Catheter identifier and method

Disclosed is an automatic means for entering into a cardiac output computer a computation constant for a given size catheter. The technique involves the placement of a capacitor across the output terminals of a thermodilution catheter assembly. This capacitor does not effect temperature measurements. By means of software controlled switching in the cardiac output computer circuity, the capacitance of this capacitor can be determined and can be used to indicate the catheter size. Disclosed also is the process used to set up and identify the catheter gauge.

BACKGROUND OF THE INVENTION 
This invention relates to thermodilution based methods for determining 
cardiac output. More specifically, to the technique used for identifying 
the particular type of the thermodilution catheter assembly connected to a 
cardiac output computer. 
Thermodilution catheters have been used to determine cardiac output and 
these catheters are typically small diameter balloon types equipped with 
distal temperature sensing means and a lumen opening a short distance 
proximal to the temperature sensor for introduction of a low-temperature 
liquid injectate into the blood stream. The change in temperature 
resulting from the introduction of the low temperature injectate is sensed 
by the temperature sensing means, usually a thermistor. The magnitude and 
duration of the temperature change over time can be used to compute the 
blood flow rate for a measure of a patient's cardiac output. U.S. Pat. No. 
3,995,623 shows a typical thermodilution catheter. 
The blood flow rate is computed from the change in blood temperature 
according to the Stewart-Hamilton dilution equation for a thermal 
indicator as described in U.S. Pat. No. 3,987,788. As per that prior 
patent, numerical values are used for a computation constant, blood 
temperature, and injectate temperature. The computation constant is 
derived from the nature of the injectate, the volume of the injectate and 
a correction factor for the rise in temperature of the injectate as it 
passes through the lumen of the catheter to the injectate orifice. It is 
the size of the lumen in the catheter which must be known in order for the 
correct computation constant to be applied. With modern microprocessor 
technology, the computation constant for any given set of operating 
conditions can be programmed into the cardiac output computer by the 
manufacturer and it, therefore, becomes possible to automatically enter 
any variables affected by the catheter once the catheter has been 
identified. 
In most cases, thermodilution blood flow measuring techniques are applied 
with a standard 5 percent glucose solution and one of a few of the 
standard injectate volumes for a given size catheter. The instructions for 
use of the computer can then specify that the injectate type and volume be 
entered. These required entries are known to the operator. The computation 
constant, on the other hand, must be looked up in the data that accompany 
the catheter and manually entered in the computer. Both the look-up and 
entering operations are prone to error. 
In the prior art, there is U.S. Pat. No. 4,407,298 which discloses a 
thermodilution catheter identifier system having a specifically designed 
connector with a plurality of electrically conductive pins. The pins 
having connective bridge members to couple them selectively and thereby 
indicate the catheter size. In that system, three extra terminals are 
needed to accomplish a binary coded number of 1 to 4 in order to provide 
the requisite signal which indicates the size of catheter used. That 
system requires a terminal with five connector pins and although more 
complex has only four codes available. 
It is accordingly an object of the present invention to provide an 
automatic means for entering the computation constant for a given size 
catheter injectate and injectate volume and temperature into the cardiac 
output computer when the catheter is connected. A manual override is 
provided in the event that a catheter of a non-standard size or a catheter 
which is not designed to automatically indicate its type by identifying 
itself is used with the cardiac output computer of the present invention. 
SUMMARY OF THE DISCLOSURE 
In order to solve the problems of the prior devices in a simplified manner, 
and to provide a technique for identifying catheters without the need for 
extra connector terminals, there is shown and disclosed herein a device 
which automatically configures the cardiac output computer in accordance 
with the catheter size used. 
The present disclosure seeks to teach a system wherein no additional pins 
are required in the connector because of circuitry provided at the distal 
end of the catheter and responsive circuitry in the cardiac output 
computer. Notwithstanding these changes, the thermistor in the 
thermodilution catheter still functions with the cardiac output computer 
without interference from the identification and automatic setting 
circuitry. 
