Abstract:
An integrated tourniquet system enables the user to establish with suitable warnings, and with timely user confirmation, individualized maximum pressure levels in the cuff that may be above normal maximum pressure levels. Also provided is a rapid and accurate method of testing for leakage in the cuff and connectors. The system is adapted for communication to remote devices for receiving and providing information relating to the pressure levels, tests, etc.

Description:
FIELD OF THE INVENTION 
     This invention pertains to pneumatic tourniquet systems commonly used for stopping the flow of arterial blood into a portion of a surgical patient&#39;s limb to facilitate the performance of a surgical procedure and for facilitating intravenous regional anesthesia. 
     BACKGROUND OF THE INVENTION 
     Surgical tourniquet systems are commonly used to stop the flow of arterial blood into a portion of a patient&#39;s limb, thus creating a clear, dry surgical field that facilitates the performance of a surgical procedure and improves outcomes. A typical surgical tourniquet system of the prior art includes a tourniquet cuff for encircling a patient&#39;s limb at a desired location, a tourniquet instrument, and flexible pneumatic tubing connecting the cuff to the instrument. In some surgical tourniquet systems of the prior art, the tourniquet cuff includes an inflatable portion, and the inflatable portion of the cuff is connected pneumatically through flexible plastic tubing and one or more connectors to a tourniquet instrument. 
     A typical tourniquet instrument of the prior art includes a pressure regulator to maintain the pressure in the inflatable portion of the cuff, when applied to a patient&#39;s limb at a desired location, near a reference pressure level that is above a minimum pressure required to stop arterial blood flow past the cuff during a time period suitably long for the performance of a surgical procedure. The reference pressure level may be set manually by a user, it may be determined automatically for an individual patient, or it may adapt automatically during a surgical procedure. Many types of such pneumatic surgical tourniquet systems have been described in the prior art, such as those described by McEwen in U.S. Pat. No. 4,469,099, No. 4,479,494, No. 5,439,477 and by McEwen et al in U.S. Pat. No. 5,607,447, No. 5,855,589 and No. 7,479,154. 
     Tourniquet instruments known in the prior art are not fully integrated with the tourniquet cuffs connected to them, or with ancillary apparatus in the operating room. As a result, typical prior-art tourniquet systems cannot change their operation in ways that could significantly improve their safety, performance and reliability. 
     The earliest surgical tourniquet systems of the prior art were entirely mechanical and thus had no integration or connectivity with other apparatus in the operating room. The invention and introduction into practice of the first surgical tourniquets employing digital technology, as described by McEwen in U.S. Pat. No. 4,469,099, enabled their integration with other apparatus in the surgical suite and with digital operating-room information systems. For example, in U.S. Pat. No. 4,479,494 McEwen describes a tourniquet system communicating with apparatus monitoring the surgical patient&#39;s blood pressure, thereby allowing the tourniquet system to receive blood pressure information and adapt tourniquet cuff pressure accordingly, and communicating with a separate printer to remotely record and display information relating to tourniquet operation. Ulrich in U.S. Pat. No. 5,569,304 describes tourniquet apparatus communicating with automatic blood pressure measuring apparatus. As another example, in U.S. Pat. No. 5,607,447 McEwen and Jameson describe a tourniquet system having an internal event register for storing certain predetermined events relating to tourniquet usage, and including connectivity allowing the recording and display of the stored events by a remote printer. 
     Typically, surgical tourniquet systems of the prior art have included means for enabling tourniquet cuff pressure to be set to levels of pressure that do not exceed a maximum limit. The earliest prior-art tourniquet systems often had maximum limits determined by the apparatus itself, for example by the maximum limit of the specific pressure regulator employed or by the maximum pressure of the source of gas supplying the pressure regulator. In one such system known in the prior art, the maximum limit that could be set by a user was 1000 mmHg. 
     Evidence from many studies published in the medical literature over the years has demonstrated that higher tourniquet pressures are associated with higher probabilities of patient injuries. Following the introduction of digital tourniquet systems such as those described by McEwen in U.S. Pat. No. 4,469,099, their increased accuracy, reliability and safety allowed users to routinely set lower and safer maximum limits in tourniquet systems based on patient safety considerations. For some prior-art surgical tourniquet systems that are widely used at present, the maximum limit is 475 mmHg. The lower maximum limit is intended to help prevent inadvertent or unintentional setting of tourniquet cuff pressure to hazardous levels higher than needed to stop arterial blood flow for the duration of a surgical procedure. A predetermined maximum limit of tourniquet pressure based on patient safety has proven to be satisfactory for almost all normal adult patients undergoing surgery in normal limbs that are encircled by standard tourniquet cuffs. 
