On-line dialysis quantification in acutely ill patients: preliminary clinical experience with a multipurpose urea sensor monitoring device.

Abstract

Direct dialysis quantification offers several advantages compared with conventional blood urea kinetic modeling, and monitoring urea concentration in the effluent dialysate with an on-line urea sensor is a practical approach. Such a monitoring device seems desirable in the short-term dialysis setting to optimize and personalize both renal replacement therapy and nutritional support of acutely ill patients. We designed a urea monitoring device consisting of a urea sensor, a multichannel hydraulic circuit, and a PC microcomputer. The sensor determines urea from catalysis of its hydrolysis by urease in liquid solution during neutral conditions. Hydrolysis of urea produces NH4+, and creates an electrical potential difference between two electrodes. Each concentration determination of urea is the average value of 10 measurements; samples are diverted and measured every 7 min. Laboratory calibration of the urea sensor has demonstrated linearity over the range 2-35 mmol/L. Urea monitoring was performed throughout the treatment course, either on the effluent dialysate or ultrafiltrate in seven acutely ill patients treated by either hemofiltration (n=5) or hemodiafiltration (n=2). The slope of the concentration of urea in the effluent over time was used to calculate an index of the dialysis dose delivered (Kt/V), urea mass removal, and protein catabolic rate. In addition, samples of the effluent were drawn every 21 min, and sent to the central laboratory for measurement of urea concentrations using an autoanalyzer. Kt/V values also were calculated with Garred's equation using pre and post session concentrations of urea in blood. Concentrations of urea in the effluent determined by the urea sensor were found to be very close to those obtained from the central laboratory over a wide range of values (3 to 42 mmol/L). In addition, Kt/V values for both hemofiltration and hemodiafiltration, when calculated with concentrations of urea in the effluent obtained by the urea sensor, did not significantly differ from Kt/V values obtained from the laboratory concentrations of urea in the effluent. On-line urea sensor monitoring of the effluent suppresses the cumbersome task of total effluent collection, and the complexity of urea kinetic analysis. The multipurpose prototype described here represents a new, simple, and direct assessment of dialysis dose and protein nutritional status of acutely ill patients, and is suitable for various modalities.