Noninvasive intracranial compliance monitoring. Technical note and clinical results

Analysis of intracranial pressure pulse waveform in studies on cerebrospinal compliance: a narrative review

Continuous monitoring of mean intracranial pressure (ICP) has been an essential part of neurocritical care for more than half a century. Cerebrospinal pressure-volume compensation, i.e. the ability of the cerebrospinal system to buffer changes in volume without substantial increases in ICP, is considered an important factor in preventing adverse effects on the patient's condition that are associated with ICP elevation. However, existing assessment methods are poorly suited to the management of brain injured patients as they require external manipulation of intracranial volume. In the 1980s, studies suggested that spontaneous short-term variations in the ICP signal over a single cardiac cycle, called the ICP pulse waveform, may provide information on cerebrospinal compensatory reserve. In this review we discuss the approaches that have been proposed so far to derive this information, from pulse amplitude estimation and spectral techniques to most recent advances in morphological analysis based on artificial intelligence solutions. Each method is presented with focus on its clinical significance and the potential for application in standard clinical practice. Finally, we highlight the missing links that need to be addressed in future studies in order for ICP pulse waveform analysis to achieve widespread use in the neurocritical care setting.

Applying video motion magnification to reveal spontaneous tympanic membrane displacement as an indirect measure of intracranial pressure in patients with brain pathologies

BACKGROUND

The observation of tympanic membrane displacement (TMD) opens up the possibility of indirect intracranial pressure (ICP) estimation. In this study, we applied a phase-based video motion magnification (VMM) algorithm to reveal spontaneous pulse TMD waveforms (spTMD) and compare them with invasively measured ICP in patients with intracranial pathologies.

METHODS

Nine adults (six traumatic brain injury and three aneurysmal subarachnoid haemorrhage; median age 44 (29-53) years admitted to the intensive care unit of Wroclaw Medical University between October 2021 and October 2022 with implanted ICP sensors were included in this retrospective study. Video recordings of the tympanic membrane were performed using a portable otoscope with a video camera and analysed by a custom-written VMM algorithm. ICP was monitored using intraparenchymal sensors and arterial blood pressure (ABP) was measured in the radial arterial lines. ICP, ABP, and spTMD videos were captured simultaneously. The pulse amplitudes of ICP (Amp_ICP), ABP (Amp_ABP) and spTMD (Amp_spTMD) were estimated using fast Fourier transform within the heart rate (HR)-related frequency range.

RESULTS

Amp_spTMD was significantly correlated with mean ICP (rS = 0.73; p = 0.025) and with Amp_ICP (rS = 0.88; p = 0.002). Age was not a significant moderator of this association. There were no significant relationships between Amp_spTMD and either mean ABP, HR, or Amp_ABP.

CONCLUSIONS

The study suggests that Amp_spTMD increases with the increase in mean ICP and Amp_ICP. Estimation of Amp_spTMD using the VMM algorithm has the potential to allow for non-invasive detection of the risk of elevated ICP; however, further investigation in a larger group of patients is required.

Estimation of Physiologic Pressures: Invasive and Non-Invasive Techniques, AI Models, and Future Perspectives

The measurement of physiologic pressure helps diagnose and prevent associated health complications. From typical conventional methods to more complicated modalities, such as the estimation of intracranial pressures, numerous invasive and noninvasive tools that provide us with insight into daily physiology and aid in understanding pathology are within our grasp. Currently, our standards for estimating vital pressures, including continuous BP measurements, pulmonary capillary wedge pressures, and hepatic portal gradients, involve the use of invasive modalities. As an emerging field in medical technology, artificial intelligence (AI) has been incorporated into analyzing and predicting patterns of physiologic pressures. AI has been used to construct models that have clinical applicability both in hospital settings and at-home settings for ease of use for patients. Studies applying AI to each of these compartmental pressures were searched and shortlisted for thorough assessment and review. There are several AI-based innovations in noninvasive blood pressure estimation based on imaging, auscultation, oscillometry and wearable technology employing biosignals. The purpose of this review is to provide an in-depth assessment of the involved physiologies, prevailing methodologies and emerging technologies incorporating AI in clinical practice for each type of compartmental pressure measurement. We also bring to the forefront AI-based noninvasive estimation techniques for physiologic pressure based on microwave systems that have promising potential for clinical practice.

