Impedance Changes during Evoked Responses

Brain impedance decreases during normal functional activity. Galambos and Velluti (1968) measured impedance and evoked potential with chronically implanted cortical electrodes in unanaesthetised cats. They recorded an impedance decrease o f 0.5 Q (<0.1 %) which accompanied a DC decrease o f 50 pV following a click-evoked auditory stimulus. The duration was o f the order o f 5 ms. They noted that an evoked potential can occur with no accompanying impedance shift.

Adey et al. (1962) also used chronically implanted electrodes to measure the brain impedance o f cats. They exposed the cats to physiological stimuli, such as the exposure o f a female cat to a male and the presentation o f milk, and saw impedance decreases o f 100 Q (2 %). These occurred over a time-course o f around 10 s and were postulated to be associated with cell swelling.

1.6.6

Impedance Changes during Spreading Depression and Epilepsy

Spreading depression and epilepsy cause similar changes in impedance, DC potential, extracellular concentration and in the size o f the extracellular space. Both are caused by an overstimulation o f neurones which disrupts cell metabolism and ionic homeostasis. The term "anoxic depolarisation" has been extended to refer to these phenomena as well as the ischaemia related effect discussed above (Holder 1993b).

One o f the defining features o f both epilepsy and SD is the slow change in DC potential over the time course o f the attack. Typically, the DC potential measured at the cortex with respect to a distant reference electrode decreases by up to 30 mV during SD (Ranck 1964) and 5 mV in epilepsy (Van Harrefeld and Schadé 1962). The DC potential shift is caused by depolarisation o f the dendrites o f the pyramidal cells in the cortex. These cells are oriented perpendicular to the surface with their cell bodies deeper than their dendrites. During stimulation, the dendrites depolarise, leaving the cortical surface negatively charged with respect to the rest o f the brain.

Ranck (1964) and Somjen et al. (1992) have identified the important events which occur during SD. Most obviously, the DC potential decreases by 1 5 - 3 0 mV for 1 - 2 minutes, and extracellular K"*" concentration increases slowly to 5 0 - 100 mM. As this occurs, the

extracellular space shrinks by 50 % (Hansen and Olsen 1980) and the impedance rises by 40 % (Holder 1992a). Recovery occurs typically 5 minutes later.

Changes in impedance and DC shift during epilepsy were investigated by Van Harrefeld and Schadé (1962). They observed two different effects following generalised chemical-induced seizures. One was associated with a large impedance change and DC shift (12 % and 5 mV respectively) and suppression o f cortical activity which was assumed to be related to spreading depression. The other change, assumed to be epilepsy because o f the distinctive EEG activity, had a smaller impedance change and DC shift (less than 5 % and 1 mV respectively). Further work (Lux et al. 1986) has shown that the changes in the extracellular space (up to 30 %) and in extracellular concentration (10 - 12 mM) are also less during epilepsy than SD. All the above work was carried out on experimental animals. Holder et al.

(1994) measured the impedance change during epilepsy in a patient with depth electrodes. The impedance increased by 5 % as the DC potential fell by 7 mV.

1.6.7

Mechanisms for impedance change during brain activity

1.6.7.1 Im pedance increase due to c e ll sw ellin g

During SD and epilepsy, large numbers o f neurones generate action potentials simultaneously. This causes the normal ionic homeostasis mechanisms to be overwhelmed. Two mechanisms cause the ionic concentration o f the cell’s interior to be greater than that o f the extracellular space (Hansen 1985, Berne and Levy 1990).

The presence o f large, impermeable “Donnan” ions inside the cell means that its volume must be maintained actively even at rest. These ions act to increase the ion concentration within the cell and cell volume is only maintained by the sodium-potassium pump which actively removes other ions from the cell. During pathological over-stimulation such as during epilepsy and SD, energy reserves run out and the sodium-potassium pump fails. This leads to the cell being hyperpolarised with respect to the extracellular space.

At the same time, waste metabolic products such as lactic acid and hydrogen ions are produced in the cell, further increasing the osmolality o f the intracellular space. The break­ down o f the sodium-potassium pump prevents these ions being removed.

These two effects combine to ensure that the ion concentration in the intracellular space rises by more than that outside the cells. Water flows along the concentration gradient by osmosis to increase the cell volume and decrease the volume o f the extracellular space. The reduction in volume o f the extracellular space can be as much as 30 % during epilepsy (Lux et al. 1986) and 50 % in SD and ischaemia (Hansen and Olsen 1980).

1.6.7.2 S p a tia l P ota ssiu m Buffering

Following a single action potential, the concentration o f extracellular increases by 0.01 - 0.04 mM (Dietzel et al. 1980). Lux et al. (1986) argued that if this is multiplied by the number o f action potentials which occur during a typical epileptic seizure, an increase o f 25 - 50 mM would be expected, leading to a decrease o f 8 % in the extracellular space. This is 4 - 5 times less than the observed reduction in the extracellular space o f 30 - 50 % discussed above. Similarly, the predicted increase o f 25 - 50 mM in extracellular K"*" is considerably more than the observed increase o f 1 0 - 1 2 mM. Thus, during epilepsy, metabolic effects alone are not sufficient to account for the observed changes in extracellular K"^, cell swelling and impedance.

