Christine Goffin

Hirndruckmodellierung und Regelung einer neuen mechatronischen externen Ventrikeldrainage

Zusammenfassung Die Ableitung von Hirnwasser durch externe Ventrikeldrainagen ist eine lebensnotwendige Maßnahme bei akutem Hirndruckanstieg und wird bisher in der Regel rein mechanisch umgesetzt. Die neu entwickelte und hier vorgestellte mechatronische Ventrikeldrainage regelt den Hirndruck und ermöglicht dem Patienten damit eine zuverlässigere Therapie. Das Drainagekonzept erlaubt dem Arzt eine umfassendere Beobachtung des Krankheitsverlaufs durch die Darstellung wichtiger Parameter wie Hirndruck, drainiertem Hirnwasserfluss und Compliance. Für die Regelung der externen Ventrikeldrainage wurde eine statische Teilkompensation des nichtlinearen Prozesses mit einem PI-Anteil kombiniert und simulativ erprobt. Im Einsatz an einem Menschmodell-Prüfstand konnte die geregelte Ventrikeldrainage die vorgegebenen Regelziele erfüllen. Abstract The drainage of cerebrospinal fluid by an external ventricular drainage is an indispensable therapy for acute brain pressure rise which at present is realized commonly with purely mechanical devices. The new drainage concept presented in this work uses a mechatronic device with automatic pressure control to enable a more reliable and automated therapy for the patient and a comprehensive examination of the disease for the doctor by the monitoring of the important parameters brain pressure, drained cerebrospinal fluid and compliance. To control the ventricular drainage, a partial static compensation of the process nonlinearity was combined with a proportional integral controller. This controller was tested in simulations and in a human-like test rig. The controlled ventricular drainage was able to achieve all brain pressure control targets.

Control of an electromechanical hydrocephalus shunt--a new approach.

Hydrocephalus is characterized by an excessive accumulation of cerebrospinal fluid (CSF). Therapeutically, an artificial pressure relief valve (so-called shunt) is implanted which opens in case of increased intracranial pressure (ICP) and drains CSF into another body compartment. Today, available shunts are of a mechanical nature and drainage depends on the pressure drop across the shunt. According to the latest data, craniospinal compliance is considered to be even more important than mean ICP alone. In addition, ICP is not constant but varies due to several influences. In fact, heartbeat-related ICP waveform patterns depend on volume changes in the cranial vessels during a heartbeat and changes its shape as a function of craniospinal compliance. In this paper, we present an electromechanical shunt approach, which changes the CSF drainage as a function of the current ICP waveform. A series of 12 infusion tests in patients were analyzed and revealed a trend between the compliance and specific features of the ICP waveform. For waveform analysis of patient data, an existing signal processing algorithm was improved (using a Moore machine) and was implemented on a low-power microcontroller within the electromechanical shunt. In a test rig, the ICP waveforms were replicated and the decisions of the ICP analysis algorithm were verified. The proposed control algorithm consists of a cascaded integral controller which determines the target ICP from the measured waveform, and a faster inner-loop integral controller that keeps ICP close to the target pressure. Feedforward control using measurement data of the patients position was implemented to compensate for changes in hydrostatic pressure during change in position. A model-based design procedure was used to lay out controller parameters in a simple model of the cerebrospinal system. Successful simulation results have been obtained with this new approach by keeping ICP within the target range for a healthy waveform.

Single pulse analysis of intracranial pressure for a hydrocephalus implant

The intracranial pressure (ICP) waveform contains important diagnostic information. Changes in ICP are associated with changes of the pulse waveform. This change has explicitly been observed in 13 infusion tests by analyzing 100 Hz ICP data. An algorithm is proposed which automatically extracts the pulse waves and categorizes them into predefined patterns. A developed algorithm determined 88%±8% (mean ±SD) of all classified pulse waves correctly on predefined patterns. This algorithm has low computational cost and is independent of a pressure drift in the sensor by using only the relationship between special waveform characteristics. Hence, it could be implemented on a microcontroller of a future electromechanic hydrocephalus shunt system to control the drainage of cerebrospinal fluid (CSF).

The Role of a Dynamic Craniospinal Compliance in NPH—A Review and Future Challenges

Despite the normal mean intracranial pressure in normal pressure hydrocephalus, the cerebral ventricles enlarge. Many hypotheses exist as to why, and these have lately been investigated in simulation. These can be grouped into one of two categories: 1) Tissue damage is caused by a transmantle pressure gradient widening the ventricles mechanically. 2) The overall cerebrospinal fluid dynamics are disturbed resulting from various pathologies. This paper reviews the literature regarding the computational simulation models investigating the development of enlarging ventricles in connection with the onset of hydrocephalus. The models are categorized by the underlying hypothesis and their results are contrasted with clinical findings in the field. Finally, open questions are identified for future modeling approaches.

