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J D Thomas

Publications and source records attributed to J D Thomas.

At least 289 records · Page 16Linked to original sources
PubMed1991

Dialysis-induced alterations in left ventricular filling: mechanisms and clinical significance.

Quantitative two-dimensional (2-D) and Doppler echocardiography were used to determine whether hemodialysis results in alterations in left ventricular (LV) diastolic filling that might contribute to dialysis-induced hypotension, as well as to assess whether any hemodynamic variables or indices of diastolic filling might be used to identify which patients were at the greatest risk of becoming hemodynamically unstable during dialysis. Sixteen male patients undergoing routine maintenance hemodialysis for end-stage renal disease were prospectively studied before and after hemodialysis. Following hemodialysis there was a significant prolongation (P less than 0.05) in LV isovolumetric relaxation time (IVRT), as well as a significant reduction in the rate and extent of early rapid ventricular filling (P less than 0.005); in contrast, late atrial-assisted filling did not change significantly. A multiple stepwise linear regression analysis of predialysis hemodynamic parameters and noninvasive indices of LV filling showed that there was a significant independent inverse relationship between the frequency of dialysis-related hypotensive episodes and the duration of early LV filling (r = -0.81; P less than 0.001). These results suggest that hemodialysis results in discrete alterations in early LV filling, with no significant compensatory increase in late atrial-assisted ventricular filling. Further, patients with the shortest early LV filling times appeared to have the greatest predilection for becoming hemodynamically unstable during dialysis.

Aged↗
PubMed1991

Ion-selective electrode studies on novel organic molecule sensors.

Researches on prospects for novel ion-selective electrodes, based on organic molecule sensors, are described. The organic molecules are large crown ethers extending from bis(metaphenylene)-26-crown-8 to bis(metaphenylene)-38-crown-12, small crown ethers, bis-crown ethers, and acyclic polyethers consisting of diphenyl ethers of tetraethylene glycol and receptor molecules of planar and tetrahedral tripodal types.

Animals↗
PubMed1991

Physical and physiological determinants of transmitral velocity: numerical analysis.

The Doppler transmitral velocity curve is commonly used to assess left ventricular diastolic function. Recent investigations, however, relating Doppler mitral indexes to ventricular compliance, relaxation, and preload have been inconclusive and at times contradictory. We used a mathematical formulation to study the physical and physiological determinants of the transmitral velocity pattern for exponential chamber pressure-volume relationships with active ventricular relaxation (2,187 combinations investigated). We showed that transmitral velocity is fundamentally affected by two principal physical determinants, the transmitral pressure difference and the net atrioventricular compliance, as well as the impedance characteristics of the mitral valve. These physical determinants in turn are specified by the compliance and relaxation parameters of physiological interest. We found that the peak mitral velocity is most strongly related to initial left atrial pressure but lowered by prolonged relaxation, low atrial and ventricular compliance, and systolic dysfunction. Peak acceleration varies directly with atrial pressure and inversely with the time constant of isovolumic relaxation, with little influence of compliance, whereas the mitral deceleration rate is approximately valve area divided by atrioventricular compliance. We then used these data to suggest possible strategies for improved analysis of noninvasive data (Doppler indexes, planimetered valve area, and isovolumic relaxation time) to estimate ventricular compliance and relaxation and atrial pressure.

Algorithms↗
PubMed1991

Noninvasive estimation of the instantaneous first derivative of left ventricular pressure using continuous-wave Doppler echocardiography.

