Extracorporeal blood circuit

Abstract

An extracorporeal blood circuit in which a blood oxygenator is placed in series between a first and second pump, the pumps being of the peristaltic or ventricular type, the inlet to the first pump being connected to patient's vein and the outlet of a second pump to a patient's artery. The useful internal volume of the body of the first and second pumps varies substantially proportional to the pressure of the blood at the inlet, between minimum and maximum values, the maximum useful volume of the first pump and the minimum useful volume of the second pump being reached at a pressure of the blood at the inlet of the respective pump within a range of atmospheric pressure .+-. 20 mm mercury.

Description

The present invention relates to an extracorporeal blood circuit connecting a membrane-containing blood oxygenating device to the vascular system of a patient, for assisting or replacing the pulmonary, cardiac or cardiopulmonary system.

Extracorporeal blood circuits are known which utilise two circulating pumps located in series on either side of a membrane-containing blood oxygenator. However, either a pipeline for partially recycling arterial blood through the oxygenator or complex control systems are required in order to maintain a definite pressure in the oxygenator, necessary for keeping the blood in the form of thin films of constant thickness.

According to the present invention there is provided an extracorporeal blood circuit comprising, a blood oxygenator; a first peristaltic pump or tubular membrane and valve pump, having an inlet connectable to a patient's blood circuit and an outlet connected in series with the oxygenator and a second peristaltic pump or tubular membrane and valve pump, having an inlet connected in series with the blood oxygenator and an outlet connectable to said patient's blood circuit, the useful internal volume of the body of the first and second pumps, varying substantially proportionally to the pressure of the blood at the inlet, between minimum and maximum values, the maximum useful volume for the first pump and the minimum useful volume of the second pump being reached at a pressure of the blood at the inlet of the respective pump within the range of atmospheric pressure .+-. 20 mm of mercury.

In order that the present invention will be better understood the following description is given, merely by way of example, reference being made to the accompanying drawings, in which:

FIG. 1 is a schematic view of one embodiment of an extracorporeal blood circuit according to the invention;

FIG. 2 shows the characteristic flow rate-pressure curve of a peristaltic pump which can be used in the circuit according to the invention; and

FIG. 3 shows the combination of the characteristic flow rate/pressure curves of two pumps located on either side of the blood oxygenator.

In the present text, the expression blood oxygenator, which has become accepted through use, denotes an exchanger of respiratory gas, that is to say not only an oxygen exchanger, but also an exchanger of carbon dioxide, water vapour and nitrogen and optionally of gases or vapours with medicinal and/or anaesthetic effects, and also possibly a heat exchanger.

Referring to FIG. 1, it is seen that the extracorporeal blood circuit connects a blood oxygenator 1 of a type which is in itself known, comprising at least one membrane 2, to the venous-arterial system of a patient.

More precisely, a cannula 3 is introduced, for example, into the inferior vena cava. A cannula which contains a non-occlusive enlargement near its end is preferably used. This enlargement can consist of three radial resilient branches 5 which press against the venous walls and keep them spaced apart locally, which clears the orifice of the cannula. Such a cannula prevents obstruction of the vein and the restrictions in flow rate which result therefrom. The branches can advantageously become smaller in order to pass through a collateral of smaller size 4 (for example, the femoral vein) sectioned for this purpose.

The cannula 3 is connected to the inlet of the blood oxygenator 1 via a flexible tube 7, for example made of silicone elastomer, on which a first pump 6, of the peristaltic type or of the type with a tubular membrane and valves (also called "a ventricular pump"), is placed.

A flexible tube 11, also made of silicone elastomer, connects the outlet of the blood oxygenator 1 to a cannula introduced into an artery 9, or preferably to a flared-out prosthesis 8 sutured to the artery, generally a femoral artery. A second pump 10, also of the peristaltic type or of the type with a tubular membrane and valves, is located on this tube 11.

Advantageously, the artery is slit longitudinally and the prosthesis 8 is sutured obliquely onto the lips of the slit, so that it slopes in the direction of preferential flow. The two sides of the prosthesis move apart under the effect of the arterial pressure. After the prosthesis has been removed, the artery is made whole again in accordance with the usual practical procedure.

