Improved cerebral circulation with trepanation - Mechanism and studies
The laws of circulation within the cranium have until recently been much less well studied than those of other sections of the cardiovascular system. This is true despite the fact that the attention of any investigators during the last decade has been sharply focused upon the problems of physiology and pathology of cerebral circulation. During recent years, thanks to the development of experimental technology, there were carried out fundamental investigations clarifying sufficiently some of the important problems of cerebral circulation. Among the important contributions by Soviet scientists are the anatomo- histological studies of the brain circulatory system; studies of the nervous and neuro-humoral control of cerebral blood supply ; studies of the local oxygen tension in the cerebral tissues. Nevertheless, many questions of the hemodynamics of cerebral circulation have found little attention and often concepts and hypotheses were advanced which are not clear because they are based on contradictory and often unverified statements. Among these are questions regarding blood movement in the cerebral blood vessels, the presence and character of cerebral pulsation and the transmission of pulse waves within the hermetical cavity of the skull. What connection is there between the cerebral blood circulation and the movements of cerebro-spinal fluid? What role does the anatomical structure of the cranial cavity play in its circulatory destructiveness? What do we know about the syphons of the internal carotid and vertebral arteries, about the numerous loops and folds of arteries and arterioles within the brain, and so forth? In connection with these problems, we now present the results of our investigations into some of the previously unclear mechanism of hemodynamics of cranial circulation. Our main attention has been given to the important question of analysis of the connection between the dynamics of blood supply to the cranial cavity and the movement of the cerebral spinal fluid. The solution to this problem is of prime importance for our understanding of the physiology and pathology of cerebral circulation.
Methods for studying cerebral circulation are difficult. In order to obtain informative data on the hemodynamic processes within the cranial cavity it is necessary to have continuous recording of fluctuations in the relationship between a ) blood volume and the cerebral spinal fluid in the cranial cavity and b) the intracranial pressure and the pressure of the arterial and venous systems. Moreover, if results are to be accurate, it is necessary to insure the hermetical sealing of the cranial cavity. For the registration of the dynamics of blood supply to the cranial-cerebral cavity the best of existing methods is electro-plethysmography which is based on an analysis of variation in the electrical conductivity of the tissues. In the application of EPG to the study of cranial circulation dynamics, it is necessary to: Determine the optimal biophysical conditions for recording the changes in electrical conductivity between the electrodes introduced into the cranial cerebral cavity; Find a criterion or criteria for qualitative evaluation of the changes in the blood supply to the cranial cerebral cavity based on the characteristics of the electroplethysmogram; Clarify the degree of error introduced in the EPG by various factors which are not connected with changes in the blood supply to the intracranial cavity. For the solution of these questions we carried out special investigations (49). Most importantly, we found that with bitemporal electrodes introduced into the cranial cavity of animals the current density in spaces filled with cerebral spinal fluid was much greater than in the other sections of the cranio-cerebral cavity. This led to the conclusion that the variation in the electrical parameters between the electrodes will depend on the changes in the volume relationship between blood and the cerebral-spinal fluid when the electrical conductivity of the media filling the cranio-cerebral cavity remains constant. Considering that blood and cerebral spinal fluid (CSF) represent ionic conductors with small internal polarization it is possible to presume that the changes in the active conductivity between the electrodes will correspond to the dynamics of blood content in the cranio-cerebral cavity. These findings enabled us to determine the relationship between the changes of the electrical conductivity of the cerebral cranial cavity and the dynamics of its blood supply. By solving a system of equations set up on a basis of a model representing approximately the cranial cavity in the form of a sphere with several layers of varied electrical conductivity, a formula was derived connecting the relative changes of the electrical conductivity of the cranial cavity and the changes in its blood content where A, B, C, Ro, Rm, are coefficient depending on the geometry of the skull cavity. As a result of calculations according to formula (1) and according to the data from direct measurements during the testing of the changes in the blood supply of the intracranial cavity a quantitative relationship between and was obtained which can be considered linear with only minor deviations. In these evaluations, we have assumed at the start a constant value of electrical conductivity for the basic circulating media (blood and CSF) filling the skull. However, as can be seen from the work of Kedrov and Naumenko (29), and others, the