Abstract:
A pressure capsule embedded in a pacemaker lead to monitor intracardiac chamber pressure is described. This pressure monitor capsule provides highly accurate pressure readings while insuring a high integrity seal against bodily fluids and tissue growth. The capsule is intended to be embedded into a pacemaker cardiac lead or a catheter with the distal (Tip) isolation diaphragm sensing pressure, coupling the pressure through an air column to a protected sensing MEMS device and providing a secure fluid seal to the lead walls. The proximal (Back) end of the capsule provides the electrical interface through the lead to the pacemaker pulse generator.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to intracardiac chamber pressure sensing and more particularly to pacemaker lead embedded pressure sensing mechanisms.  
       BACKGROUND OF THE INVENTION  
       [0002]     Intracardiac blood pressure sensing for research, diagnostic and treatment dates back to the early part of the 20th century, where early investigations utilized a canula or needle-based system with a mercury manometer. Using these techniques, pressure fluctuations in all 4 chambers of the heart have been successfully monitored. Critical diagnostic measurements of right heart systolic (pumping) and diastolic (resting) pressures can indicate disease conditions such as mitrial valve stenosis (stiffening), pulmonary artery hypertension, right heart weakness following myocardial infarction (heart attack), peripheral venus return failure (reduced preload) and electrical anomalies (arrhythmia or conduction).  
         [0003]     Blood pressure can be also monitored through a fluid-filled tube or catheter where a diaphragm in the tip of the catheter deflects to transfer pressure to a pressure sensor external to the body. This method is typically used in either canula-based or catheter-based pressure sensors. These sensors are typically Piezo Resistive Technology (PRT) sensors. In contrast, sensors based on a Wheatstone bridge topology require high power levels and are typically too large for implantation. In addition, the sensors typically need to send the signals back to a remote device to capture the measured signals, often subject to signal degradation in the transmission process.  
         [0004]     Another blood pressure sensing technology is the fiber optic blood pressure sensor. The sensor works through a small cavity embedded in the sensor tip, where the blood pressure is measured by observing the changes in length of the cavity using a measurement based on white light interferometry. Sensing light is transmitted to and reflected back from the detecting diaphragm and cavity of the sensor tip via a multimode fiber.  
         [0005]     Testing information has been published for capacitor diaphragm-based pressure sensors, coupled to pacemakers, where the sensors are an integral part of pacemaker leads. The sensors are typically implanted to monitor intracardiac right heart pressure and have demonstrated a high correlation to standard balloon catheter measurements. These devices use a capacitive-based sensor in a catheter or pacemaker lead having a titanium deflectable sensing diaphragm at the tip. The diaphragm acts as one plate of a sensing capacitor and inside the diaphragm is an air-filled cavity with a second capacitor plate. The value of the capacitance is inversely proportional to the plate distance. As the pressure changes, the titanium diaphragm deflects, changing the plate spacing and therefore the capacitance. This change in capacitance can be detected by an electronic circuit.  
         [0006]     Capacitive sensors are based on the equation: 
 
 C=K ·( A/D ) 
 
         [0007]     where K is the dielectric constant, A is the capacitor plate area, and D is the distance between the 2 capacitor plates. With a metal diaphragm, the measurement of pressure is based on the plate deflection, or the change plate distance. Thus, the capacitance change per unit pressure is limited by the macroscopic motion of the plate. For high sensitivity, the plate movement must produce a significant capacitive change. This requires a thinner plate to allow the movement per unit pressure. However, reduced plate thickness complicates the diaphragm attachment method regardless of whether the diaphragm is welded, or adhesively bonded. Also, the capacitance of the wiring to separate electronics can be orders of magnitude greater than the diaphragm capacitance. This complicates the decoding electronics for pressure measurement. In addition, having the diaphragm directly contacting the sensing media (i.e., the liquid to be measured) can cause a shift of the capacitive value of the sensor from its initial nominal value. Thus, this design is susceptible to capative changes based on the sensing media with which it comes in contact.  
