A Hybrid (Numerical-physical) Model of the Left Ventricle

Hydraulic models of the circulation are used to test mechanical devices and for training and research purposes; when compared to numerical models, however, they are not flexible enough and rather expensive. The solution proposed here is to merge the characteristics and the flexibility of numerical models with the functions of physical models. The result is a hybrid model with numerical and physical sections connected by an electro-hydraulic interface - which is to some extent the main problem since the numerical model can be easily changed or modified.

The concept of hybrid model is applied to the representation of ventricular function by a variable elastance numerical model. This prototype is an open loop circuit and the physical section is built out of a reservoir (atrium) and a modified windkessel (arterial tree). The corresponding equations are solved numerically using the variables (atrial and arterial pressures) coming from the physical circuit. Ventricular output flow is the computed variable and is sent to a servo amplifier connected to a DC motor-gear pump system. The gear pump, behaving roughly as a flow source, is the interface to the physical circuit. Results obtained under different hemodynamic conditions demonstrate the behaviour of the ventricular model on the pressure-volume plane and the time course of output flow and arterial pressure.

Mono and Bi-ventricular Assistance: Their Effect on Ventricular Energetics

When mono- and bi-ventricular mechanical assistance is used for heart recovery, its control strategy and circulatory variables affect ventricular energetics (external work-EW, oxygen consumption-VO2, cardiac mechanical efficiency-CME). This study is based on the data obtained in vitro and presents an analysis of the effects of the mono- and bi-ventricular mechanical assistance on ventricular energetics. The assistance was conducted on the principle of counterpulsation with atrio-arterial connection. It includes the following stages: 1) the characterisation of the isolated ventricle model in terms of EW, VO2 and CME as a function of the filling pressure and peripheral resistance, 2) modelling of left ventricular and pulmonary dysfunction, followed by left ventricular and bi-ventricular assistance. Experimental data enable us to draw the following conclusions:

• in general, the greatest hemodynamic improvement does not correspond to the highest energetic improvement,

• LVAD assistance deteriorates left ventricular CME while its effect on right ventricular energetics depends on the value of right ventricular elastance (Emax). Right ventricular CME is deteriorated by BVAD assistance irrespective of right Emax,

• the energetics optimisation in bi-ventricular assistance is closely related to the right Emax, which could probably be a deciding factor in the choice of the assistance mode.

Regulation of Base Balance in Bicarbonate Hemofiltration

The purpose of this study was to investigate the feasibility of bicarbonate as a substitute for acetate or lactate in hemofiltration solutions using a new bag for the bicarbonate substitution fluid. We analysed 24 hemofiltration sessions with different HCO3– concentrations (30, 35 and 40 mEq/L) in the substitution fluid. The increase in the HCO3– concentration in the substitution fluid resulted in a more positive HCO3– balance. The Net Base Gains (NBG) were, respectively, 73.7 ± 92 with 30, 138.2 ± 97 with 35 (p < 0.05 vs 30) and finally 201 ± 65.9 with 40 mEq/L (p < 0.001 vs 30). The physical separation between the base losses and gains could facilitate the modelling approach in hemofiltration. By means of a stepwise regression analysis, we studied a series of variables that could influence end-treatment HCO3–, which was significantly and directly dependent (F = 6.003, r = 0.747, p = 0.0027) on the HCO3– concentration in the substitution fluid and the apparent HCO3– space. HCO3– values predicted by the statistical model correlated well with those actually measured (r = 0.757; p < 0.001). This mathematical modelling approach allowed us to predict the quantities and concentrations of HCO3– to be infused in order to obtain an ideal acidosis correction, tailored to individual patient needs.

Computer Simulation of Hemodynamic Parameter Changes by Mechanical Ventilation and Biventricular Circulatory Support

When a Bi-Ventricular Assist Device (BVAD) is used in conjunction with mechanical ventilation (MV) of the lungs with positive intrathoracic pressure (Pt), the latter influences hemodynamics. The aim of our study was to assess the simultaneous influence of BVAD and MV on hemodynamics. We assumed ventricular pathological conditions as reduced elastances and increased rest volumes. Peripheral systemic arterial resistance was assumed to have different values. Data were obtained by computer simulation. Trends in main hemodynamic variables were compared with clinical data from literature. Simulation showed that systemic venous, pulmonary arterial and left atrial pressures are very sensitive to Pt (-2 to 5 mmHg).