In the present thermodilution type catheter assembly, the temperature 
sensing element, being a thermistor, is matched to a resistor network 
during the manufacture of the catheter assembly. The network, which may 
consist of a single resistor, is connected to the temperature sensing 
thermistor in order to maintain a constant resistance ratio 
notwithstanding manufacturing variations which cause operational 
tolerances in the thermistor. That is to say that, the resistance value of 
thermistor at a given temperature say, for example, 37.degree. C. and that 
of the network resistance must be carefully matched because thermistor 
resistance values at that temperature may vary by as much as plus or minus 
15 percent. An accurately maintained ratio between the network resistor 
and thermistor resistance at a given temperature assures linearity of the 
temperature measurement in spite of the fact that the thermistors used for 
temperature measurement usually exhibit a non-linear relationship between 
their resistance value and the measured temperature. 
In operation the thermistor and the resistor are each connected to the 
input terminals of two operational amplifiers which are part of the 
analogue front end of the cardiac output computer. These input terminals 
are maintained at equal voltages by each operational amplifier as same 
respond to current changes at their inputs. Specifically the input 
terminals are maintained at the half-way voltage level of the analogue 
front end which is zero volts. This level could be referred to as 
"analogue ground", but in reality is insulated from actual or earth 
ground. The dynamic resistance between the terminals that power the 
thermistor and resistor and analogue ground is very low, only on the order 
of a few ohms. Connected to the junction of the thermistor and resistor is 
a constant current source which supplies a fixed current to the thermistor 
and the network resistor. Across the input terminals which are at the 
half-way voltage is a capacitor of a predetermined value for each catheter 
type. The capacitor will not affect the normal temperature measuring 
operation of the thermistor resistor network circuit in the catheter 
assembly because the voltage difference between the terminals is very 
small and stays so over the entire operating temperature range. Typically 
the voltage difference at the input terminals will be about 0.001 volt and 
varies less than that. 
A typical cardiac output computer is equipped to measure the temperatures 
of blood and of a mixture of blood and injectate from thermistor data 
received from the end of the catheter. The computer described in this 
invention has in addition to the blood temperature measuring circuitry the 
capability of performing catheter identification. When the cardiac output 
computer is arranged, under software control, to measure the blood 
temperature, the instrumentation amplifiers are configured as already 
described. When it is required to measure the capacitance of the 
identification capacitor incorporated in the catheter assembly circuit, 
then the terminals of the instrumentation amplifiers are configured 
differently, again under software control. 
The process of identification is in two phases; the first set up, the 
second identification. During the set-up phase, a series of switches, 
which are normally closed, are changed simultaneously to the open 
condition. This disconnects the catheter from the constant current source; 
prevents the low dynamic input impedance at the resistor terminal from 
exceeding its dynamic range while at the same time allowing the input 
impedance at the thermistor terminal to reach a high value. Consequently, 
the thermistor terminal will attain a voltage slightly higher than that of 
the resistor terminal. This is so because a resistor in the bridge circuit 
charges the capacitor through the voltage divider formed thereby in 
conjunction with the thermistor and the matching resistor. The resistor 
terminal is kept essentially at analogue ground. 
A comparator detects this charging condition by virtue of the fact that it 
responds to very small differences in voltage between its inverting and 
noninverting inputs. Because the terminal for the thermistor is at a 
higher potential than the terminal for the resistor, the output of the 
comparator will be high. The set up duration is long enough to assure that 
the capacitor is fully charged. 
The identification phase begins after the microprocessor has allowed 
sufficient charge duration. A switch opens which changes the circuit to 
charge the capacitor in an opposite direction. At the same instant that 
switch opens, a timer is started. The thermistor terminal voltage drops at 
a rate determined by the value of the capacitor and the sum of the values 
of the thermistor and the network resistor. The comparator detects the 
condition where the voltage across the capacitor is essentially zero. At 
this point in time, the output of the comparator goes from high to low and 
stops the timer. The amount of time measured by the timer is an indication 
of the particular type of catheter used as it is a measure of the unique 
value of the capacitor selected for that type of catheter. 
