reference is now made to fig1 which is a simplified block diagram illustration of a tourniquet pressure regulating system , referenced generally 10 , which is constructed and operative in accordance with a preferred embodiment of the present invention . the tourniquet pressure regulating system 10 includes a blood pressure transducer 20 , which typically comprises a commercially available occlusive cuff and is operative to provide an indication of blood pressure . the blood pressure indication provided by blood pressure transducer cuff 20 is provided to a blood pressure monitor 30 which is operative to provide a display and / or an output indication of at least one blood pressure characteristic which may be the output of blood pressure transducer 20 or some parameter or characteristic computed or generated therefrom . for example , the blood pressure monitor 30 may provide an output indication of mean blood pressure , namely , the following weighted combination : a commercially available system including a blood pressure transducer cuff and a blood pressure monitor is the dinamap vital signs monitor , model 8100 , commercially available from critikon , inc ., tampa , fla ., usa . principles of operation of blood pressure monitors are described in monitoring in anesthesia and critical care medicine , 2nd ed ., casey d . blitt , m . d ., churchill livingstone , 1990 . blood pressure signals are provided by the blood pressure monitor 30 to an operation and control unit 40 which is operative to provide the following information : a . the operation and control unit 40 provides , to a pressurized gas supply system 50 , an estimate of a minimum effective tourniquet pressure required to maintain an established bloodless field . b . the operation and control unit 40 preferably inspects the blood pressure profile and particularly the rapidity of fluctuation thereof and provides to blood pressure monitor 30 an indication of a suitable time interval for blood pressure measurement , indicating how frequently it is necessary to measure blood pressure . c . if the operation and control unit 40 receives an indication from a user that a iv regional anesthesia ( bier &# 39 ; s block ) procedure is to be performed , the operation and control unit 40 controls operation of pressurized gas supply system 50 so as to provide appropriate pressures for each of the two tourniquet cuffs employed in a bier &# 39 ; s block procedure , according to an appropriate schedule which preferably responds to a user &# 39 ; s input . considerations in selecting appropriate timing and pressures for each stage of a bier &# 39 ; s block procedure are described in crenshaw , a . h ., ( ed . ), campbell &# 39 ; s operative orthopedics , page 2967 , 8th ed ., mosby - year book , st . louis , mo ., 1992 . pressurized gas supply system 50 typically comprises an air pump 52 , an associated valve system 54 and a pressure transducer 56 . pressurized gas supply system 50 provides inflation and deflation of a tourniquet cuff 60 . tourniquet cuff 60 may comprise any commercially available tourniquet cuff such as one of the series of reusable tourniquet cuffs , commercially available from smith & amp ; nephew richards inc ., memphis , tenn ., usa or such as one of the series of disposable tourniquet cuffs , commercially available from smith & amp ; nephew richards . alternatively , tourniquet cuff 60 may comprise a dual cuff suitable for performing bier &# 39 ; s block procedures , i . e . iv regional anesthesia procedures . dual cuffs are also commercially available from smith & amp ; nephew richards . according to a preferred embodiment of the present invention , the user of the system 10 may select any of a plurality of cuffs , varying in size , such as cuffs within a single series , to implement tourniquet cuff 60 and the operation and control unit 40 takes into account the size of the cuff as indicated by the user when estimating the minimum effective tourniquet pressure , all of which are provided integrally with the system . it is appreciated that blood pressure cuff 20 and tourniquet cuff 60 may either be on the same limb or on opposite limbs . alternatively , the two cuffs 20 and 60 may be replaced by a single double - module cuff which performs the blood pressure measurement function of cuff 20 and the tourniquet function of cuff 60 . a preferred method for implementing operation and control unit 40 is described below in further detail with reference to fig2 a and 2b . operation and control unit 40 is associated with a user input and display panel 80 . a description of a preferred configuration for panel 80 is provided below with reference to fig5 . reference is now made to fig2 a and 2b which , taken together , are a simplified flowchart showing operation of the operation and control unit 40 of fig1 for instances in which the user has indicated that a single cuff procedure , rather than a bier &# 39 ; s block double cuff procedure , is being employed . step 200 : this initialization step is preferably performed prior to any operational use of the unit 40 . in the course of experimental work , the inventors