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Contributions to the initialization of online identification algorithms
in anaesthesia
J. Almeida 1 , M. M. Silva 1 and T. Mendonça 1
1 Departamento de Matemática, Faculdade de Ciências, Universidade do Porto, Portugal.
Recent trends in medicine claim for the automation of the drug administration in clinical scenarios. Regarding
almost every measured physiological variable, since the underneath mechanisms are complex, the parameters
associated to the modeling of those systems are uncertain and must be somehow identified. Hence, for the
development of new controllers and new drug delivery system, accurate modeling and identification algorithms
must be established. For the special case of general anaesthesia, the NeuroMuscular Blockade (NMB) is one of
the measured signals and reflects the effect of the muscle relaxants administration in the human body. One of
those drugs is the atracurium. For the purpose of describing the effect of atracurium regarding the NMB level,
several overparameterized Pharmacokinetic/Pharmacodynamic (PK/PD) models were proposed in the past [1].
Nevertheless, these structures do not enable one unique estimation of the underlying parameters with the standard
system identification strategies. In [2] a new minimally parameterized Wiener model with only two parameters (α
and γ) is propose. The new modeling enabled the online identification of the model parameters, showing good
results for the NMB case study.
Most importantly, in what concerns identification algorithms, one relevant issue that should handled carefully is
parameter converge analysis. In clinical practical, most of the sufficient conditions established for the
convergence results are not fulfilled. When the algorithms are not guaranteed to converge (e.g. due to practical
conditions), initialization becomes more important. In case of insufficient prior information initial values must be
obtained from measurements on the system [3]. This paper exploits precisely the idea of using information from
the initial measured signal in order to calculate a first parameters approach for identification proposes.
NMB signals from a simulated database and from real collected patients were used to find a relationship between
the model parameter α (from the linear block) and initial NMB features response, after the administration of
atracurium bolus. The elapsed time between the bolus administration and the time where the signal drops to 50%
of its initial value (T50) was used. The correlation between the squared root of α and T50 for the simulated cases is
shown in Fig. 1, having one adjusted R squared equal to 0.9843. For the real collected cases the R squared is equal
to 0.7441 (Fig. 2).
These results indicate that the proposed methodology provides good initialization of the parameter α and
encourages its incorporation on online identification algorithms in order to enhance the initial performance of the
algorithm.
Fig.1 Fig.2
References:
[1] Lago P., Mendonça T., and Gonçalves L. (1998), On-line autocalibration of a PID controller of neuromuscular blockade,
in Proc. 1998 IEEE Int. Conf. Control Applications, Trieste, Italy, Sep. 1998, pp. 363–367.
[2] Silva M. M., Mendonça T., and WigrenT. (2009) Nonlinear identification of a minimal neuromuscular blockade model in
anaesthesia, Tech. Rep. 2009-023, Dept. Information Technology, Uppsala University, Uppsala, Sweden, available at:
http://www.it.uu.se/research/publications/reports/.
[3] WigrenT. (2003) User choices and model validation in system identification using nonlinear Wiener models, in Proc. of the
13th IFAC Symp. on System Identification, Rotterdam, the Netherlands, pp. 863–868.
50 3 rd meeting of young researchers at UP