 ) 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 ( 620 ) that compares the error signal to predetermined limits and sets an alarm flag ( 622 ) if the limits have been exceeded. Next, the signal from reservoir pressure transducer  52  is sampled ( 624 ). Control then passes to a subroutine ( 626 ) which calculates levels for the control signals for valve  28  and valve  30 . The subroutine ( 626 ) 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 ( 626 ) completes, the software task shown in FIG. 6 terminates ( 612 ). 
     FIG. 7 depicts the software task associated with the determination of the time intervals between the application of pressure waveforms having predetermined desired parameters. This software task is scheduled to execute periodically whenever channel “A” is generating pressure waveforms in foot sleeve  4 . For simplicity only the software task associated with channel “A” has been shown in FIG. 7, a similar software task to the one shown in FIG. 7 is scheduled to execute periodically whenever channel “B” is generating pressure waveforms in calf sleeve  6 . 
     As shown in FIG. 7 a subroutine ( 700 ) that determines which specific waveform parameters are to be measured is executed. This subroutine ( 700 ) uses the values of the reference waveform parameters corresponding to the channel “A” reference pressure waveform to determine which waveform parameters of the channel “A” pressure signal are to be measured. For example, if reference values for maximum pressure in a cycle period and the rate of rise of pressure during a portion of the reference waveform cycle time period are associated with the reference pressure waveform signal used in the production of pressure waveforms by channel “A”; the subroutine ( 700 ) will select these as the waveform parameters to be measured. 
     Control is next passed to a subroutine ( 702 ) which analyzes the channel “A” sleeve pressure signal and measures the values of the waveform parameters as selected by the previously executed subroutine ( 700 ). Control then passes to a subroutine ( 704 ) that calculates the absolute difference between the measured values of the pressure waveform parameters and the corresponding reference values for these parameters. If the absolute differences between the measured and reference values are above predetermined thresholds ( 706 ) the software task shown in FIG. 7 terminates ( 708 ). If the absolute differences between the measured and reference values are not above predetermined thresholds ( 706 ) the control is passed to subroutine ( 710 ) 
     This subroutine ( 710 ) retrieves the channel “A” interval time from interval timer  58 . Next control is passed to a subroutine ( 712 ) which records in therapy register  60  an interval time event. The subroutine ( 712 ) stores in therapy register  60  the time of the event as read from real time clock  64  and a value identifying that an interval time event associated with channel “A” has occurred. The subroutine ( 712 ) also stores the values of the following parameters at the time of the event: channel “A” interval time, channel “A” waveform selection signal, channel “A” reference pressure waveform and channel “A” sleeve pressure signal. 
     As shown in FIG. 7 control next passes to a subroutine ( 714 ) which resets the interval timer associated with channel “A”. The software task shown in FIG. 7 then terminates ( 708 ). 
     III. Sleeves 
     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. 
     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. 
     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 . Such measurement will reflect the effects of 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 . 
     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 plantar 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. 
     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. 
     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 , which is 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 . 
     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. 
     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. 
     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. 
     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. 
     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 . Such measurement will reflect the effects of 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 . 
     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 . 
     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 . 
     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. 
     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.