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R Burattini

Publications and source records attributed to R Burattini.

At least 19 recordsLinked to original sources
PubMed2002

Comparative analysis of aortic impedance and wave reflection in ferrets and dogs.

Our modified version of the T-tube arterial model (consisting of two parallel, loss-free transmission paths terminating in lumped loads of complex and frequency-dependent nature) was applied to experimental measurements of ascending aortic pressure and of ascending and descending aortic flows taken from dogs and ferrets. Our aim was to provide quantitative evaluation of the aortic pressure and flow pulse wave components as they relate to the distribution of arterial properties and relate to wave travel and reflection in mammalians of consistently different size and shape. Estimated effective lengths (distances to effective reflection sites) of the head-end (d(h)) and body-end (d(b)) transmission paths were approximately 12 and 30 cm, respectively, in the dog and 6.5 and 13 cm, respectively, in the ferret. These lengths and distributions of estimated arterial properties were consistent with the difference in the body size and with the more central location of the heart in the ferret's body than it is in the dog's body. In both animal species the ascending aortic pressure and flow waves could be interpreted in terms of forward and reflected components arising from the two distinct effective reflection sites, although the higher d(h)/d(b) ratio in the ferret determined the presence of one broad, indistinct minimum in the modulus of ascending aortic impedance in the frequency range from 0 to 10 Hz, rather than two distinct minima as observed in the dog.

Animals↗
PubMed2000

Insulin sensitivity and glucose effectiveness estimated by the minimal model technique in spontaneously hypertensive and normal rats.

This study was performed to compare glucose metabolism in anaesthetised spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) in an attempt to clarify whether this animal model of hypertension approximates the insulin-resistant state seen in human hypertension. With this aim the minimal model of glucose kinetics was applied to glucose and insulin data derived from a 12-sample, 120 min intravenous glucose tolerance test (IVGTT) performed in ten SHR and nine WKY rats under pentobarbital anaesthesia. This method provided two metabolic indices: the glucose effectiveness, S(G), which quantifies the ability of glucose per se to enhance its rate of disappearance and to inhibit hepatic glucose production, and the insulin sensitivity, S(I), which measures the ability of insulin to enhance plasma glucose disappearance and to inhibit hepatic glucose production. Systolic and diastolic arterial pressures in the SHR group were significantly higher (P < 0.0005) than in the WKY group. Mean S(G) and S(I) estimates from the SHR group (S(G) = 16.2 (+/- 2.0) x 10(-2) dl x min(-1) x kg(-1) and S(I) = 12.5 (+/- 1.9) x 10(-4) dl x min(-1) x kg(-1) (microU ml(-1))(-1)) were not significantly different (P > 0.05) from mean estimates that characterised the WKY group (S(G) = 13.1 (+/- 1.5) x 10(-2) dl x min(-1) x kg(-1) and S(I) = 15.8 (+/- 4.3) x 10(-4) dl x min(-1) x kg(-1) (microU ml(-1))(-1)). This result is in contrast with reported findings from humans in which insulin sensitivity is significantly reduced in the presence of hypertension.

Animals↗
PubMed2000

One- and two-compartment minimal models detect similar alterations of glucose metabolism indexes in hypertension.

A standard intravenous glucose tolerance test (IVGTT) was performed in 10 nondiabetic patients with essential hypertension (H group) and 9 normotensive control subjects (N group). A 2-compartment minimal model (2CMM) of glucose kinetics was applied to estimate indexes of glucose effectiveness, S2G and insulin sensitivity, S2I, by means of a maximum a posteriori (MAP) bayesian estimation technique. These estimates were contrasted to the S1G and S1I indexes provided by the classic minimal model (1CMM). In both the N group and the H group, the 2CMM underestimated the glucose effectiveness and overestimated the insulin sensitivity. In the H group, S2G was, on average, 63% of S1G (P > .05) and S2I was 137% of S1I (P > .05). In the N group S2G was 67% of S1G (P > .05) and S2I was 134% of S1I (P > .05). The 2CMM detected a reduction of approximately 40% (P > .05) and approximately 48% (P > .05) in S2G and S2I estimates, respectively, from the N group to the H group. Despite its reduced complexity, the 1CMM also detected a reduction of approximately 35% (P < .05) and approximately 49% (P < .05) in the S1G and in S1I indexes, respectively. Thus, the 1CMM and 2CMM showed a substantial equivalence in detecting a severe reduction in insulin sensitivity and impaired glucose effectiveness in hypertensive patients compared with normal.

