Time-varying Elastance Research Articles

Synergistic Model of Cardiac Function with a Heart Assist Device.

The breakdown of cardiac self-organization leads to heart diseases and failure, the number one cause of death worldwide. The left ventricular pressure–volume relation plays a key role in the diagnosis and treatment of heart diseases. Lumped-parameter models combined with pressure–volume loop analysis are very effective in simulating clinical scenarios with a view to treatment optimization and outcome prediction. Unfortunately, often invoked in this analysis is the traditional, time-varying elastance concept, in which the ratio of the ventricular pressure to its volume is prescribed by a periodic function of time, instead of being calculated consistently according to the change in feedback mechanisms (e.g., the lack or breakdown of self-organization) in heart diseases. Therefore, the application of the time-varying elastance for the analysis of left ventricular assist device (LVAD)–heart interactions has been questioned. We propose a paradigm shift from the time-varying elastance concept to a synergistic model of cardiac function by integrating the mechanical, electric, and chemical activity on microscale sarcomere and macroscale heart levels and investigating the effect of an axial rotary pump on a failing heart. We show that our synergistic model works better than the time-varying elastance model in reproducing LVAD–heart interactions with sufficient accuracy to describe the left ventricular pressure–volume relation.

Mathematical modeling of cardiac function to evaluate clinical cases in adults and children.

Time-varying elastance models can simulate only the pressure and volume signals in the heart chambers while the diagnosis of clinical cases and evaluation of different treatment techniques require more information. In this study, an extended model utilizing the geometric dimensions of the heart chambers was developed to describe the cardiac function. The new cardiac model was evaluated by simulating a healthy and dilated cardiomyopathy (DCM) condition for adults and children. The left ventricular ejection fraction, end-diastolic volume, end-diastolic diameter and diastolic sphericity index were 53.60%, 125 mL, 5.08 cm and 1.82 in the healthy adult cardiovascular system model and 23.70%, 173 mL, 6.60 cm and 1.40 in the DCM adult cardiovascular system model. In the healthy child cardiovascular system model, the left ventricular ejection fraction, end-diastolic volume, end-diastolic diameter and diastolic sphericity index were 59.70%, 92 mL, 4.10 cm and 2.26 respectively and 30.70%, 125 mL, 4.94 cm and 1.87 in the DCM child cardiovascular system model. The developed cardiovascular system model simulates the hemodynamic variables and clinical diagnostic indicators within the physiological range for healthy and DCM conditions proving the feasibility of this new model to evaluate clinical cases in adults and children.

Lumped parameter heart model with valve dynamics

Abstract In this work, the lumped parameter model of the left heart is presented. It is based on the concept of the time-varying elastance of myocardium and includes the model of the heart valve dynamics. Comparison of the models with instant and smooth valve opening and closing is given, as well as simulations of pathologies such as mitral valve stenosis and aortic valve insufficiency are addressed.

A 1D computer model of the arterial circulation in horses: An important resource for studying global interactions between heart and vessels under normal and pathological conditions.

Arterial rupture in horses has been observed during exercise, after phenylephrine administration or during parturition (uterine artery). In human pathophysiological research, the use of computer models for studying arterial hemodynamics and understanding normal and abnormal characteristics of arterial pressure and flow waveforms is very common. The objective of this research was to develop a computer model of the equine arterial circulation, in order to study local intra-arterial pressures and flow dynamics in horses. Morphologically, large differences exist between human and equine aortic arch and arterial branching patterns. Development of the present model was based on post-mortem obtained anatomical data of the arterial tree (arterial lengths, diameters and branching angles); in vivo collected ultrasonographic flow profiles from the common carotid artery, external iliac artery, median artery and aorta; and invasively collected pressure curves from carotid artery and aorta. These data were used as input for a previously validated (in humans) 1D arterial network model. Data on terminal resistance and arterial compliance parameters were tuned to equine physiology. Given the large arterial diameters, Womersley theory was used to compute friction coefficients, and the input into the arterial system was provided via a scaled time-varying elastance model of the left heart. Outcomes showed plausible predictions of pressure and flow waveforms throughout the considered arterial tree. Simulated flow waveform morphology was in line with measured flow profiles. Consideration of gravity further improved model based predicted waveforms. Derived flow waveform patterns could be explained using wave power analysis. The model offers possibilities as a research tool to predict changes in flow profiles and local pressures as a result of strenuous exercise or altered arterial wall properties related to age, breed or gender.

