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Clinical evaluation of a system for measuring functional residual capacity in mechanically ventilated patients

  • L Brewer1,
  • B Markewitz1,
  • E Fulcher1 and
  • J Orr1
Critical Care200812(Suppl 2):P313

Published: 13 March 2008


OxygenRepeated MeasurementEmergency MedicineClinical EvaluationBaseline Level


The ability to track the functional residual capacity (FRC) volume for a patient treated with ventilator therapy may be useful for guidance in improving or preserving gas exchange and for avoiding ventilator-associated injury to the lung. We tested the repeatability of the FRC measurement at the bedside for mechanically ventilated patients in the ICU.


All data needed for the FRC measurement were collected using a volumetric capnography system (NICO2; Respironics, Wallingford, CT, USA), which had been modified to contain a fast, on-airway oxygen sensor. The nitrogen washout and washin method was used to calculate the FRC for 13 ICU patients. The protocol for a measurement set called for increasing the FIO2 from the clinically determined baseline to 100% for 5 minutes, returning the FIO2 to the baseline level for 5 minutes, and then repeating the FIO2 change. After approximately 1 hour, the measurement set was repeated. The differences between the first and second measurements in each set were analyzed.


Bland–Altman analysis yielded a bias between repeated measurements of 85 ml (2.8%) and a standard deviation of the differences of ± 278 ml (9.0%). The r2 of the repeated measurements was 0.92 (n = 28). See Figure 1.
Figure 1

Figure 1


We have previously verified the FRC measurement accuracy of this system in bench tests and volunteer studies. The satisfactory repeatability demonstrated in this study suggests the system is clinically viable in the ICU.

Authors’ Affiliations

University of Utah Health Sciences Center, Salt Lake City, USA


© BioMed Central Ltd 2008

This article is published under license to BioMed Central Ltd.