Abstract:
A hazard monitor for surgical tourniquet systems comprises: pressure transducing means for detecting a pressure in a pneumatic tourniquet cuff; power switch means for enabling an operator in initiate an interruption in the supply of electrical power required by pressure regulator means, wherein the tourniquet instrument is connectable pneumatically to the tourniquet cuff to supply pressurized gas to the cuff, thereby producing a pressure in the cuff; pneumatic connector means for enabling an operator to connect an inflatable cuff to the pressure regulator means, and hazard detection means communicating pneumatically with the pneumatic connector means for detecting pressurized gas having a pressure greater than a predetermined pressure level when an interruption in the supply of electrical power is initiated by the operator.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to an apparatus and method for monitoring a surgical tourniquet system to detect a hazard. The invention relates more particularly, but not by way of limitation, to a hazard monitor having means to detect that a pneumatic cuff of an electrically powered surgical tourniquet system is pressurized when electrical power required for operation of one or more components of the system is not supplied to the components, and to detect whether the cuff is pressurized when an operator attempts to interrupt the supply of the electrical power required for the operation of the system. 
     BACKGROUND OF THE INVENTION 
     Surgical tourniquet systems are commonly used facilitate surgery by stopping the flow of arterial blood into a limb for a period of time sufficient for the performance of a surgical procedure, thereby allowing the surgical procedure to be performed in a dry and bloodless surgical field. 
     Published medical literature indicates that every usage of a surgical tourniquet necessarily causes some injury to the nerve, muscle and soft tissue in the limb beneath the cuff and distal to the cuff. To minimize the nature and extent of such injuries, tourniquet operators attempt to minimize the level of cuff pressure employed to establish and maintain a bloodless surgical field distal to the cuff. Also to minimize tourniquet-related injuries, tourniquet operators attempt to minimize the duration of tourniquet cuff pressurization. Cuff pressurization for an unnecessarily long period of time is hazardous because it is well established in the medical literature that the probability and severity of tourniquet-related injury to a patient&#39;s limb increase as the duration of tourniquet application increases. 
     Surgical tourniquet systems of the prior art generally include a pneumatic cuff for encircling a patient&#39;s limb at a location proximal to the surgical site, a source of pressurized gas and an instrument pneumatically connected to the cuff and the source for supplying gas to the cuff at a regulated pressure. 
     In some tourniquet systems of the prior art, the source of pressurized gas is a tank or hospital gas supply, while in other prior art systems an electrically powered air pump is integrated into the instrument. Some surgical tourniquet instruments known in the prior art incorporate electrically powered components including electronic pressure transducers, microprocessors, displays and audiovisual alarms. Although a few types of prior-art surgical tourniquet instruments having no electrically powered components are still in use, most of the surgical tourniquet instruments in common use at present are electrically powered in whole or in part. 
     One type of tourniquet instrument known in the prior art that is partially powered by electricity is the Electromedics TCPM Tourniquet Cuff Pressure Monitor (Electromedics Inc., Englewood, Colo.). This instrument includes an electrically powered display component for displaying the cuff pressure set by an operator, an electrically powered elapsed time clock to allow an operator to monitor cuff inflation time, a non-electrical pneumatic switch component for allowing an operator to inflate and deflate the cuff, and a non-electrical pressure regulator for supplying gas to the cuff at a pressure near the set pressure. An electrical power switch on the instrument controls the supply of power to the electrical components from a battery within the instrument when an operator turns on an electrical power switch on the instrument. The Electromedics instrument does not incorporate an electrically powered pump and instead requires that either a gas tank or a centralized hospital gas supply be employed as the source of pressurized gas. 
     The prior-art Electromedics instrument is designed so that, when a pressurized tourniquet cuff is no longer required near the end of a surgical procedure, an operator can first deflate the cuff using the non-electrical pneumatic switch component and the operator can then turn off power to the electrical components by using the electrical power switch. However, if an operator erroneously turns off the electrical power at some point during a surgical procedure and does not depressurize the cuff by using the separate pneumatic switch, then the cuff remains pressurized near a pressure regulated by the non-electrical pressure regulator while the electrical pressure display is unpowered and blank. This error may create a serious hazard for the patient if an untrained or inexperienced operator erroneously assumes that the cuff has been deflated because the pressure display is blank, and as a result the cuff remains pressurized for an extended period of time. Cuff pressurization for an unnecessarily long period of time is hazardous because it is well established that the probability and severity of tourniquet-related injuries to a patient&#39;s limb increase as the duration of tourniquet application increases. 