Noninvasive Quantification of Pressure-Volume Loops From Brachial Pressure and Cardiovascular Magnetic Resonance.

Pressure-volume (PV) loops provide a wealth of information on cardiac function but are not readily available in clinical routine or in clinical trials. This study aimed to develop and validate a noninvasive method to compute individualized left ventricular PV loops. The proposed method is based on time-varying elastance, with experimentally optimized model parameters from a training set (n=5 pigs), yielding individualized PV loops. Model inputs are left ventricular volume curves from cardiovascular magnetic resonance imaging and brachial pressure. The method was experimentally validated in a separate set (n=9 pig experiments) using invasive pressure measurements and cardiovascular magnetic resonance images and subsequently applied to human healthy controls (n=13) and patients with heart failure (n=28). There was a moderate-to-excellent agreement between in vivo-measured and model-calculated stroke work (intraclass correlation coefficient, 0.93; bias, -0.02±0.03 J), mechanical potential energy (intraclass correlation coefficient, 0.57; bias, -0.04±0.03 J), and ventricular efficiency (intraclass correlation coefficient, 0.84; bias, 3.5±2.1%). The model yielded lower ventricular efficiency ( P<0.0001) and contractility ( P<0.0001) in patients with heart failure compared with controls, as well as a higher potential energy ( P<0.0001) and energy per ejected volume ( P<0.0001). Furthermore, the model produced realistic values of stroke work and physiologically representative PV loops. We have developed the first experimentally validated, noninvasive method to compute left ventricular PV loops and associated quantitative measures. The proposed method shows significant agreement with in vivo-derived measurements and could support clinical decision-making and provide surrogate end points in clinical heart failure trials.

Abstract 17013: Preceding Contraction of Less Distensible Ventricular Compartment Plays a Key Role in the Pathogenesis of Mechanical Dyssynchrony in the Univentricular Heart

Background: Septal flash motion is the key to mechanical dyssynchrony in the left bundle branch block in biventricular physiology. In univentricular Fontan circulations, however, morphological variations of systemic ventricles make it difficult to evaluate the negative impact of asynchronous contraction with widened QRS duration on hemodynamics. Methods: A computational simulation of Fontan circulation using a time-varying elastance chamber model combined with a three-element Windkessel vascular model was performed. Systemic ventricles were divided into two ventricular compartments (Vnt 1 , Vnt 2 ) by large ventricular septal defects (VSD), which had various ratios of end-diastolic volumes (EDV 1 /EDV 2 , α) and contractilities (Emax 1 /Emax 2 , β). We introduced delayed contraction onset of Vnt 2 from Vnt 1 (T dys , ms) and simulated hemodynamic changes. We also studied 44 consecutive postoperative Fontan patients with wide QRS (28 male, 1-37 years; median age 16.2 years) and examined their clinical profiles including the analysis of focal ventricular wall motion with 2D speckle tracking echocardiography, hemodynamic parameters in cardiac catheterization, plasma levels of brain natriuretic peptide (BNP, pg/mL) and exercise capacities (peak VO 2 , mL/kg/min). Results: In silico , compared with the ventricles with α &gt; 1 and β &lt; 1, those with α &lt; 1 and β &gt; 1 exhibited significantly decreased cardiac output as T dys increased, and relatively impaired output reserve when tachycardia developed. These deteriorations were characterized by the increased reverse flow from Vnt 2 to Vnt 1 through VSD during mid-systole. Also in vivo , reverse flow during mid-systole via VSD, flash motion during isovolumic contraction time and lower peak systolic strain of smaller ventricular compartment were associated with higher central venous pressures, elevated BNP levels and decreased peak VO 2 (p &lt; 0.05). Conclusion: Delayed contraction in more distensible ventricular compartment can exacerbate the mechanical dyssynchrony characterized by preceding “flash” contraction and subsequent dyskinetic dilation of less distensible compartment, which contributes to the pathogenesis of heart failure in the univentricular heart.

