Authors: Poulsen, Peter; Karbing, Dan Stieper; Andreassen, Steen; Rees, Stephen Edward

Abstract: Thermodilution is the current standard for determination of cardiac output. The method is invasive and constitutes a risk for the patient. As an alternative CO² rebreathing allows non-invasive cardiac output estimation using Ficks principle. The method relies on estimation of arterial CO² partial pressure from end-tidal CO² pressure and estimation of mixed venous CO² partial pressure from end-tidal CO² during rebreathing. Presumably the oxygenation of blood in the lung capillaries increases lung capillary CO² pressure due to the Haldane effect, which during rebreathing may result in overestimation of the mixed venous CO² pressure. However, the Haldane effect is not discussed in the current litterature describing cardiac output estimation using CO² rebreathing. The purpose of this study is to construct and verify a compartmental tidal breathing lung model to investigate the physiological mechanisms that influence the CO² rebreathing technique. The model is based on an acid/base model of the blood and differential equations describing state variables for the masses of CO² and O² in the different model compartments. The model simulations show agreement with previous studies describing end-tidal to arterial differences in CO² pressure and rebreathing with high and low O² fractions in the rebreathing bag. In conclusion the simulations show that caution has to be taken when using end-tidal measurements to estimate CO² pressures, especially during rebreathing where the Haldane effect causes an overestimated mixed venous CO² partial pressure.

Keywords: Circulatory and respiratory systems; Critical care and decision support systems

Identifier: 10.3182/20090812-3-DK-2006.00024

Conference: 7th IFAC Symposium on Modelling and Control in Biomedical Systems

Location: Hvide Hus, Denmark

Start Date: Wed Aug 12 2009 - End Date: Fri Aug 14 2009

Authors: Chase, J. Geoffrey; Le Compte, Aaron; Shaw, Geoffrey M; Lin, Jessica; Pretty, Christopher; Razak, Normy; Parente, Jacquelyn Dawn; Lynn, Adrienne; Hann, Christopher E; Suhaimi, Fatanah

Abstract: Tight glycaemic control (TGC) has emerged as a major focus in critical care. However, repeating the initial successful reductions in reducing mortality and other outcomes via TGC has proven very difficult. Hence, there has been growing debate over the necessity of TGC, its goals, safety from hypoglycemia, and target cohorts. This article reviews existing knowledge and results to provide a new interpretation and explanation for the variable results in applying TGC. It then uses a validated metabolic system model to show how the root cause is the intra- and inter- patient variability, which makes TGC difficult over diverse cohorts and thus yields such variable results over many protocols.

Keywords: Critical care and decision support systems; Control of physiological and clinical variables,; Endocrine and metabolic systems

Identifier: 10.3182/20090812-3-DK-2006.00001

Conference: 7th IFAC Symposium on Modelling and Control in Biomedical Systems

Location: Hvide Hus, Denmark

Start Date: Wed Aug 12 2009 - End Date: Fri Aug 14 2009

Authors: Robert, Raymond; Micheau, Philippe, Micheau; Beaulieu, Alexandre; Avoine, Olivier; Walti, Herve

Abstract: Total liquid ventilation is an experimental method of mechanical assisted ventilation in which lungs are totally filled and then ventilated with a tidal volume of perfluorochemical liquid by using a liquid ventilator. However to apply this approach to humans, liquid ventilators must reach the same level of reliability as conventional mechanical gas ventilators. In control terms, the problem is to design a controller to perform pressure regulated ventilation in spite of large uncertainties on physiological parameters, and nonlinear behavior of the lungs. The presented pressure controlled ventilation mode is similar to those used in conventional gas ventilator excepted that the airway pressure and the positive end-expiratory pressure require two distinct controllers. Simulations, in-vitro experiments and in-vivo results with 5 healthy term newborn lambs have validated the presented controllers.

Keywords: Circulatory and respiratory systems

Identifier: 10.3182/20090812-3-DK-2006.00040

Conference: 7th IFAC Symposium on Modelling and Control in Biomedical Systems

Location: Hvide Hus, Denmark

Start Date: Wed Aug 12 2009 - End Date: Fri Aug 14 2009

Authors: Hann, Christopher E; Chase, J. Geoffrey; Desaive, Thomas; Froissart, Claire; Revie, James A; Stevenson, David; Lambermont, Bernard; Ghuysen, Alexandre; Kolh, Philippe; Shaw, Geoffrey M

Abstract: Lumped parameter approaches for modeling the cardiovascular system typically have many parameters of which many are not identifiable. The conventional approach is to only identify a small subset of parameters to match measured data, and to set the remaining parameters at population values. These values are often based on animal data or the “average human” response. The problem, is that setting many parameters at nominal fixed values, may introduce dynamics that are not present in a specific patient. As parameter numbers and model complexity increase, more clinical data is required for validation and the model limitations are harder to quantify. This paper considers the modeling and the parameter identification simultaneously, and creates models that are one to one with the measurements. That is, every input parameter into the model is uniquely optimized to capture the clinical data and no parameters are set at population values. The result is a geometrical characterization of a previously developed six chamber heart model, and a completely patient specific approach to cardiac diagnosis in critical care. In addition, simplified sub-structures of the six chamber model are created to provide very fast and accurate parameter identification from arbitrary starting points and with no prior knowledge on the parameters. Furthermore, by utilizing continuous information from the arterial/pulmonary pressure waveforms and the end-diastolic time, it is shown that only the stroke volumes of the ventricles are required for adequate cardiac diagnosis. This reduced data set is more practical for an intensive care unit as the maximum and minimum volumes are no longer needed, which was a requirement in prior work. The simplified models can also act as a bridge to identifying more sophisticated cardiac models, by providing a generating set of waveforms that the complex models can match to. Most importantly, this approach does not have any predefined assumptions on patient dynamics other than the basic model structure, and is thus suitable for improving cardiovascular management in critical care by optimizing therapy for individual patients.

Keywords: Circulatory and respiratory systems; Critical care and decision support systems; Decision support and control of biomedical systems

Identifier: 10.3182/20090812-3-DK-2006.00029

Conference: 7th IFAC Symposium on Modelling and Control in Biomedical Systems

Location: Hvide Hus, Denmark

Start Date: Wed Aug 12 2009 - End Date: Fri Aug 14 2009

Authors: None

Abstract:

Keywords:

Identifier: 10.3182/20090812-3-DK-2006.90002

Conference: 7th IFAC Symposium on Modelling and Control in Biomedical Systems

Location: Hvide Hus, Denmark

Start Date: Wed Aug 12 2009 - End Date: Fri Aug 14 2009