         [0008]     Further, thermal effects, mechanical instability and aging effects contribute to an inaccuracy in the measurement taken by the capacitive-based sensor. For example, as the sensor ages, small movement in the wiring position or compression of the insulation may significantly alter the interconnect capacitance. This is seen as a change in the zero pressure reading or a drift of the reading with time. The range, accuracy and the repeatability of pressure measurement are not only limited by the motion of the diaphragm and the capacitance of the wiring, but also any thermally induced error. Since the diaphragm dimension can change by expansion and contraction due to thermal effects, accuracy is limited. The reproducibility of these thermal effects is also determined by the precision and reproducibility of the manufacturing process.  
         [0009]     The current state of the art in intracardiac sensing is limited by the low level of signal output, remote sensing requirement, large physical size or custom fabrication for all designs. Most of the current state of the art sensors such as canula based, fluid filled catheters are not suitable for chronic (long term) unattended implantation. Others, such as the optical based sensors, require power levels too high for long term battery operation. Further, capacitor-based sensors require a secondary amplifier and detection circuit. These type of sensors may also be prone to long term drifting or lack of sensitivity.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0010]     The present invention solves the deficiencies of the existing systems by creating a pressure sensing (pressure sense) module in the form of a physically small, biologically inert package. It is intended for full implantation within the tip of a pacemaker lead or catheter. In accordance with one preferred embodiment of the present invention, the pressure sensing module is intended to be capable of chronic low power operation with high signal amplitude and long-term signal stability.  
         [0011]     In one preferred embodiment of the present invention, a pressure sensing module includes a pressure sensing capsule having a body with a distal end and a proximal end, an electrical circuit integrated into the body, a first cavity located between the distal end and the proximal end, and an isolation diaphragm coupled to the distal end of the body. The pressure sensing module further includes a Mechanical Electrical Mechanical System (MEMS) pressure sensor mounted in the first cavity of the body of the pressure sensing capsule, and a second cavity for transferring a pressure applied to the isolation diaphragm to the MEMS pressure sensor.  
         [0012]     In another embodiment, a pressure sensing capsule having a body with a distal end and a proximal end; an electrical circuit embedded into the body; a first cavity located between the distal end and the proximal end; and an isolation diaphragm coupled to the distal end of the body. The pressure sensing capsule further including a MEMS pressure sensor mounted in the first cavity of the body of the pressure sensing capsule; and, a pressure transfer cavity having a first opening operatively in communication with the isolation diaphragm and a second opening operatively in communication with the MEMS pressure sensor, the pressure transfer cavity transferring a pressure applied at the isolation diaphragm to the MEMS pressure sensor by transferring the pressure applied from the first opening to the second opening.  
         [0013]     In another preferred embodiment, a method for creating a pressure sensing capsule includes the step of providing a body with a distal end and a proximal end, the body having a first cavity located between the distal end and the proximal end, an electrical circuit embedded into the body, and a pressure transfer cavity having a first opening operatively in communication with the isolation diaphragm and a second opening operatively in communication with the MEMS pressure sensor. The method further includes coupling an isolation diaphragm to the distal end of the body; sealing the isolation diaphragm around the first opening; and, sealing the MEMS pressure sensor to the second opening; wherein the first opening is operatively in communication with the isolation diaphragm and the second opening is operatively in communication with the MEMS pressure sensor.  
