Automatic Hemostatic Tourniquet Device with Patient Danger Shield

Abstract

The automatic tourniquet device includes a tourniquet cuff, a pressure transducer, a user interface, a patient danger shield, and a pressure regulator. The pressure transducer generates a cuff pressure signal. The user interface generates a reference pressure signal. The patient danger shield is responsive to the cuff pressure signal and the reference pressure signal, and in one implementation is operable during a regulation period to generate a patient danger signal if a current pressure level in the tourniquet cuff is higher than the reference pressure level by at least a predetermined overpressure limit. The pressure regulator is responsive to the patient danger signal and has a pressure application element for increasing pressure in the cuff and a pressure reduction element for decreasing pressure in the cuff. The pressure application element is configured to be unresponsive if the patient danger signal is generated.

Description

Background Art

[0001] An automatic tourniquet system is commonly used in surgery to occlude arterial blood flow to a portion of a patient's limb, thereby facilitating the performance of surgery and improving the outcome by creating a visible and dry surgical field. A typical prior art automatic tourniquet system includes a tourniquet cuff for surrounding a desired location on the patient's limb, a tourniquet device, and a flexible tube connecting the cuff to the device. The tourniquet device includes a pressure regulator operable during an adjustment period to automatically control the pressure within the cuff to near a reference pressure desired by the user. The pressure regulator is composed of a pressurizing element that raises the pressure level within the cuff in response to a pressurizing signal. The pressure regulator is also composed of a depressurizing element that lowers the pressure level within the cuff in response to a depressurizing signal. Many types of such pneumatic surgical tourniquet systems are described in the prior art as described in Patent Documents 1, 2, and 3 by McEwen and in Patent Documents 4 and 5 by McEwen and Jameson.

[0002] McEwen described in Patent Document 1 an automatic tourniquet system that activates an audible alarm to warn the user of potential problems such as when the cuff pressure exceeds or falls below the reference pressure by more than 15 mmHg. Manes described in Patent Document 6 an automatic tourniquet device having a manually adjustable overpressure valve that limits the maximum pressure supplied to the cuff in the event of a malfunction of the device. The main drawback of Patent Document 6 is that the overpressure valve must be manually set for various reference pressures desired by the user, which is a laborious, error-prone method, or must be set to a dangerously high constant pressure far exceeding the normal safe reference pressure setting. Furthermore, Patent Document 6 could not be modified to provide an underpressure valve to safely limit the minimum pressure within the cuff in the event of a malfunction of the device.

[0003] McEwen described in Patent Document 7 a physiological tourniquet having a safety circuit that uses a digital processor for pressure adjustment and detects a specific type of malfunction including undesirable valve operation for various operating modes (cuff modes). Two major drawbacks of the prior art safety circuit limit its ability to protect patients from clinically significant malfunctions. First, the prior art safety circuit does not monitor the cuff pressure to identify a dangerous difference between the desired reference pressure and the actual cuff pressure. Second, the prior art safety circuit requires the operating mode of the cuff and the states of the pressurizing and depressurizing elements. Thus, the prior art safety circuit cannot detect possible malfunctions of the processor, its embedded software, or the valves used as the pressurizing and depressurizing elements. More specifically, in Patent Document 7, the possible cuff modes are "cuff inflation", "cuff deflation", and "cuff adjustment". Patent Document 7 has a list of a predetermined set of undesirable valve operations for each cuff mode. When the safety circuit detects any one of the undesirable valve operations specific to the current cuff mode, it immediately stops the power supply to all valves. However, in Patent Document 7, a cuff mode output signal from the processor is required to determine whether an undesirable valve operation has occurred. Thus, if the processor does not function properly or a software error occurs, an incorrect cuff mode signal may be received by the safety circuit, resulting in an incorrect positive or negative trigger. Another drawback of the prior art safety circuit is that when an undesirable valve operation is detected, the power to the valve is immediately cut off, rendering the tourniquet device inoperative during surgery.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0005] The foregoing and other objects, features, and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. In some implementations, the automatic tourniquet device includes a tourniquet cuff, a pressure transducer adapted to generate a cuff pressure signal indicative of the pressure level within the tourniquet cuff, a user interface adapted to generate a reference pressure signal indicative of a reference pressure level within the tourniquet cuff desired by the user, and a patient danger shield responsive to the cuff pressure signal and the reference pressure signal, the patient danger shield being operable during an adjustment period to generate a patient danger signal when the current pressure level within the tourniquet cuff is at least a predetermined overpressure limit higher than the reference pressure level, and a pressure regulator responsive to the patient danger signal, the pressure regulator having a pressurizing element responsive to a pressurizing signal for increasing the pressure within the tourniquet cuff and a depressurizing element responsive to a depressurizing signal for decreasing the pressure within the tourniquet cuff. The pressure regulator is operable during an adjustment period to adjust the pressure within the cuff to near a desired reference pressure level by selectively generating the pressurizing signal and the depressurizing signal. The pressurizing element is configured not to respond to the pressurizing signal when the patient danger signal is generated.

