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APPLIED PHYSIOLOGY OF THE VEINS OF THE LOWER LIMB
Superficial varices represent but one of a large group of clinical features which result from derangements of the mechanism of venous return from the lower limbs. In compression sclerotherapy the objective is restoration of the normal pattern of venous flow, rather than the ablation of obvious varicosities. In order to achieve this, a basic knowledge of the various factors concerned in the normal return of venous blood in the erect subject is essential. Furthermore, it is necessary to appreciate the relative significance of these factors in both the dynamic and the static limb. It must be realized that a great deal of work remains to be done before a satisfactory and full description of venous return from the lower limb, especially in the ambulant subject, can be obtained.
The three principal factors which influence the venous return from the lower limbs are:
(i) the heart;
(ii) gravity;
(iii) the peripheral venous pump.
While other factors, such as variation in the volume and viscosity of the blood and changes in the tone of the vein wall and the capacity of the veins, certainly influence the pressure within the veins there is, as yet, little definite information on their overall significance.
The heart
The heart acts on the venous return by its vis a tergo and vis a fronte effects.
By vis a tergo is meant the positive pressure which is transmitted from the capillaries to the venous bed. The average pressure at the venous end of a capillary at heart level is about 12 mm. Hg. This has been shown (Landis, 1930) to be increased by almost the full hydrostatic requirements in the case of capillaries situated below heart level. Thus, vis a tergo is responsible for the maintenance of a constant supply of blood to the venules in the lower limbs. It is clear, however, that the development of clinical venous insufficiency in general is unrelated to any alternations which occur in the valve of the vis a tergo factor. An exception to this generalization occurs in the rare cases in which dilated veins are present in association with major arteriovenous shunts.
Vis a fronte represents the sucking action of the right atrium on the venous system. The presence of this factor has been demonstrated in open-chest animals (Brecher, 1956). The end-result of its action is a reduction in the pressure in the varae cavae. Since the establishment of a pressure gradient, either by raising the pressure at one end of a system or by lowering it at the other, promotes flow to the area of the lower pressure, any vis a fronte contribution present must assist central flow.
In humans this contribution is negligible. Walker & Pickard (1962) have recorded directly and simultaneously the intracaval pressure above and below the diaphragm and have noted that the atrial pressure pattern disappears almost immediately below the diaphragm. We have recorded the pressures in the major intra-abdominal veins in erect and horizontal subjects (Fegan et al., 1966) and we have been unable to find any suggestion of an atrial pattern in them.
Gravity
The effects of gravity in the venous and arterial sides of the dependent limb are equal. It follows that if the arteries and veins were considered to be a rigid U-tube system, a small increment in pressure, provided by the vis a tergo, would be sufficient to return blood to the heart. This concept, however, takes no account of the influence of gravity on the formation and re-absorption of tissue fluid. The re-absorption of tissue fluid and metabolites is one of the most important functions of the veins and may be achieved both directly through the capillaries and indirectly via the lymphatics. The pressure in the capillaries at ankle level in the standing subject is the sum of the hydrostatic pressure exerted by the column of blood extending to heart level and the much smaller vis a tergo component. It is far higher than the osmotic pressure of the plasma proteins. The formation of tissue fluid depends on the pressure gradient across the capillary wall. This is equal to capillary pressure minus (colloid osmotic pressure plus tissue pressure). Little is known of the tissue pressures in the normal limb. The fact that prolonged standing in normal subjects results in swelling around the ankles suggests that there is a favourable gradient for the formation of tissue fluid. We believe that this is due to the constant high hydrostatic pressure. Tracings from the deep and superficial veins of the foot and calf in a normal subject show that marked intermittent hypertension occurs during walking exercises.
The other way in which tissue fluid can be returned to the venous system is via the lymphatics. Here again the effects of gravity must be overcome. It is possible that the muscles exert a pumping effect on the lymph vessels, because they have valves which permit only proximal flow. In addition the walls of the vessels may have the ability to contract rhythmically, squeezing lymph from lower segments to higher ones. Kinmonth et al. (1963) recorded pressures in the thoracic duct, and noted that, in the presence of obstruction, much higher pressures occurred.
The importance of lymphatic obstruction in the production of lymphoedema has been questioned recently. Burns et al. (1966) ligated the main lymphatic trunks of the thigh in dogs and found that only mild oedema of short duration resulted, despite the fact that restoration of flow along collateral and newly-developed lymphatics did not take place for two to three months. They suggested that proximal venous obstruction may be an important aetiological factor in many cases of so-called primary lymphoedema.
