This study assessed the performance of a Functional Approximation based on a Levenberg-Marquardt Algorithm (FA-LMA) to calculate airway dead space (VDaw) and the slope of phase III (S III) from capnograms.
We performed mathematical simulations to test the effect of noises on the calculation of VDaw and S III. Data from ten mechanically ventilated patients at 0, 5 and 10 cmH2O of PEEP were also studied. FA-LMA was compared with the traditional Fowler’s method (FM).
Simulations showed that: (1) The FM determined VDaw with accuracy only if the capnogram approximated a symmetrical curve (S III = 0). When capnograms became asymmetrical (S III > 0), the FM underestimated VDaw (−3.1% to −0.9%). (2) When adding noises on 800 capnograms, VDaw 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 VDaw 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 VDaw 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 cmH2O 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 cmH2O of PEEP, respectively.
The Functional Approxi- mation based on a Levenberg-Marquardt Algorithm showed less bias and dispersion compared to the traditional Fowler’s method when calculating VDaw and S III.
2009. Vol. 23, no 4, 197-206 p.