Patent Publication Number: US-2007112274-A1

Title: Wireless communication system for pressure monitoring

Description:
RELATED APPLICATIONS  
      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. 
    
    
     BACKGROUND OF THE INVENTION  
      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&#39;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&#39;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.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a wireless communication system for a vital signs system according to the present invention;  
       FIG. 2  illustrates a conceptual view of a portable unit according to the present invention;  
       FIG. 3  illustrates a conceptual view of a monitor interface unit according to the present invention;  
       FIG. 4  illustrates a conceptual view of a communications system according to the present invention;  
       FIG. 5  illustrates a conceptual view of a communications system according to the present invention;  
       FIG. 6  illustrates a conceptual view of a monitor signal conditioning unit according to the present invention;  
       FIG. 7  illustrates a conceptual view of a multiplying digital to analog converter circuit according to the present invention,  
       FIG. 8  illustrates a conceptual view of an active bridge drive circuit according to the present invention,  
       FIG. 9  illustrates a conceptual view of a synthetic bridge circuit according to the present invention,  
       FIG. 10  illustrates a conceptual view of a load adjustment circuit according to the present invention;  
       FIG. 11  illustrates a wireless communications system for a vital signs monitor according to the present invention; and  
       FIG. 12  illustrates a wireless communications system for a vital signs monitor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a preferred embodiment of a wireless pressure system  100  according to the present invention that can communicate data between a standard pressure transducer  106  (e.g. compliant with the previously described BP22 standard) and a standard vital signs monitor  108  (e.g. compliant with the previously described BP22 standard). More specifically, the wireless pressure system  100  includes a portable unit  102  that provides an excitation voltage to the transducer  106  to receive an output voltage that is proportional to the pressure of a catheter  110 . The portable unit  102  digitizes this pressure data, then transmits that data to a monitor interface unit  104  which emulates a corresponding output voltage to the vital signs monitor  108 . Consequently, the vital signs monitor  108  receives and displays a signal from the monitor interface unit  104  which corresponds to the actual pressure measured by the portable unit  102 , allowing the user to connect and therefore make use of a variety of transducers  106  and vital signs monitors  108  that are compliant with a standard (such as BP22).  
      As seen in  FIG. 1 , a standard pressure transducer  106  can be used according to the present invention, preferably supporting the 5 microvolts per volt of excitation voltage standard (5 microvolts/V EX /mmHg). This pressure transducer  106  is connected to a catheter line  110  leading to the interior of a patient, allowing the pressure transducer  106  to be in fluid communication with the cardiovasculature of the patient.  
      Additionally, a standard vital signs monitor  108  that also supports the 5 microvolts/V EX /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.  
                           TABLE 1                           Excitation   Excitation   Excitation           Type (AC,   Voltage   Frequency/       Monitor/Module   DC, Pulsed)   (nominal)   Duty                  Datascope Passport XG   DC   5.0 VDC   n/a       GE/Marquette Solar 8000 monitor &amp; Tram   DC   5.0 VDC   n/a       450SL module       GE Solar 8000 M monitor &amp; Tramrac 4A   DC   5.0 VDC   n/a       chassis &amp; Tram 450SL module       GE/Marquette Eagle 3000   DC   5.0 VDC   n/a       GE Dash 4000   DC   5.0 VDC   n/a       MDE Escort   DC   5.0 VDC   n/a       MDE Escort II (model 20100)   DC   5.0 VDC   n/a       MDE Escort Prism (model 20403)   DC   5.0 VDC   n/a       Medtronic Lifepak 12   DC   4.9 VDC   n/a       Philips/HP Merlin &amp; M1006A module   AC   3.6 Vrms   2.4 KHz       Philips/HP Merlin &amp; M1006B module   DC   5.0 VDC   n/a       Philips/HP Merlin &amp; M1006B module (new style)   DC   5.0 VDC   n/a       Philips M3046A &amp; M3000A module   DC   5.0 VDC   n/a       Philips Omnicare 24 &amp; M1041A chassis &amp;   AC   3.6 Vrms   2.4 KHz       M1006A module       Philips Omnicare 24 &amp; M1041A chassis &amp;   DC   5.0 VDC   n/a       M1006B module       Spacelabs Ultraview 1050   DC   4.0 VDC   n/a       Spacelabs 90308   DC   4.1 VDC   n/a       Spacelabs 90308 &amp; 90431 chassis/module   DC   4.4 VDC   n/a       Welch Allyn Propak CS (model 244)   Pulsed   0-5 V   170 Hz/90%                  
 
      The wireless pressure system  100  wirelessly couples the transducer  106  with the vital signs monitor  108  by preferably emulating the 5 microvolts/V EX /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  FIG. 1  only). As seen in the schematic drawing of  FIG. 2 , the portable unit  102  includes a voltage excitation circuit  120  which supplies and regulates an excitation voltage  140  through wires within the power cable  112  to the transducer  106 . This voltage excitation circuit  120 , along with the other circuits of the portable unit  102 , is powered by a power supply circuit  125  which draws its power from an external battery  126 , in this case removably secured to the transducer  106 .  