An auxiliary peristaltic pump 13, connected to the intake tube 7 of the pump 6, makes it possible to drain the peripheral end of the vein 4 and to introduce additional amounts of blood into the extracorporeal circuit from a blood source such as the bottle 16 in order to compensate for possible blood losses and optionally to introduce medicinal liquids such as heparin.

Since the extracorporeal blood circuit represented by way of example is of the venous-arterial type, it is necessary to control three different pressures, namely the blood pressure at the inlet of the pump 6, the pressure in the oxygenator and the arterial pressure of the patient, in order to keep them at the desired values.

A manometer 17 is placed on the tube 7, immediately upstream of the inlet of the pump 6. Advantageously, this manometer measures the blood pressure through the walls of the flexible tube 7, which reduces the risks of blood coagulation. As the manometer, it is possible to use, for example, that described in French Pat. No. 71/43,881. As will be seen from the text which follows, a knowledge of the blood pressure at the inlet of the pump 6 makes it possible to know its flow rate.

A manometer 14 makes it possible to check the blood pressure in the oxygenator. Moreover, a device 21 for taking the arterial pressure of the patient makes it possible to keep the latter at the desired level, by acting, when necessary, either on the volume of blood by means of the bottle 16 and the pump 13, or on the vascular resistance of the patient.

The blood must be injected into the patient at a temperature of approximately 37.degree.C. For this purpose, means for reheating and checking the temperature of the blood are provided, for example, on the tube 11. The reheating means advantageously consist of a heating element comprising an electrical resistance 18 surrounding the tube 11 or preferably embedded in its wall. This heating element is, for example, of the type described in French Pat. No. 71/46,408.

Temperature probes of types which are in themselves known 19 and 20, which are placed respectively downstream from, and at the level of, the heating element 18, are generally used. The probe 19 makes it possible to check (and if necessary to control) the reheating of the blood. The probe 20 makes it possible to avoid overheating the blood locally, which could arise if the blood flow rate were decreased or stopped momentarily.

Advantageously, the internal walls, which are in contact with the blood, of the various components forming the circuit (especially pump bodies, cannulas and connecting tubes) carry a smooth organosilicon coating applied in accordance with the process described in German patent application (DOS) No. 2,206,608.

In operation, the venous blood flows from the inferior vena cava, where it is at a pressure close to atmospheric pressure, via a cannula 3 and the tube 7 to the first peristaltic pump 6. The latter carries the blood along into the membrane-containing oxygenator 1 at a sufficient pressure to overcome the pressure drops of the apparatus. The compartments of the oxygenator reserved for the blood are kept full and the blood film of substantially constant thickness, the blood pressure being kept within a predetermined range indicated by the manometer 14. The oxygenated blood is recovered at the outlet of the oxygenator by a second peristaltic pump 10 which brings it to a pressure enabling it to be injected into the arterial system of the patient through the prosthesis 8, after suitable reheating in the tube 11.

The average blood flow rate, common to both pumps, is provided by the veins of the patient. It must be possible for this flow rate to vary in such a way as to prevent any increase in the venous pressure which could cause disturbances for the patient (and especially acute oedema of the lung); in order to prevent this, it is then convenient to increase the flow rate of the pumps 6 and 10, and in the opposite case, to decrease it if the venous pressure became too low, which could lead to the collapse of the veins or venous cavities.

For this purpose, pumps are used, the body of which provides an internal volume which varies according to the blood pressure at the inlet, which is not generally the case with pumps of the peristaltic type or of the type with a tubular membrane and valves. According to the invention, pumps are used which provide, within the range of blood pressure at the inlet effectively used, a useful internal volume which is substantially proportional to the blood pressure at the inlet of the pump. Pumps such as those described in British Pat. No. 1,287,836 are preferably employed as peristaltic pumps.

The two peristaltic pumps 6 and 10, connected in series, are generally driven synchronously, at one and the same rate or at different rates, in order to provide the same average flow rate. Preferably, they revolve at rates which are always equal to one another, and to achieve this, they are advantageously mounted on the same drive shaft. These rates can be adjustable, but it is often of value to keep the rate constant.

The peristaltic pumps can also be driven at the same rates, each by a separate motor but one which possesses the same rate/voltage characteristics, each motor being supplied by a (fixed or adjustable) common voltage source.

It is also possible to use pumps with a tubular membrane and valves, containing an inlet valve which is either automatic or preferably controlled. The controlled discharge valve can be of the same type as the inlet valve. These pumps can be connected either to separate pulse generators, synchronised on one and the same frequency, or preferably to a common pulse generator.

