Search PubMed⌕ Search

Biomedical subjects

M Malik

Publications and source records attributed to M Malik.

At least 271 records · Page 15Linked to original sources
PubMed1987

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.

Cardiac Complexes, Premature↗
PubMed1987

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.

Anthropometry↗
PubMed1987

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.

Adult↗
PubMed1986

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.

Arrhythmias, Cardiac↗
PubMed1986

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.

Atrioventricular Node↗
PubMed1985

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.

Atrioventricular Node↗
PubMed1985

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.

Computers↗
PubMed1983

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.

Arrhythmias, Cardiac↗
PubMed1982

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.

Adenine Nucleotides↗
PubMed1979

Interactions among bovine and human factor VIII, ristocetin, and DL-polylysine in the induction of guinea pig platelet aggregation.

Guinea pig and human platelets were aggregated by bovine factor VIII plus ristocetin a concentrations not allowing either substance alone to induce aggregation. Using combinations, it was shown that aggregation required bovine factor VIII and restocetin at levels where the sum of their concentrations exceeded a critical value. With guinea pig platelets, combinations of polylysine and bovine factor VIII or polylysine and human factor VIII did not show full aggregations. These results do not support the hypothesis that aggregation by mixtures of factor VIII and ristocetin arises solely from neutralization of negative charges on the platelet surface membrane.

Animals↗
PubMed1974

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.

Acetaldehyde↗