      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/V EX /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/V EX /mmHg standard), for example by using the formula Pressure (mm Hg)=(V T /(V ex ×5 μV))×(mm Hg×V), where V ex  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  FIG. 3 , the monitor interface unit  104  includes a RF antenna  118  connected to a RF transceiver  130  configured to receive the data packet transmitted by the portable unit  102 . Once received, the transceiver  130  sends the received data packet to the microcontroller  148  to extract and process the relevant information, including the pressure data.  
      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  FIG. 3 , the pressure transducer emulation circuitry  184  includes a multiplying digital to analog converter circuit  180 , an active bridge drive circuit  181 , a synthetic bridge circuit  183 , a load adjustment circuit  185 , and a monitor signal conditioning circuit  185 , all of which are responsible for converting or “translating” the digital pressure value received from the portable unit  102  into an analog form that the vital signs monitor  108  can read. Preferably, this emulation can be achieved by modifying the monitor excitation signal  147  according the digital pressure data obtained by the portable unit  102 , as will be explained in greater detail below. An example of another pressure transducer emulation circuit can be seen in U.S. Pat. No. 5,325,865, the contents of which are hereby incorporated by reference.  
      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  FIG. 3 ) to enter the monitor signal conditioning circuit  186  of the emulation circuitry  184 . The conditioning circuit  186  accepts the excitation signal  147  and “conditions” this power signal appropriately to be used by the multiplying DA converter circuit  180  and the active bridge drive circuit  181 , then supplies these circuits  180  and  181  with the conditioned power signal. More specifically, the conditioning circuit  186  converts the differential voltage signal from the monitor excitation signal  147  (i.e. Pexc and Nexc) into a reference voltage signal, or a voltage signal with a difference relative to the ground of the monitor interface unit  104 .  
       FIG. 6  illustrates a more specific schematic example of a conditioning circuit  186 . In this example, resistors R 30  and R 31  create a reference potential, or virtual ground, halfway between Pexc and Nexc labeled Vcm (for common mode voltage) and driven by amplifier U 15 . Resistors R 49 /R 51  and R 60 /R 58  form resistor dividers which create the signals 0.2 Pexc and 0.2 Nexc respectively reference to Vcm. These voltages are fed into the amplifier formed by resistors R 47 , R 48 , R 53 , R 52 , and amplifier U 19  (e.g. part number LMC7111 from National Semiconductor Corportation) which is configured to provide unity gain and differential to single ended conversion between the balanced excitation potentials 0.2 Pexc and 0.2 Nexc. The output of this amplifier is the reference input (Vref), which is supplied to the multiplying DA converter circuit  180  and the active bridge drive circuit  181 .  
      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 .  
       FIG. 7  illustrates a more specific schematic example of an AD5443 digital to analog converter circuit, as produced by Analog devices, Inc. U 13  of this specific example operates on a 3V single ended regulated power supply, while U 21  operates from a split regulated supply providing +3V and −3V. The reference voltage or Vref is supplied from U 19  (LMC7111) as previously discussed in regards to the conditioning circuit  186 . Preferably, this circuit is used in a bipolar, four quadrant multiplying design as described in the accompanying data sheet of the AD5443 and as is known in the art.  
      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 .  
       FIG. 8  illustrates a specific example of an active bridge drive circuit  181  in which an inverting amplifier is formed by resistors R 63 , R 64 , and amplifier U 20  to invert and attenuate the signal Vcm from the monitor signal conditioning circuit  186  by a factor of 2. Additional inverting is achieved with a summing amplifier formed from resistors R 50 , R 56 , R 62 , R 59 , and amplifier U 20 . The output of this circuit can be described by the following formulas: Vbridge=−(0.5Vref+(−Vref*DAC)+2(−0.5Vcm))=−0.5Vref+Vref*DAC+Vcm; OR Vbridge=Vref (DAC−0.5)+Vcm.  
