a device in accordance with an embodiment of the present invention , for use with a method in accordance with the present invention , is illustrated in fig1 through 7 . the device can be used as discussed in the preamble , to monitor changes in blood flow in a peripheral blood vessel of a human or animal subject during anaesthesia and surgery . this gives an indication of relative changes in total blood flow ( cardiac output ) as one of the indicators for enabling the anaesthetist to monitor the subjects haemodynamic condition and suitably adjust anaesthetic dose . monitoring peripheral blood flow to provide an indication of changes in cardiac output , as opposed to using blood pressure , runs contrary to anaesthesia practice over the past one hundred years where blood pressure is used in surgery to indicate changes in haemodynamic function or cardiac output . as discussed above , the present applicants believe that , because of responses to painful stimuli during surgery , blood pressure is neither a reliable or positive indicator of changes in cardiac output . they believe that either monitoring of total blood flow or , as in the preferred embodiment of the invention , monitoring of changes in blood flow in a peripheral artery during anaesthesia in surgery , will provide a much better positive indication of relative changes in total cardiac output . the method of monitoring haemodynamic function during anaesthesia and surgery in accordance with the preferred embodiment of the present invention , also preferably includes the steps of monitoring blood pressure , using standard equipment , monitoring ecg , using standard equipment and monitoring respiration using an airway thermistor . the heart rate may be monitored using the ecg device . the pulse rate may be monitored using the device in accordance with the present invention , being determined from the peripheral blood flow . these parameters , together with blood flow , provide the total “ picture ” required by the anaesthetist to enable monitoring and adjustment of anaesthetic dose to ensure the haemodynamic health of the subject . fig1 is a functional block diagram showing components of an ultrasound based device for monitoring blood flow , in accordance with an embodiment of the present invention . the device , generally indicated by reference numeral 1 , comprises a doppler transducer 2 for monitoring blood flow in a peripheral blood vessel of a human or animal subject . in operation , the transducer will be affixed to the appropriate body part of the subject eg . placed distally on the wrist or ankle of a human being , or where an animal is the subject , on the tail . note that as an alternative to a doppler transducer 2 , a pulse oximeter adapted to monitor blood flow could be used as the blood flow detector ( transducer ). in fact , any device which is capable of detecting blood flow , in the preferred embodiment in a peripheral vessel , could be used . note that a further alternative , in accordance with an alternative embodiment of the present invention , is to use a device such as a pulse oximeter in addition to using the doppler transducer 2 to monitor the changes in blood flow . the pulse oximeter is , in accordance with this embodiment , adapted to monitor blood volume in a peripheral tissue bed ( rather than oxygen saturation which is usually constant during anaesthesia where patients inspire high concentrations of oxygen ) and this may be used to improve the estimate of changes in blood flow or to enable estimation of changes in vascular resistance . in this alternative embodiment , the device of fig1 would also include a sensor and a pulse oximeter device providing an input about changes in tissue blood volume to the micro computer 4 for processing together with the perfusion input from the doppler device . the following description , however , relates to an embodiment which employs a doppler monitor only . in this embodiment , a continuous wave doppler driver / receiver 3 is connected to the doppler transducer for transmitting and receiving ultrasound signals therefrom . a microcomputer and interface 4 is arranged to process the signal from the receiver 3 , and drive the lcd display 6 to produce an output indicative of changes in cardiac output ( substantially equivalent to tissue oxygen delivery under high inspired concentrations of 0 2 ). it also controls and / or responds to the other peripherals , as follows : power is provided from the mains via a power supply regulator 11 , which is also provided with a back - up rechargeable battery 12 , in case of failure of the mains . in operation , the microcomputer of controller 4 operates to process the signal from the doppler transducer 2 to determine changes in the blood flow rate in the peripheral vessel and to control the liquid crystal display 6 to provide an indication , preferably graphical indication , of the instantaneous relative cardiac output at any time during anaesthesia and surgery . it is preferred to give an output of relative cardiac output , rather than attempting to produce an output indicative of actual cardiac output . attempting to obtain