An automated bedside method for measuring functional residual capacity by N2 washout in mechanically ventilated children

Pediatr Res. 1990 Nov;28(5):446-50. doi: 10.1203/00006450-199011000-00005.


Beside measurement of functional residual capacity (FRC) in ventilated children is impractical. Using a simple technique based on open circuit N2 washout, we measured FRC in ventilated children. The system was evaluated in the laboratory and in patients. Using a mechanical lung, the reproducibility of 200 studies over a range of 100-500 mL at each of four different flow rates (10 determinations at each level) was very high with a mean coefficient of variation of 2.3% (range 0.5-5.1%). Linearity of the integrated N2 signal for volumes of 100-500 mL washed out at different flow rates was excellent (range 7.4-17.9 L/min), r = 0.99. The mean difference between measured and preset mechanical lung volumes was 2.4% (range 0-4.6%). In vivo, reproducibility of six to 10 FRC determinations in each of 30 children gave a mean coefficient of variation of 2.7%. Comparison to the conventional Douglas bag collection method showed a high correlation (r = 0.97). We conclude that this is an easy, highly reproducible, and accurate method for FRC determination suitable to ventilated infants and children.

MeSH terms

  • Child
  • Evaluation Studies as Topic
  • Functional Residual Capacity*
  • Humans
  • Methods
  • Models, Biological
  • Nitrogen
  • Pulmonary Gas Exchange
  • Respiration, Artificial


  • Nitrogen