Because this system is temperature dependent as it includes the thermistor, 
a range of capacitor discharge times are assigned for each type of 
catheter and the selection of particular capacitors is made in accordance 
with the understanding of this effect. Various capacitor values are 
selected which will space the particular types of catheters far enough 
apart in the timing sequence to make each easily recognizable.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1, the temperature sensing circuit 10 is shown as same would appear 
in a typical thermodilution catheter. In the catheter assembly the 
temperature sensing element 11 such as a thermistor is matched to a 
resistor network. In FIGS. 1, 2 and 3, the network is shown as resistor 14 
since the network can be a single resistor 14 connected to the temperature 
sensing device 11 as shown. The reason for matching resistor 14 to 
temperature sensing device 11 involves maintaining a constant resistance 
ratio in the face of manufacturing variations of the device 11, between 
the resistance values of the device 11 at a given temperature (say 
37.degree. C.) and of resistor 14. Maintaining this resistance ratio is 
necessary because devices such as temperature sensor 11 will vary as much 
as plus or minus 15 percent and an adequately maintained ratio between 
resistance 11 and 14 over a given tolerance range assures accuracy and 
linearity of temperature measurement in spite of the fact that the devices 
such as thermistor 11 usually exhibit a strong nonlinear relationship 
between their resistance value and temperature. 
FIG. 2 is representative of the circuit in which the catheter assembly 13 
is used as the input for a current responsive instrumentation amplifier 
15. Connection to the catheter assembly 13 are made at terminals 16 (for 
the thermistor branch) and 17 (for the resistor branch). Two currents of a 
predetermined ratio are injected into points 16 and 17 in FIG. 2 through 
very high value resistors 18 and 19. These currents, and others as may be 
provided by amplifier 15, leave the catheter assembly 13 at point 22 and 
are drained in the constant current source 22a which acts as a sink. All 
connections to catheter points 16, 17 and 22 are made through resistors 
with a resistance of 1 megohm or more. Some of these resistors, 20 and 21, 
are shown in FIG. 3, most are not shown. This practice is followed to 
allow failures in active devices like integrated circuits without exposing 
the catheter assembly 13 to high leakage currents. 
In addition to measurement of temperature, circuits for catheter 
identification are also connected across terminals 16 and 17 of catheter 
assembly 13. These circuits operate at different times and independently 
of one another. 
For normal cardiac output operation the temperature range for a patient on 
which same is used can be expected to be 26.degree. to 43.degree. C. so 
the center of that range is 34.5.degree. C. The junction 22 of temperature 
sensing device 11 and resistor 14 is connected to the constant current 
source 22a. When the temperature at the thermistor 11 is 34.5.degree. C. 
no currents flow into or out of current responsive instrumentation 
amplifier 15. This is so because the ratio for resistors 18 and 19 matches 
that of the resistances of catheter assembly 13. Moreover the value of the 
resistances used makes the voltage at terminals 16 and 17 halfway between 
plus and minus five volts. If the temperature is not at 34.5.degree. C. 
then the ratio of resistances 11 and 14 does not match the ratio of 
resistance 19 and 18. Current excesses or shortages are made up by 
amplifier 15. Therefore, current flowing into or out of the current 
responsive instrumentation amplifier is indicative of the temperature 
difference at thermistor 11 of the catheter assembly 13. Over the 
operating range of 26.degree. to 43.degree. C. the currents in and out of 
current responsive instrumentation amplifier 15 are essentially 
proportional to the temperature at the thermistor 11. 
In FIG. 1, the temperature sensing circuit 10 is shown as it would appear 
in a typical catheter. The matching resistor network is a single resistor 
14. This resistor is chosen so that if the temperature of thermistor 11 is 
37.0.degree. C., the ratio of the resistance values of thermistor 11 and 
resistor 14 are optimum for the purpose of linearizing the non-linear 
response to temperature of thermistor 11. That means that, if the 
resistance of thermistor 11 at 37.0.degree. C. is about 10 percent lower 
than nominal, then the resistance of resistor 14 would also be 10 percent 
lower than its value would be if it were to match a nominal value. 
