this disclosure relates to a rate responsive leadless cardiac pacemaker or other leadless biostimulator . the leadless biostimulator can be implanted within a chamber of the patient &# 39 ; s heart . instead of measuring central venous temperature using a temperature sensor on a lead extending from the biostimulator housing , embodiments of the rate responsive leadless biostimulator of this disclosure can employ a temperature sensor supported by the biostimulator housing . the leadless biostimulator of this disclosure can use the measured temperature to adjust the rate of its electrical stimulation signals using a controller disposed within the housing . some embodiments of a leadless biostimulator may include a hermetic housing disposed in a chamber of a human heart , a battery disposed in the housing , at least two electrodes supported by the housing , a temperature sensor supported by the housing and a controller disposed in the housing . the controller can be adapted to sense intracardiac information using the two electrodes and to deliver stimulation energy from the battery to the electrode using temperature information from the temperature sensor . the temperature sensor may be supported by the leadless biostimulator housing in any manner consistent with the thermal time constant requirements of the system . the temperature sensor may be a thermistor or a semiconductor temperature sensor incorporated into the controller . in order to use central venous temperature as the metabolic parameter for a rate response algorithm , the leadless biostimulator may be able to sense and respond to changes in central venous temperatures within a clinically significant period of time , such as less than 30 seconds . since the leadless biostimulator will be disposed in contact with the patient &# 39 ; s blood within the patient &# 39 ; s heart , the biostimulator design can provide a heat conduction path from the blood to the temperature sensing element whose time constant is sufficiently small to allow the sensor to reach its final value within the chosen clinically significant time . thus , for example , if the desired clinically significant time is 30 seconds , the thermal time constant of the temperature sensing components might be chosen to be 10 seconds . fig1 shows a leadless cardiac pacemaker or leadless biostimulator 1 . biostimulator 1 can include a housing 6 having a header section 2 made from an electrically insulating material and extending from a hermetic can 3 and 4 made from , e . g ., titanium . can section 3 can be electrically insulated , and can section 4 may not insulated so that it can serve as an electrode . an electronics compartment within the cans 3 and 4 can contain the electronic components necessary for operation of the biostimulator , including a battery and a controller . a helical fixation device 5 can extend through a passage in can 3 into and through header 2 as shown . in some embodiments , the fixation device 5 can comprise an electrode , and in other embodiments a distal electrode can be separate from the helical fixation device . other details of leadless biostimulators may found in the copending applications referenced above and incorporated by reference herein . in the embodiment of fig1 , a thermistor 7 can disposed in header 2 . the thermistors can include at least two thermistors leads for electrically connecting the thermistors 7 to the controller of the leadless biostimulator . in this embodiment , at least one of the thermistor leads can extend through a feedthrough in can section 3 to a controller within the can . the other thermistor lead may be electrically connected to the can , or can alternatively pass through a feedthrough into the interior of the can . in this embodiment , thermistor 7 can be in contact with an interior surface of header 2 and thus can be in thermal contact with blood surrounding the biostimulator through header 2 . the controller inside housing 6 can be adapted to sense intracardiac information using electrodes 4 and 5 and to deliver stimulation energy from the battery to electrodes on the leadless biostimulator using temperature information from the thermistor 7 . in some embodiments , the rate of stimulation provided by a pacemaker may need to be adjusted to match the level of the patient &# 39 ; s physical activity or temperature . for example , the temperature information can determine the temperature of the patient and adjust the rate of stimulation to account for temperature variations due to fever or exercise . in the embodiment of fig2 , the temperature sensor can be a thermistor 126 disposed within a hermetic can 100 of the housing . as shown in this cross - sectional view , thermistor 126 can be bonded so as to be thermally connected to an inside surface of hermetic can 100 , and the thermistors can connect to asic controller 120 via leads 128 and substrate 124 . thus , thermistor 126 can be configured to sense the temperature of blood surrounding the biostimulator through housing 100 . other elements within hermetic can 100 include the asic substrate 124 , other electronic components 122 , and a battery ( not shown ). at least two electrodes can be supported by , and exterior to , the housing as in the embodiment of fig1 . in