Cardiac output is a central part of the hemodynamic assessment in patients having heart disease, acute hemodynamic compromise or undergoing cardiac surgery, for example. Cardiac output is a measure of the heart's effectiveness at circulating blood throughout the circulatory system. Specifically, cardiac output (measured in L/min) is the volume of blood expelled by the heart per beat (stroke volume) multiplied by the heart rate. An abnormal cardiac output is at least one indicator of cardiovascular disease.
The current standard method for measuring cardiac output is the thermodilution technique (Darovic, G. O. Hemodynamic monitoring: invasive and noninvasive clinical application. 2nd Ed. W. B. Saunders, 1995). Generally, the technique involves injecting a thermal indicator (cold or heat) into the right side of the heart and detecting a change in temperature caused as the indicator flows into the pulmonary artery.
Typically, the thermodilution technique involves inserting a flow-directed balloon catheter (such as a Swan-Ganz catheter) into a central vein (basilic, internal jugular or subclavian) and guiding it through the right atrium and ventricle to the pulmonary artery. The balloon catheter is typically equipped with a thermistor near its tip for detecting changes in blood temperature. A rapid injection of a bolus of chilled glucose solution (through a port in the catheter located in the vena cava near the right atrium) results in a temperature change in the pulmonary artery detected with the thermistor. The measured temperature change is analyzed with an external electronic device to compute the cardiac output. The algorithm implemented in this computation is typically a variant of the Stewart-Hamilton technique and is based on the theory of indicator mixing in stirred flowing media (Geddes L A, Cardiovascular devices and measurements. John Wiley & Sons. 1984).
Thermodilution measurements of cardiac output are disadvantageous for several reasons. First, placement of the thermodilution balloon catheter is an expensive and invasive technique requiring a sterile surgical field. Second, the catheter left in place has severe risks to the patient such as local infections, septicemia, bleeding, embolization, catheter-induced damage of the carotid, subclavian and pulmonary arteries, catheter retention, pneumothorax, dysrrhythmias including ventricular fibrillation, perforation of the atrium or ventricle, tamponade, damage to the tricuspid values, knotting of the catheter, catheter transection and endocarditis. Third, only specially trained physicianss can insert the balloon catheter for thermodilution cardiac output. Last, thermodilution measurements of the cardiac output are too invasive to be performed in small children and infants.
Another method used for measuring cardiac output is the dye indicator dilution technique. In this technique, a known volume and concentration of indicator is injected into the circulatory flow. At a downstream point, a blood sample is removed and the concentration of the indicator determined. The indicator concentration typically peaks rapidly due to first pass mixing of the indicator and then decreases rapidly as mixing proceeds in the total blood volume (˜10 seconds; first pass concentration curve). Further, indicator concentration slowly diminishes as the indicator is metabolized and removed from the circulatory system by the liver and/or kidneys (time depending upon the indicator used). Thus, a concentration curve can be developed reflecting the concentration of the indicator over time. The theory of indicator dilution predicts that the area under the first pass concentration curve is inversely proportional to the cardiac output.
Historically, indicator dilution techniques have involved injecting a bolus of inert dye (such as indocyanine green) into a vein and removing blood samples to detect the concentration of dye in the blood over time. For example, blood samples are withdrawn from a peripheral artery at a constant rate with a pump. The blood samples are passed into an optical sensing cell in which the concentration of dye in the blood is measured. The measurement of dye concentration is based on changes in optical absorbance of the blood sample at several wavelengths.
Dye-dilution measurements of cardiac output have been found to be disadvantageous for several reasons. First, arterial blood withdrawal is time consuming, labor intensive and depletes the patient of valuable blood. Second, the instruments used to measure dye concentrations (densitometer) must be calibrated with samples of the patient's own blood containing known concentrations of the dye. This calibration process can be very laborious and time consuming in the context of the laboratory where several samples must be run on a daily basis. Further, technical difficulties arise in extracting the dye concentration from the optical absorbance measurements of the blood samples.
A variation on the dye-dilution technique is implemented in the Nihon Kohden pulse dye densitometer. In this technique, blood absorbance changes are detected through the skin with an optical probe using a variation of pulse oximetry principles. This variation improves on the prior technique by eliminating the necessity for repeated blood withdrawal. However, as described above, this technique remains limited by the difficulty of separating absorbance changes due to the dye concentration changes from absorbance changes due to changes in blood oxygen saturation or blood content in the volume of tissue interrogated by the optical probe. This method is also expensive in requiring large amounts of dye to create noticeable changes in absorbance and a light source producing two different wavelengths of light for measuring light absorption by the dye and hemoglobin differentially. Even so, the high background levels of absorption in the circulatory system makes this technique inaccurate. Finally, where repeat measurements are desired, long intervals must ensue for the high levels of the indicator to clear from the blood stream. Thus, this technique is inconvenient for patients undergoing testing and practitioners awaiting results to begin or alter treatment.
Other approaches for measuring cardiac output exist which are not based on indicator dilution principles. These include ultrasound Doppler, ultrasound imaging, the Fick principle applied to oxygen consumption or carbon dioxide production and electric impedance plethysmography (Darovic, supra). However, these techniques have specific limitations. For instance, the ultrasound techniques (Doppler and imaging) require assumptions on the three-dimensional shape of the imaged structures to produce cardiac output values from velocity or dimension measurements.
Blood volume measures the amount of blood present in the cardiovascular system. Blood volume is also a diagnostic measure that is relevant to assessing the health of a patient. In many situations, such as during or after surgery, traumatic accident or in disease states, it is desirable to restore a patient's blood volume to normal as quickly as possible. Blood volume has typically been measured indirectly by evaluating multiple parameters (such as blood pressure, hematocrit, etc.). However, these measures are not as accurate or reliable as direct methods of measuring blood volume.
Blood volume has been directly measured using indicator dilution techniques (Geddes, supra). Briefly, a known amount of an indicator is injected into the circulatory system. After injection, a period of time is allowed to pass such that the indicator is distributed throughout the blood, but without clearance of the indicator from the body. After the equilibration period, a blood sample is drawn which contains the indicator diluted within the blood. The blood volume can then be calculated by dividing the amount of indicator injected by the concentration of indicator in the blood sample (for a more detailed description see U.S. Pat. No. 6,299,583 incorporated by reference). However, to date, the dilution techniques for determining blood volume are disadvantageous because they are limited to infrequent measurement due to the use of indicators that clear slowly from the blood.
In the dye dilution method, the dye must be injected as a rapid intravenous bolus, not as a continuous infusion, as the latter does not result in the characteristic dye dilution curve needed for the calculation of cardiac output. Choice of the injection method and volume of the injectate are relevant to measured cardiac output and the variability of sequential measurements of cardiac output obtained with transcutaneous fluorescence dye dilution. The venous system targeted by the injection is characterized by branching veins and venous valves. These present an inherent resistance to injection that contributes to a potential fragmentation of the bolus, as well as to a pooling and delayed release of any residual dye. These can be noted, respectively, by fluctuations in the morphology of the dye dilution curve and a prolongation of the tail of the dye bolus.