         [0014]     Other objects, features, and advantages will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating exemplary embodiments, are given by way of illustration and not limitation. Many changes and modifications within the scope of the following description may be made without departing from the spirit thereof, and the description should be understood to include all such modifications.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention may be more readily understood by referring to the accompanying drawings in which:  
         [0016]      FIG. 1  is a perspective view of a pressure sensing module configured in accordance with a first preferred embodiment of the present invention;  
         [0017]      FIG. 2  is a top plan view of the pressure sensing module of  FIG. 1 ;  
         [0018]      FIG. 3  is a side elevation view of the pressure sensing module of  FIG. 1 ;  
         [0019]      FIG. 4  is a cross-sectional view of the pressure sensing module of  FIG. 1 , taken along line IV-IV of  FIG. 2 ;  
         [0020]      FIG. 5  is a front elevation view of the pressure sensing module of  FIG. 1 ;  
         [0021]      FIG. 6  is a perspective view of a second pressure sensing module configured in accordance with a second preferred embodiment of the present invention;  
         [0022]      FIG. 7  is a top plan view of the second pressure sensing module of  FIG. 6 ;  
         [0023]      FIG. 8  is a cross-sectional view of the second pressure sensing module of  FIG. 6 , taken along line VII-VII of  FIG. 7 ;  
         [0024]      FIG. 9  is a perspective view of a third pressure sensing module configured in accordance with a third preferred embodiment of the present invention;  
         [0025]      FIG. 10  is a top plan view of the third pressure sensing module of  FIG. 9 ;  
         [0026]      FIG. 11  is a cross-sectional view of the third pressure sensing module of  FIG. 9 , taken along line XI-XI of  FIG. 10 ;  
         [0027]      FIG. 12  is a perspective view of a fourth pressure sensing module configured in accordance with a fourth preferred embodiment of the present invention;  
         [0028]      FIG. 13  is a top plan view of the fourth pressure sensing module of  FIG. 12 ;  
         [0029]      FIG. 14  is a cross-sectional view of the fourth pressure sensing module of  FIG. 12 , taken along line XIV-XIV of  FIG. 13 ;  
         [0030]      FIG. 15  is a perspective view of a fifth pressure sensing module having a cavity mounted pressure sensor configured in accordance with a fifth preferred embodiment of the present invention;  
         [0031]      FIG. 16  is a top plan view of the fifth pressure sensing module of  FIG. 15 ;  
         [0032]      FIG. 17  is a cross-sectional view of the fifth pressure sensing module of  FIG. 15 , taken along line XVII-XVII of  FIG. 16 ;  
         [0033]      FIG. 18  is an electronics block diagram of the sensing portion of the MEMS pressure sensor configured in accordance with one preferred embodiment of the present invention;  
         [0034]      FIG. 19  is a signal timing chart of the MEMS pressure sensor of  FIG. 18 ;  
         [0035]      FIG. 20  is a perspective view of a sixth pressure sensing module having a side-mounted pressure sensor configured in accordance with a sixth preferred embodiment of the present invention;  
         [0036]      FIG. 21  is a cross-sectional view of the sixth pressure sensing module of  FIG. 20 , taken along line XVII-XVII of  FIG. 16 ; and,  
         [0037]      FIG. 22  is a block diagram of a pressure sensor system configured in accordance with one embodiment of the present invention in which the pressure sensing modules described herein may be used. 
     
    
       [0038]     Like numerals refer to like parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0039]      FIGS. 1-3  illustrate a pressure sensing module  120  configured in accordance with a preferred embodiment of the present invention. Pressure sensing (pressure sense) module  120  includes a pressure sensor capsule  130  that houses a Mechanical Electrical Mechanical System (MEMS) pressure sensor  160  (also referred to herein as a die), in a cavity  138  within its body. In one preferred embodiment of the present invention, the body of pressure sensor capsule  130  is constructed of a three-dimensional molded ceramic structure that supports the mounting of MEMS pressure sensor  160  in a lengthwise fashion to allow pressure sensor capsule  130  to achieve a form factor small enough to fit in a catheter tubing  102 . This form factor should be small enough to fit leads used in a variety of configurations and applications, such as a pacemaker lead. It should be noted that the various descriptions for the mounting of pressure sensor module  120  to catheter tubing  102  in the various embodiments described herein are equally applicable to other lead configurations.  
