the following description should be read with reference to the attached drawings , in which like elements are numbered identically . where specific materials , dimensions , manufacturing processes and techniques for usage are described , those skilled in the field of the invention will recognize them as exemplary . however , suitable alternatives may also be utilized . as shown in fig1 the preferred embodiment of the present invention comprises an insulated elongated guidewire shaft 10 measuring about 70 inches ( 180 cm ) long and 0 . 008 inches ( 0 . 20 mm ) outside diameter ( o . d .) of spring steel wire or an alloy with a degree of flexibility and steerability that is suitable for a coronary artery intervention guidewire . a distal segment of about 1 . 3 inches ( 3 cms ) in length is tapered to about 0 . 003 inches ( 0 . 075 mm ) o . d . a radio opaque spring coil 37 ( fig1 and 3 ) surrounds this distal segment and is attached at both of its ends to the guidewire shaft by welding or soldering as indicated at 14 and 16 . in the preferred embodiment , three radio opaque reference markers 18 , 20 and 22 ( fig1 and 3 ), each about 2 mm wide , are affixed at equal intervals of 15 , 25 or 50 mm along the distal segment of the guidewire shaft 10 , beginning at a distance of about 5 to 10 mm proximal to its spring tip 12 . this segment of the guidewire shaft is then coated to a thickness of about 0 . 0008 inch ( 0 . 020 mm ) with a flexible medical grade adhesive polymer coating to serve as an electric insulating spacer 24 ( fig2 ). three thermal sensors t 26 , t 28 and t 30 ( fig1 and 3 ) are mounted over this insulating material 24 at the sites of the equally spaced radio opaque markers 10 , 11 and 12 , using an adhesive epoxy 32 ( fig2 ). in the preferred embodiment , each of the three thermal sensors comprises of a flattened thermal measuring junction of copper - constantan thermocouple ( type t ). these miniature thermal sensors do not obstruct the flow in small vessels and are considered ideal for measuring changes in localized body areas . their extremely small mass provides an accurate , fast response thermal sensor that is not affected by pressure changes . the thermal sensors may be formed by fusing , electron beam welding or simply by soldering together the two adjacent wires with a high quality solder , conductive silver ink or silver epoxy to form a common junction 34 ( fig2 ). this soldering joint should preferably extend for about 2 to 3 mm along the two adjacent wires , to cover the wires &# 39 ; circumferential winding around the thermally and electrically insulated stainless steel wire shaft 10 and radio opaque marker 12 ( fig2 ). the high thermal conductivity of silver - containing solder helps to convey the average temperature level around the guidewire to the site of the thermal sensor common junctions . for an enlarged view of one of these thermal junctions , showing its placement over a radio opaque marker , the application of insulating materials and the circumferential winding of the wires jointed at the thermal junction , refer to fig2 . two insulated conductor lead wires , measuring 0 . 0016 inch ( 0 . 04 mm ) o . d ., of copper 36 , 38 and 40 and constantan 42 , 44 and 46 in fig1 a ( 40 and 46 are also shown in fig2 ) extend from each thermal junction as adjacent helical coils which spiral the length of the guidewire shaft 10 . in the preferred embodiment , the guidewire shaft and the helical lead wires 36 - 46 are first coated with two thin layers of medical grade polymer 24 and 48 ( fig1 and 2 ), starting beyond the proximal sleeve electrodes and ending before the distal segment . coating the helical coil wires levels out the grooves between the wires . more importantly , this coating , fortified by the helical lead wires , acts as a thin tube , adding pushability and steerability to the thin guidewire shaft . the proximal segment is then coated with a close fit insulated hypotube or sheath with a wall thickness less than 0 . 0015 inch ( 0 . 04 mm ) 50 ( fig1 a , 6 and 7 ), and extending as a thin hydrophilic , silicone or a polytetrafluoroethylene ( ptfe ) coating over the distal , more flexible section 66 ( fig1 a , 6 and 7 ). sliding a hypotube 65 over the proximal section offers further protection to the helical lead wires by reducing their exposure to damaging abrasions , and provides additional pushability and steerability for advancing the guidewire into the coronary vasculature . the distal flexible coating provides a smoothly gliding surface over the distal segment of the guide - wire shaft carrying the helical lead wires and the three thermal sensors . the lead wires 36 - 46 terminate at the proximal 15 cm end segment of the guidewire shaft as three paired electrodes of copper and constantan e 54 c and e 56 k , e 58 c and e 60 k , and e 62 c and e 64 k ( fig1 a and 4 ). in the preferred embodiment , these electrode pairs take the form of sleeve electrodes as shown in fig4 a and b , with each pair spaced to match a prospective clip - on electrode connector plates 64 p , 66 p and 68 p ( fig4 