     A tourniquet instrument known in the prior art that is completely powered by electricity is that of McEwen as described in U.S. Pat. No. B1 4,469,099, which is herein incorporated by reference. McEwen &#39;099 describes a surgical tourniquet system that includes both an instrument that is electrically powered and an electrically powered air pump incorporated into the instrument as the source of pressurized gas. McEwen &#39;099 is operable from power supplied by an external AC supply supplemented by an internal battery and includes the following electrically powered components: an operator interface for allowing an operator to set the tourniquet cuff pressure and the anticipated period of time of cuff pressurization; switches to allow the operator to initiate pressurization and depressurization of the cuff; a cuff pressure display for allowing the operator to set the cuff pressure and monitor the actual cuff pressure; a microprocessor-controlled pressure regulator for regulating the cuff pressure near the set pressure; and a time display for allowing the operator to specify a surgical time and monitor the elapsed time during which the cuff has been pressurized. 
     McEwen &#39;099 also includes a variety of electrically powered audiovisual alarms for warning the operator of certain hazardous conditions that may exist during operation, including warning of any cuff over-pressurization, cuff under-pressurization or an excessive period of cuff pressurization. If the external AC power supply to McEwen &#39;099 is unexpectedly interrupted while the cuff is pressurized, the internal battery continues to provide power to the displays and alarms but the pressure regulator ceases operation and pneumatic valves in the instrument seal off the pressurized cuff to retain the pressure in the cuff for as long as possible or until external AC power is restored and normal operation can resume. Thus in the event of an interruption of external AC power during use in surgery, McEwen &#39;099 prevents hazards for the patient such as the unanticipated flow of arterial blood into the surgical field during a procedure, the loss of large amounts of blood, and in some cases the loss of intravenous anesthetic agent retained in the limb distal to the cuff. However, an unusual type of hazard may arise if the operator erroneously turns off the electrical power switch of the instrument without first deflating the tourniquet cuff, and then does not pneumatically disconnect the cuff from the instrument and remove the cuff from the patient&#39;s limb for an extended period of time. Turning off the electrical power switch of McEwen &#39;099 interrupts the supply of electrical power from both the external AC supply and the internal battery. Thus in the event of such operator errors, without the supply of any electrical power, the cuff pressure display and the time display of McEwen &#39;099 go blank and the audiovisual alarms are not functional, and an untrained or inexperienced operator may erroneously assume that the cuff has been deflated because the displays are blank. McEwen &#39;099 does not produce an audiovisual alarm to alert the operator to the hazard that the tourniquet cuff might remain pressurized and apply pressure to the patient&#39;s limb for a prolonged period of time after interruption of the electrical power to the tourniquet instrument. 
     Other surgical tourniquet systems known in the prior art are entirely powered from an external AC power supply and have no internal supplementary battery as in McEwen &#39;099. In the event of an interruption of power to these other prior-art systems during surgery, such as might arise from a disconnection of the AC supply or an operator error, any pressure and time displays included in such instruments go blank, any audio-visual alarms are non-functional, and the pressurized cuff is sealed off pneumatically to prevent the above-mentioned types of hazards that would otherwise arise for the patient if the cuff were to immediately depressurize upon power interruption. However, none of these prior-art systems produce an audiovisual alarm to alert the operator to the hazard that the tourniquet cuff might remain pressurized for a prolonged period of time after power interruption. 
     Some prior-art tourniquet instruments have a “soft” electrical power switch, typically implemented as a momentary contact membrane switch or a low current momentary pushbutton switch. Such a “soft” electrical power switch does not directly control the supply of electrical power to the operational components of the tourniquet instrument but acts to control other electrical components that directly control the supply of electrical power required for operation of the tourniquet instrument. For example, each of the prior-art A.T.S. 2000 and A.T.S. 750 tourniquet instruments manufactured by Zimmer Patient Care Division (Dover, Ohio) includes a “soft” electrical power switch which produces an interruption of electrical power required for operation of the instrument only after the operator has initiated the power interruption by actuating the “soft” power switch. 