BACKGROUND: The complete continuous-wave Doppler mitral regurgitant velocity curve should allow reconstruction of the ventriculoatrial (VA) pressure gradient from mitral valve closure to opening, including left ventricular (LV) isovolumic contraction, ejection, and isovolumic relaxation. Assuming that the left atrial pressure fluctuation is relatively minor in comparison with the corresponding LV pressure changes during systole, the first derivative of the Doppler-derived VA pressure gradient curve (Doppler dP/dt) might be used to estimate the LV dP/dt curve, previously measurable only at catheterization (catheter dP/dt). METHODS AND RESULTS: This hypothesis was examined in an in vivo mitral regurgitant model during 30 hemodynamic stages in eight dogs. Contractility and relaxation were altered by inotropic stimulation and hypothermia. The Doppler mitral regurgitant velocity spectrum was recorded along with simultaneously acquired micromanometer LV and left atrial pressures. The regurgitant velocity profiles were digitized and converted to VA pressure gradient curves using the simplified Bernoulli equation. The instantaneous dP/dt of the VA pressure gradient curve was then derived. The instantaneous Doppler-derived VA pressure gradients, instantaneous Doppler dP/dt, dP/dtmax, and -dP/dtmax were compared with corresponding catheter measurements. This method of estimating dP/dtmax from the instantaneous dP/dt curve was also compared with a previously proposed Doppler method of estimating dP/dtmax using the Doppler-derived mean rate of LV pressure rise over the time period between velocities of 1 and 3 m/sec on the ascending slope of the Doppler velocity spectrum. Both instantaneous Doppler-derived VA pressure gradients (r = 0.95, p less than 0.0001) and Doppler dP/dt (r = 0.92, p less than 0.0001) correlated well with corresponding measurements by catheter during systolic contraction and isovolumic relaxation (pooled data). The Doppler dP/dtmax (1,266 +/- 701 mm Hg/sec) also correlated well (r = 0.94) with the catheter dP/dtmax (1,200 +/- 573 mm Hg/sec). There was no difference between the two methods for measurement of dP/dtmax (p = NS). Although Doppler -dP/dtmax was slightly lower than the catheter measurement (961 +/- 511 versus 1,057 +/- 540 mm Hg/sec, p less than 0.01), the correlation between measurements by Doppler and catheter was excellent (r = 0.93, p less than 0.0001). The alternative method of mean isovolumic pressure rise (896 +/- 465 mm Hg/sec) underestimated the catheter dP/dtmax (1,200 +/- 573 mm Hg/sec) significantly (on average, 25%; p less than 0.001). CONCLUSIONS: The present study demonstrated an accurate and reliable noninvasive Doppler method for estimating instantaneous LV dP/dt, dP/dtmax, and -dP/dtmax.

Animals↗
PubMed1991

Impact of impinging wall jet on color Doppler quantification of mitral regurgitation.

BACKGROUND: In clinical color Doppler examinations, mitral regurgitant jets are often observed to impinge on the left atrial wall immediately beyond the mitral valve. In accordance with fluid dynamics theory, we hypothesized that a jet impinging on a wall would lose momentum more rapidly, undergo spatial distortion, and thus have a different observed jet area from that of a free jet with an identical flow rate. METHODS AND RESULTS: To test this hypothesis in vivo, we studied 44 patients with mitral regurgitation--30 with centrally directed free jets and 14 with eccentrically directed impinging wall jets. Maximal color jet areas (cm2) (with and without correction for left atrial size) were correlated with mitral regurgitant volumes, flow rates, and fractions derived from pulsed Doppler mitral and aortic forward flows. The groups were compared by analysis of covariance. Mean +/- SD mitral regurgitant fraction, regurgitant volume, and mean flow rate averaged 37 +/- 17%, 3.06 +/- 2.65 l/min, and 147 +/- 118 ml/sec, respectively. The maximal jet area from color Doppler imaging correlated relatively well with the mitral regurgitant fraction in the patients with free mitral regurgitant jets (r = 0.74, p less than 0.0001) but poorly in the patients with impinging wall jets (r = 0.42, p = NS). Although the mitral regurgitant fraction was larger (p less than 0.05) in patients with wall jets (44 +/- 20%) than in those with free jets (33 +/- 15%), the maximal jet area was significantly smaller (4.78 +/- 2.87 cm2 for wall jets versus 9.17 +/- 6.45 cm2 for free jets, p less than 0.01). For the same regurgitant fraction, wall jets were only approximately 40% of the size of a corresponding free jet, a difference confirmed by analysis of covariance (p less than 0.0001). CONCLUSIONS: Patients with mitral regurgitation frequently have jets that impinge on the left atrial wall close to the mitral valve. Such impinging wall jets are less predictable and usually have much smaller color Doppler areas in conventional echocardiographic views than do free jets of similar regurgitant severity. Jet morphology should be considered in the semiquantitative interpretation of mitral regurgitation by Doppler color flow mapping. Future studies of the three-dimensional morphology of wall jets may aid in their assessment.

Adult↗
PubMed1991

Forebrain ischemia induces selective behavioral impairments associated with hippocampal injury in rats.

