- Email: [email protected]
Concepts for a mechatronic device to control intracranial pressure
Concepts for a mechatronic device to control intracranial pressure S. Jetzki, M. Kiefer, M. Walter, S. Leonhardt Lehrstuhl für Medizinische Informationstechnik, Helmholtz-Institut für Biomedizinische Technik, RWTH Aachen, Pauwelsstraße 20, 52074 Aachen, phone: 0049 241 802320 Neurochirurgische Klinik, Universität des Saarlandes, Medizinische Fakultät, Gebäude 90, 66421 HomburgSaar email: [email protected]
keywords: cerebrospinal fluid (CSF), hydrocephalus, CSF-Drainage, intracranial pressure (ICP), ICP control
1
Background adjustable valve
lateral ventricles
III. ventricle aqueduct of Silvius IV. ventricle inner liquor space
Fig. 1
liquor production plexus choroidei outer liquor space
Schematic sketch of the brain
Normally, CSF production (average around 20 ml/h) and reabsorption balance each other. However, if this sensitive balance is disturbed (e.g. by overproduction, blockage of the flow pathway or reduced reabsorption) a fluid accumulation will take place leading to an increased intracranial pressure (ICP). This disease is called “hydrocephalus” and is commonly treated with an implanted pressure control valve, which drains the excessive CSF to the abdominal space. Current valves can be adjusted through the skin by a magnetic device after they have been implanted. Based on the patient’s ICP levels, which differ depending on the individual person, the physician must estimate the suitable opening pressure p0 during treatment (Fig. 2) [7].
flow
Human brain tissue is surrounded by a water-like fluid (cerebrospinal fluid, CSF). Its main purpose is to protect the brain from mechanical shocks acting on the skull and to reduce tissue weight through buoyancy. CSF is produced in the centre of the brain, mainly in a cell formation named the plexus choroidei. The CSF then flows through the ventricles, internal cavities and small openings (aqueduct of Silvius) to the outside of the brain (see Figure 1). It is reabsorbed into the venous blood system, lymph channels and the spinal channel ([5][6]).
ICP
CSF-flow p0
Fig. 2
ICP
Block model of ICP valve
The ICP depends on the orientation of the body. An upright position decreases the pressure by about 10 mmHg as CSF flows into the lumbar region of the spine due to gravity. Therefore, resent gravitational valves change the total opening pressure of the shunt depending on the body position [8]. Abrupt pressure peaks in the ICP can result from coughing or sneezing. These short pressure peaks drain off so much CSF, that it takes hours until it is reconstituted. Whereas there are limits to the ability of a purely mechanical valve to prevent such problems, a mechatronic system may be a solution to avoid too much or too little CSF drainage.
2
Model of intracranial CSFand blood dynamics
Assuming that the space inside the skull bone may be viewed as a closed cavity, this volume may be subdivided into three compartments: blood, tissue and CSF. Based on this assumption, three mass balance equations may be derived (also known as the "MonroKellie doctrine"):
Vtotal (t ) = VCSF (t ) + Vblood (t ) + Vtissue
(1)
The volume in each compartment may be balanced by itself (Eq. 2). •
•
VCSF (t ) = ∫ V prod (t ) − V res (t )dt •
•
Vblood (t ) = ∫ V bl ,in (t ) − V bl ,out (t )dt Vtissue (t ) ≈ const .