The peristaltic pumps represented in FIG. 1 advantageously consist of a flexible peristaltic tube made of silicone elastomer stretched between two fixed points and revolving wheels. The peristaltic tube, between the wheels, generally has an elliptical cross-section which is flattened to a greater or lesser extent depending on the pressure at the inlet of the pump. For a constant speed, the flow rate is a function of the pressure at the inlet of the pump, which can be seen clearly on the characteristic flow rate/intake pressure curve of such a pump, represented in FIG. 2.

It is seen that, for a functioning pressure p.sub.A between two limiting values p.sub.m and p.sub.M, at the inlet of the pump, the pump provides a flow rate Q.sub.A between two limiting values Q.sub.m and Q.sub.M ; the flow rate Q.sub.A being, within this range, substantially proportional to the pressure p.sub.A at the inlet.

In the remainder of the description, the useful minimum pressure and the useful maximum pressure at the inlet of the pump will be denoted respectively by p.sub.m and p.sub.M. Likewise, Q.sub.m and Q.sub.M will be respectively the corresponding useful minimum and useful maximum flow rates.

The useful minimum flow rate Q.sub.m is obtained when the pressure at the inlet is sufficiently low for the tube to collapse and for its opposite walls to press against one another; the cross-section of the tube assumes a dumb-bell shape. Beyond, the cross-section of the tube becomes more flattened out, but under the effect of much lower intake pressures at the entrance of the pump, which corresponds to a rapid change in the slope of the curve.

The useful maximum flow rate Q.sub.M is obtained when the pressure at the inlet of the pump is sufficiently high for the tube to assume a circular cross-section. Beyond, the tube can only expand, which requires considerably higher pressures and also corresponds to a rapid change in the slope of the curve.

In the extracorporeal blood circuit represented in FIG. 1, the flow rate of the pump 6 varies for a given level according to the venous pressure. Since the venous pressure at the level of the cannula 3 is close to atmospheric pressure, and since the pressure of the blood at the inlet of the pump 6 differs therefrom by the pressure drops in the intermediate tube 5, partially compensated for by the difference in level between the cannula 3 and the pump 6, it is generally less than atmospheric pressure. Thus a pump 6 is chosen, the characteristic flow rate/pressure curve of which extends over a region of pressures which are preferably less than atmospheric pressure. The characteristic curve of the pump 6 is shown in FIG. 3. The useful region of the curve is between the points B.sub.m and B.sub.M, and the corresponding extreme useful pressures p.sub.m6 and p.sub.M6 are, for example, in this case, both less than atmospheric pressure (point 0, of abscissa zero). The pressure p.sub.M6, like the maximum flow rate, is proportional to the speed at which the pump 6 rotates.

It is advantageous for the useful maximum pressure p.sub.M6 to be slightly less than atmospheric pressure, and generally less than 20 mm of mercury and preferably less than 10 mm of mercury below atmospheric pressure. This condition is achieved with a pump, the thin-walled peristaltic tube of which has, at rest, a circular cross-section (between the wheels, if what is involved is a rotating pump with wheels, as represented in FIG. 1); this useful cross-section is the maximum and permits a useful maximum flow rate Q.sub.M6. For a blood pressure p.sub.6 at the inlet of the pump 6, between p.sub.m6 and p.sub.M6 and less than atmospheric pressure, the peristaltic tube has a elliptical cross-section, with a surface area less than that of the circular cross-section of the same perimeter, corresponding to a flow rate Q.sub.6. When the pressure p.sub.6 becomes equal to the useful minimum pressure p.sub.m6, the peristaltic tube becomes more flattened and its cross-section becomes practically zero; the flow rate falls to the useful minimum flow rate Q.sub.m6.

Since the pump 10 is mounted in series with the pump 6, it provides strictly the same average flow rate. The flow rate of the pump 10 is thus laid down by that of the pump 6, which itself depends on the venous pressure.

The pumps 6 and 10 are generally located substantially at the same level as the oxygenator. The combination consisting of the pumps 6 and 10 and the oxygenator is generally placed below the patient, at an adjustable level, in order partially to compensate for pressure drops upstream from the pump 6 and thus to adjust the blood flow rate to the desired average value.