Two groups of rats were tested on a variety of motor and cognitive tasks after either 10 minutes of two-vessel occlusion forebrain ischemia (n = 8) or sham operative procedures (n = 6). Histological injury was absent in the sham-operated group. In the ischemic group, hippocampal injury was restricted to field CA1, while damage in the neocortex and caudoputamen was sparse. Motor tests performed on postoperative days 18 and 28 revealed no significant differences between the ischemic and sham-operated groups. Retention performance of a radial maze discrimination task was impaired, with a significant but transient increase in both working and reference memory errors. Passive avoidance acquisition and retention were not significantly affected, although conclusions concerning the utility of this task must be reserved because of variability in the behavior of the sham-operated rats. Morris maze spatial navigation (place learning) and open-field activity were insensitive to treatment group. These functional results are consistent with the observed histological injury and what is known about hippocampal injury and behavior, and they provide further guidance for the development of neurological assays appropriate for discriminating outcome from forebrain ischemia in rats.

Animals↗
PubMed1990

Influence of orifice geometry and flow rate on effective valve area: an in vitro study.

Fluid dynamics suggests that orifice geometry is a determinant of discharge properties and, therefore, should influence empiric constants in formulas (such as the Gorlin formula) to calculate stenotic valve area. An in vitro study utilizing a model of transmitral flow was conducted to investigate how the discharge coefficient changes with 1) orifice eccentricity (ratio of long to short diameter), 2) absolute area, 3) the presence of a nozzle-like inlet, and 4) varying flow. Twenty-three orifices with areas varying between 0.3 and 2.5 cm2 and eccentricities from 1:1, or circular, to 5:1, or elliptic, were tested. The calculated discharge coefficients ranged between 0.675 and 0.93. For a given area, the discharge coefficient decreased by a mean value (+/- SD) of 5.5 +/- 1.3% between circular orifices and 5:1 ellipses. Discharge coefficients increased by a mean of 8.9 +/- 3.5% from 0.3 to 2.5 cm2 area within each eccentricity class. A gradually tapering inlet (nozzle) raised the discharge coefficient by 8.8 +/- 3.9%, leading to a discharge coefficient between 0.81 and 0.93 for round orifices. The discharge coefficient did not change appreciably with flow. The concept of the discharge coefficient and its role in assessing restrictive orifices in general by hydraulic formulas (for example, the Gorlin and pressure half-time calculations) are discussed.

Blood Flow Velocity↗
PubMed1990

Analysis of the early transmitral Doppler velocity curve: effect of primary physiologic changes and compensatory preload adjustment.

Left ventricular filling (as assessed by Doppler echocardiography) has previously been shown to depend in a complex fashion on ventricular diastolic function (compliance and relaxation) as well as other variables, such as atrial pressure and compliance, ventricular systolic function and mitral valve impedance. To study the effect of isolated physiologic alterations on individual Doppler indexes, a mathematic model of mitral flow was analyzed. By varying one physiologic variable at a time, it was shown that mitral velocity acceleration is affected directly by atrial pressure and inversely by the ventricular relaxation time constant, with relatively little impact of chamber compliance. Deceleration rate was directly influenced by mitral valve area, atrial pressure and ventricular systolic dysfunction and inversely affected by atrial and ventricular compliance relations, with little impact of relaxation unless it was so delayed as to be incomplete during deceleration. Peak velocity was directly affected most strongly by initial left atrial pressure, and lowered somewhat by prolonged relaxation, low atrial and ventricular compliance and systolic dysfunction. Strikingly different filling patterns emerged when the primary physiologic alterations were accompanied by simultaneous compensatory changes in atrial pressure designed to maintain stroke volume constant. Low ventricular compliance with preload compensation produced characteristic E waves with very short acceleration and deceleration times and high peak velocity. Thus, mathematic analysis of ventricular filling helps to explain the physical and physiologic basis for the transmitral velocity curve.

Blood Flow Velocity↗
PubMed1990

Aortic regurgitation shortens Doppler pressure half-time in mitral stenosis: clinical evidence, in vitro simulation and theoretic analysis.

