Monitoring total circulating blood volume and cardiac output

Details Monitoring total circulating blood volume and cardiac output Monitoring total circulating blood volume and cardiac output

1999-07-26

2001-10-09

Jastrzab, Jeffrey R. (Department: 3762)

Surgery

Diagnostic testing

Cardiovascular

C600S341000, C600S479000

Reexamination Certificate

active

06299583

ABSTRACT:

BACKGROUND OF THE INVENTION
The determination of cardiac output, or measurement of the blood volumetric output of the heart is of substantial importance for a variety of medical situations. Intensivists utilize such information along with a number of additional pulmonary factors to evaluate heart patients within intensive care units. A variety of approaches have been developed for measuring this output, all of which exhibit certain limitations and/or inaccuracies. In effect, the volumetric aspect of cardiac output provides information as to the sufficiency of oxygen delivery to the tissue or the oxygenation of such tissue. When combined with other measurements, an important evaluation of the status of the cardiovascular system of a patient may be achieved.
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. With the indicator dilution approach, a signal is inserted into the blood upstream from the pulmonary artery, and the extent of signal dilution can then be correlated with stroke volume or volumetric output of the heart. Of these indicator dilution methods, thermodilution is the present technique of choice, and in particular, that technique employing a cold liquid injectate as the signal This approach necessarily is invasive, requiring placement of a Swan-Ganz type pulmonary artery catheter such that its tip or distal end functions to position a temperature sensor just beyond the right ventricle within the pulmonary artery. The indicator employed is a bolus of cold isotonic saline which is injected from the indwelling catheter into or near the right atrium. Downstream blood temperature then is monitored to obtain a dilution curve relating temperature deviation to time, such curves sometimes being referred to as “wash out” curves. Combining the area under this thermodilution curve with the amount of energy subtracted by cooling of the blood provides a measure of the rate at which the heart is pumping blood, such rate usually being expressed in liters per minute. If cardiac output is high, the area under the thermodilution curve for a given applied energy, Q, 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 for a given amount of applied energy, Q will be relatively large. See in this regard:
Ganz, et al., “A New Technique for the Measurement of Cardiac Output by Thermodilution in Man,”
American Journal of Cardiology
, Vol. 27, April, 1971, pp 392-396.
In a typical procedure, a cold bolus of saline at ice or room temperature in an mount 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 pulmonary 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 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. See in this regard:
Afonzo, S., et al.., “Intravascular and Intracardiac Blood Temperatures in Man,”
Journal of Applied Physiology
, Vol. 17, pp 706-708, 1962.
See also, 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 deviation approach, only 15 or 20 calories are available for the measurement. Investigators have attempted to correct for the thermal noise problem through the utilization of filtering techniques, for example, utilizing moving averages over 6 to 12 readings. However, where such corrective filtering approaches are utilized, a sudden downturn in the hemodynamic system of a patient will not be observed by the practitioner until it may be too late. The effective measurement frequency or interval for this technique is somewhat extended, for example about 10 minutes, because of the inaccuracies encountered. In this regard, a cardiac output value is achieved only as a consequence of a sequence of numerous measurements. In general, the approach does not achieve the accuracy of the above-discussed cold bolus technique. Thermodilution techniques involving the use of electrical resistance heaters are described, for example, in U.S. Pat. Nos. 3,359,974; 4,217,910; 4,240,441; and 5,435,308.
Other approaches to the elimination of an injectate in thermodilution procedures have been, for example, to introduce the thermal signal into the flowing blood by circulating a liquid within the catheter, such liquid preferably being cooler than the blood temperature. See in this regard, U.S. Pat No. 4,819,655

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