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Pressure and Flow in Arteries
201 the vertigo and tinnitus, and a 60% chance of relieving vertigo but with a gradual loss of hearing. Limited operations on the vestibular labyrinth 18 seldom succeed, since any sort of intralabyrinthine instrumentation in Meniere’s disease usually leads to complete loss of both cochlear and vestibular function. If some function is retained, both the cochlear and vestibular elements may well continue their disordered action as they did before the operation. The intracranial division of the vestibular portion of the 8th nerve is a major operation, but in most cases reported in this country 10 and elsewhere the patient has retained his hearing. It is undertaken in bilateral disease to salvage the better-hearing ear, and in some unilateral Destruction of vestibular cases with good hearing. function without operation has been achieved with
streptomycin. Unfortunately both sides are affected, making the treatment most suitable for the younger patient with bilateral disease-an unusual combination. To offset the total loss of vestibular function calls for considerable adaptation which the elderly find very hard to achieve, and the resulting incapacity may be greater than before.21 The drug may also damage further the already declining hearing, and a very careful check must therefore be kept on the state of the hearing. Any further deterioration means that the streptomycin must be stopped at once. The vestibule has also been successfully destroyed on one side by ultrasonic rays,22 but in this country the efficacy of this method has not yet been assessed. Cervico-dorsal sympathectomy is a method of treatment of which more has been heard lately. LEWIS 16 and HARRISON 18 found that injection of the stellate ganglion was not a success either in treating the attacks of giddiness or in selecting cases for operation. On the whole, patients with unilateral disease did better after sympathectomy than those with both ears involved, probably because the disease is usually more severe when both ears are affected. It is interesting that the improvement or disappearance of the vertigo was sometimes delayed for up to a month after the operation. Many of their patients have had no further loss of hearing over the course of several years ; and the operation is probably effective in preventing or retarding the progression of the deafness. The place of sympathectomy in the treatment of Meniere’s disease, however, seems to be limited to those few patients in whom the disease is bilateral and the deafness progressing in spite of medical treatment. Early operation may preserve the hearing, but improvement of hearing after late -
operation is
rare.
Pressure and Flow in Arteries THE formula devised by FRANK 23 expressed the relationship between arterial pressure and flow as resistance, and measured it in the specific terms of dynes per cm.-5 per second. This is a convenient single expression connecting pressure and flow, but it is almost certainly not a valid measure of the peripheral resistance of the cardiovascular system. FRANK’S formula is based on Poiseuille’s law, which was 21. Cawthorne, T., Hewlett, A. B. Ibid, p. 663. 22. Arslan, M. Minerva otorinolaring. 1953, 3, 141. 23. Frank, O. Z. Biol. 1928, 88, 245.
intended to define the relationship between pressure and flow of an ideal liquid passing steadily through a rigid tube of uniform calibre. But in the mammalian circulation a pulsating stream of a complex suspension flows through elastic tubes, with changes of velocity and cross-sectional area at each subdivision of the network. The relation of pressure to flow must therefore be clarified before FRANK’S formula can be applied to the circulation in anything but a casual way. The ultimate aim of many inquiries into pressure and flow has been to measure cardiac output from an aortic or arterial pressure tracing ; and with this object in view the arterial system as a whole has been investigated. Eventually, HAMILTON and REMINGTON 24 put forward an empirical formula which related various parts of the aortic-pulse contour to the volume of the blood ejected from the heart. The formula gave a reasonable prediction of the cardiac output of the dog under specified conditions, but it failed to do so when the conditions were altered.25 This and other careful investigations have made it clear that our present views on hsemodynamics cannot fully explain the pressure and flow changes observed in the animal circulation as a whole. It seems that progress will be faster if the complex problem can be broken down into its constituent parts. CouLTER and P APPENHEIMER 26 studied the seemingly straightforward matter of the physical
properties of blood flowing along a tube ;- and, as might have been feared, these turned out to be far from simple. Though the " viscosity " of blood may be measured, the measurement has meaning only if it is made under conditions which are closely and clearly defined. When blood flows along a tube, the corpuscles flow in an axial stream and the viscosity of the blood varies with the velocity of the stream and the area of cross-section of the tube. When the velocity is great, as in the aorta during systole, turbulence develops 27 and the effective viscosity of the blood is much increased. These findings upset the simpler ideas of blood viscosity : for example, it is usually held that viscosity must be increased when the red-cell count limited sense, for high velocities of flow the effective viscosity of polycythaemic blood may be less than that of normal blood, because the increased mass of the corpuscles tends to stabilise the stream and make turbulence less likely. The problem has also been simplified by investigating only a part of the circulation. LANDIS 28 measured pressure, diameter, length, and rate of flow in the arterioles and capillaries of the frog’s mesentery ; and GREEN et aJ.29 studied the connection between mean pressure and flow in the perfused hind leg of the dog. Both inquiries made it plain that many adjustments were needed before the observations could be combined in a relationship based PETERSON 30 injected a known on Poiseuille’s law. volume of blood into the aorta of a dog at selected instants during the pulse cycle, and showed that
rises ; but this is
true in
only’
a
PAPPENHEIMER has shown that at
24. 25. 26. 27. 28. 29.