electrical conductivity of blood depends, among other things, upon the speed of its movement. A special study of this question (49) showed that with changes of the blood flow velocity in cerebral vessels, the resulting alterations in the electrical conductivity of the blood effect variations in the electrical conductivity of the cranial cavity. This may be most simply expressed by saying that variation in the speed of electrical conductivity of blood is inversely related to volume variation. The analysis of partial relationships of the relative values of volume and speed fluctuations in the electrical conductivity of blood within the cranial cavity (Fig. 3) showed that one of the methods for decreasing the error caused by speed variations of the blood stream involved the correct selection of the frequency for recording the EPG. Considering also other frequency dependencies of the factors determining the optimal biophysical conditions for EPG recording (52)--the biological action of the flow, the polarization phenomena, the frequency dispersion of electrical parameters, the relationship of the electrical conductivity of blood and CSF, et.--we have concluded that the best frequencies for intracranial EPG recording vary from 15 to 30 kilocycles. At these frequencies the error caused by speed variations in the velocity of blood flow does not exceed 10 per cent. The above conclusions with exception of formula (1), are correct also for the cerebral spinal cavity. By analogy it can be shown that the relationship for the cerebral spinal EPG would also approximate a straight line if one of the electrodes is placed in the thoracic and another in the lumbar section of the vertebral column. Cerebro-spinal EPG is interesting because its relationship to the intracranial EPG can serve as a direct proof of the reverse movements of the CSF between the cavities of the brain and the spine. In the registration of intracranial EPG in man by placing electrodes on the skin of the head, there are additional difficulties in the analyses of the obtained curves because the EPG will record to a certain degree also the effects of extra cerebral blood vessels. In studies of this question several workers (5, 19), utilized electrodes at various locations on the head during the registration of the intro-cerebral EPG in man. However, in these investigations, no attention was paid to the characteristics of the electrical field distribution between the electrodes nor the frequency dispersing characteristics of living tissues. Because of this, we do not know to what degree the dynamics of blood supply of the extracerebral vessels is shown in the curves presented by the above mentioned authors. Our calculations of the effect of extracerebral vessels are based on an analysis of an equivalent electrical circuit between the electrodes which develops when the electrodes are placed on the scalp . Such an analysis reveals that the error introduced by extracranial blood vessels will reach 50 to 60 per cent while the electrical field distribution as affected by living tissues at high frequencies such as were used by the above authors will actually falsify the obtained results. However, if we select in a similar manner places for the application of the electrodes so that the elements RkCkl and RkcCka of the equivalent circuit are either excluded or reduced, then the degree of error can be reduced significantly. As we have shown (33) this can be achieved when one of the electrodes is placed on the eyeball and the second one in the area of the foramen magnum . In this location of the electrodes the current is distributed primarily inside the cranium passing through existing openings. Frequencies of 80 to 100 kilocycles appear to be optimal for the recording of intracranial EPG in man as well as for EPG registration in other parts of the body (51, 52). Such an increase in frequency as well as a special treatment of the skin surface under the electrode near the foramen magnum (removal of fat and wetting with 5 per cent salt solution) allow for a reduction of the instabilizing effect of the corneal layer of skin upon the EPG. To study the dynamics of blood supply of the intracranial cavity of man we utilized also the method of ultra high frequency EPG (47a), which allows for indirect registration by passing radio waves of 30 to 50 cm. length through the head of the patient. The above mentioned data permitted us to solve some open questions of intracranial EPG and to apply this method as one of the principal ones in the study of hemodynamics of cerebral circulation. Along with the EPG method which allows evaluation of the changes in the volume relationship between blood and CSF within the cranial cavity or in other words of the dynamics of blood supply to the brain, we have recorded on animals also the dynamics of the blood pressure in arteries at the base of the skull, in the venous sinuses and in spaces filled with CSF. During recent years, in the world literature have been described many various methods of conversion of pressure variations into electrical signals (transducers) which allowed us to construct very sensitive electromanometers. We will not deal in detail with this group of methods but here will show only the types of recording instruments utilized in our work. For the registration of pressure in acute experiments we utilized mechanical, photo-electrical and tensometric transducers (3, 48). In some experiments designed for the study of plastic changes in rapidly moving processes within the cranial cavity we utilized