Non-invasive Assessment of Systolic and Diastolic Cardiac Function During Rest and Stress Conditions Using an Integrated Image-Modeling Approach.

Background: The possibility of non-invasively assessing load-independent parameters characterizing cardiac function is of high clinical value. Typically, these parameters are assessed during resting conditions. However, for diagnostic purposes, the parameter behavior across a physiologically relevant range of heart rate and loads is more relevant than the isolated measurements performed at rest. This study sought to evaluate changes in non-invasive estimations of load-independent parameters of left-ventricular contraction and relaxation patterns at rest and during dobutamine stress.Methods: We applied a previously developed approach that combines non-invasive measurements with a physiologically-based, reduced-order model of the cardiovascular system to provide subject-specific estimates of parameters characterizing left ventricular function. In this model, the contractile state of the heart at each time point along the cardiac cycle is modeled using a time-varying elastance curve. Non-invasive data, including four-dimensional magnetic resonance imaging (4D Flow MRI) measurements, were acquired in nine subjects without a known heart disease at rest and during dobutamine stress. For each of the study subjects, we constructed two personalized models corresponding to the resting and the stress state.Results: Applying the modeling framework, we identified significant increases in the left ventricular contraction rate constant [from 1.5 ± 0.3 to 2 ± 0.5 (p = 0.038)] and relaxation constant [from 37.2 ± 6.9 to 46.1 ± 12 (p = 0.028)]. In addition, we found a significant decrease in the elastance diastolic time constant from 0.4 ± 0.04 s to 0.3 ± 0.03 s (p = 0.008).Conclusions: The integrated image-modeling approach allows the assessment of cardiovascular function given as model-based parameters. The agreement between the estimated parameter values and previously reported effects of dobutamine demonstrates the potential of the approach to assess advanced metrics of pathophysiology that are otherwise difficult to obtain non-invasively in clinical practice.

Clinical Application of Respiratory Elastance (CARE Trial) for Mechanically Ventilated Respiratory Failure Patients: A Model-based Study

Abstract Mechanical ventilation (MV) is the primary support for respiratory failure patients. MV treatment is difficult to manage due to the variable patient-specific disease state and response to MV treatment. Model-based methods have shown potential in providing unique, personalised information on patient condition for clinicians to guide MV treatment. This study presents a clinical observational trial to investigate the respiratory mechanics and quality of patient ventilator asynchrony conducted in a South-East Asia hospital intensive care unit (ICU). Pilot trial results included 7 study patients. Patient-specific respiratory mechanics Ers and Rrs were median 32.30 cmH2O/l [Interquartile range (IQR): 24.53-43.48] and 7.62 cmH2Os/l [IQR: 4.85-10.34]. The Normalised Area Under the Curve of Time-Varying Elastance (AUC-Edrs) across patients were 27.50 cmH2O/l [IQR: 24.97-28.34]. Patient ventilator interaction is assessed using an asynchrony index (AI), where AI had a median value of 27.0% [(IQR): 21.0-52.7]. Patient-specific respiratory mechanics displayed intra- and inter- patient variability, suggesting patients are different and evolved over time, and thus their MV settings should be patient-specific and time-varying. Model-based methods thus offer unique insight into patient-specific respiratory mechanics and the resulting opportunity to personalize and optimise MV based on evolving patient-specific needs.

Lumped parameter model for hemodynamic simulation of congenital heart diseases.