In FIG. 2 is shown the preferred method in which the circuit of FIG. 1 is 
used in the front end of a cardiac output computer. Terminal 22 is 
connected to the constant current source 22a which ultimately drains all 
current flowing through thermistor 11 and resistor 14 to the minus 5 volt 
rail of the analogue power supply. 
The currents through thermistor 11 and resistor 14 are sourced by the plus 
5 volt rail of the analogue power supply. The ratio of the resistance 
values of resistor 19 and 18 matches that of the resistances of thermistor 
11 and resistor 14 when thermistor 11 is at 34.5.degree. C. 34.5.degree. 
C. is the midway point of the temperature range for which most cardiac 
output computers are designed, i.e, 26.degree. to 43.degree. C. At that 
temperature the voltage at terminal 16 will be exactly halfway between the 
plus 5 volt rail and the minus 5 volt rail of the analogue power supply. 
This halfway point is also referred to as analogue ground. Similarly the 
resistance of resistor 18 is such that the current through resistor 18 
equals the current through resistor 14 when thermistor 11 is at 
34.5.degree. C. and the voltage at terminal 17 is also exactly at analogue 
ground. The input terminals 16 and 17 of the current responsive amplifier 
15 are maintained at analogue ground by the amplifier circuit. Therefore, 
when thermistor 11 is at 34.5.degree. C., no current will flow into or out 
of the terminals 16 and 17 of amplifier 15. 
If thermistor 11 is not at 34.5.degree. C. but between 26.degree. C. and 
43.degree. C., currents will flow into or out of the terminals 16 and 17 
of amplifier 15 and these currents are essentially proportional to the 
difference between the actual temperature of thermistor 11 and 
34.5.degree. C. 
In FIG. 3, additional details of the temperature measuring circuits are 
shown, as well as details of the identification circuits and the means by 
which the circuit can be switched from measuring the temperature of 
thermistor 11 to determining the identity of the catheter assembly 13. To 
permit determination of the identity of catheter assembly 13 a capacitor 
25 has been added to catheter circuit 13. This capacitor 25 will have a 
unique value that corresponds to the type of catheter. That is to say 
that, each catheter assembly 13 type is associated with a particular value 
of capacitor 25. 
As mentioned before, amplifier 15 maintains a voltage equal to analogue 
ground at its input terminals 16 and 17. This is accomplished as follows. 
Input terminals 16 and 17 are each connected to the inverting input of 
operational amplifiers 15a and 15b. These inputs draw essentially no 
current. A resistor is connected between the inverting input of each 
amplifier to its output. Therefore, any current that flows into or out of 
the inputs to amplifiers 15a or 15b, actually flows into or out of the 
resistor that connects to the respective amplifier 15a or 15b output. If 
this current causes the inverting input to differ in voltage from the 
non-inverting input, the output of the amplifier 15a or 15b swings in a 
direction that will maintain essentially zero volts between the inverting 
and non-inverting inputs. Because the non-inverting input is connected to 
analogue ground, the inputs 16 and 17 are maintained at analogue ground. 
The outputs of amplifiers 15a and 15b are combined in a conventional 
analogue adding circuit (not shown) to produce a single voltage that is 
essentially proportional to the difference between the temperature of 
thermistor 11 and 34.5.degree. C. 
In spite of the high resistance values used in the instrumentation 
amplifier 15 input stages, the impedance levels at terminals 16 and 17 are 
very low and the voltage levels are, as explained, exactly halfway between 
the plus and minus 5 volts. It should be emphasized that this "halfway" 
voltage level, which could be referred to as analogue ground, is in 
reality very well insulated from actual ground, such as may be represented 
by a metal cold water pipe. In addition, terminals 16 and 17 are not 
directly connected to the halfway point, referred to as analogue ground, 
but through very high resistance values. However, the dynamic resistance 
between terminals 16 and 17 and analogue ground is very low, only on the 
order of a few ohms. The voltages at terminals 16 and 17 are very nearly 
equal to within a fraction of a volt (i.e., 0.001 volts), and input 
impedances of the instrumentation input stages as measured at terminals 16 
and 17 are very small (i.e. 10 ohms). 