       FIG. 9  illustrates a specific example of a synthetic bridge 183 in which the signal from the active bridge circuitry of  FIG. 8 , Vbridge, is applied to resistor R 55 , while the common mode voltage, Vcm, is applied to the other side of the bridge at resistor R 27 , meeting in the middle with resistor R 26 . The final output, which has been attenuated and differentiated, can be seen as Psig and Nsig, which represents the final simulated transducer signal  150 . This final simulated transducer signal  150  can be described in this specific example by the following equations: 
 Psig−Nsig=( R 26 /R 55 +R 26 +R 27))(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  FIG. 1 ) to a signal input port on the vital signs monitor  108 , allowing the vital signs monitor  108  to process and display the pressure reading from the simulated transducer signal  150 . Thus, the vital signs monitor  108  functions as if it was directly connected to and interacting with the transducer  102 , when it is actually interacting with the monitor interface unit  104 .  
      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  FIG. 10 . In this example, the load adjustment circuitry  185  includes switched resistances R 42  (820Ω) in series with Q 15 - 1  and R 43  (430Ω) in series with Q 15 - 2 . The microcontroller  148  signal controlling Q 15 - 1  is “Load  0 ” while the signal controlling Q 15 - 2  is “Load  1 ”. If the other loads of the monitor interface unit  104  become too low, the microcontroller  148  can send a signal to turn on either Load  0  or Load  1 . Conversely, if the other loads of the monitor interface unit  104  become too high, the microcontroller  148  can send a signal to turn off either load, thus regulating the amount of current drawn by the monitor interface unit  104 .  
      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  FIGS. 2 and 3  respectively. In an alternative preferred embodiment, an RFID token can be used to transmit a “linking code” to each unit  102  and  104  through the RFID transceivers  129  and  170 , and RFID antennas  131  and  172 , as seen in  FIGS. 2 and 3  respectively. In this respect, the portable unit  102  and the monitor interface unit  104  can use the linking code to identify wireless transmission from each, while ignoring transmissions from nearby, non-linked units. A more detailed discussion of this linking or pairing process can be seen in the concurrently filed and commonly assigned U.S. Provisional Application No. 60/736,408 entitled  Wireless Communication Protocol For A Medical Sensor System , filed on Nov. 14, 2005, the contents of which are hereby incorporated by reference.  
      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  FIG. 4  with an alternate embodiment as seen in  FIG. 5 .  
      In the alternate embodiment of  FIG. 5 , the monitor excitation signal  147 , i.e. the electrical signal delivered by the vital signs monitor  108  to excite the transducer of a typical wired system, is continuously measured and recorded into a data signal which is transmitted over wireless signal  160  to the portable unit  102 . Such a continuous measurement may be desired with monitors that, for example, provide pulsing monitor excitation signals  147  and therefore expect a pulsing return signal. The portable unit  102  reads the data within the wireless signal  160  and creates transducer excitation voltage  140  accordingly. The transducer output voltage  142  from the transducer  106  passes back to the portable unit  102  where it&#39;s transmitted via a wireless signal  162  to the monitor interface unit  104 . Finally, the monitor interface unit  104  generates a simulated transducer signal  150  based on the data sent in the wireless signal  162 .  
      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  FIG. 4  functions as previously described in this specification. Namely, the portable unit  102  provides a predetermined transducer excitation voltage  140  to the transducer  106  which is returned to the portable unit  102  by transducer output voltage  142  and ultimately transmitted to the monitor interface unit  104  via wireless signal  162 . Thus, the alternate preferred embodiment of  FIG. 5  is much more complex when compared with the preferred embodiment of  FIG. 4 . Specifically, an additional emulation step occurs with the measurement of the monitor excitation signal and the reproduction or emulation of that measurement with the transducer excitation voltage  140 . In other words, the transducer excitation voltage  140  in  FIG. 4  is not derived from measurements in the monitor interface unit  104  which can allow for more accurate measurement. Therefore, the preferred embodiment of  FIG. 4  is much less likely to produce errors or increase pre-existing errors.  
      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,  FIG. 11  illustrates one preferred embodiment that connects to and communicates with such an interface module  200 . Specifically, a digital monitor interface unit  105  is provided which is generally similar to the previously described monitor interface unit  104 , except this unit  105  outputs a digital pressure signal instead of an emulated analog signal and does this over a digital interface  214  (e.g. a cable between the digital monitor interface unit  105  and the interface module  200 ). In other words, the monitor interface unit  105  does not convert the digital pressure value back to an analog value, as is the case in the previously described preferred embodiments. Instead, this digital value is transmitted, either in a raw form or a standardized form (i.e. a form understandable to the interface module  200 ) over the digital interface  214 .  
      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.  
       FIG. 12  illustrates another preferred embodiment generally similar to the previously described preferred embodiment, except a monitor interface unit  104  interfaces with an interface module  202  that produces an analog signal. Preferably, the interface module  202  is configured to interact with a pressure transducer by the BP22 standard; however other analog data transmission methods are also acceptable.  
      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.