referring to the figures , in which like reference numerals refer to like elements thereof , fig1 shows the left arm of a patient provided with a fistula suitable for hemodialysis . a fistula has shown itself to be the most effective , durable , permanent blood vessel access for extracorporeal blood treatment . a fistula is created by surgical intervention , whereby a connection is formed between an artery and a proximate vein , for example in the lower arm . the fistula is formed either by an opening being formed from the sidewall of the artery to the sidewall of the vein as shown in fig1 or by an opening in the sidewall of the artery being connected with the end of a vein . by means of the fistula , the blood flow in the artery is short - circuited to the vein , which leads to an arterializing of the vein and an increased bloodflow in the vein which allows taking out of bloodflows up to 500 ml / minute or more . as is clear from fig1 the arterial needle which leads to the extracorporeal circuit is always placed in the part of the arterialized vein which faces the hand , but at least three centimeters downstream of the connection between the artery and the vein . the arterial needle can either point towards the hand as shown in fig1 or in the other direction . the venous needle is to be inserted directed towards the heart , approximately five centimeters from the arterial needle . the expression &# 34 ; fistula &# 34 ; will be used below for the part of the arterialized vein where the needles are inserted . other types of blood vessel access can be used , such as a scribner - shunt or one or more catheters . fig2 shows an extracorporeal circuit of the type which is used in a dialysis machine . the circuit comprises an arterial needle 1 and an arterial tube 2 which connects the arterial needle 1 to a blood pump 3 which is normally of the peristaltic type , such as indicated in fig2 . at the inlet of the pump there is an arterial sensor 4 which measures the pressure immediately before the pump in the arterial tube 2 . the blood pump 3 leads the blood further , through tube 5 , to a dialyser 6 . the tube 5 can comprise an inlet 7 for heparin connected to a heparin pump 8 . many dialysis machines are additionally provided with a pressure sensor 9 which measures the pressure between the blood pump 3 and the dialyser 6 , i . e ., the so - called system pressure . the blood is lead through tube 10 from the dialyser 6 to a venous drip chamber 11 and from there back to the patient through a venous tube 12 and a venous needle 14 . the venous tube 12 is provided with a clamp device 13 which stops the blood flow upon the occurrence of a malfunction . the venous drip chamber 11 is provided with a venous sensor 15 which measures the pressure in the venous drip chamber . the arterial tube 2 can also be provided with a clamp device similar to the clamp device 13 . both the arterial needle 1 and the venous needle 14 are inserted into the fistula . when the blood passes the arterial needle 1 , which has as small a cross - sectional area as possible so as not to damage the fistula , the pressure sinks to between about - 20 to - 80 mm hg , which is measured by the arterial sensor 4 . the pressure rises in the pump 3 , said pressure being measured by the system sensor 9 . in the dialyser 6 , the pressure falls due to the flow resistance therein and the pressure downstream of the dialyser is measured with the venous sensor 15 , normally in the venous drip chamber . the pressure in the venous drip chamber is normally between about + 50 to + 150 mm hg . finally the blood is released to the fistula through the venous needle 14 , whereby a pressure drop occurs in the needle due to the flow through its small cross - section . the aforementioned pressure conditions vary considerably from patient to patient and can even vary for one and the same patient between different treatment sessions . it is therefore difficult to set up limit values for the pressure sensors which indicate different error conditions . it is particularly difficult to indicate whether the venous needle 14 is coming out of the fistula , particularly if the venous tube 12 is hanging over a position so that the venous needle is moved upwardly a long way when it comes out . in many dialysis machines one or more of said pressure detectors are not present . normally however there will be at least one veneous pressure sensor . fig3 shows a pressure curve which is obtained from the arterial sensor 4 in fig2 . this pressure curve corresponds to the pressure curve of the blood pump 3 on its suction side . the pressure pulses emanate from the periods of time when one pressure roller takes over from the other pressure roller , i . e . showing the pump stroke . the pressure curve in fig3 corresponds to the blood pump &# 39 ; s suction stroke , but also has a superimposed pulse signal obtained from the pulse in the fistula . this pulse signal is , however , very insignificant and cannot be observed with the naked eye in fig3 . in fig4 the pressure curve in fig3 has been resolved in the frequency plane ( fourier - transformation ). it can be seen that the signal consists of a base frequency , fo , at about 52 strokes per minute , as well as a large number of harmonics , of which only three