Adult↗
PubMed2000

Physiological relevance of uniform elastic tube-models to infer descending aortic wave reflection: a problem of identifiability.

A uniform, frictional elastic tube terminating in a pure resistor (model A), was compared with a uniform, frictionless elastic tube, terminating in a first-order low-pass filter load (model B). The aim was to address an identifiability problem in uniqueness of parameter estimates and to evaluate the physiological meaning of tube-length estimates obtained from these models applied to the descending aortic circulation. Measurements of high descending aortic pressure and flow were taken from three anaesthetized, open-chest dogs and used to estimate the model parameters. A simultaneous measurement of terminal aortic pressure was used to estimate the foot-to-foot pulse wave velocity. A flow-fitting procedure yielded a multiplicity of equivalent solutions for the wave transit time across the transmission tubes (tau(ai), for model A and tau(bi) for model B, i=0,1,2,...,N,...) and the related tube-lengths d(ai) and d(bi), respectively. The tube length represents the distance to an effective reflection site (effective length) of the descending aortic circulation. Assuming that this length should be no longer than the dimensions of the body, the lowest estimates (i=0) of wave transit time and tube length (average +/- SE: tau(ao)=85.7+/-10.8 ms and d(ao)=53.4+/-3.7 cm for model A; tau(bo)=46.6+/-6.7 ms and d(bo)= 29.1+/-3.5 cm for model B) were identifiable as unique and acceptable solutions. Model A located the effective reflection site a few centimeters below the terminal aortic region. This location is inconsistent with the use of a pure resistor as a tube's terminal load. Further, relatively high estimates of longitudinal frictional losses violated the assumption of small losses across the transmission path and yielded an unphysiological mean-pressure drop of 7.1+/-2.3 mmHg. The estimates of d(bo) provided by model B located the effective reflection site near the origin of the renal arteries. The model-predicted pressure wave at this location approximated the measured pressure. Thus, model B represents a significant improvement over model A as a tool to infer wave travel and reflection in the descending aortic circulation.

Animals↗
PubMed1999

Assessment of aortic pressure power components and their link to overall elastic and resistive arterial properties.

This paper reviews the analytical expressions for in-phase and quadrature aortic power components associated with the real and imaginary parts of aortic input admittance, respectively. It is shown that active power Wact, and its steady, Wstdy, and pulsatile, Wpuls, components logically follow from in-phase power. Reactive power follows from quadrature power only for sinusoidal signals. The definition of reactive power indexes for real aortic pressures and flows requires extreme care. The link between overall arterial properties and pressure power components (and indexes) is investigated, making use of a three-element windkessel model and ascending aortic pressure and flow data taken from eight anaesthetised dogs, under basal state and after treatment with a vasoconstrictor (methoxamine). Seven dogs are normotensive in the baseline state (NBA cases, n = 7), the average (+/- SE) of mean pressure being 86.5 +/- 5.2 mmHg. The eighth dog has a baseline mean pressure of 134 mmHg and is considered to be hypertensive. The two experimental cases from this dog are grouped with those from the other seven dogs after vasoconstriction, to form the NVC + H group (n = 9). On average, fitting the model to the experimental data yields a 100% increase (p < 0.05) in total peripheral resistance, a 63% decrease (p < 0.01) in total arterial compliance and a 10% decrease (p > 0.05) in aortic characteristic impedance, from the NBA group to the NVC + H. Correspondingly, the peak-to-peak amplitude of quadrature power shows a 69% increase (p < 0.02). Wact, Wstdy, and Wpuls show a 28% increase (p > 0.05), a 40% increase (p < 0.02) and a 43% decrease (p > 0.05), respectively. Energetic efficiency of the arterial system, Eart = 1 - (Wpuls/Wact), increases by 8% (p < 0.02). From analysis of the estimates of power components and arterial parameters in relation to low-frequency phase angles of aortic impedance, it is concluded that the decrease in total arterial compliance with increasing pressure reduces the power lost in pulsation. This happens at the expense of an increase in quadrature power and absolute values of related reactive power indexes.

