The ForceLAB simulator: Application to the comparison of current models of cardiomyocyte contraction.

Mathematical models are useful tools in the study of physiological phenomena. However, due to differences in assumptions and formulations, discrepancy in simulations may occur. Among the models for cardiomyocyte contraction based on Huxley's cross-bridge cycling, those proposed by Negroni and Lascano (NL) and Rice et al. (RWH) are the most frequently used. This study was aimed at developing a computational tool, ForceLAB, which allows implementing different contraction models and modifying several functional parameters. As an application, electrically-stimulated twitches triggered by an equal Ca2+ input and steady-state force x pCa relationship (pCa = -log of the molar free Ca2+ concentration) simulated with the NL and RWH models were compared. The equilibrium Ca2+-troponin C (TnC) dissociation constant (Kd) was modified by changing either the association (kon) or the dissociation (koff) rate constant. With the NL model, raising Kd by either maneuver decreased monotonically twitch amplitude and duration, as expected. With the RWH model, in contrast, the same Kd variation caused increase or decrease of peak force depending on which rate constant was modified. Additionally, force x pCa curves simulated using Ca2+ binding constants estimated in cardiomyocytes bearing wild-type and mutated TnC were compared to curves previously determined in permeabilized fibers. Mutations increased kon and koff, and decreased Kd. Both models produced curves fairly comparable to the experimental ones, although sensitivity to Ca2+ was greater, especially with RWH model. The NL model reproduced slightly better the qualitative changes associated with the mutations. It is expected that this tool can be useful for teaching and investigation.

Application based on the Canny edge detection algorithm for recording contractions of isolated cardiac myocytes.

BACKGROUND AND OBJECTIVE: The isolated cardiomyocyte preparation is amenable to several experimental approaches not suitable to the myocardial tissue, which has allowed the gain of important information on the pathophysiology of the cardiac muscle. Thus, the development of techniques for functional studies in this preparation is important. The goal of the present study was to develop a computer program to extract contraction traces generated by cyclic cell shortening from cardiomyocyte video image files. METHODS: The Canny algorithm, widely used for computer vision, was implemented for cell edge recognition and continuous tracking, so that changes in cardiomyocyte length could be monitored. The program was applied to demonstrate the effect of classical inotropic maneuvers on contraction parameters, as well as to assess the development of spontaneous activity in response to defibrillator-like electrical shocks in rat isolated cardiomyocytes. RESULTS: The method resulted in successful monitoring of variations in cell length during both electrically-triggered and post-shock spontaneous contractions, of which the rate was significantly related to shock strength. CONCLUSIONS: The proposed approach might be useful for analysis of contractile activity of isolated muscle cells, and allows detection of even the typically low-amplitude noisy spontaneous contractile events. Additionally, the experimental data suggest that the rate of spontaneous contraction could be used as an index of shock-induced electrical membrane damage.

System for open-chest, multidirectional electrical defibrillation

Abstract Introduction: Cardiomyocytes are more sensitive to stimulatory electrical fields when the latter are applied longitudinally to the cell major axis. In the whole heart, cells have different spatial orientations, which may limit the effectiveness of conventional electrical defibrillation (i.e., shock delivery in a single direction). This article describes the constructive aspects of a portable system for rapidly-switching, multidirectional stimulus delivery, composed of an electrical defibrillator and multielectrode-bearing paddles for direct cardiac defibrillation. Methods: The defibrillator delivers monophasic, truncated monoexponential waveforms with energy up to 7.3 J. Upon selection of the defibrillation modality (unidirectional or multidirectional), shock delivery is triggered through 1 or 3 outputs. In the latter case, triggering is sequentially switched to the outputs, without interval or temporal overlap. Each paddle contains 3 electrodes that define shock pathways spaced by 60°. The system was tested in vivo for reversal of experimentally-induced ventricular fibrillation in healthy swine, using 30- and 20-ms long shocks (N= 4 in each group). Results: The defibrillator delivers identical stimulus waveforms through all outputs in both stimulation modalities. In all animals, successful defibrillation required lower shock energy when 20 ms-long stimuli were applied in 3 directions, compared to a single direction. However, performance was poorer with multidirectional defibrillation for 30 ms-long shocks. Conclusion: The delivery of identical shock waveforms allowed confirmation that multidirectional defibrillation can promote restoration of sinus rhythm with lower shock energy, which may reduce myocardial electrical damage during defibrillation. Nevertheless, increase in shock duration greatly impairs the effectiveness of this defibrillation modality.

