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.