It is necessary to keep the pressure of the blood in the oxygenator within a predetermined range of pressures in order to enable the blood film to retain substantially constant thicknesses in contact with the membranes and a controlled pressure gradient across the thickness of the membrane.

Thus for a blood oxygenator consisting of an alternate stack of membranes and spacers, it can be decided to keep the relative pressure of the blood in this oxygenator, measured by means of the manometer 14 within a predetermined range, for example, between 0 and 200 mm of mercury.

In fact, if the pressure of oxygen in this oxygenator is kept below atmospheric pressure, the pressure difference between the blood and the oxygen always remains positive, and this makes it possible to use microporous membranes with a high gas flow rate which also enable bubbles to be removed satisfactorily from the blood, even in the case of foam or small bubbles; it forms a safety bubble-remover. The membranes described in French Pat. No. 1,568,130 are very particularly suitable for this purpose. Moreover, by maintaining this positive pressure difference, it is possible to prevent the membranes from sticking to one another by accident, since it is difficult to reverse this sticking.

If, on the other hand, the pressure of the blood were too much greater than that of the oxygen, the blood could rupture the membrane or overcome the hydrophobic nature of the microporous membranes and pass through them, for example under a pressure of 800 mm of mercury.

The pressure of the blood in the oxygenator can be kept within a chosen range by means of a pump 10, the characteristic flow rate/pressure curve of which extends in a region of pressures which are essentially greater than atmospheric pressure. The characteristic curve of a pump 10 is shown in FIG. 3. The useful region of the curve is between the points C.sub.m and C.sub.M, and the corresponding useful extreme pressures p.sub.m10 and p.sub.M10 are, for example, in this case, both greater than atmospheric pressure.

It is advantageous for the useful minimum pressure p.sub.m10 to be equal to or slightly greater than atmospheric pressure, and generally less than 20 mm of mercury, and preferably less than 10 mm, above atmospheric pressure. This condition is achieved with a pump, the peristaltic tube of which, at rest, has a flattened cross-section; this cross-section is practically the minimum and allows a minimum useful flow rate Q.sub.m10. The tube of the pump 10 is quite thin-walled to enable the useful maximum flow rate Q.sub.M10 to be reached for a useful maximum pressure p.sub.M10, generally less than 200 mm of mercury above atmospheric pressure and preferably of the order of 50 mm of mercury. Thus, for any flow rate imposed by the pump 6, the tube of the pump 10 will have a more or less flattened cross-section, corresponding to pressures, at the outlet of the oxygenator, of between 0 and, for example, 50 mm of mercury above atmospheric pressure. The maximum pressure at the inlet of the oxygenator depends on the pressure drops in the latter, which are generally less than 100 mm of mercury for blood flow rates of the order of 600 millilitres/minute in an oxygenator with a surface area of 0.5 m.sup.2.

In order to be certain that the average blood pressure in the oxygenator remains within the desired range, at any instant, it is necessary for two conditions to be realised simultaneously.

The first condition is that the useful maximum flow rate capacity of the pump 10 placed at the outlet of the oxygenator is greater than that of the pump 6 placed at the inlet of the oxygenator.

Since, according to the invention, the pumps 6 and 10 are generally driven synchronously, this condition can be satisfied by selecting from the following means:

The pump 10 can be driven at a speed which is greater, by a fixed percentage, than that of the pump 6. If a peristaltic pump with a rotor is used, it is possible to equip the pump 10 with a rotor of diameter greater than that of the pump 6. Preferably, the two pumps are equipped with identical rotors revolving at the same speed being mounted on a common shaft and these rotors act on different tubes, the internal perimeter of a cross-section of the tube of the pump 10 being greater than that of a cross-section of the tube of the pump 6. Of course, several of these arrangements can be combined with one another.

The second condition is that the useful minimum flow rate capacity of the pump 10 placed at the outlet of the oxygenator is less than the useful minimum flow rate capacity of the pump 6 placed at the inlet of the oxygenator. To achieve this, a tube for the pump 10 can be chosen with more flexible walls than those of the tube of the pump 6. Thus the cross-section provided by the flattened tube 10 at a pressure close to atmospheric pressure forms a dumb-bell or cross-section less than that of the tube of the pump 6 for the useful minimum pressure upstream from the latter. A tube is chosen for the pump 10 which is substantially flat at rest and which requires a smaller force to achieve the limiting dumb-bell shape than does the tube of the pump 6 which is circular at rest. It is possible to use a tube with thinner walls for the pump 10 than for the pump 6 and to combine thin walls and flat shape.