piezo-electrodes which registered the curves of speed in the pressure changes; this has allowed us to investigate phase differences (57). Both acute and chronic experiments were carried out on rats, rabbits, cats and dogs. In the conditions of an acute experiment we used intravenous medial (2%)-urethane (1 g. per Kg. of body weight) anesthesia. For the registration of intracranial EPG we introduced into the cranial cavity of animals 2 bitemporally placed silver electrodes (diameter 5-8 mm). The electrodes were inserted into polymethylmetacrylate holders which in turn had been screwed into the bones and fully covered the trepane openings. The electrodes and the surrounding bone were then covered with collodium. For the recording of cerebro-spinal EPG similar electrodes were introduced into the first cranial and third lumbar vertebrae. The scheme of electrode location is shown in Figure 6. In several experiments with cats and dogs along with electrodes for EPG registration we placed also needles for intra-cerebral and intra-arterial blood pressure readings. In chronic experiments on animals (dogs and cats) the electrodes were introduced by surgical methods under aseptic conditions. The incision was made along the central line of the scalp down to the second cervical vertebra, and the edges of the skin were pulled apart by hoods. A longitudinal section of the temporal muscle was made in the area of the occipital bone down to the bone and the edges of the muscle were pulled apart. An opening 5 mm. in diameter in cats and 8 mm. in dogs was trepanned 0.5 to 9.7 cm. from the central line. A similar operation was carried out on the opposite side. For the placement of the cervical electrode, the incision was made down to the central line of the neck muscles. The muscles were then pulled away from the first and second vertebrae. An opening in the posterior part of the first cervical vertebra was made. In the trepanation of the skull and the vertebra the dura mater was not injured. To locate the other electrode in the area from the first to the fourth lumbar vertebrae, an incision was made in this area through the skin and underlying tissues and again the sides of the wound were pulled away by hooks. On the left and the right of the spinal processes of the second and third lumbar vertebrae, the longitudinal muscles of the spine were cut and pulled away from the vertebrae and then an opening made in the body of the third lumbar vertebra. The wires from all electrodes were sewn into the skin incision along a central line at a distance from 0.5 to 1 cm. one from each other between the skull suture and the occipital protuberance. In this manner the animal had two electrodes in the cranial area and two in the spinal area. The stitches were taken out within ten to twelve days if there was no infection. The animal was taken for experimental work on the third or fourth day. Such an animal was observed for periods varying from one to three months. After such a period an acute experiment was carried out to verify the hermetical condition of the electrodes and to check by microscopic examination the condition of the tissues in the area surrounding them. In two of the operated dogs a photo-electrical pick-up for pressure recording was introduced into the cranial cavity. In addition to the experiments above mentioned, several series of observations were carried out on healthy people through the use of methods of medium frequency and super high frequency EPG.
Among the basic questions of dynamics of cerebral blood circulation that have not as yet been clarified are the methods of compensation for the variations in the blood volume within the hermetical cavity of the skull. According to many authors the most realistic mechanism advanced to explain this compensation, consists of the translocation of a certain volume of CSF from the cranial cavity into the vertebral cavity and vice versa during changes in the volume of cranial blood vessels. However, in many other studies no such movements of the CSF have been found (2,69). In view of these conflicting data, the first stages of our studies were devoted to the elaboration of possible ways of compensating for reversible shifts of CSF among the cavities of cerebrum and the spine. We carried out simultaneous EPG registration of intracranial and intra-spinal cavities both in single and repeated experiments with animals. The EPG curves obtained in single experiments (47) can be interpreted as follows: 1. Translocations of CSF connected with respiratory movements. 2. Slow translocations of CSF corresponding to waves of third order. The respiratory translocations of CSF recorded on the EPG occur in opposite directions in the cavities of the cerebrum and the spine, i.e., if, during inhalation there is a decrease in electrical resistance of the cranio-cerebral cavity and simultaneously an increase in the spinal cavity. At the time of expiration the reverse changes take place (Fig. 7). The electroplethysmogram reflects also the differences in the length of inhalation and exhalation. Since the relative change in the electrical resistance of the cerebral and spinal cavities is due to the changes in the quantitative interrelation between blood and CSF, it is possible to deduce from the EPG that during inhalation the CSF is moved from the spinal cavity into the cranio-cerebral cavity (this corresponds to the reduction in electrical resistance of the cerebrum during in halation and to its increase during exhalation). From a quantitative viewpoint, it can be seen from Figure 