Causal Cross-Spectral Analysis of Heart Rate and Blood Pressure Variability for describing the Impairment of the Cardiovascular Control in Neurally Mediated Syncope

A causal approach to the calculation of coherence and transfer function between systolic pressure (SP) and RR interval variability was applied in eight patients and eight control subjects during prolonged tilt test for investigating the impairment of cardiovascular control related to neurally mediated syncope. The causal analysis showed a depressed baroreflex regulation in resting patients, with reduced gain and increased latency from SP to RR, and a drop of the baroreflex coupling immediately before syncope. These findings, which were not elicited by traditional cross-spectral analysis, strongly suggest the use of the causal approach for the study of syncope mechanisms.

Wave Similarity Mapping Shows the Spatiotemporal Distribution of Fibrillatory Wave Complexity in the Human Right Atrium During Paroxysmal and Chronic Atrial Fibrillation

INTRODUCTION

The complexity of waveforms during atrial fibrillation may reflect critical activation patterns for the arrhythmia perpetuation. In this study, we introduce a novel concept of map, based on the analysis of the wave morphology, which gives a direct evidence in the human right atrium on the spatiotemporal distribution of fibrillatory wave complexity in paroxysmal (PAF) and chronic (CAF) atrial fibrillation.

METHODS AND RESULTS

Electrograms were recorded from a 64-electrode catheter in the right atrium of 15 patients during PAF (n = 8) and CAF (n = 7). Wave similarity maps were constructed by calculating the degree of morphological similarity of activation waves (S) at each atrial site and by following its temporal evolution. During PAF the spatiotemporal distribution of the waveforms was highly consistent across the subjects and was determined by the anatomic location. Wave similarity maps showed the existence of an extended area with low similarity index, which covered the low posteroseptal atrium (S = 0.28 +/- 0.09) and the septal region (S = 0.22 +/- 0.04), and the presence of a large tongue with high similarity index, which penetrated the lateral wall (S = 0.55 +/- 0.08) starting from the high anterolateral atrium (S = 0.54 +/- 0.06). A completely different spatiotemporal pattern was seen during CAF. No distinct regions with different similarity indexes were recognized, but a uniformly distributed low similarity index (S = 0.27 +/- 0.07) was found. The spatial pattern was highly stable in time with fluctuations of S < 0.04.

CONCLUSION

Quantification of the spatiotemporal distribution of fibrillatory wave complexity is feasible in humans by wave similarity mapping. Anatomic anchoring of waveforms during PAF and pattern destruction during CAF was determined.

Quantification of synchronization during atrial fibrillation by Shannon entropy: validation in patients and computer model of atrial arrhythmias

Atrial fibrillation (AF), a cardiac arrhythmia classically described as completely desynchronized, is now known to show a certain amount of synchronized electrical activity. In the present work a new method for quantifying the level of synchronization of the electrical activity recorded in pairs of atrial sites during atrial fibrillation is presented. A synchronization index (Sy) was defined by quantifying the degree of complexity of the distribution of the time delays between sites by Shannon entropy estimation. The capability of Sy to discriminate different AF types in patients was assessed on a database of 60 pairs of endocardial recordings from a multipolar basket catheter. The analysis showed a progressive and significant decrease of Sy with increasing AF complexity classes as defined by Wells (AF type I Sy = 0.73 +/- 0.07, type II Sy = 0.56 +/- 0.07, type III Sy = 0.36 +/- 0.04, p < 0.001). The extension of Sy calculation to the whole right atrium showed the existence of spatial heterogeneities in the synchronization level. Moreover, experiments simulated by a computer model of atrial arrhythmias showed that propagation patterns with different complexity could be the basis of different synchronization levels found in patients. In conclusion the quantification of synchronization by Shannon entropy estimation of time delay dispersion may facilitate the identification of different propagation patterns associated with AF, thus enhancing our understanding of AF mechanisms and helping in its treatment.