         [0040]     Pressure sensor capsule  130  includes an outer protrusion portion  134  to which an isolation diaphragm  150  is mounted. In one preferred embodiment of the present invention, isolation diaphragm  150  includes a raised portion  154  and a lip  152  mated to outer protrusion portion  134  and a shoulder  132 , respectively. Pressure capsule  130  is secured to catheter tubing  102  through a combination of a laser weld at an outer circumference  158  of isolation diaphragm  150 , and an adhesive support  170  that fills the gap between pressure sensor capsule  130  and catheter tubing  102 . The “top hat” design of isolation diaphragm provides a flat surface (i.e., lip  152 ) where a connection to the ceramic structure occurs. A plurality of ridges and valleys, or corrugations,  156  provides strain relief and an extended range of linearity for a larger extent of diaphragm excursion for isolation diaphragm  150 . As illustrated in  FIG. 5 , the top hat design also provides a means of isolating the pressure stress and the attachment stress because the flat center of sense area  502  of isolation diaphragm  150 , is held against the ceramic structure, with no additional stress applied from the connection joint. Further, an additional feature included in this design is that a step in the shape of outer protrusion portion  134  is provided at the distal end of the capsule. As shown in  FIG. 4 , this step provides a space to contain a gel adhesive  404 , which provides both a protective interface between the outer housing to pressure sensor capsule  130  and a way of presenting a smooth surface. Thus, the interface between isolation diaphragm  150  and the hard ceramic substrate of pressure sensor capsule  130  is protected by gel adhesive  404 . In one preferred embodiment of the present invention, an adhesive such as Room Temperature Vulcanizing (RTV) rubber may be used as gel adhesive  404  to secure isolation diaphragm  150  to pressure sensor capsule  130 . In other preferred embodiments of the present invention, epoxy, urethane, cyanoacrylic, or other suitably durable adhesive materials may be used as gel adhesive  404  to secure isolation diaphragm  150  to pressure sensor capsule  130 . In yet other preferred embodiments of the present invention, attachment methods such as metal brazing of isolation diaphragm  150  to a metal (e.g., brass) insert in the ceramic structure of pressure sensor capsule  130  may be used. Pressure sensor capsule  130  may also include a protective tube  261  attached to isolation diaphragm  150  made from such materials as titanium or nitenol. Protective tube  261  may be a metal ring attached via a laser weld or a ring made from an adhesive such as RTV.  
         [0041]     In one preferred embodiment of the present invention, MEMS pressure sensor  160  is attached in a flip chip attachment configuration. As further illustrated in  FIG. 4 , MEMS pressure sensor  160  does not directly contact the media to be measured outside pressure sensing module  120  as isolation diaphragm  150  isolates MEMS pressure sensor  160  from the media. In one preferred embodiment of the present invention, the pressure on isolation diaphragm  150  caused by the media is coupled to MEMS pressure sensor  160  through the pressure in an air filled pressure transfer cavity  148  located behind isolation diaphragm  150 . Pressure transfer cavity  148  includes a diaphragm-side opening  140  that faces isolation diaphragm  150  and a pressure sensor-side opening  142  that faces a sensing diaphragm (not shown) of MEMS pressure sensor  160 . Specifically, pressure transfer cavity  148  transfers the pressure sensed by isolation diaphragm  150  to the MEMS pressure sensor  160 . In one preferred embodiment of the present invention, to minimize package size, pressure transfer cavity  148  is shaped to transfer the pressure received at diaphragm-side opening  140  from the center of isolation diaphragm  150  and route it beneath MEMS pressure sensor  160  before it appears at the sensor at pressure sensor-side opening  142 . Thus, even though isolation diaphragm  150  is not directly facing the diaphragm of the MEMS pressure sensor  160 , the system works as pressure is transferred by an air column from isolation diaphragm  150  to the capacitive sensor on the diaphragm opening portion of MEMS pressure sensor  160 . In another preferred embodiment of the present invention, instead of air, an incompressible liquid such as oil may be used to fill pressure transfer cavity  148  and provide the transference of pressure. The incompressible liquid may include such liquids as silicon oil. In other preferred embodiments, including those described herein, the incompressible liquid may include other types of incompressible liquids and material. One benefit of using incompressible liquids instead of air is to reduce the amount of displacement by isolation diaphragm  150 , which increases the linearity of the response of the sensor. The use of the incompressible liquid also increases the dynamic pressure range of the sensor. In one preferred embodiment, the liquid is introduced into pressure transfer cavity  148  via a liquid fill opening once isolation diaphragm  150  is sealed to pressure sensing module  130 . The liquid fill opening may then be sealed using epoxy or other adhesive.  
