longitudinal sinus. A decrease in blood flow was observed when the pressure

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1 362 J. Physiol. (I942) IOI, I2.I44:6I2.824 THE EFFECT OF VARIATIONS IN THE SU.BARACHNOID PRESSURE ON THE VENOUS PRESSURE IN THE SUPERIOR LONGITUDINAL SINUS AND IN THE TORCULAR OF THE DOG BY T. H. B. BEDFORD Promn the Department of Pharmacology, Manchester University (Received 17 June 1942) Although Weed & Flexner [1933] have confirmed the earlier findings of Becht [1920] that the venous pressure in the torcular is uninfluenced by variations in the subarachnoid pressure, Wright [1938] has found that changes in venous pressure can readily be brought about by varying the subarachnoid pressure. He measured the venous pressure in the posterior part of the superior longitudinal sinus. A slight rise in the venous pressure was observed when the subarachnoid pressure was first raised; this, however, fell rapidly to a low level and remained there as long as the raised subarachnoid pressure was maintained. In another series of experiments, Wright [1938] studied the effect of a gradual rise in subarachnoid pressure on the rate of blood flow through the superior longitudinal sinus. A decrease in blood flow was observed when the pressure was first raised with a progressive reduction until complete cessation occurred, when the subarachnoid pressure exceeded the systolic arterial pressure. Astudy has been made in the following experiments of the effect of variations in subarachnoid pressure on the venous pressure in the superior longitudinal sinus and in the torcular of the dog. EXPERIMENTAL PROCEDURE Difficulty in obtaining dogs owing to the war conditions has made little selection possible; diseased and grossly undernourished animals, however, were always rejected. The subarachnoid pressure was measured with a needle in the cisterna magna. Variations in pressure were effected and maintained in the manner already described in earlier publications [Bedford, 1935, 1936]. The highest pressure employed in these experiments was 500 mm. normal saline solution and the lowest 0 mm. normal saline solution. The venous pressure in the superior longitudinal sinus was recorded in the following way.

2 SUBARACHNOID PRESSURE AND VENOUS PRESSURE 363 A cut was made with a i in. trephine through the bone over the posterior part of the sinus and continued down to stop short of the inner table. The disc of bone contained in the trephine could usually be broken away, leaving the sinus covered by the inner table and a variable amount of relatively soft diploe. The bone was then picked away with sharp forceps so as to expose a portion of sinus large enough to admit the recording needle. The sinus could usually be recognized by its dark colour. The recording needle (gauge 18) was introduced so as to point towards but not to enter the torcular. The needle was connected with a manometer of 1 mm. bore containing a 4% solution of sodium citrate. A device was also provided by which the needle could, when necessary, be flushed with citrate solution. A different technique was used to record the venous pressure in the torcular. The torcular in the dog is not an intracranial structure; it lies, like the lateral sinuses, in the substance of the bone. It is possible, therefore, to measure the pressure of the blood in its interior without opening the cranium. This was effected in the following way. A dental bur was first selected capable of making a hole through the skull bone which would just admit a gauge 18 needle and leave no space by which fluid could escape along the side of the needle. A hole was then bored through the external occipital protuberance directly into the torcular. Care was necessary to prevent the bur from striking the thin anterior wall of the torcular, which is easily perforated. The bur was immediately withdrawn after the torcular had been entered and the needle introauced. A close fit ensued if care had been taken in the choice of bur. The needle was connected with the recording manometer in the manner already described. The experiments were performed with the animals in the horizontal position. The animals were anaesthetized with intratracheal ether administered by a pump in all but two experiments when amytal (Lilly) was the anaesthetic. The amytal, in a proportion of 0-08 g. per kg. of body weight and in a 5% solution in water, was injected into the peritoneal cavity. The animals were. then kept quiet in a darkened room for 20 min. when they were found to be in a satisfactory state of anaesthesia. The animals were killed at the end of the experiments and a careful examination made of the superior longitudinal sinus, the torcular and the lateral sinus for the presence of ante-mortem clot. RESULTS The effect of variations in subarachnoid pressure on the venous pressure in the membranous superior longitudinal sinus. A rapid increase in subarachnoid pressure from the pressure level of the cerebrospinal fluid at the beginning of the experiment to 500 mm. normal saline solution was invariably accompanied by a fall in venous pressure (Fig. 1). The extent of the fall was roughly proportional to the rate at which the subarachnoid pressure was increased. If 1 mn. was spent in raising the subarachnoid pressure to 500 mm. normal saline

