Noninvasive ICP measurement methods

Modeling and estimation for non-invasive monitoring of intracranial pressure and cerebrovascular autoregulation

Author: Kashif, Faisal M. 

Brain tissue is highly vulnerable to unbalanced oxygen demand and supply. A few seconds of oxygen deficit may trigger neurological symptoms, and sustained oxygen deprivation over a few minutes may result in severe and often irreversible brain damage. The rapid dynamics coupled with the potential for severe injury necessitate continuous cerebrovascular monitoring in the populations at greatest risk for developing or exacerbating brain injury. Intracranial pressure (ICP), which is the pressure of the cerebrospinal fluid, is a vitally important variable to monitor in a wide spectrum of medical conditions involving the brain, such as traumatic brain injury, stroke, hydrocephalus, or brain tumors. However, clinical measurement of ICP is highly invasive, as it requires neurosurgical penetration of the skull and placement of a pressure sensor in the brain tissue or ventricular spaces. Measurement of ICP is thus currently limited to only those patient populations in which the benefits of obtaining the measurement outweigh the significant attendant risks, thus excluding a large pool of patients who could potentially benefit from ICP monitoring. The primary goal of our work is to address the non-invasive monitoring of ICP. A secondary aim of this work is to develop methods for the assessment of cerebrovascular autoregulation, which is the innate ability of the vasculature to maintain cerebral blood flow in the face of changes in cerebral perfusion pressure. Cerebrovascular autoregulation is often impaired in patients with brain trauma or stroke, and also in pre-term neonates, as their cerebrovascular system is not fully matured. We develop methods for non-invasive, continuous, calibration-free and patientspecific ICP monitoring. Specifically, we present a model-based approach to providing real-time estimates of ICP and cerebrovascular resistance and compliance, for each cardiac cycle, from non- or minimally-invasive time-synchronized measurements of arterial blood pressure and cerebral blood flow velocity in a major cerebral artery. In the first step, our approach exploits certain features of cerebrovascular physiology, along with model reduction ideas, to deduce a simple mathematical model of the cerebrovascular system. In the second step, we develop algorithms to compute robust estimates of model parameters by processing the measured waveforms through the constraints provided by the models dynamic equation. For validation, our non-invasive estimates of ICP were compared against invasive measurements from 45 comatose brain-injury patients, with a total of 35 hours of data (over 150,000 beats), providing more than 3,500 independent ICP estimates. Our estimates track measured ICP closely over a range of dynamic variations. Pooling all independent estimates resulted in a mean estimation error (bias) of less than 2 mmHg and a standard deviation of error of about 8 mmHg. We also suggest how variations in estimated cerebrovascular resistance and compliance in response to variations in cerebral perfusion pressure may be used to provide novel approaches for assessment of cerebrovascular autoregulation.

Non-invasive technologies for intracranial pressure/volume measurement

Authors: Ragauskas, A.,   Petkus, V.

The paper shows that innovative technologies for noninvasive monitoring, of the cerebral blood flow autoregulation, cerebral blood flow pulse and slow waves also for the registration of the reactions to the neurodiagnostic tests can be developed on the basis of the precise measurement of brain parenchyma acoustic characteristics. The innovative technological equipment for such measurement has been developed. The clinical studies proving the validity of the concepts chosen have been carried out. For the first time the innovative non-invasive method has been designed for the ICP absolute value measurement without the necessity of individual calibration of the system "non-invasive meter - patient".

Noninvasive intracranial pressure measurement using infrasonic emissions from the tympanic membrane

Authors: Stettin E, Paulat K, Schulz C, Kunz U, Mauer UM.

OBJECTIVE: We investigated whether ICP can be assessed by measuring infrasonic emissions from the tympanic membrane.

METHODS: An increase in ICP was induced in 22 patients with implanted ICP pressure sensors. ICP waveforms that were obtained invasively and continuously were compared with infrasonic emission waveforms. In addition, the noninvasive method was used in a control group of 14 healthy subjects.

RESULTS: In a total of 83 measurements, the changes in ICP that were observed in response to different types of stimulation were detected in the waveforms obtained noninvasively as well as in those acquired invasively. Low ICP was associated with an initial high peak and further peaks with smaller amplitudes. High ICP was associated with a marked decrease in the number of peaks and in the difference between the amplitudes of the initial and last peaks. The assessment of infrasonic emissions, however, does not yet enable us to provide exact figures.

CONCLUSION: It is conceivable that the assessment of infrasonic emissions will become suitable both as a screening tool and for the continuous monitoring of ICP in an intensive care environment.