7 (right hand scale of the ordinate) that approximately 10 per cent of the total quantity of the cranio-cerebral CSF is shifted from the cerebrum into the spinal cavity during each exhalation of average depth and that this returns during inhalation to it previous level. In addition to the respiratory shifts of CSF, the EPG records also indicate translocations of a third order. These are very slow changes with a frequency of thirty to forty times per hour, which are not always regular and are not always distinct. In acute experiments we observed these only on the basis of dilatation of the cerebral blood vessels caused by asphyxia (Fig. 8) and also on the basis of some other physical conditions exerting a strain on the adaptive resources of the organism. In chronic experiments we have also observed regularly uninterrupted movements of CSF between the cerebral and spinal cavities (49). Translocations of CSF in chronic experiments can be divided into two groups. The first group comprises the rhythmical shifts of CSF occurring synchronously with respiration and with waves of third degree. The second group comprises the non-periodic translocations connected with the position of the body, the movements of the head, of the body, and of the limbs. The rhythmical shifts of CSF connected with the respiratory cycles are oppositely directed as are those seen in the acute experiments. However, their shape is much more complex in unanesthetized animals as compared to anesthetized animals, although they are much closer to the latter in a sleeping animal. A main characteristic of the rhythmical translocations of CSF in chronic experiments is the fact that the third degree waves are observed continuously while in acute experiments they occur only under certain conditions. The frequency of these waves fluctuates widely from one to six per minute in various physiological states of the animal (sleeping, waking, excitement and so forth). The non-periodic translocations of CSF are also complex in their character and occur when the animal moves. The greatest CSF displacements are recorded when the animal is lifting, lowering or turning its head or when it changes its position. Besides this, the translocations of CSF were observed also when paws were lifted, when the animal stretched or yawned, and so forth . From the above mentioned facts it is indicated that a certain volume of CSF is involved in the continuous forward and backward movement of fluid between the cranial and vertebral cavities. In order to verify the correctness of this conclusion and to show once more the existence of a free connection between the cranial and vertebral cavities we have carried out a series of observations on healthy people, recording intracranial EPG in connection with several simple functional test, e.g., pressure on the abdomen, Kvekenshtedt s test, etc. In these observations we have registered the inflow of CSF into the cranial cavity when pressure was applied to the abdomen (Fig. 9) which was expressed by a decrease in the electrical conductivity of the intracranial cavity. The control here is the lower curve recording intracranial EPG during Kvekenshted s maneuver. Thus, we can conclude that under normal conditions there exist constant shifts of CSF between the cerebral and spinal cavities, which compensate for the volume changes occurring in cerebral blood vessels. Our data indicate, however, that under normal conditions, quantitative changes in CSF response depend not only upon the total change in the intracranial pressure, as one might expect, but also upon the speed with which the process occurs. Thus, in a single record of intracranial EPG and of intracranial pressure (Fig. 10), the peaks of pulse and respiratory waves do not correspond: EPG shows mainly the respiratory waves while the pulse waves appear on the pressure curve. In Figure 11 another characteristic fact is demonstrated. In conditions of a repeated experiment with a freely moving dog there were registered simultaneously the EPG and the intracranial pressure. As can be seen from the figure, this indicates that the slowly developing changes in the blood content of the intracranial cavity cause practically no changes in the intracranial pressure. These facts can be explained only if we assume that the speed of volume movements of CSF in the area of the foramen magnum is limited. Therefore, fast changes in the blood content of the cranial cavity cause an increase in the intracranial pressure which is compensated only after some time as a result of CSF translocation into the spinal cavity. In order to verify this hypothesis we have carried out calculations based on a schematical representation of the cranial cavity (Fig.12) (53). These calculations allowed for the derivation of the relation between the blood supply and the functions of pressure in the arterial (t) and venous (t) systems of the cerebrum: (2) Where: a1 and a2 are coefficients characterizing the elastic characteristics of cerebral arteries and veins. A--characterizes the passage capacity of CSF through the foramen magnum area. The solution of the equation (2) at step-wise pressure changes in either one of the systems (3) is represented graphically in Figure 13 and shows that if our assumption of the limitation CSF outflow from the cranium is correct, at different pressure changes in the arterial and venous systems of the cerebrum, the changes in its blood content (and also the changes of the EPG level) should correspond in form to the curve shown on Figure 13 i.e., they should occur