Complexity of AV nodal function: complex nodal structure or complex behavior of nodal elements?

This paper discusses the hypothesis that the complex behavior of the atrioventricular (AV) node, with particular reference to Wenckebach periods, may be explained by the complexity of nodal structure and that complicated function of individual AV nodal fibers need not necessarily be considered. To prove this hypothesis, a computer model of cardiac excitation has been employed to simulate AV nodal function for two different anisotropic AV nodal model images. The experimental computer results show that the pattern of Wenckebach periods, and decremental and concealed AV nodal conduction can be explained without considering decremental conduction properties of individual AV nodal cells.

Hysteresis of the ventricular paced QT interval in response to abrupt changes in pacing rate

The rate of QT adaptation to abrupt changes in pacing rate was studied in seven patients with newly diagnosed complete heart block with a ventricular escape rate of less than 40 beats.min-1. Their median age was 70 (range 36-84) years, and none was taking any cardioactive medication known to affect the QT interval. From a baseline pacing rate of 50 or 110 beats.min-1 the ventricular rate was increased or decreased to a new level. The time taken for the ventricular paced QT interval to complete 90% of the change secondary to the change in rate was found to be 136(16) s (mean(SEM] when the rate was increasing and 189(25) s when the rate was decreasing (p less than 0.01). This time interval was independent of the magnitude of the rate change and the baseline heart rate from which the change occurred. Furthermore, the time course of QT adaptation was found to be exponential and was characterised by a time constant of 49.1(2.2) s when the rate was increasing and 60.4(2.0) s when the rate was decreasing (p less than 0.01). It is concluded that QT measurements in response to a change in pacing rate should take into account the time dependent nature of QT changes.

Computer modeling of DDD pacemakers for use in prophylaxis of junctional reentry tachycardia

A computer simulation model of cardiac rhythm and heart-pacemaker interactions has been used to examine the response of short AV delay DDD pacing to an atrial premature depolarization in patients with a potential junctional reentry circuit. Special attention was given to the possibility of using the pacemaker to prevent atrioventricular reentry tachycardia mediated by a bidirectional accessory pathway. The computational experiments examined: (a) the differences between pacing modes with constant and quasi-Wenckebach prolongation of AV delay; (b) the effects of imposing the restrictions of a pacemaker atrial refractory period and upper rate limit; and (c) the effects of varying the ventricular refractory period so that it was either the same or longer than that of the atria. The computational results prove that some possible modes of initiation of junctional reentry tachycardia are beyond the prophylactic capabilities of current DDD pacemakers. Future possibilities involving the use of computer modeling to develop more sophisticated pacemaker modes are briefly discussed.

A one-dimensional model of atrioventricular nodal conduction

A computer model of the cardiac conduction process and an integral equation mathematical model have been employed to study the functions of the atrioventricular (AV) node. Special attention has been paid to the dependence of conduction delay on cycle length. The models have been used to evaluate the question as to whether simple cycle length dependences of AV nodal conduction could cause the oscillations of cycle length which are sometimes observed in AV reentry tachycardia. The models consider the AV node as a linear structure in which the depolarisation wavefront radiates in one dimension only. A model representing the node by two parallel linear structures has also been examined. Some results obtained do not conform with those of natural circumstance and clinical experiments, suggesting that the above hypotheses of AV nodal function are too simple and restricted to explain the natural processes.

A study on a vibratory model of a human body

This paper is concerned with the modeling of the human body as a spring mass system. Based on certain assumptions, an analysis for evaluating the mass and stiffness values of the model is developed. As an illustration of the modeling procedure, a 15-degree-of-freedom model of a male body is considered. The computed natural frequencies of the model are found to be within the range of available experimental values.

Umbilical artery catheterization: a potential cause of refractory hypoglycemia

Two infants with persistent, symptomatic hypoglycemia are reported. Both were thought to have iatrogenic hyperinsulinism due to a malpositioned umbilical artery catheter. Repositioning of the catheter to avoid direct infusion into the arterial blood supply to the pancreas resulted in prompt cessation of hypoglycemia.

Computer simulation of cardiac rhythm and artificial pacemakers using a ten-element heart model

This paper describes a universal simulation system which enables simulation models of cardiac rhythm disturbances and of artificial pacemaker actions to be constructed. Three types of heart element are introduced according to the way in which excitation impulses are propagated or produced. Cycle rate dependence of repolarization periods and of impulse transmission speeds is incorporated. Random and predetermined deviations from natural behavior are also allowed for in the model. Using the universal system, a model of simple configuration has been developed. There are five physiological and five pathological heart components joined together by uni- and bidirectional connectors. Additional bidirectional channels allow a variety of pacemakers to be "implanted" in the simulated heart.

Possible pathophysiology of torsade de pointes evaluated by a realistic heart computer model

A realistic computer model that closely simulates the behaviour of the cardiac conduction system and myocardium in producing electrocardiographic records was used to evaluate some hypotheses of the pathophysiological mechanisms causing torsade de pointes tachycardia. Five different groups of experiments showed that pathological phenomena of different natures can either directly cause the usual pattern of torsade de pointes or contribute to its occurrence. The phenomena investigated were coincidence of two or more pathological foci with slightly changing periods of impulse generation; periodical disturbances of the conduction velocities of the main branches of the conduction system; a single pathological focus changing its position by slowly moving around the whole heart or around a local area of the myocardium; combination of a pathological ventricular tachycardia focus with a one way (ventriculoatrial) direct atrioventricular accessory pathway allowing atrioventricular re-entry; and different repolarisation periods of separate areas of heart muscle in the presence of a pathological ventricular focus with a fixed or oscillating rate of impulse generation. Future extended versions of the model could be used for testing other hypotheses of the origin of torsade de pointes, such as situations in which the intramuscle or local re-entry occurs.

