Left ventricular pressure, P(t), and outflow Q(t), data were collected in anesthetized, open-chest rats and dogs. The data were used in a three-tiered validation procedure to evaluate 14 competing forms of elastance [E(t)]-resistance (R) left ventricle (LV) pump models. Competing models arose from considering two forms of parameterization of E(t), time variation versus no time variation in LV unstretched volume (Vd) and dependence versus no dependence of R on P(t) and isovolumic P(t). A descriptive test based on the normalized root-mean-square errors in the fit to P and, separately, in the fit to Q was used to distinguish between models. The best of the competing models was the one that treated Vd as a function of time and R as a constant. Models of this form fitted the data very well and were said to be descriptively valid. The best of the competing models were then asked to predict the observed responses to changes in afterload, preload, and prior-beat history. The models did not predict these conditions well and failed to pass the test for predictive validity. Additionally, the model parameters were judged not to represent their supposed physical homologs and, thus, failed the test for explanative validity. One cause for E(t)-R model failure was an inadequate representation of events at end systole. This deficiency was apparently due to not accounting for deactivation in the model. Other features may also be needed before a comprehensive LV model can be formulated. Identical conclusions were made from data from the rat and the dog.
An asymmetric T-tube model of the arterial system with complex terminal loads was formulated in the time domain. The model was formulated to allow it to be fitted to the aortic pressure waveform, the aortic flow waveform, or simultaneously to both the aortic and descending aortic flow waveforms. Pressure and flow measurements were taken in anesthetized open-chest dogs under basal, vasoconstricted, and vasodilated states. It was found that the T-tube model fitted the data well in all formulations and in all vasoactive states. However, all parameters were estimated accurately in all vasoactive states only with the formulation that fitted to both aortic and descending aortic flow simultaneously. The T-tube model was compared with the three-element windkessel model with regard to the respective models' ability to recreate specific aspects of the pressure waveform and with regard to the estimates of global arterial parameters. The T-tube model recremated those features of the pressure waveform, such as diastolic waves, that the windkessel model could not. Also, the T-tube model systematically estimated lower global arterial compliance and higher characteristic impedance than the windkessel. It was argued that the T-tube model accurately represented important wave transmission features of the arterial loading system. The model is recommended for use in characterizing the arterial load and for merging with representations of the left ventricle in studies of left ventricle-systemic arterial interaction.
Elastance-resistance [E(t)-R] representations of the left ventricle (LV)were evaluated for their ability to reproduce instantaneous pressure [P(t)J and outflow [Q(t)J. Experiments were performed in open-chest rats. P(t) and Q(t) were measured during steady-state ejecting beats and during a beat in which the aorta was suddenly clamped. The degree of clamping varied from partial to total occlusion. The total occlusion beat was considered an isovolumic beat that generated an isovolumic pressure [Pis,(t)J with a characteristic time to maximal P1,0(t) [Tpisomaxl. In ejecting beats, 34% of stroke volume was delivered after Tpisomax. P(t) and Q(t) from the steady-state ejecting beats and P,so(t) from the clamped beat were then used to estimate parameters of an E(t)-R model. Components of P(t) and Q(t) not accounted for by E(t)-R were identified and termed extra-pressure [Pext(t)I and extra-outflow [Qe,x(t)]. Pext(t) and Qext(t) were near-zero valued until Tpisomax; then they became systematically positive and finally negative valued after end ejection. During partial aortic occlusion, P(t) was elevated and Q(t) was reduced. However, the time of ejection was extended, and the fraction of stroke volume delivered after Tpisomax increased as P(t) was made higher. Partial occlusion also prolonged the positive phase of PeX,(t) and Qe,d(t). Elements possessing "active" and "deactive" properties were added to the E(t)-R model in an attempt to account for Pe,xt(t) and Qext(t) during partial occlusion.Optional forms of these elements were considered. These expanded E(t)-R models were fitted to basal ejecting data and then asked to predict data from a partial occlusion beat. All expanded models failed to adequately predict the partial occlusion pressure and/or outflow. It was concluded that 1) late ejection was quantitatively important to LV pumping, 2) behavior during late ejection was inconsistent with E(t)-R, and 3) ad hoc modification of E(t)-R models was not likely to yield LV pumping models that could satisfactorily reproduce instantaneous P(t) and Q(t) behavior over the entire ejection period. (Circulation Research 1990;66:218-233) It has become popular to relate instantaneous left ventricular pressure [P(t)], volume [V(t)], and outflow [Q(t)] by using mathematical models based on time-varying elastance [E(t)] and resistance (R).1-8 Such representations are appealing because, if valid, they allow identification of instantaneous left ventricular (LV) pump properties from data recorded from only one or two heart beats. In previous study,3 however, we found serious deficiencies in the performance of E(t)-R models: 1) Errors in the prediction of P(t) increased progressively as end ejection was approached. 2) E(t)-R models failed to predict P(t) and Q(t) data other than that to which these models were fitted. 3) The estimated parameter values of the E(t)-R model were unrealistic estimates of physical entities the parameters supposedly represented. 4) Interdependencies between parameters were found and resulted in nonuniq...
