Correlation of rheoencephalography and laser Doppler flow: a rat study

The goal of neuromonitoring in both the neurosurgery intensive care unit, during transport of wounded military service members is to prevent brain damage that may occur due to failure of CBF AR. Currently, there is no single measuring modality capable of monitoring for such brain injuries as hypoxia, ischemia, elevated ICP, edema, intracranial hemorrhage, vasospasm.

CBF AR reflects the ability of the brain to keep brain blood flow relatively constant despite changes in arterial pressure. Failure of autoregulation is associated with a worse outcome in various acute neurological diseases [5]. The suitability of REG as a potential brain monitoring modality is due to its potential to detect vasoreactivity. Continuous CBF AR assessment would have prognostic value, indicate pathological states, and potentially influence therapy with the use of the ‘optimal cerebral perfusion pressure’ paradigm (CPP) [6]. Pressure autoregulation is an important hemodynamic mechanism, which protects the brain against inappropriate fluctuations in CBF in the face of changing CPP. Pressure autoregulation can be monitored with ICM+ software [2].

Few studies have compared results of measurements made with REG to LDF measurements.

Our study established a correlation between REG and LDF during transient CO₂ inhalation. The physical basis of REG is that blood and cerebrospinal fluid are better conductors than brain tissue. Consequently, increases in REG amplitude reflect increases in the amount of blood in the brain. The usefulness of REG for neuromonitoring compared to LDF is that (when used to measure humans), REG is noninvasive. A disadvantage of REG is its sensitivity to artifacts; to overcome this disadvantage in our study, we used data pre-processing to clean the REG signal before quantification (see Methods).

This study is part of an ongoing investigation of brain electrical impedance (rheoencephalography/REG) and its potential for use as a noninvasive, continuous brain monitor. The suitability of REG for neuro-monitoring is due to its capability to detect vasoreactivity, the change in cerebral blood flow autoregulation (CBF AR) in response to a vasodilatory or vasoconstrictive stimulus. CBF AR is an instrinsic neuroprotective physiological phenomenon, which reflects the ability of the brain to maintain relatively constant CBF despite changes in cerebral perfusion pressure. This study compared results measured simultaneously by REG and laser Doppler flowmetry (LDF), a commonly used neuromonitoring modality. Using LDF, Lee et al. showed that breaching lower limits of autoregulation was associated with a significant increase of the pressure reactivity index (PRx) in a model of arterial hypotension [9]. The rat study reported here measured CBF AR during CO₂ inhalation (a vasodilatory stimulus).

A strong correlation between cerebrovascular reactivity (CVR) and ventilatory response to CO₂ has been demonstrated [1]. CBF and its distribution are highly sensitive to changes in the partial pressure of arterial CO₂ (pCO₂). This physiological response (cerebrovascular CO₂ reactivity) is a vital homeostatic function that helps regulate and maintain central pH, which in turn affects the respiratory central chemoreceptor stimulus. As a response to hypercapnia, CBF increases to wash out CO₂ from brain tissue, thereby attenuating the rise in central pCO₂; in contrast, hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue pCO₂ [1]. Carbon dioxide is the most suitable vasoactive stimulus [7]. Acetazolamide and CO₂-induced stimulation of the cerebral vasomotors have been shown to be valid techniques to measure reduction in perfusion reserve due to extracranial cerebrovascular occlusive disease [12].

Our hypothesis was that both LDF and REG reflect CBF AR.

An international consensus has been published on the usefulness of multimodality monitoring in neurocritical care [10]. Invasive neuro-monitoring modalities typically measured are blood flow by LDF, intracranial pressure (ICP), brain tissue O₂ and temperature, and tissue perfusion. Noninvasive modalities used are near infrared spectroscopy (NIRS), transcranial Doppler (TCD) and neurophysio-logical methods (EEG, evoked potentials).

Brain imaging methods such as single-photon emission computed tomography (SPECT), Positron Emission Tomography (PET) scan, Magnetic Resonance Imaging (MRI), and cerebral perfusion computed tomography are not suitable for continuous monitoring and detecting the onset of neurological deterioration.

A NASA Technical Reports search for REG [11] found 90 publications containing REG as a keyword, most referring to bibliographies and indexes. However, a report related to REG from 1 979 stated that there was an increase in the blood volume in the head that was retained for the entire 96-day flight and leveled off at the end the 140-day flight [17].

One of the reports describes that +Gz acceleration without straining maneuvers decreases CBF velocity to half of normal and probably caused a caudal fluid shift from both intra- and extracranial tissues [8]. A similar effect was described in a rabbit study [14]. Impaired cerebrovascular autoregulation and reduced CO₂ reactivity after long duration spaceflight was reported [16].