The Impact of Rotary Blood Pump in Conjunction with Mechanical Ventilation on Ventricular Energetic Parameters

Objectives: Aim of this work is to study the impact of left ventricular rotary blood pump assistance, on energetic variables, when mechanical ventilation (MV) of the lungs is applied.

Methods: Computer simulation was used to perform this study. Lumped parameter models reproduce the circulatory system. Variable elastance models reproduce the Starling’s law of the heart for each ventricle. After the reproduction of ischemic heart disease left ventricular assistance was applied using a model of rotary blood pump. The pump speed was changed in steps and was assumed to be constant during each step. The influence of mechanical ventilation was introduced by different values of positive mean thoracic pressure.

Results: The increase of the rotational speed has a significant influence on some ventricular energetic variables. In fact it decreased left ventricular external work, left and right ventricular pressure-volume area and the left ventricular efficiency. Finally, it increased the right ventricular efficiency but had no influence on the right ventricular external work. The increase of thoracic pressure from –2 to +5 mmHg caused a significant decrease of external work, pressure-volume area (right ventricular pressure-volume area dropped up to 50%) and an increase of right ventricular efficiency (by 40%) while left ventricular efficiency remained almost stable.

Conclusions: Numerical simulation is a very suitable tool to predict changes of not easily measurable parameters such as energetic ventricular variables when mechanical assistance of heart and/or lungs is applied independently or simultaneously.

In Vivo and Simulation Study of Artificial Ventilation Effects on Energetic Variables in Cardiosurgical Patients

Objectives: The analysis of energetic ventricular variable changes during artificial ventilation, obtained by numerical simulation was done. Twenty-one sets of hemodynamic parameters for eight cardiosurgical patients were used to estimate left and right stroke work. The data were collected for three methods of ventilation: conventional, lung-protective (with minute ventilation diminished by half) and high frequency ventilation (with frequency 5, 10, or 15 Hz).

Methods: The computer simulator (CARDIOSIM©) of the cardiovascular system, was used as a tool to calculate values of energetic ventricular variables for conditions that corresponded to these during in vivo measurements. Different methods of ventilation caused differences of intrathoracic pressure, haemodynamic and finally energetic ventricular variables. The trends of these variable changes were the same in in vivo and simulation studies, in the whole range of intrathoracic pressure changes (Pt = 1.5-3.5 mmHg).

Results: As values of main hemodynamic variables like cardiac output or arterial, systemic and pulmonary pressures were very close in both studies. Cardiac index and left ventricular stroke work also differed less than 10% for all examined patients and computer simulation. In a case of right ventricular stroke work the difference between in vivo data and simulation was a bit greater than 10% for two of eight patients under study.

Conclusions: Our comparative analysis proved that numerical simulation is a very useful tool to predict changes of main hemodynamic and energy-related ventricular variables caused by different levels of positive Pt. It means that it can help an anesthesiologist to choose an appropriate method of artificial ventilation for cardiosurgical patients.

Application of a user-friendly comprehensive circulatory model for estimation of hemodynamic and ventricular variables

Purpose

Application of a comprehensive, user-friendly, digital computer circulatory model to estimate hemodynamic and ventricular variables.

Methods

The closed-loop lumped parameter circulatory model represents the circulation at the level of large vessels. A variable elastance model reproduces ventricular ejection. The circulatory model has been modified embedding an algorithm able to adjust the model parameters reproducing specific circulatory conditions. The algorithm reads input variables: heart rate, aortic pressure, cardiac output, and left atrial pressure. After a preliminary estimate of circulatory parameters and ventricular elastance, it adjusts the amount of circulating blood, the value of the systemic peripheral resistance, left ventricular elastance, and ventricular rest volume. Input variables and the corresponding calculated variables are recursively compared: the procedure is stopped if the difference between input and calculated variables is within the set tolerance. At the procedure end, the model produces an estimate of ventricular volumes and Emaxl along with systemic and pulmonary pressures (output variables). The procedure has been tested using 4 sets of experimental data including left ventricular assist device assistance.

Results

The algorithm allows the reproduction of the circulatory conditions defined by all input variable sets, giving as well an estimate of output variables.

Conclusions

The algorithm permits application of the model in environments where the simplicity of use and velocity of execution are of primary importance. Due to its modular structure, the model can be modified adding new circulatory districts or changing the existing ones. The model could also be applied in educational applications.

