Model fitting of volumetric capnograms improves calculations of airway dead space and slope of phase III.
BACKGROUND
This study assessed the performance of a Functional Approximation based on a Levenberg-Marquardt Algorithm (FA-LMA) to calculate airway dead space (VD(aw)) and the slope of phase III (S(III)) from capnograms.


METHODS
We performed mathematical simulations to test the effect of noises on the calculation of VD(aw) and S(III). Data from ten mechanically ventilated patients at 0, 5 and 10 cmH(2)O of PEEP were also studied. FA-LMA was compared with the traditional Fowler's method (FM).


RESULTS
Simulations showed that: (1) The FM determined VD(aw) with accuracy only if the capnogram approximated a symmetrical curve (S(III) = 0). When capnograms became asymmetrical (S(III) > 0), the FM underestimated VD(aw) (-3.1% to -0.9%). (2) When adding noises on 800 capnograms, VD(aw) was underestimated whenever the FM was used thereby creating a bias between -5.54 and -1.28 ml at standard deviations (SD) of 0.1-1.8 ml (P < 0.0001). FA-LMA calculations of VD(aw) were close to the simulated values with the bias ranging from -0.21 to 0.16 ml at SD from 0.1 to 0.4 ml. The FM overestimated S(III) and showed more bias (0.0041-0.0078 mmHg/ml, P < 0.0001) than the FA-LMA (0.0002-0.0030 mmHg/ml). When calculating VD(aw) from patients, variability was less with the FA-LMA leading to mean variation coefficients of 0.0102, 0.0111 and 0.0123 compared to the FM (0.0243, 0.0247 and 0.0262, P < 0.001) for 0, 5 and 10 cmH(2)O of PEEP, respectively. The FA-LMA also showed less variability in S(III) with mean variation coefficients of 0.0739, 0.0662 and 0.0730 compared to the FM (0.1379, 0.1208 and 0.1246, P < 0.001) for 0, 5 and 10 cmH(2)O of PEEP, respectively.


CONCLUSIONS
The Functional Approximation based on a Levenberg-Marquardt Algorithm showed less bias and dispersion compared to the traditional Fowler's method when calculating VD(aw) and S(III).
Tusman
Gerardo
G
Department of Anesthesiology, Hospital Privado de Comunidad, Mar del Plata, Argentina. gtusman@hotmail.com
Scandurra
Adriana
A
Böhm
Stephan H
SH
Suarez-Sipmann
Fernando
F
Clara
Fernando
F
D000403:Airway Resistance / Q000502:physiology*; D000465:Algorithms; D019296:Capnography / Q000379:methods*; D002245:Carbon Dioxide / Q000737:chemistry; D003198:Computer Simulation; D004867:Equipment Design; D006801:Humans; D015233:Models, Statistical; D008962:Models, Theoretical; D011175:Positive-Pressure Respiration / Q000379:methods*; D011650:Pulmonary Alveoli / Q000503:physiopathology*; D011659:Pulmonary Gas Exchange; D012123:Pulmonary Ventilation; D015203:Reproducibility of Results; D012126:Respiratory Dead Space; D012984:Software
D002245:Carbon Dioxide