Modelling and Understanding Normal Pressure Hydrocephalus

Up to 10% of all dementia patients may actually suffer from Normal Pressure Hydrocephalus (NPH), a disease whose pathogenesis is not yet understood. The ventricles enlarge in this particular form of hydrocephalus, although the mean intracranial pressure (ICP) is not elevated. In this paper, the two main biomechanical hypotheses for the formation of NPH are examined by critically reviewing biomechanical models investigating the onset of NPH: 1) A transmantle pressure gradient between ventricles and subarachnoid space (SAS) widens the ventricular space mechanically, stresses the ventricular walls and causes edema leading to tissue damage. 2) Disturbed dynamics caused either by reduced compliance, vascular disease, malabsorption or obstructions of cerebrospinal fluid (CSF) pathways cause tissue damage. The different models are analyzed and contrasted with clinical findings in order to summon what could be gained by biomechanical model based analysis. It is the goal of this article to identify open questions that can be answered by biomechanical models.

Robust Control of Intracranial Pressure with an Electromechanical Extra-ventricular Drainage

The drainage control of cerebrosprinal fluid, as needed in the treatment of increased intracranial pressure, is to this day achieved manually by using an external ventricular drainage system. The manual control procedure bears several risks for the patient among which the rapid decrease of intracranial pressure induced by patients upper body inclination angle change is the most prominent. An automatic controller is suggested to increase patients safety and the quality of the therapy by delivering a continuous pressure control and the rejection of disturbances like cerebrospinal fluid production rate and inclination angle changes. In this contribution an intracranial pressure controller is designed using the robust control methodology and subsequently validated in nonlinear simulations and in a hydrodynamic human cerebral simulator. The controller is designed using a mixed uncertainty approach accounting for uncertainties in the technical and the physiological system. A self-scheduled implementation of the controller guarantees the compensation of the nonlinear input function for the process, being of Hammer stein structure. The controller shows stable response in simulations and Mock experiments for various operating points and disturbance rejection tests.

Functional modeling of the craniospinal system for in-vitro parameter studies on the pathogenesis of NPH

Normal Pressure Hydrocephalus (NPH) has become a common disease in the elderly coming along with typical symptoms of dementia, gait ataxia and urinary incontinence, which make the differential diagnosis with other forms of dementia difficult. Furthermore the pathogenesis of NPH is still not understood. About 10% of all demented patients might be suffering from NPH [1]. Many hypotheses suggest that modified biomechanical boundary conditions affect the craniospinal dynamics inducing the pathogenesis of NPH. We present a novel approach for an in-vitro model of the craniospinal system to investigate important hydrodynamic influences on the system such as (dynamic) compliance of the vascular system and especially the spinal subarachnoid space (SAS) as well as reabsorption and hydrostatics. The experimental set-up enables the individual adjustment of relevant parameters for sensitivity analyses regarding the impact of resulting CSF dynamics on the pathogenesis of NPH.

Biomechanical model of cerebral vascular dynamics and their effect on CSF dynamics

With advancing age venous wall thickness increases going along with a loss of elastic properties, the arterioles curl and capillary density decreases leading to a reduction of cross sectional area [1]. Even in healthy individuals these changes influence vascular and CSF dynamics, as a reduction of total cerebral blood flow, aqueduct and cervical stroke volume are reported [2]. In NPH these dynamics seem to be altered in a different way, as aqueduct stroke volume [2] and ICP amplitude are increased and arteriovenous delay is drastically reduced compared to normal aging [3]. However it is not clear in what way the described vascular alterations influence the pressure propagation inside the vessels and impact CSF dynamics. So far no biomechanical model exists that investigates the influence of macroscopic and microscopic changes of cerebral blood vessels. That is why we put a model up for discussion that simulates vascular pressure propagation and enables the investigation of altered vascular properties in the context of NPH. A Matlab Simulink model was developed reproducing each vessel section by a distensible compartment. Therefore the cerebral vascular tree was divided into 13 sections from carotid artery to venous sinuses and the pulsatile carotid artery and sinus pressure were inputted as Fourier series. The cross sectional area was varied according to literature data and flow resistance was implemented taking into account the rheological characteristics of blood. The Windkessel function and relaxation properties of vascular walls were integrated by a Voigt model, enabling the variation of wall properties for each section individually. Due to the distensible vessel walls each section interacts with the CSF compartment and autoregulation was implemented by a simple proportional controller. After parameterisation mean pressure and pressure amplitude in the vessel sections showed good accordance with literature values [4]. We have proposed a model of vascular dynamics that is able to identify the impact of altered vascular wall properties and structural changes. Furthermore the effect of different arterial and venous input pressure profiles can be analysed. These parameter analyses are part of our ongoing research.

FIRST RESULTS OF A NEW ELECTROMECHANICAL CONTROLLED EXTERNAL VENTRICULAR DRAINAGE IN A PORCINE MODEL

Acute increase of intracranial pressure (ICP) usually has to be treated with an external ventricular drainage (EVD). Current standard mechanical EVD carry a lot of disadvantages, which hypothetically could be bet- ter managed by a newly developed electromechanical EVD. In this report our first preliminary results of such an elec- tromechanical EVD applied in a porcine animal model are presented. The drainage was demonstrated to be both suc- cessful in monitoring and controlling elevated ICP, and able to detect slit ventricles due to overdrainage, if the indented target ICP was set too low.