MONITORING An analysis of electrode performance in 100 patients

A lengthened umbilical artery catheter incorporating an oxygen electrode (Searle) was flow-guided to the pulmonary artery in 48 out of 62 attempts (77.4%). In the last 70 catheters used, electrode performance was evaluated over a total of 3687 h by comparison with an ABL2 (Radiometer) blood-gas analyser; 654 comparisons were made, of which 255 resulted in recalibration of the electrode output. When sampling intervals were within 6 h, in more than 80 % of the comparisons the difference between in vivo and in vitro measurement was within +0.4 kPa (±3 mm Hg). Twenty-four electrodes were removed sooner than clinically indicated (mean useful life 48.5 h) because of failure of oxygen or pressure measurements. The measurement of mixed venous oxygen (CvOj) has been described by several workers as an index of body oxygen supply, useful in the management of cardiorespiratory disorders. This is based on the relationship expressed by the re-arranged Fick equation: = arterial O2 content- oxygen consumption cardiac output From this equation it can be seen that alteration of mixed venous oxygen content will follow any imbalance between oxygen supply and demand. In this paper we have analysed the results of a technique used to monitor mixed venous oxygen tension (PvOj) continuously with flow-guided catheter oxygen electrodes (probes) inserted percutaneously. APPARATUS The oxygen sensor (Parker, Key and Davies, 1971) was a small Clark-type polarographic cell made of a silver anode and cathode separated by dried potassium chloride electrolyte (fig. 1). This cell was mounted at the end of a 5-F double-lumen p.v.c. catheter and the tip dip-coated in poly-styrene. Wires from the electrode traversed one lumen of the catheter, the other being used for sampling or pressure monitoring (fig. 2). Onc