On-line arterial blood gas analysis with optodes: current status.

OBJECTIVES: To summarize the rationale for and the principles of blood gas and pH measurement with photochemical sensors (optodes) placed in the arterial line--either intravascularly (in vivo) or extravascularly (ex vivo). To review the specific problems that occur with in vivo measurement; the clinical data that have been obtained with continuous intravascular and on-demand extravascular systems; and, the role of this technology in the intensive care unit. METHODS AND
RESULTS: The principles of absorbance and fluorescent optical sensors are described. The accuracy of intravascular PO2 optodes can be affected by thrombosis, the wall effect (if the sensor touches the arterial wall it may read tissue values) and reduced blood flow past the sensor. Current optical pH, PCO2 and PO2 probe/cannula designs, including hybrid probes with electrochemical PO2 sensors, have not yet fully overcome these problems of the intravascular milieu. On-demand blood gas monitors that locate the optodes extravascularly, within the radial artery line, avoid these intravascular measurement problems. On-demand systems can have accuracy comparable to conventional laboratory blood gas analyzers. With either intravascular or extravascular measurement large patient studies are lacking and the relevant cost benefit ratios are not known.
CONCLUSION: Before intravascular monitors can be used routinely for clinical care, reliability, consistency and accuracy will have to be demonstrated in large and widely divergent patient groups. Extravascular on-demand blood gas analysis is accurate, allows trend monitoring of blood gases and decreases the risk of infection, the therapeutic decision time and patient blood loss. As large patient studies are lacking the clinical role of on-line blood gas analysis cannot be clearly delineated.