30.
Hamilton, W. F., Remington, J. W. Amer. J. Physiol. 1947, 148, 14. Remington, J. W. Fed. Proc. 1952, 11, 750. Coulter, N. A. jun., Pappenheimer, J. Amer. J. Physiol. 1949, 159, 401. McDonald, D. A. J. Physiol. 1955, 127, 533. Landis, E. M. Amer. J. Physiol. 1933, 103, 432. Green, H. D., Lewis, R. N., Nickerson, N. D., Heller. A. L. Ibid, 1944, 141, 518. Peterson, L. H. Circulation Res. 1954, 2, 127.
202
equal injections did not cause identical or even predictable superimpositions on the pressure pulse curve. In his view, the systolic force needed to impart linear acceleration to the blood-mass in the arterial tree is an important consideration in pressure-flow changes within the aorta. Mc]DONALD 27 and WOMERSLEY 31 have re-examined pressure and flow in the dog’s femoral artery under standard conditions. They measured the changes of pressure gradient between two points along the artery ; and the coincident changes in blood-flow velocity were recorded by means of highspeed photographs of an oxygen bubble in the artery. They found .that the oscillating pressure-gradient was related -to the volume-flow which it generated after a time lag, in rather the same way that an alternating voltage is related to the corresponding current. In fact, WOMERSLEY has devised an " alternatingcurrent " theory of flow, to be distinguished from the relationship based on Poiseuille’s" law (which can be considered as the " direct-current theory). According to the " direct " theory, it may be permissible to think and speak of the sum of the resistances of the branches of a network of vessels ; but when there is an oscillating pressure in a branching system it would be more correct to consider the effect of branching as analogous to a sum of impedances. Using the standard of flow and the "’ alternating " theory of be flow to the expected principles hydrodynamics, over a known pressure-gradient in the femoral artery was calculated, and the agreement between the observed and calculated volume of flow was within the limits of observation error. Thus, the theory seems to have been justified. Observations by BURCH 32 encourage the hope that similar fundamental work may be possible in man, using relatively simple methods. BURCH used venousocclusion plethysmography to measure the phasic changes of blood-flow in the finger. The finger-tip is enclosed in an airtight rigid container which does not obstruct the circulation. When the venous outflow is briefly occluded by a pressure cuff on the finger, the volume distal to the cuff increases by the amount of blood flowing into it during the period of occlusion ; and equal volumes of air are displaced to and from the plethysmograph into a continuous recording system. Until now it has been customary to measure the mean rate of blood-flow over a brief period, though the original description of the plethysmographic method by HEWLETT and VAN ZWALUWENBURG 33 was much concerned with observations of the phasic changes in the rate of flow. BURCH has adapted the finger plethysmograph to record the rapid changes of flow into the finger during the phases of the pulse cycle. To this technique he has given the cumbersome name of electro-rheoplethysmography ; but perhaps the title is justified since he observes the time-course of the volume changes, the rate and acceleration of inflow and outflow, and the difference between inflow and outflow of a single pulse cycle. Tracings are taken from two fingers simultaneously, and the venous outflow of one is intermittently occluded, the other serving as a control for spontaneous changes in the finger-volume. The rate of volume-change can then be measured over any portion of the pulse cycle. The 31. Womersley, J. R. J. Physiol. 1955, 127, 553. 32. Burch, G. E. Circulation, 1956, 13, 641. 33. Hewlett, A. W., Van Zwaluwenburg, J. G. Arch. intern. Med.
