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G J Uijen

Publications and source records attributed to G J Uijen.

At least 19 recordsLinked to original sources
PubMed2001

Geometrical aspects of the interindividual variability of multilead ECG recordings.

The electrocardiogram (ECG) as measured from healthy subjects shows a considerable interindividual variability. This variability is caused by geometrical as well as by physiological factors. In this study, the relative contribution of the geometrical factors is estimated. In addition a method aimed at correcting for these factors is described. First, a measure (RV) for quantifying the overall variability is presented, and for healthy individuals its value is estimated as 0.52. Next, based on a simulation study using the individual (heart-lung-torso) geometry of 25 subjects, the variability caused by geometrical factors is estimated as 0.40, indicating that in healthy subjects the RV for healthy individuals resulting from electrophysiology is of the order of 0.33. In an evaluation of the correction procedure, applied to realistic, simulated body surface potentials, it is shown that RV caused by geometrical factors can be reduced from 0.40 to 0.06. When applying the correction procedure to measured ECG data no reduction of the RV value could be demonstrated. These results indicate that the involved procedure of the inverse computation of a cardiac equivalent source, at the present time, is of insufficient quality to cash in on the substantial reduction of RV values from 0.52 down to 0.33 that might be obtainable.

Body Surface Potential Mapping↗
PubMed2000

Geometrical factors affecting the interindividual variability of the ECG and the VCG.

Various measures for quantifying the interindividual variability of the electrocardiogram and the vectorcardiogram in healthy subjects are presented. An analysis of factors that may cause this variability is performed, in particular of the geometrical factors of body size, heart size, heart position, and orientation. The results indicate that the variations in the magnitude of the electrocardiogram as observed through leads placed on the anterior thorax are dominated by the solid angle at which the outline of ventricular mass is seen from points on the thorax. Heart size and body size as such play only a secondary role. The limited spatial sampling of the anterior thorax directly overlaying the heart causes the mean values of all measures of amplitudes in women to be lower than in men. The vectorcardiogram magnitude was found to be much less dependent on overall geometry and heart position, and, hence, also to be less dependent on gender.

Adult↗
PubMed1999

On selecting a body surface mapping procedure.

Throughout the world, various procedures related to body surface mapping have evolved. The large differences in these procedures make multicenter studies difficult. This paper discusses the problems involved in selecting the number of leads, lead placement, and map format. Methods are highlighted that have been developed for pooling of the data as obtained by different centers. Recommendations are included to newcomers in the field. (The work stems from an international study, the Noninvasive Evaluation of the Myocardium, a study group sponsored by the European Commission, which has as one of its objectives the standardization of body surface mapping procedures.)

Body Surface Potential Mapping↗
PubMed1999

Interindividual variability of multilead electrocardiographic recordings: influence of heart position.

The electrocardiogram (ECG) of normal, healthy subjects shows a large interindividual variability. Part of this variability is due to the heart position and orientation relative to the electrodes. In this report, the interindividual variability is quantified using the relative variability measure, computed as the averaged standard deviation in the ECGs, scaled by the average root mean square of the ECGs. The relative variability in the QRS complex is estimated as 0.52. The heart position and orientation relative to the lead positions is documented in 25 normal subjects. The long axis angle varies considerably among the subjects (27.1+/-8.8 degrees to the transversal plane and 38 degrees +/-5 degrees to the frontal plane). Moving the electrodes in the frontal plane to a position relative to a common reference point at the base of the heart (shift: 0.8+/-0.7 cm leftward and 2.4+/-2.3 cm downward) did not reduce the interindividual variability.

Adult↗
PubMed1998

Comparison of coronary flow velocity and regional myocardial perfusion for functional evaluation of coronary artery disease in the setting of angioplasty.

Two essentially different methods for physiological evaluation of coronary artery disease were compared in the setting of angioplasty and related to quantitative coronary angiography. Forty-five patients, referred for percutaneous transluminal coronary angioplasty (PTCA), were examined by digital subtraction angiography (DSA) and by coronary flow velocity measurements distal to the target stenosis. Before PTCA, hyperemic mean transit time (HMTT) was correlated with % area stenosis r = 0.56*, coronary flow velocity reserve (CFVR) r = 0.58* and with CFVRN (CFVR normalized to a mean blood pressure of 100 mmHg) r = 0.68*. The correlation between CFVR and % area stenosis was r = 0.72* (*P < 0.001). After PTCA, all correlations between these measurements disappeared. HMTT and CFVR remained abnormal in 18% and 32 % of the patients, respectively. Pre-PTCA, distal coronary flow velocity measurements were reasonably well related to the assessment of regional myocardial perfusion. Flow velocity parameters, however, were better related to angiographic stenosis parameters. After PTCA, HMTT showed a more consistent improvement compared to CFVR. Flow velocity measurements appear to be more useful for the evaluation of local coronary stenoses, whereas the assessment of regional myocardial perfusion by DSA may be used for a more general evaluation of vessel territories.

