Novel laparoscopic training system with continuously perfused ex-vivo porcine liver for hepatobiliary surgery.

OBJECTIVE: To introduce a novel laparoscopic training system with a continuously perfused ex-vivo porcine liver for hepatobiliary surgery. BACKGROUND: Existing models for laparoscopic training, such as box trainers and virtual reality simulators, often fail to provide holistic training and real haptic feedback. We have formulated a new training system that addresses these problems. METHODS: Real-Liver Laptrainer consists of a porcine liver, customized mannequin, ex-vivo machine perfusion system, and monitoring software. We made a detailed comparison of Real-Liver Laptrainer with the LapSim virtual reality simulator and the FLS Trainer Box systems. Five laparoscopic surgeons assessed the new system on multiple features. We assessed the performances of 43 trainees who used the new system to perform laparoscopic cholecystectomy (LC) three times. RESULTS: Real-Liver Laptrainer offered more functions and better tactile feedback than the FLS or LapSim system. All five surgeons graded the quality of the new system as realistic. The utility of the system for training was scored as 3.6 ± 1.1 on a scale of 1-5. Between the first and third attempts, the number of successfully performed LCs increased (9 vs 14 vs 23; P = .011), while the numbers of liver damage incidents (25 vs. 21 vs. 18, P = .303) and gallbladder perforations decreased (17 vs. 12 vs. 9, P = .163). The mean LC operation time significantly decreased (63 vs. 50 vs. 44, P < .0001). CONCLUSION: Real-Liver Laptrainer is a feasible, stable, and practical training model that has potential for improving the laparoscopic skills of surgeons.

Effects of electrical heterogeneity on transmural reentry during acute global ischemia.

Ventricular arrhythmias are commonly observed in patients with ischemia. It is reported that the electrophysiological changes evoked by ischemia are greater in the epicardium than in the endocardium. To investigate the effects of this heterogeneity on transmural reentry, the computer simulation method is used. A two-dimensional model which can reproduce the endocardial, epicardial and middle cell types, approximate the ischemic characteristics and distribution of the ischemic severity is developed by setting different ratios of the maximum conductance of the rapid and slow inward rectifier potassium currents and considering the three major component conditions of acute ischemia at the ionic level. The results demonstrate that action potentials of the ischemic cells have elevated resting potential, shortened duration, slowed upstroke and declined amplitude. Conduction velocity is much more depressed in the epicardium because of the ischemia-induced transmural gradient of excitability. The epicardially initiated activation has wider vulnerable window and more possibility to cause unidirectional propagation even reentry. Dispersion of the excitability is proposed to be the underlying mechanism.

[Short-term memory study of the ventricular mathematical model].

Ventricular fibrillation (VF) is the main cause of sudden death in cardiovascular diseases. The effects of electrophysiological heterogeneity and dynamic factors bring a lot of difficulties in understanding and revealing the mechanisms of VF. Cardiac short-term memory is one of the factors that make invalidation of restitution curve slope as the criteria for transition from ventricular tachycardia to VF. Therefore, investigation of inherent properties of short-term memory and its role in VF has great significance. In this paper we took advantage of the perturbed down-sweep protocol to measure dynamic and local S1S2 restitution curves on three widely used mathematical models to reveal their memory property. And by making abrupt change of the stimulation cycle length, we examined the attenuation process of the memory. The results showed, the rate-dependent action potendial duration (APD) is related with S1 pacing. The APD difference under different S2 cycle length is more pronounced with short S1 cycle length. Except the Luo-Rudy 1991 model, the Luo-Rudy 1994 and Noble 1991 models can at least reflect some of the short-term memory. And the memory attenuated in exponential way. Therefore, in quantitative electrophysiological study, these two models can be used in the future for investigating the characteristics of the short-term memory and its contribution to VF.

[Algorithm study on the three-dimensional cardiac tissue based on the model of ventricular action potential].