It is thus seen that it is possible, according to the invention, simultaneously to satisfy the two conditions listed above.

In practice, when a stable state is reached and when the flow rate of the extracorporeal circulation is satisfactory, it is advantageous, in order to reduce blood traumas, to decrease the speed of the pump 6 in order to bring its actual flow rate to approximately four-fifths of the maximum flow rate. The readings of the manometer 17 which indicates the actual flow rate, are used for this purpose.

Peristaltic pumps are generally preferred during cardiac or cardiopulmonary replacements. However, during assistances, there is the danger of competition between the more or less pulsed flow rate of such pumps and that originating from the heart beats of the patient.

This is why, if it is desired to take advantage of the diastolic pause in order to inject the blood originating from the extracorporeal circuit and to avoid competition between the latter and the natural heart, it is preferable to resort to pumps, the ejection of which can be synchronised relative to the cardiac cycle. These pumps are generally chosen from amongst membrane pumps adapted for blood flow. They are known by the name of ventricular pumps or pumps with a tubular membrane and valves. Pumps such as those described in French Pat. No. 72/07,863 can advantageously be used.

These pumps are equipped with controlled valves upstream and downstream. The maximum volume of the arterial ventricle forming the pump 10 is greater by 50 percent at most and preferably by 20 percent at most than the maximum volume of the venous ventricle forming the pump 6. The generally rigid casings of these pumps are advantageously connected, in opposite phases, to one and the same pulse generator.

The start of systole can be induced either by an electrocardiographic signal, or preferably by the passage of the arterial pressure of the patient below a definite threshold. The control of the intake and discharge pressures of either pump can be effected by controlling the pressures of the drive gaseous fluid. This is thus again a method of functioning comparable to that of peristaltic pumps with a flow rate which is a function of the upstream pressure.

As with the latter, it is even possible to achieve protection against injections of air by placing them vertically, the blood passing through them in a downwards direction and the air thus being trapped at the level of the inlet valve which may not provide a rigorous seal.

The use of two pumps with a tubular membrane and valves and a fluid pulse generator such as that which is the subject of French Pat. No. 72/06,812 is particularly advantageous for reviving patients in ambulances or helicopters, because it does not require any electrical energy and can function solely by means of the expansion of the oxygen used for flushing the oxygenator. In this case, the gas monitor can be dispensed with and replaced by a simple open circuit flushing, after expansion for driving the pumps. With a 0.5 m.sup.3 bottle of oxygen, independent operation is satisfactory for minimum weight and bulk.

A combination connected to the venous-arterial system of a patient has been described, but the same combination can be used in venous-venous or arterial-venous extracorporeal circulation. In the latter case, the pump 6 can be operated so that it functions at its maximum flow rate (it acts as a flow rate limiting device) and the latter is adjusted by controlling the speed of rotation of the pump.

The extracorporeal circuit can comprise several auxiliary combinations, arranged in parallel, each consisting of a blood oxygenator located between two pumps 6 and 10. Such auxiliary combinations are connected by taps 22 and 23 to the main circuit represented in FIG. 1. They can be brought into operation or short-circuited in order rapidly to meet variable requirements of the patient.

Since the extracorporeal blood circuit according to the invention uses pumps which cause a very small degree of haemolysis and since it avoids any direct recycling of the blood, it can be used advantageously for a long period. Since the pumps are self-regulating, it provides great simplicity, reliability and safety, particularly with respect to injection of air, since the pumps act as bubble traps.

If the blood oxygenating device is equipped with microporous membranes and if a gas stream passes through it under a pressure less than atmospheric pressure, it can reabsorb the bubbles introduced accidentally into the blood before the inlet of the oxygenator, and does so the better, the finer are these bubbles. In this function, the oxygenator behaves better than the bubble traps usually employed (operating by gravity or by Archimedes thrust), the latter acting the better, the larger are the bubbles; thus, under these preferential conditions, there is no need to place such a bubble trap in the circuit.

