Jörn Bajorat

Comparison of invasive and less-invasive techniques of cardiac output measurement under different haemodynamic conditions in a pig model.

Background and objective: Despite the introduction of various less‐invasive concepts of cardiac output measurement, pulmonary arterial thermodilution is still the most common measurement technique. Methods: This prospective controlled study was designed to compare different methods of cardiac output measurement simultaneously. Pulmonary arterial thermodilution, transpulmonary thermodilution (PiCCO™), trans‐oesophageal echo‐Doppler probe (HemoSonic™) and partial carbon dioxide rebreathing technique (NICO™ monitor) were evaluated against a peri‐aortic transit‐time flow‐probe as reference method in a clinically relevant animal model. After approval from the Local Ethics Committee on Animal Research, the investigations were conducted in nine anesthetized domestic pigs. Systemic haemodynamics were modulated systematically by the application of catecholamines, caval occlusion and exsanguination. Statistical analysis was performed with Bland‐Altman and linear regression. Results: A total of 366 paired cardiac output measurements were carried out at a reference cardiac output between 0.5 and 7 L min−1. The correlation coefficients for pulmonary arterial and transpulmonary thermodilution against reference were 0.93 and 0.95, for trans‐oesophageal Doppler and partial rebreathing technique 0.84 and 0.77. Pulmonary arterial thermodilution and transpulmonary thermodilution showed comparable bias and limits of agreement. Where HemoSonic™ showed an overestimation of cardiac output at a higher precision, NICO™ overestimated low and underestimated higher cardiac output values. Conclusions: Our data suggest that pulmonary arterial thermodilution and PiCCO™ may be interchangeably used for cardiac output measurement even under acute haemodynamic changes. The method described by Bland and Altman demonstrated an overestimation of cardiac output for both thermodilution methods. HemoSonic™ and NICO™ offer non‐invasive alternatives and complementary monitoring tools in numerous clinical situations. Trend monitoring and haemodynamic optimizing can be applied sufficiently, when absolute measures are judged critically in a clinical context. The use of the NICO™ system seems to be limited during acute circulatory changes.

The benefits of using Guyton's model in a hypotensive control system

In order to improve the intraoperative applications, this paper presents the advantages of using Guytons model in hypotensive control system development. In this system, the mean arterial pressure is decreased and maintained at a low level during anaesthesia by controlling sodium nitroprusside infusion rate. The key of the study is to develop a physiological model of cardiovascular dynamics to present the mean arterial pressure response to sodium nitroprusside, which was considered as a linear model in most of known blood pressure control systems. Being linear, the previous models cannot accurately mimic a physiological system of human circulation, especially at deep hypotensive control with strong reaction of the body. The enhanced model in this study was modified based on Guytons model of human circulation. It is useful to design a PID controller, which allows studying and handling the wide range of the body sensitivities. This model is also helpful for studying the behaviors of patients under anaesthesia conditions, such as the perfusion of organs and the reaction of the body at hypotensive state. A fuzzy gain scheduler and a supervising algorithm were also developed for online tuning the controller to handle the behavior of the body. The control system was tested on 25 experiments on seven pigs in the animal laboratory. Simulation and experiment results proved the usefulness of Guytons model in control system design which can present the dynamical response of blood pressure in the circulation under and after hypotensive control. The results also indicated the safety and stability of the controller.

Design and implementation of a control system reflecting the level of analgesia during general anesthesia.

Abstract Introduction: Measuring and ensuring an adequate level of analgesia in patients are of increasing interest in the area of automated drug delivery during general anesthesia. Therefore, the aim of this investigation was to develop a control system that may reflect the intraoperative analgesia value. Our hypothesis was that a feedback controller could be applied in clinical practice safely and at an adequate quality of analgesia. The purpose of this study was to evaluate the practical feasibility of such a system in a clinical setting. Methods: The control system for the level of analgesia described in this paper relies on a parameter combination of heart rate variability (HRV), heart rate (HR), and blood pressure (mean arterial pressure, MAP), which serve as input variables for an expert system. For this fuzzy system, the experience of the participating anesthesiologists was translated into a set of fuzzy rules. In a pilot trial, the control system for automated titration of remifentanil, a short-acting opioid, was tested combined with a closed-loop propofol infusion system for hypnosis. Ten adult patients (4 women, 6 men), aged 22–52 years (median, 45 years; range, 29–49 years), with an American Society of Anesthesiologists physical status class I or II and who were scheduled for elective trauma surgery in a supine position were enrolled in this prospective trial. The precision of the system was calculated using internationally defined performance parameters. Results: There was no human intervention necessary during the computer-controlled administration of propofol and remifentanil, and operating conditions were satisfactory in all patients. All patients assessed the quality of anesthesia as “good” to “very good”. Median performance error, median absolute performance error, and wobble for HR and MAP during maintenance of anesthesia were -8.98 (5.32), 10.08 (4.17), and 2.68 (1.29) and -4.51 (12.73), 13.63 (2.27), and 3.90 (2.08) [mean (SD)], respectively. Conclusion: The control system, reflecting the level of analgesia during general anesthesia designed and evaluated in this study, allows for a clinically practical, nearly fully automated infusion of an opioid during medium-length surgical procedures with acceptable technical requirements and an adequate precision.