[0006] The pressurizing element can further be configured not to respond to the pressurizing signal after a predetermined danger period if the patient danger signal is detected at the end of the predetermined danger period. The depressurizing element can be adapted not to respond to the depressurizing signal if the patient danger signal is detected at the end of the predetermined danger period.

[0007] The predetermined overpressure limit can be set to a level that can indicate a malfunction of the pressure regulator. In some implementations, the predetermined overpressure limit is set to 50 mmHg.

[0008] The user interface can be adapted so that the user can select a predetermined overpressure limit from a plurality of overpressure limits.

[0009] The predetermined danger period can be a function of the predetermined response time of the pressure regulator. In some implementations, the predetermined danger period is 200 milliseconds.

[0010] In some implementations, a display that a patient danger signal has been generated is stored in the non-volatile memory of the patient danger shield, and the user interface is adapted to prevent the start of the adjustment period if the stored display exists in the non-volatile memory.

[0011] In some implementations, the automatic tourniquet device includes a tourniquet cuff, an automatic tourniquet device having a pressure transducer adapted to generate a cuff pressure signal indicative of the pressure level within the tourniquet cuff, a user interface adapted to generate a reference pressure signal indicative of the reference pressure level within the tourniquet cuff desired by the user, a pressure regulator responsive to the reference pressure signal and further responsive to the cuff pressure signal, the pressure regulator being operable during an adjustment period to maintain the pressure within the tourniquet cuff near the reference pressure level, and a patient danger shield responsive to the cuff pressure signal and the reference pressure signal. The patient danger shield is operable to issue a patient danger alarm if the pressure within the tourniquet cuff differs from the reference pressure level by at least a predetermined pressure difference and is operable independently of the pressure regulator during the adjustment period.

[0012] In some implementations, the automatic tourniquet device includes a tourniquet cuff, a pressure transducer adapted to generate a display of the pressure level within the tourniquet cuff, a pressure regulator operable during an adjustment period to adjust the pressure level within the tourniquet cuff near a reference pressure level, and a patient danger shield operable during the adjustment period to issue a patient danger alert when the pressure level within the tourniquet cuff is at least equal to an excessive pressure limit. The pressurizing element of the pressure regulator is rendered inoperable when a patient danger alert is issued.

[0013] The excessive pressure limit can be set to correspond to a pressure level that can indicate a malfunction of the pressure regulator. In some implementations, the excessive pressure limit is 450 mmHg.

[0014] The excessive pressure limit can be set to correspond to a pressure level that is higher than the reference pressure level by an amount that can indicate a malfunction of the pressure regulator. In some implementations, the excessive pressure limit is 50 mmHg.

[0015] In some implementations, if a patient danger alert is detected at the end of a predetermined danger period, the pressure regulator can be rendered inoperable after the end of the predetermined danger period.

[0016] In some implementations, the predetermined danger period is a function of a predetermined response time of the pressure regulator.