     However, for some surgical patients, limbs and situations, the predetermined maximum limit to which pressure can be set in known prior-art tourniquet systems may be insufficient to stop arterial blood flow and thus establish a bloodless field to facilitate surgery. Examples include: patients who are very obese; patients who have certain abnormal medical conditions such as hypertension; patients who have abnormal physiology or anatomy, including calcified arterial vessels or limbs of large circumference; and situations where certain non-standard types of tourniquet cuffs are used. Alternatively, for some patients and limbs and situations, the predetermined maximum limit of known prior-art tourniquet systems may be much higher than required to stop blood flow, and thus may allow tourniquet pressure to be set to levels that are unnecessarily and hazardously high. For example, lower tourniquet pressure settings are typically sufficient and safer for many pediatric patients, for adult patients who are of small physical size or who have limbs of small circumference, and when tourniquet cuffs having variable-contour shapes and greater widths are employed. 
     Leakage of pressurized gas from the tourniquet cuff, from pneumatic tubing between the instrument and cuff, and from connectors that attach the tubing to the cuff and instrument may affect tourniquet safety, performance, and reliability. Accordingly, the 2007 Recommended Practices for the Use of the Pneumatic Tourniquet in the Perioperative Practice Setting (RPs) of the US Association of periOperative Nurses (AORN) recommend that the tourniquet cuff, tubing, and connectors should be kept clean and in good working order. The AORN RPs further recommend, based on published literature, that the tourniquet cuff, tubing and connectors should be inspected for cracks and leaks because unintentional pressure loss can result from loose tubing connectors, deteriorated tubing, or cuff bladder leaks, and may result in patient injury. At present, because tourniquet systems of the prior art are not fully integrated with the cuffs connected to them through tubing and connectors, such inspections and checking are performed manually and often inconsistently, or only after a hazardous incident or patient injury has occurred. 
     To best comply with the 2007 AORN Recommended Practices regarding inspection and checking of tourniquet cuffs, connectors, and tubing, their pneumatic integrity should be routinely checked between surgical procedures and surgical staff should be alerted to any potential hazards found so that remedial action can be taken promptly. If this is not done, then leaking and potentially hazardous tourniquet cuffs, connectors, and tubing may be used for surgery, and may remain in use for long periods of time. Also, users may not be alerted to defects which may be small initially but which may increase to become significant hazards for patients, either slowly or very rapidly. Additionally, unauthorized reprocessing and reuse of cuffs manufactured to be single-use disposable cuffs may introduce leakage hazards if such cuffs are not carefully inspected before each reuse, or after each reuse, because improper, uncontrolled and unlimited reprocessing may impair the shape and integrity of the pneumatic seals of cuff connectors. Even if disposable tourniquet cuffs are used as single-use products, and if it is assumed that such cuffs are not leaking at time of first use, the tubing and connectors that connect the disposable cuffs to the tourniquet instrument may leak and such leakage may go undetected, allowing the leaking tubing or connectors to remain in use until an obvious patient hazard or injury occurs, and during which time other limitations in tourniquet safety, performance and reliability are produced. 
