Investigation of the Relationship Between Transcranial Impedance and Intracranial Pressure

Abstract

Studies on piglets have shown that cranial bioimpedance (Z) measurements correlate well with invasively measured intracranial pressure (ICP). We have tested the feasibility of collecting transcranial impedance from a clinical device for measuring whole-body water content (ImpediMed SFB7). In the clinical study, 50 normal healthy volunteers had transcranial impedance measured using nine different head montages (forehead to mastoid (left/right), temporal to mastoid (left/right), forehead to temporal (left/right), forehead to occipital (left/right) and temporal to temporal). Impedance was measured 20 times over a frequency range per montage and ANOVA used to test for effects of electrode position upon recorded value. For the experimental study, five sedated and ventilated Marino sheep were instrumented for intraventricular ICP and transcranial impedance measurement. Measures of ICP were recorded while ICP was increased from baseline to greater than 50 mmHg in five steps using an intraventricular infusion of mock CSF. There is a significant effect of electrode position and gender upon transcranial impedance (p < 0.001). The temporal-mastoid electrode position had significantly lower impedance values in keeping with its shorter path length. ICP correlated with craniospinal compliance measurements and Impedance vs Freq by ICP step shows a clear ICP dependence (p = 0.007) across the sheep.

Transcranial bioimpedance measurement in horses: a pilot study

“…When used in piglets to quantify hypoxia-induced cerebral oedema (Lingwood et al 2002) increases in TCBI correlated with directly measured intracranial pressure (ICP). Strong correlation was also found between directly measured ICP and TCBI in anaesthetised sheep over a range of blood and intracranial pressures (Shaw et al 2012).…”

“…TCBI can vary directly with raised ICP when caused by hypoxia (Lingwood et al 2002), but TCBI varies indirectly with ICP when ICP is influenced by either iatrogenic hypertension or increased intracellular volume (Shaw et al 2012). ICP was not measured in the current study; however, as intracranial pressure can reach values of 55 mmHg in horses under general anaesthesia positioned with the head down (Brosnan et al 2008), the effect of altered ICP on TCBI in anaesthetised horses has to be considered.…”

“…Despite the effective measurement of total body water in horses using bioimpedance technology (Forro et al 2000;Fielding et al 2007), and the use of TCBI to reflect changes in intracranial homeostasis in other species (Lingwood et al 2003;Shaw et al 2012) the use of TCBI has not been reported in horses. The objective of this pilot study was to investigate the feasibility of TCBI measurement and the quality of the resulting signal in conscious horses.…”

“…Bioimpedance refers to the resistance measured to alternating current flow through living tissue (Shaw et al 2012) and has been used to study body composition (Cornish et al 1993) and acute fluid shifts (Bordelon & Wingfield 2002). Bioimpedance is measured in people using commercial devices, such as the Impedimed SBF7, which do so by measuring the impedance (Ω) recorded from the tissues lying between electrodes.…”

“…The resistance (the opposition to current flow produced by the tissue pathway) and the reactance (the resistance offered to alternating current by capacitors -in this case, the cell membrane) are then plotted against each other and mathematically extrapolated. This allows the values of resistance to be calculated for infinite (R ∞ ) and zero frequencies (R 0 ) from which the tissue's characteristic impedance (Z c ) may be estimated (Shaw et al 2012).…”