title: Modifying the baricity of local anesthetics for spinal anesthesia by temperature adjustment: model calculations.
abstract: BACKGROUND
  Although local anesthetics (LAs) are hyperbaric at room temperature, density drops within minutes after administration into the subarachnoid space. LAs become hypobaric and therefore may cranially ascend during spinal anesthesia in an uncontrolled manner. The authors hypothesized that temperature and density of LA solutions have a nonlinear relation that may be described by a polynomial equation, and that conversion of this equation may provide the temperature at which individual LAs are isobaric.
  
  
  METHODS
  Density of cerebrospinal fluid was measured using a vibrating tube densitometer. Temperature-dependent density data were obtained from all LAs commonly used for spinal anesthesia, at least in triplicate at 5 degrees, 20 degrees, 30 degrees, and 37 degrees C. The hypothesis was tested by fitting the obtained data into polynomial mathematical models allowing calculations of substance-specific isobaric temperatures.
  
  
  RESULTS
  Cerebrospinal fluid at 37 degrees C had a density of 1.000646 +/- 0.000086 g/ml. Three groups of local anesthetics with similar temperature (T, degrees C)-dependent density (rho) characteristics were identified: articaine and mepivacaine, rho1(T) = 1.008-5.36 E-06 T2 (heavy LAs, isobaric at body temperature); L-bupivacaine, rho2(T) = 1.007-5.46 E-06 T2 (intermediate LA, less hypobaric than saline); bupivacaine, ropivacaine, prilocaine, and lidocaine, rho3(T) = 1.0063-5.0 E-06 T (light LAs, more hypobaric than saline). Isobaric temperatures (degrees C) were as follows: 5 mg/ml bupivacaine, 35.1; 5 mg/ml L-bupivacaine, 37.0; 5 mg/ml ropivacaine, 35.1; 20 mg/ml articaine, 39.4.
  
  
  CONCLUSIONS
  Sophisticated measurements and mathematic models now allow calculation of the ideal injection temperature of LAs and, thus, even better control of LA distribution within the cerebrospinal fluid. The given formulae allow the adaptation on subpopulations with varying cerebrospinal fluid density.
authors.affiliation: Department of Anesthesiology and Intensive Care Medicine, University Hospital Carl Gustav Carus, Dresden, Germany. axel.heller@mailbox.tu-dresden.de
authors.forename: Axel R
authors.initials: AR
authors.lastname: Heller
authors.forename: Katrin
authors.initials: K
authors.lastname: Zimmermann
authors.forename: Kristin
authors.initials: K
authors.lastname: Seele
authors.forename: Thomas
authors.initials: T
authors.lastname: Rössel
authors.forename: Thea
authors.initials: T
authors.lastname: Koch
authors.forename: Rainer J
authors.initials: RJ
authors.lastname: Litz
mesh_terms: D000399:Air Pressure; D000465:Algorithms; D000775:Anesthesia, Spinal; D000779:Anesthetics, Local / Q000134:cerebrospinal fluid / Q000737:chemistry*; D002045:Bupivacaine / Q000134:cerebrospinal fluid / Q000737:chemistry; D002555:Cerebrospinal Fluid / Q000737:chemistry; D016208:Databases, Factual; D006801:Humans; D015233:Models, Statistical; D013696:Temperature
chemical_list: D000779:Anesthetics, Local; D002045:Bupivacaine