discovered that a good tourniquet cuff pressure value for maintaining an already established bloodless field may be computed by multiplying ongoing readings of blood pressure , such as mean blood pressure readings , by a factor which depends on the type of limb , the size of tourniquet cuff and , typically , the age group ( adult / pediatric ) in step 200 , a table is constructed which stores values for the above constant , termed herein a , for a variety of limb types , tourniquet cuff sizes , and , preferably age groups . a preferred method for performing step 200 is described below with reference to fig3 . step 210 : determine which of the following modes has been selected by the user : set - up / manual / automatic . the flowchart below assumes that the automatic mode has been selected . if the manual mode has been selected , a non - automatic procedure for controlling tourniquet pressure may be employed by the present apparatus , such as providing control signals which are based on user - selected or system default values . typically , the user may select a manual work mode at any point in the course of using the system , at which point the system abandons the computations described below and makes a gradual transition to user - selected constant tourniquet pressure values . if the set - up mode has been selected , unit 40 prompts the user through a set - up session in which the user is prompted to define each of a plurality of general parameters which are stored in a parameter table 82 ( fig1 ), such as the following parameter table : ______________________________________parameter table______________________________________i . generalbloodless field establishing pressure -- arm -- 300bloodless field establishing pressure -- leg -- 500bloodless field establishing pressure -- majorartery -- 300operator requested pressure increments ( orpi &# 39 ; s ): arm -- single cuff : 0 proximal dual cuff : 0 distal dual cuff : 0leg -- single cuff : 0 proximal dual cuff : 0 distal dual cuff : 0major artery single cuff : 0 proximal dual cuff : 0 distal dual cuff : 0time to elapse until reperfusion of arm 60time to elapse until reperfusion of leg 60ii . procedure - specificlimb ( arm / leg / major artery ) cuff ( single / dual ) cuff sizepatient age group ( adult / pediatric ) ______________________________________ during set - up , the user is prompted to select a pressure with which a bloodless field is to be established in the arm , in the leg and in a major artery . the user is also preferably prompted to select &# 34 ; operator requested pressure increments &# 34 ; ( orpi &# 39 ; s ), both for single cuff procedures and for each of the proximal and the distal cuffs employed in bier &# 39 ; s block procedures . the orpi &# 39 ; s may be selected differently for the arm , for the leg and for a major artery . these increments are intended to allow a user to partially override the system by providing a higher bloodless field maintaining tourniquet pressure than that which the system would otherwise provide . specifically , the system adds the appropriate orpi to its own estimation of the minimum tourniquet pressure required to maintain a bloodless field . the user also preferably selects a maximum time after which it is believed desirable to perform reperfusion . the system preferably provides an audio alarm to indicate that this time has elapsed . until the user chooses to select a value for any of the above parameters , the most recently selected previous value is stored in the parameter table 82 . initially , system default values are stored in the parameter table 82 , such as the values indicated in the above parameter table . pressure values are given in units of mm hg . time values are given in minutes . step 230 : if a start key has been punched on the display and user input panel 80 of fig1 the system prompts the user to provide at least one procedure - specific value which characterizes an individual operation on an individual patient , such as the following : ii . type of cuff -- single or dual . a dual cuff procedure is also termed herein a bier &# 39 ; s block procedure . iv . patient age group . typically age groups include a single adult age group and one or more pediatric age groups . step 240 : update the parameter table 82 according to the inputs received in step 230 . step 250 : provide a control indication to blood pressure monitor 30 including an indication of a time interval between blood pressure measurements , such as the minimum time interval supported by the blood pressure monitor 30 , or approximately 2 - 2 . 