The recent development of computer technology has made it possible to simulate the hemodynamics of congenital heart diseases on a desktop computer. However, multi-scale modeling of the cardiovascular system based on computed tomographic and magnetic resonance images still requires long simulation times. The lumped parameter model is potentially beneficial for real-time bedside simulation of congenital heart diseases. In this review, we introduce the basics of the lumped parameter model (time-varying elastance chamber model combined with modified Windkessel vasculature model) and illustrate its usage in hemodynamic simulation of congenital heart diseases using examples such as hypoplastic left heart syndrome and Fontan circulation. We also discuss the advantages of the lumped parameter model and the problems for clinical use.

Noninvasive evaluation of left ventricular elastance according to pressure-volume curves modeling in arterial hypertension.

End-systolic left ventricular (LV) elastance (Ees) has been previously calculated and validated invasively using LV pressure-volume (P-V) loops. Noninvasive methods have been proposed, but clinical application remains complex. The aims of the present study were to 1) estimate Ees according to modeling of the LV P-V curve during ejection ("ejection P-V curve" method) and validate our method with existing published LV P-V loop data and 2) test the clinical applicability of noninvasively detecting a difference in Ees between normotensive and hypertensive subjects. On the basis of the ejection P-V curve and a linear relationship between elastance and time during ejection, we used a nonlinear least-squares method to fit the pressure waveform. We then computed the slope and intercept of time-varying elastance as well as the volume intercept (V0). As a validation, 22 P-V loops obtained from previous invasive studies were digitized and analyzed using the ejection P-V curve method. To test clinical applicability, ejection P-V curves were obtained from 33 hypertensive subjects and 32 normotensive subjects with carotid tonometry and real-time three-dimensional echocardiography during the same procedure. A good univariate relationship (r2 = 0.92, P < 0.005) and good limits of agreement were found between the invasive calculation of Ees and our new proposed ejection P-V curve method. In hypertensive patients, an increase in arterial elastance (Ea) was compensated by a parallel increase in Ees without change in Ea/Ees In addition, the clinical reproducibility of our method was similar to that of another noninvasive method. In conclusion, Ees and V0 can be estimated noninvasively from modeling of the P-V curve during ejection. This approach was found to be reproducible and sensitive enough to detect an expected increase in LV contractility in hypertensive patients. Because of its noninvasive nature, this methodology may have clinical implications in various disease states.NEW & NOTEWORTHY The use of real-time three-dimensional echocardiography-derived left ventricular volumes in conjunction with carotid tonometry was found to be reproducible and sensitive enough to detect expected differences in left ventricular elastance in arterial hypertension. Because of its noninvasive nature, this methodology may have clinical implications in various disease states.

Negative Lung Elastance in Mechanically Ventilated Spontaneously Breathing Patient

Mathematical modelling of respiratory system can guide clinicians in better monitoring and decision making for mechanically ventilated (MV) patients in intensive care unit (ICU). However, most mathematical models are develop for fully sedated patients and not particularly reliable to be applied for spontaneous breathing (SB) patients. Monitoring respiratory mechanics of SB patients requires invasive clinical protocols and equipment that are clinically too intensive to carry out. Previous study hypothesized that negative elastance occurred in SB patients due to the SB effort produced by the patient. Thus, this paper aims to further investigate the distribution of negative elastance in SB patients by extending the noninvasive time-varying elastance model. By capturing and reviewing the distribution of the negative elastance in SB patient, it can provide more consistent monitoring and decision making particularly for SB patients. Clinical data from 5 MV patients from Christchurch Hospital were used in this study. The area under the curve (AUC) for the time-varying elastance, Edrs, is estimated and analysed in each SB patient. The results are reported as median and interquartile range (IQR) for continuous data with a total of 82 hours. From the result, it was found that all patients have distribution of negative elastance with Patients 1 and 3 have higher distribution of negative elastance due to the SB effort. The median vaue for the negative elastance for all patients’ ranges from -0.66 cmH20.s/l to -2.27 cmH20.s/l. Negative elastance occurs when negative pressure is generated in the patient’s pleural space causing air volume to enter the lung. Thus, by capturing and reviewing the distribution of the negative elastance in SB patient, it can provide more consistent monitoring and decision making particularly for SB patients.

