Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-09-05
2004-07-06
Hindenburg, Max F. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C600S485000, C600S481000
Reexamination Certificate
active
06758822
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for determining the stroke volume and, hence, the cardiac output of a patient, as well as to a system that implements the method.
BACKGROUND ART
Accurate measurement of the cardiac output (CO) of a patient has proven to be a valuable diagnostic tool. Accordingly, several methods for determining CO have been developed, of which thermodilution, the direct oxygen Fick method, and the pulse contour method (PCM) are at present the most prevalent. These known methods for measuring CO, however, are affected by several drawbacks that greatly limit their application in the clinical setting as well as for purposes of research.
Measurement of CO using thermodilution, which is described, for example in Ganz, W. and Swan, H. J. C. (1972), “Measurement of blood flow by thermodilution,” Am. J. Cardiol. 29, pp. 241-246, has become routine in the hemodynamic evaluation and management of critically ill patients. As is well known, this method is based on the law of conservation of energy and on the application of the Stewart-Hamilton equation, for which a number of conditions must be fulfilled. These conditions include complete mixing of the thermal indicator with blood, no loss of indicator within the dilution volume, and constant blood flow during the dilution time.
Inaccuracy in the determination of CO may result from the inconsistency of these assumptions in many clinical conditions. In particular, variability of blood flow may occur as a consequence of hemodynamic instability related to changes in heart rate, cardiac arrhythmia, valvular or congenital heart disease, and application of mechanical ventilation. Additional limitations of the thermodilution method are its invasiveness and the impossibility of monitoring CO beat-to-beat in critical conditions and during the course of acute pharmacological interventions.
The direct oxygen Fick approach is the standard reference technique for CO measurement. See, for example, Fagard, R. and Conway, 3 (1990), “Measurement of cardiac output: Fick principle using catheterization,” Eur. Heart J. 11, Suppl. I, pp. 1-5. According to the Fick principle, CO can be determined by the ratio of oxygen uptake to the difference in oxygen content between arterial and mixed venous blood. The validity of the principle depends upon the assumption that pulmonary blood flow is approximately identical to systemic blood flow and that the lungs themselves do not extract oxygen. Although this method appears to be the most accurate among those currently available, its use is limited by a series of practical problems. These problems include the need for right heart catheterization to obtain truly mixed venous blood, the assumption of the availability of appropriate analytical techniques for measuring oxygen uptake and content, and the attainment of a steady state in which apparent oxygen consumption matches tissue oxygen utilization. The fulfillment of these conditions makes the method unsuitable for repeated measurements and, consequently, not apt to follow rapid changes in flow over time.
The pulse contour method (PCM), which has been developed from an original idea by J. A. Herd et al. dating back to 1864 and from a theory commonly referred to as the “Windkessel” (German for “air chamber”) theory of Franck (Franck O., 1930), derives CO from the arterial pressure pulse wave. The PCM method is based on the existence of a relationship between the volume of blood expelled by the left ventricle (LSV) or the volume of blood expelled by the right ventricle (RSV) and the area under the pressure curve P(t). Unlike the thermodilution and Fick methods, which measure mean CO over a limited time span, the PCM operates on a beat-to-beat basis. The primary assumption of PCM is that the pressure rise during systole is related, in a complex way, to the systolic filling of the aorta and proximal large arteries. Various approaches have therefore been devised to approximate, by means of different models of the arterial system, the relationship between aortic pressure and flow.
One of the most famous models used in PCM was developed by Wesseling and his co-workers and is described in, among many other references:
Wesseling, K. H., Dc Wit, B., Weber, J. A. P. and Smith, N. T. (1983), “A simple device for the continuous measurement of cardiac output. Its model basis and experimental verification,” Adv. Cardiol. Phys. 5, Suppl II, pp.16-52;
Wesseling, K. H., Jansen, J. R. C., Settels, J. J. and Schreuder, J. J. (1993), “Computation of aortic flow from pressure in humans using a nonlinear, three-element model,” J. Appl. Physiol. 74, pp. 2566-2573;
Jansen, J. R. C., Wesseling, K. H., Settels, J. J. and Schreuder, J. J. (1990), “Continuous cardiac output monitoring by pulse contour during cardiac surgery,” Eur. Heart J. 11, Suppl 1, pp. 26-32;
Sprangers, R. L., Wesseling, K. H., lmholz, A. L., Imholz, B. P. and Wieling, W. (1991), “Initial blood pressure fall on stand up and exercise explained by changes in total peripheral resistance,” J. Appl. Physiol. 70, pp. 523-530;
Jellema, W. T., Imholz, B. P. M., van Goudoever, J., Wesseling, K. H. and van Lieshout, J. J. (1996), “Finger arterial versus intrabrachial pressure and continuous cardiac output during head-up tilt testing in healthy subjects,” Clin. Sci. 91, pp.193-200;
Stock, W. J., Baisch, F., Hillebrecht, A., Schulz, H. and Karemaker, J. M. (1993), “Noninvasive cardiac output measurement by arterial pulse analysis compared to inert gas rebreathing,” J. Appl. Physiol. 74, pp. 2687-2693;
Harms, M. P. M., Wesseling, K. H., Pott, F., et al. (1999), “Continuous stroke volume monitoring by modelling flow from non-invasive measurement of arterial pressure in humans under orthostatic stress,” Clin. Sci. 97, pp. 291-301;
Houtman, S., Oeseburg, B. and Hopman, M. T. E. (1999), “Non-invasive cardiac output assessment during moderate exercise: pulse contour compared with C02 rebreathing,” Clin. Physiol. 19, pp. 230-237;
Jellema, W. T., Wesseling, K. H., Groeneveld, A. B. J, Stoutenbeek, C. P., Thjis, L. G. and van Lieshout, J. J. (1999), “Continuous cardiac output in septic shock by simulating a model of the aortic input impedance. A comparison with bolus injection thermodilution,” Anesthesiology 90, pp.1317-1328;
Langewouters, G. J., Wesseling, K. H. and Goedhard, W. J. A. (1984), “The static elastic properties of 43 human thoracic and 20 abdominal aortas in vitro and the parameters of a new model,” J. Biomech. 17, pp. 425-435; and
Stock, W. J., Stringer, R. C. 0. and Karemaker, J. M. (1999), “Noninvasive cardiac output measurement in orthostasis: pulse contour analysis compared with acetylene rebreathing,” J. Appl. Physiol. 87, pp. 2266-2273.
The Wesseling method is based on a model of the elastic properties of the aorta and has been found to be satisfactory under certain hemodynamic circumstances. According to the “Modelflow” method developed by Wesseling and coworkers, uncalibrated values of CO are obtained by relating the area under the pulsatile systolic portion of the pressure wave to parameters derived from a nonlinear three-element model of the arterial system. In PCM, in order to establish a relationship between pressure and flow, the mechanical properties of the arteries, as function of arterial pressure, are approximated either by several empirical formulae or by using a model based on age- and sex-predicted values not directly pertaining to the subject under study.
The three elements of the model used in Modelflow are aortic characteristic impedance (i.e., the relationship between the rise of pressure in the aortic root in opposition to the flow of blood ejected from the left ventricle), arterial compliance (i.e., the relationship between changes in blood volume and changes in pressure in the aorta), and peripheral vascular resistance (i.e., the relationship between mean pressure and mean flow). The first two elements of the model—impedance and compliance—depend mostly on the elastic properties of the aorta. In Modelflow, these elements are predicted by an experimentally derived arcta
Hindenburg Max F.
Natnithithadha Navin
Young & Thompson
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