The peripheral venous pump
The peripheral muscular pumps form the essential mechanism responsible for the return of venous blood from the lower limb in the dynamic state. Considered as a whole the peripheral pump consists of a number of separate but functionally integrated components. In fact, each myofascial compartment acts as a muscle pump unit. For ease of description these may be considered in regional groups, as follows:
(i) The plantar, or foot pump.
(ii) The calf pump (including the anterior tibial and peroneal pumps).
(iii) The thigh pump.
(iv) The abdominal pump.
The returning blood is passed from the capillaries to venules and from these into larger veins. This collection occurs in both the superficial and deep systems of veins, which are connected by the perforating veins. Above the ankle the flow of blood is from the superficial to the deep via the perforating veins. The deep veins are compressed by the muscles and the contained blood is then pumped towards the heart. During the exercise the peripheral pump must return a greatly increased volume of blood from the lower limb. The ability of the pump to do this may be called its physiological reserve, and depends upon the development of the muscles, the capacity of the veins and the number and distribution of the venous valves.
Derangements of the venous return from the lower limb can be properly understood only in the light of the mechanisms responsible for returning blood to the heart. A series of investigations involving continuous pressure recordings in the superficial and deep veins has been carried out to elucidate these mechanisms.
Detailed results of these investigations have been published elsewhere (Fegan et al., 1964; Fegan et al., 1966; Pegum & Fegan, 1967; Keane & Fegan, 1966). The following is a summary of the findings. These are best considered in the regional groups described above.
The foot pump
Venous return from the foot was studied (Pegum & Fegan, 1967) by cannulating the deep and superficial veins through an incision over the proximal end of the first interosseous space. One cannula was introduced into the perforating vein at this site, and another into the medial limb of the dorsal venous arch, with its tip 2-5 cm. distal to the tip of the medial malleolus. Each cannula was connected to a Sanborn physiological pressure transducer, and the transducers were connected through strain-gauge amplifiers to a pen recorder. Continuous pressure records were obtained in ten experiments. Waves were recorded while a subject stood on the uncannulated foot, with the ball (metatarsal heads) of the cannulated foot lightly touching the floor, and the heel 3-7 cm. above the floor. He swung his weight on to the cannulated foot, without allowing the heel to drop, and then returned to the resting position.
The deep veins of the sole are connected with the superficial veins of the dorsum of the foot by perforating veins, which are either un-valved or have valves which allow flow from deep to superficial veins (Chapter I). These facts allow an interpretation of the pressure readings presented here in terms of blood flow.
Many of the records show that when the deep pressure (i.e. the pressure in the deep veins) rose to exceed the superficial pressure, the superficial pressure then began to rise also, but to a lesser extent. This suggests that pressure in the deep veins was being transmitted to the superficial veins by the perforating veins, and it can be assumed that blood then flowed through the perforating veins along the pressure gradient. This evidence that pressure is transmitted from the deep veins of the sole to the dorsal venous arch adds strength to the suggestion that a function of the deep layer of the superficial fascia of the foot is to support the dorsal venous arch against this pressure.
The increase in the pressure in the deep veins during exercise may be due to their compression either by the weight of the body or by the contraction of the plantar muscles. There is evidence that both factors play a part.
In the walking exercise any effect of the two forces occurred simultaneously, since the plantar muscles contracted at the same time as the body weight compressed the sole.
In exercise in which the body weight was swung on to the ball of the foot compression of the sole, and particularly of the region in which the deep veins are situated, was much reduced. This suggests that the rise in pressure recorded during this exercise was due to compression of the deep veins by the plantar muscles, which contracted as the foot was placed under strain.
The intermediate fall in the pressure in the deep veins in the exercise of lifting the heel from the floor may be due to removal from the sole of the compressing effect of the body weight, since the pressure fell as the heel rose, and rose as the heel was lowered again.
The pressure in the superficial veins did not always fall when the deep vein pressure did, and this is evidence of the action of valves in the superficial and perforating veins, which retained the blood, and maintained pressure, in the superficial veins. The deep plantar veins appear to form the chamber of a pumping mechanism, in which the movements of the foot, and the weight of the body, actively aid venous return from the foot. This 'plantar venous pump' is usually compressed at the same time as its deep outflow along the posterior tibial veins is obstructed by contraction of the calf muscles. The perforating veins and dorsal venous arch act as an alternative outflow channel, thus allowing the stroke volume of the plantar venous pump to be greater than it otherwise would be, with a consequent greater effectiveness in venous return.
The calf or leg pump. (This includes the anterior tibial and peroneal muscular venous pumps.)
Twelve young adult male volunteers were investigated in order to determine the normal pattern of venous pressures in the leg.
Method. The technique used was similar to that described earlier in relation to the investigations of the plantar venous pump.
Polythene catheters were passed into the deep venous system through perforating veins and a catheter was inserted into the long saphenous vein at the same level. The pressure changes were recorded with the subject standing, at rest and during exercise.