Quantifying patient effort in spontaneously breathing patient using negative component of dynamic Elastance

Respiratory mechanics of fully sedated patients can be easily estimated as the ventilator has full control of patient’s work of breathing. However, in spontaneously breathing patients or patients whose work of breathing is only partially assisted, respiratory mechanics estimation is much more difficult. This difficulty is caused by un-modelled and variable patient effort introduced into the system. The Time-varying elastance model is a model that can estimate respiratory mechanics of spontaneously breathing patients with use of dynamic elastance. The model has a negative elastance component where measured airway pressure is decreasing, yet the air is flowing into the lungs due to patient effort. In this study, quantification of the negative elastance component is studied. Airway flow, pressure and Electrical Activity of Diaphragm signals from 22 invasively ventilated patients using Pressure Support mode are used for this analysis. Two methods have been used to quantify negative elastance, the zero-crossing method and trapezoidal method. The estimated median values of negative elastance using the zero-crossing method is: -3.289 [Interquartile range (IQR: -4.803~-2.504] cmH2O/L and for trapezoidal method is: -1.899 [IQR: -2.362-1.664] cmH2O/L. The correlation between electrical activity of the diaphragm and negative elastance is very weak as the R values across patients are: -0.0697 [IQR: -0.4972~ -0.0255] for zero-crossing method and 0.0939 [IQR: -0.0293-0.3003] for trapezoidal method. Negative elastance is a conceptual component of the model and can be used to quantify patient demand. However, in this study, quantifying patient effort using negative elastance has little similarity to electrical activity of the diaphragm, as negative elastance appears to capture more than only patient effort. Further research is needed to use this metric to observe patient effort.

A Novel Mean-Value Model of the Cardiovascular System Including a Left Ventricular Assist Device.

Time-varying elastance models (TVEMs) are often used for simulation studies of the cardiovascular system with a left ventricular assist device (LVAD). Because these models are computationally expensive, they cannot be used for long-term simulation studies. In addition, their equilibria are periodic solutions, which prevent the extraction of a linear time-invariant model that could be used e.g. for the design of a physiological controller. In the current paper, we present a new type of model to overcome these problems: the mean-value model (MVM). The MVM captures the behavior of the cardiovascular system by representative mean values that do not change within the cardiac cycle. For this purpose, each time-varying element is manually converted to its mean-value counterpart. We compare the derived MVM to a similar TVEM in two simulation experiments. In both cases, the MVM is able to fully capture the inter-cycle dynamics of the TVEM. We hope that the new MVM will become a useful tool for researchers working on physiological control algorithms. This paper provides a plant model that enables for the first time the use of tools from classical control theory in the field of physiological LVAD control.

Minimally invasive, patient specific, beat-by-beat estimation of left ventricular time varying elastance

BackgroundThe aim of this paper was to establish a minimally invasive method for deriving the left ventricular time varying elastance (TVE) curve beat-by-beat, the monitoring of which’s inter-beat evolution could add significant new data and insight to improve diagnosis and treatment. The method developed uses the clinically available inputs of aortic pressure, heart rate and baseline end-systolic volume (via echocardiography) to determine the outputs of left ventricular pressure, volume and dead space volume, and thus the TVE curve. This approach avoids directly assuming the shape of the TVE curve, allowing more effective capture of intra- and inter-patient variability.ResultsThe resulting TVE curve was experimentally validated against the TVE curve as derived from experimentally measured left ventricular pressure and volume in animal models, a data set encompassing 46,318 heartbeats across 5 Piétrain pigs. This simulated TVE curve was able to effectively approximate the measured TVE curve, with an overall median absolute error of 11.4% and overall median signed error of −2.5%.ConclusionsThe use of clinically available inputs means there is potential for real-time implementation of the method at the patient bedside. Thus the method could be used to provide additional, patient specific information on intra- and inter-beat variation in heart function.