Medical equipment classification according to corrective maintenance data: a strategy based on the equipment age

INTRODUCTION: Decision-making on medical equipment management is a daily task for clinical engineers, but it may prove difficult to easily extract relevant information from the large amount of data from computerized maintenance management systems. This article describes a simple method of medical equipment classification based on corrective maintenance indicators. METHODS: Three indicators were calculated based on the number of events, duration and cost of corrective maintenance. Three classes were defined according to the indicator values of different equipment ages: class A for 0-4 years, class B for 5-9 years, and class C for equipment older than 10 years. The method was applied to 2,134 pieces of equipment from the Health Service system of the University of Campinas. RESULTS: From the total, 51.7% of the equipment were classified as C, 4.2% as B and 44.1% as A. The infusion pump for general use was the type of equipment of which most units were in the C class (84.7%), even though almost 50% of them were acquired within less than 9 years, and would thus be expected to be classified as A and B. Among the pumps in class C, 39.5% were from a single manufacturer, although the equipments were acquired recently. CONCLUSION: The developed classification may be an important tool for raising alerts about equipment more prone to maintenance problems, as well as for identification of equipments with acceptable maintenance history, supporting decision-making on equipment replacement.

The influence of cell dimensions on the vulnerability of ventricular myocytes to lethal injury by high-intensity electrical fields / Influência das dimensões celulares sobre a vulnerabilidade de miócitos ventriculares ao efeito letal de campos elétricos de alta intensidade

Application of high intensity electric fields (HIEF) to the myocardium is commonly used for cardiac defibrillation/cardioversion. Although effective at reversing life-threatening arrhythmias, HIEF may cause myocyte damage due to membrane electropermeabilization. In this study, the influence of cell length and width on HIEF-induced lethal injury was analyzed in isolated rat cardiomyocytes in parallel alignment with the field. The field-induced maximum variation of membrane potential (ΔVmax) was estimated with the Klee-Plonsey model. The studied myocyte population was arranged in two group pairs for comparison: the longest vs. the shortest cells, and the widest vs. narrowest cells. Threshold field intensity was significantly lower in the longest vs. shortest myocytes, whereas cell width influence was not significant. The threshold ΔVmax was comparable in all groups. Likewise, a significant leftward shift of the lethality curve (i.e., relationship of the probability of lethality vs. field intensity) of the longest cells was observed, evidencing greater sensitivity to HIEF-induced damage. However, the lethality curve as a function of ΔVmax was similar in all groups, confirming a prediction of the Klee-Plonsey model. The similar results for excitation and injury at threshold and HIEF stimulation, respectively, indicate that: a) the effect of cell length on the sensitivity to the field would be attributable to differences in field-induced membrane polarization that lead to excitation or lethal electroporation; b) the Klee-Plonsey model seems to be reliable for analysis of cell interaction with HIEF; c) it is possible that increased cell length in hypertrophied hearts enhances myocyte fragility upon defibrillation/cardioversion.