1913, 12, 1.
first derivative of the volume-time chart gives the time-course of the inflow and outflow rate, and the second derivative the acceleration-rate of the blood inflow. The form of such curves has yet to be related to the form of the arterial and venous pressure pulses. They vary considerably in different physiological and clinical states, and the study of these changes is likely to further our knowledge of the function of the central and peripheral circulation in health and disease. But it is doubtful whether the finger-tip is the most suitable part in which to study the circulation. The smaller vessels of the digits are specialised to take part in. the control of body-temperature, and they are particularly rich in arteriovenous anastomoses,34 which are readily opened and shut by thermal and nervous stimuli. For this reason blood-flow fluctuates widely in the finger-tips, and it might also be expected that ; there would be an unusual range of pressure-gradient between the arterial and venous ends of the capillaries. Observations on these vessels, therefore, cannot be a: interpreted as typical of the circulation as a whole. Furthermore, plethysmographic records from the finger-tips may be unsatisfactory for technical reasons. The part of the inflow curve from which reliable readings may be made is limited by the artefacts caused by the inflation of the cuff at the beginning of the curve and by venous congestion at the end. When the vessels are dilated and the arteriovenous anastomoses widely open, the artefacts may so approach each other that no reliable estimation of the blood-flow can be made. BURCH considers possible methods of with these limitations, but his suggestions are dealing not entirely convincing ; and the forearm or calf might be a better part of the anatomy to investigate. These important and interesting investigations I bristle with technical difficulties. Electromanometers are not hard to find, so that the recording of phasic pressure changes in the cardiovascular system is relatively simple ; but the simultaneous recording of phasic flow changes is a different matter. Improved methods, however, will probably simplify the present abstruse picture of pressure and flow. Meanwhile, there should be no serious objection to expressing the relationship between mean pressures and mean flow in terms of peripheral resistance," so long as it is 1 clear that the term is no more than an arithmetical arrangement of pressure and flow measurements, and that a change of " peripheral resistance " does not necessarily denote a change of calibre of arterioles or
.
.
I
I ’
I’
-
"
"
resistance vessels."
The New Minister of Health Mr. DENNIS VOSPER has been appointed Minister of Health in Mr. Macmillan’s Government. Mr. VosPEx, who is 41, was educated at Marlborough College and Pembroke College, Cambridge. During the late war he served with the CheshireRegiment and held the rank of major. He was returned to Parliament as member for the Runcorn division of Cheshire in 1950, and the following year he became a Lord Commissioner of the Treasury. From 1950 to 1954 he was a whip. As parliamentary secretary to the Ministry of Education since 1954 he has taken a special interest in the welfare of handicapped children.
Mr. J. K. VAUGHAN-MORGAN, who has represented Reigate since 1950, has been appointed parliamentary secretary to the ministry. He has been chairman of the
Conservative health committee. 34.
Grant,
R.
T., Bland, E.
-
Heart, 1931, 15, 385.
streptomycin. Unfortunately both sides are affected, making the treatment most suitable for the younger patient with bilateral disease-an unusual combination. To offset the total loss of vestibular function calls for considerable adaptation which the elderly find very hard to achieve, and the resulting incapacity may be greater than before.21 The drug may also damage further the already declining hearing, and a very careful check must therefore be kept on the state of the hearing. Any further deterioration means that the streptomycin must be stopped at once. The vestibule has also been successfully destroyed on one side by ultrasonic rays,22 but in this country the efficacy of this method has not yet been assessed. Cervico-dorsal sympathectomy is a method of treatment of which more has been heard lately. LEWIS 16 and HARRISON 18 found that injection of the stellate ganglion was not a success either in treating the attacks of giddiness or in selecting cases for operation. On the whole, patients with unilateral disease did better after sympathectomy than those with both ears involved, probably because the disease is usually more severe when both ears are affected. It is interesting that the improvement or disappearance of the vertigo was sometimes delayed for up to a month after the operation. Many of their patients have had no further loss of hearing over the course of several years ; and the operation is probably effective in preventing or retarding the progression of the deafness. The place of sympathectomy in the treatment of Meniere’s disease, however, seems to be limited to those few patients in whom the disease is bilateral and the deafness progressing in spite of medical treatment. Early operation may preserve the hearing, but improvement of hearing after late -
operation is
rare.