[0017] In some implementations, a display indicating that a patient danger alert has been issued is stored in the non-volatile memory of the patient danger shield, and the user interface connected to the automatic tourniquet device is configured to prevent the start of the adjustment period if the stored display is present in the non-volatile memory.

[0018] According to the implementation of the method, a method for issuing a patient danger alarm in an automatic tourniquet device includes providing a tourniquet cuff, a pressure transducer adapted to generate a cuff pressure signal indicating the pressure level in the tourniquet cuff, a user interface adapted to generate a reference pressure signal indicating the reference pressure level desired by the user in the tourniquet cuff, providing an automatic tourniquet device having a pressure regulator responsive to the reference pressure signal and the cuff pressure signal, providing a patient danger shield responsive to the cuff pressure signal and the reference pressure signal, operating the pressure regulator to maintain the pressure in the tourniquet cuff near the reference pressure level during the adjustment period, and operating the patient danger shield independently of the pressure regulator during the adjustment period. The patient danger shield issues a patient danger alarm when the pressure in the tourniquet cuff differs from the reference pressure level by at least a predetermined pressure difference.

[0019] The predetermined pressure difference can be determined from the pressure in the tourniquet cuff compared to at least one of a predetermined overpressure limit and a predetermined underpressure limit.

Brief Description of the Drawings

[0020]

Figure 1

[0021]

Figure 2

Embodiments for Carrying Out the Invention

[0022] FIG. 1 shows a block diagram of a preferred embodiment of an automatic tourniquet device 2 having a patient danger shield 4. A tourniquet cuff 6 having an inflatable airbag is shown, pneumatically connected to the automatic tourniquet device 2 and in place on the patient's limb. The automatic tourniquet device 2 includes a processor 10, a pressure transducer 20, a user interface 30, a pressure regulator 40, and a cuff mode safety circuit 50.

[0023] The patient risk shield 4 is used in conjunction with the automatic tourniquet device 2 and operates independently of the automatic tourniquet device 2, as will be described in more detail below. The patient risk shield 4 is connected to a pressure regulator 40, a user interface 30, and a pressure transducer 20.

[0024] The processor 10 communicates with the pressure transducer 20, the user interface 30, the pressure regulator 40, and the cuff mode safety circuit 50 to control the operation of the automatic tourniquet device 2.

[0025] The pressure transducer 20 generates a cuff pressure signal indicative of the pressure level inside the tourniquet cuff 6 and transmits it to the processor 10 and the patient risk shield 4.

[0026] The user interface 30 communicates with the processor 10 and the patient risk shield 4. The user interface 30 can selectively display any of the following information, namely, the pressure level (cuff pressure) within the tourniquet cuff 6, the pressure level (reference pressure) to be maintained within the tourniquet cuff 6 when the tourniquet cuff 6 is inflated, the length of time (adjustment period) during which the tourniquet cuff 6 is inflated, the inflation time alarm limit, an alarm message explaining the detected alarm event, and other information regarding the operation of the automatic tourniquet device 2. The user interface 30 includes a control device for the user to adjust the reference pressure or the inflation time alarm limit, a control device for the user to inflate or deflate the tourniquet cuff 6 to start or end the adjustment period respectively, and other control devices related to the operation of the automatic tourniquet device 2. The adjustment period starts when the user starts the inflation of the tourniquet cuff 6 through the user interface 30. The adjustment period ends when the pressure level within the tourniquet cuff 6 is reduced to a pressure near 0 mmHg. The user interface 30 can include an audio transducer and a visual indicator that issue an audible and visual alarm to the user during various alarm states, such as when the adjustment period exceeds the inflation alarm limit time. In a preferred embodiment, the user interface 30 is an LCD graphical display with an integrated touch screen, LED indicators, and an audio transducer, but it will be understood that other types of user interfaces that can receive user input and communicate information can be used.

