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Biomedical subjects

D N Ghista

Publications and source records attributed to D N Ghista.

At least 19 recordsLinked to original sources
PubMed2005

Perfusion studies of steady flow in poroelastic myocardium tissue.

The behaviour of the heart has always elicited interest and particularly the study of its myocardium, as 5-10% of the blood pumped by the heart is passed through the coronary arteries to the myocardium itself. An in-depth investigation of the myocardium behaviour is useful. The present work aims to investigate how myocardium perfusion is influenced by myocardial stress and diseased states, and in general by LV pumping abnormalities. LV myocardial perfusion can then serve as a possible index of the capacity of the LV to respond to its work demand, and thus of the risk of heart failure. The poroelastic analysis of the myocardium based on finite element method (FEM) for regional perfusion through a rectangular element with various physiological ranges of loading conditions was studied.

Animals↗
PubMed2005

Numerical study on the pulsatile flow characteristics of proximal anastomotic models.

Haemodynamics was widely believed to correlate with anastomosis restenosis. Utilizing the haemodynamic parameters as indicator functions, distal anastomosis was redesigned by some researchers so as to improve the long-term graft patency rate. However, there were few studies upon the proximal anastomosis. Therefore, in this study, flow characteristics and distributions of the haemodynamic parameters in proximal anastomosis under physiological flow condition have been investigated numerically for three different grafting angles: namely, 45 degrees forward facing, 45 degrees backward facing, and 90 degrees anastomotic joints. The simulation results showed a flow separation region along the graft inner wall immediately after the heel at peak flow phase and it decreased in size with the grafting angle shifting from 45 degrees forward facing to 45 degrees backward facing. At the same time, a pair of vortex was found in the cross-sectional planes of the 45 degrees backward facing and 90 degrees grafts. In addition, stagnation point was found along the graft outer wall with small shifting during the physiological cycle. High spatial and temporal wall shear stresses gradients (WSSG) were observed around the anastomotic joint. Low time-averaged wall shear stress (WSS) with elevated oscillation shear index (OSI) was found near the middle of anastomosis at the aorta wall and along the graft inner wall respectively, while high time-averaged WSS with low OSI was found at the heel, the toe, and the region downstream of the toe. These regions correlated to early lesion growth. Elevated time-averaged WSSG was found at the same region, where the elevated low-density lipoprotein (LDL) permeability was observed as reported in the literature. The existence of nearly fixed stagnating location, flow separation, vortex, high time-averaged WSS with low OSI, low time-averaged WSS with elevated OSI, and high time-averaged WSSG may lead to graft stenosis. Moreover, the simulation results obtained were consistent with those of experimental measurements. Based on the validated simulation results, the 45 degrees backward-facing graft was found to have the lowest variation range of time-averaged WSS and the lowest segmental average of WSSG among the three models investigated. The 45 degrees backward-facing graft is thus recommended for the bypass operation with expected higher patency rate.

Anastomosis, Surgical↗
PubMed1993

Asymmetrical loads and lateral bending of the human spine.

The human spine is modelled as a cantilever-type beam column. Under the influence of static asymmetrical loads, muscle and low-back forces are predicted from a hypothetical but revealing model. Such forces produced by asymmetrical loads are much larger than for a corresponding symmetrical load. Asymmetrical loads can encourage, especially in young schoolchildren, lateral bending of the spine by alleviating muscle and low-back forces. This could possibly be a factor contributing to the surprisingly high percentage of schoolchildren with measurable scoliotic curves. The wearing of knapsack-type bags is advocated.

Adolescent↗
PubMed1993

Biomechanical simulations of scoliotic spinal deformity and correction.

A new approach to surgical correction of scoliosis has been advanced by us, in the form of simulation of the surgical correction system and technique. For this purpose, we developed a finite-element model of the spinal column (SFEM), applied tractions to it and determined the model stiffness so as to watch the actual spinal geometry. Having patient-simulated this SFEM, we applied to this SFEM corrective forces and determined the optimal set of forces to gain the best correction of the spinal deformity. We then developed a special instrumentation to measure the applied corrective forces during surgery using a particular fixation system. The SFEM corrected geometry was shown to compare favourably with the post-surgical curve. We have now developed an elastic beam-column model (EBCM) to which muscle activation forces, representing asymmetrical paralysis of the vertebral column muscles, can be applied to generate a given scoliotic curve. In that process the stiffness properties of the patient-simulated EBCM are determined. Now on these patient-simulated EBCM(s), identical corrective force systems are applied as developed by the finite-element model (SFEM) and implemented surgically for these patients. It is shown that the EBCM corrected geometries compare favourably with both SFEM corrected geometries as well as with the post-surgical curves for similar corrective force systems. Thus the EBCM can be employed to presurgically simulate scoliolic correction, specify the optimal corrective system of forces so as to gain the best surgical correction.