FIG. 3 shows the specifics of the invention. Capacitor 25 connected between 
terminal 16 and 17 of the catheter assembly 13 does not affect normal 
temperature measurement operation of the catheter assembly 13 during a 
thermodilution cardiac output analysis. This is so because the voltage 
difference between terminals 16 and 17 is very small and stays so over the 
entire thermodilution temperature range. The low dynamic input impedances 
of the current responsive instrumentation amplifiers 15a and 15b create a 
time constant when combined with capacitor 25 that is very short when 
compared to the rate of which the blood temperature changes. 
The identification of the catheter size is done by means of charging and 
discharging (charging in the opposite direction) of capacitor 25. The time 
for discharge is directly related to the capacitor value and each size of 
catheter includes a specifically valued capacitor 25. The time for 
discharge or decay of the full charged capacitor 25 will vary with 
temperature of thermistor 11, but the variance is within a known range for 
any specifically selected capacitor value. Each different value of 
capacitor 25 can be spaced on a time diagram such that its time for decay 
values for the entire temperature range is well separated from the range 
of any other selected capacitor 25. For the present and preferred 
embodiment this approach is adequate for the number of catheter types 
which need to be identified. 
Each different gauge of catheter includes a specifically predetermined 
value for the capacitor 25, shown in FIG. 3. The capacitor 25 works in 
combination with the resistances of 11 and 14 to form a time constant. A 
different capacitance value will be read as a different decay time 
interval, and although the time interval for a specific gauge will vary 
with temperature; the range of variance can be accommodated. The 
particular gauges can be determined without concern for the variations due 
to temperature and tolerances. In the preferred embodiment that is the 
means by which a catheter gauge is identified. Should there be the need to 
determine a greater number of catheter gauges then an approach in which 
the temperature of thermistor 11 is also determined and applied to achieve 
a reduction in the variation of the time constant due to temperature can 
be used. In the present situation that is not necessary. 
FIG. 3 shows that four switches, 26, 27, 28, and 29 are connected in the 
circuit. When the circuit must serve to measure the temperature of 
thermistor 11 all switches are closed as shown. When it is desired to 
determine the identity of the catheter, switches 27, 28 and 29 are opened 
simultaneously; switch 26 is retained as shown. 
The opening of switches 27, 28 and 29 results in the following changes. 
Switch 27 disconnects the current flowing through resistor 18, which 
assures that amplifier 15a will not be overloaded and will be able to 
maintain terminal 17 at analogue ground. Switches 28 and 29 disconnect the 
catheter assembly 13 from the rest of the analogue circuit allowing the 
current through resistor 19 to charge capacitor 25. The final voltage to 
which capacitor 25 is charged is determined by the voltage divider formed 
by resistor 19; thermistor 11 and resistor 14. 
Switch 26 is arranged to be either connected to plus 5 volts or to minus 5 
volts. In the former condition, current flows through resistor 19 for a 
period of time controlled by the digital section microprocessor circuit 30 
which includes the counter or timer 24. Timer 24 permits current to flow 
for a specified time period t.sub.o sufficient to permit current flow 
through capacitor 25 until same is fully charged and has reached its 
equilibrium, see FIG. 4. At equilibrium terminal 16 will assume a small 
voltage (typically 30 millivolts) above the voltage at terminal 17. 
In the preferred embodiment, this period of time t.sub.0 is approximately 
0.2 seconds. After the full charge of capacitor 25 has been attained, 
digital section microprocessor circuitry 30 moves switch 26 to 5 volts 
negative and starts the timer 24 from a reset condition. Timer 24 measures 
the amount of time t.sub.1 minus to for the charged capacitor 25 to 
discharge to zero volts, see FIG. 4. The time required for this discharge 
is indicative for the particular capacitor value and therefore of the 
catheter gauge because the value of the capacitor 25 is such that same is 
specifically selected for each given catheter gauge. 