Pulsatile tympanic membrane displacement is associated with cognitive score in healthy subjects

To test the hypothesis that pulsing of intracranial pressure has an association with cognition, we measured cognitive score and pulsing of the tympanic membrane in 290 healthy subjects. This hypothesis was formed on the assumptions that large intracranial pressure pulses impair cognitive performance and tympanic membrane pulses reflect intracranial pressure pulses. 290 healthy subjects, aged 20-80 years, completed the Montreal Cognitive Assessment Test. Spontaneous tympanic membrane displacement during a heart cycle was measured from both ears in the sitting and supine position. We applied multiple linear regression, correcting for age, heart rate, and height, to test for an association between cognitive score and spontaneous tympanic membrane displacement. Significance was set at P < 0.0125 (Bonferroni correction.) A significant association was seen in the left supine position (p = 0.0076.) The association was not significant in the right ear supine (p = 0.28) or in either ear while sitting. Sub-domains of the cognitive assessment revealed that executive function, language and memory have been primarily responsible for this association. In conclusion, we have found that spontaneous pulses of the tympanic membrane are associated with cognitive performance and believe this reflects an association between cognitive performance and intracranial pressure pulses.

Intracranial pulse pressure waveform analysis using the higher harmonics centroid

Background

The pulse waveform of intracranial pressure (ICP) is its distinctive feature almost always present in the clinical recordings. In most cases, it changes proportionally to rising ICP, and observation of these changes may be clinically useful. We introduce the higher harmonics centroid (HHC) which can be defined as the center of mass of harmonics of the ICP pulse waveform from the 2nd to 10th, where mass corresponds to amplitudes of these harmonics. We investigate the changes in HHC during ICP monitoring, including isolated episodes of ICP plateau waves.

Material and methods

Recordings from 325 patients treated between 2002 and 2010 were reviewed. Twenty-six patients with ICP plateau waves were identified. In the first step, the correlation between HHC and ICP was examined for the entire monitoring period. In the second step, the above relation was calculated separately for periods of elevated ICP during plateau wave and the baseline.

Results

For the values averaged over the whole monitoring period, ICP (22.3 ± 6.9 mm Hg) correlates significantly (R = 0.45, p = 0.022) with HHC (3.64 ± 0.46). During the ICP plateau waves (ICP increased from 20.9 ± 6.0 to 53.7 ± 9.7 mm Hg, p < 10⁻¹⁶), we found a significant decrease in HHC (from 3.65 ± 0.48 to 3.21 ± 0.33, p = 10⁻⁵).

Conclusions

The good correlation between HHC and ICP supports the clinical application of pressure waveform analysis in addition to the recording of ICP number only. Mean ICP may be distorted by a zero drift, but HHC remains immune to this error. Further research is required to test whether a decline in HHC with elevated ICP can be an early warning sign of intracranial hypertension, whether individual breakpoints of correlation between ICP and its centroid are of clinical importance.

Intracranial Pressure and Intracranial Elastance Monitoring in Neurocritical Care

Patients with acute brain injuries tend to be physiologically unstable and at risk of rapid and potentially life-threatening decompensation due to shifts in intracranial compartment volumes and consequent intracranial hypertension. Invasive intracranial pressure (ICP) monitoring therefore remains a cornerstone of modern neurocritical care, despite the attendant risks of infection and damage to brain tissue arising from the surgical placement of a catheter or pressure transducer into the cerebrospinal fluid or brain tissue compartments. In addition to ICP monitoring, tracking of the intracranial capacity to buffer shifts in compartment volumes would help in the assessment of patient state, inform clinical decision making, and guide therapeutic interventions. We review the anatomy, physiology, and current technology relevant to clinical management of patients with acute brain injury and outline unmet clinical needs to advance patient monitoring in neurocritical care.