Adult↗
PubMed1998

Lead system transformation of body surface map data.

Multicenter application of body surface map data (multilead electrocardiographic [ECG] data) is hampered by the fact that the centers involved in body surface mapping use lead systems differing in lead placement as well as in the number of leads. In this study, the performance of two methods for converting multilead ECGs from one lead system to another is evaluated in their application to the major lead systems presently in use throughout the world. The first method is based on Laplacian interpolation, and the second method is derived from the correlations between the signals in an extensive lead system. Through analyzing the representation errors, it was found that, for lead systems incorporating over 60 leads, both methods work well, yielding errors comparable to interbeat differences in individuals. For lead systems incorporating fewer leads, the correlation method is to be preferred.

Adult↗
PubMed1997

Functional evaluation of lipid-lowering therapy by pravastatin in the Regression Growth Evaluation Statin Study (REGRESS)

BACKGROUND: Lipid-lowering therapy during 2 years in the Regression Growth Evaluation Statin Study (REGRESS) was associated with less progression of coronary atherosclerosis in the pravastatin group compared with the placebo group. The effect of lipid-lowering therapy on the functional state of the coronary circulation is less well known. The purpose of this study was to evaluate this effect. METHODS AND RESULTS: In a substudy of REGRESS, 69 patients were randomized to pravastatin or placebo. Thirty-seven of these patients were allocated to the medical management stratum. Quantitative coronary angiography, regional myocardial perfusion, exercise testing, and classification of angina pectoris were assessed at baseline and after 2 years of therapy. Regional myocardial perfusion was assessed by digital subtraction angiography after intracoronary papaverine with video-densitometric calculation of the hyperemic mean transit time (HMTT) of contrast. In the medical management stratum, regional myocardial perfusion was assessed in 31 regions in the pravastatin group and 25 regions in the placebo group. The change in HMTT in the pravastatin group was -0.18 seconds (-5%) and in the placebo group +0.52 seconds (+18%), a difference of 0.70 seconds (P=.004). The mean difference in change in classification of angina pectoris (scale, 1 to 4) between pravastatin and placebo was 0.7 (P=.03) in favor of the pravastatin-treated patients. The change in HMTT was correlated with the change in exercise time (r=-.65, P=.002). CONCLUSIONS: In patients with symptomatic coronary artery disease, treatment with the HMG-coenzyme A reductase inhibitor pravastatin during 2 years resulted in a preserved regional myocardial perfusion, whereas patients on placebo deteriorated. The classification of angina pectoris improved only in patients receiving pravastatin. In lipid-lowering therapy, the evaluation of myocardial perfusion by assessment of the HMTT reveals a combined measure of functional and structural changes in the coronary circulation.

Adult↗
PubMed1996

LDL-Apheresis Atherosclerosis Regression Study (LAARS). Effect of aggressive versus conventional lipid lowering treatment on coronary atherosclerosis.

BACKGROUND: Intensive lipid lowering may retard the progression of coronary atherosclerosis. LDL-apheresis has the potential to decrease LDL cholesterol to very low levels. To assess the effect of more aggressive lipid lowering with LDL-apheresis, we set up a randomized study in men with hypercholesterolemia and severe coronary atherosclerosis. METHODS AND RESULTS: For 2 years, 42 men were treated with either biweekly LDL-apheresis plus medication or medication alone. In both groups a dose of simvistatin of 40 mg per day was administered. Baseline (mean+/-SD) LDL cholesterol was 7.8+/-1.9 mmol x L(-1) and 7.9+/-2.3 mmol x L(-1) in the apheresis and medication groups, respectively. The mean reduction in LDL cholesterol was 63% (to 3.0 mmol x L(-1)) and 47% (to 4.1 mmol x L(-1)), respectively. Primary quantitative coronary angiographic end points were changes in average mean segment diameter and minimal obstruction diameter. No differences between the apheresis and medication groups were found in mean segment diameter (-0.01+/-0.16 mm versus 0.03+/-0.16 mm, respectively) or in minimal obstruction diameter (0.01+/-0.13 mm versus 0.01+/-0.11 mm, respectively), expressed as means per patient. On the basis of coronary segment, mean percent stenosis of all lesions showed a tendency to decrease; only in the apheresis group more minor lesions disappeared in comparison to the medication group. On bicycle exercise tests, the time to 0.1 mV ST-segment depression increased significantly by 39% and the maximum level of ST depression decreased significantly by 0.07 mV in the apheresis group versus no changes in the medication group. CONCLUSIONS: Two years of lipid lowering both with medication alone or LDL-apheresis with medication showed angiographic arrest of the progression of coronary artery disease. However, more aggressive treatment induced functional improvement, which may precede anatomic changes.