Campos elétricos de alta intensidade (HIEF) são aplicados ao miocárdio durante desfibrilação e cardioversão. Embora eficazes na reversão de arritmias potencialmente letais, HIEF podem lesar cardiomiócitos por eletropermeabilização da membrana. Neste estudo, a influência das dimensões celulares sobre o efeito letal de HIEF foi estudada em cardiomiócitos isolados de rato alinhados paralelamente ao campo. A máxima variação do potencial de membrana induzida pelo campo (ΔVmax) foi calculada com o modelo de Klee-Plonsey. As células estudadas foram distribuídas em dois pares de grupos de acordo com seu comprimento e largura. A intensidade limiar do campo não dependeu da largura celular, mas sim do comprimento (menor nas células mais longas, p < 0.001), enquanto ΔVmax no limiar foi comparável entre os grupos. Nas células mais longas, observou-se desvio à esquerda (p < 0.01) da curva que descreve a relação entre probabilidade de letalidade e a intensidade do campo, evidenciando maior sensibilidade à ação deletéria de HIEF. Porém, a curva de letalidade em função de ΔVmax foi semelhante em todos os grupos, o que confirma a predição pelo modelo de Klee-Plonsey. A similaridade de resultados com estimulação limiar e com HIEF indica que: a) o efeito do comprimento celular sobre a sensibilidade ao campo poderia ser atribuído a diferenças no grau de polarização da membrana durante a aplicação do estímulo; b) o modelo de Klee-Plonsey parece ser confiável para a análise da interação espacial da célula com HIEF; c) é possível que o maior comprimento celular em miócitos hipertrofiados os torne mais susceptíveis a lesão durante desfibrilação/cardioversão.

Lethal effect of electric fields on isolated ventricular myocytes.

Defibrillator-type shocks may cause electric and contractile dysfunction. In this study, we determined the relationship between probability of lethal injury and electric field intensity (E in isolated rat ventricular myocytes, with emphasis on field orientation and stimulus waveform. This relationship was sigmoidal with irreversible injury for E > 50 V/cm . During both threshold and lethal stimulation, cells were twofold more sensitive to the field when it was applied longitudinally (versus transversally) to the cell major axis. For a given E, the estimated maximum variation of transmembrane potential (Delta V(max)) was greater for longitudinal stimuli, which might account for the greater sensitivity to the field. Cell death, however, occurred at lower maximum Delta V(max) values for transversal shocks. This might be explained by a less steep spatial decay of transmembrane potential predicted for transversal stimulation, which would possibly result in occurrence of electroporation in a larger membrane area. For the same stimulus duration, cells were less sensitive to field-induced injury when shocks were biphasic (versus monophasic). Ours results indicate that, although significant myocyte death may occur in the E range expected during clinical defibrillation, biphasic shocks are less likely to produce irreversible cell injury.

New method for minimally invasive urodynamic assessment in men with lower urinary tract symptoms.

OBJECTIVES: Urodynamic studies are considered the reference standard to diagnose bladder outlet obstruction. However, the procedure is invasive, expensive, and time-consuming. The purpose of this study was to evaluate a new minimally invasive urodynamic assessment model and compare the results with those of conventional urodynamic evaluation. METHODS: The study included 50 male patients who presented with lower urinary tract symptoms. Their mean age was 62 years (range 34 to 82). After undergoing a conventional urodynamic study, they underwent the minimally invasive evaluation. The urethral device is a conical apparatus that adapts to the urethral meatus and fossa navicularis. The isometric bladder pressure and interrupted flow were recorded. The results of the conventional assessment were classified according to the Abrams-Griffiths number and a logistic regression fit was applied to the minimally invasive method. RESULTS: Only two variables demonstrated the predictive capacity: the isometric pressure and interrupted flow. In addition to selecting the relevant variables, logistic regression analysis is a more adequate model that provides a binary result of obstructed and unobstructed, used to predict the normal and equivocal categories of the Abrams-Griffiths classification, and taken as the reference standard. The sensitivity and specificity of the new method was 67% and 79%, respectively. CONCLUSIONS: The urethral device proved to be simple and easy to use. The minimally invasive method was able to detect most patients with bladder outlet obstruction; thus, the conventional urodynamic assessment could be avoided. We consider this method to have a place as a first-line noninvasive examination.

