Patent Application
20070112274
The measurement of blood pressure is an important technique used by medical personnel for diagnosing and treating a wide range of injuries and conditions. By measuring and especially monitoring a patient's blood pressure, medical personnel can be alerted to problems at an early stage, increasing the likelihood of successful treatment.
While indirect methods of blood pressure monitoring, such as with a pressure cuff and stethoscope, are often desired for quick pressure readings, these methods can be inaccurate by as much as 10 percent, making them undesirable for longer term blood pressure monitoring of more critical patients. Consequently, direct blood pressure monitoring methods are preferred for patients with serious or critical conditions due to their improved accuracy and easier long-term implementation.
The most popular direct blood pressure monitoring method is performed through catheterization, in which a fluid-filled catheter is inserted into a patient at a desired location, such as within a blood vessel. The catheter is filled with a solution, such as saline, and is connected via a tube to a pressure transducer. As the blood pressure within the patient changes, the pressure on the solution within the tube changes proportionally, allowing the connected transducer to accurately measure the pressure within the patient. The pressure transducer is in turn connected to a vital signs monitor which displays the blood pressure readings to the medical personnel. A representative pressure transducer can be seen in U.S. Pat. No. 4,576,181, the contents of which are hereby incorporated by reference.
Typically, transducers have utilized a pressure responsive diaphragm mechanically coupled to piezo-resistive strain gauges arranged in a Wheatstone bridge arrangement. In this respect, the amount of strain placed on the strain gauge can be determined by applying an excitation voltage to the Wheatstone bridge arrangement, then monitoring the output voltage of the bridge. Thus, as the strain varies, the output voltage from the transducer also varies proportionately.
The vital signs monitor connected to the pressure transducer is responsible for providing this excitation voltage to the bridge arrangement and measuring the output voltage to determine the blood pressure within the patient. Currently, most medical device manufacturers recognize a standard in the proportionality of the excitation voltage provided to a transducer and the output voltage in which five microvolts of signal per volt of excitation voltage is equivalent to one millimeter of mercury applied pressure. This standard is also known as standard BP22 “Blood Pressure Transducers” from the Association for the Advancement of Medical Instrumentation (AAMI). The widespread use of this standard allows sensors from many different manufacturers to be interchanged with monitors from other manufactures, enabling the user the flexibility to mix and match components as desired.
One disadvantage to these systems is the cumbersome cable connecting the transducer to the vital signs monitor. These cords can easily tangle, can accidentally pull out from the vital signs monitor, and can be easily confused when multiple pressure monitoring lines are used. Further, the length of these cords limits the distance the patient can move from the vital signs monitor and must be disconnected and secured when a patient is transported within the hospital.
Currently, some wireless transducer products are available, eliminating the use of a cord between the transducer and a visual display. For example, some wireless pressure transducers are available from Memscap, which transmit sensor data to a computer. However, these wireless transducer systems have integrated permanent transducers and wireless functionality to communicate with only a remote personal computer. In this respect, the current wireless transducer systems cannot connect to standard transducers or standard vital signs monitors. Since vital signs monitors are integrated with hospital information systems and represent a significant expense, hospitals are reluctant to switch to these wireless systems which would require the use of only that company's transducer system equipment.
The most common wireless sensor system currently available in some hospitals are wireless ECG transmitters and monitors. ECG telemetry utilize a standard method which transmits. from a portable patient-attached module to a hospital infrastructure, such as a dedicated network of antennas and display monitors. However, unlike invasive blood pressure, ECG does not utilize an artificial transducer or an excitation voltage.
What is needed is a wireless pressure transducer system that can easily connect with the vital signs monitors and ordinary transducers used by many hospitals today.
It is an object of the present invention to overcome the limitations of the prior art.
It is another object of the present invention to provide a wireless communication system for a pressure transducer system.
It is another object of the present invention to provide a wireless communication system that can function with most pressure transducer systems currently used in hospitals.