     No surgical tourniquet system or monitoring apparatus is known in the prior art that can detect the presence of a pressurized pneumatic cuff of a surgical tourniquet system when electrical power required for proper operation of the surgical tourniquet system is not supplied to the system. Furthermore, no electrically powered tourniquet instrument is known in the prior art that can prevent an operator from interrupting the supply of the electrical power required for the operation of the tourniquet instrument if the operator initiates an interruption of the electrical power while a pneumatic cuff connected to the tourniquet instrument is pressurized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial representation and block diagram of the preferred embodiment in a surgical application. 
     FIG. 2 is a circuit schematic of the preferred embodiment. 
     FIG. 3 is a circuit schematic of an embodiment of the invention adapted for use with tourniquet instruments that have a soft power switch. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred 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 depicts hazard monitor  2  configured to monitor the pressure in tourniquet cuff  4  positioned on limb  6 . Tourniquet instrument  8  is used to inflate and pressurize tourniquet cuff  4 , thereby occluding blood flow in limb  6  during surgical procedures. Tourniquet instrument  8  is connected pneumatically to tourniquet cuff  4  via pneumatic tubing  10 , pneumatic T-connector  12 , and pneumatic tubing  14 . Tourniquet instrument  8  has a number of components that are electrically powered during normal operation, including pressure transducer, pressure display, time display, alarms and indicators. 
     As shown in FIG. 1, hazard monitor  2  connects pneumatically to tourniquet cuff  4  via pneumatic tubing  16 , pneumatic T-connector  12 , and pneumatic tubing  14 . In addition, hazard monitor  2  connects electrically with tourniquet instrument  8  via electrical cable  18 , in order to permit hazard monitor  2  to monitor the voltage applied to an electrical component within tourniquet instrument  8  that requires electrical power for operation, as described below. 
     As shown in FIG. 1, tourniquet cuff  4  communicates pneumatically with pressure transducer  20  through pneumatic tubing  16 , pneumatic T-connector  12 , and pneumatic tubing  14 . In the preferred embodiment, pressure transducer  20  is a normally-closed single-pole single-throw pressure switch (MPL-600 Series, Micro Pneumatic Logic, Pompano Beach, Fla.); the contacts of this pressure switch open when the sensed pressure is greater than a predetermined pressure of 15 mmHg. Pressure transducer  20  is specified for operating pressures up to 2000 mmHg, well above the typical maximum pressure of 450 mmHg used in normal tourniquet cuff procedures. It will be apparent to those skilled in the art that, in place of the pressure switch employed in the preferred embodiment, pressure transducer  20  may be implemented by employing an analog pressure transducer which outputs a pressure signal proportional to the sensed pressure, and that the resulting pressure signal can be compared to a reference signal indicative of a predetermined reference pressure to detect when the sensed pressure in cuff  4  in is greater than the predetermined reference pressure level. 
     In the preferred embodiment, the supply of electrical power to a component of tourniquet instrument  8  requiring electricity for operation is monitored by monitoring the voltage level at the component; the preferred embodiment determines that power is not supplied to the component if the monitored voltage level at the component is below a predetermined voltage level. It will be appreciated that the supply of electrical power to the component could alternately be monitored by monitoring the level of current passing through the component. In the preferred embodiment, as can be seen in FIG. 1, voltage detector  22  connects via electrical cable  18  to an electrical component of tourniquet instrument  8  that requires electrical power in order for tourniquet instrument  8  to operate normally during a surgical procedure. Examples of such electrical components of tourniquet instrument  8  are: a pressure transducer used for sensing the pressure in tourniquet cuff  4 ; a display for producing an indication for an operator of the sensed pressure in cuff  4 ; a pressure regulator or individual electrically powered elements of the pressure regulator such as electro-pneumatic valves or microprocessors; an electrical pump for generating compressed air for use by a pressure regulator, and a display for providing an operator with an indication of the time during which pressurized gas has been supplied to cuff  4  by the tourniquet instrument  8 . In the preferred embodiment, voltage detector  22  monitors the voltage at any selected one of such electrical components via electrical cable  18 . When the voltage applied to the monitored electrical component is above a predetermined threshold, voltage detector  22  produces a signal and when the voltage is below the threshold no signal is produced. 