[0027] The pressure regulator 40 is pneumatically connected to the tourniquet cuff 6 and communicates with the processor 10 to adjust the pressure inside the pressure tourniquet cuff 6 near the reference pressure during the adjustment period. The pressure regulator 40 includes a pressurizing element 42 and a depressurizing element 44. The pressurizing element 42 raises the pressure level within the tourniquet cuff 6 in response to a pressurizing signal. The depressurizing element 44 lowers the pressure level within the tourniquet cuff 6 in response to a depressurizing signal. In a preferred embodiment, the pressure regulator 40 generates the pressurizing signal and the depressurizing signal based on a cuff pressure signal and a reference pressure signal indicating a desired pressure level within the tourniquet cuff 6. However, it will be apparent to those skilled in the art that the pressure regulator 40 can include a pressure transducer for generating the cuff pressure signal. It will also be apparent to those skilled in the art that the processor 10 can generate the pressurizing signal and the depressurizing signal and transmit them to the pressure regulator 40.

[0028] The cuff mode safety circuit 50 communicates with the processor 10 and the pressure regulator 40 to monitor and detect an undesirable combination of the operation of the pressurizing element 42 and the depressurizing element 44 and the cuff mode. The cuff mode safety circuit 50 is described in Patent Document 7 incorporated herein by reference.

[0029] The patient danger shield 4 communicates with the pressure transducer 20, the user interface 30, and the pressure regulator 40 to monitor the cuff pressure and identify a dangerous difference between the reference pressure and the actual cuff pressure. The patient danger shield 4 is independent of the adjustment of the pressure level inside the tourniquet cuff 6 by the processor 10 and the pressure regulator 40. The patient danger shield 4 does not require any information from the processor 10 or the pressure regulator 40 to function, and enables the patient danger shield 4 to protect the patient enclosed by the tourniquet cuff 6 from a malfunction of the processor 10 and / or the pressure regulator 40 that causes a dangerous difference between the reference pressure and the actual cuff pressure.

[0030] In some implementations, at least a portion of the patient risk shield 4 is an electrical circuit operable independently of the processor 10, the pressure regulator 40, the cuff mode safety circuit 50, and the pressure transducer 20. In some implementations, the patient risk shield 4 is implemented as a circuit component separate from the processor 10, the pressure regulator 40, the cuff mode safety circuit 50, and the pressure transducer 20. In other implementations, the patient risk shield 4 may be implemented in one or more separate regions of the same circuit components as the processor 10, the pressure regulator 40, the cuff mode safety circuit 50, and the pressure transducer 20.

[0031] Figure 2 shows a block diagram of the patient risk shield 4. The patient risk shield 4 includes a non-volatile memory 402, a comparator 404, a patient risk circuit element 406, and a clock 408.

[0032] The non-volatile memory 402 generates an overpressure limit signal based on a reference pressure signal received from the user interface 30. The overpressure limit signal indicates an overpressure limit that is considered undesirable, dangerous, and / or indicative of a malfunction of the pressure regulator 40 when the cuff pressure exceeds it. The overpressure limit can be a predetermined pressure value higher than the reference pressure, such as a value 50 mmHg higher than the reference pressure. Alternatively, the overpressure limit can be a value obtained by subtracting the reference pressure from a predetermined pressure value, such as a value obtained by subtracting the reference pressure from 450 mmHg (i.e., the overpressure limit is 450 mmHg). Alternatively, the overpressure limit can be selected by the user from a plurality of overpressure limits via the user interface 30. It will be apparent to those skilled in the art that the overpressure limit can vary depending on the reference pressure. For example, when the reference pressure is less than 400 mmHg, the overpressure limit is 450 mmHg, and when the reference pressure is greater than 400 mmHg, the overpressure limit is 650 mmHg.