Animals↗
PubMed1999

Viscoelasticity modulates resonance in the terminal aortic circulation.

We used an inertance-viscoelastic windkessel model (IVW) to interpret aortic impedance patterns as seen in the terminal aortic circulation of the dog, and to explain evident oscillatory phenomena in flow measurements. This IVW model consists of an inertance, L, connected in series with a viscoelastic windkessel (VW) where the peripheral resistance, Rp, is connected in parallel with a Voigt cell (a resistor, Rd, in series with a capacitor, C) to account for viscoelasticity. Pressure and flow measurements were taken from the terminal aorta, just downstream of the origin of renal arteries, in three anaesthetised open-chest dogs, under a variety of haemodynamic conditions induced by administering a vasoconstrictor agent (methoxamine) and a vasodilator (sodium nitroprusside). Mean pressure ranged from 40 to 140 mm Hg. The resistance Rp was calculated as the ratio of mean pressure to mean flow. Parameters L, C and Rd were estimated by fitting measured to model predicted flow waves. We found that prominent oscillations observed in flow waves, from midsystole to diastole, are related to resonance that occurs at a frequency, f(o), where reactance of inertance of blood motion matches the reactance of arterial compliance. Estimates of f(o) increased from 2.4 to 10 Hz with increasing pressure and showed a correlation with values of static elastic moduli plotted against mean pressure of dogs' peripheral arteries previously reported by others. Viscous losses, Rd, of arterial wall motion limited the amplitude of resonance peak. We conclude that viscoelasticity, rather than pure elasticity, is a key issue to interpret terminal aortic impedance as it relates to resonance.

Animals↗
PubMed1998

Complex and frequency-dependent compliance of viscoelastic windkessel resolves contradictions in elastic windkessels.

Based on simulated data, recent studies by others showed that fitting measured pulse pressure with the pulse pressure predicted by the two-element windkessel (W2-based pulse pressure method, PPM) yielded estimates of total arterial compliance closer to simulated values than other estimation methods that use either the W2 model or the three-element windkessel (W3). A later experimental application of the PPM, made by us, however, yielded relatively non pressure dependent estimates of compliance that were in contradiction with pressure dependent estimates obtained from the W2 model by fitting to the full aortic pressure wave (full pressure method, FPM). To explain these contradictory findings, in the present study we interpreted the aortic input impedance in terms of a viscoelastic windkessel (VW), where total peripheral resistance is connected in parallel to a complex and frequency dependent compliance, Cc(j omega), described by the Voigt cell. Using ascending aortic pressure and flow taken from four dogs, under a variety of haemodynamic states, we compared the estimates of compliance obtained from the W3 and VW models and from different W2-based estimation methods: the FPM (Cw2), the PPM (Cpp), the decay time method, DTM (Cdt), and the area method, AM (C(am)). The VW-based estimates of complex compliance resolved contradictions in the W2-based estimates. Static compliance of VW-model, Cvw = Cc(0), showed a good correlation (p = 0.999) with Cw2. Correlation of static compliance with C(am) and Cdt estimates was affected by distortions in diastolic pressure decay. The modulus of VW model's dynamic compliance, ¿Cc(omega(h))¿, at the heart pulsation omega(h), was well correlated (p = 0.975) with Cpp. Analysis of data fit and compliance estimates indicated that the VW model yields an improvement over the W3 in the physical interpretation of the overall arterial properties.

Animals↗
PubMed1997

Identification and physiological relevance of an exponentially tapered tube model of canine descending aortic circulation.

