Biomedical subjects

K B Chandran

Publications and source records attributed to K B Chandran.

78 records · Page 5Linked to original sources

PubMedDate not supplied

Subcortical EEG changes in rhesus monkeys following experimental hyperextension-hyperflexion (whiplash).

A controlled study, involving EEG recordings from the scalp and chronically implanted electrodes in the cortex (ECoG), as well as from selected subcortical nuclei, was undertaken to investigate the neurophysiologic effects on rhesus monkeys following experimental whiplash (hyperextension of the head and neck). Sixteen animals, equally divided into four groups, were studied through the following protocol: (1) two animals within each group were whiplashed and then electrodes were implanted into the brain of one; (2) the second two animals were implanted with deep electrodes and then one was whiplashed. Weekly EEG follow-ups showed hippocampal spiking in three of the four whiplashed and then electrode-implanted animals and in one of implanted and then whiplashed animals 6 to 8 weeks postwhiplash. Several results deserve attention. (1) The "whiplash syndrome" owes part of its symptoms to EEG disturbances in the brain. (2) Prior to the onset of spiking, ie, 6 to 8 weeks postwhiplash, practically all scalp, cortical, and subcortical EEG recordings were normal. (3) When hippocampal EEG spiking did take place, only normal and mildly abnormal changes were seen in either the electrocorticogram (ECoG) or scalp electroencephalogram (EEG). (4) The growth and development of this trauma-induced hippocampal spiking followed the classic sequence for the spread of an epileptogenic focus. (5) This apparent subclinical form of posttraumatic epilepsy may be due to the combined effects of the whiplash plus the subcortical electrode placements further decreasing the already well-known, low-spiking threshold of the hippocampi.

Animals↗

PubMedDate not supplied

Negative pressure transients with mechanical heart-valve closure: correlation between in vitro and in vivo results.

Negative pressure transients (NPT) recorded in a single closing event of mechanical valves in the mitral position in an in vitro setup are compared with data recorded in the left atrium in vivo with the valves implanted in the mitral position in an animal model. The loading at valve closure (dP/dtCL) computed from the in vivo ventricular pressure recording (ranging from 700 to 2300 mm Hg/s) agreed with the magnitudes predicted in our earlier in vitro experiments (750-3000 mm Hg/s). The NPT signals and the corresponding power spectral density plots from the in vivo data were in qualitative agreement with those recorded in vitro. The NPT magnitudes were found to be below the vapor pressure for blood in mechanical valves with rigid occluders suggesting a potential for the valve to cavitate in vivo. Our in vivo results also suggest that the valves with flexible occluders are less likely to cavitate. The correlation of the in vitro and in vivo data also suggests that the flexibility of valve housing used in the in vitro studies is not an important factor in the dynamics of mechanical valve closure in vivo.

Animals↗

PubMedDate not supplied

Steady flow analysis in the vicinity of an end-to-end anastomosis.

A study using both experimental and numerical methods was conducted to determine flow dynamics in the vicinity of an end-to-end anastomosis. In vitro experiments were performed to determine the variations in the conduit cross-section at the anastomosis in specimens under static loading. The experimentally determined geometries were used to simulate steady flow through an end-to-end anastomosis using the finite analytic (FA) numerical solution technique in order to obtain solutions for the governing axisymmetric laminar flow equations. Wall shear stresses in the vicinity of end-to-end anastomoses under steady flow conditions were measured using a flush-mounted hot-film anemometer (FMHFA) probe. The experimental measurements were in agreement with numerical results, except in flow with low Reynolds numbers. Wall shear stresses increased proximal to the anastomosis in flow from the Penrose tubing (simulating an artery) to the PTFE graft. In flow from the PTFE graft to the Penrose tubing, low wall shear stresses were observed distal to the anastomosis. The present study suggests a correlation between regions of low wall shear stress and the development of anastomotic neointimal fibrous hyperplasia (ANFH) in end-to-end anastomoses.

Anastomosis, Surgical↗

PubMedDate not supplied

Dependence of tissue valve leaflet motion on the viscosity of blood analogue fluid.

The dependence of the leaflet motion of bioprosthetic heart valves on the viscosity of the blood analogue fluid was studied in this work. A pericardial and a porcine tissue valve were mounted in a pulse duplicator and high-speed films were taken to record the motion of the valve leaflets. The blood analogue fluids used were physiological saline with a viscosity coefficient of 1.0 cP, and glycerol solution with a viscosity of 3.5 cP. The transvalvular pressure drop and percentage of regurgitation were also measured with the time-averaged flow rate maintained at 6.00 +/- 0.05 litres/min. Our results show that the leaflets did not stiffen with up to 15 days' exposure to glycerol. Also, there was no substantial difference in the time of opening of the leaflets or in the area of opening of the valves with the two blood analogue fluids. However, the leaflets closed substantially later in the cardiac cycle in the case of glycerol solution, owing to the interaction between the leaflets and the viscosity of the fluid. For proper comparison of the flow dynamics past prosthetic valves at comparable Reynolds number and Womersley number, our results suggest that glycerol solution should be used as the blood analogue fluid for tissue valves also.

Animals↗

PubMedDate not supplied

Arterial enhancement in acute cerebral ischemia: clinical and angiographic correlation.

PURPOSE: To investigate the cause and clinical significance of arterial enhancement (AE) in contrast-enhanced T1-weighted MR examinations after acute cerebral ischemia. METHODS: Contrast MR examinations and conventional angiograms of 17 patients studied following an acute ischemic event or an internal carotid occlusion were retrospectively reviewed. MR and angiographic studies were performed within 1 day of each other. The presence of AE was correlated with both angiographic findings and patient clinical status. RESULTS: AE was not confined to patients with angiographic evidence of complete arterial occlusion. Only 64% of patients demonstrating AE had complete occlusion angiographically. Complete arterial occlusion did not always correlate with AE. In two of nine patients with complete occlusion, no AE was identified. In five of 10 patients with AE, angiographic slow flow was identified. In patients without AE, no angiographic slow flow was identified. In the 64% of patients with AE, significant symptoms were identified. Patients without AE were either asymptomatic or had mild symptoms at the time of the MR study. CONCLUSIONS: Our data support the hypothesis that arterial slowing is the cause of AE, which appears to be an indicator of decreased brain perfusion. Such MR findings may add important supplemental information to those provided by conventional angiography.

Acute Disease↗

PubMedDate not supplied

Mechanical valve closing dynamics: relationship between velocity of closing, pressure transients, and cavitation initiation.

In this study, the closing dynamics of mechanical heart valves was experimentally analyzed with the valves mounted in the mitral position of an in vitro flow chamber simulating a single closing event. The average linear velocity of the edge of the leaflet during the final 2.065 degrees of the traverse before closing was measured using a laser sweeping technique, and the negative pressure transients at 2 mm from the leaflet inflow surface in the fully closed position was recorded at the instant of valve closure. The cavitation number was computed for the various mechanical valves at a range of load at valve closure. The data were correlated with cavitation bubble visualization previously obtained with the same experimental set up. Cavitation incipience with mechanical valves was found to be independent of the flexibility of the valve holder. For the same loading rate at valve closure, valves with flexible (polyethylene) leaflets were found to close with comparable velocity to those with rigid (pyrolytic carbon) leaflets, but the negative pressure transients did not reach magnitudes close to the vapor pressure for the fluid with flexible leaflets. For the same leaflet closing velocity (and hence the cavitation number), valves with a seat stop or a seating lip in the region of maximum leaflet velocity were observed to cavitate earlier, suggesting that the effect of "squeeze flow" may be an important factor in cavitation incipience.

Compliance↗