title: Partial carbon dioxide rebreathing: a reliable technique for noninvasive measurement of nonshunted pulmonary capillary blood flow.
abstract: OBJECTIVE
  To determine the validity and clinical utility of the partial CO2 rebreathing technique for measurement of nonshunted pulmonary capillary blood flow and cardiac output.
  
  
  METHODS
  Prospective, controlled animal laboratory investigation and clinical trial.
  
  
  METHODS
  Animal research facility and intensive care unit of a university hospital.
  
  
  METHODS
  Fifteen adult sheep, weighting 58 to 78 kg.
  
  
  METHODS
  Mechanically ventilated patients with different underlying diseases (n = 12) and with adult respiratory distress syndrome (ARDS) (n = 8).
  
  
  METHODS
  CO2 elimination rate (VCO2) was measured breath-by-breath with a system developed for the study and also by gas collection (validation procedure in patients with different underlying diseases). Partial CO2 rebreathing maneuvers, cardiac output by thermodilution, and blood gas analysis were performed in sheep with lung atelectasis and in patients with ARDS.
  
  
  RESULTS
  The degree of correlation between VCO2 measured with the system developed and gas collection was very good (r2 = .95, p < .0001), and bias and precision calculations (1 +/- 9 mL/min) showed close agreement between methods. The overall degree of correlation between partial CO2 rebreathing measurements and cardiac output was moderate (r2 = .54, p < .0001), the noninvasive method tending to underestimate cardiac output, as shown by bias and precision calculations (-1.69 +/- 1.90 L/min). In contrast, the overall degree of correlation between partial CO2 rebreathing measurements and nonshunted pulmonary capillary blood flow was good (r2 = .73, p < .0001). Bias and precision calculations (0.25 +/- 0.83 L/min) showed a tendency for the partial CO2 rebreathing technique to slightly overestimate pulmonary capillary blood flow. Variance differences between partial CO2 rebreathing measurements and cardiac output could be mostly explained by intrapulmonary right-to-left shunt fraction (r2 = .51, p < .0001).
  
  
  CONCLUSIONS
  Our results support the use of the system developed for breath-by-breath VCO2 measurements. The lack of agreement between partial CO2 rebreathing measurements and cardiac output was mostly explained by intrapulmonary right-to-left shunt, suggesting that this technique may not be appropriate for monitoring cardiac output in patients with increased venous admixture. In contrast, our results demonstrate that the partial CO2 rebreathing technique is reliable for measurement of the effective nonshunted pulmonary capillary blood flow. This technique may prove useful to guide ventilatory therapy adjustments in an attempt to optimize nonshunted pulmonary capillary blood flow.
authors.affiliation: Clinic of Anesthesiology and Intensive Care Medicine, University Clinic Carl Gustav Carus, Technical University Dresden, Germany.
authors.forename: M G
authors.initials: MG
authors.lastname: de Abreu
authors.forename: M
authors.initials: M
authors.lastname: Quintel
authors.forename: M
authors.initials: M
authors.lastname: Ragaller
authors.forename: D M
authors.initials: DM
authors.lastname: Albrecht
mesh_terms: D000328:Adult; D000368:Aged; D000818:Animals; D002196:Capillaries / Q000503:physiopathology; D002245:Carbon Dioxide / Q000378:metabolism; D002302:Cardiac Output; D004195:Disease Models, Animal; D005260:Female; D006801:Humans; D008168:Lung / Q000098:blood supply* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods; D001261:Pulmonary Atelectasis / Q000503:physiopathology; D011652:Pulmonary Circulation; D011659:Pulmonary Gas Exchange; D015203:Reproducibility of Results; D012119:Respiration; D012121:Respiration, Artificial; D012128:Respiratory Distress Syndrome / Q000503:physiopathology* / Q000628:therapy; D012756:Sheep
chemical_list: D002245:Carbon Dioxide