The aim of this study was to evaluate the effect of incorporating aortic tapering in a tube model of descending aortic circulation. We described the descending aorta and its peripheral load by an exponentially tapered transmission tube terminating in a first-order, low-pass filter load. Under the assumption of adaptation between the transmission tube and the terminal load, the input impedance of this model was characterized by five free parameters, the characteristic impedance, Zce(0), at the tube entrance; the product, qde, between the tapering factor q and the tube length, de, the product ce(0)de, between the compliance, ce(0), at the tube entrance and the tube length; the time constant, tau ne, of the load and the peripheral resistance, Rp. We estimated these parameters making use of experimental pressure and flow measurements taken from the high descending aorta of three anaesthetized dogs. We contrasted the behaviour of this model with that of a competing model constituted by a uniform transmission tube also terminating in a first-order low-pass filter load. We compared the data fits and, with the aid of an extra measurement of pressure in the abdominal aorta, we tested the congruence between the estimates of the transmission tubes' parameters and the physical and geometrical properties of descending thoracic aorta. The tapered tube model showed a slightly better ability in fitting to experimental flow and reproducing input impedance data. However, the estimates of the transmission tube parameters failed to assess the physical properties of descending aorta. By contrast, the estimates of tube parameters provided by the uniform model allowed location of the junction between the tube and its terminal load in the abdominal aorta at level of major branches. These estimates were well correlated with the real system's properties. In conclusion, the complexity added to the uniform tube model by accounting for exponential aortic tapering gave rise only to a better curve fitting, but did not show any identifiable benefits regarding physiological interpretation of the physical properties of the descending aorta.

Animals↗
PubMed1996

Fit to diastolic arterial pressure by third-order lumped model yields unreliable estimates of arterial compliance.

The pressure pulse contour analysis method uses a third-order lumped model to evaluate the elastic properties of the arterial system and their modifications with adaptive responses or disease. A fundamental assumption underlying this method is that the estimates of model parameters (two compliances, an inertance, and a peripheral resistance) obtained from a measurement of cardiac output, and a simultaneous measurement of an arterial pressure, are independent of the pressure measurement site. If true, this hypothesis would provide a minimally invasive method for estimation of arterial compliance. The aim of the present study was to test the validity of this assumption and the ability of the method to assess changes of compliance in response to vasoactive drug administration. In five anaesthetised, open-chest dogs we measured pulsatile pressure and flow in the ascending aorta and pulsatile pressure in the terminal aorta, under basal, vasoconstricted (methoxamine), and vasodilated (sodium nitroprusside) conditions. Model peripheral resistance was assumed equal to the ratio of mean pressure to cardiac output. Estimates of inertance and compliances, and the associated estimation errors, were determined by fitting the model output to either the diastolic portions of ascending aortic pressure, P(adt), or terminal aortic pressure, Ptd(t). Results showed that the assumption of independency of model parameter estimates on the arterial pressure measurement site was not verified. Different images of the vasoactive drug-induced changes in vascular compliance were obtained from fits to P(adt) and Ptd(t). Model parameter estimates were associated with high estimation errors and were very sensitive to the choice of the period of diastolic pressure to be fitted. Model predicted aortic pressure, over the entire heart cycle, did not compare well with experimental ascending aortic pressure. Our results question the reliability of the pressure pulse contour analysis method for evaluating arterial compliance.

Animals↗
PubMed1996

Comparison of linear and nonlinear formulations of the three-element windkessel model.

The three-element windkessel model incorporating a constant compliance (model A) was compared with two nonlinear versions of the same model (models B1 and B2) incorporating a pressure-dependent compliance. The aim was to test whether nonlinear elasticity yielded better model behavior in describing ascending aortic pressure-flow relationships and interpreting the physical properties of the arterial system. Exponential and bell-shaped compliance vs. pressure curves were assumed in models B1 and B2, respectively. To test these models, we used measurements of ascending aortic pressure and flow from three dogs under a wide variety of hemodynamic states obtained by administering vasoactive drugs and by pacing the heart. These data involved pressure waves with and without an evident oscillation during diastole. Model parameters were estimated by fitting experimental and model-predicted ascending aortic pressures. Our results indicated that only models A and B1 were identifiable. Fits to ascending aortic pressure obtained from model B1 were significantly better than fits obtained from model A. However, 1) the accuracy of parameter estimates, as judged from parameter estimation error analysis, was better in model A than in model B1, 2) the estimates of characteristic parameters of the compliance vs. pressure relation in model B1 were inconsistent with expected physiological trends of this relation, and 3) model B1 did not improve the approximation of diastolic pressure in the presence of an evident oscillation. We conclude that, even in the presence of better data fit, the nonlinear three-element windkessel cannot be preferred over the traditional linear version of this model.