     As can be seen in FIG. 1, power supply  24  supplies the electrical power necessary for the electrically powered components in hazard monitor  2 . Power supply  24  is independent of any external sources of power, including the electrical power supply found in tourniquet instrument  8 . Power supply  24  is monitored by low power detector  26  which detects when the voltage produced by power supply  24  has fallen below a predetermined threshold, as described further below. In the preferred embodiment, power supply  24  is a 3 volt lithium-ion battery capable of supplying power to hazard monitor  2  for up to 10 years before requiring replacement. 
     Low power detector  26  monitors the voltage output by power supply  24 . When the voltage output by power supply  24  drops below a predetermined threshold required for normal operation of hazard monitor  2  and requires replacement, low power detector  26  produces a signal. 
     Alarm control  28  responds to the signals produced by low power detector  26  and voltage detector  22 , and to the closed or open circuit provided by pressure transducer  20 , and produces an alarm signal when an alarm condition is present. An alarm condition exists when either: (a) pressure in tourniquet cuff  4  is above the predetermined pressure of 15 mmHg as sensed by pressure transducer  20  and the voltage applied to the monitored electrical component within tourniquet instrument  8  is below a predetermined threshold as sensed by voltage detector  22 ; (b) the voltage output of power supply  24  is below a predetermined threshold as sensed by low power detector  26 . In the preferred embodiment, the alarm condition logic is implemented via low-power CMOS logic gates. It is obvious to those skilled in the art that the alarm condition logic in alarm control  28  could be implemented in a number of ways, including the use of a microcontroller-based system, a network of diode and transistor logic gates, or the use of analog switches and relays. 
     When an alarm signal is produced by alarm control  28  the operator is alerted to the alarm condition by both audible and visual alarms via visual indicator  30  and audible indicator  32 . In the preferred embodiment, audible indicator  32  is a low-power piezoelectric pulse-tone generator, while visual indicator  30  is a low-power electromagnetically-actuated status indicator (Status Indicator Model 30-ND, Mark IV Industries, Mississauga, Ontario, Canada). Visual indicator  30  is a bi-stable indicator which requires no power during steady-state and minimal power when changing state from inactive (reset—alarm condition not indicated) to active (set—alarm condition indicated). In the preferred embodiment, visual indicator  30  remains in its last state indefinitely after power supply  24  has been depleted. By operating in this way, visual indicator  30  alerts the operator of a persisting alarm condition, such as low power in power supply  24  sensed by low power detector  26 , even after power supply  24  has been fully depleted. 
     When tourniquet cuff  4  is applied to a patient&#39;s limb and tourniquet instrument  8  is supplying pressurized gas to cuff  4  during a surgical procedure and hazard monitor  2  is configured as shown in FIG. 1, hazard monitor  2  senses both the voltage applied to the monitored electrical component within tourniquet instrument  8  and the pneumatic pressure in tourniquet cuff  4 . In the event that the sensed pneumatic pressure in tourniquet cuff  4  exceeds a predetermined pressure level when electrical power is not supplied to the monitored electrical component in tourniquet instrument  8 , hazard monitor  2  detects this hazardous condition and produces a alarm signal and an audio-visual alarm perceptible to the operator via visual indicator  30  and audible indicator  32 . The alarm signal continues to be produced, and both visual indicator  30  and audible indicator  32  continue to indicate the alarm condition, until the pressure in tourniquet cuff  4  drops below the predetermined pressure level, or until electrical power is supplied to the component in tourniquet instrument  8 . 
     When cuff  4  is not pressurized above the predetermined pressure level, the switch contacts of pressure transducer  20  are closed, and hazard monitor  2  does not produce any alarm unless low power detector  26  senses that power supply  24  is below a predetermined minimum voltage and requires replacement; in that event, hazard monitor  2  responds to low power detector  26  by producing a low-power alarm perceptible to the operator via visual indicator  30  and audible indicator  32 . Visual indicator  30  continues to produce the low-power alarm until power supply  24  is replaced with another power supply having a voltage level greater than the predetermined minimum voltage, while audible indicator continues to produce the low-power alarm until power supply  24  is completely depleted. 
     FIG. 2 is a simplified schematic diagram of the preferred embodiment that shows the interconnections of the major components of the preferred embodiment. 