Cardiac reentry is one of the important factors to induce arrhythmias. It could lead to ventricular tachycardia (VT) or even fibrillation (VF), resulting in sudden cardiac death. With the wide use of computer in the quantitative study of electrophysiology, the three-dimensional virtual heart for simulations needs to be developed imminently in computer. In this paper, numerical algorithm of the model was studied. The three-dimensional model was constructed by integrating Luo-Rudy 1991 ventricular cell model and diffusion equation. The operator splitting method was employed to solve the model. The alternate direction iterative (ADI) format and seven-point centered difference method were used for the partial differential equation. And the discrete format with second-order accuracy was taken for the boundary conditions. The results showed that the ADI format and seven-point centered difference method both could successfully figure out the membrane potential and electrical activities with good numerical stability. However, computing consumption could be greatly reduced with the ADI format, implying that the ADI method with large time step was more powerful in numerical simulations.

[The effects of hypokalemia on the Na+ channel in cardiac tissue--a computer simulation study].

In order to explore the reason why hypokalemia could increase the vulnerable window (VW) for unidirectional conduction block in Long QT Syndromes (LQTS), we observed the effect of hypokalemia on the spatial gradients of Na channel conductance (G(Na)) and gating factors by using the LR91 1-dimensional heterogeneous virtual cardiac ventricular tissue model quatitively. The computer simulation experiments were divided into two groups, namely control and LQTS groups. The action potential was elicited after the basic stimulus S1 (-70 microA/microF, 1.5 ms) was given 10 times with basic cycle length (BCL) 500, 1000 and 2000 ms. To test the VW in unit of time (VWtime), the S1-S2 programmed stimuli were used with shortening S1S2 interval at the decrement of 1 ms. At the same time, the spatial gradients of Na channel conductance (G(Na)) and gating factors, m, h, j, were investigated. The APD and ionic channel currents were also detected under the conditions of normal and lower concentration of K+ outside of cell. We found that hypokalemia, LQTS and slow pacing rate enhanced the spatial gradient of G(Na) by increasing the spatial gradient of inactive gating factors h x j. The results also showed that hypokalemia deduced the peak values of I(K) and I(K1), which prolonged the action potential duration and enlarged the repolarization dispersion in this 1-D tissue cable model. Possibly these are the important factors to cause the spatial gradient of h x j and G(Na). enlargement. These changes increase the incidence of unidirectional conduction block of VW, and are vital reasons to increase the possibility of ventricular arrhythmia generation.

[An optical mapping system based on spectral shift of voltage-sensitive dyes].

Recently, non-invasive optical methods to monitor transmembrane electrical potential using voltage sensitive dyes have been applied widely in the studies of normal and pathological heart rhythms and defibrillation. In the present paper, the authors measured the excitation and the emission spectra of the voltage-sensitive dyes di-4-ANEPPS bound to phospholipid bilayer membranes. And according to the spectral shift of di-4-ANEPPS, the authors presented an optical mapping system combining a DALSA CCD camera and a LED light source. Using this optical mapping system, the authors could record the action potential duration of the heart cells with high spatial and temporal resolutions. It can be a powerful tool in the study of cardiac arrhythmia mechanisms.

Study for relevance of the acute myocardial ischemia to arrhythmia by the optical mapping method.

