EMERGING TECHNOLOGIES Continuous Intravascular Blood Gas Monitoring

N AVERAGE adult at rest consumes approximately A 100 million trillion molecules of oxygen per second; with virtually no capacity for oxygen storage, cessation of oxygen delivery quickly results in irreversible cellular damage. The human cardiopulmonary system functions remarkably well in healthy individuals, maintaining the mitochondrial oxygen tension (greater than 1 mmHg) required for aerobic metabolism. In the critically ill and in anesthetized patients, however, any acute dysfunction may result in hypoxia. The need for oxygen measurement in blood was long apparent, but it was not until Clark developed the polarographic electrode in the 1950s that this became routinely available in the clinical setting.’ With the addition of the carbon dioxide electrode by John Severinghaus in 1958, the blood gas analyzer was developed.2 Today it is difficult to imagine treating patients in an acute care setting without the immediate availability of blood gas analysis. Still, there are limitations in using intermittent measurement to evaluate such a dynamic system. Ideally, blood gas data would be provided continuously so that oxygenation, ventilation, and acid-base status could be monitored and adverse changes treated before impacting the patient’s overall condition. Decreasing the number of blood samples drawn would provide other benefits such as decreasing the risk of iatrogenic infection or anemia, exposure of personnel to blood-borne diseases, and the possibility of analysis error. Continuous noninvasive monitoring of oxygenation and ventilation with pulse oximetry and capnography represents a substantial improvement in patient monitoring but does not replace arterial blood gas and pH analysis. For nearly two decades efforts have been made to adapt the electrochemical sensors used in in vitro blood gas measurements to continuous in vivo monitoring, but these efforts have met with limited success.3-5 Over the past 5 years, significant advances in continuous blood gas monitoring have been made using photochemical sensors, often called “optodes.” This article will first briefly review the electrochemical technology used in blood gas analyzers, and then describe the development and design of optically based sensors. This