b ). an alternative type of electrode connector 70 , with a cylindrical receptor that would allow freer steering of the guidewire , is envisaged in fig1 b , 3 a , 6 and 7 . the six sleeve electrodes e 54 c and e 56 k , e 58 c and e 60 k , and e 62 c and e 64 k are externally connectable through extension cables 72 , 74 and 76 ( fig3 b and 5 ) to three respective reference junctions 78 , 80 and 82 ( fig3 b ) that are maintained at a constant temperature medium . further external connections of the device are illustrated in fig5 . these include a servomotor - driven infusion pump 84 which feeds into the guiding catheter 86 used to position the guidewire within the coronary artery . the pump is adjusted to inject a slow steady infusion of 1 - 3 ml / sec room temperature saline at the ostium of the coronary artery over a period of 10 - 15 seconds . the external connections of the device preferably should also include a manual or automatic feedback circuit 88 to adjust the rate of saline or other miscible infusion so as to produce a standard average temperature gradient , whether at basal conditions or during measures of coronary flow reserve . output from thermal sensors t 26 , t 28 and t 30 ( fig . 1 ) and their reference junctions 78 , 80 and 82 ( fig3 b ) is transmitted to three separate thermocouple amplifiers 90 , 92 and 94 ( fig3 b and 5 ), a high speed color - coded monitor 96 ( fig5 ), a recorder 98 , and an online programmed microprocessor , or computer 100 with its associated printer 102 . the computer should be programmed to calculate various values derived from the measured transmit time of coronary blood flow passing between each pair of thermal sensors , according to the formulae described below . a push button , electronically calculated standard 104 serves to calibrate the temperature gradient induced by the saline infusion . ideally , electrocardiographic , intra - aortic pressure and blood pressure tracings , as well as the patient &# 39 ; s breathing , should be continuously monitored throughout the procedure described below . in the preferred embodiment , the thermodilution guidewire is introduced through a judkin &# 39 ; s guiding catheter 86 ( fig5 ) into the ostium of the coronary artery , and the guidewire &# 39 ; s distal segment is steered forward through the coronary branch of interest . if there is a segment with suspected coronary stenosis , the flexible radio opaque spring tip of the guidewire 37 is gently manipulated through this segment and pushed forward for 3 - 5 cms to reach the distal coronary branches . baseline temperature level of blood flow through the coronary artery is first detected by the three sequentially mounted thermal sensors t 26 , t 28 and t 30 , which are electrically connected to their three respective d . c . amplifiers 90 , 92 and 94 . output from these amplifiers appears as three separate tracings on the color monitor 96 , which should preferably be a fast - sweep monitor with adjustable speed up to 1000 mm / sec . a manual or automatic zero suppress adjusts the three readings closer together . use of the push button predetermined standard 104 , which is connected online with the three thermal sensor amplifiers , produces a standard deflection equivalent to 0 . 1 - 1 . 0 degrees centigrade below baseline temperature level of coronary blood flow . utilizing the constant rate perfusion pump 84 connected to the guiding catheter 86 , a slow , steady rate perfusion of room temperature ( approximately 22 degrees c .) normal saline or ringer &# 39 ; s solution is started at 0 . 5 - 2 . 0 ml / sec and continued for 10 - 15 seconds according to the request of the operator . this constant rate saline infusion flows from the tip of the guiding catheter at the coronary ostium into the coronary artery at a temperature about 12 - 15 degrees centigrade below that of the coronary blood flow . mixing between the steady , slow saline infusion and the phasic , pulsatile flow of the coronary blood induces a temperature gradient within 0 . 5 degree centigrade that flows downstream to the distal epicardial coronary branches at the same velocity as coronary blood flow . rhythmic changes of the phasic pulsatile coronary blood flow with each phase of the cardiac cycle modulate the degree of thermodilution induced by the steady slow infusion , creating periodic oscillations of temperature gradient . these oscillations that are produced by the characteristic biphasic coronary flow pattern with its large diastolic and small systolic components , simulate rectified since waves , as illustrated by tw 26 , tw 28 and tw 30 in fig8 and 10 . these cyclical temperature changes reflect the different degrees of dilution of the cool saline infusion in the relatively warm coronary blood during the different phases of the cardiac cycle with the less diluted diastolic phase producing warmer temperatures than that of the more fully diluted systolic phase . the size of these waves do not affect the accuracy of the proposed method , since the method is based on the transit time of the phase shift between successive