Ventricular arrhythmias are commonly observed in patients with acute coronary occlusion and ischemia. The purpose of the present study is to determine ischemic electrophysiological effects and their role in ischemia-induced arrhythmia. Optical mapping of the membrane potential with voltage-sensitive dyes was used in the study. The mapping was performed with di-4-ANEPPS in Langendorff-perfused rabbit hearts. The excitation-contraction decoupler 2,3-butanedione monoxime was used to suppress motion artifacts caused by contraction of the heart. The acute global ischemia was developed by a rapid reduction of the flow rate. The experiments revealed that ischemic tissues were characterized by an obvious reduction in action potential duration and action potential upstroke, slower conduction velocity (CV) and the property of post-repolarization refractoriness. Moreover, the magnitude of CV reduced both in control and ischemia when the pacing cycle length was short. CV reduction was even early in ischemia, resulting in a broader curve during ischemia. Moreover, the dominant frequency of ventricular tachycardia/ventricular fibrillation (VT/VF) in ischemia was less than that in control, implying a decreasing tendency of VT/VF frequency for low excitability. Therefore, combined with our previous simulation study, the dynamic changes of CV and longer refractory period were suggested to play an important role in the ischemia-related arrhythmia. Low excitability in ischemic tissue was the fundamental mechanism in it.

Examination of depth-weighted optical signals during cardiac optical mapping: a simulation study.

Optical mapping has become a powerful tool to explore complex cardiac propagation. Many experiments and studies claimed that the fluorescence obtained from tissue surface is the averaged response of the transmembrane potential upon probing depth rather than only on the surface. With the electrical propagation model and the photon transport model, the effects of depth-weighted optical signals are examined both during a normal excitation wave and a spiral wave. Our results indicate that depth-weighted optical signals may infer cardiac activation dynamics, such as the mode and the direction of the propagation, the spatial distribution of depolarization or repolarization.

Mechanisms of the acute ischemia-induced arrhythmogenesis--a simulation study.

The underlying ionic mechanisms of ischemic-induced arrhythmia were studied by the computer simulation method. To approximate the real situation, ischemic cells were simulated by considering the three major component conditions of acute ischemia (elevated extracellular K(+) concentration, acidosis and anoxia) at the level of ionic currents and ionic concentrations, and a round ischemic zone was introduced into a homogeneous healthy sheet to avoid sharp angle of the ischemic tissue. The constructed models were solved using the operator splitting and adaptive time step methods, and the perturbation finite difference (PFD) scheme was first used to integrate the partial differential equations (PDEs) in the model. The numerical experiments showed that the action potential durations (APDs) of ischemic cells did not exhibited rate adaptation characteristic, resulting in flattening of the APD restitution curve. With reduction of sodium channel availability and long recovery of excitability, refractory period of the ischemic tissue was significantly prolonged, and could no longer be considered as same as APD. Slope of the conduction velocity (CV) restitution curve increased both in normal and ischemic region when pacing cycle length (PCL) was short, and refractory period dispersion increased with shortening of PCL as well. Therefore, dynamic changes of CV and dispersion of refractory period rather than APD were suggested to be the fundamental mechanisms of arrhythmia in regional ischemic myocardium.

[Computer simulation study of the re-entry mechanisms in one-dimensional ischaemic myocardium].

Torsades de Pointes is a kind of severe ventricular arrhythmia. Myocardial ischaemia is one of the major causes leading to TdP. In this paper the mechanisms of the TdP were quantitatively studied under the condition of ischaemia based on the Noble98 dynamic model of the ventricular action potential. The study was conducted on one-dimensional homogeneous myocardium with the method of computer simulation. The models were firstly developed to simulate the lower excitability, extracellular accumulation of the K+ concentration or the decreased gap junctions in ischaemic myocardium. By separately reducing the Na+ conductance, increasing the extracellular K+ concentration or decreasing the conductance of the gap junctions enabled us to study the effect of each change in isolation. Then different degrees of ischaemic models were established to study their physiological features. The study showed that the conduction velocity became slower with the ischaemia aggravation, the action potential duration became shorter and the width of the vulnerable window obviously became larger than the normal conditions. The results illustrated that ischaemia was easily leading to unidirectional conduction block and resulted in re-entry and arrhythmias.

[Computer simulation methods of cardiac electrophysiology].