A comparator 23 is used to stop the timer 24 when the discharge curve, FIG. 
4, passes through zero volts at t .sub.1, after decay of the charge of 
capacitor 25. The comparator 23 has a high common mode rejection ratio to 
help ignore the presence of noise such as hum, from the transduced signal 
of catheter assembly 13 as seen at terminals 16 and 17. The voltage across 
capacitor 25 does not change due to hum and therefore the comparator 23 is 
accurate even with the presence of noise. 
When the voltage at terminal 16 is equal to that at terminal 17, this 
condition can be detected by the comparator 23. When the voltage at 
terminal 16 drops below the voltage at terminal 17, the output of the 
comparator 23 goes from high to low and stops a timer 24. Timer 24 is 
connected to the output 23a of comparator 23. The value read on the timer 
24 is indicative of the type of catheter assembly 13 used. Each type of 
catheter assembly 13 has a capacitor 25 connected between terminals 16 and 
17 and the value of capacitance for capacitor 25 is unique for each type 
of catheter assembly 13. 
It is to be expected that for a given type of catheter assembly 13, the 
indicative time interval will vary with the temperature as sensed by 
temperature sensing device 11 and with the value of resistor 14 as the 
result of the matching process. Therefore, it is necessary to assign a 
range of times for each size of catheter assembly 13 and to space the 
values of capacitance of capacitor 25 (used for each size) far enough 
apart so that each catheter assembly 13 type can be positively recognized 
in spite of these expected variables. The preferred factor of 3 allows for 
all these variables, but different applications may require another 
factor. 
In particular, the time needed to discharge the capacitor 25 can be 
calculated knowing the resistances of the temperature sensing device 11 
and the resistor 14 and the capacitance of capacitor 25. The following 
chart shows the differences in the time function as a result of different 
capacitances. 
______________________________________ 
Added Capacitance Approximate Time 
Picofarads (pF) (Microseconds) 
______________________________________ 
0 &lt;100 
10,000 180 360 
33,000 600 1200 
100,000 1800 3600 
330,000 6000 12000 
______________________________________ 
Zero added capacitance will be a competitive catheter or a catheter for 
which no constants have been included in the software. The four 
capacitance values given above can be assigned to four different catheter 
gauges as follows: 
______________________________________ 
Catheter Gauge: 1 
Capacitance .01 
microfarad 
2 .033 microfarads 
3 .1 microfarads 
4 .33 microfarads 
______________________________________ 
Once this determination has been made, the time measured by timer 24 can be 
used as an input signal to the current responsive instrumentation 
amplifier 15 and more specifically to set the value of catheter lumen size 
into the cardiac output computer of which amplifier 15 is a portion. 
The invention consists of the addition of a capacitor 25 between terminals 
16 and 17 of the catheter assembly 13 and the additional of 
instrumentation circuitry and software to allow the measurement of a time 
constant created by the capacitor 25 under special circumstances. These 
special circumstances are set up by switches 26 through 29 (see FIG. 3 for 
placement of these switches in the preferred circuit). The input signals 
at comparator 23 from terminals 31 and 32 are measured only as a 
difference between them thus effectively filtering any hum from the 
catheter assembly 13. An optical coupler 33 is connected to the output 23a 
as a safety device to electrically isolate the catheter assembly 13 from 
the digital section microprocessor circuitry 30. 
While a specific and preferred embodiment has been shown and described for 
use in connection with thermodilution catheters and cardiac output 
computers, skilled artisans will no doubt appreciate that the technique 
disclosed herein can be used in connection with other equipment so long as 
it is understood that the timing of a capacitor discharge is the requisite 
element used for identification of the nature of the remote element. More 
specifically, this technique can be used in conjunction with other 
circuitry without the need of additional connecting terminals, wires, 
leads and the like. Thus, a simplified approach to automatic 
identification is provided without the addition of a complex network or 
wiring schemes and without interference with the measurement of the 
transduced signal, in this case a thermodilution wave form. In the claims 
which follow, the basic concept is sought to be protected apart from the 
specific preferred disclosure.