The characteristics and the advantages of the circuit according to the invention will become more apparent from the following examples:

EXAMPLE 1

The circuit is the same as that represented in FIG. 1, with the exception of the means for driving the pumps 6 and 10. The pumps are in effect driven at speeds which can be adjusted between 0 and 40 revolutions/minute by direct current motors supplied by a common voltage source. The pumps 6 and 10 are of the type described in British Pat. No. 1,287,836 the peristaltic tubes being made of silicone elastomer. The tube of the pump 6 has an internal diameter of 10 mm and an external diameter of 12.6 mm; the periphery of the three-wheel rotor describes a circle of diameter 140 mm. The tube of the pump 10 has an internal diameter of 11.25 mm and an external diameter of 14.1 mm; the periphery of the rotor describes a circle of diameter 140 mm. The pump 10 is placed at a height of 50 cm above the blood oxygenator. The latter consists of two identical combinations each comprising 16 microporous membranes and seven spacers stacked and clamped between two end plates; it is of the type described in French Pat. No. 1,597,874. Its surface area of exchange is 1 m.sup.2. At constant load, it opposes the blood with a pressure drop of 50 mm of mercury.

This circuit was used for cardiopulmonary assistance for a period of 12 hours. At the end of the treatment, it was found that the degree of haemolysis of the blood was less than 0.5 percent. The blood pressure inside the blood oxygenator was kept within the range 50-150 mm of mercury. For an average blood flow rate of 800 millilitres/minute, 40 millilitres/minute of oxygen and 60 millilitres/minute of carbon dioxide were transferred.

EXAMPLE 2

The circuit and the pumps 6 and 10 are similar to those of Example 1 and are the same as in FIG. 1. The rotors of the pumps 6 and 10 are mounted on a common shaft driven by a single motor 12 at a speed which can be adjusted between 0 and 40 revolutions/minute. Each rotor comprises three wheels at 120.degree., and the two rotors are 60.degree. apart from one another.

The tube of the pump 6 has a circular cross-section at rest. The internal diameter of the tube is 15.8 mm and the external diameter is 20 mm. The tube of the pump 10 is elliptical at rest. The internal long axis and short axis of the ellipse are respectively 24 and 4 mm. When deformed under pressure, this tube assumes a circular cross-section of internal diameter 16.8 mm and of external diameter 20 mm. The useful diameter of the rotors of the pumps 6 and 10 is 190 mm. The oxygenator has a membrane surface area of 3 m.sup.2.

This combination is used for subtotal cardiopulmonary replacement on an adult patient for a period of 52 hours. The blood flow rate is adjusted to an average value of 2 litres/minute and average transfers of 130 millilitres/minute of oxygen and 150 millilitres/minute of carbon dioxide are observed. The pressure and haemolysis conditions are the same as in Example 1. A second identical combination is ready for use if the transfers prove to be momentarily insufficient.

Claims

We claim:

1. An extracorporeal blood circuit comprising, in combination:

a. a blood oxygenator;

b. a first peristaltic pump including a variable tube means of a type having a capacity variable between maximum and minimum values in proportion to the pump inlet pressure, the maximum capacity being reached at an inlet pressure of atmospheric pressure .+-. 20 mm of mercury;

c. an inlet of said first pump connectable to a patient's blood circuit;

d. an outlet of said pump connected in series with said oxygenator;

e. a second peristaltic pump including a variable tube means of a type having a capacity variable between maximum and minimum values in proportion to the pump inlet pressure, the minimum capacity being reached at an inlet pressure of atmospheric pressure .+-. 20 mm of mercury;

f. an inlet of said second pump connected in series with said blood oxygenator; and

g. an outlet of said second pump connectable to said patient's blood circuit.

2. A blood circuit as claimed in claim 1, and further comprising means for driving said first and second pumps synchronously.

3. A blood circuit as claimed in claim 1, wherein the capacity of said first pump reaches a maximum value for a liquid pressure at its inlet which is less than atmospheric pressure.

4. A blood circuit as claimed in claim 1, wherein the capacity of said second pump reaches a minimum value for a liquid pressure at its inlet which is greater than atmospheric pressure.

5. A blood circuit as claimed in claim 1, wherein the useful maximum flow rate capacity of said second pump is greater than the useful maximum flow rate capacity of said first pump.

6. A blood circuit as claimed in claim 1, wherein the useful minimum flow rate capacity of said second pump is less than the useful minimum flow rate capacity of said first pump.

