title: Measurements of optical pathlength using phase-resolved spectroscopy in patients undergoing cardiopulmonary bypass.
abstract: BACKGROUND
  Near infrared spectroscopy (NIRS) has been used during cardiac surgery to monitor cerebral oxygenation although the validity of this technique has yet to be established. Although optical pathlength included in the algorithm for calculating NIRS values is supposed to be constant, recent evidence has suggested that optical pathlength could be affected by acute hemodilution in animals. We conducted the present study to investigate whether optical pathlength changes during cardiopulmonary bypass (CPB), and whether these changes affect NIRS values in adult patients.
  
  
  METHODS
  Nine patients undergoing elective cardiac surgery with CPB were enrolled in this study. Optical pathlength and cerebral NIRS values (oxyhemoglobin [DeltaO(2)Hb] and tissue oxygen index) were measured by phase-resolved spectroscopy and NIRO 100, respectively. Optical pathlength, hemoglobin concentration, and NIRS values were measured at the following points: 1) after the induction of anesthesia, 2) 10 min after the start of CPB, 3) 60 min after the start of CPB, and 4) 1 h after CPB. The associations between optical pathlength and other variables were analyzed by Pearson correlation coefficients and multiple regression analysis.
  
  
  RESULTS
  Optical pathlength significantly increased starting at 27.7-30.8 cm at 10 min, and 31.3 cm at 60 min after the start of CPB (P < 0.0001). Hemoglobin concentrations significantly decreased (from 11.2 to 7.1 g/dL at 10 min and 7.7 g/dL at 60 min P < 0.0001). There was a significant correlation (r = 0.55, P < 0.001) between percentage changes in pathlength and hemoglobin concentration. Multiple regression analysis showed that optical pathlength was a significant determinant of DeltaO2Hb.
  
  
  CONCLUSIONS
  The results indicate that optical pathlength can change during CPB and its changes may affect DeltaO2Hb.
authors.affiliation: Department of Anesthesiology, National Cardiovascular Center, Suita, Osaka, Japan. kenji.yoshitani@duke.edu
authors.forename: Kenji
authors.initials: K
authors.lastname: Yoshitani
authors.forename: Masahiko
authors.initials: M
authors.lastname: Kawaguchi
authors.forename: Takashi
authors.initials: T
authors.lastname: Okuno
authors.forename: Tomoko
authors.initials: T
authors.lastname: Kanoda
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Ohnishi
authors.forename: Masakazu
authors.initials: M
authors.lastname: Kuro
authors.forename: Mitsunori
authors.initials: M
authors.lastname: Nishizawa
mesh_terms: D000368:Aged; D002315:Cardiopulmonary Bypass; D006801:Humans; D008875:Middle Aged; D055095:Optics and Photonics / Q000295:instrumentation; D010108:Oxyhemoglobins / Q000032:analysis / Q000378:metabolism; D019265:Spectroscopy, Near-Infrared / Q000295:instrumentation / Q000379:methods*
chemical_list: D010108:Oxyhemoglobins