Ultrasonic Assessment of the Medial Temporal Lobe Tissue Displacements in Alzheimer’s Disease

We aim to estimate brain tissue displacements in the medial temporal lobe (MTL) using backscattered ultrasound radiofrequency (US RF) signals, and to assess the diagnostic ability of brain tissue displacement parameters for the differentiation of patients with Alzheimer’s disease (AD) from healthy controls (HC). Standard neuropsychological evaluation and transcranial sonography (TCS) for endogenous brain tissue motion data collection are performed for 20 patients with AD and for 20 age- and sex-matched HC in a prospective manner. Essential modifications of our previous method in US waveform parametrization, raising the confidence of micrometer-range displacement signals in the presence of noise, are done. Four logistic regression models are constructed, and receiver operating characteristic (ROC) curve analyses are applied. All models have cut-offs from 61.0 to 68.5% and separate AD patients from HC with a sensitivity of 89.5% and a specificity of 100%. The area under a ROC curve of predicted probability in all models is excellent (from 95.2 to 95.7%). According to our models, AD patients can be differentiated from HC by a sharper morphology of some individual MTL spatial point displacements (i.e., by spreading the spectrum of displacements to the high-end frequencies with higher variability across spatial points within a region), by lower displacement amplitude differences between adjacent spatial points (i.e., lower strain), and by a higher interaction of these attributes.

Assessment of a Non Invasive Brain Oximeter in Volunteers Undergoing Acute Hypoxia

INTRODUCTION

Research in traumatic brain injury suggests better patient outcomes when invasive oxygen monitoring is used to detect and correct episodes of brain hypoxia. Invasive brain oxygen monitoring is, however, not routinely used due to the risks, costs and technical challengers. We are developing a non-invasive brain oximeter to address these limitations. The monitor uses the principles of pulse oximetry to record a brain photoplethysmographic waveform and oxygen saturations. We undertook a study in volunteers to assess the new monitor.

PATIENTS AND METHODS

We compared the temporal changes in the brain and skin oxygen saturations in six volunteers undergoing progressive hypoxia to reach arterial saturations of 70%. This approach provides a method to discriminate potential contamination of the brain signal by skin oxygen levels, as the responses in brain and skin oxygen saturations are distinct due to the auto-regulation of cerebral blood flow to compensate for hypoxia. Conventional pulse oximetry was used to assess skin oxygen levels. Blood was also collected from the internal jugular vein and correlated with the brain oximeter oxygen levels.

RESULTS

At baseline, a photoplethysmographic waveform consistent with that expected from the brain was obtained in five subjects. The signal was adequate to assess oxygen saturations in three subjects. During hypoxia, the brain's oximeter oxygen saturation fell to 74%, while skin saturation fell to 50% (P<0.0001). The brain photoplethysmographic waveform developed a high-frequency oscillation of ~7 Hz, which was not present in the skin during hypoxia. A weak correlation between the brain oximeter and proximal internal jugular vein oxygen levels was demonstrated, R2=0.24, P=0.01.

CONCLUSION

Brain oximeter oxygen saturations were relatively well preserved compared to the skin during hypoxia. These findings are consistent with the expected physiological responses and suggest skin oxygen levels did not markedly contaminate the brain oximeter signal.

Review: pathophysiology of intracranial hypertension and noninvasive intracranial pressure monitoring

Measurement of intracranial pressure (ICP) is crucial in the management of many neurological conditions. However, due to the invasiveness, high cost, and required expertise of available ICP monitoring techniques, many patients who could benefit from ICP monitoring do not receive it. As a result, there has been a substantial effort to explore and develop novel noninvasive ICP monitoring techniques to improve the overall clinical care of patients who may be suffering from ICP disorders. This review attempts to summarize the general pathophysiology of ICP, discuss the importance and current state of ICP monitoring, and describe the many methods that have been proposed for noninvasive ICP monitoring. These noninvasive methods can be broken down into four major categories: fluid dynamic, otic, ophthalmic, and electrophysiologic. Each category is discussed in detail along with its associated techniques and their advantages, disadvantages, and reported accuracy. A particular emphasis in this review will be dedicated to methods based on the use of transcranial Doppler ultrasound. At present, it appears that the available noninvasive methods are either not sufficiently accurate, reliable, or robust enough for widespread clinical adoption or require additional independent validation. However, several methods appear promising and through additional study and clinical validation, could eventually make their way into clinical practice.