3 364 T.H.B.BEDFORD solution, a fall in venous pressure which averaged 20 mm. normal saline solution was observed. When the subarachnoid pressure was raised more slowly and 1 min. allowed for each increase in 100 mm. normal saline solution at intervals of 3 min., no change was observed in the venous pressure in a quarter of the animals while a fall of pressure which averaged 10 mm. occurred in the remainder. When the subarachnoid pressure was raised still more slowly, a slightly greater proportion of animals presented no change in venous pressure. Minutes Fig. 1. The effect of variations in subarachnoid pressure on the venous pressure in the superior longitudinal sinus. - Subarachnoid pressure. -*-0- Venous pressure Systolic pressure in femoral artery. The arrow indicates the point at which the subarachnoid pressure was artificially varied. Ether anaesthesia. It was found, however, that the animals which presented no change in venous pressure when the subarachnoid pressure was first raised, frequently did so after it had been raised and lowered for the second or third occasion; thereafter a rise in subarachnoid pressure was accompanied by a fall in venous pressure. Lowering the subarachnoid pressure from 500 mm. normal saline solution to its original level was accompanied by corresponding changes in the opposite direction and the venous pressure assumed its original level. Occasionally, however, a level slightly higher than the original was assumed but this rarely persisted and the original level was generally regained within 15 min. The

4 SUBARACHNOID PRESSURE AND VENOUS PRESSURE 365 extent of response of the venous pressure in any particular animal remained fairly constant on each subsequent elevation of pressure. Except for minor variations, the venous pressure level assumed on raising the subarachnoid pressure was generally maintained (Fig. 2); in a few instances, however, the pressure rose gradually until the original level was regained after 5-20 min. The above changes in venous pressure occurred independently of variations in systemic blood pressure; the systemic arterial blood pressure was uninfluenced by changes in subarachnoid pressure of the magnitude employed in It I Fig Minutes The effect of prolonged variations in subarachnoid pressure on the venous pressure in the torcular. Constants as in Fig. 1. Ether anaesthesia. these experiments. The results obtained in two experiments in which the animals were anaesthetized with amytal were similar to those in which the animals were anaesthetized with ether. Experiments were performed on seventeen dogs. The effect of variations in subarachnoid pressure on the venous pressure in the torcular. The effects of variations in subarachnoid pressure on the venous pressure in the torcular were identical with those observed when the venous pressure was recorded in the posterior part of the membranous superior longitudinal sinus. Experiments were performed on six dogs.

5 366 T. H. B. BEDFORD DISCUSSION Although the results of the above experiments fail to confirm the finding of Weed & Flexner [1933] that the venous pressure in the torcular is uninfluenced by changes in subarachnoid pressure, they do, however, provide evidence for the existence of a compensating mechanism by which the cerebral circulation adapts itself to the changes in subarachnoid pressure. The efficiency of this mechanism varies considerably in different animals. In some instances compensation appeared to be complete and the venous pressure remained uninfluenced by changes in subarachnoid pressure. Nearly every animal, however, which presented no change in venous pressure when the subarachnoid pressure was first raised, showed a fall after the subarachnoid pressure had been raised, for the second or third time. Wright [1938] has provided strong experimental evidence that no compression or collapse of the superior longitudinal sinus occurs when the subarachnoid pressure is increased to levels even greater than the systolic arterial blood pressure; it is evident, therefore, that in the absence of variations in the systemic circulation, any change observed in the sinus is a direct consequence of variations in the cerebral circulation. A fall in venous pressure invariably followed a sudden rise in subarachnoid pressure, and the new venous pressure level generally persisted as long as the raised subarachnoid pressure was maintained. It was also found that, on lowering the subarachnoid pressure, a reduced venous pressure frequently accompanied a lower subarachnoid pressure level than was observed when the pressure was increased. These findings are probably related to Wright's observation that cerebral vessels which have been collapsed as a result of raising the subarachnoid pressure require a lower pressure level for the reversal of the process than that at which they first collapsed. Wright [1938] considers the phenomenon is in conformity with the law that resistance to the passage of fluid through a tube varies inversely as the fourth power of the radius. The diameter of the collapsed vessels is negligible at the beginning of filling, and as no active expansion can take place, the resistance to filling is considerable. Becht [1920] measured the venous pressure by screwing a brass tube 4 mm. in diameter into a drill hole made directly into the torcular through the external occipital protuberance. As might be anticipated, considerable difficulty was encountered from clotting and this method of recording the venous pressure was abandoned by Weed & Hughson [1921] who exposed the superior longitudinal sinus over the posterior third of its course by carefully removing the bone in a sagittal groove by means of a rongeur. A shortened lumbar puncture needle was then introduced into the sinus and passed backwards into the torcular. This technique appears to have been employed by Weed and his co-workers without modification throughout their experiments. It is open,