     Pneumatic leakage in tourniquet systems that is not detected by routine inspections and checking is undesirable in surgery and may be hazardous. In the past, undetected pneumatic leakage led users of prior-art systems to set tourniquet pressures at reference levels that were substantially higher than required physiologically to compensate for intra-operative reductions in cuff pressure that users had observed but had not been able to attribute to obvious leakage. However, setting unnecessarily high pressures is hazardous because in the medical literature higher tourniquet pressure levels have been associated with higher probabilities of patient injuries to nerves and soft tissues. More recently, some surgical tourniquet systems of the prior art have attempted to compensate for undetected levels of pneumatic leakage in the design of their pressure regulators. In typical systems, the pressure regulator is designed to maintain cuff pressure within a predetermined pressure range from a reference pressure, and any fluctuations beyond that range are offset by actuation of a pump, reservoir, or valve in an effort to bring the cuff pressure back within the range. If there is pneumatic leakage sufficient to cause the cuff pressure to decrease beyond the predetermined pressure range, actuation of the pressure regulator may bring it back within range, and if not a pressure-regulation alarm is produced. Such systems of the prior art may compensate for significant levels of sustained, undetected leakage without producing any indication of leakage or alarm for the user. Further, sustained leakage may produce an error in the indicated tourniquet cuff pressure in single-port tourniquet systems of the prior-art which estimate cuff pressure by measuring pneumatic pressure within the tourniquet instrument. For typical surgical tourniquet systems of the prior art, three limitations in the performance and reliability of their pressure regulators exist in the presence of undetected pneumatic leakage. First, tourniquet cuff pressure fluctuates unnecessarily as decreases in cuff pressure are offset by the actuations of the pressure regulator. Second, unnecessarily frequent actuation of the pressure regulator reduces the operational life and reliability of its mechanical components, increases the cost of maintaining and replacing those components, and may increase capital costs by necessitating early replacement of the entire tourniquet instrument. Third, operation of prior-art tourniquet systems on battery power is impaired. Typical tourniquet systems of the prior art may be powered either by external AC power or by an internal battery, so that they can continue to operate safely in the event of a sudden interruption of external power, and so that they can operate independently of external AC power for a prolonged period of time, for example during transportation of a patient from a pre-operative room to the operating room, or to facilitate surgery under emergency or battlefield conditions. However, in the presence of sustained leakage pneumatic leakage, the operational time of a tourniquet system when powered by an internal battery for surgery may be substantially reduced due to unnecessary actuations of the pressure regulator. Additionally, the overall life of the internal battery may be significantly reduced, reducing the performance and reliability of the tourniquet system and thereby increasing costs and hazards. 
     There is a need for a surgical tourniquet system that overcomes the above-described limitations of the prior art. For example, no system is known in the prior art that prevents the inadvertent or unintentional setting of tourniquet pressure to a level substantially higher than needed for one individual surgical patient, and yet allows such high pressure levels to be set if needed to stop blood flow in another individual patient. As another example, no tourniquet system known in the prior art includes means for automatically checking the integrity of its pneumatic components prior to each use, or after each use, or for identifying, recording and alerting the user to possible hazards identified by such checking. As yet another example, no known prior-art tourniquet system communicates information about the results of such pneumatic integrity checking, or information on individualized maximum pressure limits, to a remote display, printer or other apparatus to inform the user, to record the information for quality assurance, or for other purposes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial representation of the preferred embodiment in a surgical application. 
         FIG. 2  is a block diagram of the preferred embodiment. 
         FIG. 3  is a flow chart showing the operation of the safe extended pressure interlock. 
         FIG. 4  is a flow chart showing the sequence of operation of the leakage test. 
     
    
    
     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. 
       FIG. 1  shows an inflatable tourniquet cuff  2  applied to a limb  4  of patient  6  and pneumatically connected to instrument  8 . Cuff  2  is supplied with pressurized gas from instrument  8  to occlude the flow of arterial blood in limb  4  past cuff  2 . In the preferred embodiment the gas is air, but it will be apparent that other gases or fluids may be used to pressurize cuff  2 . A pneumatic passageway to cuff  2  is provided by cuff tubing  10 . Cuff tubing  10  is shown to be of sufficient length to allow a pneumatic connection to cuff  2  to be made outside of a sterile surgical field. Cuff tubing  10  is fitted with a male locking connector  12 , and mates to form a releasable pneumatic connection with female locking connector  14 . Female locking connector  14  is fitted to flexible plastic tubing  16  which connects to instrument  8 . Additional connectors may be used to connect tubing  16  to instrument  8  or be otherwise included in the pneumatic passageway between cuff  2  and instrument  8 . 
