Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-10-27
2002-05-28
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S505000, C600S549000
Reexamination Certificate
active
06394961
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to medical procedures and apparatus to perform medical procedures to determine the flow rate of blood. In particular, the invention relates to the determination of cardiac output characteristics for diagnosis purposes.
BACKGROUND OF THE INVENTION
The determination of cardiac output, or measurement of the blood volumetric output of the heart is substantially important for a variety of medical situations. Healthcare professionals utilize such information along with a number of additional pulmonary factors to evaluate the condition of their subject's heart. Even with the variety of approaches developed for measuring this output, each exhibit certain limitations and/or inaccuracies. The volumetric aspect of cardiac output provides information about the sufficiency of oxygen delivery to tissue or the oxygenation of the tissue. When used in combination with other measurements it provides important status and evaluation information of the cardiovascular system.
Methods for determining cardiac output as the thermodilution method are discussed, for example in U.S. Pat. Nos. 3,651,318, 4,217,910, and 4,236,527. As conventionally employed, this method involves either injecting a bolus of liquid into the bloodstream at a temperature which is cooler or warmer (usually cooler) than blood temperature, or heating a segment of the blood indirectly with electrical resistance heaters, and monitoring the temperature deviation of the blood as a function of time at a place downstream from the place at which the temperature deviation is caused. The area under the resulting temperature deviation vs. time curve (known as the thermodilution curve) is a measure of the rate at which the heart is pumping blood (usually expressed in liters per minute). If cardiac output is high, the area under the thermodilution curve will be relatively small in accordance with the well-known Stewart-Hamilton relationship. Conversely, if cardiac output is low, the area under the thermodilution curve will be relatively large.
Currently, the more accepted approach for deriving cardiac output values is an indicator dilution technique which takes advantage of refinements made earlier in pulmonary catheter technology. The standard of cardiac output measurement from pulmonary artery catheterization are described in for example, in U.S. Pat. Nos. 3,915,155, 3,726,269 and 3,651,318 involve periodic injection into the subject's bloodstream of a bolus, during which thermodilution measurements are performed to determine cardiac output. Such techniques cannot generally be used for continuous monitoring. Moreover, such catheterization techniques pose significant risk to the subject, including malignant arrhythmias, pulmonary artery rupture, and in rare cases, death. However since knowledge of cardiac output is crucial in the care of critically ill subjects, as well as subjects with chronic heart disease requiring monitoring of medication work has been underway to develop less invasive apparatus and methods for monitoring cardiac output.
Advances in the art now require only central venous and arterial catheters as opposed to such invasive methods. Additionally, processes and devices have been developed to determine the fill level of the circulatory system of a patient disclosed in WO93/21823. Additionally, WO93/21823 provides a process for determining the end diastolic heart volume, the pulmonary blood volume, the extravascular thermovolume and/or the global cardiac function index. Extravascular thermovolume correlates, if there is no significant perfusion defect in the lungs (e.g., pulmonary embolism), closely to the degree of extravascular lung water. However, the clinical value of that measurement has not been shown explicitly yet.
In a typical procedure, a cold bolus of saline at ice or room temperature in an amount of about 5-10 milliliters is injected through the catheter as a measurement procedure which will require about two minutes to complete. For purposes of gaining accuracy, this procedure is repeated three or four times and readings are averaged. Consequently, the procedure requires an elapsed time of 4-5 minutes. In general, the first measurement undertaken is discarded inasmuch as the catheter will have resided in the bloodstream of the body at a temperature of about 37° C. Accordingly, the first measurement procedure typically is employed for the purpose of cooling the dilution channel of the catheter, and the remaining measurements then are averaged to obtain a single cardiac output value. Thus, up to about 40 ml of fluid is injected into the intravascular system of the patient with each measurement which is undertaken. As a consequence, this procedure is carried out typically only one to two times per hour over a period of 24 to 72 hours. While practitioners would prefer that the information be developed with much greater frequency, the procedure, while considered to be quite accurate, will add too much fluid to the cardiovascular system if carried out too often.
Of course, the accuracy of the procedure is dependent upon an accurate knowledge of the temperature, volume, and rate of injection of the liquid bolus. Liquid volume measurements during manual infusions are difficult to make with substantial accuracy. For example, a syringe may be used for injecting through the catheter with the result that the volume may be identified only within several percent of its actual volume. Operator error associated with volume measurement and rate of injection also may be a problem. Because the pulmonary catheters employed are somewhat lengthy (approximately 30 to 40 inches), it is difficult to know precisely the temperature of the liquid injectate at the point at which it enters the bloodstream near the distal end of that catheter. Heat exchange of the liquid dispensing device such as a syringe with the catheter, and the blood and tissue surrounding the catheter upstream of the point at which the liquid is actually released into the blood may mean that the injectate temperature is known only to within about five percent of its actual temperature. Notwithstanding the slowness of measurement and labor intensity of the cold bolus technique, it is often referred to as the “gold standard” for cardiac output measurement by practitioners. In this regard, other techniques of determining cardiac output typically are evaluated by comparison with the cold bolus approach in order to determine their acceptability.
Another technique of thermodilution to measure cardiac output employs a pulse of temperature elevation as the indicator signal. In general, a heating coil is mounted upon the indwelling catheter so as to be located near the entrance of the heart. That coil is heated for an interval of about three seconds which, in turn, functions to heat the blood passing adjacent to it. As is apparent, the amount of heat which can be generated from a heater element is limited to avoid a thermocoagulation of the blood or damage to tissue in adjacency with the heater. This limits the extent of the signal which will be developed in the presence of what may be considered thermal noise within the human body. In this regard, measurement error will be a result of such noise phenomena because of the physiological blood temperature variation present in the body. Such variations are caused by respirations, coughing, and the effects of certain of the organs of the body itself. For further discussion see Afonzo, S., et al., “Intravascular and Intracardiac Blood Temperatures in Man,” Journal of Applied Physiology, Vol. 17, pp 706-708, 1962 and U.S. Pat. No.4,595,015.
This thermal noise-based difficulty is not encountered in the cold bolus technique described above, inasmuch as the caloric content of a cold bolus measurement is on the order of about 300 calories. By contrast, because of the limitations on the amount of heat which is generated for the temperature approach, only 15 or 20 calories are available for the measurement. Investigators have attempted to correct for the thermal noise problem th
Burger Thorsten
Pfeiffer Ulrich J.
Mallari Patricia C.
Nasser Robert L.
Pulsion Medical Systems AG
Studebaker Donald R.
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