     Power supply  24  is a 3 volt lithium-ion battery. In FIG. 2, the positive terminal of power supply  24  is shown labeled as Vbatt and the negative terminal is shown connected-to the ground. Power supply  24  is connected to voltage regulator  34 , which produces a reference voltage of 1.5 volts, labeled as Vref, which is used by voltage detector  22  and low power detector  26 , as described below. 
     As is common practice when describing logic circuits the terms “high” and “low” are used to describe the states of signals in the following description of the circuit schematic shown in FIG.  2 . When a signal is described has “high” its voltage is near the level of the voltage produced by power supply  24 . When a signal is described as low it has a voltage level near zero. 
     The normally closed electrical contacts of pressure transducer  20  are shown in FIG. 2 by the symbol for a switch. One of the switch contacts is connected to ground and the other switch contact is connected to both high-impedance pull-up resistor  36  in series with Vbatt, and to one of the inputs of AND gate  38 . When the pressure sensed by pressure transducer  20  is less than the predetermined pressure the switch contacts of pressure transducer  20  are in the closed position and the level of the signal at the input of AND gate  38  is low. When the pressure sensed by pressure transducer  24  is greater than the predetermined pressure, the switch contacts of pressure transducer  20  open and the level of the signal at the input of AND gate  38  is high. 
     Voltage detector  22  is comprised of analog comparator  40  and high-impedance resistors  42  and  44  configured as a voltage divider network. The voltage signal from the monitored component within tourniquet instrument  8  is shown in FIG. 2 with the label Vtourn. Vtourn as conducted by electrical cable  18  is communicated to the voltage divider network formed by resistors  42  and  44 . Analog comparator  40  compares the level of the voltage-divided Vtourn signal at the junction of resistor  42  and  44  with the level of the reference voltage Vref. Analog comparator  40  is configured so that when the level of the voltage-divided signal from Vtourn is less than the level of Vref, the signal level at the output of analog comparator  40  will be low. When the level of the voltage-divided signal from Vtourn is greater than level of Vref, the signal level at the output of analog comparator  40  will be high. Analog comparator  40  has hysteresis to prevent oscillations in its output signal when the level of the voltage-divided signal from Vtourn is near the level of Vref. 
     Low power monitor  26  is comprised of analog comparator  46  and high-impedance resistors  48  and  50  configured as a voltage divider network. Vbatt is connected to the voltage divider network formed by resistors  48  and  50 . Analog comparator  46  compares the level of the voltage-divided Vbatt signal at the junction of resistor  48  and  50  with the level of the reference voltage Vref. Analog comparator  46  is configured so that when the level of the voltage-divided signal from Vbatt is less than the level of Vref, the signal level at the output of analog comparator  46  is low. When the level of the voltage-divided signal from Vbatt is greater than level of Vref, the signal level at the output of analog comparator  46  is high. Analog comparator  46  has hysteresis to prevent oscillations in its output signal when the level of the voltage-divided signal from Vbatt is near the level of Vref. 
     Alarm control  28  is implemented via low-power CMOS logic gates, AND gate  38 , OR gate  52 , and NOT gates  54  and  56 . As shown in FIG. 2 the logic gates comprising alarm control  28  are configured such that the output of alarm control  28  is an alarm signal which is at a high level when either: (a) the signal from voltage detector  22  is at a low level and the signal from pull-up resistor  36  connected to pressure transducer  20  is at a high level; or (b) the signal from low power detector  26  is at a low level. 
     As shown in FIG. 2, the output of alarm control  28  is communicated to the clock input of positive-edge triggered mono-stable multi-vibrator  58 , the clock input of negative-edge triggered mono-stable multi-vibrator  60 , and audible indicator  32 . Positive-edge triggered mono-stable multi-vibrator  58  has its output connected to the set input of visual indicator  30 , while negative-edge triggered mono-stable multi-vibrator  60  has its output connected to the reset input of visual indicator  30 . In this configuration, when the alarm signal makes a transition from low (alarm condition not present) to high (alarm condition present), positive-edge triggered mono-stable multi-vibrator  58  applies a pulse to the set input of visual indicator  30 , changing the display on visual indicator  30  from the inactive to active state which indicates to the operator that an alarm condition is present. When the alarm signal changes makes a transition from high to low, negative-edge triggered mono-stable multi-vibrator  60  applies a pulse to the reset input of visual indicator  30 , changing the display on visual indicator  30  from the active to inactive state. The pulse-width and amplitude of the pulses produced by positive-edge triggered mono-stable multi-vibrator  58  and negative-edge triggered mono-stable multi-vibrator  60  are configured so the current and voltage supplied to the set and reset inputs of visual indicator  30  is sufficient to cause visual indicator  8  to change state. As shown in FIG. 2, the alarm signal output from alarm control  28  is also communicated to audible indicator  32 , a piezoelectric pulse-tone generator which generates an audible alarm when the alarm signal is high. 