A congenital syndrome of long, thin limbs, severe joint and tendon laxity, microspherophakia, ectopia lentis, heart murmurs and aortic dilatation was identified in 7 calves. All affected calves were sired by a single phenotypically normal bull suspected of germline mosaicism for a new mutation resulting in this disease. One of the calves subsequently died with ruptured aorta at age 16 months. Histopathologic and electron microscopic studies of the aortic media of affected calves demonstrated disorganized elastin and narrowed elastic lamina separated by widened spaces. This bovine disease provides a unique animal model of the human Marfan syndrome. A herd of cattle with this disease is being developed for further studies.
The relation between reflected waves and features of ascending aortic pressure waveforms and impedance patterns was investigated with a modified T-tube model of the systemic arterial circulation. Ascending aortic pressure and flow and descending aortic flow were measured in 10 dogs under basal conditions and under the effect of an agent (methoxamine) that caused vasoconstriction and an increase of mean aortic pressure. A broad range of aortic pressure amplitudes and features was obtained. These waveshapes were classified into four groups. Under basal conditions, cases for which a prominent diastolic fluctuation was present (n = 8) were grouped in A. Cases for which this fluctuation was absent (n = 2) were grouped in B. Groups C (n = 4) and D (n = 3) included cases that, under vasoconstricted conditions, did or did not display, respectively, a diastolic fluctuation in pressure. Arterial T-tube model parameters were estimated by simultaneously fitting the model to both ascending and descending aortic flow with aortic pressure as input. A good fit was obtained in any case considered. After parameter estimation, forward and reflected waves and impedance patterns at the entrance of head circulation (head and upper limbs) and body circulation (trunk and lower limbs) as well as their merger in the ascending aorta were determined. T-tube input impedance compared well with impedance data points obtained from the ratio of corresponding harmonics of ascending aortic pressure and flow. In some cases (group A), modulus and phase spectra displayed two distinct minima, in the range from 0 to 10 Hz. In some other circumstances, these minima were less distinct (groups B and C) and could even appear as one (group D). Whether one or two minima appeared in the ascending aortic impedance spectra at low frequency and whether a prominent diastolic fluctuation did or did not appear in aortic pressure, pressure and flow waveshapes proximal to the heart were explained by the presence of two effective reflecting sites in the systemic circulation. In group B, a diastolic fluctuation in pressure was absent despite the fact that head-end and body-end reflected waves were distinct. This happened because body-end reflected waves peaked corresponding to a minimum of the head-end reflected wave. In group D, a diastolic fluctuation in aortic pressure was absent because the body-end reflected wave moved into systole and superimposed on the head-end reflected wave. This superimposition was due to increased pulse wave velocity in the body transmission path as a result of decreased arterial distensibility.(ABSTRACT TRUNCATED AT 400 WORDS)
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