Cerutti, C., M. P. Gustin, P. Molino, and C. Z. Paultre. Beat-to-beat stroke volume estimation from aortic pressure waveform in conscious rats: comparison of models. Am J Physiol Heart Circ Physiol 281: H1148-H1155, 2001.-Several methods for estimating stroke volume (SV) were tested in conscious, freely moving rats in which ascending aortic pressure and cardiac flow were simultaneously (beat-to-beat) recorded. We compared two pulse-contour models to two new statistical models including eight

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... ameters extracted from the pressure waveform in a multiple linear regression. Global as well as individual statistical models gave higher correlation coefficients between estimated and measured SV (model 1, r ϭ 0.97; model 2, r ϭ 0.96) than pulse-contour models (model 1, r ϭ 0.83; model 2, r ϭ 0.91). The latter models as well as statistical model 1 used the pulsatile systolic area and thus could be applied to only 47 Ϯ 17% of the cardiac beats. In contrast, statistical model 2 used the pressure-increase characteristics and was therefore established for all of the cardiac beats. The global statistical model 2 applied to data sets independent of those used to establish the model gave reliable SV estimates: r ϭ 0.54 Ϯ 0.07, a small bias between Ϫ8% to ϩ10%, and a mean precision of 7%. This work demonstrated the limits of pulse-contour models to estimate SV in conscious, unrestrained rats. A multivariate statistical model using eight parameters easily extracted from the aortic waveform could be applied to all cardiac beats with good precision. cardiac output; hemodynamics; statistical model ESTIMATION OF CARDIAC OUTPUT from the arterial pressure (AP) waveform is an interesting challenge, but it remains difficult to accurately achieve. Since the 1970s, several groups have regularly proposed new methods or improvements of already-published strategies (1, 2, 6, 7, 21, 22) . Most of these have been implemented in humans, owing to a clear clinical interest in the noninvasive assessment of cardiovascular function. In animals, the assessment of cardiac output from the AP waveform is of great interest for physiological or pharmacological experiments. Several techniques have been developed for continuous measurement of cardiac blood flow using electromagnetic or pulsed Doppler flow probes placed around the ascending aorta. However, these methods require critical open-chest surgery and a long recovery period after the surgery. Only a few old works have described methods that use pulsewave analysis in dogs (8) or in rats (5, 13, 14) via AP measured in the ascending aorta. Recently, Yang and Kuo (24) used a similar model in anesthetized rats via the femoral AP. Most of the methods developed for humans are based on pulse-contour analysis, which relies on the windkessel arterial model, and several groups have proposed analytical models linking AP and cardiac blood flow (16, 21, 22) . In the windkessel model, pulsatile systolic area and stroke volume (SV) are related by means of the characteristic impedance of the aorta (Z aort ). Several methods have been proposed for estimating Z aort (2, 6, 8, 22) . Many clinical applications have been made using aortic (22), radial (7, 19, 22), brachial (1), or finger (2, 17, 23) pressure waveforms. However, the validation of models in humans is quite difficult owing to the existing techniques for measuring cardiac output. Gold standard methods are thermodilution or dyedilution methods, which cannot provide continuous measurement of cardiac output. In addition, in clinical research projects, data are generally recorded in controlled conditions during short periods of time. It appeared interesting to manage a project with rats because of the possibility of recording and processing beat-to-beat data over long periods of spontaneous activity and continuously measuring cardiac blood flow. The aim of the present work was to test the validity of pulse-contour methods in conscious unrestrained rats for a beat-to-beat analysis using AP values that were measured in the ascending aorta and recorded during several hours. We compared two methods derived from human hemodynamic models that use pulse-contour analysis to two new methods that use multivariate statistical models including a large number of parameters extracted from the arterial pulse contour. METHODS Animals Experiments were performed with male Sprague-Dawley rats (Iffa Credo; L'Arbresle, France) that weighed 300-400 g and were housed in controlled conditions (21 Ϯ 1°C and a