Electrotonic Inhibition and Active Facilitation of Excitability in Ventricular Muscle

Inhibition and Facilitation in Cardiac Muscle. Introduction: The effects of subthreshold electrical pulses on the response to subsequent stimulation have been described previously in experimental animal studies as well as in the human heart. In addition, previous studies in cardiac Purkinje fibers have shown that diastolic excitability may decrease after activity (active inhibition) and, to a lesser extent, following subthreshold responses (electrotonic inhibition). However, such dynamic changes in excitability have not been explored in isolated ventricular muscle, and it is uncertain whether similar phenomena may play any role in the activation pal-terns associated with propagation abnormalities in the myocardium. Methods and Results: Experiments were performed in isolated sheep Purkinje fibers and papillary muscles, and in enzymatically dissociated guinea pig ventricular myocytes. In all types of preparations introduction of a conditioning subthreshold pulse between two subthreshold pulses was followed by a transient decay in excitability (electrotonic inhibition). The degree of inhibition was directly related to the amplitude and duration of the conditioning pulse and inversely related to the postconditioning interval. Yet, inhibition could be demonstrated long after (> 1 sec) the end of the conditioning pulse. Electrotonic inhibition was found at all diastolic intervals and did not depend on the presence of a previous action potential. In Purkinje fibers, conditioning action potentials led to active inhibition of subsequent responses. In contrast, in muscle cells, such action potentials had a facilitating effect (active facilitation). Electrotonic inhibition and active facilitation were observed in both sheep ventricular muscle and guinea pig ventricular myocytes. Accordingly, during repetitive stimulation with pulses of barely threshold intensity, we observed: (1) bistability (i.e., with the same stimulating parameters, stimulus: response patterns were either 1:1 or 1:0, depending on previous history), and (2) abrupt transitions between 1:1 and 1:0 (absence of intermediate wenckebach-like patterns). Simulations utilizing an ionic model of cardiac myocytes support the hypothesis that electrotonic inhibition in well-polarized ventricular muscle is the result of partial activation of I_k following subthreshold pulses. On the other hand, active facilitation may be the result of an activity-induced decrease in the conductance of I_K1. Conclusion: Diastolic excitability of well-polarized ventricular myocardium may be transiently depressed following local responses and transiently enhanced following action potentials. On the other hand, diastolic excitability decreases during quiescence. Active facilitation and electrotonic inhibition may have an important role in determining the dynamics of excitation of the myocardium in the presence of propagation abnormalities.     

单发室性早搏中RT间期规律的分析

目的探讨室性早搏前后RT间期变化的规律。方法选取单发、有完全代偿间期的室性早搏心电图,测量室早及其前后相邻波群(简称室早前、室早、室早后)的RT间期,进行对比。对室早前心室舒张时间与室早RT间期变化的关系进行分析。结果RT间期由短到长的顺序为:室早<室早前<室早后。室早前心室舒张时间与室早RT间期无线性相关关系。结论室早时RT间期最短,室早后RT间期最长,这一变化规律与心室舒张时间无关。     

Mental stress-induced myocardial ischemia in coronary artery disease patients with left ventricular dysfunction

Background  Reduced left ventricular ejection fraction (LVEF) is a risk factor for poor outcomes in patients with coronary artery disease (CAD). Mental stress-induced myocardial ischemia (MSIMI) also identifies a subset of CAD patients at increased risk for future cardiovascular events. Susceptibility to MSIMI in patients with CAD and reduced LVEF is unknown. Methods and Results  We enrolled 182 patients (67 women) with a mean age of 64 years and a documented history of CAD in this study. Baseline resting ejection fraction was determined by use of technetium 99m sestamibi gated single photon emission computed tomography. Abnormal LVEF was defined as less than 45% for men and less than 50% for women (based on published norms for our software [Cedars-Sinai Medical Center]). All participants underwent mental stress testing with a public speaking task. Rest/stress myocardial perfusion single photon emission computed tomography was performed via conventional methodology. Images were visually compared for number and severity of perfusion defects by use of a scoring method from 0 to 4. A summed difference score was calculated as the difference between summed stress and rest scores. A score of greater than 3 was considered abnormal. MSIMI developed in 19% of patients with normal LVEF and 31% of those with reduced LVEF. There is no statistically significant difference between the two groups (P=.11). Conclusions  CAD patients with left ventricular dysfunction are equally susceptible to MSIMI as those with normal LVEF. This study was supported by grants HL 070265 and HL 072059 from the National Heart. Lung, and Blood Institute. This material is also the result of work supported by resources and with the use of facilities at the Department of Veterans. Affairs Medical Center, Gainesville, Fla.     

The vanishing vast ventricular thrombus.

A 54-year old man presented with multiple pulmonary emboli and an incidental finding of a huge left ventricular thrombus. Transthoracic echo images demonstrated a globally dilated heart with very poor left ventricular function. It was elected to manage the patient medically, and he was commenced on warfarin therapy, resulting in completed resolution of the thrombus over 10 weeks. No underlying cause was found and he did not experience any further embolic events. This illustrates a rare case of a large ventricular thrombus in a patient with no underlying risk factors.     