It is another object of the present invention to provide a wireless communication system that reduces errors introduced by electrical signal measurement and reproduction.
The present invention attempts to achieve these objects, in one embodiment, by providing a wireless communication system for use with a vital signs monitor system. The wireless communication system includes a portable unit that connects to a typical pressure transducer and a monitor interface unit that connects to a typical vital signs monitor. The portable unit obtains a pressure reading from the transducer by providing an excitation voltage to the transducer, digitizing the output, and then wirelessly transmiting the pressure data to the monitor interface unit. The monitor interface unit receives the digitized voltage supplied by the portable unit and converts the pressure data into a format recognizable by the vital signs monitor.
As seen in
Additionally, a standard vital signs monitor 108 that also supports the 5 microvolts/VEX/mmHg BP22 standard can be used according to the present invention. While the voltage proportion is standardized, the excitation voltage (i.e. the electricity provided for excitation purposes) provided by different manufacturers widely vary in format such as voltage magnitude, timing (e.g. AC or DC), and other characteristics, and therefore the transducers and other equipment that connect to these monitors must be capable of handling the excitation voltage provided. For example, Table 1 illustrates examples of BP22 compliant monitors and some selected characteristics of their excitation voltage.
The wireless pressure system 100 wirelessly couples the transducer 106 with the vital signs monitor 108 by preferably emulating the 5 microvolts/VEX/mmHg standard. More precisely, this emulation includes two discrete actions: emulation of the excitation voltage of the vital signs monitor 108 to the transducer 106 and emulation of the voltage output of the transducer 106 to the vital signs monitor 108.
The excitation voltage emulation is performed by the portable unit 102, which is connected to the transducer 106 by power cable 112 (seen in
Since the portable unit 102 ultimately generates a pressure reading in a digital form, the excitation voltage 140 can be in a variety of different formats or voltages. Preferably, excitation voltages 140 with minimal power requirements are preferred to maximize the life of the battery 126. In a preferred embodiment, the excitation voltage 140 is equal to about 1.225 Volts.
As previously described, the excitation voltage 140 travels through a resistive bridge 143 (such as a Wheatstone bridge) within the transducer 106 and provides an output voltage 142 according to the 5 microvolts/VEX/mmHg standard through additional wires in cable 112 to the portable unit 102.
Once in the portable unit 102, the output voltage 142 initially passes through a differential amplifier 141 which “cleans up” the voltage signal by applying amplification and filtering. Next, an analog to digital converter 122 (AD converter) produces a digital value based on both the amplified and filtered output voltage 142 and the original excitation voltage 140 (i.e. a reference voltage), transmitting these digital measurements on to a microcontroller 132. The microcontroller 132 converts these digital voltage values into a pressure reading according to the BP22 standard (5 microvolts/VEX/mmHg standard), for example by using the formula Pressure (mm Hg)=(VT/(Vex×5 μV))×(mm Hg×V), where Vex is equal to the excitation voltage 140 and VT is equal to the output voltage 142. In a preferred embodiment the digital value attributed to the pressure reading will be between 0 and 4095. Alternately, the digital value can simply be maintained without conversion, allowing the monitor interface unit 104 to manipulate the digital value appropriately. It should also be understood that a variety of software techniques can be used in this regard so that the monitor interface unit 104 can interpret this digital data and produce an emulated analog transducer signal.
Next, the microcontroller 132 readies this pressure data to be sent to the monitor interface unit 104 by creating a data packet appropriate for wireless transmission. For example, this may include adding time stamp information for the pressure data, CRC error detection data, unit identification data, and other relevant information.
After its assembly, the data packet is communicated to the RF transceiver 128, which transmits the data packet via antenna 116. This wireless RF transceiver 128 may transmit and receive with a variety of different frequencies and protocols, such as radio frequencies, infrared frequencies, Bluetooth protocols, and TDMA protocols.