a measurement giving actual cardiac output is very difficult because a ) vessel diameter is required or b ) it assumes that changes in blood flow or vessel diameter in one vessel similarly reflect changes in the whole animal . monitoring changes in blood flow to provide an output relative to a reference , such as the signal output by the blood flow monitor when the patient is at rest prior to anaesthesia and surgery , is much more convenient , and provides sufficient indication to the anaesthetist to guide him to control anaesthetic depth . the loudspeaker 9 is controlled by the controller 4 to provide an audible pulse signal and alarms should the blood flow fall below or rise above pre - set levels . preferably , the display 6 also provides a visual alarm indication . the control panel 8 can be used to pre - set the blood flow display and alarm limits , depending upon , for example , the size of the subject and the species of the subject . it is envisaged that a device would be provided suitable for operation on a human subject and a separate device suitable for operation on animal subjects , the animal subject device preferably being adapted for use with a number of animal species , control limits being pre - set for species and animal size by the control panel 8 . the microcomputer and interface 4 is arranged to process the doppler signal output to give an indication of blood flow changes based on the strength of the signal . fig2 shows the external appearance of an example device 1 . equivalent items to fig1 are given the same reference numerals . the entire device 1 is housed within a robust housing 13 . brackets 14 are provided to hold a reference manual giving operating instructions on the device 1 . the device is mounted on rubber feet 15 and has a carrying handle 16 . a plug 17 is provided for connection to a mains power supply . in operation , before a subject is anaesthetised , the doppler transducer ( sensor ) 2 is positioned on the skin surface , overlying a peripheral artery such as located in the human forearm at the level of the wrist ( radial or ulna artery ), on the plantar surface of the foot of a dog or cat ( pedal artery ) or on the ventral surface of the tail ( coccygeal artery ). the device is attached to the subject at rest while conscious and a flow rate determined . the control pad 8 is then used to set a “ base line flow ” rate and a base bar ( reference number 20 , fig3 ) will appear on the operating display . the base bar will be used as a reference by the anaesthetist as the “ normal ” flow rate of the conscious resting subject ( ie . prior to induction of anaesthesia ). as an alternative , the device may also be arranged to store a series of “ standard ” base bars , being default settings for a particular animal species / size of animal . this would be necessary for animals which may not tolerate attachment of the transducer while conscious . for a human subject , however , it is preferable to pre - set the levels and the display by monitoring of the individual subject . fig3 shows an example operating display for a human subject during anaesthesia and surgery . the left hand side of the display , indicated by reference numeral 21 , is taken up by a bar graph which graphically continuously indicates peripheral blood flow rate based on the signal obtained from the peripheral vessel , processed by the controller 4 to provide the display . the base bar 20 is permanently in place on the graphical display and is pre - set by monitoring the flow rate of the conscious subject at rest , prior to the induction of anaesthesia . all flow rates and flow alarms are determined relative to this base bar 20 . a high limit bar 22 and low limit bar 23 are also displayed . these can either be pre - set by the anaesthetist or pre - stored in memory to automatically be displayed depending upon the set base bar level and other subject factors , eg . weight , age , etc . for example , appropriate limits could be determined by clinical trials and then stored in the memory of the device . a moving flow marker 24 is also displayed . this shows the actual real - time flow rate ( relative to the base bar ). it is this marker 24 that the anaesthetist will watch carefully to obtain an indication of changes in haemodynamic function . preferably , the flow marker is arranged to flash . should the rate fall to the lower limit bar 23 or rise to the high limit bar 22 an audible alarm will sound and the numeric flow display 26 will flash . the anaesthetists attention will thus be drawn to the alarming level of perfusion or blood flow and appropriate action can be taken ( eg . altering anaesthetic dose , administration of iv fluid , inotropic drugs etc .). note that it is unlikely during appropriate levels of anaesthesia during surgery in normal , healthy patients that blood flow will ever rise much above the base bar . this is because standard anaesthetics tend to depress ( rather than stimulate ) cardiac output in a dose dependent fashion . such a monitoring device can also be used for monitoring haemodynamic function during critical care such as post cardiac surgery . on this point , a novel device such as this is likely to provide precise clinical data on the effect of anaesthetics and surgical manipulation on peripheral blood flow in humans and animals . however , there are applications of this device , such as cardiac stress testing ( treadmill testing ) or conscious humans or race horses , where blood flow could increase above the base line measurement . referring again to fig3 , the controller 4 also determines the pulse rate of the subject from the doppler flow signal . this is displayed in the top right hand portion 25 of the display 6 . the anaesthetist can also therefore view pulse rate , at a glance . the bottom right hand corner of the display 26 displays the actual ( instantaneous ) peripheral blood flow rate in alphanumeric . should the probe signal change caused by transducer or skin movement relative to the artery or loss of acoustic coupling or otherwise malfunction , a “ probe error ” display will flash 27 . switching the device on and taking no further action defaults the screen to the monitoring display ( fig2 ). all input and control of the device is set by rotating knob 80 ( fig2 ) to select function or value and pressing enter to accept function or value . upper and lower limit thresholds may also be set for pulse rate , such that if the thresholds are reached audible alarms / visual alarms are provided . a breath to breath audible output and a numeric display of respiratory rate , may also be provided if an airway thermistor is employed . fig4 through 7 show examples of screen displays which may appear during initial set up of the apparatus prior to operation on a human or animal subject . the example screens are based on the device as designed for animal use . this is generally the same as what would appear in the device as designed for human subjects , except that it is envisaged that there would be no screen for default species settings ( fig5 ) although default settings based on body size could be introduced . alternatively , all the settings for the alarm function could be entered manually ( fig4 ). after selecting either the default settings ( fig5 ) or entering the alarm settings manually ( fig4 ), the device will then display the result and settings as selected ( fig6 ) before reverting to the “ running ” display associated with the continuous monitoring function ( a running display is shown in fig3 for a human being , but a similar display would be shown for animal ). the boxed items of display ( fig4 ) (“ run ”, “ pause ” etc ) are what can be selected by turning the knob 80 . a selected function displays as inverse display ( ie . white letters on black background ). depressing the knob will then cause the numerical value to increase in magnitude to a maximum number . subsequently turning the knob by 10 ° will move the selection to the next boxed item in a left to right , top to bottom flow with wrap - around at bottom . turning the knob counter clockwise will reverse the selection highlighting . fig4 shows a typical data entry display for manual entry of the alarm settings , which enables entry of pulse rate high / low limits and flow rate high / low limits ie . minimum , base and maximum levels for each item . these values can be set manually based on the preference / clinical experience of the anaesthetist . alternatively , selection of alarm limits may be based on default settings as shown for animals . fig5 shows a display for default settings which can be selected , which will be based on clinical trials for the particular species / weight of animal ( note that manually five entered default settings may be stored by the user in memory ). fig6 illustrates the screen with the default settings which were either entered manually ( fig4 ) or selected ( fig5 ). devices may obviously be designed with different default settings for different species and animal sizes , depending upon application . fig6 is a diagram of the entered / selected alarm setting display , also showing the rest of the control panel from fig2 , incorporating screen selection knob 80 , mode button 31 , enter button 32 and on / off switch 33 . for this example ( 10 - 20 kg dog ) using fig6 “ enter ” can be pressed while the selection knob is set on “ animal class ” to display the animal class display from fig5 . a 10 - 20 kg dog will be class “ 3 ”, the knob is turned 10 ° clockwise to highlight the numerical animal class function number 3 which results in the various high / low default limits shown in fig6 . enter button is then pressed which now selects the default settings ( for class number 3 ) and changes the display screen to fig6 . turning the knob 5 ° will increment by one value resulting in the display value being 1 . thus turning the knob to approximately 55 ° clockwise will set the value to 11 ( a 15 kg dog ). the knob can be rotated counter - clockwise to decrement the values . again the “ enter ” button is pressed which records and accepts the value . at this point all the values on the data enter display will change to the default values for a 15 kg dog . the highlighted box will move to the run box assuming the “ enter ” button will be