多普勒超声心动图对儿童急性肾炎早期左室舒张功能的研究

目的　采用多普勒超声心动图评价儿童急性肾炎 (AGN)早期左室舒张功能 (LVDF)损害的严重程度及疗效观察。方法　将 76例急性肾炎患儿按病情程度分为轻度、重度两组与 5 4例健康对照组进行对照。应用HP - 10 0 0型超声诊断仪 ,分别于治疗前和治疗后 2周测定二尖瓣舒张早期峰值流速 (EV)、二尖瓣舒张晚期峰值流速 (AV)、EV AV比值、舒张早期充盈分数(EI)、舒张晚期充盈分数 (AI)、EI AI比值、E峰加速时间 (AT)、E峰减速时间 (DT)、DT AT比值、肺静脉收缩期最大前向血流速度(S)、舒张期最大前向血流速度 (D)、D S比值、左房内径 (LA)、主动脉内径 (AO)、LA AO比值等 11项指标。结果　EV AV、EI AI、LA AO、D S是判断LVDF损害的重要指标 ;LVDF损害与病情严重程度有关 ,其中 ,EV、EV AV、EI、EI AI、AT、D S随病情加重而降低 ,AV、AI、DT、DT AT、LA AO随病情加重而增高 ;EI、AI、AT、DT、DT AT是评价其疗效的重要指标 ;治疗后 ,LVDF恢复时间晚于临床症状恢复时间。结论　上述指标可动态检测LVDF ,对指导临床诊治具有重要意义。同时该检测方法操作简便易行、重复性强、无痛苦 ,结果较为精确。     

Cardiovascular malformations and organic solvent exposure during pregnancy in Finland

In order to investigate the possible association between cardiovascular malformations and maternal exposure to organic solvents during the first trimester of pregnancy, 569 cases and 1,052 controls were retrospectively studied. The cases represented all infants with diagnosed cardiovascular malformations born in Finland in 1982-1984, and the controls were randomly selected from all normal births in the country during the same period. All mothers were interviewed approximately 3 months after delivery by a midwife using a structured questionnaire. Exposures to organic solvents at work during the first trimester of pregnancy were slightly more prevalent among the mothers of affected infants (10.4%) than among those of controls (7.8%). Logistic regression analysis of exposure to organic solvents showed an adjusted relative odds ratio of 1.3 (95% confidence interval, 0.8-2.2). In the analysis of ventricular septal defect, exposure to organic solvents showed an adjusted relative odds ratio of 1.5 (95% confidence interval, 1.0-3.7).     

核素、多普勒和心尖搏动图反映冠心病左室舒张功能的比较——Fish判别分析

用电子计算机对核素、多普勒和心尖搏动图中的8项左室舒张功能指标进行判别分析,比较各指标与冠心病的密切程度。结果显示患者年龄和各项舒张功能指标明显不同于正常组。判别方程为:Z=0.21X_1+23.86X_2-22.88X_3-0.18X_4+2.83X_5+2.06X_6+66.86X_7+1.66X_8。临界值为42.99。判别正确率为100％。各指标的贡献率大小依次为EV/AV,1/3FF,A/E-O,PFR,DATI,EDC,IRT和1/3FFd。提示:多普勒EV/AV与冠心病的关系最为密切,核素的1/3FF次之,心尖搏动图的A/E～O亦是一个较好的指标。     

高血压患者左室构型和房性心律失常的关系

目的了解高血压患者左室构型和房性心律失常关系。方法应用超声心动图及动态心电图，测定左室结构及房性心律失常发生率。结果Ｋｌｅｉｇｅｒ３～６发生和构型有联系（Ｐ＜０．００１）。结论左室构型影响房性心律失常。     

左室肥厚对急性心肌梗塞时心肌酶的影响

通过２０例有左室肥厚和２３例无左室肥厚患者发生急性心肌梗塞时ＣＰＫ值的比较，发现在梗塞部位相同的情况下，心肌肥厚组的酶值水平高于非心肌肥厚组（１３．２±４．８比９．０±４．６Ｕ／ＬＰ＜０．０１），这可能与肥厚组酶活力增强和心肌梗塞体积增大有关。     

Acute Efficacy and Chronic Follow-Up of Patients with Non-Thoracotomy Third Generation Implantahle Defibrillators

Non-thoracotomy implantation of implantable cardioverter defibrillators (ICDs) has simplified the process of device inserfion, promising to decrease associated procedural coniplications while providing sudden death protection at least equal to epicardial systems. This study presents the acute and chronic results of 110 patients who underwent attempted non-thoracotomy ICD impiuntation wiih the Medtronic Transvene lead system and PCD model 7217 or 7219. Of the 110 patients attempted, 100 (91%) had the system successfully implanted without the need for an epicar-dial patch. One patient died 1 week postoperatively of septic shock related to the implantation (0.9% perioperative mortality). During folloiv-up of 16 ± 11 months, 45% of the patients had an event detected as ventricular tachycardia; 26% of these detections were felt clinically to be due to supraventricular rhythms. Of the remainder, 87% were successfully treated with the first VT therapy, and 98% were terminated by the final therapy; 66% of the patients had at least one episode of ventricular fibrillation, of which 5% were felt to be inappropriate detections; 65% of the appropriate episodes were successfully treated with the first VF therapy, and all were converted by the final therapy. Total mortality at 6, 12, and 24 months was 3%, 11%, and 19% respectively. Only one patient had sudden cardiac death, occurring at 13 months postimplant. Overall, the non-thoracotomy lead system for this ICD displayed infrequent implant complications and proved to be reliable ai terminating arrhythmias and maintaining a low rate of sudden cardiac death in this high risk popuiation.