         [0042]     The chamber defined by pressure transfer cavity  148  is sealed by an underfill material  408  that surrounds pressure sensor-side opening  142  and provides a seal for a plurality of electrical solder bumps  406  that is used to attach, as well as provide electrical connection between, MEMS pressure sensor  160  and pressure sensor capsule  130 . In one preferred embodiment, underfill material  408  is a specially engineered epoxy that is designed to both fill any undesired areas between MEMS pressure sensor  160  and pressure sensor capsule  130  and control the stress on the solder joints at the plurality of electrical bumps  406 . The stress may be caused by either a difference in thermal expansion between MEMS pressure sensor  160  and pressure sensor capsule  130 , or physical stresses caused by vibration or drop shock. Once cured, underfill material  408  absorbs the stress, reducing the strain on electrical bumps  406 , greatly increasing the life of the finished package. Underfill material  408  is typically applied using a capillary flow process where material is dispensed next to a bonded flip chip such as MEMS pressure sensor  160  and allowed to “wick” under the die. The bumped MEMS device is placed in cavity  138  of pressure sensor capsule  130 , with the sensing diaphragm (not shown) of MEMS pressure sensor  160  mated to pressure sensor-side opening  142 . Thus, the underfill material seals the pressure within pressure transfer cavity  148  and provides stability for die attach and corrosion resistance.  
         [0043]     As discussed above, in one preferred embodiment of the present invention, provisions for mounting MEMS pressure sensor  160  are made through solder bumping a plurality of contacts  136 . Electrical coupling of MEMS pressure sensor  160  to external devices such as pacemakers are made using coupling of: (i) a plurality of wire contacts  104  from a plurality of wires  106 , to (ii) an internal electrical circuitry  402 . Specifically, the ceramic structure of pressure sensor capsule  130  is a molded piece, with integral electrical connections of gold, tin or comparable electrical connective material forming internal electrical circuitry  402 . These connections pass through the ceramic structure to the location of MEMS pressure sensor  160 . In one preferred embodiment of the present invention, the location of the electrical connections to plurality of wire contacts  104  are on the proximal end of pressure sensor capsule  130 , which is the end opposite to the end on which isolation diaphragm  150  is located. It should be noted that any suitable type of electrical connections could be used, including an electrical connection made through twisted pair wires, flex circuits, single wires or similar means of electrical connection. In addition, connection between the various electrical contacts described herein may be made through surface solder, solder cups or conductive adhesives. Strain relief  107  may be provided to plurality of wires  106  through application of a flexible RTV. This seal, placed over the conductive connection of wires or flex circuits, provides both strain relief and additional corrosion protection. In another preferred embodiment, a flexible circuit carrying material (flex-circuit) may be used to provide the circuitry needed to connect MEMS pressure sensor  160  to plurality of wires  106 . In this embodiment, pressure sensor capsule  130  is separated into two or more pieces, MEMS pressure sensor  160  may be directly mounted on the flexible circuit carrying material and pressure sensor capsule  130  would then be mounted to the flexible circuit carrying material to create a sandwiched, layered construction.  
         [0044]      FIGS. 6-8  illustrate a second pressure sensing module  620  configured in accordance with a second preferred embodiment of the present invention. It should be noted that, unless otherwise noted, the description provided above for the embodiment of the pressure sensing module exemplified by pressure sensing module  120  is equally applicable to this second and other preferred embodiments.  
