Dehydration and fluid volume kinetics before major open abdominal surgery

IntroductionAssessment of dehydration in the preoperative setting is of potential clinical value. The present study uses urine analysis and plasma volume kinetics, which have both been validated against induced changes in body water in volunteers, to study the incidence and severity of dehydration before open abdominal surgery begins. MethodsThirty patients (mean age 64 years) had their urine analysed before major elective open abdominal surgery for colour, specific weight, osmolality and creatinine. The results were scored and the mean taken to represent a dehydration index'. Thereafter, the patients received an infusion of 5ml/kg of Ringer's acetate intravenously for over 15min. Blood was sampled for 70min and the blood haemoglobin concentration used to estimate the plasma volume kinetics. ResultsDistribution of fluid occurred more slowly (P<0.01) and the elimination half-life was twice as long (median 40min, not significant) in the 11 patients (37%) diagnosed to be moderately dehydrated as compared with euhydrated patients. The dehydration index indicated that the fluid deficit in these patients corresponded to 2.5% of the body weight, whereas the deficit in the others was 1%. In contrast, the 11 patients who later developed postoperative nausea and vomiting had a very short elimination half-life, only 9min (median, P<0.01). These patients were usually euhydrated but had microalbuminuria (P<0.03) and higher natriuresis (P<0.01). ConclusionsThe degree of dehydration before major surgery was modest as evidenced both by urine sampling and volume kinetic analysis.