     Cuff  2  is generally similar in design and construction to the cuffs described by McEwen in U.S. Pat. No. 5,741,295, U.S. Pat. No. 5,649,954, and by Robinette-Lehman in U.S. Pat. No. 4,635,635. Cuff  2  may be formed of plastic coated fabric materials that can withstand, and that can be sterilized by techniques normally used to sterilize medical devices to a level of sterility that allows them to be safely used within a sterile surgical field. Cuff  2  may also be formed of materials that can withstand multiple cleaning and disinfection cycles by techniques normally used to clean and disinfect medical devices which are used during surgical procedures. The pneumatic passageway formed by the inflatable portion of cuff  2 , the connections made by connectors  12  and  14 , and tubing  16  does not normally permit the escape of gas at the pressures supplied by instrument  8 . Accidental damage caused by sharp objects, damage during sterilization or cleaning, wear, and manufacturing defects may cause the leakage of gas from cuff  2 , connectors  12  and  14  and tubing  16  when cuff  2  is pressurized. 
     Instrument  8  includes a user interface  18  that comprises a color graphic display panel  20 , a keypad  22 , and an alarm indicator  24 . A similar user interface, employing a monochromatic graphic display panel has been described in U.S. Pat. No. 5,556,415. 
     Display panel  20  is employed for the selective display of any of the following information: the level of pressure within cuff  2  as measured by instrument  8  (cuff pressure); desired tourniquet cuff pressure values input by the user; the pressure level to be maintained in cuff  2  when cuff  2  is pressurized (reference pressure level); indicators of potential hazards; pressure warning indicators; alarm reference “limits” or values; alarm messages describing detected alarm events; menus of user selectable commands for the operation of instrument  8 ; and other information and instructions pertinent to the operation of instrument  8 . To facilitate a clear and rapid understanding of the information presented to the user of instrument  8 , alphanumeric text, graphic symbols, and color are all used to convey information. 
     Keypad  22  provides a means for the user of instrument  8  to control the operation of instrument  8 . Keypad  22  has an “inflate” key to initiate the pressurization of cuff  2 , a “deflate” key to initiate the deflation of cuff  2 . Keypad  22  has other keys to permit the user of instrument  8  to input desired tourniquet cuff pressure values, set alarm limits, confirm desired tourniquet cuff pressure values, respond to alarms, and initiate leakage testing of the attached cuff and tubing. 
     Instrument  8  signals the presence of hazards and alarm conditions via alarm indicator  24  and symbols and text messages describing the alarm condition displayed upon display panel  20 . Alarm indicator  24  includes a visual indicator in the form of a red lamp and a speaker for generating audio tones. 
     It will be appreciated that other types of user interface known in the art may be used by the invention, for example keypad  22  could be replaced by a touch screen interface to display panel  20  allowing the user to interact with instrument  8  by touching selected areas of display panel  20 ; or the keys of keypad  22  could configured as “soft keys” located adjacent to display; or the user interface could be provided by another remote device in communication with instrument  8 . 
     Instrument  8  maintains a register of events, similar to that described in U.S. Pat. No. 5,911,735, to record events and store the values of relevant parameters at the time of the event such as cuff pressure, reference pressure level, pressure change during cuff leakage testing, inflation time, and alarm thresholds. Events that are recorded and stored by event register  26  shown in  FIG. 2  include: the completion of a test for leakage from cuff  2  and the pneumatic passageway between cuff  2  and instrument  8 ; the pressurization of cuff  2 ; the deflation of cuff  2 ; changes made to the reference pressure level; detected alarm conditions, changes made to alarm limits and other events related to the operation of instrument  8 . 
     Event printer  28  is connected to instrument  8  via interface cable  30 . Event printer  28  provides a hard copy printout of recorded events and the values of parameters associated with each event as recorded and stored by instrument  8 . 
     In addition to communicating with printer  28 , event register  26  also manages communications with an external operating room information network  32 . Operating room information network  32  may be either a single device or a collection of devices in communication with instrument  8 . 
     Event register  26  will respond to a request for an event record received from network  32  by transmitting data indicative of any recorded events and the values of parameters associated with each event to network  32  for subsequent remote display along with data collected from other instrumentation in the operating room. 
     In addition, event register  26  will respond to a request for the value of an operating parameter of instrument  8  received from network  32  by transmitting the current value of the requested operating parameter to network  32 . Some parameters for which values may be requested include: cuff pressure; reference pressure level; inflation time; and other parameters indicative of the operational states of instrument  8 . 