[0033] The excessive pressure limit signal from the non-volatile memory 402 and the cuff pressure signal from the pressure transducer 20 are compared by a comparator 404, and the result is transmitted to a patient danger circuit element 406. The patient danger circuit element 406 generates a patient danger signal indicating a patient danger when the cuff pressure signal is greater than the excessive pressure limit signal (i.e., the cuff pressure is greater than the excessive pressure limit), and transmits a pressure danger signal to the pressure regulator 40. In response to the patient danger signal, the pressurizing element 42 is adapted to be non-responsive (not respond) to the pressurizing signal, thereby preventing the pressure regulator 40 from inadvertently further increasing the cuff pressure, and only allowing the pressure regulator 40 to attempt to lower the pressure level in the tourniquet cuff 6 below the excessive pressure limit by the operation of the pressure reducing element 44.

[0034] When the cuff pressure signal is greater than the excessive pressure limit signal, the patient danger circuit element 406 can also start a timer for the danger period using a clock 408. If the patient danger signal is present at the end of the danger period, a potential problem is confirmed, and the pressurizing element 42 and the pressure reducing element 44 are each adapted to be non-responsive to the pressurizing signal and the pressure reducing signal, respectively. As a result, the pressure regulator 40 becomes inoperable, thereby keeping the pressure level inside the tourniquet cuff 6 stable. The danger period is a predetermined period that is long enough for the pressure regulator 40 to resolve a potential problem and adjust the cuff pressure below the excessive pressure limit using the pressure reducing element 44 under normal operation. In a preferred embodiment, the predetermined period is 200 ms (milliseconds) or based on the response time of the pressure regulator 40. The response time of the pressure regulator 40 is a predetermined measurable duration for the pressure regulator 40 to adjust the pressure level inside the tourniquet cuff 6 from a first reference pressure to a second reference pressure. In a preferred embodiment, the response time is determined by measuring the time required for the pressure regulator 40 to adjust the cuff pressure from a first reference pressure of 300 mmHg to a second reference pressure of 250 mmHg. It will be apparent to those skilled in the art that the response time can be measured from alternative first and second reference pressures.

[0035] If the patient danger signal continues to exist even after the danger period, the patient danger circuit element 406 can store an indication that a danger has occurred in the non-volatile memory 402. The patient danger shield 4 communicates the indication that a danger has occurred to the user interface 30. In response, the user interface 30 can be adapted to prevent the user from starting an adjustment period. During maintenance, the stored indication is erased from the non-volatile memory 402.

[0036] In some implementations, the patient danger shield comprises a CCLD (Complex Programmable Logic Device) configured to have a first input device, a second input device, a memory, a comparator, and an output device, as described above in connection with FIG. 2. Other devices such as FPGAs can also be used.

[0037] An example of the implementation of a preferred embodiment will be described below.

[0038] a.) The tourniquet cuff 6 is attached to the patient for a surgical procedure by the user. The user interacts with the user interface 30 to set a desired reference pressure of 300 mmHg in the tourniquet cuff 6, thereby inflating the tourniquet cuff 6 and starting an adjustment period.

[0039] b.) The pressure regulator 40 generates a pressurizing signal to the pressurizing element 42 to raise the pressure level in the tourniquet cuff 6 to the reference pressure. The pressure regulator continues to adjust the pressure level in the tourniquet cuff 6 near the reference pressure using the pressurizing element 42 and the depressurizing element 44 during the surgical procedure.

[0040] c.) During surgery, the cuff pressure exceeds the excessive pressure limit of the reference pressure + 50 mmHg due to the manipulation of the limbs experienced during normal surgery. The patient risk shield 4 generates a patient risk signal, starts a timer for a 200 - millisecond risk period, communicates with the pressure regulator 40 to make the pressurizing element 42 not respond to the pressurizing signal, thereby preventing the pressure regulator 40 from inadvertently further increasing the pressure level in the cuff 6. In this example, the pressure regulator 40 can solve potential problems and adjust the cuff pressure to be lower than the excessive pressure limit using the pressure - reducing element 44. For this reason, the patient risk signal is no longer generated by the patient risk shield 4, and the automatic tourniquet device 2 remains functional.

