OPTICAL SENSORS FOR CLINICAL MONITORING

Technical progresses make it now possible to monitor well known or new parameters in vivo or in the laboratory with high accuracy. Especiall y optical sensors can advantageously be used for many medical applicat ions. To understand advantage and limitation of a measuring technique the basic processes will be shortly discussed. There are two types of optical sensors: 1) optical sensors which use intrinsic indicators (as for example haemoglobin or cytochromes). In this chapter tissue photo metry and evaluation methods for multicomponent scattering systems are discussed; nearinfrared and NADH fluorescence measurements are shortl y mentioned. 2) Optical sensors using extrinsic indicators (optodes). As extrinsic indicators absorbant as well as luminescent indicators ar e used. Luminescence indicators are especially sensitive. Microoptodes and two dimensional imaging is possible. From the basic molecular rea ctions of the sensing mechanisms follows that for most of the indicato r reactions there is a non-linear, almost hyperbolic relationship betw een optical signal and concentration of the analyte. Consequently, acc uracy as well as sensitivity of the optode is changing in a given meas uring range. Therefore, the optical indicator must be carefully select ed. Lifetime (or phase angle) measurements have the advantage that the ir accuracy is independent of indicator concentration, intensity of th e light source and light transport between the sensing element and the photometric setup. Optodes can be manufactured as flexible membranes permeable for the analyte. This facilitates the construction of fibreo ptic sensors. As practical examples oxygen optodes, ion optodes, optic al pCO(2) sensors, and bench-top as well as intra-arterial blood gas m easurements are discussed in detail.