Measuring intracranial pressure by invasive, less invasive or non-invasive means: limitations and avenues for improvement

Sixty years have passed since neurosurgeon Nils Lundberg presented his thesis about intracranial pressure (ICP) monitoring, which represents a milestone for its clinical introduction. Monitoring of ICP has since become a clinical routine worldwide, and today represents a cornerstone in surveillance of patients with acute brain injury or disease, and a diagnostic of individuals with chronic neurological disease. There is, however, controversy regarding indications, clinical usefulness and the clinical role of the various ICP scores. In this paper, we critically review limitations and weaknesses with the current ICP measurement approaches for invasive, less invasive and non-invasive ICP monitoring. While risk related to the invasiveness of ICP monitoring is extensively covered in the literature, we highlight other limitations in current ICP measurement technologies, including limited ICP source signal quality control, shifts and drifts in zero pressure reference level, affecting mean ICP scores and mean ICP-derived indices. Control of the quality of the ICP source signal is particularly important for non-invasive and less invasive ICP measurements. We conclude that we need more focus on mitigation of the current limitations of today's ICP modalities if we are to improve the clinical utility of ICP monitoring.

Natural resonance frequency of the brain depends on only intracranial pressure: clinical research

To understand and control intracranial pressure (ICP) is required for treatments in various clinical situations. To establish non-invasive ICP prediction method, we focused on the natural resonance frequency (NRF) of the brain. The ICP value, pulse waveform of intracranial pressure (PWICP) and cervical carotid pulse waveform (CCPW) were simultaneously collected from patients who underwent neurosurgical treatment. A total of 43 data were obtained from 27 patients. The total measured time was 29,653 seconds and the measured mean ICP value in each data ranged from 3.82 to 69.39 (mean 25.9) hPa. Respiratory synchronized cardiac pulses were selected and following CCPW and PWICP were collected. The transfer characteristics from collected CCPW to PWICP were calculated. The initial negative peak was judged as the NRF of the brain. The relationship between the ICP value and the NRF of the brain was presented on the quadratic functions graph (ICP = 0.0329(NRF)² + 0.0842NRF; R² = 0.9952). It means that the individual NRF only depends on their ICP value. The ICP value will be predicted by checking NRF of the brain from somewhere.

Assessment of a Non-Invasive Brain Oximeter in a Sheep Model of Acute Brain Injury

Introduction

Evidence suggests treatments guided by brain oxygen levels improve patient outcomes following severe traumatic brain injury; however, brain oxygen levels are not routinely monitored as an effective non-invasive method has not been established. We undertook a study, in a sheep model of acute brain injury, to assess a new non-invasive brain oximeter. The monitor uses the principles of pulse oximetry to record a pulse and oxygen levels.

Methods

We studied 8 sheep. An acute increase in intracranial pressure was induced with an injection of blood into the cranial vault. The temporal changes in the brain oximeter, intracranial pressure and cerebral perfusion pressure were recorded. Simultaneous conventional skin pulse oximetry was also recorded to assess the possible influence of skin blood flow on the brain oximeter signal.

Results

At baseline, a pulsatile waveform consistent with the brain circulation was obtained in 7 animals. The baseline brain pulse was quite distinct from the simultaneous conventional skin pulse and similar in shape to a central venous pressure waveform. Injection of blood into the cranial vault triggered an immediate increase in intracranial pressure and fall in cerebral perfusion pressure, by 60-s cerebral perfusion pressure recovered. The brain oximeter oxygen levels demonstrated similar changes with an immediate fall and recovery by 60 s. Periods of high intracranial pressure were also associated with high-frequency oscillations in the brain pulse waveform; there was, however, no change in the conventional skin pulse oximeter pulse waveform.

Conclusion

The brain oximeter detected acute changes in both oxygen levels and the brain pulse waveform following an increase in intracranial pressure levels. The brain oximeter could assist clinicians in the management of acute brain injury.