[0041] d.) At another point during surgery, due to an electrical failure inside the automatic tourniquet device 2, the pressurizing element 42 operates unnecessarily. As a result, the pressure level inside the tourniquet cuff 6 gradually increases to a level that also exceeds the excessive pressure limit of 350 mmHg (reference pressure + 50 mmHg) as before. The patient risk shield 4 generates a patient risk signal, starts a timer for a 200 - millisecond risk period, and communicates with the pressure regulator 40 to make the pressurizing element 42 not respond to the pressurizing signal. Since the pressurizing element has a hardware malfunction, the pressure regulator 40 cannot lower the pressure level in the tourniquet cuff 6 below the excessive pressure limit within the 200 - millisecond risk period. As a result, the patient risk shield 4 confirms the dangerous excessive pressure state and identifies the occurrence of a malfunction, so it communicates with the pressure regulator 40 to make the pressurizing element 42 and the pressure - reducing element 44 not respond to the pressurizing signal and the pressure - reducing signal respectively, resulting in the pressure regulator 40 becoming inoperable, thereby keeping the pressure level inside the tourniquet cuff 6 stable. The patient risk shield 4 successfully identifies a malfunction that the prior - art cuff - mode safety circuit 50 described in Patent Document 7 could not identify because the operation of the pressurizing element 42 is permitted during the adjustment - cuff mode as described in Patent Document 7.

[0042] e.) The patient risk shield 4 transmits and stores in the non-volatile memory an indication that a patient risk signal has been generated for a time longer than the risk time. The stored indication is transmitted to the user interface 30 to prevent the user from starting a subsequent adjustment period, thereby preventing the use of the automatic tourniquet device 2 while a malfunction is detected. In this example, the user interface 30 alerts the user when attempting the next inflation of the tourniquet cuff. The alert can notify the user of a past detected malfunction and the need to repair the automatic tourniquet device 4.

[0043] f.) After repair by a trained person, the stored indication is erased from the non-volatile memory 402 to enable the user to start an adjustment period using the user interface 30.

[0044] It will be understood that instead of monitoring and dealing with a dangerous overpressure condition as described above, the patient risk shield 4 can monitor a dangerous underpressure condition. For this purpose, the non-volatile memory 402 generates an underpressure limit signal based on a reference pressure signal from the user interface 30. The underpressure limit signal indicates an underpressure limit which is a pressure level at which a malfunction of the pressure regulator 40 is considered undesirable, dangerous, when the cuff pressure falls below it. The underpressure limit can be a predetermined pressure value lower than the reference pressure, such as a value 50 mmHg lower than the reference pressure. Alternatively, the underpressure limit can be selected by the user from a plurality of underpressure limits via the user interface 30. It will be apparent to those skilled in the art that the underpressure limit can vary depending on the reference pressure.

[0045] The underpressure limit signal from the non-volatile memory 402 and the cuff pressure signal from the pressure transducer 20 are compared by a comparator 404, and the result is transmitted to a patient danger circuit element 406. The patient danger circuit element 406 generates a patient danger signal indicating a patient danger when the cuff pressure signal is smaller than the underpressure limit signal (i.e., the cuff pressure is smaller than the underpressure limit), and transmits it to the pressure regulator 40. In response to the patient danger signal, the pressure reducing element 44 is adapted not to respond to the pressure reducing signal, thereby preventing the pressure regulator 40 from inadvertently further reducing the cuff pressure and only allowing the pressure regulator 40 to attempt to increase the pressure level in the tourniquet cuff 6 above the underpressure limit by the operation of the pressure increasing element 42.