(2)
25
3 Note that a fluid exchange between the blood and CSF compartment can only occur in two spots. First, CSF is transferred out of the arterial blood to the CSF compartment by the plexus choroideus. The amount is determined by the pressure difference between the arterial blood pressure (ABP) and the ICP and an absorption constant. •
V prod (t ) = k prod ⋅ ( ABPplexus (t ) − ICP (t ))
(3)
It is assumed that venous reabsorption starts above a certain threshold (RG) . After this point reabsorption is described as a linear function of the pressure difference between ICP and venous pressure (VBP) and a reabsorption constant. • 0 ICP < RG ⎫ ⎧ V res (t ) = ⎨ ⎬ ⎩k res ⋅ ( ICP (t ) − VBP (t )) else ⎭
(4) To model intracranial fluid dynamics, the cerebral blood flow has to be considered as well. As in other vascular areas of the body, cerebral blood flow is locally auto-regulated. Thus, the brain tissue is able to control the local blood flow depending on physiological demands (eg. activity, metabolism, CO2) [1]. Moreover, blood volume and CSF volume affect each other, described in [2]. As a last requirement, a constitutive equation linking intracranial volume to ICP is needed. Brain tissue is an elastic substance. Its properties may be described best by a Bingham plastic [3]. If the shear stress is below a certain limit, elastic deformation takes place. Above this limit, the deformation is plastic. Measurements have shown that the static relation between ICP and volume is approximately exponential in the range of moderate ICP values [4] leading to
Concept for ICP Control
An implanted shunt system works in parallel to the natural CSF absorption pathway (Fig. 4). The aim of a shunt is normalization of the ICP on a desired value given by the physician. The following conditions must be considered with the draft of the automatic controller depending upon treatment concept: - which lasting offset ICP values can be tolerated? - which dynamic deviations of the controlled variable can be tolerated? - how fast is a deviation compensated? - which mean current rates by the valve are to high or low? After discussions with different physicians the following basic data were specified for the shunt system: - For a short time the brain pressure increased up to 10mmHg can be tolerated also by seriously ill patients. - The average value should not be more than 2 mmHg above the given target pressure. - Compensating of a pressure disturbance within 30 seconds is sufficient. - A maximum drainage capacity of 500 ml/h is sufficient for the drainage (more than the double maximum natural production rate). Additional to the actual pressure control also the change of the body position (upright or lying down) of the patient must be considered in the automation concept, because it changes the physiological desired value. With an angle sensor the body situation is seized and the desired ICP value given by the physician is adapted automatically. In addition, a part of the controlled system changes (the drainage height and thus also the discharge during given valve position), which has to be compensated by a correction of the automatic controller characteristics. ICP Control Concept desired ICP
ICP (t ) = k 0 + k elast ⋅ e
+
aVtotal ( t )
(5)
Combining the equations given above, the following model for the CSF circulation system may be composed, see Fig. 3. tissue volume ~const.
blood volume
CSF production ABP ICP
Prod
ICP
+
-
∫ dt
CSF volume
ICP
+ V
CSF resorption V ICP
Fig. 3
VBP
Model of CSF production and resorption
-
+
-
GR (z)
VProd valve
-
Vtissue
∫
+
-
VRes
angle sensor ICP sensor
Vblood ICP
ICP
+ V
V ICP
Fig. 4 Block diagram of the control circuit and set point tracing for the implanted valve system With the selection of a suitable automatic controller structure, the characteristics of the actuator must be considered accordingly. With switching valves with only two switch conditions, a controller with hysteresis can be used, in order to avoid too much switching of the valve.
26
Clinical Trial
In order to develop and test the control algorithms, tests were performed using an external drainage system. Patients who receive a commonly used external CSF drainage before a shunt implantation are put in an intensive care station. Their ICP is recorded for three to seven days. During this period, excess CSF is drained through an external tube to a container. Adjusting the height of the container determines the ICP value at which CSF starts to drain. In order to test the control concepts in clinical tests, a system was developed for brain pressure control through external CSF drainage (Fig.5).