Pressure and Flow in Arteries THE formula devised by FRANK 23 expressed the relationship between arterial pressure and flow as resistance, and measured it in the specific terms of dynes per cm.-5 per second. This is a convenient single expression connecting pressure and flow, but it is almost certainly not a valid measure of the peripheral resistance of the cardiovascular system. FRANK’S formula is based on Poiseuille’s law, which was 21. Cawthorne, T., Hewlett, A. B. Ibid, p. 663. 22. Arslan, M. Minerva otorinolaring. 1953, 3, 141. 23. Frank, O. Z. Biol. 1928, 88, 245.
intended to define the relationship between pressure and flow of an ideal liquid passing steadily through a rigid tube of uniform calibre. But in the mammalian circulation a pulsating stream of a complex suspension flows through elastic tubes, with changes of velocity and cross-sectional area at each subdivision of the network. The relation of pressure to flow must therefore be clarified before FRANK’S formula can be applied to the circulation in anything but a casual way. The ultimate aim of many inquiries into pressure and flow has been to measure cardiac output from an aortic or arterial pressure tracing ; and with this object in view the arterial system as a whole has been investigated. Eventually, HAMILTON and REMINGTON 24 put forward an empirical formula which related various parts of the aortic-pulse contour to the volume of the blood ejected from the heart. The formula gave a reasonable prediction of the cardiac output of the dog under specified conditions, but it failed to do so when the conditions were altered.25 This and other careful investigations have made it clear that our present views on hsemodynamics cannot fully explain the pressure and flow changes observed in the animal circulation as a whole. It seems that progress will be faster if the complex problem can be broken down into its constituent parts. CouLTER and P APPENHEIMER 26 studied the seemingly straightforward matter of the physical
properties of blood flowing along a tube ;- and, as might have been feared, these turned out to be far from simple. Though the " viscosity " of blood may be measured, the measurement has meaning only if it is made under conditions which are closely and clearly defined. When blood flows along a tube, the corpuscles flow in an axial stream and the viscosity of the blood varies with the velocity of the stream and the area of cross-section of the tube. When the velocity is great, as in the aorta during systole, turbulence develops 27 and the effective viscosity of the blood is much increased. These findings upset the simpler ideas of blood viscosity : for example, it is usually held that viscosity must be increased when the red-cell count limited sense, for high velocities of flow the effective viscosity of polycythaemic blood may be less than that of normal blood, because the increased mass of the corpuscles tends to stabilise the stream and make turbulence less likely. The problem has also been simplified by investigating only a part of the circulation. LANDIS 28 measured pressure, diameter, length, and rate of flow in the arterioles and capillaries of the frog’s mesentery ; and GREEN et aJ.29 studied the connection between mean pressure and flow in the perfused hind leg of the dog. Both inquiries made it plain that many adjustments were needed before the observations could be combined in a relationship based PETERSON 30 injected a known on Poiseuille’s law. volume of blood into the aorta of a dog at selected instants during the pulse cycle, and showed that
rises ; but this is
true in
only’
a
PAPPENHEIMER has shown that at
24. 25. 26. 27. 28. 29.
30.
Hamilton, W. F., Remington, J. W. Amer. J. Physiol. 1947, 148, 14. Remington, J. W. Fed. Proc. 1952, 11, 750. Coulter, N. A. jun., Pappenheimer, J. Amer. J. Physiol. 1949, 159, 401. McDonald, D. A. J. Physiol. 1955, 127, 533. Landis, E. M. Amer. J. Physiol. 1933, 103, 432. Green, H. D., Lewis, R. N., Nickerson, N. D., Heller. A. L. Ibid, 1944, 141, 518. Peterson, L. H. Circulation Res. 1954, 2, 127.
