Investigation of the relationship between transcranial impedance and intracranial pressure

Transcranial bioimpedance measurement in horses: a pilot study

OBJECTIVE

This pilot study aimed to evaluate the feasibility of transcranial bioimpedance (TCBI) measurement and variability of TCBI values in healthy conscious horses and to study effects of body position and time on TCBI in anaesthetized horses.

STUDY DESIGN

Prospective, observational study.

ANIMALS

A total of four research horses and 16 client-owned horses presented for surgery.

METHODS

After establishing optimal electrode position using computed tomography scans of cadaver heads, TCBI [described using impedance at zero frequency, R_0, (Ω)] was measured in four conscious, resting horses to investigate the feasibility and changes in TCBI over time (80 minutes). Data were compared using a paired t test. TCBI was then measured throughout anaesthesia (duration 92 ± 28 minutes) in 16 horses in dorsal and lateral recumbency. Data were analysed using a general linear model; gamma regression was chosen as a model of characteristic impedance [Z_c; (Ω)] against time. Data are presented as mean ± standard deviation.

RESULTS

No change in R₀ was seen in conscious horses (age = 15.3 ± 7.3 years, body mass = 512 ± 38 kg) over 80 minutes. The technique was well tolerated and caused no apparent adverse effects. In 16 horses (age = 7.4 ± 4.7 years; body mass = 479 ± 134 kg) anaesthetized for 92 ± 28 minutes, Z_c fell during anaesthesia, decreasing more in horses in lateral recumbency than in horses in dorsal recumbency (p = 0.008). There was no relationship between Z_c and body mass or age.

CONCLUSIONS AND CLINICAL RELEVANCE

TCBI is readily measured in horses. TCBI did not change with time in conscious horses, but decreased with time in anaesthetized horses; this change was greater in horses in lateral recumbency, indicating that TCBI changes in anaesthetized horses may be related to the effects of recumbency, general anaesthesia, surgery or a combination of these factors.

Transcranial Bioimpedance Measurement as a Non-invasive Estimate of Intracranial Pressure

OBJECTIVES

We have previously demonstrated a relationship between transcranial bioimpedance (TCB) measurements and intracranial pressure (ICP) in an animal model of raised ICP. The primary objective of this study was to explore the relationship between non-invasive bioelectrical impedance measurements of the brain and skull and ICP in traumatic brain injury (TBI) patients.

MATERIALS AND METHODS

Included patients were adults admitted to the Neurological Intensive Care Unit with TBI and undergoing invasive ICP monitoring as part of their routine clinical care. Multi-frequency TCB measurements were performed hourly through bi-temporal electrodes. The bioimpedance parameters of Z _c (impedance at the characteristic frequency) and R ₀ (resistance to a direct current) were then modelled against ICP using unadjusted and adjusted linear models.

RESULTS

One hundred and sixty-eight TCB measurements were available from ten study participants. Using an unadjusted linear modelling approach, there was no significant relationship between measured ICP and Z_c or R₀. The most significant relationship between ICP and TCB parameters was found by adjusting for multiple patient specific variables and using Z_c and R₀ normalised per patient (p < 0.0001, r ² = 0.32).

CONCLUSIONS

These pilot results confirm some degree of relationship between TCB parameters and invasively measured ICP. The magnitude of this relationship is small and, on the basis of the current study, TCB is unlikely to provide a clinically useful estimate of ICP in patients admitted with TBI.

Accuracy, Precision, Sensitivity, and Specificity of Noninvasive ICP Absolute Value Measurements

An innovative absolute intracranial pressure (ICP) value measurement method has been validated by multicenter comparative clinical studies. The method is based on two-depth transcranial Doppler (TCD) technology and uses intracranial and extracranial segments of the ophthalmic artery as pressure sensors. The ophthalmic artery is used as a natural pair of "scales" that compares ICP with controlled pressure Pe, which is externally applied to the orbit. To balance the scales, ICP = Pe a special two-depth TCD device was used as a pressure balance indicator. The proposed method is the only noninvasive ICP measurement method that does not need patient-specific calibration.

Research on simulation and experiment of noninvasive intracranial pressure monitoring based on acoustoelasticity effects

The real-time monitoring of intracranial pressure (ICP) is very important for craniocerebrally critically ill patients, but it is very difficult to realize long-time monitoring for the traditional invasive method, which very easily infects patients. Many noninvasive methods have emerged, but these have not been able to monitor ICP for long periods in real time, and they are not ready for clinical application. In order to realize long-time, online, real-time, noninvasive monitoring for ICP, a new method based on acoustoelasticity of ultrasound is herein proposed. Experimental models were devised to research the new method for experiment and simulation. Polymethyl methacrylate and hydrogel were adopted for the experiment, and their mechanical properties were very close to the real brain. A numerical solution for acoustoelasticity theory was acquired by simulating calculation based on a finite-element method. This was compared to the experimental value. The results showed a consistent match between theoretical solution and experimental value, with maximum error at most 5%. Thus, the effectiveness of the new method was verified. Theoretical and practical foundation is provided for this new method, and it could be used for animal experimentation or clinical testing in further research.