         [0045]     In this embodiment, a pressure sensor  660  is mounted within a cavity  638  in a pressure sensor capsule  630  via a plurality of bump connectors  636 . Plurality of bump connectors  636  is connected to a plurality of wire connectors  604  from a plurality of wires  606  via an electrical circuit  802 . Once connected, second pressure sensing module  620  may be mounted in interior  608  of a catheter  602   
         [0046]     Further, in this embodiment the attachment of an isolation diaphragm  650 , which includes a flat border  652 , is to a flat surface  632  within a rimmed portion  634  on pressure sensor capsule  630 . The pressure from the media to be measured is transferred from isolation diaphragm  650  to pressure sensor  660  through an air filled pressure transfer cavity  648  having a diaphragm-side opening  640  and a sensor-side opening  642 . Strain relief at the attachment surface is provided by a plurality of corrugations  656  stamped into the surface of isolation diaphragm  650 . Plurality of corrugations  656  also provides extended linearity for larger diaphragm displacement excursion. This design presents a simpler architecture, but does not isolate the adhesive junction between isolation diaphragm  650  to the ceramic structure of pressure sensor capsule  630  nor provide a smooth transition to the outer case. In this embodiment, isolation diaphragm  650  may be made larger, as compared to the top hat design of isolation diaphragm  150  of pressure sensor module  120 , to extend its active area to the maximum diameter of the ceramic structure of pressure sensor capsule  630 . Like the top hat design described above, isolation diaphragm  650  may be attached to the ceramic structure using adhesives or brazing methods. Further, in this embodiment, isolation diaphragm  650  may be hermetically sealed to catheter tubing  602  through a weld operation and be in an end mount configuration. Isolation diaphragm  650  may also include a protective ring  661  at its tip. Protective ring  661  may be a metal ring or a ring made from an adhesive such as RTV. Thus, isolation diaphragm  650  is located between the outer wall of catheter tubing  602  and protective ring  661 .  
         [0047]      FIGS. 9-11  illustrate a third pressure sensing module  920  configured in accordance with a third preferred embodiment of the present invention. In this embodiment, a pressure sensor  960  is mounted within a cavity  938  in a pressure sensor capsule  930  via a plurality of bump connectors  936 . Plurality of bump connectors  936  is connected to a plurality of wire connectors  904  from a plurality of wires  906  via an electrical circuit  1102 . Once connected, third pressure sensing module  920  may be mounted in interior  908  of a catheter  902   
         [0048]     In this embodiment, the attachment of an isolation diaphragm  950 , which includes a flat border  952 , is to a flat ceramic surface  932  on pressure sensor capsule  930 . The pressure from the media to be measured is transferred from isolation diaphragm  950  to pressure sensor  960  through an air filled pressure transfer cavity  948  having a diaphragm-side opening  940  and a sensor-side opening  942 . Similar to isolation diaphragm  650 , stress relief from the attachment surface is provided by a plurality of corrugations  956  stamped into the surface of isolation diaphragm  950 . This design, similar to the design described above for second pressure sensing module  620 , presents a simple architecture, and allows for the welding of isolation diaphragm  950  to the ceramic structure of pressure sensor capsule  930  because the diameter of isolation diaphragm  950  is as large as the diameter of the ceramic structure.  
         [0049]      FIGS. 12-14  illustrate a fourth pressure sensing module  1220  configured in accordance with a fourth preferred embodiment of the present invention. In this embodiment, a pressure sensor  1260  is mounted within a cavity  1238  in a pressure sensor capsule  1230  via a plurality of bump connectors  1236 . Plurality of bump connectors  1236  are connected to a plurality of wire connectors  1204  from a plurality of wires  1206  via an electrical circuit  1402 . Further, in this embodiment, instead of confining all the functionality of the pressure sensor into a single die, a separate die  1262  may be included in fourth pressure sensing module  1220  to provide such functionality as additional pre-processing, amplification, further post-processing of the measured signals, or additional sense function (temperature, chemical, &amp; biological).  