Optical micromanipulations inside yeast cells

We present a combination of nonlinear microscopy and optical trapping applied to three-dimensional imaging and manipulation of intracellular structures in living cells. We use Titanium-sapphire laser pulses for nonlinear microscopy of the nuclear envelope and the microtubules marked with green fluorescent protein in fission yeast. The same laser source is also used to trap small lipid granules naturally present in the cell. The trapped granule is used as a handle to exert a pushing force on the cell nucleus. The granule is moved in a raster-scanning fashion to cover the area of the nucleus and hence displace the nucleus away from its normal position in the center of the cell. Such indirect manipulations of an organelle (e.g., nucleus) can be useful when direct trapping of the chosen organelle is disadvantageous or inefficient. We show that nonlinear microscopy and optical manipulation can be performed without substantial damage or heating of the cell. We present this method as an important tool in cell biology for manipulation of specific structures, as an alternative to genetic and biochemical methods. This technique can be applied to several fundamental problems in cell biology, including the mechanism of nuclear positioning and the spatial coordination of nuclear and cell division.

Second-harmonic generation sensitivity to transmembrane potential in normal and tumor cells

Second-harmonic generation (SHG) is emerging as a powerful tool for the optical measurement of transmembrane potential in live cells with high sensitivity and temporal resolution. Using a patch clamp, we characterize the sensitivity of the SHG signal to transmembrane potential for the RH 237 dye in various normal and tumor cell types. SHG sensitivity shows a significant dependence on the type of cell, ranging from 10 to 17% per 100 mV. Furthermore, in the samples studied, tumor cell lines display a higher sensitivity compared to normal cells. In particular, the SHG sensitivity increases in the cell line Balb/c3T3 by the transformation induced with SV40 infection of the cells. We also demonstrate that fluorescent labeling of the membrane with RH 237 at the concentration used for SHG measurements does not induce any measurable alteration in the electrophysiological properties of the cells investigated. Therefore, SHG is suitable for the investigation of outstanding questions in electrophysiology and neurobiology.

Combined intracellular three-dimensional imaging and selective nanosurgery by a nonlinear microscope

We use near-IR femtosecond laser pulses for a combination of microscopy and nanosurgery on fluorescently labeled structures within living cells. Three-dimensional reconstructions of microtubule structures tagged with green fluorescent protein (GFP) are made during different phases of the cell cycle. Further, the microtubules are dissected using the same laser beam but with a higher laser power than for microscopy. We establish the viability of this technique for the cells of a fission yeast, which is a common model to study the mechanics of cell division. We show that nanosurgery can be performed with submicrometer precision and without visible collateral damage to the cell. The energy is primarily absorbed by the GFP molecules, and not by other native structures in the cell. GFP is particularly suitable for multiphoton excitation, as its excitation wavelength near 900 nm is benign for most cellular structures. The ability to use GFP to label structures for destruction by multiphoton excitation may be a valuable tool in cell biology.

Exploring directionality in spontaneous heart period and systolic pressure variability interactions in humans: implications in the evaluation of baroreflex gain

Although in physiological conditions RR interval and systolic arterial pressure (SAP) are likely to interact in a closed loop, the traditional cross-spectral analysis cannot distinguish feedback (FB) from feedforward (FF) influences. In this study, a causal approach was applied for calculating the coherence from SAP to RR (K(s-r)) and from RR to SAP (K(r-s)) and the gain and phase of the baroreflex transfer function. The method was applied, compared with the noncausal one, to RR and SAP series taken from 15 healthy young subjects in the supine position and after passive head-up tilt. For the low frequency (0.04-0.15 Hz) spectral component, the enhanced FF coupling (K(r-s) = 0.59 +/- 0.21, significant in 14 subjects) and the blunted FB coupling (K(s-r) = 0.17 +/- 0.17, significant in 4 subjects) found at rest indicated the prevalence of nonbaroreflex mechanisms. The tilt maneuver recovered FB influences (K(s-r) = 0.47 +/- 0.16, significant in 14 subjects), which were stronger than FF interactions (K(s-r) = 0.34 +/- 0.19, significant in 9 subjects). At the respiratory frequency, the RR-SAP regulation was balanced at rest (K(s-r) = 0.30 +/- 0.18 and K(r-s) = 0.29 +/- 0.20, significant in 11 and 8 subjects) and shifted toward FB mechanisms after tilt (K(s-r) = 0.35 +/- 0.19 and K(r-s) = 0.19 +/- 0.11, significant in 14 and 8 subjects). The causal baroreflex gain estimates were always lower than the corresponding noncausal values and decreased significantly from rest to tilt in both frequency bands. The tilt-induced increase of the phase lag from SAP to RR suggested a shift from vagal to sympathetic modulation. Thus the importance of nonbaroreflex interactions pointed out the necessity of accounting for causality in the cross-spectral analysis of the interactions between cardiovascular variables in healthy humans.