according to exponential dependence. Later, we carried out experiments in order to clarify whether or not the theoretical data correspond to the experimental results. Experiments recording intracranial EPG in conditions of sharp, split-second changes in the animal s body position on a special stand show that during the first few seconds, until the compensatory reaction of the cerebral vessels begins, the change of the EPG level of the intracranial cavity corresponds in form to the curve obtained by our calculations (Fig. 14). This allowed us to conclude that when the blood supply of the cranial cavity changes within a few seconds of time (e.g., respiratory waves or waves of III order) the excess volume of blood in the cranial cavity is compensated by translocation of CSF into the spinal cavity without a significant change in the intracranial pressure, while, on the other hand, a fast increment in blood content within a second or so (e.g., pulse waves) increases primarily the intracranial pressure and the compensatory translocation of CSF is not significant. This conclusion, clarifying the mechanism of respiratory waves, waves of IIIrd order and of slow non-periodic changes in the blood content in the intracranial cavity due to nervous, humoral and some other--yet unknown--mechanisms, leaves open the important question of pulsation in the hermetical cavity of the cranium. The question of cerebral pulsation in the hermetical cavity of the cranium has a long history. During several decades of study of the hemodynamics within the closed cranium there have developed two theories, one of which recognizes the possibility of cerebral pulsation while the other denies it. Among the supporters of the theory of pulsation within a close cranium area, among others. From our current knowledge of the hemodynamics of cerebral circulation it follows that to prove one or the other theory we will have to clarify the following questions: 1. Whether or not there exist pulse fluctuations in the intracranial pressure inside the hermetically closed skull; 2. Are there or are there not pulse translocations of CSF of the cerebral arteries to the venous system since compensatory pulsatory displacements of CSF to the spinal cavity have not been recorded; 3. Is there a compensatory, pulsating outflow of venous blood from the cranium. In relation to the first of these questions, through our experimental data (50, 56), it is shown that, in the closed cavity of the skull, there are constant pulsations of the intracranial pressure (Fig. 15a), in the order of 2-6 mm. of H2O in anesthetized animals. When the hermetical sealing of the skull is impaired these fluctuations decrease markedly (Fig. 15b). Similar data were obtained by Bering (12) from a human being with mechanotronic electromanometers. D. I. Parolla (62), using thermoelectrometry, showed that blood circulation in the vessels of the cerebral meninges has a pulsating character. De la Torre, Netsky and Meshan (17), using the method of speed kymography as devised by Schneider, also demonstrated the pulsating character of circulation in the cerebral vessels. The low value for the pulse fluctuations of the intracranial pressure--correctly measured only recently--had caused many authors to doubt its existence. Among them were those who used the "transparent skull" method. Our comparison of the data obtained with the method which uses an air bubble and transparent skull and those which employ manometric devices (50) showed that the first of these cannot detect cerebral pulsation where the skull is "sealed" because of its low sensitivity. The lack of sensitivity depends upon the fact that the existing pulse fluctuations within the intracranial cavity compress the edges of the air bubble not more than 2 micra. Nevertheless, as we have shown, an improvement in the air bubble method permits the detection of pulse changes in the radius of the bubble and therefore also of fluctuations in intracranial pressure (Fig. 16). For the clarification of this problem, we utilized the data obtained by the EPG method. Using cats and rabbits we introduced electrodes into various areas of the skull cavity and showed that the form, amplitude and phase of the pulse waves depends on the location of EPG registration (Fig. 17). Analogous data (using ultra high frequency EPG) was obtained also in man(52). All this shows the existence of pulsating translocations of CSF between the various areas of the cranial cavity. The pulse movements of CSF in the cranial cavity can be observed only when the sensitivity of the method used is sufficiently high. Thus, Sigwart (74) using contrast röntgenoscopy, could not register any movements in the sealed cranial cavity of human beings. On the basis of his data, Sigwart concluded that pulsation is absent in the sealed cranial cavity of man and states that contrast röntgenoscopy can register movements over 0.5 mm. Using data observed with low frequency electroplethysmography we can calculate the volume of the CSF involved in the reversible movement during the cardiac cycle. This volume does not exceed 10-15 ml. These data enables to calculate the "thickening" of the pia mater due to inflow of blood, a change which does not exceed 0.22 mm. Since Sigwart, in his experiments, considered it impossible to observe intracranial movements smaller than 0.5 mm, his method of contrast x-ray fluoroscopy cannot be employed in the detection of cerebral pulsation. The answer to the third question concerning the existence of a pulsating outflow of venous blood from the