ICP [mmHg]
4
hysteresis
10 8 6 4 2
closed valve open valve
0
1
2
3
4
time [min]
Fig. 6: Characteristic of a low pass filtered process during ICP Control
PC (optional) squeeze valve
ICP-measurement security drainage
Monro-Niveau
Fig. 5 Structure of the experimental system for CSF drainage and ICP control In addition to the conventional drainage system, the setup was equipped with a squeeze valve used to switch CSF flow on and off. The brain pressure measurement was performed with a conventional ICP measurement device from the manufacturer Spiegelberg. A microcontroller controls, displays and enters the desired parameter values. A PC can be attached by an electrically isolated interface for logging of the test sequence. As a security measure, another drainage container was fixed parallel, in a higher position, thus limiting the maximum brain pressure in case of a system crash. Because the control valve can take only two discrete switch steps, a two-step action controller with adjustable hysteresis was implemented. Hysteresis in combination with a low-pass filter prevents too much switching of the valve. At the Universitätsklinik Homburg/Saar, the controlled drainage could be improved on five patients. In one case the controlled drainage worked during a period of one week without interruption. Figure 6 exemplifies the function of the automatic controller with hysteresis.
The desired ICP value amounted to 7 mmHg, the switching hysteresis was adjusted to 1mmHg. During short pulses the valve was opened for two to three seconds. In each case, the ICP exceeded the upper trigger level of the hysteresis range for only a very short time. On the average, the ICP lied somewhere below the lower trigger level because the drainage capacity of the system is much higher than the physiological CSF production rate of the patient. The drainage volume is expressed in equation 6. •
V = ( ICP + ( hbrain − hcontainer )) ⋅ G current
(6)
A test sequence gave the drainage conductance .G current ≈ 275
ml / h mmHg
(7)
With this clinical trial the container of the drainage was fixed at Monro level. With a sampling time of 1 second, 0.6 ml of CSF per sampling step flowed through the opened valve. Since the brain pressure reacts with a time-delay, the valve is opened usually for 2 to 3 scanning steps. The drainage of 1.2 to 1.8 ml leads to sustained drop below the lower trigger level. By using a smaller tube diameter in the drainage, the automatic controller action could be improved clearly. ICP [mmHg]
control unit
15
10
5
Fig. 7a Characteristic of the ICP Control over many hours with one coughing attack
0
2
3
4
5
6
7
8 9 time [h]
27
ICP [mmHg]
valve switching
ICPsoll = ICP0 + k ⋅ sin( x ⋅ π )
80
(8)
coughing
ICP [mmHg]
60 40 20 desired ICP 0 0
interpolated with the sine-like equation 8. Figure 8 shows the automatic desired value adjusting in the clinical trial.
10
20
30
40
50 60 time [sec]
valve switching 50 40
Fig. 7b: Recognition of pressure peaks caused by coughing
20
Figure 7a shows the curve characteristic of controlling over several hours with the same patient. During the process of the trial, the patient had a cough attack after 3.5 hours. With strong coughing, the extremely high pressure in the chest area spreads through the blood vessels and the connecting soft tissue parts into the head, causing the brain pressure to rise to exorbitantly high pressures (90 mmHg, range of values in the figure is cut off). As a result, the control algorithm opened the valve and too much CSF ran off, so that after the coughing episode the ICP was at a very low level. It took 1.5 hours until the ICP was in the desired range again. This clearly demonstrates that a reliable artefact recognition procedure is required to detect coughing and deactivate the control loop during this time. A good criterion to detect a cough attack is the abrupt rise of ICP. If the ICP doubles itself in one sampling step, there is probably an artefact and the drainage will be closed for a certain time (e.g. 10 sec.), illustrated in figure 7b. If the brain pressure returns back to a low level this time, a cough attack is present and the valve stays closed with a repeated rise again. If the ICP remains on the high value, the increase in pressure will be attributed to other causes and the drainage opens. In case of a changed body position of the patient, the desired value of the brain pressure changes. In order to adjust this accordingly, a measurement of the patient position is necessary. At the adjustable heading of the bed, an angle sensor was attached. Position changes, which are caused by turning the body or pillows, cannot be recognized. At the beginning of the drainage a calibration of the angle sensing is necessary. Hereby, the patient is brought into a horizontal position and the desired ICP value is adjusted. After the ICP has reached the desired value, the drainage is closed and the patient is seated up as far as possible. After fading away the transients, the desired ICP value for the upper body position adjusts itself. Between these two values the angle dependent ICP is