Adolescent↗
PubMed1989

Performance assessment of the Terry Fox jogging prosthesis for above-knee amputees.

The Terry Fox jogging (TFJ) prosthesis was developed at Chedoke-McMaster Hospital to alleviate the asymmetric jogging pattern experienced by above-knee amputees when attempting to jog with conventional walking prostheses. This prosthesis features a spring-loaded, telescoping shank designed to eliminate any vaulting action and control the trunk motion during stance. The spring is intended to attenuate the impact forces and release its stored energy at push-off to provide momentum transfer to the jogger. This prosthesis was comprehensively assessed in the gait laboratory, by evaluating the kinematics, energy and power flow patterns of an above-knee amputee jogger wearing the TFJ prosthesis. Included in the assessment is the ability of the prosthesis to satisfy a set of relevant design criteria that have been established from non-amputee jogging patterns. An increased swing phase time for the prosthetic limb and the need to have the knee hyperextended throughout the stance phase contributed to an asymmetric jogging style. The telescoping action did lower the amputee's centre of mass, thereby reducing the vaulting effect. However, the spring only imparted a lifting action to the jogger and the ground reaction forces were double those of a non-amputee jogger. These findings clearly indicate a need to redesign the TFJ prosthesis and are being incorporated in the design of a new physiological jogging prosthesis.

Adult↗
PubMed1989

Presurgical finite element simulation of scoliosis correction.

For surgical correction of scoliotic spinal deformity, internal fixation systems apply lateral and distractive corrective forces. In order to gain maximal correction, a finite--element analysis of the spinal deformity correction technique has been carried out preoperatively, after first employing the spinal deformity correction finite--element model to determine the in vivo spinal stiffness. The presurgical analysis also gives us an appreciation of how the parameters of deformity, stiffness and corrective forces jointly contribute to the value of the correction index. The paper presents the methodology and clinical application. It also summarizes the results for ten patients, whereby the efficacy of presurgical analysis is assessed by comparing the corrective index values by presurgical simulation with the surgical results for equivalent levels of corrective forces.

Adolescent↗
PubMed1988

Microcomputerized on-line evaluation of heart rate variability power spectra in humans.

The heart rate variability power spectra is computed using an on-line microcomputer system. In normal subjects (n = 10), standing and post-exercise states demonstrated statistically significant increases (p less than 0.05) in the peak power at 0.1 Hz. During exercise, the peak power at 0.1 Hz was reduced significantly (p less than 0.05). In patients with myocardial infarction, either the peak at 0.1 Hz or the peak associated with breathing frequency was augmented. Our system is mobile and is useful especially when on-line heart rate variability power spectra are required in experimental or clinical situations.

Adult↗
PubMed1988

Statistical pattern classification of clinical brainstem auditory evoked potentials.

The brainstem auditory evoked potentials (BAEPs) recorded in the neurological clinic were classified using the Bayes classifier (BC) and Fisher's linear discriminant function (FLD). The latencies of initial five peaks, interpeak intervals were examined for optimum features to develop classifiers. The accuracy of classification was 85.3% when absolute latencies of peaks III, IV and V were used as features. The BC gave better performance than FLD, indicating that second order statistics of BAEPs for normal and pathological classes are different. The results of this study indicate that latencies alone give enough information for recognizing normal from pathological BAEPs, using physician's evaluation of BAEPs as the reference.

Adult↗
PubMed1988

Biomechanical basis of optimal scoliosis surgical correction.

For an optimal approach to surgical correction of scoliosis, it was deemed desirable to biomechanically simulate the set of corrective forces applied by alternative internal fixation systems, so as to determine and apply the internal fixation system producing the best correction under safe levels of forces applied by the fixation systems to the spinal structures. To this end, we have developed, and presented here, (1) a spinal finite-element model relating the applied corrective forces to the corrected spinal configurations, (2) a method for determining the stiffness of the patient's spine prior to surgery, (3) computerized finite-element analysis simulation of alternative internal correction-fixation systems, so as to determine the most efficacious system, (4) instrumentations for surgically implementing the recommendations of the surgical simulation analysis and (5) comparisons of the model-simulated and surgically-obtained corrected spinal configurations. These procedures together constitute the biomechanical foundations of scoliosis surgical correction.