Left ventricular function: time-varying elastance and left ventricular aortic coupling.

Many aspects of left ventricular function are explained by considering ventricular pressure–volume characteristics. Contractility is best measured by the slope, Emax, of the end-systolic pressure–volume relationship. Ventricular systole is usefully characterized by a time-varying elastance (ΔP/ΔV). An extended area, the pressure–volume area, subtended by the ventricular pressure–volume loop (useful mechanical work) and the ESPVR (energy expended without mechanical work), is linearly related to myocardial oxygen consumption per beat. For energetically efficient systolic ejection ventricular elastance should be, and is, matched to aortic elastance. Without matching, the fraction of energy expended without mechanical work increases and energy is lost during ejection across the aortic valve. Ventricular function curves, derived from ventricular pressure–volume characteristics, interact with venous return curves to regulate cardiac output. Thus, consideration of ventricular pressure–volume relationships highlight features that allow the heart to efficiently respond to any demand for cardiac output and oxygen delivery.

Contribution of the Arterial System and the Heart to Blood Pressure during Normal Aging - A Simulation Study.

During aging, systolic blood pressure continuously increases over time, whereas diastolic pressure first increases and then slightly decreases after middle age. These pressure changes are usually explained by changes of the arterial system alone (increase in arterial stiffness and vascular resistance). However, we hypothesise that the heart contributes to the age-related blood pressure progression as well. In the present study we quantified the blood pressure changes in normal aging by using a Windkessel model for the arterial system and the time-varying elastance model for the heart, and compared the simulation results with data from the Framingham Heart Study. Parameters representing arterial changes (resistance and stiffness) during aging were based on literature values, whereas parameters representing cardiac changes were computed through physiological rules (compensated hypertrophy and preservation of end-diastolic volume). When taking into account arterial changes only, the systolic and diastolic pressure did not agree well with the population data. Between 20 and 80 years, systolic pressure increased from 100 to 122 mmHg, and diastolic pressure decreased from 76 to 55 mmHg. When taking cardiac adaptations into account as well, systolic and diastolic pressure increased from 100 to 151 mmHg and decreased from 76 to 69 mmHg, respectively. Our results show that not only the arterial system, but also the heart, contributes to the changes in blood pressure during aging. The changes in arterial properties initiate a systolic pressure increase, which in turn initiates a cardiac remodelling process that further augments systolic pressure and mitigates the decrease in diastolic pressure.

Assessing respiratory mechanics using pressure reconstruction method in mechanically ventilated spontaneous breathing patient

BackgroundRespiratory system modelling can aid clinical decision making during mechanical ventilation (MV) in intensive care. However, spontaneous breathing (SB) efforts can produce entrained “M-wave” airway pressure waveforms that inhibit identification of accurate values for respiratory system elastance and airway resistance. A pressure wave reconstruction method is proposed to accurately identify respiratory mechanics, assess the level of SB effort, and quantify the incidence of SB effort without uncommon measuring devices or interruption to care. MethodsData from 275 breaths aggregated from all mechanically ventilated patients at Christchurch Hospital were used in this study. The breath specific respiratory elastance is calculated using a time-varying elastance model. A pressure reconstruction method is proposed to reconstruct pressure waves identified as being affected by SB effort. The area under the curve of the time-varying respiratory elastance (AUC Edrs) are calculated and compared, where unreconstructed waves yield lower AUC Edrs. The difference between the reconstructed and unreconstructed pressure is denoted as a surrogate measure of SB effort. ResultsThe pressure reconstruction method yielded a median AUC Edrs of 19.21 [IQR: 16.30–22.47]cmH2Os/l. In contrast, the median AUC Edrs for unreconstructed M-wave data was 20.41 [IQR: 16.68–22.81]cmH2Os/l. The pressure reconstruction method had the least variability in AUC Edrs assessed by the robust coefficient of variation (RCV)=0.04 versus 0.05 for unreconstructed data. Each patient exhibited different levels of SB effort, independent from MV setting, indicating the need for non-invasive, real time assessment of SB effort. ConclusionA simple reconstruction method enables more consistent real-time estimation of the true, underlying respiratory system mechanics of a SB patient and provides the surrogate of SB effort, which may be clinically useful for clinicians in determining optimal ventilator settings to improve patient care.