Mitral valve areas determined by Doppler pressure half-time were compared with areas obtained by planimetry in two groups of patients with mitral stenosis: 24 patients without aortic regurgitation and 32 patients with more than grade 1 aortic regurgitation. The severity of aortic regurgitation was assessed by color flow mapping; 17 patients had grade 2, 10 had grade 3 and 5 had grade 4 aortic regurgitation. Regression equations for pressure half-time area versus planimetry mitral valve area were calculated separately for the aortic regurgitation (r = 0.88) and the nonaortic regurgitation group (r = 0.86); analysis of covariance revealed a significant (p less than 0.001) difference between the two groups leading to overestimation of planimetry area by the pressure half-time method in the aortic regurgitation group. The mitral valve areas in the group without regurgitation were best calculated with the expression 239/T1/2 (r = 0.77) as compared with a best fit of 195/T1/2 (r = 0.85) for the aortic regurgitation group. To elucidate the mechanisms affecting pressure half-time in aortic regurgitation, an in vitro model of mitral inflow in the presence of varying regurgitant volumes and different ventricular chamber compliances was used. Aortic regurgitation shortened directly measured pressure half-time proportional to the regurgitant fraction but an increase in left ventricular compliance could offset this effect. Finally, in a mathematic model of mitral inflow the competing effects of aortic regurgitation and chamber compliance could be confirmed. In conclusion, aortic regurgitation results clinically in a significant net shortening of pressure half-time leading to mitral valve area overestimation. However, the effect is moderate and individually unpredictable because of changes in chamber compliance.

Adult↗
PubMed1990

Multi-enzyme electrodes for the determination of starch by flow injection.

A tri-enzyme electrode has been developed for determining starch in a flowing stream based on amperometric monitoring of hydrogen peroxide at a potential of +600 mV versus a silver--silver chloride reference electrode. The nylon-based starch-sensing membranes (over a platinum electrode) were prepared from an enzyme cocktail containing various ratios of amyloglucosidase (AMG), mutarotase (MUT) and glucose oxidase (GO). The best starch-sensing membrane (to give the type A electrode) was made from an enzyme cocktail of AMG--MUT--GO (2000 + 100 + 100 U; where 1 U = 16.67 nanokatals), i.e., containing a high level of AMG. In this system, starch samples were first incubated with soluble alpha-amylase (alpha-AMY) (1000 U) for 1 h at room temperature prior to analysis with the tri-enzyme electrode. Attempts were also made to immobilize alpha-AMY on to nylon net, either alone or as a component of a four-membrane starch-enzyme electrode but starch signals were weak compared with those generated by starch pre-treated with soluble alpha-AMY. This system, associated with the type A starch electrode, not only exhibited a calibration of wide linear range (1 x 10(-4)-0.1% m/v starch) but also showed promising operational properties. It has excellent thermal stability over the range 30-70 degrees C.

Electrodes↗
PubMed1990

Quantification of jet flow by momentum analysis. An in vitro color Doppler flow study.

Previous investigations have shown that the size of a regurgitant jet as assessed by color Doppler flow mapping is independently affected by the flow rate and velocity (or driving pressure) of the jet. Fluid dynamics theory predicts that jet momentum (given by the orifice flow rate multiplied by velocity) should best predict the appearance of the jet in the receiving chamber and also that this momentum should remain constant throughout the jet. To test this hypothesis, we measured jet area versus driving pressure, flow rate, velocity, orifice area, and momentum and showed that momentum is the optimal jet parameter: jet area = 1.25 (momentum).28, r = 0.989, p less than 0.0001. However, the very curvilinear nature of this function indicated that chamber constraint strongly affected jet area, which limited the ability to predict jet momentum from observed jet area. To circumvent this limitation, we analyzed the velocities per se within the Doppler flow map. For jets formed by 1-81-mm Hg driving pressure through 0.005-0.5-cm2 orifices, the velocity distribution confirmed the fluid dynamic prediction: Gaussian (bell-shaped) profiles across the jet at each level with the centerline velocity decaying inversely with distance from the orifice. Furthermore, momentum was calculated directly from the flow maps, which was relatively constant within the jet and in good agreement with the known jet momentum at the orifice (r = 0.99). Finally, the measured momentum was divided by orifice velocity to yield an accurate estimate of the orifice flow rate (r = 0.99). Momentum was also divided by the square of velocity to yield effective orifice area (r = 0.84). We conclude that momentum is the single jet parameter that best predicts the color area displayed by Doppler flow mapping. Momentum can be measured directly from the velocities within the flow map, and when combined with orifice velocity, momentum provides an accurate estimate of flow rate and orifice area.

Animals↗
PubMed1990

Prediction of successful outcome in 130 patients undergoing percutaneous balloon mitral valvotomy.