Comparison of heart rate variability response in children undergoing elective endotracheal intubation with and without neuromuscular blockade: a randomized controlled trial

The routine use of neuromuscular blocking drugs (NMBD) for endotracheal intubation in children is the subject of much controversy. The analysis of heart rate variability (HRV) can reveal information about the functional state of the autonomic nervous system (ANS).

Automatic drug delivery in anesthesia - the design of an anesthesia assistant system ⋆

Abstract The main goals of general anesthesia are adequate hypnosis, analgesia and maintenance of vital functions. For a number of surgical procedures neuromuscular block is essential. Furthermore patient safety and cost reduction as minimized drug consumption and shortened post-operative recovery are main issues and motivation of automation efforts in anesthesia. Since the beginning of the eighties engineers and physician are working together in the field of the development of closed-loop systems for drug delivery. The work gives only a short overview about the development of the automation in drug delivery systems over the last years without the claim of completeness and expressed the much more vision. In the final stage, the designed control system, the so called “Rostocker assistant system for anesthesia control (RAN)” should be fitted out with the possibility to control four different drugs automatically. Currently the multiple-input multiple-output (MIMO) control of the depth of hypnosis and neuromuscular blockade is realized as well as the closed-loop control of deep hypotension. A pilot study for the closed-loop control of analgesia is currently running. The paper points some general remarks and the designed MIMO-system for controlling the depth of hypnosis and the neuromuscular blockade

Online Process Identification of Hypnosis

For most kinds of surgeries an adequate anaesthesia is necessary. Three main components characterise an anaesthesia - the relaxation, the analgesia and the hypnosis. This paper treats the topic of the Propofol/ Remifentanil induced hypnosis. We try to find a suitable model structure for the online identification of this process during a surgery. After a review of known models from literature an own model approach was developed and validated. By an offline optimisation study the parameter estimation problem of the defined cost function was examined. An online approach based on a recursive, iterative gradient algorithm was implemented. The quality of the identified parameters could be verified using measurements of surgeries.

A surgical technique for a terminal intracranial hypertension model in pigs

The life-threatening effects of intracranial hypertension on brain perfusion and cerebral metabolism are the subject of current research in different animal models. The purpose of this study was to describe an efficient, reliable and inexpensive surgical method for temporary elevation of intracranial pressure (ICP) in acutely instrumented pigs in a research setting. Therefore, a balloon catheter was inserted into the left lateral ventricle and an ICP sensor was placed in the parenchyma of the right cerebral hemisphere. Ten acutely instrumented pigs were studied while under deep terminal general anaesthesia. The step-by-step inflation of the intraventricular balloon allows one to achieve the desired ICP up to 46 mmHg and maintain it at this level. ICP values ranged from a median of 2 (1–2) mmHg to 43 (29–45) mmHg. To the authors’ knowledge, this is the first detailed description of a minimally invasive surgical technique for temporary ICP elevation in pigs via stepwise inflation of an intraventricular balloon.

A fuzzy system for regulation of the analgesic remifentanil during general anaesthesia

During the last decade, the research group Anaesthesia Control at the University of Rostock (Germany) developed an assistant system to support anaesthetists in the operating theatre. The main objectives during general anaesthesia are appropriate hypnosis, analgesia, relaxation, and stable vital functions. The development of our controllers for hypnosis and relaxation were completed in the last years. The systems were established in the clinical routine in the university hospital. Until now there exists no standard method to measure the state of analgesia or stress response of the patient. In this paper a fuzzy approach is presented to control the application of the analgesic drug remifentanil based on the expert knowledge of anaesthetists. The anaesthetists monitor vital parameters for example arterial blood pressure and heart rate to apply the analgesic drug. Additionally, the heart rate variability was involved as input for the fuzzy analgesia system, which is realized with two controllers, one active at a time. The main controller regulates the continuous rate of remifentanil based on the change in the mean arterial blood pressure, the heart rate and its standard deviation. The second controller calculates a bolus amount of remifentanil based on the absolute values of input parameters, if the heart rate is higher than 90 bpm. Both fuzzy controllers are Mamdani inference systems. The usual time regime for general anaesthesia as well as the minimal and maximal dosage of remifentanil are regarded in the system. The development options of the systems and the first results by controller simulations are discussed.

