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Biomedical subjects

U Kiencke

Publications and source records attributed to U Kiencke.

11 recordsLinked to original sources
PubMed2002

Observer design for haemodynamics in patients undergoing cardiac surgery.

During extracorporeal circulation, many important circulatory parameters are unknown, being inaccessible for measuring probes, as e.g. the perfusion of the brain. An observer system, which estimates such patient variables continuously throughout the operation, can extend the information basis for the decisions of the perfusionist regarding the control of the heart-lung machine and thus contribute to adjust this operation procedure to the actual patient situation. The observer design is based on a mathematical model of the human circulatory system. Beside the classical Luenberger observer design, a rule-based approach has been tested, which is also based on the structure of a Luenberger observer, however instead of an observer matrix a correction algorithm has been used in the feedback loop. A prototype of this system for animal experimental and clinical evaluation has been realised.

Algorithms↗
PubMed2002

[Multi-value regulatory systems for extracorporeal circulation].

Extracorporeal perfusion is the standard technique in cardiac surgery. It is controlled by perfusionists on the basis of their clinical experience and on the available data collected pre- and intra-operatively. But in spite of intensive monitoring postoperative complications occur. An appropriate control of the heart-lung machine (HLM) using an "autopilot" might improve the quality of heart-surgery and decrease postoperative complications. Hence, a mathematical model of a human circulatory system has been developed which provides much more information about haemodynamics, blood gases and acid-base status than standard monitoring. It has been implemented on a system which is capable of integrating measured data as input parameters in real-time in the simulation. Now, soft- and hardware control concepts based on the human circulatory system have to be developed which are able to control the HLM.

Coronary Disease↗
PubMed1999

Simulation of arterial hemodynamics after partial prosthetic replacement of the aorta.

BACKGROUND: Replacing parts of the aorta with a non-compliant vascular prosthesis results in marked alterations of the aortic input impedance and influences arterial hemodynamics. We propose a mathematical model of circulation that can predict hemodynamic changes after simulation of vascular grafting. METHODS: A new mathematical model of the human arterial system was developed on a 75-MHz Pentium personal computer using Matlab software. The human arterial tree was delineated according to a 128-branch design encompassing bifurcations and physical properties of the arterial wall. A digitized aortic flow wave was chosen as the input signal to the system. After determination of the modules of elasticity of native vascular tissue and standard prostheses in technical experiments, replacement of any part of the aorta with a prosthesis was simulated by increasing the elasticity in the parts desired. RESULTS: During control conditions, the model displayed a physiologic distribution of flow and pressure waves throughout the arterial system. Simulated replacement of the aorta resulted in an increase in pressure amplitude and a partial loss of the aortic "Windkessel" function. Calculation of the aortic input impedance showed an increase in the characteristic impedance, whereas the peripheral resistance remained unaltered. CONCLUSIONS: This mathematical model of the arterial circulation is useful for simulating hemodynamic changes after implantation of vascular grafts. The results of the model analysis are consistent with those in previous experimental work.

Aorta↗
PubMed1999

[Signal analysis of the hemodynamics of extracorporeal circulation for the evaluation of patient status].

The present paper describes the intra-operative evaluation of circulation dynamics during open-heart surgery, with the aim of providing the anaesthetist with objective data for assessing patient status during the procedure. For this purpose, the parameters pulse wave velocity and biological impedance were used. With the aid of these parameters it is possible, in the extracorporeal model, to detect different kinds of stenosis, volume losses, vessel dilatation and vessel constriction. In addition, the influence of arteriosclerosis on circulation dynamics was investigated. The assignment of the parameters to various events is effected using a "decision tree" and a neural network. Finally, the technique is verified using data obtained from animal experiments.

Animals↗
PubMed1998

[Simulation of metabolic processes in human circulation: evaluation of pH value and CO2 balance].

This article describes a model of the human circulatory system, with emphasis on metabolic processes in particular the calculation of CO2 balance and the pH of venous blood. The model overall is capable of simulating both physiological and pathological circulatory systems. Simulated parameters are locally distributed blood flows, and pressures in the arterial system. This modelling of haemodynamics serves, among other things, to determine the oxygen supply situation of individual organs. The four most important control mechanisms of the human circulatory system for maintaining haemodynamics--the renin-angiotensin system, the autonomic nervous system, local arterial autoregulation and vascular stress relaxation--have been incorporated into the model. Another important parameter for estimating the oxygen supply situation of the human body is the acid-base status of arterial and venous blood. The calculation of the pH of blood plasma and erythrocytes is described in the second part of the article. The results achieved with our simulation model correspond well with those reported in the literature. The behaviour of the human circulatory system can be reproduced at rest and under conditions of loading.

Acid-Base Equilibrium↗
PubMed1997

Hemodynamic consequences of replacing the aorta by vascular grafts simulated in a mathematical model.