6 SUBARACHNOID PRESSURE AND VENOUS PRESSURE 367 however, to the grave objection that the cranium can be no longer regarded as a closed chamber after the sinus has been exposed. Nevertheless, it has been shown in the present series of experiments that if care be taken to expose a portion of the sinus sufficient only to admit the recording needle, the results are identical with those obtained when the venous pressure is recorded directly from the torcular. * No attempt has been made in this investigation to study the effect of subarachnoid pressures greater than 500 mm. normal saline solution; pressures of greater magnitude than this were considered to depart too far from the physiological. Weed & Flexner [1933] also restricted themselves mainly to the study of the effect of pressure variations up to 450 mm. normal saline solution, although Wright [1938], at times, used pressures which even exceeded the systolic arterial pressure. Ether was the anaesthetic used by Becht [1920] and by Weed & Flexner [1933] throughout their experiments and, except for two experiments in which intraperitoneal amytal was the anaesthetic, all the remaining experiments in the present series were performed on animals anaesthetized with intratracheal ether. It is noteworthy that similar results were obtained whether ether or amytal was the anaesthetic. Weed & Flexner [1933] state enmphatically that even profound changes in subarachnoid pressure are unaccompanied by changes in venous pressure in the superior longitudinal sinus. In a later paper, however, by Mortenson & Weed [1934] on 'The absorption of isotonic fluids from the subarachnoid space' a figure (Fig. 3, p. 463) is presented illustrating an experiment in which a study was made of the effect of raised subarachnoid pressure on the venous pressure in the superior longitudinal sinus and on the rate of inflow of normal saline solution into the subarachnoid space of the dog. The subarachnoid pressure was raised after an initial control period of 30 min. in multiples of the normal pressure and at intervals of 15 min. It is significant that a fall in venous pressure of some 20 mm. normal saline solution accompanied the raising of the subarachnoid pressure from 360 to 540 mm. normal saline. The reduced venous pressure level was maintained with slight variation for 15 min. when, on increasing the subarachnoid pressure to 720 mm. normal saline, it showed a tendency to assume a slightly lower level. Although no commen1 is made by Mortenson & Weed on the fall in venous pressure, it would appear to be in harmony with the findings in the experiments now under consideration. To conclude, it would appeat that although the cerebral circulation possesses considerable power of adaptation to changes in subarachnoid pressure, a rise in subarachnoid pressure from the normal cerebrospinal fluid pressure level of approximately mm. normal saline solution, is accompanied in the majority of animals by a slight but persistent fall in the venous pressure in the sinus and in the torcular.

7 368 T. H. B. BEDFORD SUMMARY 1. A study has been made of the effect of variations in subarachnoid pressure on the venous pressure in the membranous superior longitudinal sinus and in the torcular of the dog. 2. A rapid increase in subarachnoid pressure from the pressure level of the cerebrospinal fluid to 500 mm. normal saline solution was. invariably accompanied by a fall in venous pressure which averaged 20 mm. normal saline solution. Identical results were obtained whether the venous pressure was recorded in the membranous superior longitudinal sinus or in the torcular. 3. When the subarachnoid pressure was raised more slowly and 1 mi. allowed for each increase of 100 mm. normal saline solution at intervals of 3 min., a fall in venous pressure which averaged 10 mm. normal saline solution was observed in the majority of the animals. 4. The fall in venous pressure generally persisted as long as the raised subarachnoid pressure was maintained; the original level was, however, regained when the subarachnoid pressure was lowered. 5. It is concluded that, although the cerebral circulation possesses considerable power of adaptation to changes in subarachnoid pressure, a rise in subarachnoid pressure from the pressure level of the cerebrospinal fluid to 500 mm. normal saline solution even when effected slowly, is generally accompanied by a small but persistent fall in the venous pressure in the superior longitudinal sinus and in the torcular. REFERENCES Becht, F. C. [1920]. Amer. J. Phy8iol. 51, 1. Bedford, T. H. B. [1935]. Brain, 58, 427. Bedford, T. H. B. [1936]. Brain, 59, 324. Mortensen, 0. A. & Weed, L. H. [1934]. Amer. J. Phy8iol. 108, 458. Weed, L. H. & Hughson, W. [1921]. Amer. J. Physiol. 58, 101. Weed, L. H. & Flexner, L. B. [1933]. Amer. J. Phy8iol. 105, 266. Wright, R. D. [1938]. Aust. New Zeal. J. Surg. 7, 215.