Animals↗
PubMed1994

On the approximation of static open-loop characteristics of baroreceptor reflex.

We investigated the adequacy of three different functions that have been used in the literature to characterize the relationship between mean systemic arterial pressure (SAP) and mean baroreceptor pressure (BP) in open-loop experimental preparations. These curves are the normal cumulative distribution function (CDF), the logistic (or growth) function (LF), and the third-order polynomial (TOP). Ten sets of experimental data from isolated carotid sinus preparations were selected from the literature as being representative of all baroreflex curves. Then, the Levenberg-Marquardt method was used to obtain best least-square approximation to these data and parameter estimates for each of the three approximating curves. The first derivative of each best-fit SAP-BP curve yielded a curve for the open-loop gain (G) as a function of BP. Our analysis indicated that both the CDF and the LF were superior to the TOP in fitting the SAP-BP data and in giving a realistic description of the G-BP curve. The operative range of BP was evaluated by estimating threshold (BPth) and saturation (BPsat) BPs using arbitrary definitions reported in the literature. The TOP did not allow this evaluation. The other two curves showed some disagreement due to different definitions of BPth and BPsat. After modifying the definition of these parameters associated with the LF, we could conclude that the analytical descriptions of SAP-BP and G-BP curves as obtained from the LF and the CDF were practically equivalent. Approximation method using the TOP should be avoided.

Animals↗
PubMed1994

Relationship between strength of short-term systemic autoregulation and initial resistance.

The relationship between strength of short-term whole body autoregulation and peripheral resistance in the reference state (initial resistance) was investigated in 9 anesthetized closed-chest dogs and 18 anesthetized open-chest cats. Baroreflex regulation was abolished in one of three ways: barodenervation, ganglionic blockade, or setting pressure constant in the isolated carotid sinuses after vagotomy. Ascending aortic pressure and flow and venous pressure were measured in the reference state and 1-3 min after partial occlusions of the inferior vena cava. Cardiac output and peripheral resistance (ratio between arteriovenous pressure difference and cardiac output) were normalized for body weight. Strength of autoregulation was quantified by a resistance gain (Gra), defined as the ratio between change in normalized peripheral resistance and corresponding change in normalized cardiac output. A broad range of values for peripheral resistance in the reference state (Ro) was obtained as a result of the different interventions used to abolish baroreflex regulation. Arteriovenous pressure difference and normalized cardiac output during multiple vena cava occlusions in the 9 dogs and in 8 of the cats were fitted with a parabola convex to the flow axis. From the best fit, Gra was estimated. In the remaining 10 cats Gra was estimated from a single occlusion of vena cava. When data of all dogs and cats were taken together, we found a linear relationship between Gra and Ro: Gra = K1.Ro + K2. The constants K1 and K2 were 17.9 x 10(-3) min.kg.ml-1 and -14.5 x 10(-3) mmHg.min2.kg2.ml-2, respectively. The correlation coefficient was 0.9.

Animals↗
PubMed1993

Effective distributed compliance of the canine descending aorta estimated by modified T-tube model.