Computer simulation is one of the powerful protocols to study electrophysiology theoretically. In this paper,the algorithm of Rush and Larsen was used to solve the ordinary differential equations (ODE's) in the Luo-Rudy models of mammalian ventricular cell. The operator splitting and adaptive time step methods were used to solve the partial differential equations (PDE's) in cardiac tissue conduction models. Using these methods we accomplished the simulation programs of single ventricular cell model and two-dimensional (2-D) tissue model. The methods of initiating spiral waves were studied with this software. The data obtained from 2-D simulation can be used for further study on isopotential contour lines, spiral wave tip trajectories, and pseudo-ECG.

[Study of cellular electrophysiology based on Noble98 dynamic model of ventricular action potential].

On the basis of mammalian ventricular action potential model Noble98, and with the use of Runge-Kutta for solution, the Wenckebach periodicity phenomenon, the transmural heterogeneity of the ventricular myocardium and its rate dependence are studied. The results indicate that these inherent properties may, lead to temporal-space disorganized in the normal heart,and may become the underlying factors for arrhythmias. At the same time, in this study are established the basic methods for quantitative cellular electrophysiology which is essential for future studies on the mechanism of arrhythmia.

Computer simulation of the reentrant cardiac arrhythmias in ischemic myocardium.

Computer simulation was performed to determine how reentrant activity could occur due to the spatial heterogeneity in refractoriness induced by the regional ischemia. Two regional ischemic models were developed by decreasing the intracellular ATP concentration, reducing conductance of the inward Na+ current and increasing the extracellular K+ concentration on the two-dimensional sheet. Operator splitting method was used to integrate the models. The vulnerability to reentry was estimated from the timings of premature stimuli on the constructed models, which could result in unidirectionally propagating action potentials. Two kinds of sustained spiral waves and their Pseudo-Electroscardiograms were observed in numerical simulation. The results showed that the dispersion of refractory period increased with ischemic aggravation, and led to augment of the vulnerable window. A permature stimulation within the vulnerable window could easily induce spiral reentry. The Pseudo-Electrocardiograms of the spiral waves exhibited monomorphic tachycardiac waveforms. Thus, the spatial heterogeneity in refractoriness could be a substrate for reentrant ventricular tachyarrhythmias on the regional ischemic tissue.

[Mathematical model of cardiac action potential and its computer simulations].

Malignant arrhythmias and ventricular fibrillation are generally accepted as one of the major causes of death in cardiovascular diseases. Based on the H-H equations, the mathematical model of the cardiac cell action potential consists of the ion channels, pumps, exchangers and transporters that are closely connected with intra- and extra-cellular ion concentrations, the channel's conditions, nerve transductors and drugs. It can build the link between cell electrophysiology and clinical pathophysiology. By altering the cellular environments the computer simulating study on this kind of model can help us look into the electrophysiological changes of the cardiac tissue and even the whole heart and investigate the mechanisms of the cardiac arrhythmias as well. The components of the model and its computer simulating study are introduced in the paper.

Vulnerability during short-term memory induced response in canine ventricle.

Cardiac short-term memory is an intrinsic property which can make the action potential duration produce a transient response after a sudden change in heart rate. The change of vulnerability was investigated by using computer simulation method during the transient period which was created by abruptly shortening the cycle length from 800ms to 300ms. The study was performed on a heterogeneous fiber consisting of endo-, mid-, and epi-cardiac canine myocytes. An OpenMP parallel algorithm was used to accelerate the calculation. The study shows that the vulnerable window (VW) relied on both pacing times and locations. At the cycle length of 300ms, compared with the situation of 500th beat, there was a large transmural dispersion of repolarization (TDR) at the 30th beat. For most of the sites along the fiber, VW consistently demonstrated widely at the beginning of the transient period. Generally, with sustained pacing, VW tended to become small. The results suggested that during a memory-induced transient response, the probability of an occurrence of reentrant wave increased immediately after an abrupt change in pacing rate because of the relatively large TDR and VW within this period. Therefore, avoidance of a sudden heart rate variation was indicated to be helpful for the suppression of reentrant arrhythmias.