202
equal injections did not cause identical or even predictable superimpositions on the pressure pulse curve. In his view, the systolic force needed to impart linear acceleration to the blood-mass in the arterial tree is an important consideration in pressure-flow changes within the aorta. Mc]DONALD 27 and WOMERSLEY 31 have re-examined pressure and flow in the dog’s femoral artery under standard conditions. They measured the changes of pressure gradient between two points along the artery ; and the coincident changes in blood-flow velocity were recorded by means of highspeed photographs of an oxygen bubble in the artery. They found .that the oscillating pressure-gradient was related -to the volume-flow which it generated after a time lag, in rather the same way that an alternating voltage is related to the corresponding current. In fact, WOMERSLEY has devised an " alternatingcurrent " theory of flow, to be distinguished from the relationship based on Poiseuille’s" law (which can be considered as the " direct-current theory). According to the " direct " theory, it may be permissible to think and speak of the sum of the resistances of the branches of a network of vessels ; but when there is an oscillating pressure in a branching system it would be more correct to consider the effect of branching as analogous to a sum of impedances. Using the standard of flow and the "’ alternating " theory of be flow to the expected principles hydrodynamics, over a known pressure-gradient in the femoral artery was calculated, and the agreement between the observed and calculated volume of flow was within the limits of observation error. Thus, the theory seems to have been justified. Observations by BURCH 32 encourage the hope that similar fundamental work may be possible in man, using relatively simple methods. BURCH used venousocclusion plethysmography to measure the phasic changes of blood-flow in the finger. The finger-tip is enclosed in an airtight rigid container which does not obstruct the circulation. When the venous outflow is briefly occluded by a pressure cuff on the finger, the volume distal to the cuff increases by the amount of blood flowing into it during the period of occlusion ; and equal volumes of air are displaced to and from the plethysmograph into a continuous recording system. Until now it has been customary to measure the mean rate of blood-flow over a brief period, though the original description of the plethysmographic method by HEWLETT and VAN ZWALUWENBURG 33 was much concerned with observations of the phasic changes in the rate of flow. BURCH has adapted the finger plethysmograph to record the rapid changes of flow into the finger during the phases of the pulse cycle. To this technique he has given the cumbersome name of electro-rheoplethysmography ; but perhaps the title is justified since he observes the time-course of the volume changes, the rate and acceleration of inflow and outflow, and the difference between inflow and outflow of a single pulse cycle. Tracings are taken from two fingers simultaneously, and the venous outflow of one is intermittently occluded, the other serving as a control for spontaneous changes in the finger-volume. The rate of volume-change can then be measured over any portion of the pulse cycle. The 31. Womersley, J. R. J. Physiol. 1955, 127, 553. 32. Burch, G. E. Circulation, 1956, 13, 641. 33. Hewlett, A. W., Van Zwaluwenburg, J. G. Arch. intern. Med.
1913, 12, 1.
first derivative of the volume-time chart gives the time-course of the inflow and outflow rate, and the second derivative the acceleration-rate of the blood inflow. The form of such curves has yet to be related to the form of the arterial and venous pressure pulses. They vary considerably in different physiological and clinical states, and the study of these changes is likely to further our knowledge of the function of the central and peripheral circulation in health and disease. But it is doubtful whether the finger-tip is the most suitable part in which to study the circulation. The smaller vessels of the digits are specialised to take part in. the control of body-temperature, and they are particularly rich in arteriovenous anastomoses,34 which are readily opened and shut by thermal and nervous stimuli. For this reason blood-flow fluctuates widely in the finger-tips, and it might also be expected that ; there would be an unusual range of pressure-gradient between the arterial and venous ends of the capillaries. Observations on these vessels, therefore, cannot be a: interpreted as typical of the circulation as a whole. Furthermore, plethysmographic records from the finger-tips may be unsatisfactory for technical reasons. The part of the inflow curve from which reliable readings may be made is limited by the artefacts caused by the inflation of the cuff at the beginning of the curve and by venous congestion at the end. When the vessels are dilated and the arteriovenous anastomoses widely open, the artefacts may so approach each other that no reliable estimation of the blood-flow can be made. BURCH considers possible methods of with these limitations, but his suggestions are dealing not entirely convincing ; and the forearm or calf might be a better part of the anatomy to investigate. These important and interesting investigations I bristle with technical difficulties. Electromanometers are not hard to find, so that the recording of phasic pressure changes in the cardiovascular system is relatively simple ; but the simultaneous recording of phasic flow changes is a different matter. Improved methods, however, will probably simplify the present abstruse picture of pressure and flow. Meanwhile, there should be no serious objection to expressing the relationship between mean pressures and mean flow in terms of peripheral resistance," so long as it is 1 clear that the term is no more than an arithmetical arrangement of pressure and flow measurements, and that a change of " peripheral resistance " does not necessarily denote a change of calibre of arterioles or
.
.
I
I ’
I’
-
"
"
resistance vessels."
The New Minister of Health Mr. DENNIS VOSPER has been appointed Minister of Health in Mr. Macmillan’s Government. Mr. VosPEx, who is 41, was educated at Marlborough College and Pembroke College, Cambridge. During the late war he served with the CheshireRegiment and held the rank of major. He was returned to Parliament as member for the Runcorn division of Cheshire in 1950, and the following year he became a Lord Commissioner of the Treasury. From 1950 to 1954 he was a whip. As parliamentary secretary to the Ministry of Education since 1954 he has taken a special interest in the welfare of handicapped children.
Mr. J. K. VAUGHAN-MORGAN, who has represented Reigate since 1950, has been appointed parliamentary secretary to the ministry. He has been chairman of the
Conservative health committee. 34.
Grant,
R.
T., Bland, E.
-
Heart, 1931, 15, 385.