5 minutes . step 260 : receive mean blood pressure measurements from blood pressure monitor 30 and conduct an ongoing analysis thereof , including continuous computations of the average mean blood pressure and of the variance of the blood pressure , over a moving window of , typically , 5 measurements . step 262 : if the parameter table 82 indicates that a single cuff procedure is being followed , go to step 270 . otherwise , go to fig4 a . step 270 : provide an initial inflating control indication to pressurized gas supply system 50 including the bloodless field establishing pressure stored in the table 82 , also termed herein the ep , and a time interval during which the establishing pressure is to be applied . typically , the time interval is predetermined . a suitable value is , for example , 3 minutes . a particular feature of the present invention is that , once a bloodless field has been established , the tourniquet cuff pressure is reduced so that a lower value is used to maintain the bloodless field than is used to establish the bloodless field . step 280 : an appropriate a value is read from the a table constructed in step 200 , according to the following procedure - specific parameters read from the parameter table 82 : limb , cuff size , patient age group . step 290 : on an ongoing basis , such as at the same frequency at which blood pressure is being measured , an inflating or deflating control signal is provided to pressurized gas supply system 50 of fig1 . the inflation / deflation control signal indicates the extent of inflation / deflation required to move from the current tourniquet pressure in tourniquet 60 to a newly computed tourniquet pressure . the system - computed tourniquet pressure comprises an estimated minimum pressure required to maintain a bloodless field ( mp ), to which is preferably added the user - selected orpi value . a . the current mean blood pressure is multiplied by the a value accessed in step 280 . b . optionally , the product computed in substep a is incremented by a blood pressure fluctuation ( bpf ) factor . the bpf factor may be established during initialization for each of a plurality of blood pressure variances and for each of a plurality of blood pressure measurement frequencies , and may be stored in a table . the bpf factor table may then be accessed as a function of the current blood pressure measurement frequency ( step 300 ) and of the current blood pressure variance as computed in step 260 . typically , the bpf factor is large to the extent that the blood pressure variance is large and the blood pressure measurement frequency is small . for example , the bpf factor may be a constant which is determined in step 422 of fig4 for each age group , limb type and cuff size . each entry in the bpf factor table is selected such that fluctuations of more than a predetermined number of standard deviations will not occur , at a predetermined statistical confidence level . for example , the bpf factor value may be selected so as to prevent fluctuations of more than two standard deviations at a level of confidence of 95 %. fluctuations in blood pressure may be forecasted by use of exponential smoothing , which is a known statistical technique . step 300 : ( optional ) on an ongoing basis , such as once per blood pressure measurement , starting from after the first 3 or 5 blood pressure measurements , a signal indicative of an appropriate blood pressure measurement frequency is transmitted to blood pressure monitor 30 of fig1 . if the variation of the current blood pressure measurement , as computed in step 260 , is large , the signal indicates that the blood pressure measurement frequency should increase , and conversely , if the variation is small , the blood pressure measurement frequency may be decreased . preferably , the recommended blood pressure measurement frequency is computed as follows : a . fluctuations in blood pressure are forecasted by use of exponential smoothing . b . a frequency is selected such that fluctuations of more than a predetermined number of standard deviations will not occur , at a predetermined statistical confidence level , before the next blood pressure measurement . for example , the frequency may be selected so as to prevent fluctuations of more than two standard deviations at a level of confidence of 95 %. step 310 : preferably , status information is displayed on display panel 80 of fig1 such as indications of current tourniquet cuff pressure , current mean blood pressure , time remaining until reperfusion . step 320 : if the time elapsed since the second time the start button is pressed exceeds the reperfusion time interval defined in the parameter table 82 , an alarm is provided , such as an audio alarm . step 330 : once the user indicates , as by using stop key 530 of panel 80 of fig5 that the surgical procedure has been completed , a suitable deflation control signal is provided to pressurized gas supply system 50 . for example , the deflation control signal may comprise a signal to deflate the cuff . fig3 is a preferred method for implementing initialization step 200 of fig2 a in which a look - up table for the &# 34 ; a &# 34 ; parameter is constructed . the following steps are performed for each entry in the look - up table , i . e . for each type of limb ( arm / leg / major artery ), for each tourniquet cuff size , and for each age group ( adult and at least one pediatric age group ). step 400 : select a sample of approximately 400 patients which is representative of the patients within the age group as to sex , age and medical condition . step 410 : take approximately 3 measurements , separated by 15 minute time intervals , of blood pressure and of doppler opening pressure ( dop ). step 420 : conduct a linear regression analysis ( n = 3 × 400 = 1200 ), using the doppler opening pressure as the predicted variable and the average blood pressure as the predicting variable . the a value for the individual entry is the slope of the linear regression . step 422 : it is believed that a suitable method for computing the bpf factor employed in step 290 is by computing the standard error of the y axis intercept for each age group , limb type and cuff size . step 430 : store the a value of step 420 in the location in the a table which corresponds to the individual limb , cuff size and , preferably , age group . for example , a suitable value of a , for the arm , for a cuff of width 5 . 