REPORT rialization of the blood in the vascular region being sensed. Typical instruments average the values obtained from five successive heartbeats before displaying calculated 02-saturation values. Good correlation between this noninvasive approach and the standard laboratory method (optical measurement of a lysed blood sample) is observed at (^-saturated values greater than 70%. Below this value, considerable error has been documented (17). Nonetheless, the use of pulse oximetry is now commonplace in critical care and surgical units, providing reliable and vital continuous monitoring information. Extracorporeal in-line and online sensing systems. A third approach for performing bedside chemical testing is to recirculate the blood via extracorporeal tubing to a sensor, or an array of sensors, placed outside the body. To be considered a closed-loop "in-line" system, the blood that passes from the patient must return through an appropriate vein. Alternatively, "on-line" systems may be used to classify arrangements in which blood is continuously or intermittently sampled (either directly from an in-dwelling sampling probe inserted into the
patient or from an extracorporeal blood loop such as a heart-lung machine), passed through the measurement system, and then on to waste. The latter approach makes it possible to perform frequent one- and two-point calibrations of the sensors simply by diverting appropriate calibrating solutions and/or gas-phase mixtures through the sensor array. To date, the development of in-line extracorporeal sensing techniques has focused primarily on the continuous measurement of Po2, Pco2, and pH. For example, a disposable optical sensorbased system intended for use during cardiopulmonary bypass procedures is now available. This three-sensor system is calibrated prior to insertion into the blood line of a heart-lung machine. The stability of the sensors' calibration is critical to the long-term analytical performance of the system. However, because this in-line system is intended primarily for short-term heart bypass procedures (4-5 h), reasonably good correlation with conventional methods can be obtained over this finite period without a repeated two-point calibration of the optical sensors (18). To compensate for sensor drift, periodic one-point calibrations can be ac-
complished (without removing the sensors from the loop) by performing discrete sample blood gas determinations on conventional laboratory instruments and updating the output display of the in-line monitor accordingly. A similar system involves the use of a hybrid detection system; optical sensors for Pco2 and pH and electrochemical measurement of Prj2· Because the patient's blood is often cooled considerably during open-heart surgery, both systems allow simultaneous measurement of the temperature (via a thermocouple) of the blood in the extracorporeal loop so that, if desired, values can be corrected to those that would be obtained at 37 °C, as reported by most conventional blood gas instruments. The on-line measurement approaches are more accurate than in-line systems. The ability to calibrate these systems frequently enhances long-term analytical performance. One commercial system incorporates a disposable all-electrochemical sensor array that measures blood gases, electrolytes, and hematocrit in a single sample of whole blood drawn either on demand or at preprogrammed intervals from a sampling device placed in the blood loop of a heart-lung machine. Between sam-
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434 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 7, APRIL 1, 1990
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