0
artefact
desired ICP
30
10 0
10
20
30
40
50 time [min]
Fig. 8 Adaptation of the set value to the patient position
5
Mechatronic integration
Raised, destructive ICP demands a therapy. To decrease and control ICP, valves can drain excessive CSF. ICP and body position are prior parameters. Running a closed control loop is possible with the valve as an electronic, controllable actuator. The concept of integrating all components into a micro mechanic system is illustrated in Fig. 9. catheter and pressure sensor
angle sensor telemetry control unit
valve
Fig. 9 Implant ICP Control
for
The control unit handles information processing and fulfils the following tasks: - sensor data processing - actuator control - process parameters examination - adaption of changed circumstances (e.g. body position) - energy management - communication - data recording
28
The sensors for pressure, flow and body position are attached over analog or digital interfaces to the control unit. The measurement of the actual control parameter, i.e. the pressure in the ventricle, takes place at the tip of the drainage catheter. Due to air pressure fluctuations (750 mbar...1400 mbar) which are measured inside the body as well, it is essential to measure a reference pressure in another place of the body. Some possibilities would be the measurement of the tissue near the implant, a measurement of the periatonal pressure or a pressure measurement in another compartment, which is affected as little as possible by the body (e.g. by muscle power or respiration). A measurement of the body position is necessary, since ICP depends on it. Like disturbance variable compensation, the desired ICP value can be adapted. In order to ensure the working reliability of the system, all sensors should be redundantly present. With a telemetry unit, sensor measurements can be transferred outward and data (e.g. desired ICP, control parameter, etc....) into the implant. The power supply of the implant should, where possible, be independent of any external power supply, and preferably run on a battery source. If the stored energy does not suffice, an additional method of supplying power by telemetric means and an energy storage must be planned as well.
6
[3] J. E. Galford, J. H. McElhaney, “A viscoelastic study of scalp, brain and dura“, Journ. Biomech., vol. 3, 1970. [4] C. J. J. Avezaat, J. H. M. VanEijndhoven, D. J. Wyper, “Cerebrospinal fluid pulse pressure und intracranial volume-pressure relationships”,Journal of Neurology, Neurosurgery and Psychiatry, vol. 42, pp. 687-700, 1979. [5] M. Johnston, A. Zakharov, C. Papaiconomou, G. Salmasi, D. Armstrong, “Evidence of connections between cerebrospinal fluid and nasal lymphatic vessels in humans, non-human primates and other mammalian species”, Dec 10;1(1):2 Cerebrospinal Fluid Res, 2004. [6] C.P. Maurizi, “A cycle of cerebrospinal fluid: supporting evidence and theoretical considerations”, Med Hypotheses, Mar; 54(3):417-22, 2000. [7] G. Zemack, B. Romner, “Adjustable valves in Normal-Pressure Hydrocephalus: A retrospective study of 218 patients”, Neurosurgery vol. 51, no. 6, Dec., 2002. [8] M. Kiefer, R. Eymann, M. Strowitzki, W. I. Steudel, „Gravitational shunts in longstanding overt ventriculomegaly in adults“, Neurosurgery vol. 55, no. 3, Sept., 2004.
Summary and Outlook
A systematic approach to model intracranial hydrodynamics has been presented. The model may be used for training purposes or to evaluate the influence of diagnostic (e.g. infusion tests) or therapeutic measures (e.g. the implantation of a bypass valve) on intracranial fluid dynamics. A concept was presented on an ”intelligent” shunt system. The goal of the system is the optimal regulation of brain pressure, so that a desired value given by the physician can be sufficiently kept. Some the components of such a system proved their suitability already in clinical tests. For the testing of the regulation concepts, a system for the pressure control at external CSF drainages was developed and tested successfully in several clinical trials.