Noninvasive assessment of intracranial pressure with venous ophthalmodynamometry

Authors: Raimund Firsching, Prof. Dr. med.1, Claudia Müller, Dr. med.2, Steffen-Ulrich Pauli1, Benjamin Voellger, Dr. med.1, Friedrich-Wilhelm Röhl, Dr. rer. nat.3, and Wolfgang Behrens-Baumann, Prof. Dr. med.2

Object. Venous ophthalmodynamometry is a technique used to register the pressure within the central retinal vein. Because the outflow of the central retinal vein is exposed to the intracranial pressure (ICP), the pressure of the central retinal vein may be correlated with the ICP. In the absence of adequate statistical evidence, the authors compared the pressure of the central retinal vein with results of simultaneous invasive monitoring of ICP in neurosurgical patients.

Methods. The pressure within the central retinal vein was recorded in 102 patients, in whom invasive continuous monitoring of ICP had become necessary for various reasons, mostly because of suspected hydrocephalus and intracranial hemorrhage.

Results. A highly significant correlation of the pressure in the central retinal vein and the intracranial cavity was confirmed statistically. An increased pressure of the central retinal vein indicated an elevated ICP, with a probability of 84.2%, whereas a normal pressure of the central retinal vein indicated a normal ICP in 92.8% of patients.

Conclusions. Venous ophthalmodynamometry is a valuable technique for the noninvasive assessment of ICP.

Transcranial Doppler Pulsatility Index: Not an Accurate Method to Assess Intracranial Pressure

Authors: Behrens, Anders MD, MSc; Lenfeldt, Niklas MSc, PhD; Ambarki, Khalid PhD; Malm, Jan MD, PhD; Eklund, Anders PhD; Koskinen, Lars-Owe MD, PhD.

BACKGROUND: Transcranial Doppler sonography (TCD) assessment of intracranial blood flow velocity has been suggested to accurately determine intracranial pressure (ICP).

OBJECTIVE: We attempted to validate this method in patients with communicating cerebrospinal fluid systems using predetermined pressure levels.

METHODS: Ten patients underwent a lumbar infusion test, applying 4 to 5 preset ICP levels. On each level, the pulsatility index (PI) in the middle cerebral artery was determined by measuring the blood flow velocity using TCD. ICP was simultaneously measured with an intraparenchymal sensor. ICP and PI were compared using correlation analysis. For further understanding of the ICP-PI relationship, a mathematical model of the intracranial dynamics was simulated using a computer.

RESULTS: The ICP-PI regression equation was based on data from 8 patients. For 2 patients, no audible Doppler signal was obtained. The equation was ICP = 23*PI + 14 (R2 = 0.22, P < .01, N = 35). The 95% confidence interval for a mean ICP of 20 mm Hg was −3.8 to 43.8 mm Hg. Individually, the regression coefficients varied from 42 to 90 and the offsets from −32 to +3. The mathematical simulations suggest that variations in vessel compliance, autoregulation, and arterial pressure have a serious effect on the ICP-PI relationship.

CONCLUSIONS: The in vivo results show that PI is not a reliable predictor of ICP. Mathematical simulations indicate that this is caused by variations in physiological parameters.

Noninvasive detection of elevated intracranial pressure using a portable ultrasound system

Authors: Prunet B, Asencio Y, Lacroix G, Montcriol A, Dagain A, Cotte J, Esnault P, Boret H, Meaudre E, Kaiser E.

OBJECTIVE: The aim of this study is to prospectively compare the accuracies of transcranial color-coded sonography (TCCS) and transcranial Doppler (TCD) in the diagnosis of elevated intracranial pressure.

METHODS: A prospective, blinded, head-to-head comparison of TCD and TCCS methods using intracranial pressure (ICP) measured continuously via an intraparenchymal catheter as the reference standard in 2 groups of 20 neurocritical care patients each: high ICP (group 1) and normal ICP (group 2). Middle cerebral artery (MCA) pulsatility index (PI) recordings from all patients' sonographic reports were selected based on the highest left or right recorded MCA PI. Transcranial Doppler was performed using a dedicated TCD device, and TCCS was performed using a portable ultrasound system.

RESULTS: The PI values obtained did not differ significantly between the 2 methods (group 1, P = .46; group 2, P = .11). Linear regression analysis identified a significant relationship between PI obtained with both methods (r = 0.897; P < .0001). The duration of PI measurement was statistically longer with TCCS than TCD (group 1, P < .01; group 2, P < .01). Diagnostic accuracies were good and similar for both methods (TCD area under curve, 0.901; TCCS area under curve 0.870; P = .69).

CONCLUSIONS: This work is a pilot study comparing TCCS and TCD in the detection of elevated ICP. This study suggests that a bedside portable ultrasound system may be useful to determine MCA PI with accuracy similar to that of a dedicated TCD device.

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