Thus, by supplying excitation voltage 140 to the transducer 106 and comparing the output voltage 142 to the original excitation voltage 140, the portable unit 102 interacts with the pressure transducer 106 in a similar manner as a vital signs monitor 108, but instead obtains digital pressure data that can be transmitted wirelessly.
The emulation of the voltage output from the pressure transducer 106 is performed by the monitor interface unit 104, which is connected to the vital signs monitor 108 by cable 114. As seen in the schematic drawing of
To determine the voltage value which is appropriate to communicate the pressure reading to the vital signs monitor 108, the monitor interface unit 104 must also be “aware” of the format (e.g. voltage magnitude, A.C. or D.C., etc.) of the monitor excitation signal 147 that is produced by the vital signs monitor 108, or at least couple to and manipulate this signal 147. As previously discussed, most vital signs monitors supply their own transducer excitation signal 147 and expect a BP22 standardized transducer signal based on the monitor excitation signal 147. Additionally, the monitor excitation signal 147 can also server as a source of power for the monitor interface unit 104, as described in more detail below. In other words, the monitor excitation signal 147 can be used both as a source of power and also as a reference for the transducer emulation circuit.
In the present embodiment, this monitor excitation signal 147 is supplied through isolated wires within cable 114 to a pressure transducer emulation circuit 184 and a monitor power harvesting circuit 187. The monitor power harvesting circuit 187 converts the monitor excitation signal 147 into a format appropriate for use by the circuits of the monitor interface unit 104. Preferably, the monitor power harvesting circuit 187 is responsible for converting AC power, if present, into DC power since DC power is typically used by the circuits and chips of electronic devices. This AC to DC conversion can be achieved, for example, by rectifying the AC power with a diode bridge. Thus, the monitor power harvesting circuit 187 supplies DC power, despite an input power of either AC or DC power from the monitor excitation signal 147. The monitor power harvesting circuit 187 supplies this DC power to a power supply 174 which reduces the voltage to a level appropriate for use by the chips and circuits of the portable unit 102, such as 3.5 volts, then distributes this power to the circuits of the monitor interface unit 104. In this manner, the monitor interface unit 104 can power itself, and therefore all of its circuits exclusively by the excitation signal 147 produced by the vital signs monitor 108. In an alternative preferred embodiment, the power supply circuit 174 can draw power from an A.C. adapter or a battery.
Returning again to
The analog emulation process (i.e. producing a pressure signal recognizable to the vital signs monitor 108) allows the excitation signal 147 (represented as “Pexc” and “Nexc” in
The multiplying digital to analog converter circuit 180 accepts and modifies this referenced power signal (Vref in the specific example) according to the digital pressure value obtained from the microcontroller 148. More specifically, the DA converter circuit 180 outputs a differential current that is proportionally lower than the referenced power voltage value, based on the value of the digital pressure value. For example, this conversion can be achieved by first determining the ratio by which the conditioned analog signal should be reduced (e.g. dividing the current digital pressure value by the maximum digital pressure value of the AD converter circuit 122 in the portable unit 102), then reducing the voltage of the referenced analog signal by that ratio (e.g. multiplying the ratio by the value of the referenced analog signal). Generally speaking, this ratio acts as a “conversion factor” for the digital pressure value since the digital value alone is not absolute, but rather an arbitrary digital number that differs with different types of analog to digital circuits. Thus, the DA converter circuit 180 modifies the value of the voltage output proportionally based on this ratio. It should be noted that such conversion factors may differ, depending on how the digital pressure data is provided to the monitor interface unit 104. However, no matter what the format of the digital pressure data is, it can be converted into a form usable by the digital to analog converter circuit 180.