7
Literature
[1] A. C. Guyton, J. E. Hall, “Textbook of Medical Physiology”, 10th ed., W. B. Saunders Company, 2000. [2] M. Walter, S. Jetzki, S. Leonhardt, „A model for Intracranial Hydrodynamics“,27th Annual International Conference IEEE Engineering in Medicine and Biology Society, Shanghai, 2005.
29
keywords: cerebrospinal fluid (CSF), hydrocephalus, CSF-Drainage, intracranial pressure (ICP), ICP control
1
Background adjustable valve
lateral ventricles
III. ventricle aqueduct of Silvius IV. ventricle inner liquor space
Fig. 1
liquor production plexus choroidei outer liquor space
Schematic sketch of the brain
Normally, CSF production (average around 20 ml/h) and reabsorption balance each other. However, if this sensitive balance is disturbed (e.g. by overproduction, blockage of the flow pathway or reduced reabsorption) a fluid accumulation will take place leading to an increased intracranial pressure (ICP). This disease is called “hydrocephalus” and is commonly treated with an implanted pressure control valve, which drains the excessive CSF to the abdominal space. Current valves can be adjusted through the skin by a magnetic device after they have been implanted. Based on the patient’s ICP levels, which differ depending on the individual person, the physician must estimate the suitable opening pressure p0 during treatment (Fig. 2) [7].
flow
Human brain tissue is surrounded by a water-like fluid (cerebrospinal fluid, CSF). Its main purpose is to protect the brain from mechanical shocks acting on the skull and to reduce tissue weight through buoyancy. CSF is produced in the centre of the brain, mainly in a cell formation named the plexus choroidei. The CSF then flows through the ventricles, internal cavities and small openings (aqueduct of Silvius) to the outside of the brain (see Figure 1). It is reabsorbed into the venous blood system, lymph channels and the spinal channel ([5][6]).
ICP
CSF-flow p0
Fig. 2
ICP
Block model of ICP valve
The ICP depends on the orientation of the body. An upright position decreases the pressure by about 10 mmHg as CSF flows into the lumbar region of the spine due to gravity. Therefore, resent gravitational valves change the total opening pressure of the shunt depending on the body position [8]. Abrupt pressure peaks in the ICP can result from coughing or sneezing. These short pressure peaks drain off so much CSF, that it takes hours until it is reconstituted. Whereas there are limits to the ability of a purely mechanical valve to prevent such problems, a mechatronic system may be a solution to avoid too much or too little CSF drainage.
2
Model of intracranial CSFand blood dynamics
Assuming that the space inside the skull bone may be viewed as a closed cavity, this volume may be subdivided into three compartments: blood, tissue and CSF. Based on this assumption, three mass balance equations may be derived (also known as the "MonroKellie doctrine"):
Vtotal (t ) = VCSF (t ) + Vblood (t ) + Vtissue
(1)
The volume in each compartment may be balanced by itself (Eq. 2). •
•
VCSF (t ) = ∫ V prod (t ) − V res (t )dt •
•
Vblood (t ) = ∫ V bl ,in (t ) − V bl ,out (t )dt Vtissue (t ) ≈ const .