New aspects of the rostocker assistant system of anesthesia control: Invited Paper for the session ‘Current trends and challenges in closed loop anesthesia’

The research group medical control at the Universities of Rostock (Germany) and Wismar has developed an assistant system for anesthesia to support anesthetists in controlling and maintaining the state of the patient in the operating theatre. The main objectives during general anesthesia are adequate level of hypnosis, analgesia, relaxation, and stable vital functions. During the last 20 years many controllers for the automatic drug delivery in anesthesia were developed. Starting with controllers for keeping a constant level of neuromuscular blockade, controllers for the hypnosis and analgesia were performed. Our research group developed a control system with an adaptive Generalized Controller for the neuromuscular blockade, a fuzzy controller for the control of the level of hypnosis and a fuzzy-system for analgesia control. The current contribution summarizes two studies, the MIMO control of the neuromuscular blockade and the depth of anesthesia which was done with 22 patients and the MIMO control of the depth of anesthesia and the level of analgesia. A model-based predictor for the level of neuromuscular blockade (NMB), to predict the level of NMB after stopping the drug infusion was integrated as a new feature for improving the clinical benefit.

Blutdruckregelung im Umfeld chirurgischer Interventionen / Blood pressure control in the area of surgical interventions

Zusammenfassung Für spezifische chirurgische Interventionen, wie zum Beispiel die Implantation eines Aortenstents, wünschen einige Kardiologen die Absenkung des physiologischen mittleren arteriellen Blutdruckes auf tiefe Bereiche um 40 mm Hg. Dadurch können mögliche intra- sowie postoperative kardiale und cerebrale Komplikationen vermieden werden. Für die Absenkung des Blutdruckes wird von den Anästhesisten oftmals das Medikament Nitroprussidnatrium genutzt. Da die Patienten sehr individuell auf dieses Medikament reagieren, benötigt der Arzt einige Minuten, bis er durch empirische Variation der Applikationsrate den Zielblutdruck erreicht hat. In der Arbeitsgruppe „Anaesthesia Control“ an der Universität Rostock wurde ein adaptives Regelungssystem entwickelt, das selbstständig die Absenkung des mittleren arteriellen Blutdruckes auf den gewünschten Zielwert vornimmt. Um dieses Regelungssystem simulativ testen zu können, wurde das von Guyton et al. entwickelte Kreislaufmodell in MATLAB/Simulink implementiert. In dieser Arbeit wird sowohl das Simulink-Guyton-Modell als auch der Entwurf eines selbsteinstellenden Reglers zur automatischen Medikamentenapplikation behandelt. Der entwickelte Regler zur Blutdruckabsenkung wurde in 25 Versuchen an 7 Hausschweinen experimentell validiert. Das entwickelte Regelungssystem wurde anschließend um eine Komponente zur Blutdruckanhebung erweitert. Mittels Noradrenalin kann der mittlere arterielle Druck nun auf vorgegebene Werte angehoben werden, um z.B. den cerebralen Perfusionsdruck bei Patienten mit Schädel-Hirn-Trauma aufrecht zu erhalten. Der Regler wurde in einer Pilotstudie an 10 Schweinen getestet. Abstract For specific surgical interventions, such as aortic stent implantation, it might be temporarily necessary to decrease mean arterial pressure to rather low levels (around 40 mm Hg). Such hypotensive pressure levels are necessary to avoid intra- and postoperative intricacies. Traditionally, the drug Nitroprussidnatrium is used for this task. To adjust the correct amount of drug to reach the target pressure as fast as possible and without overshoot, the anaesthetists typically use empirical knowledge and might need several minutes until the target point is reached. In our research group, an adaptive control system was developed for this task which is able to compute and set the transient drug release automatically. For the design and testing of the adaptive control strategy, the well known Guyton model was implemented into the MATLAB/Simulink development environment. This paper describes the implementation and adaption of the Guyton model to hypotensive pressure control and provides some algorithmic details of the adaptive control strategy for automatic drug delivery in deep hypotension. The designed control system was successfully validated in animal trials (25 trials on 7 pigs). Following this, an additional controller component for increase of blood pressure with the help of the drug Noradrenalin was implemented. It is now possible to increase blood pressure to a specific value to save defined cerebral perfusion pressure for patients with craniocerebral injury. In a second pilot trial, this controller extension was tested in 10 pigs.