Biomechanical Phenomena↗
PubMed1988

Spectral analysis of heart rate variability following human heart transplantation: evidence for functional reinnervation.

To determine the status of innervation in long-term human donor allografts, the power spectrum of heart rate variability was analysed in 9 post-transplant patients and 7 healthy control subjects. The mean post-transplant follow-up was 17.8 months (range: 2-37 months). Continuous ECG signals were recorded throughout a 15-min rest period. An R-R interval tachogram was generated and an autoregressive model using linear predictive coding, was applied to the heart rate variability data. In 8 transplant patients the frequency oscillations were irregular, broad based and widely dispersed from 0 to 1 Hz. The patterns resembled white noise and were consistent with dissociation of the donor allograft from the recipient's central nervous system. In contrast, one patient displayed a heart rate variability spectrum indistinguishable from that of control subjects. This pattern contained two distinct spectral bands; one corresponding to the patient's respiratory rate at 0.2 Hz and a low frequency Mayer wave at 0.1 Hz. Atropine abolished the respiratory (vagal) peak. Except for this patient's post-transplant time (33 months compared to the group mean of 17.6 months), there were no clinical characteristics which distinguished this patient from the others. While the mean heart rate for the remaining 8 allografts was significantly higher than controls (95.3 vs 64.5 bt/min; P less than 0.001) the standard deviation of heart rate variability for the 8 patients was significantly narrower than controls (0.7 vs 4.86; P less than 0.01). The variance of heart rate for the patient with the normal power spectrum was fourfold greater than the mean SD of the other transplant patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Radiological changes in rheumatoid arthritis: measurement of area of juxta-articular bone as outcome event in clinical therapeutic trials of antirheumatic drugs.

A high degree of intra - and inter - rater reproducibility was obtained in measurement of a standardized area of the head of the first right metacarpal using a precision digitizer. This instrument had an accuracy for every point digitized of 0.127 mm. Intra-rater difference between readings at approximately a month's interval showed no significant differences. The method is reasonably simple to perform and is applicable to small joints such as the proximal interphalangeal joints. The method may prove useful in following progression of joint damage in rheumatoid arthritis.

Anti-Inflammatory Agents↗
PubMed1988

Power spectrum of heart rate variability: a non-invasive test of integrated neurocardiac function.

Under steady state conditions, frequency specific oscillations in the heart rate record reflect beat-to-beat autonomic control of sinus node activity. Using an autoregressive model, samples (2.5 min) of continuous ECG records were analyzed in 36 healthy young adults during supine rest (45 min), orthostatic stress and controlled respiration. In the supine state, constancy of heart rate was achieved (mean HR 62.8 bt/min +/- 4.88 SD). However, the 0.1 Hz peak spectral power varied considerably: average coefficient of variation was 34% compared to only 8% for heart rate. When breathing rates were synchronized to a metronome there was a small insignificant decrease in the peak power at 0.1 Hz compared to spontaneous respiration. Standing produced a significant increase in the peak power at 0.1 Hz especially during synchronized breathing. There was a maximum increase in the low frequency (0.1 Hz) peak power of 40 (bt/min)2. Hz-1 at a controlled breathing frequency of 0.25 Hz in the standing compared to supine position. The data show that reproducibility of the power spectrum heart rate variability is best achieved at controlled but physiologic respiratory rates and, preferably, in the upright position.

Adult↗
PubMed1987

Heart rate variability power spectrogram as a potential noninvasive signature of cardiac regulatory system response, mechanisms, and disorders.

This paper attempts to provide evidence that the heart rate variability power spectrum (HRVPS) reflects the presence of neural control of cardiac regulation. A normal individual is seen to have a characteristic HRVPS (comprising a 0.1-Hz peak and a respiratory peak at 0.25-0.34 Hz), which is altered in a predictable manner in response to orthostatic stress and exercise, while in two patients with autonomic neuropathy, the HRVPS failed to demonstrate such a characteristic alteration in response to orthostatic stress. Postinfarct HRVPS signatures were studied in two patients with anterior and inferior infarcts so as to lend insight using non-invasive means into both the healing process and the dominant deliterious sympathetic or protective vagal tone due to the infarct. When subjects with transplanted hearts were studied, their HRVPS did not exhibit the characteristic pattern of a normal individual; rather, the HRVPS energy was spread over a wider and higher frequency range. However, one of the transplanted patients surprisingly but consistently revealed the characteristic HRVPS; the post-transplant time at the time of the study was 33 months and the patient had the typically high resting heart rate of a transplant recipient but a wide standard deviation like that of a normal individual. This could be the first noninvasively demonstrated evidence in humans of reinnervation of a transplanted heart. Thus, the HRVPS constitutes a simple non-invasive method to assess cardiac neuroregulatory response and disorders and it is proposed that it be referred to as the heart rate variability cardiogram (HRVC).