     It will be apparent to those skilled in the art that hazard monitor  2  may be adapted to integrate with differing designs of prior-art tourniquet systems. For example, if desired, transducer  20  of hazard monitor  2  may be adapted to connect directly in line with the pneumatic tubing between instrument  8  and cuff  4 , rather than via a T-piece adapter as in the preferred embodiment, such that tourniquet instrument  8  is pneumatically connected through hazard monitor  2  to tourniquet cuff  4 . 
     If desired, hazard monitor  2  may be physically integrated into a prior-art tourniquet instrument, sharing the same physical housing but having separate circuitry, power supply and alarms. The hazard monitor may be further adapted by being more fully integrated into certain types of prior-art tourniquet instruments, by sharing a common battery or some common audio-visual alarms or other components to simplify the overall design and reduce overall costs. For example, the prior-art tourniquet of McEwen &#39;099 produces a cuff over-pressurization alarm when the difference between the actual pressure that is sensed in a tourniquet cuff and a reference pressure level selected via the tourniquet instrument exceeds a cuff over-pressurization limit; in such a prior-art tourniquet, some audible and visible alarm indicators could be used in an adaptation of hazard monitor  2 . Also, McEwen &#39;099 employs a tourniquet cuff having two pneumatic ports; for overall simplicity and to reduce overall costs, hazard monitor  2  could be adapted to employ one of these two ports to communicate pneumatically with the cuff to determine cuff pressurization. 
     Some prior-art tourniquet instruments have a “soft” electrical power switch (“SP” in FIG.  1 ), typically implemented as a momentary contact membrane switch or a low current momentary pushbutton switch. With reference to FIG. 3, such a “soft” electrical power switch SP does not directly control the supply of electrical power to the operational components of the tourniquet instrument but acts to control other electrical components, such as shown at  25 , that directly control the supply of electrical power required for operation of the tourniquet instrument. The hazard monitor of the present invention may be adapted and integrated with such tourniquet instruments to prevent the power required for the operation of the tourniquet instrument from being interrupted if the “soft” power switch SP is actuated by an operator in an attempt to turn the power off at a time when the cuff is pressurized. For example, each of the prior-art A.T.S. 2000 and A.T.S. 750 tourniquet instruments manufactured by Zimmer Patient Care Division (Dover, Ohio) includes a “soft” electrical power switch which produces an interruption of electrical power required for operation of the instrument only after the operator has initiated the power interruption by actuating the “soft” power switch, and in the case of the A.T.S. 2000 has continued to actuate the “soft” power switch for a continuous period of at least 2 sec. The hazard monitor of the present invention could be readily adapted and integrated with these prior-art tourniquet instruments so that initiation of a power interruption by the operator actuating the “soft” power switch SP does not produce an interruption of the electrical power required for the operation of the tourniquet instrument (as shown in FIG. 3) if the presence of pressurized gas in the cuff is detected by, for example, the above described pressure transducer  20  of the adapted and integrated hazard monitor at the time of switch actuation by the operator. The presence (or lack thereof of pressurized gas in the cuff can be signaled by the transducer  20  as a high (or low) input to an AND gate  38 ′ as explained above in connection with the gate  38  of the alarm control  28  to which the transducer output is also applied (FIG. 2.) The switch SP output serves as the other input to gate  38 ′, and the output of the gate  38 ′ controls the power-interruption component  25  mentioned above. 
     It will also be apparent to those skilled in the art that hazard monitor  2  may be adapted to simultaneously monitor two cuffs and one tourniquet instrument controlling both cuffs, and it will also be apparent that hazard monitor  2  may be adapted to monitor dual-port cuffs and tourniquet instruments connected to those dual-port cuffs. Additionally, it will be appreciated by those skilled in the art that LEDs, LCDs and audio speakers may be used to implement other forms of visual and audible alarms perceptible to a human operator of a tourniquet instrument and others in the vicinity.