It has been proposed that spatial potassium buffering (Lux et al. 1986, Orkand et al. 1966, Dietzel et al. 1980) is responsible for the movement o f away from the site o f stimulus. This reduces the measured extracellular and therefore the osmotic potential o f the extracellular fluid, causing more water to flow into the cells, increasing cell swelling and impedance. The extracellular concentration o f is high following a stimulus. therefore diffuses along its concentration gradient firstly into glial cells and then along their axons. Away from the site o f stimulus, the concentration o f is greater inside the glial cell than out and so K leaves the cell. The current loop (K^ into the glial cell, along the cell and out again) is completed by Na^ (flowing towards the stimulus) and Cl" ions (away from the stimulus) in the extracellular space.

At the site o f stimulus, and Cl" are removed and only partially replaced by Na^. The osmotic potential o f the extracellular fluid is therefore reduced further, the cell swells more and impedance increases further. Moreover, at distant sites (up to 1 mm from the stimulus), leaves glial cells, increasing the osmotic potential o f the extracellular space there and cells

shrink. Lux et al. (1986) estimated that sufficient can be removed to account for the difference between the observed changes in extracellular and the predicted changes due to

1.6.7.3 Im pedance d ecrea se due to increase in b lo o d volum e

During brain metabolism, the concentration o f metabolites in the blood builds up. Increasing concentrations o f CO2, and nitrous oxide and decreasing concentrations o f O2 and ATP all have vasodilative properties so local blood flow increases. Blood has a lower impedance than the brain and so an increase in blood volume leads to a reduction in impedance. Blood flow has been shown to increase by 260 % in dogs during generalised epilepsy (Plum et al. 1968). In addition, an increase in blood velocity can decrease longitudinal impedance as erythrocytes align parallel to the vessel walls (Coulter and Pappenheimer 1949). The effect on brain impedance o f any impedance change due to blood flow is reduced by the fact that, in rabbits, the impedance o f blood accounts for only 11 % o f the total brain impedance (Van Harrefeld and Ochs 1956).

The reduction in impedance due to an increase in blood flow can be most readily observed during normal brain function and evoked potentials when the increase in impedance due to cell swelling is negligible or absent (Holder 1993b).

1.6.7.4 O th er m echanism s f o r im pedance decrease

Two other mechanisms have been suggested to contribute to the impedance decrease during activity. Firstly, metabolism is associated with a local increase in temperature and the impedance o f tissue can decrease by 2 % per degree centigrade temperature rise (Foster and Schwann 1989). An increase in blood volume can also change temperature, although the brain temperature can be either more or less than core temperature depending on the species and preparation used (La Manna et al. 1989, Henker et al. 1998). The impedance change due to temperature changes is generally considerably smaller than those due to other mechanisms.

An increase in ion concentration in the extracellular space will cause a decrease in impedance. At low concentrations, the impedance is proportional to the ion concentration (Webster 1990) and the resulting impedance change could be significant. This effect does not appear to have been mentioned in the literature and it is discussed in more detail in section 2.4.4.

1.6.7.5 Sum m ary

Different mechanisms for the impedance change occur depending on the effect being studied. During normal physiological activity such as evoked responses, the cell volume is largely maintained by the sodium-potassium pump and the dominant impedance change is a decrease

due to an increase in blood volume. Pathological brain activity can be so severe that the sodium-potassium pump fails and cell swelling causes an overall impedance decrease.

Both the impedance decrease due to blood flow and the increase due to cell swelling occur because the applied current is largely excluded from the cells. At high frequencies, current can cross the capacitative membrane o f neurones and blood cells and the current path is not affected by cell swelling. At low frequencies, the current is restricted to the extracellular space and is therefore more sensitive to cell swelling. Also, the resistance o f blood is lower than that o f the brain because the proportion o f extracellular space in the blood (the plasma) is much greater than in the brain, so it too is frequency dependent. This has two implications: impedance imaging o f the brain is likely to be more sensitive at low frequencies and brain imaging may be particularly suitable for multi-frequency measurements.

1.6.8

Human functional brain imaging

Similar mechanisms to those discussed above are expected to cause impedance changes in humans. Non-invasive brain imaging has been performed by single photon emission computed tomography (SPECT), positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)^. Studies have shown, for example, that blood flow increases during somatosensory evoked responses by about 5 % using PET depending on the area o f brain studied (Ibanez et al. 1995). In an early fMRI study, visual stimulation caused a blood flow increase o f 32 ± 10 % in the visual cortex (Belliveau et al. 1991). SPECT has been used during and between epileptic seizures {ictal and interictal recording) to image the blood volume change (Stefan et al. 1990). Interpretation was difficult due to the delay between the injection o f the radioisotope and features appearing in the image, but they concluded that the localisation o f the activity corresponded with EEG and ictal recordings were associated with an increase in blood flow and inter-ictal with a blood flow decrease.

In addition to measuring indices related to blood flow and volume, fMRI can be used to give a measure o f diffusion. Diffusion-weighted imaging (DWI) has been used to measure the freedom o f water to diffuse through a tissue. This is related to the conductivity and has been shown to correlate with ischaemia and epilepsy (Halpem and Huang 1995).