pressed to accept all the default values and change the display to fig7 — if a run display of particular value is to be changed , eg . warning tone to off , the knob is turned either clockwise or counter - clockwise to the desired box . pressing enter will toggle the value ( to on / off etc ) and move the selection to next value ( left to right , top to bottom ). when all values on the data enter display are set and run is entered , the display changes to the run display . in the pause mode ( fig7 ), the display will be inverse . all data enter values will be display on the run display format . run is selected by turning the knob counter - clockwise approximately 10 ° and “ enter ” button pressed . the monitor will now start to function , updating the display approximately every 15 seconds , showing heart rate , flow , and moving the flow marker above or below the base value . at any time during operation the knob can be turned to highlight any value on the run display . during the procedure , the base value may need to be adjusted . such as with re - positioning the patient for surgery . turn the knob to highlight the base value eg 2 . 0 fig3 , press enter , turn the knob ( clockwise or counter - clockwise ) to display the desired base flow , then press enter . the monitor will accept the new base flow number and readjust the high / low limit bars . with regard to the embodiments discussed above , the output signal from the doppler transducer is a signal the amplitude and / or frequency of which varies depending upon the rate of blood flow in the peripheral vessel being monitored . as discussed above , the signal can therefore be processed by the micro computer 4 to control a display to give an output indicative of changes in total blood flow as the changes in blood flow in the peripheral vessel correlate with changes in total cardiac output ( co ). in a clinical situation , such as during anaesthesia in surgery , the accuracy of this correlation is important , i . e ., it is important that the displayed changes correlate well with the actual changes in cardiac output or tissue oxygen delivery . if the display gives an inaccurate reading , particularly in the critical range ( i . e ., in the region of the alarm levels ) then information given to the anaesthetist can be misleading and ultimately lead to a dangerous situation . the present applicants have found that the accuracy of the correlation between the changes in the output signal from the doppler transducer and changes in cardiac output can be much improved by further processing of the signal to adjust the signal by a factor which is based on regression analysis of actual experimental subjects . they have also found that the correlation can be even further improved by adjusting the processed signal by employing a co - variant factor , in the preferred embodiment being heart rate . adjustment of the signal using these factors preferably leads to a more accurate output and the microprocessor is preferably arranged to process the signal from the doppler transducer by including adjustments based on these factors . fig9 is a schematic plot of “ perfusion index ” in relation to cardiac output ( co ) or tissue oxygen delivery , for a notional experimental subject , to illustrate how regression analysis may be applied in accordance with this embodiment of the invention . perfusion index is a term the applicants have chosen to represent the processed output of the doppler device ( or where another device is being used to monitor blood flow , the output from that device ). the processed signal from the doppler device , which is a voltage output proportional to doppler frequency change , whether it be amplitude or frequency , provides an output known as the perfusion index . ideally , this output will be directly proportional to cardiac output or tissue oxygen delivery ( curve a of fig9 ). during anaesthesia , high inspired amounts of oxygen are applied so that the arterial oxygen content is relatively constant . changes in cardiac output can be taken to be substantially the same as changes in tissue oxygen deliver , therefore , in these circumstances . the ideal , unfortunately , is not the case . from experiments with subjects , however , it is possible to plot perfusion index against co or tissue oxygen delivery , by monitoring cardiac output with another device arranged to directly monitor cardiac output , and by applying a device such as a doppler monitor to monitor “ perfusion index ”, on an experimental subject , to give a realistic plot , plot a in fig9 . the equation for the curve is : where y is in this case cardiac output or tissue oxygen delivery , x is perfusion index , a is the slope and b is the intercept ( see fig9 ). by adjusting the output of the doppler device by modifying it by a factor corresponding to a and b , i . e ., modifying it by using a regression analysis employing a experimental subject , a more accurate correlation of perfusion index ( i . e ., the new adjusted perfusion index ) with cardiac output or tissue oxygen deliver can be obtained . in the preferred embodiment , therefore , the micro computer 