waves as detected by the sequentially located thermal sensors . in addition , the length of intervals between the diastolic and systolic components provides a useful indication of the relative size of the two components . each phase of the simulated rectified sine waves of temperature gradients is detected in sequence by the three serially mounted thermal sensors t 26 , t 28 and t 30 as it arrives at the site where the sensor is mounted . in normal coronary flow , these waves are expressed as three consecutive rectified sine waves fig8 with two equal transit times between them expressed as phase shifts . the degree of phase shift between recorded waves is an expression of the transit time of the average flow velocity of coronary flow at the time of measurement . v = δ   s δ   t where v = velocity , s = the distance between each pair of thermal sensors , and t = transit time . using this formula the online computer 100 generates the mean blood flow velocity during a single cardiac cycle . since room temperature infusion extends over a predetermined measuring time of 10 - 15 seconds covering several cardiac cycles , the average flow velocity during the measuring procedure is also determined . values of volume flow may be calculated when the angiographically determined vessel diameter is supplied to the online programmed microprocessor according to the following equation : q = π 4 × d × δ   s δ   t where q is calculated when the diameter d is measured by angiography . the thermal time constant of the fast response thermal sensors in a running fluid medium such as the bloodstream is less than 20 milliseconds . this thermal time constant affects all the consecutive oscillations equally and therefore has no effect on the transit time of the phase shift between them . thorough mixing of the infused saline and the coronary flow is not essential since it is the transient time between identical phases and not the degree of temperature gradient which is of crucial value . similarly , insignificant thermal conduction through the arterial walls does not affect the transit time of temperature changes . determination of coronary flow reserve may be carried out by repeating room temperature saline infusion after increasing the rate of coronary flow by means of pharmacologically - induced maximal dilatation of the coronary arteries . under basal conditions normal coronary flow velocity induces a standard range of transit time between consecutive waves , and is accurately calculated from the degree of phase shift between consecutive waves ( fig8 ). during measures of coronary flow reserve , the increased flow velocity produces narrow phase shifts denoting short transit times . the degree of narrowing is inversely proportional to the size of coronary flow reserve . referring to fig9 the increased rate of coronary flow is also expressed by smaller temperature gradients , since the infused saline is diluted in the increased volume of coronary flow reserve . post stenotic reduction of the mean velocity prolongs the transit time between successive waves and is expressed by widening of the phase shifts between them ( fig1 ). evaluation of the degree of coronary stenosis which is a valuable parameter in decision making is determined by the degree of widening of the phase shift at basal conditions and during measures of coronary reserve . for optimal patient care , this procedure may be repeated to evaluate the immediate and late outcome of coronary intervention . the forward direction of coronary flow is indicated by the order of colors shown on the color coded monitor 96 . the order of colors would be reversed in certain types of coronary stenosis with reversed flow from adjacent collateral vessels . after the saline infusion is stopped , the trailing part of the transient temperature drop with its characteristic rectified sine wave pattern returns either gradually or rapidly to its original straight baseline level . the rate of its return to its original baseline level is an indication of the transmyocardial flow velocity , which may reveal functional or structural alterations in the microvascular circulation . fig1 illustrates that copper and constantan lead wires 36 - 46 may also extend beyond the thermal sensors t 26 , t 28 and t 30 to distally in a continued helical coil winding until the beginning of the spring tip 12 . these extensions of the helical coil lead wires beyond the thermal sensors serve to maintain the smooth surface of the distal segment of the guidewire , which will assist easy introduction of the guidewire through the distal branches of the coronary artery . in addition , these distal extensions may help to secure the thermal sensors in position during the winding process . the extensions have no electric function and do not interfere with the function of the thermal sensors . after the thermal sensors and adjacent wires leads have been fixed to the insulated stainless steel shaft , the distal extensions may be electrically interrupted by any cutting instrument . the terminal comprising 3 mm of these helical coils and the proximal