We studied 130 patients undergoing percutaneous balloon mitral valvotomy. The relation between valvular morphology according to a previously described echocardiographic scoring system and hemodynamic outcome expressed as qualitative ("good" and suboptimal) and as absolute change in valve area was analyzed. The relative importance of the individual components of this echocardiographic score (valvular thickening, mobility, calcification, and subvalvular disease) to the change in valve area after valvotomy was also examined. Mean transmitral pressure gradient decreased from 16 +/- 6 to 6 +/- 3 mm Hg (p less than 0.0001), and mitral valve area increased from 0.9 +/- 0.3 to 1.8 +/- 0.7 cm2 (p less than 0.0001). Results in individual patients were variable. Eighty-four percent (61 of 73) of patients with an echocardiographic score of 8 or less had a "good" outcome (final valve area greater than or equal to 1.5 cm2 and an increase in valve area of greater than or equal to 25%), whereas 58% (33 of 57) of patients with an echocardiographic score of 8 or more had a suboptimal result (p less than 0.001). The sensitivity of an echocardiographic score of 8 or less for predicting a "good" outcome was 72%, and the specificity was 73%. The echocardiographic score correlated negatively (r = -0.40, p less than 0.0001) with the absolute increase in mitral valve area after valvotomy, but there was substantial scatter in the data. Of the four components of the total echocardiographic score, valvular thickening correlated best with the absolute change in value area (r = -0.47, p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1989

Prevalence of valvular regurgitation by Doppler echocardiography in patients with structurally normal hearts by two-dimensional echocardiography.

The prevalence of valvular regurgitation in a large population with structurally normal hearts remains unknown. From the computer database of the echocardiography laboratory of the hospital, 7209 records containing results of both two-dimensional and Doppler echocardiographic examinations were identified, from which 867 (12%) with no structural abnormality on two-dimensional echocardiograms were obtained for analysis. Of these 867 records, 291 (34%) had evidence of regurgitation by Doppler technique in at least one cardiac valve. Mitral regurgitation was found in 167 (19%), tricuspid regurgitation in 151 (17%), pulmonic regurgitation in 45 (5%), and aortic regurgitation in 29 records (3%). Regurgitation of just one valve was the most common and occurred in 207 records (24%). This was followed by regurgitation of two valves (69 records, 8%), three valves (13 records, 2%), and four valves (two records, 0.2%). The prevalence of mitral, tricuspid, and aortic regurgitation was found to increase significantly with increasing age, as was the prevalence of regurgitation involving multiple valves. In 98% and 95% of mitral and tricuspid regurgitations, respectively, the jets were confined to the proximal one fourth of the atria, suggesting only trivial or mild regurgitation. Thus valvular regurgitation occurs not uncommonly in patients with structurally normal hearts referred for echocardiographic evaluation. These findings caution against the inappropriate diagnosis of clinical disease in the many patients who fall into this category. The increasing prevalence of valvular regurgitation with increasing age suggests that a wear-and-tear phenomenon rather than a congenital cause is involved in most instances.

Adolescent↗
PubMed1989

The biochemical ecology of Biomphalaria glabrata, a snail host of Schistosoma mansoni: short chain carboxylic and amino acids as phagostimulants.

1. A buccal mass olfactometer was used to investigate the responses of the fresh-water pulmonate snail Biomphalaria glabrata to carboxylic and amino acids. 2. The snails proved very discriminating as only 6 (14.6%) of the 41 chemical species tested were effective as phagostimulants. These are ranked as follows in order of potency:- butanoate greater than propanoate greater than D-malate greater than 2-hydroxybutanoate = L-tartrate = L-aspartate. 3. The structure-activity relationships of the active compounds, and their significance to the ecology and control of the snails are discussed.

Amino Acids↗
PubMed1989

Fluid dynamics model of mitral valve flow: description with in vitro validation.

A lumped variable fluid dynamics model of mitral valve blood flow is described that is applicable to both Doppler echocardiography and invasive hemodynamic measurement. Given left atrial and ventricular compliance, initial pressures and mitral valve impedance, the model predicts the time course of mitral flow and atrial and ventricular pressure. The predictions of this mathematic formulation have been tested in an in vitro analog of the left heart in which mitral valve area and atrial and ventricular compliance can be accurately controlled. For the situation of constant chamber compliance, transmitral gradient is predicted to decay as a parabolic curve, and this has been confirmed in the in vitro model with r greater than 0.99 in all cases for a range of orifice area from 0.3 to 3.0 cm2, initial pressure gradient from 2.4 to 14.2 mm Hg and net chamber compliance from 16 to 29 cc/mm Hg. This mathematic formulation of transmitral flow should help to unify the Doppler echocardiographic and catheterization assessment of mitral stenosis and left ventricular diastolic dysfunction.

Animals↗