     Event register  26  will also respond to a request to change the value of an operating parameter of instrument  8  received from network  32  by attempting to change the value of the operating parameter to the value received from network  32 . For example, network  32  may request that the reference pressure level be changed to a new desired tourniquet cuff pressure level. 
     As described further below event register  26  is inhibited from transmitting values of parameters and inhibited from changing the value of a parameter during certain predetermined operational states of instrument  8 . For example, event register  26  is inhibited from transmitting the level of pressure in cuff  2  when testing the magnitude of leakage from cuff  2 . This prevents devices in communication with instrument  8  that typically associate pressurization of cuff  2  with the performance of a surgical procedure from misinterpreting the cuff pressure level during a leakage test. 
     It will be appreciated that instrument  8  may be configured to communicate wirelessly over a radio frequency communication link with printer  28  and operating room information network  32 . 
     A block diagram of instrument  8  is shown in  FIG. 2 . Referring  FIG. 2 , controller  34  is a microcontroller typical of those known in the art with associated memory, analog, and digital peripheral interface circuitry, and other support components. Controller  34  executes software programs that control the operation of instrument  8  as described below. For clarity, and to enable a better understanding of the principles of the invention, some functions that are performed by controller  34  are described and shown in  FIG. 2  as separate functional blocks. These function blocks are pressure regulator  36  and event register  26 . 
     A source of pressurized gas for supply to cuff  2  is generated by pneumatic pump  38  which is pneumatically connected to reservoir  40  by tubing  42 . In response to control signals from controller  34 , pump  38  operates to pressurize reservoir  40 . Reservoir pressure transducer  44  is pneumatically connected by tubing  46  to reservoir  40  and generates a signal indicative of the pressure within reservoir  40  which is communicated to controller  34 . Controller  34  activates pump  38  to maintain the pressure in reservoir  40  near a predetermined level. It will be appreciated that an external source of pressurized gas for the pressurization of cuff  2  could be provided to instrument  8  eliminating the necessity for pump  38  and reservoir  40 . 
     Controller  34  receives a cuff pressure signal indicative of the pressure within cuff  2  from pressure transducer  48 . Pressure transducer  48  is pneumatically connected to cuff  2  via manifold  50  and the pneumatic passageway formed by tubing  16 , connectors  12  and  14 , and cuff tubing  10 . The cuff pressure signal is communicated to pressure regulator  36  by controller  34 . As is typical in the art, the pressure within cuff  2  is measured as a gauge pressure (relative to ambient pressure) and expressed in units of mmHg. As shown if  FIG. 2 , cuff pressure transducer  48  shares a common pneumatic connection to cuff  2  with pressure increase valve  52  and pressure decrease valve  54 . Other configurations of pneumatic connection to cuff  2  may be employed. For example, an additional port may be included in cuff  2  for direct connection to transducer  48 , or transducer  48  may be incorporated into cuff  2 . 
     When enabled by controller  34 , pressure regulator  36  operates to maintain the pressure in cuff  2  (cuff pressure) near the reference pressure level by selectively actuating pressure increase valve  52  and pressure decrease valve  54 . 
     Preferably, the pressure regulator  36  may be disabled by the controller  34  so that the pressure regulator  36  no longer actuates valves  52  and  54  in response to fluctuations in the pressure in cuff  2  and changes in the reference pressure level. The connections among the controller  34 , pressure regulator  36  and valves  52 ,  54  are such that when pressure regulator  36  is disabled, controller  34  may directly actuate valve  52  and  54  in order to pressurize and depressurize cuff  2  for reasons explained more below. Alternatively, the pressure regulator  36  could be effectively disabled by other means, such as diagrammed at dashed block  53  in  FIG. 2 , which provides for closing the pneumatic passageway to cuff  2  near valves  52  and  54 . 