OBJECTIVE: Replacing parts of the aorta by a noncompliant vascular prosthesis results in marked alterations of the aortic input impedance and influences arterial hemodynamics. We propose a mathematical model of circulation able to predict hemodynamic changes after simulation of vascular grafting. METHODS: Using a mathematical 128-branch model of the human arterial system a digitized aortic flow wave was chosen as the input signal to this system. After determination of the modules of elasticity of native vascular tissue and customary prostheses in technical experiments, replacement of any part of the aorta with a prosthesis was simulated by increasing the elasticity in the parts desired. RESULTS: During control conditions, the model displayed a physiologic distribution of flow and pressure waves throughout the arterial system. Simulated replacement of the aorta resulted in an increase of pressure amplitude and a partial loss of the aortic "Windkessel" function. Calculation of the aortic input impedance showed an increase of the characteristic impedance, while the peripheral resistance remained unaltered. CONCLUSION: This mathematical model of the arterial circulation proves to be useful to simulate hemodynamic changes after implantation of vascular grafts. The results of the model analysis are consistent with previous work done in experimental setups.

Aorta↗
PubMed1997

Description of the ventriculoarterial interaction dynamics using recurrence plot strategies.

OBJECTIVE: The classical description of ventriculoarterial coupling by calculating the ratio between the effective arterial elastance Ea to the end-systolic elastance Ees does not give insight into the underlying dynamics of the interaction between left-ventricular pressure (LVP) and aortic pressure (AOP) and flow (AOF). The aim of this study was to introduce a state space representation for the ventriculoarterial coupling and to quantify changes of the coupling state. METHODS: A ventriculoarterial state space orbit VAO was defined to be dependent on three variables: VAO = [LVP(t), AOP(t + delta t), AOF(t + delta t)]. Changes in the coupling effect directly or indirectly on the time series of these parameters. They reflect the actual state of the cardiovascular system. The time delay delta t between the LVP and the aortic signals takes respect to the short delay between the heart action and the resulting waves in the arterial tree. The recurrence map of the VAO(i) (i = 1 .. N, N = number of points) is constructed by plotting the index i of every single point on the orbit (x-axis) against the indices of his 10 nearest neighbors (y-axis) in distance. The data were recorded in 9 anaesthetized pigs with a sample frequency of 512 Hz over a period of 6 seconds using piezoelectric pressure sensors and a Doppler flowmeter. A control condition was compared to a total occlusion of the descending aorta as a strong artificial disturbance of ventriculoarterial interaction. The nonlinear parameters percent recurrence, percent determinism and the entropy were calculated from the plot. RESULTS: Periodic crossing points and forbidden zones in all plots identify the nonlinear character of the chosen variables. The recurrent patterns are less rigid for control conditions than for total occlusion. Entropy (2.3% rise) and determinism (24% rise) are significantly (p < 0.003) increased. Total aortic occlusion leads to more complex time correlation patterns. CONCLUSIONS: These results may reflect the loss of an ideal coupling state leading to a more complex deterministic behavior of the overall regulatory system. Because recurrence plots do not impose rigid constraints on data set size, stationarity, or statistical distribution, we hypothesize that this technique might be useful to describe the nonlinear dynamics between left ventricle and arterial system.

Animals↗
PubMed1997

Simulation of human circulation at cardiopulmonary bypass.

This article describes a simulation model of the human circulatory system under extracorporeal circulation. The model is supposed to enable the design of a controller for heart-lung-machines, and to perform simulations of the controllers' behaviour. The controller should be able to control extracorporeal circulation based on the oxygen supply to individual organs. The model contains all important control mechanisms in the human circulation that maintain hemodynamics, and the influences of extracorporeal circulation to them. We considered the renin-angiotensin-system, the arterial local autoregulation and the autonomic nervous system. The behaviour of the most important hormone levels is reproduced. Furthermore, hemodynamics in the arterial tree is modeled with locally distributed pressure and flow parameters in 128 vessel segments to gain knowledge about the oxygen supply position of single internal organs. The influences of the special circumstances of an operation under extracorporeal circulation such as anesthesia or hypothermia are implemented in the model. Simulation results show that our model describes the behaviour of human circulation detailed enough to validate the designed controllers.

Arteries↗
PubMed1997

A mathematical high time resolution model of the arterial system under extracorporeal circulation.

OBJECTIVE: The purpose of the following study was to establish a computer-generated model of the hemodynamic effects of pulsatile extracorporal perfusion describing flow and pressure parameters in the body for any given input flow patterns. METHODS: The human arterial tree was delineated according to a 128-branch model encompassing bifurcations and linear physical properties of the arterial walls. The distribution of flow and pressure waves was calculated based on a refined 3-element windkessel model. Autoregulatory mechanisms of brain and kidneys were implemented. RESULTS: By providing a simulated, "pump-generated" flow curve as the input signal to the system, the model was able to create and display flow and pressure curves at a high time resolution in each part of the systemic circulation including reflection phenomena throughout any observation period chosen. The hemodynamic effects of different pump-flow patterns, age, variations in hematocrit, hypothermia and occlusion of arterial branches, like the renal artery, could be simulated. CONCLUSION: In an attempt to get closer to a mathematically based regulation of heart-lung machines, this model of computer generated extracorporeal circulation provides an initial step. Ongoing research is required for implementation of metabolic conditions and continuous approximation of the model of the real physiologic or pathologic situation.

Arteries↗