Right Ventricular Assistance by Continuous Flow Device

Objectives·. This work is another step in the development of the circulatory model CARDIOSIM® and of its model library. Continuous flow assistance is often used to support the right ventricular failure. Computer simulation is one of the methods to study the effect of this assistance on the failing ventricle. The purpose of this study was to evaluate the effect of this support on some hemodynamic variables, when different right ventricular end-systolic elastance and pump speed values were applied.

Methods: The rotary blood pump model was included into the software package CARDIOSIM®, which reproduces the cardiovascular system. Lumped parameters models were used to reproduce the circulatory phenomena. Variable elastance models reproduced the Starling’s law of the heartfor both ventricles. In the study right ventricular end-systolic elastance (EmaxraT) and the rotational speed of the pumptookthree different values. All the other parameters of the model were constants.

Results: The rotational speed of the pump had a significant influence on right ventricular end-diastolic and end-systolic volumes, right atrial pressure (Pra), right ventricular (Qro) and pump flows. The effects on pulmonary arterial pressure (Pap) were more evident when the right ventricular end-systolic elastance was low. When the speed of the device increased the mean value of Pra decreased for each value of EmaxMT. The total flow (Qro + pump flow) increased when the speed of the pump increased.

Conclusions: Our simulation (in good agreement with the results presented in literature) showed that Hemo-pump produces a rise in total flow, a drop in blood flow pumped out by the right ventricle and a drop in right atrial pressure.

Towards a Personalized and Dynamic CRT-D

Background: In spite of cardiac resynchronization therapy (CRT) benefits, 25 – 30% of patients are still non responders. One of the possible reasons could be the non optimal atrioventricular (AV) and interventricular (VV) intervals settings. Our aim was to exploit a numerical model of cardiovascular system for AV and VV intervals optimization in CRT.

Methods: A numerical model of the cardiovascular system CRT-dedicated was previously developed. Echocardiographic parameters, Systemic aortic pressure and ECG were collected in 20 consecutive patients before and after CRT. Patient data were simulated by the model that was used to optimize and set into the device the intervals at the baseline and at the follow up. The optimal AV and VV intervals were chosen to optimize the simulated selected variable/s on the base of both echocardiographic and electrocardiographic parameters.

Results: Intervals were different for each patient and in most cases, they changed at follow up. The model can well reproduce clinical data as verified with Bland Altman analysis and T-test (p > 0.05). Left ventricular remodeling was 38.7% and left ventricular ejection fraction increasing was 11% against the 15% and 6% reported in literature, respectively.

Conclusions: The developed numerical model could reproduce patients conditions at the baseline and at the follow up including the CRT effects. The model could be used to optimize AV and VV intervals at the baseline and at the follow up realizing a personalized and dynamic CRT. A patient tailored CRT could improve patients outcome in comparison to literature data.

Modelling of Cardiovascular System: Development of a Hybrid (Numerical-Physical) Model

Physical models of the circulation are used for research, training and for testing of implantable active and passive circulatory prosthetic and assistance devices. However, in comparison with numerical models, they are rigid and expensive. To overcome these limitations, we have developed a model of the circulation based on the merging of a lumped parameter physical model into a numerical one (producing therefore a hybrid). The physical model is limited to the barest essentials and, in this application, developed to test the principle, it is a windkessel representing the systemic arterial tree. The lumped parameters numerical model was developed in LabVIEW environment and represents pulmonary and systemic circulation (except the systemic arterial tree). Based on the equivalence between hydraulic and electrical circuits, this prototype was developed connecting the numerical model to an electrical circuit--the physical model. This specific solution is valid mainly educationally but permits the development of software and the verification of preliminary results without using cumbersome hydraulic circuits. The interfaces between numerical and electrical circuits are set up by a voltage controlled current generator and a voltage controlled voltage generator. The behavior of the model is analyzed based on the ventricular pressure-volume loops and on the time course of arterial and ventricular pressures and flow in different circulatory conditions. The model can represent hemodynamic relationships in different ventricular and circulatory conditions.

Computer Simulation of Coronary Flow Waveforms during Caval Occlusion

Objectives: Mathematical modeling of the cardiovascular system is a powerful tool to extract physiologically relevant information from multi-parametric experiments. The purpose of the present work was to reproduce by means of a computer simulator, systemic and coronary measurements obtained by in vivo experiments in the pig.