     Pressure increase valve  52  is an electrically operated normally closed pneumatic valve. The inlet of valve  52  is pneumatically connected via tubing  56  to reservoir  40 , the outlet of valve  52  is connected to cuff  2  via the pneumatic passageway formed by manifold  50 , tubing  16 , connectors  14  and  12 , and cuff tubing  10 . A pressure increase signal from pressure regulator  36  supplies electrical power for the operation of pressure increase valve  52 . When supplied with electrical energy valve  52  opens to allow gas to flow from reservoir  40  to cuff  2 , thereby increasing the pressure of gas in the inflatable portion of cuff  2 . The amount of electrical power supplied by pressure regulator  36  to valve  52  controls the average rate of gas flow through valve  52 . The electrical characteristics of the pressure increase signal are adapted to be appropriate for the operating requirements of valve  52 . Valve  52  may be configured as an electrically operated proportional valve wherein the rate of gas flow through valve  52  varies as a function of the electrical current supplied to the valve. Otherwise, valve  52  may be configured as an electrically operated solenoid valve that may be either fully open or fully closed; the average rate of gas flow through the valve may be controlled by pulse width modulating the electrical current supplied to the valve 
     A pneumatic pump may be used in place of pressure increase valve  52  to directly supply gas to increase the pressure in cuff  2  in response to the pressure increase signal from pressure regulator  36 . 
     Pressure decrease valve  54  is also an electrically operated two position normally closed valve similar to valve  52 . The inlet of valve  54  is pneumatically connected to cuff  2  via the pneumatic passageway formed by manifold  50 , tubing  16 , connectors  14  and  12 , and cuff tubing  10 , the outlet of valve  54  is open to atmosphere. A pressure decrease signal from pressure regulator  36  supplies electrical power for the operation of pressure decrease valve  54 . Pressure regulator  36  sets the level of the pressure decrease signal to control the opening of valve  54 . Pressure decrease valve  54  responds to the control signal from pressure regulator  36  to allow gas to be vented from cuff  2  to atmosphere, thereby decreasing the pressure of gas in cuff  2 . 
     A proportional integral control algorithm is used by pressure regulator  36  to calculate and set the levels of the pressure increase and pressure decrease control signals for valves  52  and  54  necessary to maintain the cuff pressure near the reference pressure level. It will be appreciated by those skilled in the art that other pressure regulation control algorithms could be employed by pressure regulator  36  to set the levels of pressure increase and pressure decrease control signals for valves  52  and  54 . 
     When enabled, pressure regulator  36  will respond to a difference in pressure between the reference pressure level and the cuff pressure caused by transient volume changes in cuff  2  due to manipulation of limb  4  during surgery or to gas leakage from cuff  2 ; to add or remove gas from cuff  2  by adjusting the level of the control signals for valve  52  and valve  54  thereby increasing or decreasing the gas pressure within cuff  2  and maintaining the cuff pressure near the reference pressure level. 
     If during limb manipulation or at other times pressure regulator  36  can not maintain the cuff pressure within the operating limits of pressure regulator  36 , controller  34  will indicate a high or low pressure alarm condition to the user via user interface  18  and event register  26  will record a corresponding alarm event. In the preferred embodiment an alarm will be indicated if the pressure regulator  36  cannot maintain the cuff pressure with a predetermined regulation limit of plus or minus 15 mmHg of the reference pressure level. It will be appreciated that other regulation limits may be selected and that they need not be symmetrical around the reference pressure level. 
     In the preferred embodiment, the reference pressure level may be set to pressure values that are equal to or below a normal safe pressure limit of 475 mmHg. The reference pressure level may also be set to pressure values that are greater than the normal safe pressure limit when additional steps are taken as described below. Controller  34  will not permit the value of the reference pressure level to exceed a predetermined system maximum limit of 600 mmHg and sets the initial value of the reference pressure level to a value that does not exceed the normal safe pressure limit. Values of the reference pressures level that are greater than the normal safe limit and equal to or less than the system maximum limit are extended pressure values. 
     It will be appreciated that other predetermined pressures may be chosen as safe and system maximum pressure limits and that the limits could be set by the user of instrument  8 , set automatically by instrument  8 , or set in response to limits received from operating room information network  32  via event register  26 . 
     Setting the reference pressure level to a value greater than the normal safe pressure limit may be necessary in some circumstances to occlude blood flow in limb  4 , such as when attempting to occlude blood flow in the limb of a very obese patient. The use of higher cuff pressures such as those above the normal safe pressure limit are associated with higher probabilities injury to limb  4 . To reduce the risk of the user increasing the reference pressure level to a pressure value that is above the normal safe limit inadvertently or unintentionally the preferred embodiment has a safe extended pressure interlock. The safe extended pressure interlock sets the value of the reference pressure level. The safe extended pressure interlock produces a hazard indication when a desired tourniquet cuff pressure value exceeds the normal safe limit and requires user confirmation before setting the value of the reference pressure level to a desired tourniquet cuff pressure value that is greater than the normal safe limit. 