The “proportioned” reference analog signal is next supplied to the active bridge drive circuit 181 which creates a voltage bridge that “drives” the synthetic bridge 183 to produce the final simulated transducer signal 150. Specifically, the active bridge drive circuit 181 modifies the analog reference signal, for example by summing multiple voltage signals such as the reference signal, the proportioned reference signal and the common mode signal, to achieve an appropriate value for the synthetic bridge 183. The synthetic bridge 183, in turn, attenuates the output voltage of the bridge drive circuit 181, then converts this referenced analog signal back into a differential signal, creating the simulated transducer signal 150.
Psig−Nsig=(R26/R55+R26+R27))(Vbridge−Vcm)=1/51(Vref(DAC−0.5))
Since Vref=0.2(Pexc−Nexc)
Psig−Nsig=1/255(Pexc−Nexc)(DAC−0.5)
The digital value written to “DAC” in the previous equation can range from 0 to 4095 in the previous specific example. As previously discussed, the differential signal output expected by the vital signs monitor 108 is scaled to 5 μV per volt of excitation per mmHg. Thus, with 1 volt of excitation (Pexc−Nexc=1), a digital value of 4095 (full scale) would correspond to a differential signal (Psig−Nsig) of 0.5/255=0.00196 which is equivalent to 392 mmHg. A digital value of 0 would be equivalent to −392 mmHg. A digital value of 2048 would be equivalent to 0 mmHg.
This simulated transducer signal 150 is supplied by the synthetic bridge 183 through wires within cable 114 (seen only in
The transducer emulation circuit 184 also includes the load adjustment circuitry 185 which allows the microcontroller to emulate a “load” or resistance amount on the monitor excitation signal 147. Specifically, the load adjustment circuitry 185 monitors the load being drawn from the excitation signal 147, and when this load deviates from an amount that would normally be drawn by a typical pressure transducer, the microcontroller 148 causes the circuit 185 to increase or decrease resistance. In this respect, the monitor interface unit 104 draws a similar amount of power in a similar way to a standard pressure transducer. Since some monitors 108 have alarms that may be triggered if the load is outside of a specified range, the load adjustment circuitry 185 can maintain the load at a normal level, preventing false alarms.
A more specific example of such an emulation circuit can be seen in the schematic illustration in
In operation, the user first connects the portable unit 102 to the pressure transducer 106, then connects the monitor interface unit 104 to the vital signs monitor 108. Next, the user activates the portable unit 102 and monitor interface unit 104 and “links” these units 102 and 104 together so that they recognize the RF signals transmitted by each other. In one preferred embodiment, the user can enter a “linking code” into each unit 102 and 104 by way of the user inputs 133 and 178 and the user outputs 137 and 176, as seen in
After recognizing each other, the portable unit 102 begins sending an excitation signal 140 to the pressure transducer 106, measuring and converting the output signal 142 into a digital pressure reading. The microcontroller 132 encodes this pressure data into a data packet suitable for wireless transmission and ultimately transmits this data packet with RF transceiver 128. This process is continually repeated, creating a stream of data packets that are wirelessly transmitted.
The monitor interface unit 104 receives the data packets with transceiver 130 and the microcontroller 148 parses out the relevant data, including the digital pressure values. Each digital pressure value is sent to the pressure transducer emulation circuit 184 which produces an analog signal based on the BP22 standard and communicates this simulated transducer signal 150 back to the vital signs monitor 108. The vital signs monitor 108 interprets the simulated transducer signal 150 as a pressure reading and displays the value according to its functionality.
It should be noted that the overall architecture of this preferred embodiment of the wireless pressure system 100 acts to minimize errors that may distort the pressure reading displayed at the vital signs monitor 108 when compared with alternative embodiments. The reasons for this error minimization can be more clearly appreciated by comparing the present embodiment as seen in
In the alternate embodiment of
Thus, this alternate embodiment includes multiple measurements and voltage emulations in series, creating a more complex series of steps. Further, the transducer excitation voltage 140 is directly derived from the measurement of the monitor excitation voltage 147. Since almost all electrical measurements and voltage reproductions introduce at least some error or inaccuracy, multiple measurements and reproductions in series may increase these errors, possibly combining and magnifying them. Additionally, emulating the exact monitor excitation signal 140 at the portable unit requires many different circuitries to achieve such a wide range of voltage. Further, a larger battery will typically be necessary since most monitors 108 provide a relatively high excitation voltage 147. In other words, the increased complexity of this embodiment can more easily lead to errors and additional demands on the components of each unit 102 and 104.
