title: The carbon monoxide re-breathing method can underestimate Hbmass due to incomplete blood mixing.
abstract: OBJECTIVE
  Hemoglobin mass (Hbmass) is commonly assessed using the CO re-breathing method with the subject in the seated position. This may lead to an underestimation of Hbmass as blood in lower extremity veins while seated may not be tagged with carbon monoxide (CO) during the re-breathing period.
  
  
  METHODS
  To test this hypothesis, CO re-breathing was performed on four occasions in nine male subjects, twice in the seated position and twice in combination with light cycle ergometer exercise (1 W/kg body-weight) intending to accelerate blood circulation and thereby potentially allowing for a better distribution of CO throughout the circulation as compared to in the seated position. Blood samples were drawn from an antecubital vein and the saphenous magna vein following the re-breathing procedure.
  
  
  RESULTS
  In the seated position, CO re-breathing increased the percent carboxyhemoglobin (%HbCO) in the antecubital vein to 8.9 % (7.8-10.7) [median (min-max)], but less (P = 0.017) in the saphenous magna vein [7.8 % (5.0-9.9)]. With exercise, no differences in %HbCO were observed between sampling sites. As a result, CO re-breathing in combination with exercise revealed a ~3 % higher (P = 0.008) Hbmass, i.e., 936 g (757-1,018) as compared to 908 g (718-940) at seated rest.
  
  
  CONCLUSIONS
  This study suggests an uneven distribution of CO in the circulation if the CO re-breathing procedure is performed at rest in the seated position and therefore can underestimate Hbmass.
authors.affiliation: Zurich Center of Integrative Human Physiology, Institute of Physiology, University of Zürich, Winterthurerstrasse 190, 8057 Zurich, Switzerland.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Keiser
authors.forename: Christoph
authors.initials: C
authors.lastname: Siebenmann
authors.forename: Thomas Christian
authors.initials: TC
authors.lastname: Bonne
authors.forename: Henrik
authors.initials: H
authors.lastname: Sørensen
authors.forename: Paul
authors.initials: P
authors.lastname: Robach
authors.forename: Carsten
authors.initials: C
authors.lastname: Lundby
mesh_terms: D000328:Adult; D002248:Carbon Monoxide / Q000097:blood*; D002263:Carboxyhemoglobin / Q000378:metabolism; D015444:Exercise / Q000502:physiology*; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D008297:Male; D012119:Respiration; D014680:Veins / Q000502:physiology*
chemical_list: D006454:Hemoglobins; D002248:Carbon Monoxide; D002263:Carboxyhemoglobin