Abstract
BackgroundInduction of anesthesia causes a drop in arterial pressure that might change the kinetics of infused crystalloid fluid. The aim of this report is to provide a mathematical view of how fluid distributes in this setting.MethodsData were retrieved from three studies where 76 patients (mean age 63 years, mean body weight 66 kg) had received approximately 1.1 L of Ringer’s solution over 60 min by intravenous infusion before and during induction of spinal, epidural, or general anesthesia. A population kinetic model was used to analyze the fluid distribution and its relationship to individual-specific factors. Frequent measurements of blood hemoglobin and the urinary excretion served as dependent variables.ResultsBefore anesthesia induction, distribution to the extravascular space was threefold faster than elimination by urinary excretion. Both distribution and elimination of infused fluid were retarded in an exponential fashion due to the anesthesia-induced decrease in the mean arterial pressure (MAP). A decrease in MAP from 110 to 60 mmHg reduced the rate of distribution by 75% and the rate of elimination by 90%. These adaptations cause most of the infused fluid to remain in the bloodstream. Age, gender, type of anesthesia, and the use of ephedrine had no statistically significant effect on plasma volume expansion, apart from their possible influence on MAP.ConclusionThe decrease in MAP that accompanies anesthesia induction depresses the blood hemoglobin concentration by inhibiting both the distribution and elimination of infused crystalloid fluid. The report provides mathematical information about the degree of these changes.
Highlights
Crystalloid fluid is often administered intravenously (i.v.) during induction of anesthesia (McCrae and Wildsmith 1993), this fluid is considered to have a poor plasma volume–expanding effect (Jacob et al 2012)
Volume loading before the induction does not prevent a drop in arterial pressure (Coe and Revenäs, Correspondence: robert.hahn@ki.se; r.hahn@telia.com 1Research Unit, Södertälje Hospital, 152 86 Södertälje, Sweden 2Karolinska Institutet at Danderyds Hospital (KIDS), Stockholm, Sweden 1990; Ewaldsson and Hahn 2005) while providing the fluid during the onset of anesthesia can reduce the magnitude of the drop, which is best studied in parturients (James and Dyer 2016)
Besides being of physiological interest, the excessive volume expansion is mirrored by a reduction of the blood hemoglobin (Hb) concentration that affects oxygen delivery, the planning of deliberate hemodilution, and estimates of the blood loss allowed before initiating erythrocyte transfusion
Summary
Crystalloid fluid is often administered intravenously (i.v.) during induction of anesthesia (McCrae and Wildsmith 1993), this fluid is considered to have a poor plasma volume–expanding effect (Jacob et al 2012). Volume loading before the induction (pre-loading) does not prevent a drop in arterial pressure (Coe and Revenäs, 1990; Ewaldsson and Hahn 2005) while providing the fluid during the onset of anesthesia (co-loading) can reduce the magnitude of the drop, which is best studied in parturients (James and Dyer 2016). One reason for why co-loading maintains the arterial pressure better than pre-loading might be that more volume remains in the plasma in close connection with the infusion. Hahn Perioperative Medicine (2021) 10:34 the fluid was given only after the induction showed an arrested distribution that resolved only 20 min later (Hahn and Nemme 2020), resulting in a plasma volume expansion that temporarily approached 100% of the infused amount. Induction of anesthesia causes a drop in arterial pressure that might change the kinetics of infused crystalloid fluid. The aim of this report is to provide a mathematical view of how fluid distributes in this setting
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