4 is arranged to modify the output of the doppler receiver 3 by a factor relating to the regression analysis . this has been found to provide a much improved output , i . e ., a more accurate indication of the cardiac output . in application , therefore , regression analysis is carried out by a monitoring perfusion index against cardiac output or tissue oxygen delivery for a plurality of subjects . the results of the regression analysis are then used to calculate a weighting factor to be applied to the output from the doppler monitor , by the device in accordance with the embodiment of the present invention , in order to adjust that output to create a more accurate output indicative of cardiac output or tissue oxygen delivery . in the example given in fig9 , a and b are calculated and y with the new adjusted output , is produced in accordance with the formula y = ax + b . note that tissue oxygen delivery = tissue blood flow ( cardiac output ) x arterial oxygen content . a further improvement to the correlation of perfusion index to cardiac output can be made by further modifying the output signal from the doppler transducer by making an adjustment for a co - variate factor . there are therefore a number of variants which influence cardiac output and which may also determine the accuracy of an output signal from the doppler monitor . the applicants have found that , in patients anasethetised for surgery , including a co - variate factor based on heart rate also results in an increase in the accuracy of the final output of the device . a co - variate factor relating to mean arterial pressure does not improve the output and in fact degrades it . preferably , therefore , in accordance with the preferred embodiment of the invention , the output of the doppler monitor is also adjusted by applying a co - variate factor , based on the heart rate of the patient . again , a number of experimental subjects are monitored to see what variation of the output of the doppler monitor ( perfusion index ) occurs with pulse rate . a weighting factor is then applied to the output from the signal in accordance with detected heart rate for a patient , to further improve the response of the device . a further modification which may be made to the device is to process the output to provide an indication of the “ trend ” of the output and also provide a display of the trend . all measurements are stored periodically , for example every one to five seconds , and a display which gives the direction that the output is taking , i . e ., either up or down , is provided for the anaesthetist . this “ trend ” display can be useful in anaesthesia , and will generally provide more direction to an anaesthetic as far as anaesthetic dose required is concerned , than a straight forward “ number ” display not indicating any trend . as discussed above , the preferred doppler device to be used with the present invention is a continuous wave doppler . these are preferably cheap , easy to build and portable . in operation , the ultra sound beam is transmitted from one crystal and the reflected wave received by another . the change in frequency of the reflected signal is in part due to the velocity of the red blood cell flow . the change in the amplitude of the signal depends on the vessel , distance and tissue density differences . vessel wall motion alters the high amplitude , of the signals which influences the shape of the amplitude / time spectrum of the reflected wave . this problem can be minimised by using doppler crystals with higher sound frequencies ( 8 to 10 mhz ). in addition use of front end clutter filters designed to optimise the illumination of reflected sound from skin , subcutaneous tissue and fat can be employed , and this is preferred . since the amplitude and time lay of the reflected noise depends on the depth and size of the blood vessel being analysed , the filters are preferably specific for either body size ( e . g ., adult human , child or neonate ) or species ( e . g ., cat , dog , horse ). a toggle switch preferably enables the operator to select the desired clutter filter ( not shown in the figures ). the change in time difference between the reflected signal from the proximal and distal wall of the blood vessel can be analysed and will indicate changes in blood vessel diameter . an estimate of blood vessel diameter combined with the estimate of velocity of blood flow , can be used to give index of blood flow , which can be modified in accordance with the factors discussed above to give the desired output ( perfused index ) which accurately correlates with cardiac output . as discussed in the preamble of the specification , other devices which are capable of monitoring blood flow could be used instead of continuous wave dopplers . as discussed above , a pulse oximeter may also be used to provide a monitoring device in accordance with the present invention . pulse oximeters are currently designed to measure the transmission of red and infra - red light from haemoglobin of the arterial blood and estimate the arterial oxygen saturation . however , changes in the reflective wavelength of the light