Compact cell image projector: application to study the relationship between stimulus interval and contraction amplitude in isolated rat cardiomyocytes / Projetor compacto de imagem celular: aplicação ao estudo da relação entre intervalo entre estímulos e amplitude de contração em cardiomiócitos isolados de rato

Cardiac inotropy depends, among other factors, on the interval between contractions. In this study, we developed instrumentation for cell shortening recording, which was used to investigate the influence of stimulatory rhythm on contraction amplitude of isolated rat ventricular myocytes. Peak cell shortening amplitude was recorded during electric stimulation at the average rate of 0.5 Hz with different stimulatory patterns: regular and pseudo-random rhythms, as well as double pulse stimulation. Cells were perfused at 23 ºC with modified Tyrode’s solution with or without 10 nM isoproterenol (ISO). The main advantages of the developed microscopy system were its relatively low cost(~US$ 1,000.00), small size (150 × 170 × 300 mm), and absence of detectable optic distortions. We observed that average contraction amplitude was similar for all stimulatory patterns, in the absence and presence of ISO (p > 0.05), although the amplitude of individual contractions was highly dependent on the previous interval, and was significantly increased by ISO (p < 0.05). With the double pulse patterns, the amplitude ratio of contractions following the shorter and the longer intervals was ~0.55. ISO positive inotropic effect was more prominent for contractions after short intervals, which increased the ratio to ~0.80. This might be explained by acceleration of the recovery of sarcoplasmic reticulum Ca2+ release channels from the adapted state, possibly by proteinkinase A-dependent phosphorylation, which would resultin enhanced systolic Ca2+ release.

O inotropismo cardíaco depende de inúmeros fatores, entre eles o intervalo entre contrações. Neste trabalho, desenvolvemos instrumentação para registro de encurtamento celular e investigamos a influência do ritmo estimulatório sobre a atividade contrátil de miócitos ventriculares isolados de rato. A amplitude do encurtamento celular foi registrada durante estimulação elétrica à freqüência média de 0,5 Hz, com ritmo regular, ritmo pseudoaleatório e pulsos duplos. Os miócitos foram perfundidos a 23 ºC com solução de Tyrode modificada contendo ou não 10 nMde isoproterenol (ISO). O sistema de microscopia desenvolvido é de custo relativo baixo (~US$ 1.000,00), dimensões reduzidas(150 × 170 × 300 mm) e apresenta boa qualidade óptica (sem distorções ou paralaxe detectáveis). Observamos que a amplitude média das contrações foi semelhante em todos os ritmos estimulatórios na ausência e presença de ISO (p > 0,05), embora a amplitude de contrações individuais fosse dependente do intervalo precedente, e ISO tenha causado aumento da amplitude média das contrações (p < 0,05). Nos padrões com pulso duplo, a razão de amplitude das contrações que seguem o menor e o maior intervalo foi ~0,55. O efeito inotrópico positivo de ISO foi mais pronunciado para contrações após intervalos curtos, o que levou a razão para ~0,80. Isto poderia ser explicado por aceleração da recuperação dos canais de liberação de Ca2+ do retículo sarcoplasmático do estado adaptado, causada possivelmente por fosforilação pela proteína quinase A, o que aumentaria a quantidade de Ca2+ liberada durante a sístole.

Resposta contrátil de miócitos cardíacos isolados a níveis fisiológicos de pressão hidrostática / Contractile response of isolated cardiac myocytes to physiological levels of hydrostatic pressure

Com o advento da técnica de isolamento de células cardíacas por digestão enzimática, tornaram-se possíveis inúmeros estudos sobre a atividade contrátil cardíaca. Muito se tem progredido no estudo do acoplamento excitação-contração no coração sadio e doente. Contudo, na situação fisiológica, as células do coração estão submetidas a pressões da ordem de 100 mmHg durante parte do cliclo cardíaco, condição na qual se desconhece o comportamento dos miócitos, provavelmente pela dificuldade de montagem de preparação adequada. Com a possibilidade de medição de encurtamento celular por meio da detecção de borda de sinal de vídeo, realizamos um estudo do efeito de níveis diferentes de pressão, na faixa fisiológica, sobre a atividade contrátil de miócitos ventriculares isolados de ratos adultos, usando para isto uma câmara de perfusão desenvolvida no nosso laboratório. Os resultados indicaram que, para pressões supra-atmosféricas de até 140 mm-Hg, o limiar estimulatório, amplitude do encurtamento, tempo para encurtamento máximo e duração total da contração não foram afetados significativamente. Concluímos que a pressão hidrostática aplicada não deve submeter os miócitos a qualquer tensão, indicando que estes aparentemente comportam-se como recipientes homogeneamente preenchidos de fluido incompressível.