Reassessing Diagrams of Cardiac Mechanics: From Otto Frank and Ernest Starling to Hiroyuki Suga.

This article explores the importance of diagrams in the history of the understanding of cardiac function, by comparing Ernest Starling's famous "Law of the Heart" (1918) with the mathematically based view of cardiac mechanics put forward by Otto Frank (1897). Whereas Frank's diagrams gained influence in German cardio-physiological publications, they were widely unknown abroad until 1969, when Hiroyuki Suga began to present similar approaches for warm-blooded animals as Frank had done for the frog. Suga succeeded in correlating the pressure volume area (PVA)-a composite of Frank's work loop plus the area of remaining potential energy-with the oxygen consumption of the beating heart. With the concept of time-varying elastance as an index of cardiac contractility, Suga's approach became attractive for clinical applications, and Daniel Burkhoff and colleagues were able to use these insights for real-time, interactive simulations of the cardiovascular system. Such tools can be used for exploring basic hemodynamic principles and, thanks to technical developments of miniature pumps within the same time frame (Καιρός, the "right moment," or "the opportune"), to test the effects of device-based treatment for heart failure. These outcomes confirm that old analyses of the heart's activity may still be useful today.

Partial cavopulmonary assist from the inferior vena cava to the pulmonary artery improves hemodynamics in failing Fontan circulation: a theoretical analysis.

Cavopulmonary assist (CPA) for failing Fontan patients remains a challenging issue in the clinical setting. To evaluate the effectiveness of a partial CPA from the inferior vena cava (IVC) to the pulmonary artery (PA), we performed a theoretical analysis using a computational model of the Fontan circulation. Cardiac chambers and vascular systems were described as the time-varying elastance model and the modified three-element Windkessel model, respectively. A rotational pump described as a non-linear function was inserted between the IVC and the PA. When pulmonary vascular resistance index varied from 2.1 to 5.9 Wood units m(2), the partial CPA maintained cardiac index as efficiently as total CPA and markedly reduced the IVC pressure compared with total CPA. However, the partial CPA increased the superior vena cava pressure substantially. The modification from total to partial CPA is potentially an effective alternative in failing Fontan patients suffering from high IVC pressure.

A hybrid (hydro-numerical) circulatory model: investigations of mechanical aortic valves and a numerical valve model

Abstract In most cases of diseased heart valves, they can be repaired or replaced with biological or mechanical prostheses. Biological prostheses seem to be safer than mechanical ones and are applied with good clinical outcomes. Their disadvantage, when compared with mechanical valves, is durability. In the development and application of mechanical and biological heart valves, a significant role can be played by a Hybrid (Hydro-Numerical) Circulatory Model. The aim of this paper is to demonstrate the opportunities created by the hybrid model for investigations of mechanical heart valves and their computer models under conditions similar to those of the circulatory system. A diode-resistor numerical valve model and three different design mechanical aortic valves were tested. To perform their investigations, computer applications were developed under RT LabView to be run on a PC. Static and dynamic characteristics of the valves were measured and registered - pressure in the numerical time-varying elastance left ventricle (pLV), in the aorta (pas) and flow (f), proving, among other factors, that 1) time delay of pas with respect to pLV is mainly related to the valve’s opening time, and 2) the valves of substantially different designs tested under identical hydrodynamic conditions reveal nearly the same dynamic performance.