Causal transfer function analysis to describe closed loop interactions between cardiovascular and cardiorespiratory variability signals

Although the concept of transfer function is intrinsically related to an input-output relationship, the traditional and widely used estimation method merges both feedback and feedforward interactions between the two analyzed signals. This limitation may endanger the reliability of transfer function analysis in biological systems characterized by closed loop interactions. In this study, a method for estimating the transfer function between closed loop interacting signals was proposed and validated in the field of cardiovascular and cardiorespiratory variability. The two analyzed signals x and y were described by a bivariate autoregressive model, and the causal transfer function from x to y was estimated after imposing causality by setting to zero the model coefficients representative of the reverse effects from y to x. The method was tested in simulations reproducing linear open and closed loop interactions, showing a better adherence of the causal transfer function to the theoretical curves with respect to the traditional approach in presence of non-negligible reverse effects. It was then applied in ten healthy young subjects to characterize the transfer functions from respiration to heart period (RR interval) and to systolic arterial pressure (SAP), and from SAP to RR interval. In the first two cases, the causal and non-causal transfer function estimates were comparable, indicating that respiration, acting as exogenous signal, sets an open loop relationship upon SAP and RR interval. On the contrary, causal and traditional transfer functions from SAP to RR were significantly different, suggesting the presence of a considerable influence on the opposite causal direction. Thus, the proposed causal approach seems to be appropriate for the estimation of parameters, like the gain and the phase lag from SAP to RR interval, which have a large clinical and physiological relevance.

Multiphoton multifocal microscopy exploiting a diffractive optical element

Multiphoton multifocal microscopy (MMM) usually has been achieved through a combination of galvo scanners with microlens arrays, with rotating disks of microlens arrays, and cascaded beam splitters with asynchronous rastering of scanning mirrors. Here we describe the achievement of a neat and compact MMM by use of a high-diffraction-efficiency diffractive-optic element that generates a multiple-spot grid of uniform intensity to achieve higher fidelity in imaging of live cells at adequate speeds.

A method for quantifying atrial fibrillation organization based on wave-morphology similarity

A new method for quantifying the organization of single bipolar electrograms recorded in the human atria during atrial fibrillation (AF) is presented. The algorithm relies on the comparison between pairs of local activation waves (LAWs) to estimate their morphological similarity, and returns a regularity index (rho) which measures the extent of repetitiveness over time of the detected activations. The database consisted of endocardial data from a multipolar basket catheter during AF and intraatrial recordings during atrial flutter. The index showed maximum regularity (rho = 1) for all atrial flutter episodes and decreased significantly when increasing AF complexity as defined by Wells (type I: rho = 0.75 +/- 0.23; type II: rho = 0.35 +/- 0.11; type III: rho = 0.15 +/- 0.08; P < 0.01). The ability to distinguish different AF episodes was assessed by designing a classification scheme based on a minimum distance analysis, obtaining an accuracy of 85.5%. The algorithm was able to discriminate among AF types even in presence of few depolarizations as no significant rho changes were observed by reducing the signal length down to include five LAWs. Finally, the capability to detect transient instances of AF complexity and to map the local regularity over the atrial surface was addressed by the dynamic and multisite evaluation of rho, suggesting that our algorithm could improve the understanding of AF mechanisms and become useful for its clinical treatment.