7. A blood circuit as claimed in claim 1, wherein the two pumps further comprising an adjustable speed single motor, a common shaft driven by said motor and identical rotors of said peristaltic pumps mounted on said common shaft.

8. A blood circuit as claimed in claim 1, wherein, in the oxygenator, the blood is separated from the oxygenating gas stream by at least one microporous membrane.

9. A blood circuit as claimed in claim 8, in which the microporous membrane is water-repellent.

10. A blood circuit as claimed in claim 1, wherein the internal wall in contact with the blood of at least one component of the circuit carries a smooth organosilicon coating.

11. A blood circuit as claimed in claim 1, and further comprising at least one cannula connected to the inlet of the first pump for removing the venous blood and a non-occlusive enlargement near its end.

12. A blood circuit as claimed in claim 1, and further comprising a flared-out prosthesis, connected to the outlet of the second pump, and wherein said prosthesis can be stitched to a patient's artery.

13. A blood circuit as claimed in claim 1, and further comprising means for reheating the blood and means for sensing the temperature of the blood located at the level of and downstream from the said means for reheating the blood.

14. A blood circuit as claimed in claim 1, and further comprising an auxiliary pump and at least one other source of blood or medicinal liquid connected to the inlet of said first pump by said auxiliary pump.

15. A blood circuit as claimed in claim 1, and further comprising a manometer upstream from said first pump.

16. A blood circuit as claimed in claim 15, wherein said manometer is of the type which measures the pressure across the wall of a flexible tube.

17. A blood circuit as claimed in claim 1, and further comprising a plurality of blood oxygenators connected in parallel and a further one of said first and second pumps connected in series on either side of each blood oxygenator.

18. An extracorporeal blood circuit comprising, in combination:

a. a blood oxygenator;

b. a first tubular variable membrane and valve pump of a type having a capacity variable between maximum and minimum values in proportion to the inlet pressure of the pump, the maximum capacity being reached at an inlet pressure of atmospheric pressure .+-. 20 mm of mercury;

c. an inlet of said first pump connectable to a patient's blood circuit;

d. an outlet of said pump connected in series with said oxygenator;

e. a second tubular variable membrane and valve pump of a type having a capacity variable between maximum and minimum values in proportion to the inlet pressure, the minimum capacity being reached at an inlet pressure of atmospheric pressure .+-. 20 mm of mercury;

f. an inlet of said second pump connected in series with said blood oxygenator; and

g. an outlet of said second pump connectable to a patient's blood circuit.

19. An extracorporeal blood circuit comprising; in combination:

a. a blood oxygenator;

b. a first peristaltic pump of a type having a capacity which is variable in proportion to the pump inlet pressure and having a tube of circular cross-section at rest;

c. an inlet of said first pump connectable to a patient's blood circuit;

d. an outlet of said pump connected in series with said oxygenator;

e. a second peristaltic pump of a type having a capacity which is variable in proportion to the pump inlet pressure and having a tube of substantially ellipitical cross-section at rest, the internal perimeter of the tube of the second pump being greater than that of the tube of the first pump, so that the maximum capacity of the first pump and the minimum capacity of the second pump is reached at a pump inlet pressure of atmospheric pressure .+-. 20 mm of mercury;

f. an inlet of said second pump connected in series with said blood oxygenator; and

g. an outlet of said second pump connectable to said patient's blood circuit.

20. An extracorporeal blood circuit comprising, in combination:

a. a blood oxygenator;

b. a first tubular membrane and valve pump of a type having a capacity which is variable in proportion to the pump inlet pressure and having a tubular membrane of circular cross-section at rest;

c. an inlet of said first pump connectable to a patient's blood circuit;

d. an outlet of said pump connected in series with said oxygenator;

e. a second tubular membrane and valve pump of a type having a capacity which is variable in proportion to the pump inlet pressure and the tubular membrane having a substantially elliptical cross-section at rest, the internal perimeter of the tube of the membrane of the second pump being greater than that of the tubular membrane of the first pump, so that the maximum capacity of the first pump and the minimum capacity of the second pump is reached at a pump inlet pressure of atmospheric pressure .+-. 20 mm of mercury;

f. an inlet of said second pump connected in series with said blood oxygenator; and

g. an outlet of said second pump connectable to said patient's blood circuit.