title: Bedside red cell volumetry by low-dose carboxyhaemoglobin dilution using expiratory gas analysis.
abstract: BACKGROUND
  We developed a non-invasive, continuous, high-resolution method of measuring carboxyhaemoglobin fraction (COHb%) using expiratory gas analysis (EGA). We assessed whether application of EGA to carboxyhaemoglobin dilution provides red cell volume (RCV) measurement with accuracy equivalent to that of CO-haemoximetry, with a smaller infusion volume of carbon-monoxide-saturated autologous blood (COB). Method. We assessed the agreement between repeated COHb% measurements by EGA and simultaneous measurement by CO-haemoximetry, using Bland and Altman plot, in healthy subjects and patients with artificially controlled ventilation and no radiological evidence of pulmonary oedema or atelectasis. We assessed the agreement between RCV measurements by EGA with infusion of 20 ml of COB (RCVEGA) and RCV measurements by CO-haemoximetry with infusion of 100 ml of COB (RCVHEM), in healthy subjects.
  
  
  RESULTS
  The 'limits of agreement' between COHb% measurement by EGA (1 min average) and CO-haemoximetry were -0.09 and 0.08% in healthy subjects, and -0.11 and 0.09% in patients. Given the resolution of CO-haemoximetry (0.1%), the accuracy of EGA was equivalent to or greater than that of CO-haemoximetry. The 'limits of agreement' between RCVEGA and RCVHEM were -0.14 and 0.15 litre. Given the average resolution of RCVHEM (0.14 litre), the accuracy of RCVEGA was equivalent to that of RCVHEM.
  
  
  CONCLUSIONS
  EGA provided non-invasive, accurate, continuous, high-resolution COHb% measurements. Applying EGA to carboxyhaemoglobin dilution, we achieved RCV measurements with accuracy equivalent to that of CO-haemoximetry, with one-fifth of the COB infusion volume. However, clinical application of the method is limited to patients with no radiological evidence of pulmonary oedema or atelectasis.
authors.affiliation: Department of Emergency Medicine and Critical Care, Saitama Medical Center, 1981 Tsujido-machi, Kamoda, Kawagoe-shi, Saitama 350-8550, Japan. sawanom-tky@umin.ac.jp
authors.forename: M
authors.initials: M
authors.lastname: Sawano
authors.forename: T
authors.initials: T
authors.lastname: Mato
authors.forename: H
authors.initials: H
authors.lastname: Tsutsumi
mesh_terms: D000328:Adult; D001811:Blood Volume Determination / Q000379:methods; D001944:Breath Tests / Q000379:methods; D002248:Carbon Monoxide / Q000378:metabolism*; D002263:Carboxyhemoglobin / Q000378:metabolism*; D003422:Critical Care / Q000379:methods; D004911:Erythrocyte Volume; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods; D019095:Point-of-Care Systems; D011659:Pulmonary Gas Exchange; D015203:Reproducibility of Results
chemical_list: D002248:Carbon Monoxide; D002263:Carboxyhemoglobin