The evolution of the modern pacemaker lead may be best understood through a review of the development of the pacemaker itself.  The earliest pacemaker simply delivered stimulation pulses at a fixed repetition rate.  These were known as "asynchronous" or fixed rate pacemakers. " Unipolar stimulation" was achieved by delivering electrical pulses between the tip electrode of the lead and the pacemaker case.  Due to their asynchronous nature, the stimulation pulse often competed with natural rhythms.  The "demand" pacemaker included sense amplifiers to enable sensing of natural rhythms.  In the presence of natural cardiac signals, the demand pacemaker would inhibit a stimulation pulse.  In the absence of natural cardiac signals, the demand pacemaker would deliver stimulation pulses.  However, sensing between the tip and the case (referred to as "unipolar sensing") sometimes detected myopotentials; that is, the electrical signals generated by the pectoral muscle tissue.  The sensing of myopotentials can falsely inhibit the demand pacemaker. 
To solve this problem, bipolar leads were developed.  A bipolar lead has two electrodes located within the heart: a tip electrode and a ring electrode.  The ring electrode is located approximately one-half inch proximally from the tip electrode.  This configuration enabled a significant reduction of myopotential sensing, as well as eliminating any pectoral stimulation.  However, depending on the orientation of the lead and the direction of the wavefront, bipolar sensing of cardiac signals would sometimes result in signals that are smaller than unipolar signals.  The arrival of unipolar/bipolar programmability in demand pacemakers enabled the physician to noninvasively reprogram the pacemaker's polarity to accommodate the patient's changing conditions. 
Modern pacemakers can now alter their stimulation rate to accommodate the patient's exercise or stress needs.  These rate-responsive pacemakers employ a variety of sensors to determine the physiological condition of the patient.  Physiologic sensors may be located on the pacemaker lead or within the pacemaker itself.  Physiologic sensors in use today include: minute volume, temperature, oxygen saturation of the blood, respiration, stroke volume, ventricular gradient, activity, and pre-ejection period (PEP), etc. 
The ideal physiologic sensor would be one that provides information about the patient's exercise level or workload, and ideally, will operate in a closed loop fashion.  In other words, it should operate to minimize the divergence from the ideal operating point.  For this reason, the development of a sensor for monitoring blood oxygen saturation for use with an implantable pacemaker is desirable.  Oxygen saturation of the blood provides a direct indication of oxygen consumption of the patient during exercise.  Furthermore, oxygen saturation has an inverse relationship with pacing rate.  That is, as oxygen saturation decreases due to exercise, the pacing rate will increase so that the divergence from the optimum point is minimized. 
The development of an oxygen saturation sensor and circuitry for operating such a sensor incorporated into a pacemaker lead is shown in several references.  See, for example, U.S. Pat.  No.  4,399,820, to Wirtzfeld et al.;  U.S. Pat.  No.  4,750,495, to Moore et al.;  and U.S. Pat.  No.  4,815,469, to Cohen et al. 
Unfortunately, problems still exist which have heretofore hindered a widespread clinical use of such a pacing system.  One of the major difficulties in developing an oxygen sensing system has been to develop a pacemaker lead having a reliable, hermetically enclosed sensor that can be located within the heart.  The typical oxygen sensor in combination with a pacemaker lead includes one or more light-emitting diodes (LEDs), phototransistors and resistors.  The prior art suffers from complex circuit designs, which designs are difficult to miniaturize and hermetically encapsule.  Also, the process of providing a reliable weld to a relatively large area without damaging the sensor electronics is not an easy task. 
Another problem is protecting the oxygen sensor circuitry from overvoltages, such as those seen during cardioversion, defibrillation and electrosurgery.  In the event of a high voltage cardioversion or defibrillation pulse, the integrated circuits could be destroyed losing all rate-responsive functionality. 
Another potential problem occurs when using one or both of the stimulation conductors as the sensor return conductor.  Should the sensor fail or interfere with the stimulation electrodes' functionality, pacing of the heart may be jeopardized.  For example, bodily fluids may intrude into the sensor circuitry or a lead fracture may occur at the sensor connection (particularly given the periodic forces that are regularly placed on the lead as it moves or flexes with the heart).  Under these failure modes the stimulation electrodes could be impaired or even destroyed, thus losing all the functionality of the lead. 
Another disadvantage of oxygen sensor designs that use the same conductors as for stimulating, is that they exhibit rectification of electrosurgery signals.  Thus, the current oxygen sensor designs do not meet the proposed international Cenelac standard.  Therefore, it is an objective of the present invention to provide a simple hermetic packaging technique for a physiological sensor in a pacemaker lead, particularly an oxygen saturation sensor. 
It is an objective of the present invention to provide a packaging technique for a physiological sensor in a pacemaker lead that does not interfere with basic operation of the pacemaker. 
It is an objective of the present invention to provide a physiological sensor in a pacemaker lead that is not affected by electrosurgery signals, a cardioversion pulse, or a defibrillation pulse. 
It is an objective of the present invention to provide a physiological sensor in a pacemaker lead that permits either unipolar or bipolar stimulation. 
It is further an objective of the present invention to provide a reliable sensor circuit with minimum components which will minimize the overall diameter of the lead. 
Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.