can be identified in fig4 . by eliminating the frequency fo and its harmonics , the effect of the blood pump &# 39 ; s pressure pulses on the pressure in the arterial sensor 4 can be eliminated . such elimination can be done with the aid of notch filters . if the frequency and phase of the interference are known , notch - equivalent filters can advantageously be used . one example is the generation of sinus signals at the known frequency together with its harmonics and the subtraction of these from the signal at suitable phase . with an adaptive filter , the amplitude and the phase of the generated signals can be determined . this filter technique is known . the calculations and the subtraction suitably occur in a signal processor . the signal processor and its analogue / digital converter must however have high resolution since the pulse signal is very weak . fig5 shows the signal in fig3 in the frequency plane after subtraction of the interference due to the blood pump &# 39 ; s pressure waves , i . e . subtraction of the base frequency fo and its harmonics . from fig5 it can be seen that a half base frequency , i . e . 0 . 5 fo , is also represented in the frequency plane together with the corresponding harmonics 1 . 5 fo , 2 . 5 fo , 3 . 5 fo etc . ( fo , 2fo , 3fo etc . have already been eliminated ). this half base frequency is due to the fact that the blood pump used is of the peristaltic type , with two rollers which act on the tube segment in the blood pump . the rollers are probably not entirely symmetrical , which gives rise to the half base frequency ( 0 . 5 fo ). half the base frequency is also the same as the motor &# 39 ; s rotational speed . this rotational speed is known since it is generated by the dialysis machine . the motor which drives the blood pump can be constituted by a stepping motor which is driven at predetermined frequency . by using this known frequency signal or the known rotational speed of the blood pump , the frequency fo can be determined very accurately , which results in an accurate removal of these frequency components . fig6 shows the signal which is obtained after the above - mentioned adaptive filtering and elimination of the pump frequency and its harmonics . moreover , the pulse signal has passed a band - pass filter which lets through the frequencies 30 - 180 strokes / minute ( 0 . 5 - 3 hz ). as is clear from fig6 the amplitude of the pulse signal is dependent on many factors , such as damping in the tubes , etc . other factors can be changes in the height position of the arm , or that the needle has temporarily come closer to the wall of the fistula . even though fig3 - 6 relate to the pressure conditions of the arterial needle , the conditions are similar with a venous needle . an indication that the needle has fallen out is that the amplitude of the pulse signal sinks to zero . in practice , an alarm signal can be emitted if the amplitude sinks below 20 % of an earlier determined normal amplitude . this normal amplitude can be determined during the first stage of the treatment when the dialyser is being observed by a nurse , for example during the first half hour of the treatment . the pulse signal can disappear temporarily for other reasons than the needle having fallen out , such as the patient moving . the adaptive signal processing then re - adjusts the settings to the new situation , after which the pulse signal can be recovered and separated . such an adaptive adjustment to normal , but changed situations , takes a certain amount of time . it is therefore suitable if the emitting of an alarm signal is delayed by a short period of time on the order of a number seconds . another way of determining when an alarm signal is to be emitted is to determine the relationship between the amplitude of the pulse signals from the venous sensor 15 and the pulse signal from the arterial sensor 4 . due to the different damping in , for instance , the blood tube 2 and the blood tube 12 , respectively , as well as the venous drip chamber 11 , the amplitude from these sensors is different , whereby the venous sensor 15 generally has a lower amplitude . if the pulse signal from the venous sensor 15 disappears more or less completely at the same time as the pulse signal from the arterial sensor 4 is still present and substantially unchanged , this is a certain sign of a problem with the venous needle 14 ; either that it has come too close to the blood vessel wall or fallen out completely . according to the present invention it is proposed that the alarm signal is emitted when the relationship between the amplitudes for the pulse signals from the venous sensor 15 and the arterial sensor 4 , respectively , are changed substantially , such as the relationship between the amplitudes sinking below a limit value which is 50 % of the original value . if it is desired to obtain greater accuracy for the detection , said limit value can instead be set at 30 %. if there is a patient who has weak blood vessels , whereby it can easily happen that the venous needle 14 comes too close to the blood vessel wall , or if problems arise in another way which can be acceptable and would not lead to an alarm , the limit value should be set even lower , such as at 20 %. if the amplitude of the pulse signal from the arterial sensor 4 reduces greatly , this is probably an indication of a problem with the arterial needle 1 which can also give rise to an alarm signal . from fig5 it can be seen that if the frequency of the pulse lies close to the half base frequency ( 0 . 