referring now to fig1 an implantable pacemaker 10 constructed in accordance with this invention includes an impedance circuit 12 , an atrial sense circuit 14 , an atrial pace circuit 16 , a ventricular sense circuit 18 and a ventricular pace circuit 20 . these circuits are connected to a microprocessor 22 by a bus 24 . the microprocessor 22 is further connected by another bus 26 to a telemetry circuit 28 . the pacemaker is connected to the heart 30 of a patient by two bipolar leads , atrial lead 32 and ventricular lead 34 . in this embodiment , the pacemaker 10 is operated in a dual chamber mode , and in this mode it senses cardiac activity in the atrium and ventricle of the heart 30 via the bipolar leads 32 , 34 . the pacemaker 10 further senses through these electrodes , as described more fully below , an impedance indicative of the metabolic demand of the patient . the microprocessor process the information descriptive of the cardiac activity and the impedance and , when required , generates cardiac pacing pulse commands to the pace circuits 16 and 20 . these circuits then generate appropriate pacing pulses . details of this operation are described for example , in u . s . pat . no . 5 , 441 , 523 , incorporated herein by reference . typically , in the prior art , the impedance circuit was coupled to one of the leads . for example , in the above - mentioned u . s . pat . no . 5 , 441 , 523 the impedance measurement was made using the ventricular lead . however , as described above , because of problems with one or both electrodes of these leads , the impedance circuit 12 may not derive a proper metabolic demand signal . in order to overcome his problem , the present invention is arranged and constructed to allow the impedance circuit 12 to determine the metabolic demand by making other measurements . more particularly , as seen in fig2 the impedance circuit 12 includes a switch network 40 , an electrode selection control 42 , an impedance determinator circuit 44 and a current source 46 . the switch network 40 is connected to the atrial tip electrode 32t and the atrial ring electrode 32r by lead 32 , the ventricular ring electrode 34r and the ventricular tip electrode 34t by lead 34 . in order to make an impedance measurement , in response to control signals from electrode control 42 , the switch network 40 applies a current from current source 46 to one of the electrodes and the parameter case , and provides the response sensed between another of the electrodes and the pacemaker casing to impedance determinator 44 . the determinator 44 then generates an impedance parameter indicative of this impedance . the impedance parameter is sent to the microprocessor 22 via bus 24 . the microprocessor 22 includes an impedance / minute volume converter 50 to generate for each impedance parameter a corresponding mv parameter . the mv parameter is transmitted to a mapping circuit 52 which convert the mv into a corresponding metabolic indicated rate , mir . the mir is fed to a state machine 54 . the state machine 54 also receives as inputs , the atrial and ventriculars sense signals as and vs from the atrial sen circuit 14 and the ventricular sense circuit 18 , respectively . the state machine 54 generates , when required , atrial pulse commands ap and ventricular pulse commands vp for the atrial and ventricular pace circuits 16 , 20 , respectively . referring now to fig3 the switching network 40 consists of a multiplexer 40a and a demultiplexer 40b . these are connected to the four electrodes 32t , 32r , 34t , 34r as shown . the multiplexer 40a receives its input from the current source 46 . the output of demultiplexer 40b is fed to impedance determinator 44 . the determinator 44 stores readings from the electrodes into a memory 48 , as described below . memory 48 may be part of the circuit 12 or may be a memory associates with microprocessor 22 . the operation of the invention shall now be described in conjunction with the flow chart of fig4 . initial programming values and other parameters are loaded into the microprocessor 22 during the initializing of the device ( step 200 ). this initial set up also includes designation a preferred set of electrodes for the impedance measurement . for example , as discussed above , initially the impedance parameter may be measured by applying current to the ventricular ring electrode 34r . in other words , initially the electrode controller 42 sets the multiplexer 40a so that the current from source 46 flows to the electrode 34r . from this electrode 34r current flows through the cardiac tissues and across the pleural cavity to the casing 62 of pacemaker 10 . multiplexer 40b is set by controller 42 so that the ventricular tip electrode 34t is connected to the impedance determinator 44 . the determinator 44 determines the corresponding transthoracic impedance as described above . following the set step 200 , a timer 60 is initiated . after timer 60 times out ( step 202 ) a signal is generated by the impedance determinator to perform a reliability check on the electrode system . the timer 60 may be set for example to initiate the reliability check every 30 days . this reliability check is performed as follows . in step 204 the multiplexer 40a is set to feed current i to a first electrode and all or some of the other electrodes are connected by the multiplexer 40b sequentially to obtain corresponding impedance measurements by impedance determinator 44 ( step 206 ). the results are stored in step 208 into memory 48 . in step 210 a check is performed as to whether the multiplexer 40a is at the last electrode . if not , than in step 212 the multiplexer 40a is advanced to the next electrode and steps 204 - 210 are repeated . after all the data has been collected and stored in memory 48 , then in step 214 the microprocessor 22 looks up all the impedance values and selects the best impedance value and the corresponding electrode pair that was the source of this value ( step 216 ) determined in accordance with preselected criteria . in step 218 a check is performed to determine if the best value is above a certain preselected threshold th . if it is , then in step 220 the multiplexer 40a and the multiplexer 40b are set to the pair of electrodes that resulted in the best impedance reading and then the process is started over again . if in step 218 it is determined that even the best impedance value is too low , then in step 222 a signal is sent to the state machine 54 to turn the rate responsive feature off and to pace at a preselected standby rate . this mode is continued for a preselected time after which the reliability check is repeated . this may be continued until a suitable electrode pair is found or suspended after a suitable electrode pair cannot be found for n ( e . g ., 5 ) times ( step 224 ). fig5 shows another embodiment invention . in this embodiment an implantable cardioversion / defibrillation device 110 is provided consisting of impedance circuit 112 , atrial sense circuit 114 , atrial pace circuit 116 , ventricular sense circuit 118 , ventricular pace circuit 120 , microprocessor 122 , bus 124 , bus 126 , telemetry circuit 128 and defibrillator 170 . device 110 is coupled to the heart 30 via electrodes 132 , 134 and 172 . the elements of fig5 which are common to pacemaker 10 operate in the manner described above . the cardioversion / defibrillator circuit 170 is controlled by microprocessor 122 to generate defibrillator pulses to electrode ( s ) 172 . advantageously , the electrode ( s ) 172 is also connected to the impedance circuit 112 . the multiplexer 40a and multiplexer 40b are modified to accept an extra connection from this electrode 172 . in this manner , the impedance circuit is able to determine impedance readings not only from the atrial and ventricular leads but also through the defibrillation lead ( s ). although the invention has been described with reference to several particular embodiments , it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention . accordingly , the embodiments described in particular should be considered exemplary , not limiting , with respect to the following claims .
in an implantable pacemaker or cardioversion / defibrillation device , the optimal electrodes for determining a pacing parameter are found . a test signal is sequentially switched and a response from each of the remaining electrodes is determined . from those responses an optimal electrode or electrode pair is selected and used for pacing parameter determination . if an optimal electrode is not found , then a fall back pacing rate is used .