(2)
25
3 Note that a fluid exchange between the blood and CSF compartment can only occur in two spots. First, CSF is transferred out of the arterial blood to the CSF compartment by the plexus choroideus. The amount is determined by the pressure difference between the arterial blood pressure (ABP) and the ICP and an absorption constant. •
V prod (t ) = k prod ⋅ ( ABPplexus (t ) − ICP (t ))
(3)
It is assumed that venous reabsorption starts above a certain threshold (RG) . After this point reabsorption is described as a linear function of the pressure difference between ICP and venous pressure (VBP) and a reabsorption constant. • 0 ICP < RG ⎫ ⎧ V res (t ) = ⎨ ⎬ ⎩k res ⋅ ( ICP (t ) − VBP (t )) else ⎭
(4) To model intracranial fluid dynamics, the cerebral blood flow has to be considered as well. As in other vascular areas of the body, cerebral blood flow is locally auto-regulated. Thus, the brain tissue is able to control the local blood flow depending on physiological demands (eg. activity, metabolism, CO2) [1]. Moreover, blood volume and CSF volume affect each other, described in [2]. As a last requirement, a constitutive equation linking intracranial volume to ICP is needed. Brain tissue is an elastic substance. Its properties may be described best by a Bingham plastic [3]. If the shear stress is below a certain limit, elastic deformation takes place. Above this limit, the deformation is plastic. Measurements have shown that the static relation between ICP and volume is approximately exponential in the range of moderate ICP values [4] leading to
Concept for ICP Control
An implanted shunt system works in parallel to the natural CSF absorption pathway (Fig. 4). The aim of a shunt is normalization of the ICP on a desired value given by the physician. The following conditions must be considered with the draft of the automatic controller depending upon treatment concept: - which lasting offset ICP values can be tolerated? - which dynamic deviations of the controlled variable can be tolerated? - how fast is a deviation compensated? - which mean current rates by the valve are to high or low? After discussions with different physicians the following basic data were specified for the shunt system: - For a short time the brain pressure increased up to 10mmHg can be tolerated also by seriously ill patients. - The average value should not be more than 2 mmHg above the given target pressure. - Compensating of a pressure disturbance within 30 seconds is sufficient. - A maximum drainage capacity of 500 ml/h is sufficient for the drainage (more than the double maximum natural production rate). Additional to the actual pressure control also the change of the body position (upright or lying down) of the patient must be considered in the automation concept, because it changes the physiological desired value. With an angle sensor the body situation is seized and the desired ICP value given by the physician is adapted automatically. In addition, a part of the controlled system changes (the drainage height and thus also the discharge during given valve position), which has to be compensated by a correction of the automatic controller characteristics. ICP Control Concept desired ICP
ICP (t ) = k 0 + k elast ⋅ e
+
aVtotal ( t )
(5)
Combining the equations given above, the following model for the CSF circulation system may be composed, see Fig. 3. tissue volume ~const.
blood volume
CSF production ABP ICP
Prod
ICP
+
-
∫ dt
CSF volume
ICP
+ V
CSF resorption V ICP
Fig. 3
VBP
Model of CSF production and resorption
-
+
-
GR (z)
VProd valve
-
Vtissue
∫
+
-
VRes
angle sensor ICP sensor
Vblood ICP
ICP
+ V
V ICP
Fig. 4 Block diagram of the control circuit and set point tracing for the implanted valve system With the selection of a suitable automatic controller structure, the characteristics of the actuator must be considered accordingly. With switching valves with only two switch conditions, a controller with hysteresis can be used, in order to avoid too much switching of the valve.
26
Clinical Trial
In order to develop and test the control algorithms, tests were performed using an external drainage system. Patients who receive a commonly used external CSF drainage before a shunt implantation are put in an intensive care station. Their ICP is recorded for three to seven days. During this period, excess CSF is drained through an external tube to a container. Adjusting the height of the container determines the ICP value at which CSF starts to drain. In order to test the control concepts in clinical tests, a system was developed for brain pressure control through external CSF drainage (Fig.5).
ICP [mmHg]
4
hysteresis
10 8 6 4 2
closed valve open valve
0
1
2
3
4
time [min]
Fig. 6: Characteristic of a low pass filtered process during ICP Control
PC (optional) squeeze valve
ICP-measurement security drainage
Monro-Niveau
Fig. 5 Structure of the experimental system for CSF drainage and ICP control In addition to the conventional drainage system, the setup was equipped with a squeeze valve used to switch CSF flow on and off. The brain pressure measurement was performed with a conventional ICP measurement device from the manufacturer Spiegelberg. A microcontroller controls, displays and enters the desired parameter values. A PC can be attached by an electrically isolated interface for logging of the test sequence. As a security measure, another drainage container was fixed parallel, in a higher position, thus limiting the maximum brain pressure in case of a system crash. Because the control valve can take only two discrete switch steps, a two-step action controller with adjustable hysteresis was implemented. Hysteresis in combination with a low-pass filter prevents too much switching of the valve. At the Universitätsklinik Homburg/Saar, the controlled drainage could be improved on five patients. In one case the controlled drainage worked during a period of one week without interruption. Figure 6 exemplifies the function of the automatic controller with hysteresis.