some embodiments , the asic 120 can be adapted to sense intracardiac information using the electrodes and to deliver stimulation energy from the battery to one of the electrodes using temperature information from the thermistor 126 . in the embodiment of fig3 , the temperature sensor can be a semiconductor temperature sensor integrated into asic 124 . a thermally conductive pad 125 can extend from the temperature sensor in asic 124 to an interior surface of housing 100 . thus , the temperature sensor can sense the temperature of blood surrounding the biostimulator through housing 100 with conductive pad 125 . as in the embodiment of fig2 , at least two electrodes can be supported by , and exterior to , the housing . the asic 120 can be adapted to sense intracardiac information using the electrodes and to deliver stimulation energy from the battery to one of the electrodes using temperature information from the integrated temperature sensor . the embodiment of fig4 is similar to that of fig3 , but omits the thermally conductive pad . thus , the temperature sensor integrated into asic 120 senses the temperature of blood surrounding the biostimulator via the thermal resistance between the asic 120 and the can 100 . similarly , in this embodiment , the asic 120 can be adapted to sense intracardiac information using the electrodes and to deliver stimulation energy from the battery to one of the electrodes using temperature information from the integrated temperature sensor . tests were conducted to see how thermal response times compared among some of these embodiments . fig5 a and 5b show a first prototype assembly having a housing 200 made from a tube capped off at ends 202 and 204 with silicone . the tube can be an 8 mm stainless steel tube , for example . a thermistor 206 was encapsulated with cyanoacrylate to bond it to the inside of housing 200 within the silicone at end 204 . silicone grease was applied between the thermistor and the housing wall contact point . wires 208 extending from thermistor 206 were insulated . the cavity 210 within housing 200 was filled with water . housing 200 had a 7 mm diameter and 25 . 5 mm length . the silicone at end 204 extended 6 . 5 mm into housing 200 . two beakers were filled with 500 ml of distilled water and immersed a thermistor in each beaker to monitor temperature . the second beaker was then placed on a hot plate / stirrer and the temperature was adjusted approximately 10 ° c . higher than the first beaker . the stirrer ran to agitate the solution . the prototype assembly was immersed in the first beaker for at least 5 minutes and transferred the prototype assembly to the second beaker in less than 1 second . the temperature was recorded from all three sensors ( one on each beaker and one on the prototype assembly ) for a sample rate greater or equal to 1 second / sample for at least 1 minute after transferring the prototype assembly to the second beaker . it was verified that the temperature in the second beaker does not change by more than 5 % during the course of the procedure . the measured temperatures were compared with a thermal model based on the thermal circuit shown in fig6 . the results are shown in fig7 . the model time constants are derived by minimizing the rms error in tm ( t )− tt ( t ) over all time . for each experiment the derived time constants are given . in this case the thermal time constant between the bath and thermistor was determined to be 4 . 3 seconds . let : t = time since immersion in bath ; th = bath temperature ; tc = start temperature ; tm ( t )= thermistor temperature , measured ; tt ( t )= thermistor temperature , simulated ; te ( t )= adhesive + silicone temperature , simulated ; τbt = bath - to - thermistor time constant ; τbe = bath - to - adhesive + silicone time constant ; τte = thermistor - to - adhesive + silicone time constant . another test was conducted using a prototype similar to that of fig5 a - 5b but using much less cyanoacrylate adhesive to bond the thermistor to the can . the same test protocol was used as in example 1 . the results are shown in fig8 . the thermal time constant between the bath and thermistor was determined to be 3 . 0 seconds . a test was conducted using the test protocol of example 1 with a prototype similar to that of fig5 a - 5b but filled with air instead of water . the results are shown in fig9 . the thermal time constant between the bath and thermistor was determined to be 4 . 0 seconds and therefore the thermal mass of the battery is not expected to greatly change these results . a test was conducted using the test protocol of example 1 with a prototype similar to that of fig5 a - 5b but with an air gap between the silicone plug and the adhesive / thermistor , and using only a very small amount of cyanoacrylate adhesive to bond the thermistor to the can . the results are shown in fig1 . the thermal time constant between the bath and thermistor was determined to be 3 . 4 seconds . a test was conducted using the test protocol of example 1 with a prototype similar to that of fig5 a - 5b but with the thermistor floating in , not bonded to , the can and with the can filled with air instead of water . the results are shown in fig1 . the thermal time constant between the bath and thermistor was also determined to be 5 . 