[0046] When the cuff pressure signal is smaller than the underpressure limit signal, the patient danger circuit element 406 can also start a timer for a danger period using a clock 408. If a patient danger signal exists at the end of the danger period, a potential problem is confirmed, and the pressure increasing element 42 and the pressure reducing element 44 are each adapted not to respond to the pressure increasing signal and the pressure reducing signal, as a result of which the pressure regulator 40 becomes inoperative, thereby keeping the pressure level inside the tourniquet cuff 6 stable. The danger period is a predetermined period long enough for the pressure regulator 40 to resolve a potential problem and adjust the cuff pressure above the underpressure limit using the pressure increasing element 42 under normal operation. The predetermined period can be 200 milliseconds or based on the response time of the pressure regulator 40. The response time of the pressure regulator 40 is a predetermined measurable duration for the pressure regulator 40 to adjust the pressure level inside the tourniquet cuff 6 from a first reference pressure to a second reference pressure. In a preferred embodiment, the response time is determined by measuring the time required for the pressure regulator 40 to adjust the cuff pressure from a first reference pressure of 250 mmHg to a second reference pressure of 300 mmHg. It will be apparent to those skilled in the art that the response time can be measured from alternative first and second reference pressures.

[0047] It will be apparent to those skilled in the art that the patient risk shield 4 can generate a patient risk signal by comparing the cuff pressure with both the overpressure limit and the underpressure limit.

[0048] Considering the many possible embodiments to which the principles of the disclosed invention can be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be construed as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. Accordingly, we claim as our invention all that comes within the scope and spirit of these claims.

Claims

1. An automatic tourniquet device, a tourniquet cuff, a pressure transducer adapted to generate a cuff pressure signal indicative of the pressure level within the tourniquet cuff, a user interface adapted to generate a reference pressure signal indicative of a reference pressure level within the tourniquet cuff desired by the user, a patient danger shield responsive to the cuff pressure signal and the reference pressure signal, the patient danger shield being operable to generate a patient danger signal if the current pressure level within the tourniquet cuff is at least a predetermined overpressure limit higher than the reference pressure level during an adjustment period for adjusting the pressure within the tourniquet cuff to near the desired reference pressure level, a pressure regulator responsive to the patient danger signal, the pressure regulator having a pressurizing element responsive to a pressurizing signal for increasing the pressure within the tourniquet cuff and further having a depressurizing element responsive to a depressurizing signal for decreasing the pressure within the tourniquet cuff and comprising, the pressure regulator being operable during the adjustment period by selectively generating the pressurizing signal and the depressurizing signal, the pressurizing element being configured not to respond to the pressurizing signal if the patient danger signal is generated, a predetermined danger period starting when the patient danger signal is generated, the predetermined danger period being a predetermined period sufficient for the pressure regulator to adjust the pressure within the tourniquet cuff below the predetermined overpressure limit using the depressurizing element, the pressurizing element further being configured not to respond to the pressurizing signal after the predetermined danger period if the patient danger signal is detected at the end of the predetermined danger period, and the depressurizing element being adapted not to respond to the depressurizing signal if the patient danger signal is detected at the end of the predetermined danger period, an automatic tourniquet device.

2. The automatic tourniquet device according to claim 1, wherein the predetermined overpressure limit is set to a value obtained by subtracting the reference pressure level from a pressure level within the tourniquet cuff that is regarded as indicating a malfunction of the pressure regulator when the pressure level exceeds it.

3. The automatic tourniquet device according to claim 2, wherein the predetermined overpressure limit is set to 50 mmHg.

4. The automatic tourniquet device according to claim 1, wherein the user interface is further adapted to enable the user to select the predetermined overpressure limit from a plurality of overpressure limits.

5. The predetermined danger period is based on a predetermined response time of the pressure regulator, The predetermined response time of the pressure regulator is the time for the pressure regulator to adjust the pressure level of the tourniquet cuff from a first reference pressure to a second reference pressure. The automatic tourniquet device according to claim 1.

6. When the first reference pressure is 300 mmHg and the second reference pressure is 250 mmHg, the predetermined danger period is 200 milliseconds. The automatic tourniquet device according to claim 5.

7. When the patient danger signal continues to exist even after the predetermined danger period, a display indicating that a danger has occurred is stored in the non-volatile memory of the patient danger shield, and the user interface is adapted to prevent the start of the adjustment period if the stored display exists in the non-volatile memory. The automatic tourniquet device according to claim 1.

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