In contrast, the preferred embodiment of
In some patient monitoring systems, multiple sensors can be connected to a single vital signs monitor. Instead of including many different types of sensor ports on a single vital signs monitor (i.e. one blood pressure sensor port, one EKG sensor port, etc.), some monitors provide a plurality of generic ports into which different vital signs sensors can be connected. Since each sensor may have a different physical connection port, different power requirement, and a different data transmission scheme, these vital signs monitors rely on interface modules to “interface” between the generic ports of the monitor and one particular sensor type (e.g. a blood pressure transducer).
Thus, the interface module accommodates the specific connection, power, and data requirements of the sensor, then transmits the sensor data on to vital signs monitor. In this respect, interface modules can greatly simplify the amount of equipment used in a typical hospital room by allowing many different types of patient sensors to connect and therefore display on a single vital signs monitor.
For example, an interface module for a wired pressure transducer may provide an excitation signal to the pressure transducer while measuring the output voltage of the transducer. The interface module may then convert this pressure reading into a proprietary format understood by the vital signs monitor and further communicate this data via one of the generic ports on the vital signs monitor. The interface module may also provide additional information to the vital signs monitor to facilitate proper display of the data, such as the measurement units, how to graph the data, or critical sensor levels that signal an alarm (e.g. very low blood pressure may automatically cause an alarm to sound on the vital signs monitor).
In order to accommodate different sensor types, each interface module must be customized to work with a specific type (and sometimes brand) of sensor. Thus, different sensor types require a different interface module. In one respect, such customizations are directed to including a port on the interface module that will connect to a port or connector on the sensor. In other words, the sensor must be able to physically plug into the interface module.
In another respect, such customizations are directed to drivers or software specific to each sensor type. These drivers allow the interface module to interpret the raw data received from each sensor and convert it (e.g. with a conversion routine) into a format that can be displayed on a monitor. Additionally, these drivers also provide the communication format of the monitor which allows the interface module to communicate this sensor data in a form understandable to the vital signs monitor. While most processing of the sensor data occurs in the interface module, additional conversions and calculations of the data can also occur in the vital signs monitor.
One example of such an interface module is the Philips M1032A Vuelink Module which can connect to the generic sensor ports of a Philips IntelliVue line of vital sign monitors. More information regarding the Philips Vuelink Module can be found the Vuelink brochure entitled “Vuelink Device Interfacing Module” document number 452298291381 printed in the Netherlands in August of 2003, the contents of which are hereby incorporated by reference. Additional examples and interface module discussion can be seen in U.S. Pat. Nos. 6,477,424 and 5,666,958; and U.S. Pub. No. 2003-0028226, the contents of which are hereby incorporated by reference.
The wireless pressure system 100 described in this specification can be adapted to connect with such an interface module. For example,
The interface module 200 is connected to the digital interface 214 and includes software that can interpret or convert the digital data as a standard pressure value (e.g. mmHg). Since the monitor interface unit 105 can supply digital pressure data in a many different digitals forms, the interface module 200 must understand how this digital data relates to the actual pressure data measured on the patient. In other words, the interface module 200 must know how to convert this digital data into a meaningful form. Preferably, this conversion is fully performed in the interface module 200, however part or all of these interpretations or conversion routines can be performed in the monitor interface unit 105 or in a connected adapter unit.
Since these conversions may vary depending on the make and type of the sensor, the conversion algorithms or routines can be automatically selected based on detection of specific sensors (e.g. plug and play devices) or can be manually selected by way of inputs (buttons, switches, touch screens, etc.). Thus, the conversion routine or algorithm needed for the interface module 200 to “understand” the digital data can be selected for different sensors.