Some aspects of computer modelling in cardiac electrophysiology

A computer model simulating the electrical activity of the heart is described. The model is designed for use in clinical cardiological research. Its implementation is based on the methods of discrete process simulation. The simulation program is written in FORTRAN, has approximately 12,000 lines and requires 160 kilobytes of memory to run. The whole heart is modelled, including a realistic description of the conduction system and a 'hollow shell' structure with a central plane representing the atrial, ventricular and septal components of the cardiac musculature. The myocardium itself is represented by simple hexagonal elements, which are pieced together to form the 'hollow shell'. The ideas used to develop the model are briefly described, but emphasis is given to clinical applications. The model is first validated by examining its output under well-known pathological conditions. Clinical applications, including predictive value, testing of hypotheses, and evaluation of pacemaker function, are then described, results being presented in the form of orthogonal lead projections.

Shell computer model of cardiac electropotential changes

A discrete process computer model has been developed to simulate the electropotential changes of heart musculature and the operation of the cardiac conduction system. The model is implemented on an ICL-4/72 computer and is oriented to cardiac rhythm studies, allowing practically all rhythm pathologies, including pacemaker applications, to be simulated. The paper describes in detail the principles on which the model is based, compares the model with other models of the same system and shows concisely some results of simulation experiments in the form of computer generated ECG records.

Computer simulation of dual chamber pacemaker algorithms using a realistic heart model

Single and dual chamber pacing algorithms have been incorporated into a realistic computer model of cardiac electrical activation. The model enables different pacemaker algorithms to be tested, it allows prediction of their behavior, and it produces a simulated ECG record for each case. The computer model has been used to test eight different modifications of a simple DDD mode to prevent or terminate pacemaker-mediated "endless loop" tachycardia: (1) constant prolongation of the atrial channel refractory period; (2) prolongation of the atrial refractory period after a ventricular premature beat (VPB); (3) atrial pacing synchronously with a VPB; (4) simple rate control; (5) rate control in which the VA counter is not reset; (6) no ventricular pacing after an atrial premature beat; (7) rate limitation of atrial sensing; and (8) a combination of DDD and high frequency atrial stimulation modes. These modifications were tested with VPBs, atrial premature beats, atrial stimulation without capture, and accelerating sinus tachycardia. Only the pacemaker designed not to pace the ventricles following an atrial premature beat behaves satisfactorily in all four circumstances. Further possibilities for the development and use of a pacemaker-oriented computer heart model are discussed.

Computer simulation of the cardiac conduction system

A discrete computer model simulating the operation of the cardiac impulse transmission apparatus and the electropotential changes of the heart musculature has been developed on a NORD-100 minicomputer. The model is written in NORD-FORTRAN and allows description of practically all basic pathologies of the transmission apparatus. A simulated ECG curve is produced in each case. The current version of the model is especially suited for studies of heart rhythms. Pacemaker descriptions (of most important pacemaker types) have been introduced into the model and the computer computations make it possible to assess the effects of different pacemaker modes under different pathological situations. The principles on which the model is based is described and the paper shows concisely several examples in the form of simulated ECG records.

X-ray structure of a dinucleoside monophosphate A2'p5'C that contains a 2'-5' link found in (2'-5')oligo(A)s induced by interferons: single-stranded helical conformation of 2'-5'-linked oligonucleotides

In order to understand why DNA and RNA have the 3'-5' and not the 2'-5' link and to delineate the stereochemistry of the 2'-5' phosphodiester links, we crystallized and carried out a very accurate x-ray diffraction analysis of A2 p5'C, an analog of A2' p5'A. Contrary to numerous reports in the literature that conclude that the tendency for 2'-5' nucleotides to stack intramolecularly is stronger than for 3'-5' counterparts, we find hardly any intramolecular base stacking for this molecule but find an intramolecular "stacking" of the ribose oxygen-4' of cytidine on top of the adenine ring. Although A2' p5'C shows the standard conformational features usually found for 3'-5' nucleotides, the overall stereochemistry of 2'-5' nucleotides is quite different because the 2' link orients the backbone inwards to the bases unlike the 3' and 5' links that orient it away from the bases. With the conformational features found for A2' p5'C, it is possible to build a very compact right-handed single-stranded helix but not a double helix. Such a preference for single-stranded helices may be the reason for the absence of 2'-5' bonds in DNA and RNA even though the 2'-5' bonds are formed more readily then 3'-5' bonds.

Development of a toxicity evaluation program for dental materials and products. II. Screening for systemic toxicity

A series of systemic toxicity tests for various types of compounds are presented with illustrative data and discussed in regard to their relevance as potential screening tests for dental compounds and products. In developing and marketing a new dental material, it is important not only to ensure safety for the patient but also for the dentist, dental assistant, and laboratory worker who routinely handles and uses the product. Data are presented on a number of commercially available dental products as well as various chemical entities, which were tested by one or more of the methods described.