skull is given clearly in the studies of M. G. Belekhova (7, 8), carried out in our laboratory. Using methods of intracranial EPG, piezography and tensography, Belekhova showed the presence of pulse fluctuations of pressure and volume in venous sinuses, in diploic veins and in the jugular veins as they leave the cranium. The increase in the pulsating outflow from the skull is well seen in the EPG of the jugular vein, when the modified method of Gartner and Wagner (Fig. 18) (30) is employed in conjunction with asphyxia. Answers obtained to all three of our questions fully confirm the existence of pulsation within the hermetical cavity of the cranium. Since the presence of pulsations in the hermetical cavity of the skull was unequivocally demonstrated, this has opened the way to an analysis of the mechanisms involved. This analysis is essentially based on the relationship between the pulsations in the various regions of the cranium and the continuity of the capillary blood flow. To reconcile these two apparently contradictory sets of experimental data is possible only if we assume that the pulse wave in the sealed cavity of the skull is transmitted according to hydraulic laws directly from the arteries into the veins, thus bypassing the capillary bed. This transmission can be carried out by the means of CSF and also possibly as a result of transmission through the entire mass of the brain. However this hypothesis requires specific experimental verification which should cover two of the following points: 1. Demonstration that the speed of distribution of a pulse wave in the hermetic cavity of the cranium is different from that which is specific for the circulatory system and that, as can be seen from the laws of hydraulics, the speed of distribution of the pulse wave in the skull should markedly exceed the speed of its distribution through the blood vessels, if this assumption is correct. 2. Proof that the transmission of pressure from CSF into the venous system, in contrast to the pressure transmission from the arterial system to the CSF occurs with small losses, i.e., that small fluctuations of only several mm. of water within the intracranial pressure can cause similar pressure changes in the venous system of the brain. This assumption demands specific clarification because some authors (23), consider that a similar transmission should be accompanied by a significant reduction in pressure. For verification of the first of the above assumptions--we measured the speed of the pulse wave in the skull cavity using the method of Hürtle (25) with a few changes (58). In acute experiments with dogs and cats using piezomanometers we registered in pairs the pulse fluctuations of pressure in the peripheral and the central portion of the common carotid artery (external carotid was tied off), in the subarachnoidal space of the brain and in the longitudinal venous sinus (Fig. 19). The pulse waves in the peripheral sections of the carotid artery differ somewhat from each other in amplitude and differ greatly in form (Fig. 19a, b). This allows us to conclude that first, pulsation in the vessels at the base of the skull exists, and second, that the passage of the pulse wave across the right and left "syphon" of the internal carotid does not eliminate it. The differences in the form of these curves and especially the "flattened" form of the pulse wave in the vessels at the base of the skull most probably shows the limitation of the speed of volume translocation of CSF which appears to be one of the adaptations protecting the brain from sharp mechanical injuries. (A similar characteristic in the pulse wave form within the hermetical cavity of the skull is seen also on the tracings recorded by other methods.) The time relationship of the curves recorded by electrodes 1-2, 1-3, 1-4 (Fig. 19a) allows us to calculate the time of passage of the pulse wave from the artery to the vein and the speed of its distribution in the hermetically sealed cranium. The average results of ninety-six measurements of the speed of passage of the pulse wave in the skull are shown on Table 2. From Table 2 it follows that the pulse wave reaches the subarachnoid space and the sinuses simultaneously within 0.0003 seconds after the entry of the pulse wave into the skull. These figures are in agreement with the data of M. G. Belekhova and A. I. Naumenko (9). The pulse wave is distributed in the skull (Fig. 19a) approximately twenty times faster than in the vessels of the circulatory system. We will now discuss the second principle necessary for the understanding of the mechanism of brain pulsation in the closed cranium, namely the condition of the close connection between the pressure changes in the subarachnoidal space of the venous system and those in the venous sinuses. Such a connection can be substantiated from the following evaluations. If the section of the arteries in the brain represents a circle then the section of the venous vessels basically represents an elipsoid (large veins) and a triangle (sinuses). As a result of this, an increase of the volume of the arterial blood in the skull or the widening of the arteries is accompanied by an expansion of their walls and the degree of this expansion depends on the elasticity of the wall and on the expanding pressure, which can be calculated according to Young s law. Since the elasticity of the wall is great (1330 dynes/cm 2) a great force is expended on the extension of the blood vessels and the change of the intra-arterial pressure is greatly reduced as transmitted to the