Adult↗
PubMed1996

Low density lipoprotein apheresis improves regional myocardial perfusion in patients with hypercholesterolemia and extensive coronary artery disease. LDL-Apheresis Atherosclerosis Regression Study (LAARS).

OBJECTIVES: In a randomized study we evaluated the effect of biweekly low density lipoprotein (LDL) apheresis plus simvastatin versus medication alone on regional myocardial perfusion. BACKGROUND: In patients with severe hypercholesterolemia, diet and lipid-lowering drugs are often insufficient to achieve optimal LDL cholesterol values. Low density lipoprotein apheresis is a very effective lipid-lowering therapy. Assessment of regional myocardial perfusion enables evaluation of the functional state of the coronary circulation. METHODS: We studied 42 patients with severe hypercholesterolemia and extensive coronary artery disease who were randomized to diet and simvastatin with or without biweekly LDL apheresis. Regional myocardial perfusion was assessed by digital subtraction angiography with videodensitometric calculation of hyperemic mean transit time (HMTT) of contrast medium at baseline and after 2 years of therapy. RESULTS: Low density lipoprotein cholesterol decreased by 63% (to 3.0 mmol/liter) in the LDL apheresis group and by 47% (to 4.1 mmol/liter) in the medication group. Paired HMTT measurements were assessed in 43 regions in the LDL apheresis group and 35 regions in the medication group. In the LDL apheresis group, regional HMTT decreased over 2 years from 3.35 +/- 1.18 (mean +/- SD) to 2.87 +/- 0.82 s (-14%, p = 0.001), whereas no change in the medication group was observed: 2.95 +/- 1.06 to 2.96 +/- 0.90 s (p = NS). In the patient-based comparison, the mean change in HMTT was -0.45 s (-14%, p = 0.01) in the LDL apheresis group and -0.05 s (-2%, p = NS) in the medication group, respectively. Only exercise-induced ischemia improved in the LDL apheresis group. CONCLUSIONS: Biweekly LDL apheresis plus simvastatin decreased time-averaged LDL cholesterol levels by an additional 31% (1.1 mmol/liter) compared with medication alone. After 2 years of therapy, regional myocardial perfusion improved in the LDL apheresis group and remained unchanged in the medication group. Thus, aggressive reduction of LDL cholesterol has a favorable effect on regional myocardial perfusion and alleviates ischemia.

Adult↗
PubMed1995

Lead system transformation for pooling of body surface map data: a surface Laplacian approach.

In this paper, a method is described to transform Body Surface Map (BSM) data from one lead system to that of another. This enables pooling of BSM data between different centres. The transformation tool is based upon Laplacian interpolation. It is evaluated by inspecting transformations from lead systems having few leads to one having many leads.

Body Surface Potential Mapping↗
PubMed1993

The number of signals in multilead ECGs in individuals.

The number of independent signals in multilead electrocardiograms (ECGs) (body surface maps) of individuals have been determined by using two different methods. Both methods are based on an analysis of the eigenvalues of the spatial covariance matrix corresponding to the individual multilead ECG. One method takes only the noise level into account (threshold method). The other method is based on an information theoretic criterion (minimum description length) and uses explicitly the statistical properties of the noise (noise covariance). The analysis was performed on the QRS complexes and ST-T complexes of the multilead ECG of 61 subjects. The noise was estimated from three isoelectric intervals (PQ interval). For the different subjects, 0-14 signals (with a mean of 7 signals) were detected in the QRS interval and 0-9 signals (with a mean of 3 signals) were detected in the ST-T interval.

Electrocardiography↗
PubMed1992

On the detection of the number of signals in multi-lead ECGs.

New methods which have been developed in the field of signal processing to detect the number of intrinsic source signals in a set of multiple signals have been studied in their application to the analysis of multilead ECGs (body surface maps). All these methods are based on the analysis of the eigenvalues of the covariance matrix related to the observed data. By using artificial multilead ECGs in which the properties of the signal and noise parts were known, the effect of taking into account these properties was investigated. It was found that the method based on information-theoretic criteria was superior in detecting the number of signal components, in the case that the noise in the data is non-white, as compared to a simple threshold method.