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Utility of the Tympanic Membrane Pressure Waveform for Non-invasive Estimation of The Intracranial Pressure Waveform

Time domain analysis of the intracranial pressure (ICP) waveform provides important information about the intracranial pressure-volume reserve capacity. The aim here was to explore whether the tympanic membrane pressure (TMP) waveform can be used to non-invasively estimate the ICP waveform. Simultaneous invasive ICP and non-invasive TMP signals were measured in a total of 28 individuals who underwent invasive ICP measurements as a part of their clinical work up (surveillance after subarachnoid hemorrhage in 9 individuals and diagnostic for CSF circulation disorders in 19 individuals). For each individual, a transfer function estimate between the invasive ICP and non-invasive TMP signals was established in order to explore the potential of the method. To validate the results, ICP waveform parameters including the mean wave amplitude (MWA) were computed in the time domain for both the ICP estimates and the invasively measured ICP. The patient-specific non-invasive ICP signals predicted MWA rather satisfactorily in 4/28 individuals (14%). In these four patients the differences between original and estimated MWA were <1.0 mmHg in more than 50% of observations, and <0.5 mmHg in more than 20% of observations. The study further disclosed that the cochlear aqueduct worked as a physical lowpass filter.

Advances in neuro-monitoring

Neuromonitoring aims to detect harmful physiologic events, early enough to guide the treatment instituted. Evidences encourage us to implement multimodal monitoring, as no single monitor is capable of providing a complete picture of dynamic cerebral state. This review highlights the role of intracranial pressure monitoring, cerebral oxygenation (jugular venous oximetry, brain tissue oxygenation, near infrared oximetry, cerebral microdialysis) and cerebral blood flow monitoring (direct and indirect methods) in the management of neurologically injured patients. In this context, the recent developments of these monitors along with the relevant clinical implications have been discussed. Nevertheless, the diverse range of data obtained from these monitors needs to be integrated and simplified for the clinician. Hence, the future research should focus on identification of a most useful monitor for integration into multimodal system.

Update in intracranial pressure evaluation methods and translaminar pressure gradient role in glaucoma

Glaucoma is one of the leading causes of blindness worldwide. Historically, it has been considered an ocular disease primary caused by pathological intraocular pressure (IOP). Recently, researchers have emphasized intracranial pressure (ICP), as translaminar counter pressure against IOP may play a role in glaucoma development and progression. It remains controversial what is the best way to measure ICP in glaucoma. Currently, the 'gold standard' for ICP measurement is invasive measurement of the pressure in the cerebrospinal fluid via lumbar puncture or via implantation of the pressure sensor into the brains ventricle. However, the direct measurements of ICP are not without risk due to its invasiveness and potential risk of intracranial haemorrhage and infection. Therefore, invasive ICP measurements are prohibitive due to safety needs, especially in glaucoma patients. Several approaches have been proposed to estimate ICP non-invasively, including transcranial Doppler ultrasonography, tympanic membrane displacement, ophthalmodynamometry, measurement of optic nerve sheath diameter and two-depth transcranial Doppler technology. Special emphasis is put on the two-depth transcranial Doppler technology, which uses an ophthalmic artery as a natural ICP sensor. It is the only method which accurately and precisely measures absolute ICP values and may provide valuable information in glaucoma.

A provocative test to determine brain compliance in the management of patients with hydrocephalus

Non-invasive techniques to explore intracranial compliance and pressure have been extensively explored in recent years. Previous techniques have used expensive technologies to make these measurements, often with difficulty. We present a novel, inexpensive method and algorithm to observe trends in intracranial compliance measurement targeted towards the treatment and management of hydrocephalus. The technique uses two photo-plethysmographic sensors to record arterial pulse perfusion, a common tilt table apparatus to methodically and artificially increase intracranial pressure, and a digital signal processing algorithm to determine phase difference between the waveforms. A secondary phase-difference disease signature approach is also hypothesized.

Intracranial Pressure Monitoring: Invasive versus Non-Invasive Methods-A Review

Monitoring of intracranial pressure (ICP) has been used for decades in the fields of neurosurgery and neurology. There are multiple techniques: invasive as well as noninvasive. This paper aims to provide an overview of the advantages and disadvantages of the most common and well-known methods as well as assess whether noninvasive techniques (transcranial Doppler, tympanic membrane displacement, optic nerve sheath diameter, CT scan/MRI and fundoscopy) can be used as reliable alternatives to the invasive techniques (ventriculostomy and microtransducers). Ventriculostomy is considered the gold standard in terms of accurate measurement of pressure, although microtransducers generally are just as accurate. Both invasive techniques are associated with a minor risk of complications such as hemorrhage and infection. Furthermore, zero drift is a problem with selected microtransducers. The non-invasive techniques are without the invasive methods' risk of complication, but fail to measure ICP accurately enough to be used as routine alternatives to invasive measurement. We conclude that invasive measurement is currently the only option for accurate measurement of ICP.

