The kinetics of Ringer’s solution in young and elderly patients during induction of general anesthesia with propofol and epidural anesthesia with ropivacaine

BACKGROUND: Different fluid regimens are often adopted for elderly patients, but it is not known whether they handle infused fluids differently during the induction of anesthesia than young patients. METHODS: Mean arterial pressure (MAP), plasma dilution (based on hemoglobin) and volume kinetics were assessed during an intravenous (i.v.) infusion of 1000 ml of lactated Ringer's solution over exactly 60 min in 29 patients given general anesthesia with propofol (15 < 65 years of age, and 14 > 65 years old) and in 16 patients receiving lumbar epidural anesthesia with titrated doses of ropivacaine (7 patients < 65 years of age and 9 > 65 years old). RESULTS: General anesthesia caused a greater decrease in MAP than epidural anesthesia (mean 15% vs. 9%; P < 0.001) and was followed by a more pronounced plasma dilution (30% vs. 18%; P < 0.001); the maximum values reaching 50%. The fluid-induced hemodilution increased after the onset of anesthesia. The distribution rate constant (k(t)), which governs the preference for infused fluid to retain the plasma, was significantly reduced, with the lowest values being reached during general anesthesia (P < 0.002). However, only the reduction of MAP, and not the patient's age group or the type of anesthesia per se, had a statistically significant influence on k(t) after the induction. CONCLUSION: Induction of anesthesia greatly increases fluid-induced hemodilution, the magnitude of which can be expressed as an acute reduction of the distribution rate constant for infused fluid. The post-induction value of this parameter was closely associated with MAP but not with the patient's age.