from the tissue bed depend on : b . changes in the total mass of tissue including red blood cells . once a pulse oximeter is functioning on a patient , it assumes that the background tissue and blood “ mass ” is constant ( fixed ), it focuses on the pulsatile part of perfusions or blood flow wave form and therefore assumes that changes in the wavelength of the light are due to changes in oxygenation . typically during anaesthesia , patients breath high inspired concentrations of oxygen . therefore , changes in light absorption are far more commonly due to changes in the mass of red blood cells ( i . e ., the assumed to be constant light absorption ) than to changes in arterial oxygenation . to modify a pulse oximeter , we need to work form the principle that using two light wavelengths ( one in the visible red spectrum and one in the infra - red spectrum ): at the isobestic wavelength , the absorbing power of oxyhaemoglobin in the reduced haemoglobin is the same . therefore total absorbency depends only on the sum of the two and not the state of oxygenation . therefore the total absorbency depends only on the total amount of blood present . as tissue blood flow increases or decreases , the total absorbency at the isobestic point will change and this can be used to give a measure of the relative change in blood ( mass ) flow in the tissue bed . such a device can therefore be used to monitor changes in blood flow in peripheral tissue beds . electromagnetic flow meters have been designed to be surgically implanted around large blood vessels such as the aorta and renal artery . it is possible that such a device may be adapted to be placed around a peripheral tissue bed , such as a finger or tail , to provide an indication of relative changes in blood flow . this may not be accurate , however . there is no reason that an electromagnetic flow meter could not be used in the present invention , by implantation of a cuff type flow meter around a blood vessel . this is , however an invasive technique , and although it falls within the scope of the present invention it is not preferred . other available devices which could be adapted in accordance with the present invention are non - invasive optical flow meters . these devices measure the absorption characteristics of light scattered by blood flowing through tissues such as skin surface , detecting this reflected light , analysing the frequency of the wave forms to obtain the mean peak light frequencies in estimating blood flow . problems with this approach are that the device only measures very superficial ( i . e ., skin surface ) blood flow , which during anaesthesia is altered by vaso constriction such as caused by changes in body temperature . the device is also subject to movement artefacts / vibration such as caused by patient positioning , movement by surgical manipulations , restorations , vibrations from re - circulating water beds , etc . it is therefore difficult to get a continuous measure from a wave ( pre - anaesthesia ) through to anaesthesia when positioned for surgery . further , the signal requires considerable damping to get a stable measurement , which sacrifices the accuracy of the “ real time ” measurement . it also relies on estimating the doppler signal change in the scattered light to obtain the peak frequency and fails to measure perfusion of deeper tissues . nevertheless , although not preferred , it is quite possible that such a device could be used in the present invention . the above description is of a relatively sophisticated device which can be used with the method in accordance with the present invention . as discussed in the preamble , a primitive device , in the form of a “ colour chart ” can also be used . colours indicating various flow rates would be established by clinical trials for various species in order to produce the colour chart . an anaesthetist will then have reference to the colour chart and compare with the colour of the part of the body concerned such as the oral mucosa , in order to monitor flow rate in the subject . an example colour chart is schematically illustrated in fig8 . it will be appreciated by persons skilled in the art that numerous variations and / or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit and scope of the invention as broadly described . the present embodiments are , therefore , to be considered in all respects as illustrative and not restrictive .
the present invention relates to a method and apparatus for monitoring haemodynamic function in animals and humans during anaesthesia and surgery . during anaesthesia and surgery the subject &# 39 ; s haemodynamic , respiratory , neuromuscular and neurological functions are monitored as indicators of the condition of the health of the subject . commonly , variations in blood pressure are used to imply corresponding variations in cardiac output , i . e . good blood pressure equals good cardiac output . the present invention utilizes a device to monitor changes of blood flow in peripheral blood vessels of the subject as an indicator of cardiac output . this is believed to provide a much more accurate indicator .