The perforating vein selected was usually about 10-15 cm. above the medial malleolus, and was located either by ascending phlebography or by palpating for fascial defects. The latter method was found more reliable. Pressure recordings were taken: (i) at rest; (ii) during raising and lowering the heels; (iii) during walking; (iv) during quadriceps drill.
Findings
1. The deep and superficial pressures are equal in the resting limb in the erect subject.
2. At rest. The pressure observed at rest corresponds to that of the hydrostatic pressure of a column of blood rising to heart level.
3. During exercise. The records obtained from the deep veins during a single step show that the onset of muscular contraction is accompanied by a sharp increase in the pressure, which rises to a peak. The pressure levels off and may fall slightly during maintained muscular contraction. Finally, it falls rapidly to its lowest level as the weight is taken off the foot and the muscles relax. When the foot is lowered again a smaller peak is produced as the muscles contract to maintain balance. This peak is followed by a fall, as the resting position is resumed. The pressure then gradually rises to the resting level.
Superficial pressures. The changes in the superficial veins are less marked and are slightly later than those in the deep. The lowest pressure in the superficial veins occurs about 0.1 to 0.2 seconds later than in the deep.
During muscular contraction the pressure in the deep veins rises above that in the superficial veins. Blood from the deep veins is pumped towards the heart, and the valves in the perforating veins are closed. For a short period during muscular relaxation the pressure in the deep veins falls considerably below that in the superficial veins. It is probable that blood then passes from the superficial to the deep veins via the perforating veins, along the pressure gradient.
The thigh pump
Each of the three groups of muscles in the thigh, with its enveloping fascia and contained veins, is a pumping unit. Two of the groups were studied, the adductors and the quadriceps.
The exercises in the standing subject were: (i) quadriceps drill; (ii) adduction. Table 1 gives the pressures recorded in the upper, middle and lower parts of the femoral vein, and in the long saphenous vein at the level of the knee. It is probable that the transfer of blood from the superficial to the deep veins, and towards the heart, is achieved by mechanism similar to that in the calf.
The abdominal pump
Records were made of the intra-abdominal venous pressures, and of the effect of respiration on these pressures, in the horizontal and erect positions (Fegan et al., 1966).
Results
Ten recording were made with the patients standing and horizontal. The levels at which the pressures were obtained ranged from the external iliac vein to the inferior vena cava. The detailed data are in Tables 2 and 3.
TABLES 1, 2 AND 3. Copies available free of charge. E-mail: fegan@fegan.com.
The pressure recorded at the end of expiration was taken as representing the basic value at any particular level. This was, on average, 7.5 mm. Hg with the subject horizontal and 26.0 mm Hg when erect. Variation of the individual basic values obtained in the horizontal position was small (range 3 to 10 mm Hg) and was independent of the site of the catheter tip. With the patient in the erect position the basic pressure varied with the level of the catheter tip, and was found to be 7 mm. Hg in the inferior vena cava (one reading), 22 mm. Hg in the common iliac vein (three readings), and 31 mm. Hg in the external iliac vein (six readings).
Inspiration produced a rise in pressure in all cases. The average values of this rise in the horizontal and erect positions were 6.3 and 8.7 mm. Hg, respectively. It is noteworthy that in all but two instances the increase in pressure produced with the subject standing exceeded that produced in the horizontal position, although the basic pressure was considerably higher.
Figure 22 - available free of charge, e-mail: fegan@fegan.com. - shows the method of action of the abdominal pump, on expiration with the femoral valve open, and on inspiration when the valve is closed because of the increased intra-abdominal pressure. Available free of charge
These observations demonstrate the existence of an abdominal component of the peripheral venous pump and support the conclusion that the efficiency of its action is greater when the subject is erect.
References
Breecher, G. A. (1956), Venous Return. New York: Grune & Stratton.
Burns, J. I., Rivero, O. R., Pentecost, B. L. & Caine, J. S. (1966), Brit. J. Surg. 53, 634.
Fegan, W. G., Milliken, J. C. & FitzGerald, D. E. (1966), Arch. Surg. 92, 44.
Fegan, W. G., FitzGerald, D. E. & Milliken, J. C. (1964), Irish J. Med. Sci. 464, 363.
Keane, T. F. L. & Fegan, W. G. (1966), to be published.
Kinmonth, J. B., Sharpey-Schafer, E. P. & Taylor, E. W. (1963), Lancet, 1, 1425.
Landis, E. M. (1930), Heart, 15, 209.
Pegum, J.M. & Fegan, W. G. (1967), Cardiovascular Research, 1, 249.
Walker, W. G. & Pickard, C. (1962), Bull, Soc. Int. Chir. 21, 257.