The interface module 200 then sends the appropriate data over monitor interface 212 (e.g. a direct connection, cable, etc.) to cause the digital pressure value to be displayed on the vital signs monitor 108. In this respect, the pressure value remains a digital value after being communicated to the interface module 200, allowing the interface module 200 to appropriately communicate the patient pressure data to the vital signs monitor 108 for display.
In addition to including the ability to display on a specific type of vital signs monitor (e.g. with a specific proprietary monitor driver), the interface module 200 also preferably includes the ability to display sensor data on many different types of vital signs monitors 108 (e.g. by including many different vital signs monitor drivers). In such situations, different monitors may be automatically detected, or a user may select an appropriate monitor from an input (e.g. buttons or switches) on the interface module 200. Additionally, the interface module 200 preferably includes the hardware and software components necessary to connect to standard vital signs monitors, either through a typical display port or through a sensor input port using an emulated sensor signal, as described previously in this specification.
The monitor interface unit 104 is connected via analog interface 216 (e.g. a cable) to the interface module 202, allowing the monitor interface unit 104 to provide analog pressure data in a standard format (e.g. BP22) or any proprietary format. The interface module 202 converts this simulated analog pressure signal into an appropriate data format (either analog or digital), then communicates the pressure data over monitor interface 212 to the vital signs monitor 108 (as described in the previous example). In other words, the preferred embodiment of the wireless pressure system 100 initially described in this specification is essentially connected to an interface device and displayed on a vital signs monitor.
While the term interface module has been described in this specification, it should be understood that this term can be more broadly understood to mean any device that can connect between a patient sensor and a vital signs monitor. It should also be understood that there may be a variety of different arrangements that can facilitate connection of the wireless pressure system 100 to a vital signs monitor 108 (i.e. a wireless connection that ultimately results in a pressure display on a vital signs monitor 108). While a few of these arrangements have been described (e.g. a direct connection to the vital signs monitor 108, an interface module, etc.), other arrangements are contemplated as falling within the scope of this invention.
For example, the interface modules 200 and 202 may directly connect to a port on the vital signs monitor 108.
In another example, the interface modules 200 and 202 can be incorporated into monitor interface units 104 and 105 respectively. In this respect, each monitor interface unit 104 or 105 includes a generic connector (to connect to the vital signs monitor) and an interface circuit that is configured or customized to communicate with a specific vital signs monitor 108 or series of monitors from a particular manufacture (i.e. a proprietary communications protocol is used). Thus, the monitor interface units 104 or 105 include the software, drivers, protocols, connection ports, and similar elements that allow these monitor interface units 104 or 105 to directly connect to and interact with the communications bus of the vital sign monitor 108, a vital signs monitor chassis, or a vital signs monitor rack.
Further, such direct connection allows the vital signs monitor 108 to more easily communicate with and control the interface modules 104 and 105. For example, a user can actuate an input device (e.g. buttons) on the vital signs monitor 108 to shut down the monitor interface unit 104 or 105.
This direct connection between the vital signs monitor 108 may also facilitate the communication of other data types to be displayed on the vital signs monitor 108. For example, the monitor interface unit 104 or 105 can transmit wireless signal strength data and a battery level of the portable unit 102, in addition to the pressure data. This additional data can be displayed on the vital signs monitor 108 or used as the basis for alarms (e.g. an audible noise when the battery level of the portable unit 102 is critical).
Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.
This application claims priority to U.S. Provisional Application Ser. No. 60/736,428, filed Nov. 14, 2005 entitled Wireless Communication System For Pressure Monitoring; and U.S. Provisional Application Ser. No. 60/736,408, filed Nov. 14, 2005 entitled Wireless Communication Protocol For A Medical Sensor System, and are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60736428 | Nov 2005 | US | |
| 60736408 | Nov 2005 | US |