Data Interpretation, Statistical↗
PubMed1992

The performance of information-theoretic criteria in detecting the number of independent signals in multi-lead ECGs.

Three different methods to detect the number of independent signals in multilead ECGs were evaluated by using different ECG realizations with known specifications for signal and noise: a threshold method (TM), the minimum description length (MDL) and Akaike's information criterion (AIC). The fundamental assumption in this kind of signal processing is that both the signal and the noise stem from independent stochastic generators. The consequence is that the detection of the number of signals is only possible if the noise is white, or if the noise properties have been specified. The evaluation was performed with respect to the QRS complex of individual multilead ECG simulations and to the entire ensemble. In the simulated ECGs the number of independent signals was fixed (eight). It was found that, out of the three methods studied, the performance of MDL was the best, especially when the number of available observations in the noise (used to estimate the noise specifications) was moderate.

Data Collection↗
PubMed1991

Concept of maximal flow ratio for immediate evaluation of percutaneous transluminal coronary angioplasty result by videodensitometry.

BACKGROUND: In the setting of percutaneous transluminal coronary angioplasty (PTCA), immediate information about the result of the intervention is important, whereas morphological parameters are often less reliable than in diagnostic coronary arteriography. Recently, a new videodensitometric method was introduced and validated in animal experiments, which allows accurate comparison of maximal myocardial perfusion between situations with different degrees of stenosis. This method uses mean transit time (Tmn) of the contrast agent at maximal hyperemia as a parameter for maximal flow and is strictly in accordance with indicated dilation theory. METHODS AND RESULTS: In 40 patients with angina pectoris, single-vessel disease, and a positive exercise test at the time of acceptance for PTCA, this approach was applied for evaluation of the improvement of maximal flow achieved by the PTCA. Maximal vasodilation was induced immediately before and 15 minutes after PTCA by intracoronary administration of papaverine, and digital angiographic studies were performed. By special breath-holding instruction, almost motionless, triggered image acquisition was possible during 15-20 heartbeats. Excellent subtraction images could be obtained, and reliable determination of Tmn at maximal hyperemia was possible in 33 patients both before and after PTCA. The ratio between maximal flow after and before PTCA, called maximal flow ratio (MFR), was represented by the ratio between Tmn before and after the intervention and compared with the results of exercise testing 24-48 hours before and 7-10 days after the procedure. After correction for pressure changes, MFR was 2.2 +/- 1.5 for the 33 dilated vessels and 1.0 +/- 0.2 for 25 normal vessels serving as a control. In 94% of all patients, an MFR value of more than 1.6 or less than 1.6 discriminated between presence or absence of reversal of exercise test result from positive to negative. If on-line judgment of success was based upon angiographic parameters or measurement of trans-stenotic pressure gradient, the relation with noninvasive functional improvement was present only in 66% and 74% of all patients, respectively. A definite range of what can be called normal Tmn at maximal hyperemia could be distinguished, and post-PTCA values for successfully dilated arteries returned completely to this normal range. CONCLUSIONS: Accurate comparison of maximal myocardial perfusion before and after PTCA is possible in man, improvement of maximal flow is highly related to functional improvement as indicated by exercise test results, and, therefore, this method provides a straightforward way for on-line evaluation of the result of the intervention.

Absorptiometry, Photon↗
PubMed1990

Mean transit time for videodensitometric assessment of myocardial perfusion and the concept of maximal flow ratio: a validation study in the intact dog and a pilot study in man.

Over the last decade it has become more and more obvious that besides anatomical information about the severity of coronary artery stenoses, information about coronary and myocardial blood flow is necessary to understand the functional significance of these obstructions and to evaluate the result of an intervention. Several methods have been proposed for this purpose, each of these having their particular limitations. In this study a new method is shortly described which allows the accurate calculation of relative maximal myocardial perfusion by ECG-triggered digital radiography (videodensitometry), using mean transit time (Tmn) as time parameter; this technique is based on the original physiologic principles of indicator dilution theory. This method was validated in 8 instrumented dogs in which an excellent linear relation was present between 1/Tmn and flow (r = 0.96 +/- 0.03). Although this method does not allow assessment of resting flow and therefore coronary flow reserve (CFR), it provides a means for the reliable comparison of maximal myocardial flow in different situations and it is independent of most factors affecting coronary flow reserve. The ratio between maximal flow after and before an intervention is called maximal flow ratio (MFR) and this concept was applied in a pilot study in man to evaluate PTCA results in 10 patients undergoing elective angioplasty. MFR was compared with the result of exercise testing 24 hours before and 10 days after the angioplasty. MFR greater than or equal to 1.5 was always accompanied by reversal of exercise test result from positive to negative. We conclude that the accurate calculation of relative maximal perfusion of the myocardium is possible by videodensitometry and suggest that comparison of maximal flow after and before an intervention can be valuable in man for functional evaluation of the result of the intervention.