         [0050]     Once pressure sensor  1260  is connected with separate die  1262 , fourth pressure sensing module  1220  is then mounted in the interior  1208  of a catheter  1202 .  
         [0051]     In this embodiment, the attachment of an isolation diaphragm  1250  to pressure sensor capsule  1230  is similar to the connection of isolation diaphragm  652  to pressure sensor capsule  630 , as discussed above. In addition, the functioning of pressure sensor capsule  1230  is also similar to pressure sensor module  630 , with the exception that pressure sensor capsule  1230  may contain additional functionality, as noted above.  
         [0052]     A MEMS pressure sensor device can also be electrically coupled to a pressure sensing module through wire bonds as compared to the use of solder bumps as described for the above embodiments. Because the MEMS pressure sensor device has both its pressure sense diaphragm and electrical circuitry—including contacts for the electrical circuitry—on the same surface of a silicon die, the MEMS pressure sensor device is mounted with its pressure sense diaphragm/contact-side up. The contacts are then wire bonded to contacts on the pressure sensing module. In one preferred embodiment of the present invention, the cavity in which the MEMS pressure sensor is mounted is covered by the isolation diaphragm. Thus, this cavity needs to be sealed to prevent loss of transfer of the pressure affecting isolation diaphragm.  
         [0053]      FIGS. 15-16  illustrate a fifth pressure sensing module  1520  configured in accordance with a fifth preferred embodiment of the present invention, where a MEMS pressure sensor  1560  is mounted under an isolation diaphragm  1550  within an cavity  1570  of a pressure sensor capsule  1530 . This mounting configuration results in a smaller geometry device as neither a separate chamber for mounting MEMS pressure sensor  1560  nor an air pressure transfer cavity for transferring the pressure measured by isolation diaphragm  1550  are required. Further, this configuration also allows for the electrical coupling of MEMS pressure sensor  1560  to a plurality of contacts  1536  using either wire bonds or solder bumps. However, when MEMS pressure sensor  1560  is connected to pressure sensing module  1520  in a solder bumped configuration and an underfill material is used to adhere MEMS pressure sensor  1560 , a pressure channel that extends beyond the body of MEMS pressure sensor  1560  should be provided. This is because the side of MEMS pressure sensor  1560  on which sense diaphragm is located is both facing down and layered between the body of MEMS pressure sensor  1560  and pressure sensing module  1520 . The pressure channel provides a coupling of the pressure from isolation diaphragm  1550  through the underfill material to the sensing diaphragm of MEMS pressure sensor  1560 . Similar to the other embodiments described herein, to connect MEMS pressure sensor  1560  to an external device, a plurality of electrical connections  1702  are passed through to the proximal end of the ceramic body of pressure sensor capsule  1530 , as illustrated in  FIG. 17 . These electrical connections contact a plurality of wire contacts  1504  belonging to a plurality of wires  1506  in a catheter tubing  1502 .  
         [0054]      FIGS. 20-21  illustrate a sixth pressure sensing module  2020 , configured in accordance with a sixth preferred embodiment of the present invention, where a MEMS pressure sensor  2060  is mounted within a cavity  2070  of a pressure sensor capsule  2030  to implement a side-mounted pressure sensor. Cavity  2070  is displaced on the side of pressure sensor capsule  2030 , and an isolation diaphragm is  2050  is mounted on a ledge  2080 . This configuration does not require a separate cavity nor air pressure transfer chamber and allows for the MEMS pressure sensor  2060  to be electrically coupled to a plurality of contacts  2036  using either wire bond connections or a plurality of bumps, as discussed for MEMS pressure sensor  1560  of fifth pressure sensing module  1520 . A plurality of electrical connections  2002 , located in a notched portion  2082  of the body at the proximal end of pressure sensing capsule  2030  is used to contact a plurality of wire contacts from a plurality of wires in a catheter or lead for an external device (not shown). In one preferred embodiment, the plurality of wire contacts are soldered onto plurality of electrical connections  2002 .  