5 cm and for an adult population is 2 . 04 . reference is now made to fig4 a - 4b which , taken together , comprise a preferred method for implementing operation and control unit 40 for instances in which the user has indicated , in step 262 of fig2 a , that a bier &# 39 ; s block dual cuff procedure is being employed . if the user has so indicated , the method of fig4 a may be employed . if the flow of fig4 a is interrupted by a patient discomfort indicator , such as via the &# 34 ; start &# 34 ; button 520 of fig5 the method of fig4 b may be employed . fig5 is a pictorial illustration of user input and display panel of fig1 . the display and control panel of fig5 preferably includes the following elements : a . an on / off button 510 . when the button 510 is in its on position , the system is in stand - by mode and awaits further inputs . d . a &# 34 ; manual &# 34 ; button 540 enabling the user to select a manual mode of operation ; e . a &# 34 ; set - up &# 34 ; button 542 enabling the user to select a set - up mode of operation ; g . an input value selecting mechanism such as a scroll 550 which allows a user to indicate whether he wishes to increase or decrease an existing numerical value . h . a prompt scroll 560 which allows a user to indicate that the next prompt should be displayed on display 546 . it is appreciated that , more generally , all references above to &# 34 ; mean blood pressure &# 34 ; may be replaced by any suitable combination of the systolic and diastolic blood pressures . it is appreciated that the invention shown and described herein is useful for providing tourniqueting for any or all limbs of a patient and / or occlusion of one or more arteries of a patient during any vascular surgical procedure which requires tourniqueting or occlusion . it is appreciated that the apparatus of the present invention may be employed to provide computerized control of any suitable tourniqueting schedule . for example , it may be desired to provide a bloodless field using two or more adjacent occlusive cuffs , according to a schedule shown in fig6 : b . after a predetermined or user - selected time interval , inflate the second cuff ; c . once the second cuff is inflated , deflate the first cuff , thereby relieving pressure on the first cuff body site ; d . after a predetermined or user - selected time interval , inflate the first cuff ; e . once the first cuff is inflated , deflate the second cuff , thereby relieving pressure on the second cuff body site ; g . upon receipt of an interrupt indicating that the operation is terminated , provide a deflation control signal to the inflated cuff or cuffs . preferably , once an individual cuff has been inflated , an ongoing control signal is provided , as described above with reference to fig3 according to which a relatively low tourniquet pressure is provided , just enough to maintain the bloodless field . this relatively low pressure is typically determined according to an ongoing indication of blood pressure , as described above with reference to fig3 . the above schedule has the advantage of allowing a bloodless field to be maintained for a long time period , however , pressure on each individual tourniqueted location is provided only a portion of the time period , such as half of the time period . the above schedule is particularly useful in applications wherein anesthetic is provided proximally of the tourniqueted areas , such as in procedures carried out under general anesthesia . it is appreciated that various features of the invention which are , for clarity , described in the contexts of separate embodiments may also be provided in combination in a single embodiment . conversely , various features of the invention which are , for brevity , described in the context of a single embodiment may also be provided separately . it will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove . rather , the scope of the present invention is defined only by the claims that follow :
an automatic tourniquet system including variable pressure cuff apparatus for applying a variable pressure to a limb or artery of a patient in order to occlude blood flow thereat and control apparatus for determining the operative pressure of the variable pressure cuff apparatus and including apparatus for estimating the minimum effective cuff pressure required for complete occlusion .