The desired ICP value amounted to 7 mmHg, the switching hysteresis was adjusted to 1mmHg. During short pulses the valve was opened for two to three seconds. In each case, the ICP exceeded the upper trigger level of the hysteresis range for only a very short time. On the average, the ICP lied somewhere below the lower trigger level because the drainage capacity of the system is much higher than the physiological CSF production rate of the patient. The drainage volume is expressed in equation 6. •
V = ( ICP + ( hbrain − hcontainer )) ⋅ G current
(6)
A test sequence gave the drainage conductance .G current ≈ 275
ml / h mmHg
(7)
With this clinical trial the container of the drainage was fixed at Monro level. With a sampling time of 1 second, 0.6 ml of CSF per sampling step flowed through the opened valve. Since the brain pressure reacts with a time-delay, the valve is opened usually for 2 to 3 scanning steps. The drainage of 1.2 to 1.8 ml leads to sustained drop below the lower trigger level. By using a smaller tube diameter in the drainage, the automatic controller action could be improved clearly. ICP [mmHg]
control unit
15
10
5
Fig. 7a Characteristic of the ICP Control over many hours with one coughing attack
0
2
3
4
5
6
7
8 9 time [h]
27
ICP [mmHg]
valve switching
ICPsoll = ICP0 + k ⋅ sin( x ⋅ π )
80
(8)
coughing
ICP [mmHg]
60 40 20 desired ICP 0 0
interpolated with the sine-like equation 8. Figure 8 shows the automatic desired value adjusting in the clinical trial.
10
20
30
40
50 60 time [sec]
valve switching 50 40
Fig. 7b: Recognition of pressure peaks caused by coughing
20
Figure 7a shows the curve characteristic of controlling over several hours with the same patient. During the process of the trial, the patient had a cough attack after 3.5 hours. With strong coughing, the extremely high pressure in the chest area spreads through the blood vessels and the connecting soft tissue parts into the head, causing the brain pressure to rise to exorbitantly high pressures (90 mmHg, range of values in the figure is cut off). As a result, the control algorithm opened the valve and too much CSF ran off, so that after the coughing episode the ICP was at a very low level. It took 1.5 hours until the ICP was in the desired range again. This clearly demonstrates that a reliable artefact recognition procedure is required to detect coughing and deactivate the control loop during this time. A good criterion to detect a cough attack is the abrupt rise of ICP. If the ICP doubles itself in one sampling step, there is probably an artefact and the drainage will be closed for a certain time (e.g. 10 sec.), illustrated in figure 7b. If the brain pressure returns back to a low level this time, a cough attack is present and the valve stays closed with a repeated rise again. If the ICP remains on the high value, the increase in pressure will be attributed to other causes and the drainage opens. In case of a changed body position of the patient, the desired value of the brain pressure changes. In order to adjust this accordingly, a measurement of the patient position is necessary. At the adjustable heading of the bed, an angle sensor was attached. Position changes, which are caused by turning the body or pillows, cannot be recognized. At the beginning of the drainage a calibration of the angle sensing is necessary. Hereby, the patient is brought into a horizontal position and the desired ICP value is adjusted. After the ICP has reached the desired value, the drainage is closed and the patient is seated up as far as possible. After fading away the transients, the desired ICP value for the upper body position adjusts itself. Between these two values the angle dependent ICP is
0
artefact
desired ICP
30
10 0
10
20
30
40
50 time [min]
Fig. 8 Adaptation of the set value to the patient position
5
Mechatronic integration
Raised, destructive ICP demands a therapy. To decrease and control ICP, valves can drain excessive CSF. ICP and body position are prior parameters. Running a closed control loop is possible with the valve as an electronic, controllable actuator. The concept of integrating all components into a micro mechanic system is illustrated in Fig. 9. catheter and pressure sensor