Methods: We monitored in anesthetized open-chest pig the phasic blood flow of the left descending coronary artery, aortic pressure, left ventricular pressure and volume. Data were acquired before, during, and after caval occlusion.

Inside the software simulator (CARDIOSIM©) of the cardiovascular system, coronary circulation was modeled in three parallel branching sections. Both systemic and pulmonary circulations were simulated using a lumped parameter mathematical model. Variable elastance model reproduced Starling’s law of the heart.

Results: Different left ventricular pressure-volume loops during experimental caval occlusion and simulated cardiac loops are presented. The sequence of coronary flow-aortic pressure loops obtained in vivo during caval occlusion together with the simulated loops reproduced by the software simulator are reported. Finally experimental and simulated instantaneous coronary blood flow waveforms are shown.

Conclusions: The lumped parameter model of the coronary circulation, together with the cardiovascular system model, is capable of reproducing the changes during caval occlusion, with the profound shape deformation of the flow signal observed during the in vivo experiment. In perspectives, the results of the present model could offer new tool for studying the role of the different determinants of myocardial perfusion, by using the coronary loop shape as a “sensor” of ventricular mechanics in various physiological and pathophysiological conditions.

Evolution of Ventricular Energetics in the Different Stages of Palliation of Hypoplastic Left Heart Syndrome: A Retrospective Clinical Study

Hyperplastic left heart syndrome (HLHS) patients are palliated by creating a Fontan-type circulation passing from different surgical stages. The aim of this work is to describe the evolution of ventricular energetics parameters in HLHS patients during the different stages of palliation including the hybrid, the Norwood, the bidirectional Glenn (BDG), and the Fontan procedures. We conducted a retrospective clinical study enrolling all HLHS patients surgically treated with hybrid procedure and/or Norwood and/or BDG and/or Fontan operation from 2011 to 2016 collecting echocardiographic and hemodynamic data. Measured data were used to calculate energetic variables such as ventricular elastances, external and internal work, ventriculo-arterial coupling and cardiac mechanical efficiency. From 2010 to 2016, a total of 29 HLHS patients undergoing cardiac catheterization after hybrid (n = 7) or Norwood (n = 6) or Glenn (n = 8) or Fontan (n = 8) procedure were retrospectively enrolled. Ventricular volumes were significantly higher in the Norwood circulation than in the hybrid circulation (p = 0.03) with a progressive decrement from the first stage to the Fontan completion. Ventricular elastances were lower in the Norwood circulation than in the hybrid circulation and progressively increased passing from the first stage to the Fontan completion. The arterial elastance and Rtot increased in the Fontan circulation. The ventricular work progressively increased. Finally, the ventricular efficiency improves passing from the first to the last stage of palliation. The use of ventricular energetic parameters could lead to a more complete evaluation of such complex patients to better understand their adaptation to different pathophysiological conditions.

Switched ratiometric lock-in amplifier enabling sub-ppm measurements in a wide frequency range

Lock-in amplifiers (LIAs) are extensively used to perform high-resolution measurements. Ideally, when using LIAs, it would be possible to measure a minimum signal variation limited by the instrument input equivalent noise at the operating frequency and the chosen filtering bandwidth. Instead, digital LIAs show an unforeseen 1/f noise at the instrument demodulated output, proportional to the signal amplitude that poses a fundamental limit to the minimum detectable signal variation using the lock-in technique. In particular, the typical resolution limit of fast operating LIAs (>1 MHz) is of tens of ppm, orders of magnitude worse than the expected value. A detailed analysis shows that the additional noise is due to slow fluctuations of the signal gain from the generation stage to the acquisition one, mainly due to the digital-to-analog and analog-to-digital converters. To compensate them, a switched ratiometric technique based on two analog-to-digital converters alternately acquiring the signal coming from the device under test and the stimulus signal has been conceived. A field-programmabale gate array-based LIA working up to 10 MHz and implementing the technique has been realized, and results demonstrate a resolution improvement of more than an order of magnitude (from tens of ppm down to sub-ppm values) compared to standard implementations working up to similar frequencies. The technique is generally applicable without requiring calibration nor ad hoc experimental arrangements.