     A flowchart showing the sequence of operation of the safe extended pressure interlock is shown in  FIG. 3 . Referring to  FIG. 3 , the operation of the safe extended pressure interlock begins with an the receipt of a new desired tourniquet cuff pressure value ( 300 ), the new desired tourniquet cuff pressure value may be received from user interface  18  in response to the user inputting a value or received from event register in response to a communication from a remote device. If the reference pressure level is equal to or below the normal safe pressure limit ( 302 ) and the new desired tourniquet cuff pressure value is greater than the normal safe pressure limit ( 304 ); a hazard is indicated by user interface  18  by warning message is shown on display panel  20  and activation of alarm indicator  24  ( 306 ). The user is prompted to depress a confirmation key on keypad  22  ( 308 ) to permit the reference pressure level to be set to the new desired tourniquet cuff pressure value which is above the normal safe pressure limit. The confirmation key is distinct from the keys used to input the desired tourniquet cuff pressure; if the confirmation key is depressed within a predetermined time interval of 2 seconds ( 310 ), the reference pressure level is set to the new desired tourniquet cuff pressure value ( 312 ); if the confirmation key is not depressed within the time interval the interlock resets ready to receive a desired tourniquet cuff pressure value and no change is made to the reference pressure level. When the value of the reference pressure level is greater than the normal safe pressure limit an extended pressure warning indicator is shown on display panel  20  ( 314 ), to indicate to the user that the reference pressure level has an extended pressure value. The indication that the reference pressure level has an extended pressure value is also communicated by event register  26  to network  32  for possible remote display. 
     Event register  26  records events related to the operation of the safe extended pressure interlock and communicates parameters related to the operation of the safe extended pressure interlock to printer  28  and network  32 . For example, an event is recorded when the new desired tourniquet cuff pressure value is greater than the normal safe pressure limit. 
     In summary, to increase the value of the reference pressure level to a desired tourniquet cuff pressure value that is greater than the normal safe pressure limit, user confirmation must be received within a predetermined time limit after a hazard warning has been indicated to the user. When the reference pressure level exceeds the normal safe pressure limit, an extended pressure warning is indicated to the user. 
     The continuous leakage of gas from cuff  2  and the gas passageway between cuff  2  and instrument  8  may prevent pressure regulator  36  from maintaining the cuff pressure near the desired reference pressure level, cause excessive wear of the components comprising instrument  8 , and increase the power consumption of the system which will result in a shorter operating time when instrument  8  is powered from a battery supply. Gas leakage may result, for example, from wear or damage to the inflatable portion of cuff  2 , needle or towel clip punctures, wear or damage to the sealing surfaces of connectors  12  and  14  and damage to tubing  16 . 
     During periods when cuff  2  is not pressurized to occlude blood flow in limb  4  for the performance of a surgical procedure, the preferred embodiment allows the user to perform a leakage test to conveniently estimate the magnitude of pneumatic leakage from cuff  2  and the pneumatic passageway between cuff  2  and instrument  8 . The leakage test rapidly estimates the magnitude of gas leakage from cuff  2 , cuff tubing  10  connectors  12  and  14 , and tubing  16 . 
     During the leakage test, controller  34  completes a sequence of operations described further below and shown in  FIG. 4  to pressurize and depressurize cuff  2  and the pneumatic passageway between cuff  2  and instrument  8 . To prevent the inadvertent or unintentional initiation of a leakage test at time when cuff  2  is pressurized for a surgical procedure, user interface  18  is configured to only permit the initiation of a leakage test when the pressure level in cuff  2  and the pneumatic passageway between cuff  2  and instrument  8  is estimated to be near zero. The pressure level in cuff  2  and the pneumatic passageway between cuff  2  and instrument  8  may be estimated by the pressure level sensed by cuff pressure transducer  48  or by the value of the reference pressure level. Only when cuff  2  is fully depressurized can a leakage test be initiated by the user of instrument  8 . This protection mechanism prevents the user from initiating a leakage test when cuff  2  is pressurized to occlude blood flow in limb  4  of patient  2 , as the sequence of operations preformed during a leakage test would be potentially hazardous to patient  2 . 