         [0055]     By mounting isolation diaphragm  2050  on the side of pressure sensing capsule  2030 , a slot, or wire channel,  2084  may be created in pressure sensing capsule  2030  to allow wires or other leads (such as pacing leads) to pass beneath pressure sensing capsule  2030 . These wires may include wires for other devices mounted downstream of the distal end of pressure sensing capsule  2030  on the catheter tubing. In one preferred embodiment, pressure sensing capsule  2030  is mounted on the end of the catheter tubing, and any leads may extend through slot  2084 . In another embodiment, the body of pressure sensing capsule  2030  may be completely enclosed in the catheter tubing, which will have a cut-out for isolation diaphragm  2050 .  
         [0056]     In one preferred embodiment, as shown in  FIG. 18 , the MEMS pressure sensors described herein includes a circuit  1800  containing both a sensing capacitor (Cp) and a reference capacitor (Cr). Pressure changes sensed by isolation diaphragm  150  will cause changes in the sensing capacitor Cp. This variation in capacitance is amplified by an Op-Amp in circuit  1800 . Circuit  1800  converts the capacitive pressure sensed into a signal, which in one embodiment is a pulse width modulated (PWM) digitally coded signal for a range of voltages (e.g. 0.5 to 4.5 volts) that represents the sensed pressure.  
         [0057]     As illustrated in  FIG. 19 , in one preferred embodiment the digital output is pulse width modulated. The communications is initiated with a marker pulse of a specific length. This is followed by a null pulse of predefined level (a logical one or zero). The pulse width temperature signal is transmitted, with a predefined minimum and maximum width proportional to a minimum and maximum temperature. This is followed by a second null pulse. The pressure signal is transmitted next with a minimum and maximum pulse width proportional to pressure. The sequence is then repeated. In other embodiments, digital output methods such as pressure only, re-sequenced timing, coded digital signals, Frequency Shift Keyed (FSK), and Pulse Amplitude Coded (PAC) signals can be used.  
         [0058]      FIG. 22  illustrates one exemplary application of the pressure sensor modules described herein, where a pressure sensor module  2220 , which includes a diaphragm  2250  configured similarly to the isolations diaphragms described above is coupled to a sense diaphragm  2262  in a MEMS pressure sensor  2260 . An amplification and processing circuitry  2264  reads the signals from sense diaphragm  2262 , and after processing them, delivers the processed signals to a device  2264 . For example, device  2264  may be a pacing device. Device  2264  may have other wires for use in providing pacing that are not shown in the figure, but which may be integrated into the lead in which pressure sensor module  2220  is mounted.  
         [0059]     The MEMS pressure sensor of the present invention provides the following features:  
         [0060]     1. Accuracy: MEMS devices are micro-machined structures controlled through precise physical and chemical attributes. The accuracy achieved from using these techniques provide highly accurate reproduction and consistency.  
         [0061]     2. Parasitic effects: Since the MEMS capacitive sensing diaphragm is integrated with the sensing detector and amplifier, the parasitic wiring connection is eliminated or rendered insignificant.  
         [0062]     3. Stability: The MEMS system utilizes a combination reference diaphragm and sensing diaphragm. The ratio of the two elements reduces manufacturing variations, long-term drift and thermal effects.  
         [0063]     4. Material fatigue-based drift: The MEMS silicon structure shows insignificant drift with time as compared to metal diaphragm and wired interconnect.  
         [0064]     The embodiments described above are exemplary embodiments. Those skilled in the art may now make numerous uses of, and departures from, the above-described embodiments without departing from the inventive concepts disclosed herein. Various modifications to these embodiments may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments. Thus, the scope of the invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Accordingly, the present invention is to be defined solely by the scope of the following claims.