And it is clear that hyperventilation starts a two-phase process: alkalosis in the free hand corresponds to the negative phase, after the end of hyperventilation, the potential difference is gradually restored. After removing the cuff and blood flow with a more alkaline pH in the right hand, the SCP increases, apparently due to the alkalization of blood in the region of the reference electrode. The duration of SCP changes is longer than the hyperventilation process itself. So, with hyperventilation and the location of the reference electrode on the arm, and active – on the head, a negative shift in potential in the region of the reference electrode may play some role in the shifts of the SCP. 2 The shifts of extra-cranial origin SCP were studied during hyperventilation of 1.5 min duration. For this purpose, one of the active electrodes was placed on the earlobe, the rest in standard recording areas. To prevent shifts of the SCP in the region of the reference electrode due to alkalization of blood in the arm, this arm was pressed with a tonometric cuff during hyperventilation with a pressure higher than systolic. During hyperventilation, a shift in the SCP of positive polarity was recorded on the earlobe, approximately the same as in other monopolar leads on the head .
Changes in SCP during hyperventilation. Registration of SCP from the areas of the head and left ear.
F, C, O, Td – areas of registration of SCP, Es – abduction from the left ear. The right hand, on which the reference electrode is located, is pinched by a tonometric cuff. The vertical bars correspond to the beginning and end of hyperventilation.
And it is clear that with hyperventilation on the head, including the ear, a positive shift of SCP to 7 mV is recorded. This means that the pH of the blood of the head relative to the peripheral blood is biased in the acidotic direction. Such a pH shift occurs in the vascular system of the head, and not the hand, where the CRR was stabilized due to its short-term clamping. The reaction of AMR to hyperventilation significantly exceeds the time during which hyperventilation was carried out. As follows from fig. 6.4, the origin of SCP shifts on the head can be either intracranial, associated with the accumulation of lactate at the BBB as a result of spasm of cerebral vessels, or extracranial, due to the dynamics of pH at the histohematological barrier during spasm of the skin vessels of the head during hyperventilation. It is all the more difficult to differentiate changes in SCP of extra- and intracranial origin, since in both cases shifts of SCP of positive polarity should be observed. In addition, there is a connection between the blood vessels of the external integument of the head, including the auricle, with venous sinuses. In particular, the posterior ear vein connects through the posterior temporal diploic vein with the transverse sinus. Changes in the actual skin potentials are unlikely, since when recording the SCP during hyperventilation there was no dynamics of skin resistance, which would indicate a skin-galvanic reaction.
According to published data (S. Tomita Gotoh, Y. Hayashida, 1996), when the reference electrode is located on the earlobe, and the active electrode is located on the vertex under hyperventilation, a small negative shift of the SCP of 0.8 mV is observed. The authors suggest that it is caused by depolarization of cortical neurons. However, in our opinion, it cannot be ruled out that with such registration such small changes in SCP may be due to the difference in unidirectional, but unequal in magnitude and time of development of shifts of constant potentials on the head and on the ear. The resulting potential difference is associated with unequal pH dynamics in various parts of the vascular system. The results of A. Lehmenkuhler et al. (1999). The authors investigated the shifts of SCP recorded from the surface of the brain, skull and skin in rats with changes in the gas composition of the blood. It was shown that changes in the SCP with an amplitude of up to 2 mV are detected on the skin, on the skull the shift of the SCP has the same polarity and a large amplitude (up to 4 mV), while on the brain the shifts of the SCP have an opposite polarity with a significant amplitude (up to 5 mV). The reversal of the potential sign occurred when passing through the BBB. The authors believe that the dynamics of SCP during registration from the surface of the head is determined by the potentials of the BBB, which have a diffusive nature.
Despite the fact that the dynamics of SCP on the surface of the head does not depend on changes in the membrane potentials of neurons and glia, it is closely related to neuronal activity, since it reflects the nature of brain tissue metabolism under the influence of vasoconstriction of the cerebral vessels of the head. An indirect confirmation of this is the presence of a correlation between SCP and EEG before and during hyperventilation (see section “The relationship of EEG parameters and brain energy metabolism”).
However, pronounced shifts of SCP in the ear, which have the same orientation as on the head, indicate the possible participation of extracranial vascular reactions in the genesis of changes in SCP in hyperventilation.
A careful examination of the dynamics of SCP in hyperventilation allows us to distinguish two types of reactions. The reaction of the first type can be called “weak.” It is characterized by unsynchronized shifts of SCP in different polarity. The reaction often begins with a negative phase, which then turns into a positive one. In this group, the increase in SCP is significant only in the central and frontal areas (shifts of 1.5 + 0.68 mV and 1.3 + 0.57 mV, respectively).
The second type of reaction is “strong.” It is a shift of SCP of positive polarity synchronized across all the leads. The values of these shifts are highly reliable and amount to 4.69 + 0.99 mV in the frontal region; in the central region – 3.61 + 1.03 mV; in the occipital – 4.46 + 0.87 mV; in the right temporal region – 3.46 + 0.94 mV. The reaction of the “strong” type occurs more often in young people. In addition, while the standard procedure of hyperventilation in some subjects develops a “weak” type reaction, then forcing hyperventilation can lead to the development of a “strong” type reaction.