Resonant and notch behavior in intracranial pressure dynamics

OBJECT

The intracranial pulse pressure is often increased when neuropathology is present, particularly in cases of increased intracranial pressure (ICP) such as occurs in hydrocephalus. This pulse pressure is assumed to originate from arterial blood pressure oscillations entering the cranium; the fact that there is a coupling between the arterial blood pressure and the ICP is undisputed. In this study, the nature of this coupling and how it changes under conditions of increased ICP are investigated.

METHODS

In 12 normal dogs, intracarotid and parenchymal pulse pressure were measured and their coupling was characterized using amplitude and phase transfer function analysis. Mean intracranial ICP was manipulated via infusions of isotonic saline into the spinal subarachnoid space, and changes in transfer function were monitored.

RESULTS

Under normal conditions, the ICP wave led the arterial wave, and there was a minimum in the pulse pressure amplitude near the frequency of the heart rate. Under conditions of decreased intracranial compliance, the ICP wave began to lag behind the arterial wave and increased significantly in amplitude. Most interestingly, in many animals the pulse pressure exhibited a minimum in amplitude at a mean pressure that coincided with the transition from a leading to lagging ICP wave.

CONCLUSIONS

This transfer function behavior is characteristic of a resonant notch system. This may represent a component of the intracranial Windkessel mechanism, which protects the microvasculature from arterial pulsatility. The impairment of this resonant notch system may play a role in the altered pulse pressure in conditions such as hydrocephalus and traumatic brain swelling. New models of intracranial dynamics are needed for understanding the frequency-sensitive behavior elucidated in these studies and could open a path for development of new therapies that are geared toward addressing the pulsation dysfunction in pathological conditions, such as hydrocephalus and traumatic brain injury, affecting ICP and flow dynamics.

A signal analysis algorithm for determining brain compliance non-invasively

Patients with increased intracranial pressure (ICP) caused by hydrocephalus or brain injury have poor brain compliance or increased brain stiffness. The condition is commonly treated by a surgical diversion of cerebrospinal fluid (CSF) through placement of a ventriculoperitoneal (VP) shunt. These inserted devices frequently fail and require replacement. Assessment of failed devices typically requires an invasive surgical procedure to implant an ICP sensor. Brain compliance can be determined non-invasively by comparing the intracranial pressure (ICP) waveform to the digital artery waveform. The ICP waveform is derived from a piezo sensor snugged into the external ear canal and worn as a headset. The digital artery waveform is derived from a stand pulse oximeter. Digital signal processing performed on sampled data from these two sensors shows a time-lag or phase relationship between the two waves which widens with worsening brain stiffness or compliance. An algorithm is presented that shows how these signals can be used to compute brain compliance. An instrument designed to calculate real-time brain compliance to aid healthcare professionals is described.

Noninvasive assessment of intracranial pressure waveforms by using pulsed phase lock loop technology. Technical note

Elevated intracranial pressure (ICP) is a major factor associated with incidences of morbidity and mortality in patients with neurological disorders. The use of conventional methods for ICP monitoring is currently limited to patients with severe neurological conditions because of the methods' invasive nature. The authors have developed an ultrasonic device capable of monitoring ICP waveforms noninvasively by using a pulsed phase lock loop (PPLL) technique. The PPLL device records skull movement (several meters) associated with ICP pulsations. The purpose of this study was to correlate PPLL waveforms with invasively measured ICP waveforms in patients at the University of California San Diego Medical Center (13 patients). A linear regression analysis revealed a high correlation between PPLL waveforms and invasively measured ICP waveforms during the same time domain (r2 = 0.88). A coherence function analysis, which provides the fractional portion of the mean square value at the output that is contributed by the input at a certain frequency, showed medium to high correlations (r2 = 0.50-0.90) between PPLL waveforms and invasively measured ICP waveforms at each harmonic wave components. Furthermore, there was a significant correlation (r2 = 0.680, p < 0.01) in the harmonic distortion ratio (an index representing how much a given waveform is distorted from a pure sine wave) between PPLL waveforms and invasively measured ICP waveforms. In conclusion, PPLL technology enables the noninvasive evaluation of ICP dynamics in clinical settings.