Adult↗
PubMed1987

Power spectral analysis of normal and pathological brainstem auditory evoked potentials.

The brainstem auditory evoked potential (BAEP) recording has become a powerful investigational tool in neurological diagnosis. The BAEPs of patients have different latencies and morphologies when compared to those of normals. In this paper the power spectra (PS) of BAEPs of 21 normals, 17 patients with multiple sclerosis (MS) and 12 patients with head injury (HI) computed by Blackman-Tukey (BT) and Maximum Entropy (ME) methods are examined for their frequency composition. Three major peaks appear at approximately 170 Hz, 520 Hz and 950 Hz in PS of normal BAEPs. The average power contained in the frequency bands spread around these frequency bands for BAEPs of patients differed significantly (P less than 0.05) from those of normal BAEPs. The peaks observed in ME spectra were found to match those computed using BT method. The model order for representing both normal and patient BAEPs is greater than 40 and data compression afforded by modelling the BAEPs is of the order of 5:1.

Adolescent↗
PubMed1987

Intrinsic indices of the left ventricle as a blood pump in normal and infarcted left ventricles.

To assess the left ventricle as a blood pump, data are collected from contrast angiograms and analysed by computer, using two-dimensional finite element analysis, to provide instantaneous distributions of intra-LV flow and differential pressure during the diastolic and ejection phases. Characteristic indices are derived for normal and infarcted LVs, and for cases before and after administration of nitroglycerin. These indices may be used to assess the degree and nature of dysfunction in coronary artery disease.

Biomedical Engineering↗
PubMed1986

Mechanics of left ventricular aneurysm.

When a coronary artery is significantly occluded, the left ventricular myocardial segment, which is perfused by that coronary artery, will become ischaemic and even irreversibly infarcted. An acute infarct has very low stiffness and if it involves the entire wall there is a risk of rupture; however, in the absence of such a critical situation, fibrous tissue is laid into the infarcted myocardial segment. Such an infarcted fibrotic myocardial segment will not be able to contract, and so generate tensile stress. The surrounding intact myocardium will contract and generate wall stress, thereby developing a high intra-chamber systolic pressure; the chronically infarcted and fibrotic segment will have to sustain this high chamber pressure. Its loss of contractility and the resulting reduced systolic stiffness relative to the intact segment, will cause it to deform into a bulge; this is an aneurysm. When a left ventricular chamber with an aneurysm contracts during the systolic phase, some blood also goes into the aneurysm, and this decreases the stroke volume; since the aneurysm wall is passive, stagnant blood flow prevails in the aneurysm itself, which in turn can give rise to the formation of a mural thrombus. These serious consequences provide a justification for the analysis of an infarcted left ventricular chamber, in order to predict the size of the aneurysmic bulge. Such an analysis is presented in this paper. To determine the left ventricular wall deformation, and the stress arising from infarction of a wall segment (which leads to a ventricular aneurysm) the left ventricle is modelled here as a pressurized ellipsoidal shell. Deformations of infarcted wall segments are computed for several damaged wall-thicknesses in left ventricles of different shapes. The analysis involves a derivation of equations for wall-stress equilibrium with the chamber pressure, and myocardial incompressibility before and after infarct formation. The equations are solved by the Newton-Raphson method (using elliptical integrals of the first and second kind). Of significance are the prognostic implications of the results, presented in the form of graphs, showing the dependence of tensile stress and the bulge of infarcted wall-segments, on the extent of damaged wall-thickness and the angle of infarct. Scaled illustrations of the bulge shapes, for various degrees of infarcts, are provided. The results indicate that for rupture of the ventricle, the percentage of infarcted wall-thickness and the shape of the ellipsoidal left ventricular chamber play more dominant roles than the angle-of-damage, or the extent of the infarct.

Biomechanical Phenomena↗