Evidence of unbalanced regulatory mechanism of heart rate and systolic pressure after acute myocardial infarction

The interactions between systolic arterial pressure (SAP) and R-R interval (RR) fluctuations after acute myocardial infarction (AMI) were investigated by measures of synchronization separating the feedback from the feedforward control and capturing both linear and nonlinear contributions. The causal synchronization, evaluating the ability of RR to predict SAP (chi(s/t)) or vice versa (chi(t/s)), and the global synchronization (chi) were estimated at rest and after head-up tilt in 35 post-AMI patients, 20 young and 12 old. Significance and nonlinearity of the coupling were assessed by surrogate data analysis. Tilting increased the number of young subjects in which RR-SAP link was significant (from 17 to 19) and linear (from 11 to 18). In AMI, both significance and linearity of the coupling were low at rest (26 significant and 24 nonlinear) and further reduced after tilt (17 significant and 16 nonlinear). Old subjects showed a partial recovery of linearity after tilt (rest: 1 linear of 7 significant; tilt: 5 linear of 8 significant). In young subjects, the causal synchronization indexes were balanced and increased from rest (chi(t/s) = 0.072 +/- 0.037 and chi(s/t) = 0.054 +/- 0.028) to tilt (chi(t/s) = 0.125 +/- 0.071 and chi(s/t) = 0.108 +/- 0.053). On the contrary, in old subjects and AMI patients, the feedforward was prevalent to the feedback coupling at rest (old: chi(t/s) = 0.041 +/- 0.023 and chi(s/t) = 0.069 +/- 0.042; AMI: chi(t/s) = 0.050 +/- 0.030 and chi(s/t) = 0.089 +/- 0.053). Tilting blunted the unbalance in old subjects (chi(t/s) = 0.065 +/- 0.052 and chi(s/t) = 0.069 +/- 0.044) but not in AMI patients (chi(t/s) = 0.040 +/- 0.019 and chi(s/t) = 0.060 +/- 0.040). Thus, after AMI, nonlinear mechanisms are elicited in RR-SAP interactions. Furthermore, the neural regulation of the cardiovascular system resulted in imbalance as a consequence of impaired feedback and enhanced feedforward control mechanisms.

Experimental approach for testing the uncoupling between cardiovascular variability series

In cardiovascular variability analysis, the significance of the coupling between two series is commonly assessed by defining a zero level on the magnitude-squared coherence (MSC). Although the use of the conventional value of 0.5 does not consider the dependence of MSC estimates on the analysis parameters, a theoretical threshold Tt is available only for the weighted covariance (WC) estimator. In this study, an experimental threshold for zero coherence Te was derived by a statistical test from the sampling distribution of MSC estimated on completely uncoupled time series. MSC was estimated by the WC method (Parzen window, spectral bandwidth B = 0.015, 0.02, 0.025, 0.03 Hz) and by the parametric autoregressive (AR) method (model order M= 4, 8, 12, 16), on time series with length L = 180, 300, 420, 540 s. Te decreased with increasing B and L and with decreasing M (range: 0.11-0.54 for WC estimator, 0.06-0.46 for AR estimator). Values for the typical parameter settings of WC and AR estimation (B = 0.025 Hz; M = 8; L = 300 s) were, respectively, 0.24 and 0.17. Moreover, Tt was always higher (range: 0.12-0.65) and the results were less dependable than those for Te in defining the zero level of MSC. Thus, with the proposed method, the hypothesis of uncoupling is rejected by accounting for the parameters that affect the confidence of spectral and cross-spectral estimates. The broad applicability of this approach should favour its introduction for assessing the significance of the coupling between cardiovascular variability series.

Continuous monitoring of myocardial acid-base status during intermittent warm blood cardioplegia

OBJECTIVE

Intermittent warm blood cardioplegia (IWBC) is a well-established technique for myocardial protection during cardiac operations. According to standardized protocols, IWBC administration is currently performed every 15-20 min regardless of any individual variable and in the absence of any instrumental monitoring. We devised a new system for continuous measurement of the acid-base status of coronary sinus blood for on-line evaluation of myocardial oxygenation during IWBC.

