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Abstract: Rapid determinations of bloodgas parameters are desirable in today's clinical care. This calls for continuousin vivo measurements with miniaturised electrodes. In this study we have replaced the commonly used glass pH sensor in a pCO2-electrode of the conventional type with a monocrystalline antimony electrode. The disadvantages of the glass electrode, such as instability and fragility when miniaturised, are of minor importance for an antimony electrode. The results, which confirm that the antimony pCO2-electrode does not need frequent calibration, have been obtained using macroscale electrodes. However, the favourable characteristics of the antimony electrode are known to be maintained even after miniaturisation. Therefore, it seems possible that in the future we shall be able to measure bloodgas parameters such as pCO2 in vivo with reasonable accuracy.

Based on monocrystalline antimony we have developed a multipoint tissue surface pH electrode. The six electrodes were produced by spark cutting from a large antimony single crystal. The electrodes were then cast in epoxy resin in a ring shaped structure which fitted around the MDO oxygen electrode. The antimony electrode was ground and polished to expose an undisturbed closely packed crystal plane of antimony to the measuring solution. Before and after monitoring periods standardization was performed in TRIS buffers of pH 6.72, 7.32 and 7.74 at 37° C.

The membranes of the pO2 sensor in a conventional blood gas analyser and a commercially available catheter pO2electrode were coated with a glutardialdehyde stabilized heparin layer. It was analysed whether or not the "non-thrombogenic" heparin surface could improve the accuracy of in vitro pO2 registration and continuous in vivo paO2 monitoring. The in vivo experiments were performed on anaesthetized and non-anticoagulated dogs. Two identical blood gas analysers were used, one furnished with a heparin coated pO2 membrane and the other with a standard pO2 membrane. In vitro pO2 determinations exhibited - when simultaneously analysing the same blood sample - identical mean values. For repeated pO2 determinations, however, the standard deviation was significantly lower on the analyser furnished with a heparinized membrane. The non-heparinized catheter pO2 electrode showed a marked deterioration in accuracy with time as compared with blood-gas analysis on reference blood samples. Falsely low paO2 values were presented. The heparinized catheter electrode monitored values in full agreement with those of the conventional blood gas analyser using standard membranes.It is concluded that the heparin surface on the oxygen diffusible membranes diminishes the variability of the diffusion characteristics both at contact with anticoagulated blood in vitro and during in vivo paO2 monitoring. The "non-thrombogenic" heparin surface prevents diffusion impeding clot formations when an arterial catheter pO2 sensor is used.

Surface-heparinized catheter Po2electrodes were used for continuous intraarterial monitoring in non-heparinized dogs. During one-day experiments the read-out of the electrode monitoring unit was compared with that of conventional blood gas analysis. Furthermore, the electrodes were studied after long-term implantation. The intra-arterial Po2electrodes have previously been reported to be linear in the gas phase, water phase and in heparinized blood. The present study demonstrates that the catheter electrodes remain linear after surface heparinization. As compared with the conventional blood gas analysis it was found that the desk analyser in the range Pao2> 18 kPa underestimated the actual Pao2as compared to the invasively monitored Pao2, but in the range below 7 kPa the desk analyser overestimated the Pao2. Surface-heparinized Po2catheter electrodes were studied after up to 23 days of implantation. No coagulation phenomenon was found around the catheters, neither macroscopically nor by scanning electron microscopy. The electrical function withstood 7 days of implantation. After more extended periods the electrical leads were broken without any damage to the catheter itself. When compared with acutely implanted surface-heparinized electrodes, there was no difference in monitoring characteristics between the acutely implanted and the long-term implanted Po2electrodes.

Surface-heparinized catheter Po2 electrodes were used for continuous intraarterial monitoring in non-heparinized dogs. During one-day experiments the read-out of the electrode monitoring unit was compared with that of conventional blood gas analysis. Furthermore, the electrodes were studied after long-term implantation. The intra-arterial Po2 electrodes have previously been reported to be linear in the gas phase, water phase and in heparinized blood. The present study demonstrates that the catheter electrodes remain linear after surface heparinization. As compared with the conventional blood gas analysis it was found that the desk analyser in the range Pao2 > 18 kPa underestimated the actual Pao2 as compared to the invasively monitored Pao2, but in the range below 7 kPa the desk analyser overestimated the Pao2. Surface-heparinized Po2 catheter electrodes were studied after up to 23 days of implantation. No coagulation phenomenon was found around the catheters, neither macroscopically nor by scanning electron microscopy. The electrical function withstood 7 days of implantation. After more extended periods the electrical leads were broken without any damage to the catheter itself. When compared with acutely implanted surface-heparinized electrodes, there was no difference in monitoring characteristics between the acutely implanted and the long-term implanted Po2 electrodes.