angle sensor telemetry control unit
valve
Fig. 9 Implant ICP Control
for
The control unit handles information processing and fulfils the following tasks: - sensor data processing - actuator control - process parameters examination - adaption of changed circumstances (e.g. body position) - energy management - communication - data recording
28
The sensors for pressure, flow and body position are attached over analog or digital interfaces to the control unit. The measurement of the actual control parameter, i.e. the pressure in the ventricle, takes place at the tip of the drainage catheter. Due to air pressure fluctuations (750 mbar...1400 mbar) which are measured inside the body as well, it is essential to measure a reference pressure in another place of the body. Some possibilities would be the measurement of the tissue near the implant, a measurement of the periatonal pressure or a pressure measurement in another compartment, which is affected as little as possible by the body (e.g. by muscle power or respiration). A measurement of the body position is necessary, since ICP depends on it. Like disturbance variable compensation, the desired ICP value can be adapted. In order to ensure the working reliability of the system, all sensors should be redundantly present. With a telemetry unit, sensor measurements can be transferred outward and data (e.g. desired ICP, control parameter, etc....) into the implant. The power supply of the implant should, where possible, be independent of any external power supply, and preferably run on a battery source. If the stored energy does not suffice, an additional method of supplying power by telemetric means and an energy storage must be planned as well.
6
[3] J. E. Galford, J. H. McElhaney, “A viscoelastic study of scalp, brain and dura“, Journ. Biomech., vol. 3, 1970. [4] C. J. J. Avezaat, J. H. M. VanEijndhoven, D. J. Wyper, “Cerebrospinal fluid pulse pressure und intracranial volume-pressure relationships”,Journal of Neurology, Neurosurgery and Psychiatry, vol. 42, pp. 687-700, 1979. [5] M. Johnston, A. Zakharov, C. Papaiconomou, G. Salmasi, D. Armstrong, “Evidence of connections between cerebrospinal fluid and nasal lymphatic vessels in humans, non-human primates and other mammalian species”, Dec 10;1(1):2 Cerebrospinal Fluid Res, 2004. [6] C.P. Maurizi, “A cycle of cerebrospinal fluid: supporting evidence and theoretical considerations”, Med Hypotheses, Mar; 54(3):417-22, 2000. [7] G. Zemack, B. Romner, “Adjustable valves in Normal-Pressure Hydrocephalus: A retrospective study of 218 patients”, Neurosurgery vol. 51, no. 6, Dec., 2002. [8] M. Kiefer, R. Eymann, M. Strowitzki, W. I. Steudel, „Gravitational shunts in longstanding overt ventriculomegaly in adults“, Neurosurgery vol. 55, no. 3, Sept., 2004.
Summary and Outlook
A systematic approach to model intracranial hydrodynamics has been presented. The model may be used for training purposes or to evaluate the influence of diagnostic (e.g. infusion tests) or therapeutic measures (e.g. the implantation of a bypass valve) on intracranial fluid dynamics. A concept was presented on an ”intelligent” shunt system. The goal of the system is the optimal regulation of brain pressure, so that a desired value given by the physician can be sufficiently kept. Some the components of such a system proved their suitability already in clinical tests. For the testing of the regulation concepts, a system for the pressure control at external CSF drainages was developed and tested successfully in several clinical trials.
7
Literature
[1] A. C. Guyton, J. E. Hall, “Textbook of Medical Physiology”, 10th ed., W. B. Saunders Company, 2000. [2] M. Walter, S. Jetzki, S. Leonhardt, „A model for Intracranial Hydrodynamics“,27th Annual International Conference IEEE Engineering in Medicine and Biology Society, Shanghai, 2005.
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