Sero-prevalence and epidemiology of peste des petits ruminants in Libya

We conducted a cross-sectional study during 2013 to quantify the serological prevalence of peste des petits ruminants (PPR) infection and to investigate host factors associated with PPR infection in small ruminants in Libya. A two-stage sampling design was carried out. A total number of 148 flocks owning at least 100 heads each were randomly selected. Sixteen to forty-eight samples were collected from each selected flock. A total number of 3,508 serum samples from unvaccinated animals were collected and analysed at IZSLER Brescia, Italy, by using competitive ELISA, IDvet innovative diagnostics (IDvet 310, France). The overall serological prevalence among SR was 33% (95% CI: 31.4-34.5). Significant differences between the prevalence in the geographical branches were observed. The lowest prevalence level was observed in Zawiyah branch (16.1%), whereas the highest value was obtained for the Sabha branch (56.8%). Considering the age, a serological prevalence of 24.7%, 31.5% and 42.1% was observed in SR <1 year, between 1 and 2 years and more than 2 years, respectively. Statistically significant differences (p < .001) in the sero-prevalence levels were also observed between the age groups. Our findings suggest that the southern part of Libya could be more exposed to the infections coming from the neighbouring countries and this should be better investigated to correctly identify wherever specific entry points can be considered at higher risk than others. The results also confirmed the endemic status of PPR in Libya, with a constant exposure to the infection of the animals during their life. In the framework of the global strategy for control and eradication of PPR, our results, even if obtained by a preliminary study, can contribute to the assessment of the epidemiological situation of PPR in Libya as required by the Stage 1 of the plan.

Occurrence and impact of domestic violence and abuse in gay and bisexual men: A cross sectional survey

This cross-sectional survey measured adult experience and perpetration of negative and potentially abusive behaviours with partners and its associations with mental and sexual health problems, drug and alcohol abuse in gay and bisexual men attending a UK sexual health service. Of 532 men, 33.9% (95% CI: 29.4-37.9) experienced and 16.3% (95% CI: 13.0-19.8) reported carrying out negative behaviour. Ever being frightened of a partner (aOR 2.5; 95% CI: 2.0-3.1) and having to ask a partner's permission (aOR 2.7; 95% CI: 1.6-4.7) were associated with increased odds of being anxious. There were increased odds of cannabis use in the last 12 months amongst men who reported ever being physically hurt (aOR 2.4; 95% CI: 1.7-3.6). Being frightened (aOR 2.2; 95% CI: 1.5-3.2), being physically hurt (aOR 2.3; 95% CI: 1.4-3.8), being forced to have sex (aOR 2.5; 95% CI: 1.3-4.9) and experiencing negative behaviour in the last 12 months (aOR 1.7; 95% CI: 1.2-2.5) were associated with increased odds of using a Class A drugs in the last 12 months. Sexual health practitioners should be trained with regards to the risk indicators associated with domestic violence and abuse, how to ask about domestic violence and abuse and refer to support.

Selective extraction of intracellular components from the microalga Chlorella vulgaris by combined pulsed electric field-temperature treatment

The synergistic effect of temperature (25-65 °C) and total specific energy input (0.55-1.11 kWh kgDW(-1)) by pulsed electric field (PEF) on the release of intracellular components from the microalgae Chlorella vulgaris was studied. The combination of PEF with temperatures from 25 to 55 °C resulted in a conductivity increase of 75% as a result of cell membrane permeabilization. In this range of temperatures, 25-39% carbohydrates and 3-5% proteins release occurred and only for carbohydrate release a synergistic effect was observed at 55 °C. Above 55 °C spontaneous cell lysis occurred without PEF. Combined PEF-temperature treatment does not sufficiently disintegrate the algal cells to release both carbohydrates and proteins at yields comparable to the benchmark bead milling (40-45% protein, 48-58% carbohydrates).

Note: Differential configurations for the mitigation of slow fluctuations limiting the resolution of digital lock-in amplifiers

The resolution of digital lock-in amplifiers working with a narrow bandwidth (<100 Hz) is limited by slow fluctuations, which can be two orders of magnitude larger (μV range) than the noise of the input amplifier (tens of nV). In order to tackle this issue, affecting state-of-the-art laboratory instrumentation and here systematically quantified, three differential sensing configurations are presented. They adapt to different setup conditions and are based on manual and automatic tuning of dummy references, allowing a 25-fold resolution improvement for enhanced long-term tracking of impedance sensors.