The knowledge of the partial pressure of oxygen (PO2), the pH, and the partial pressure of carbon dioxide (PCO2) in arterial blood is very important in the management of critically ill patients. Sensing elements with specific reagent chemistries for each of the above measurands are attached to the tips of each of three optical fibers. When used as intravascular transducers, the three fibers are enclosed in a single catheter which is then inserted into the artery through an incision for real-time continuous monitoring of the blood gases and pH. Light from a laser diode source at a specific wavelength enters the fiber at one end (proximal), propagates through the core, and irradiates the sensing element at the distal end inside the artery.
The sensing element contains a dye with fluorescent molecules which, on absorption of the photons, are excited to a higher-energy state. In that state, the molecule loses some of its energy to the lattice structure. As it returns with lower energy from an intermediate state of excitation, it fluoresces at a different wavelength.
Since the energy of a photon is inversely proportional to the wavelength, the energy in the emitted light is of a longer wavelength. The presence of an analyte (measurand) modifies the intensity of the reemitted (fluorescent) light. It can be measured using wavelength-selective devices and photonic detectors, whose output is proportional to the concentration of the measurand. An analyte such as oxygen quenches the fluorescence intensity.
This is the basic transduction mechanism in PO2 transducers. Based on the fluorescence quenching effect, intravascular PO2 transducers have been built as shown in Fig. 5.47a. The relationship between PO2 (measured) and the fluorescence characteristics of the dye molecule is given by the rearranged form of the Stern-Voltmer equation
where I is the fluorescence intensity after quenching by oxygen and I0 is the intensity in the absence of oxygen. K is the quenching constant, and it represents the slope of the PO2 vs. I0/I plot, which characterizes the performance of the oxygen transducer.108 By measuring I0 at the excitation wavelength and at the fluorescence wavelength , the partial pressure of oxygen can be determined.
The sensing element in a pH transducer is a pH-sensitive dye, whose acid from (HA) and the base form (A_) fluoresce at different excitation wavelengths, but the longer wavelength of fluorescence emission is the same for both forms, |
| Figure 5.47 Fiber-optic fluorescence chemical transducers. (a) Oxygen transducer based on the fluorescence quenching effect. (b) pH transducer. |
This is the basic transduction mechanism. In this case, the pH can be expressed as
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