Monitoring functional residual capacity (FRC) by quantifying oxygen/carbon dioxide fluxes during a short apnea.
BACKGROUND
Clinically applicable methods for measuring FRC are currently lacking. This study presents a new method for FRC monitoring based on quantification of metabolic gas fluxes of O2 and CO2 during a short apnea.


METHODS
Base line exchange of oxygen and carbon dioxide was measured with indirect calorimetry. End-tidal ( approximately alveolar) O2 and CO2 concentrations were measured before and after a short apnea, 8-12 s, and FRC was calculated according to standard washin/washout formulas taking into account the increased solubility of CO2 in blood when the tension is increased during the apnea. The method was tested in a lung model with CO2 excretion and O2 consumption achieved by combustion of hydrogen and implemented in six ventilator-treated patients with acute respiratory failure (ARF).


RESULTS
In the lung model the method showed excellent correlation (r = 0.98) with minimal bias (34 ml) and a good precision, limits of agreement being 160 and -230 ml, respectively, compared to the reference method. In six ARF patients changes in FRC induced by increase or decrease in PEEP and measured with the O2/CO2 flux FRC method corresponded well with changes in reference values of FRC (r = 0.76-0.94).


CONCLUSIONS
A new method has been proposed in which FRC could be monitored from measurements of physiological fluxes of gases during a short apnea with the use of standard ICU equipment and some calculations. We anticipate that with further development, this technique could provide a new tool for monitoring respiratory changes and ventilator management in the ICU.
Stenqvist
Ola
O
Department of Anesthesiology and Intensive Care, Sahlgrenska University Hospital, Göteborg, Sweden. ola.stenqvist@aniv.gu.se
Olegård
C
C
Søndergaard
S
S
Odenstedt
H
H
Kárason
S
S
Lundin
S
S
D000328:Adult; D000368:Aged; D001049:Apnea / Q000097:blood*; D001784:Blood Gas Analysis; D002153:Calorimetry, Indirect; D005652:Functional Residual Capacity / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D008991:Monitoring, Physiologic; D012121:Respiration, Artificial; D012131:Respiratory Insufficiency / Q000097:blood* / Q000503:physiopathology*; D013990:Tidal Volume / Q000502:physiology