Adult↗
PubMed1990

Mean transit time for the assessment of myocardial perfusion by videodensitometry.

The intrinsic limitations of coronary arteriography to predict the physiological effects of coronary obstructions are well known. Therefore, more direct assessments of the functional significance of coronary stenoses are becoming increasingly important. Study of contrast passage by electrocardiogram-triggered digital radiography has been proposed as a way of assessing changes in myocardial perfusion. The main problems in this approach are the limited time for motionless image acquisition, the potential alteration of vascular volume between different states, and the changing flow pattern induced by contrast agents. This has led to empiric substitution of mean transit time (Tmn) by other time parameters and to representation of vascular volume by maximal contrast intensity (Dmax). To avoid these problems, intact dogs were studied during almost motionless image acquisition of 20-25 consecutive paced heart beats obtained with synchronous radiographic pulses. In this way, unequivocal and reproducible determination of Tmn was possible. Constant and maximal vascular volume was created by continuous infusion of dipyridamole, and it was proved that coronary flow in this model was not influenced by contrast injections. Flow in the circumflex artery was measured by a ring mounted and calibrated Doppler probe. In each dog, flow in the circumflex artery was varied by a balloon occluder in 12 small steps (range, 0-174 +/- 42 ml/min). Inverse appearance time (1/Tapp), Dmax, Dmax/Tapp, inverse time of maximal intensity (1/Tmax), and 1/Tmn were calculated and the relations of these parameters to measured flow were investigated. Tmn proved to be the most reliable parameter for this purpose (r = 0.97 +/- 0.02; mean +/- SD), followed by Tmax (r = 0.93 +/- 0.04). Dmax failed to represent vascular volume but, in fact, showed a moderate correlation with flow (r = 0.78 +/- 0.22), as did Tapp (r = 0.64 +/- 0.18, 0.75 +/- 0.27, and 0.59 +/- 0.26 for the three definitions of Tapp used in this study). Dmax/Tapp correlated better with flow than either component separately. Our results indicate that the mean transit time calculated by videodensitometry can be used to accurately assess changes in myocardial perfusion strictly according to the original principles of indicator dilution theory.

Animals↗
PubMed1988

Is nonionic isotonic iohexol the contrast agent of choice for quantitative myocardial videodensitometry?

All currently used contrast media in coronary angiography induce a considerable hyperemic response interfering with the interpretation of circulation times derived from myocardial time-density curves. Aim of this study therefore, was to find a contrast agent with minimal hyperemic response. For this purpose 2, 4 and 6 ml of the nonionic isotonic low iodinated contrast agent iohexol (Omnipaque 140) and 6 ml of a similarly low iodinated but still hypertonic solution of the ionic diatrizoate (Urographin 30%) were administered into the left coronary artery of 8 anesthetized instrumented dogs. Heart rate was held constant by atrial pacing and left ventricular pressure, left ventricular dP/dt and mean and phasic coronary blood flow were recorded. To test the hypothesis that the hyperemic response to nonionic contrast media is partly due to an increase in inotropic state mediated by Ca++ion influx, all measurements were repeated 30 minutes after intracoronary administration of 0.5 mg verapamil. For iohexol the increase in coronary blood flow was small but significant: 12 +/- 7%, 25 +/- 11% and 38 +/- 16% for the 2, 4 and 6 ml administrations, respectively (mean +/- s.d.; p less than 0.01). For the diluted diatrizoate the increase in coronary blood flow was 65 +/- 23%. Increases for currently used contrast agents are on the order of 200-300%. After verapamil, the hyperemic response to iohexol decreased significantly to 9 +/- 5%, 20 +/- 8% and 29 +/- 12% for the 2, 4 and 6 ml administrations, respectively (p less than 0.01). The reaction to diatrizoate was not affected by verapamil. Moreover, there was a significant positive correlation between the increase in coronary blood flow and left ventricular dP/dt max under all conditions for all but one dog. We conclude that the isotonic, low iodinated nonionic contrast agent iohexol has only a moderate influence on coronary blood flow, which can be further attenuated by verapamil. By this approach, a more reliable assessment of circulation times from myocardial time-density curves obtained by digital subtraction angiography and videodensitometry becomes possible.

Angiography↗