5 fo ) of the blood pump or multiples thereof , difficulties will occur in separating the pulse signal from the blood pump signal . in particular , there will be difficulties in such a separation if the difference between the pulse and any of the blood pump &# 39 ; s frequencies is less than about 5 - 10 %. in accordance with the present invention , it is suggested that the blood pump is adapted so that the pulse always lies at at least about 10 % from any of the blood pump &# 39 ; s frequency components . this can be done by making the blood pump increase or decrease its speed by about ± 10 % when the pulse detection system according to the invention senses that there is a risk of collision . such a change of the blood pump &# 39 ; s speed will hardly be noticed by the patient . in order to reduce the risk of exceeding any maximum possible bloodflow speed , said regulation can be - 15 % to 5 % or - 20 % to 0 %, or something similar . the frequency of the pulse signal can be used for other purposes such as are known per se . thus , a great rise in the pulse implies that there is a risk for shock , etc . since the pressure pulses of the blood pump 3 are strong , these pressure pulses can be transmitted to the venous sensor 15 through a path which comprises the tube 2 , the arterial needle 1 , the fistula , the venous needle 14 and the tube 12 to the venous sensor 15 . if the arterial needle 1 and / or the venous needle 14 comes out , said path for the pressure pulses will be broken and thus will cease . this characteristic can be used in order to detect the integrity of both the arterial needle and the venous needle simultaneously . fig2 shows a pressure sensor 9 for the system pressure . the pressure wave from the blood pump 3 passes through the system sensor 9 and the dialyser 6 to the venous sensor 15 . in this way there is both a time delay from the system sensor 9 to the venous sensor 15 and a damping . the system sensor 9 is positioned so that the pulse signal is very small or completely absent . by comparing the signals from the arterial sensor 4 , the venous sensor 15 and the system sensor 9 , suitable conditions for emitting an alarm signal can be determined . fig7 shows a schematic circuit similar to fig2 for single - needle dialysis , whereby the same reference numerals have been used for the same components as in fig2 . the difference compared to two - needle dialysis is merely that one needle is used . furthermore , expansion vessels 21 and 22 are required and often a second pump 23 . the system pressure sensor 9 is often placed after the dialyser 6 . apart from this , the function is basically the same as described above , in as far as concerns the present invention . frequencies between about 0 . 2 - 20 hz have been quoted above . the reason for the use of these frequencies is that they are in the infrasound range and do not give rise to audible sound . it is useful to use frequencies of about 1 hz since many patients find this frequency calming , presumably due to the fact that it is close to the frequency of the heart . normally , however , it is preferable to use frequencies for the blood pump which differ from the heart frequency if the pulse is to be used as an indication , for example 1 . 5 hz and upwards or below about 0 . 8 hz . an ultrasound generator can also be used as the pressure wave generator , it being coupled to the blood vessel by an arm band as described above , or to the extracorporeal blood circuit for transmission through the blood vessel access as described above . a suitable ultrasound frequency ought to lie at just above 20 khz , for instance 20 - 40 khz . in principle , it is possible to use frequencies within the range of 20 - 20 , 000 hz , but this is not preferred since it has apparently been found to be disturbing to the patients and personnel . the principles of the present invention can also be applied for detecting the condition of another component in the extracorporeal circuit , such as the dialyser , by letting a pressure wave pass through that component , and detecting the changed condition with a pressure sensor . the invention can also be used for other applications than those described in detail above , such as those mentioned in the introduction , like hemofiltration etc . the various electronic means for obtaining the desired function have not been described above although a skilled man will realise various possibilities and can practice the invention without a detailed account of any embodiments .
methods for detecting the condition of a catheter providing access to a patient &# 39 ; s blood vessel are disclosed in which the blood is supplied to an extracorporeal blood flow circuit . the method includes generating a pressure wave on one side of one of the access points for supplying blood to the extracorporeal blood flow circuit or returning the blood to the patient , then sensing the pressure wave on the other side of the access points . apparatus for detecting the condition of the catheter are also provided .