surrounding medium (CSF). Therefore, if the pulsation of the arterial pressure amounts to several mm. Hg. than corresponding pressure fluctuations of CSF would equal about 2-4 mm. of a column of water. In the transmission of the pressure from the CSF to the cerebral veins this will be even further reduced. In this case there occurs no expansion in the vessels of the venous system as a whole because the form of their section is not circular and permits pressure transmission in a similar fashion to pressure transmission through a corrugated membrane, with only a small loss (10-15%). From these evaluations we can understand the mechanism of the close connection between the pressure in the CSF system and the venous system. Since the literature contains a large body of experimental data showing indirectly the connection between the pressure in veins and the pressure of CSF, we will limit ourselves here only to one example of a direct experiment demonstrating close connection of the pressure in the CSF system and the system of cranial veins. In an acute experiment with a dog using pressure recording electrodes we recorded the intracranial pressure, the pressure in the longitudinal sinus and in the cervical artery, during decrease of the arterial pressure due to stimulation of the stellate ganglion and the subsequent increase after cessation of the irritation (Fig. 10a, b). There occurred no significant change in the venous pressures, there was only a cessation of the pulse waves due to the cardiac arrest while respiratory waves continued. Later, when cardiac contractions were resumed the pulse waves reappeared on curves 1 and 2 (Fig. 20), however, a small indirect increase (Fig. 20b, c) in the venous pressure caused a distinct increase in the intracranial pressure. All these facts point to the correctness of the hypothesis that, in the hermetical cavity of the cranium, the pulse wave is transmitted indirectly from the arterial system into the veins and in doing so, bypasses the capillary bed. CONCLUSION Summarizing the data we have obtained with animals and man we can conclude that under normal conditions there are continuous, rather complex changes in the CSF volume of the hermetical cavity of the cranium. The translocations of CSF between the cavities of the cranium and of the spine have both a periodic (corresponding to respiration and waves of IIIrd order) and also a non-periodic (connected with slow changes in the blood volume in the cranial cavity) character. These shifts of CSF as a rule are associated with small changes in the intracranial pressure. Since the speed of volume CSF translocations between the brain and the spine cavities is limited, rapid changes in the blood supply of the cranio-cerebral cavity are compensated for by re-redistribution between the CSF and the venous blood in the cranial cavity. This is associated with appreciable fluctuations in the intracranial pressure and represents the phenomenon corresponding to the term "cerebral pulsation". Pulsation actually exists in the hermetic cavity of the skull due to the presence of periodic fluctuations of the intracranial pressure and of pressure in the brain veins due to the translocation of CSF between the various areas of the cranial cavity as well as due to the presence of a pulsating outflow of venous blood from the skull. The speed of distribution of the pulse wave in the hermetical cavity of the skull is twenty times greater than that in the general circulatory system. The close connection between the intracranial and venous pressure indicates a direct transmission of the pulse wave from the arterial system to the venous system, which bypasses the capillary bed. This reconciles two apparently contradictory sets of data in regard to the presence of cerebral pulsation on the one side and uninterrupted circulation in the brain capillaries on the other. All these facts point out discrepancies to be found in the Monroe-Kelly theory. In view of all the facts, we can conclude that one of the basic characteristics of the hemodynamics of the cerebral circulation is the uninterrupted fluctuating shifts of CSF which appear to serve as an active mechanism to assure the necessary level of cerebral circulation. These CSF shifts are the basis of the mechanism allowing the realization of neuro-humoral regulation of cerebral blood circulation under the conditions of an hermetical and non-expandable cranial cavity. The limitation of the speed of CSF volume shifts plays a leading role in the protection of the central nervous system tissues from mechanical injury because it reduces fast and unexpected shocks. The participation of CSF in the direct transmission of the pulse wave from the arteries of the brain into the veins facilitates the formation of optimum conditions for the utilization of oxygen from the capillary bed in the brain because if facilitates the smooth and uninterrupted blood flow. The functions of the most important organ of human beings and animals--the brain--are to a great degree determined by the condition of its blood supply which assures constant and ample inflow of nutritive materials and the removal of waste products. Without exact knowledge of all facets of the normal physiology of cerebral blood supply and without some understanding the laws of hemodynamics of the cerebral circulation, to which we have dedicated this review, we cannot comprehend the mechanisms which give rise to many pathological conditions in the cerebrum and its vascular system.
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