end of the radio opaque spring tip may be fixed to the guidewire shaft with an anchor ring 40 ( fig1 ) of epoxy resin or polytetrafluoroethylene ( ptfe ). the foregoing description of the preferred embodiment of the present invention applies to a guidewire with two intervals between three thermal sensors . the advantage of having two intervals is to allow simultaneous comparative measures along both intervals . a simple guidewire designed on the same principles but with a single interval separating two thermal sensors , could be less expensive to manufacture , but would not provide two simultaneous measures , rendering it less useful for comparative studies . it is envisaged that a more complex guidewire could comprise three intervals between four thermal sensors . by positioning the proximal interval before and the distal after a stenotic lesion , this configuration would provide the advantage of determining simultaneously the blood flow velocity both proximal from and distal to a coronary stenotic segment . the middle interval could provide further information about the velocity of blood flow through the stenosed segment . in the preferred embodiment of the present invention , type t copper and constantan thermocouples are used as thermal sensors . other types of thermal sensors may be used when preferred . a significant advantage of using thermocouples is that they require no electric current to be introduced along the lead wires . this is an important consideration , given the unpredictable flow velocity which a stenosed segment of the coronary artery may experience , and its sensitivity to the heat generated by the electric current utilized by other types of thermal sensors . alternate types of thermal sensors may take the form of thin film of platinum foil , or of suitable sized thermistors configured as resistor thermal sensors over one arm of a three lead thermometer bridge . in the description of the preferred embodiment of the present invention , the most distal of the three sequentially mounted thermal sensors is located just proximal to the terminal radio opaque spring copil . when desired , this thermal sensor may be placed at the tip of the radio opaque spring coil ( not shown ). this location of the distal thermal sensor at the extreme tip of the guidewire offers the advantage of providing values of average flow velocity along more distal coronary branches and collateral vessels , but it may limit the guidewire &# 39 ; s much - needed high degree of flexibility . it is further contemplated that the helical coils of the thermal sensor lead wires described in the preferred embodiment could be replaced with different types of coated conductor paths of the same thermocouple materials running the length of the guidewire shaft . fig6 and 7 illustrate one type of alternative conductor paths applied to both two and three sensor configurations of the device . in this version , alternative successive copper and constantan conductor film coatings would be painted , sprayed or printed over an insulated guidewire shaft . each conduction film would be insulated from the next by a thin film of insulating ink , except at both ends of the conductor film , where they would form sequentially placed thermal sensors at the guidewire &# 39 ; s distal segment f 118 , f 120 , f 122 and sleeve electrodes e 124 c , e 126 k , e 128 c , e 130 k , e 132 c , e 134 k at its proximal segment . assuming each of the conductor films and their insulating layers do not exceed 0 . 01 mm , they would have a total thickness of 0 . 06 mm adding 0 . 12 mm on both sides of an insulated guidewire shaft about 0 . 15 mm o . d . giving a total thickness of 0 . 27 mm . a close fit thin coating of 0 . 04 mm thickness 65 giving a total thickness of 0 . 35 mm ( 0 . 014 inch ) to the guidewire , would protect these films over most of their length , except the distal 25 mm segment , where the hypotube would be extended as a thin flexible polymer coating 66 . if practicable , this arrangement would offer considerable advantages in ease of manufacturing .
the system for measuring the velocity of blood flow in the coronary artery , comprises a guidewire adapted for coupling with conventional coronary intervention devices and has two or more temperature sensors mounted at equally spaced intervals along its distal segment . with the guidewire positioned at a point of interest in the artery , a steady infusion of a room - temperature saline solution is injected into the blood stream at the coronary ostium , lowering the local blood temperature slightly . warmer pulsatile flow from the aorta mixes with this inflow , producing phasic temperature oscillations that are detected in sequence by the guidewire &# 39 ; s temperature sensors . the elapse of time between the detected phase shifts indicates the velocity of the flowing blood . an external monitoring system connected to the guidewire provides instant read - outs of this value , which may be secured before , during and after intervention procedures , or after induced coronary hyperemia to determine coronary reserve .