     The sequence of operations carried out by controller  34  to in performing a leakage test is shown  FIG. 4 . Referring to  FIG. 4 , the pressure level in cuff  2  and the pneumatic passageway between cuff  2  and instrument  8  is estimated ( 400 ). If the estimated pressure level in cuff  2  and the pneumatic passageway between cuff  2  and instrument  8  is near zero ( 402 ) user interface  18  is configured to enable the user to initiate the performance of a leakage test. In the preferred embodiment a menu selection for the leakage test is made visible on display panel  20  and selectable via keypad  22 . It will be appreciated that other methods appropriate for a chosen type of user interface could be used to enable the initiation of a leakage test such as enabling or disabling a the operation of dedicated leakage test key or other enabling or disabling menu choices on a touch screen. 
     If a request to initiate a leakage test is received ( 404 ), controller  34  disables pressure regulator  36  ( 406 ) in the preferred a manner as described above. This ensures that pressure regulator  36  does not act to modify the pressure level within cuff  2  and the passageway during the period of time that the leakage test is being performed. Event register  26  is inhibited from transmitting the value of the pressure level within cuff  2  during this operational state when pressure regulator  36  is disabled. Controller  34  then pressurizes cuff  2  and the pneumatic passageway between cuff  2  and instrument  8  ( 408 ) by actuating pressure increase valve  52  until the pressure sensed by pressure transducer  48  is near a test pressure level of 250 mmHg. Alternatively, the controller first pressurizes cuff and passageway until the pressure sensed by the pressure transducer is near the test pressure level, and then effectively disables the pressure regulator by closing the pneumatic passageway to cuff  2  near valves  52  and  54 , as also described above. 
     Controller  34  then records the level of pressure within cuff  2  and the pneumatic passageway as sensed by pressure transducer  48 . This first pressure level is maintained in the memory of controller  34 . After a leakage test period of 30 seconds has elapsed controller  34  again determines the level of pressure within cuff  2  and the pneumatic passageway as sensed by pressure transducer  48  ( 410 ). This second pressure level is maintained in the memory of controller  34 . Controller  34  next estimates the magnitude of leakage of gas from cuff  2  and the passageway as predetermined function of the first and second pressure levels ( 412 ). In the preferred embodiment, the predetermined function evaluates the magnitude of difference between the first and second pressure levels. The estimated magnitude of leakage is indicated to the user via user interface  18 . Pressure regulator  36  is enabled and the leakage test is completed ( 414 ). After an estimation of the magnitude of leakage has been completed and prior to enabling pressure regulator  36 , controller  34  may act to deflate cuff  2  by actuating pressure decrease valve  54 . 
     If the estimated magnitude of leakage exceeds a predetermined magnitude, an alarm indication is produced by user interface  18 . 
     The estimate of the magnitude of leakage ( 412 ) may be made any time after the second pressure level stored in the memory of controller  34 , and need not immediately precede the step of enabling the pressure regulator ( 414 ) as described above. 
     To obtain a more accurate estimation of the magnitude of leakage from cuff  2  and passageway, controller  34  can, at multiple predetermined intervals during the leakage test period, determine and store the pressure level within cuff  2  and passageway. 
     For clarity, predetermined values have been used in the description above for the test pressure level and estimation time interval. The estimation time interval need not be predetermined and could be dependent upon a predetermined magnitude of difference between the first and second test pressure levels. Also, values for test pressure level and estimation time interval could set by a user of instrument  8  via user interface  18  or set automatically by instrument  8  dependent upon certain characteristics of the cuff to be tested. 
     It will be apparent that an estimate of the magnitude gas leakage from cuff  2  could also be made from pressure changes caused by gas leaking into cuff  2  and the passageway if they are initially pressurized to a negative pressure (below atmospheric) by using alternate apparatus to that described above. 
     An event associated with the completion of a leakage test is stored by event register  26  along with the levels of parameters associated with the test including first and second test pressures, test time interval, and estimated magnitude of leakage. When requested, event register  26  communicates with printer  28  to provide a hardcopy print out of the test results and with network  32  to permit test results to be stored and displayed elsewhere.