To estimate descending thoracic aortic compliance in anesthetized open-chest dogs, a modified T-tube arterial model was used. This model consists of two uniform and lossless elastic tubes, one representing arteries going toward the head and upper limbs and the other (body tube) representing descending aortic circulation to the trunk and lower limbs. Each tube terminates with a generalized first-order low-pass filter load. Pressure and flow in the ascending aorta and flow in the upper descending aorta were measured and used to estimate model parameters. Using the estimated model parameters, we calculated the pressure waveshape at the termination of the body tube. Comparison of this model-predicted pressure with pressure measured in the abdominal aorta near the origin of renal arteries suggested that the end of the body tube (effective reflecting site of the body circulation) corresponds to this major branching site of the abdominal aorta. To calculate the length of the body tube, we used aortic pulse wave velocity estimated from the measurements of pressure in ascending and abdominal aorta. Calculated body tube length averaged 30.3 +/- 2.8 cm and approximated the measured length (30.6 +/- 3.0 cm) of the aorta from the arch to the region of the origin of renal arteries. Compliance of the body tube averaged 123 +/- 20 x 10(-6) g-1.cm4.s2 and was interpreted as the descending thoracic aortic compliance. The ratio of this compliance to the body tube length gave an estimate of the effective distributed compliance, i.e., the compliance per unit length that would be observed in the absence of tapering. This ratio averaged 4.10 +/- 0.86 x 10(-6) g-1.cm3.s2 and fell in between the values of local aortic compliance independently estimated along the descending thoracic aorta from measurements of pressure and diameter. Thus tube compliance resulted in a physically identifiable property. This property was contrasted with the ill-defined effective compliances of the terminal loads.

Animals↗
PubMed1992

Evaluation of hypercholesterol diet-induced changes in viscoelastic properties of carotid circulation in pigs.

Measurements of pulsatile pressure and flow at the input of the left and right carotid arteries and a new lumped parameter model were used to quantify changes in the overall dynamic mechanical properties of the carotid circulation between five control diet-fed pigs and five pigs fed a hyperlipidemic diet for 16 wk. The model represents the portion of the circulation supplied by either the left or the right carotid artery and is characterized by five parameters: peripheral resistance (Rp), an overall inertance (L), and an overall frequency-dependent compliance constituted by a capacitor C (static compliance) in series with a Maxwell section, i.e., a capacitor Cd, in parallel with a resistor Rd. Rp was calculated as the ratio between mean pressure (P) and mean flow (Q). The other four parameters were estimated by fitting measured to model predicted flows. The average static compliance was reduced by 40% (P = 0.01) between normal (P = 62.0 +/- 4.3 mmHg) and hyperlipidemic diet-fed pigs (P = 62.7 +/- 4.7 mmHg). A significant reduction in the overall cross-sectional area was inferred from a 53% increase (P = 0.05) in L, whereas resistance vessel tone was unchanged as judged from estimates of Rp. No signs of occlusive disease were found in any of the animals.

Animals↗
PubMed1991

Systemic autoregulation counteracts the carotid baroreflex.

The interaction between autoregulation and baroregulation and its effect on the gains of the short-term pressure regulatory system was studied by performing both open- and closed-loop experiments in the same five anesthetized, vagotomized dogs, and by analyzing the data making use of a new model. With carotid pressure constant (no baroregulation) the pressure-flow data were convex to the flow axis, thus indicating the presence of autoregulation. When baroregulation was present the data were convex to the pressure axis. Our model was able to fit the data as measured in both cases. From the fitting procedure the zero-flow pressure intercept Pzf, the autoregulation resistance gain Gra, and the baroregulation resistance gain Grb were estimated. Pzf was about 20 mmHg in three dogs and about zero in the other two. Average values of Gra and Grb were 13.0 +/- 3.5 mmHg min2/L2 and 0.83 +/- 0.25 min/L, respectively. The two curves which fitted the data points collected in the presence and in the absence of baroreflex intersected at a point (Qo, Po) generally different from the control point. We determined the open-loop gain, Goc = GrbQo, about the point (Qo, Po). The averaged value was 2.23 +/- 0.84. When autoregulation was neglected, the resistance gain Grb and the open-loop gain Goc obtained from the same closed-loop method were underestimated (0.32 +/- 0.15 min/L and 0.88 +/- 0.48, respectively). In the open-loop preparation the carotid sinuses were isolated and the aortic (P) versus carotid (Pca) pressure data were collected. A third-order polynomial was fitted to these data.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Two arterial effective reflecting sites may appear as one to the heart.

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)

Animals↗