Long-term in vivo experience of an electrochemical sensor using the potential step technique for measurement of mixed venous oxygen pressure

An implantable amperometric blood oxygen sensor was developed to improve ra te adaptation of heart pacemakers. Two different working electrode material s in direct contact with the blood were rested, smooth glassy carbon and go ld. Reference electrodes of Ag/AgCl and porous pyrolytic carbon were evalua ted. A counter electrode being the titanium housing of the pulse generator was partly coated with carbon. An implantable pacemaker system with chronoc oulometric oxygen detection was developed. Heart synchronous potential step s were periodically applied to the 7.5 mm(2) working electrode in the atriu m. Both single and double potential step techniques were evaluated. The oxy gen diffusion limited current was used to calculate the stimulation rate. B ench tests and studies on 31 animals were performed to evaluate long-term s tability and biocompatibility. In five dogs, the AV node was destroyed by R F ablation to create a realistic animal model of a pacemaker patient. Senso r stability and response to exercise was followed up to a maximum implantat ion time of 4 years. Post-mortem examinations of the electrode surfaces and tissue response were performed. The results show that a gold electrode is more stable than glassy carbon. The Ag/AgCl reference was found not to be b iocompatible, but activated carbon was stable enough for use as reference f or the potentiostat. Double potential steps stabilize the sensor response i n comparison to single steps. Blood protein adsorption on the gold surface decreased the oxygen transport but not the reaction efficacy. No adverse ti ssue reactions were observed. (C) 1998 Elsevier Science S.A. All rights res erved.