5 seconds . a test was conducted using the test protocol of example 1 with a prototype similar to that of fig5 a - 5b but with the thermistor floating in the can , with the insulated wires leading from the thermistor contained within a straw to further insulate the wires from the bath temperature , and with the can filled with air instead of water . the results are shown in fig1 . the thermal time constant between the bath and thermistor was determined to be 11 . 8 seconds . a test was conducted using the test protocol of example 1 with a prototype similar to that of fig5 a - 5b but with the thermistor bonded to a semiconductor chip within the can . the chip dimensions were 4 mm × 5 mm × 20 mils . the semiconductor / thermistor assembly was wrapped in one layer of polyimide tape , and the thermistor wires were thermally insulated from the bath using a straw . the can was filled with air , not with water . in this simulation , the model was altered to allow the semiconductor chip ( simulating an asic ) to gain heat from the bath at a first time constant , the thermistor to gain heat from the asic at a second time constant , and the thermistor to lose heat to the wires at a third time constant . the thermal model is shown in fig1 . the results of this test are shown in fig1 . the bath to asic time constant was determined to be 12 . 9 seconds . this test suggests that the thermal time constant between an integrated aisc thermal sensor with no specific thermal connection between the can and asic provides acceptable thermal results within the housing of a leadless cardiac pacemaker . in some embodiments , the temperature sensor may be a thermistor , a semiconductor temperature sensor , or part of an asic containing the controller . the sensed temperature is used by the leadless stimulator control circuitry to adjust a rate of electrical stimulation provided by the biostimulator to the patient &# 39 ; s heart . the temperature sensor may sense temperate in a range between 36 ° c . to 42 ° c . the low end of the temperature range allows for normal body temperature ( 37 ° c . ), less circadian variations and less a dip in temperature due to exercise . the high end of the temperature range allows for normal body temperature , plus fever , plus the increase in temperature due to exercise . the resolution may be about 0 . 023 ° c . this represents better than ⅕ th of the smallest anticipated dip amplitude during exercise ( 0 . 15 ° c .). unless defined otherwise , all technical terms used herein have the same meanings as commonly understood by one of ordinary skill in the art of cardiac technologies . specific methods , devices , and materials may be described in this application , but any methods and materials similar or equivalent to those described herein can be used in the practice of the present invention . while embodiments of the invention have been described in some detail and by way of exemplary illustrations , such illustration is for purposes of clarity of understanding only , and is not intended to be limiting . various terms have been used in the description to convey an understanding of the invention ; it will be understood that the meaning of these various terms extends to common linguistic or grammatical variations or forms thereof . it will also be understood that when terminology referring to devices , equipment , or drugs that have been referred to by trade names , brand names , or common names , that these terms or names are provided as contemporary examples , and the invention is not limited by such literal scope . terminology that is introduced at a later date that may be reasonably understood as a derivative of a contemporary term or designating of a hierarchal subset embraced by a contemporary term will be understood as having been described by the now contemporary terminology . further , while some theoretical considerations have been advanced in furtherance of providing an understanding of the invention , the claims to the invention are not bound by such theory . moreover , any one or more features of any embodiment of the invention can be combined with any one or more other features of any other embodiment of the invention , without departing from the scope of the invention . still further , it should be understood that the invention is not limited to the embodiments that have been set forth for purposes of exemplification , but is to be defined only by a fair reading of claims that are appended to the patent application , including the full range of equivalency to which each element thereof is entitled .
a leadless cardiac pacemaker comprises a housing , a plurality of electrodes coupled to an outer surface of the housing , and a pulse delivery system hermetically contained within the housing and electrically coupled to the electrode plurality , the pulse delivery system configured for sourcing energy internal to the housing , generating and delivering electrical pulses to the electrode plurality . the pacemaker further comprises a temperature sensor hermetically contained within the housing and adapted to sense temperature information , wherein the pacemaker can control electrical pulse delivery at least partly based on the temperature information .