title: Evaluation of a CO2 partial rebreathing functional residual capacity measurement method for use during mechanical ventilation.
abstract: OBJECTIVE
  There is a need for an automated bedside functional residual capacity (FRC) measurement method that does not require a step change in inspired oxygen fraction. Such a method can be used for patients who require a high inspired oxygen fraction to maintain arterial oxygenation and for patients ventilated using a circle breathing system commonly found in operating rooms, which is not capable of step changes in oxygen. We developed a CO(2) rebreathing method for FRC measurement that is based on the change in partial pressure of end-tidal carbon dioxide and volume of CO(2) eliminated at the end of a partial rebreathing period. This study was designed to assess the accuracy and precision of the proposed FRC measurement system compared to body plethysmography and nitrogen washout FRC.
  
  
  METHODS
  Accuracy and precision of measurements were assessed by comparing the CO(2) rebreathing FRC values to the gold standard, body plethysmography FRC, in twenty spontaneously breathing volunteers. The CO(2) rebreathing FRC measurements were then compared to nitrogen washout FRC in twenty intensive care patients whose lungs were mechanically ventilated. For each subject, an average value of CO(2) rebreathing FRC was compared to the average gold standard method. Measurements were accepted for statistical analysis if  they had been recorded from periods of stable tidal ventilation, defined as a coefficient of variation of tidal volume of <0.13.
  
  
  RESULTS
  Compared to body plethysmography, the accuracy (average error) for the CO(2) rebreathing method during stable ventilation (n = 8) was 0.03 L and precision (1 standard deviation of the error) was 0.29 L (0.8 ± 7.6% of body plethysmography). During stable mechanical ventilation (n = 9), the accuracy was  -0.02 L and precision was 0.26 L (-1.1 ± 12.6% of nitrogen washout).
  
  
  CONCLUSIONS
  The CO(2) rebreathing method for FRC measurement provides acceptable accuracy and precision during stable ventilation compared to the gold standards of body plethysmography and nitrogen washout. The results based on periods of stable ventilation best approximate the performance of the system in the likely areas of application during controlled mechanical ventilation. Further study of the CO(2) rebreathing method is needed to evaluate accuracy in a larger group of controlled mechanical ventilation patients, including patients with respiratory insufficiency and significant lung injury.
authors: University of Utah Health Sciences Center, Salt Lake City, UT 84132, USA. Lara.Brewer@hsc.utah.edu
authors: Lara
authors: L
authors: Brewer
authors: Joseph
authors: J
authors: Orr
authors: Earl
authors: E
authors: Fulcher
authors: Boaz
authors: B
authors: Markewitz
mesh_terms: D000328:Adult; D001944:Breath Tests / Q000295:instrumentation*; D002245:Carbon Dioxide / Q000032:analysis*; D003936:Diagnosis, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D005652:Functional Residual Capacity; D006801:Humans; D008176:Lung Volume Measurements / Q000295:instrumentation*; D008297:Male; D015203:Reproducibility of Results; D012121:Respiration, Artificial / Q000295:instrumentation*; D012680:Sensitivity and Specificity
chemical_list: D002245:Carbon Dioxide