METHODS

In 19 patients undergoing cardiac surgery for coronary artery bypass graft and/or valve surgery and receiving IWBC (34-37 degrees C) by antegrade induction (3 min) and retrograde or antegrade maintenance (2 min) every 15 min, continuous monitoring of myocardial oxygenation and acid/base status was performed by means of a multiparameter PO(2), PCO(2), pH, and temperature sensor (Paratrend7 (R), Philips Medical System) inserted into the coronary sinus.

RESULTS

Mean cross-clamping time was 76+/-26 min; ischemic time was 13+/-0.2 min. pH decline was not linear, showing an initial fast decline, a point of flexus, and a progressive slow decline. After every ischemic period, the pH adaptation curve showed a complex pattern reaching step-by-step lower minimum levels (7.28+/-0.14 during the first ischemic period, to 7.16+/-0.19 during the third ischemic period - P=0.003). PO(2) decreased rapidly at 90% in 5.0+/-1.2 min after every reperfusion. During ischemia, PCO(2) increased steadily at 1.6+/-0.1 mmHg per minute, with progressively incomplete removal after successive reperfusion, and progressive increase of maximal level (42+/-12 mmHg during the first ischemic period, to 53+/-23 mmHg during the third ischemic period - P=0.05).

CONCLUSIONS

Myocardial oxygen, carbon dioxide, and pH show marked changes after repeated IWBC. Myocardial ischemia is not completely reversed by standardized reperfusions, as reflected by steady deterioration of PCO(2) and pH after each reperfusion. Progressive increase of reperfusion durations or direct monitoring of myocardial oxygenation could be advisable in cases of prolonged cross-clamping time.

Principal component analysis and cluster analysis for measuring the local organisation of human atrial fibrillation

The distribution of atrial electrogram types has been proposed to characterise human atrial fibrillation. The aim of this study was to provide computer procedures for evaluating the local organisation of intracardiac recordings during AF as an alternative to off-line manual classification. Principal component analysis (PCA) reduced the data set to a few representative activations, and cluster analysis (CA) measured the average dissimilarity between consecutive activations of an intracardiac signal. The data set consisted of 106 bipolar signals recorded on 11 patients during electrophysiological studies for catheter ablation. Performances of PCA and CA in distinguishing between organised (type I) and disorganised (type II/III, Wells criteria) were assessed, in comparison with manual reading, by evaluating the predictive parameters of the classification analysis. Both methods gave high accuracy (92% for PCA and 89% for CA), confirming the feasibility of on-line characterisation of AF. Sensitivity was lower than specificity (81% against 98% for PCA, and 77% against 97% for CA), with seven out of eight misclassifications of PCA in common with CA. Differences between manual and computer analysis may be related to the higher resolution of PCA and CA in the measurement of the organisation of atrial activations. These procedures are suitable for providing automatic (by CA) or semi-automatic (by PCA) measures of the extent of local organisation of AF in the pre-ablation treatment phase.

Different patterns of atrial activation in idiopathic atrial fibrillation: simultaneous multisite atrial mapping in patients with paroxysmal and chronic atrial fibrillation

OBJECTIVES

We aimed to evaluate: 1) the behavior of electrical activity simultaneously in different atrial regions during atrial fibrillation (AF); 2) the difference of atrial activation between paroxysmal and chronic AF; 3) the atrial refractoriness dispersion; and 4) the correlation between the effective refractory periods (ERPs) and the FF intervals.

BACKGROUND

Little data exist on the electrophysiologic characteristics of the different atrial regions in patients with AF. A more detailed knowledge of the electrical activity during AF may provide further insights to improve treatment of AF.

METHODS

Right and left atria were extensively mapped in 30 patients with idiopathic AF (18 paroxysmal and 12 chronic). In different atrial locations, we analyzed 1) the FF interval duration; and 2) the grade of organization and, in case of organized electrical activity, the direction of atrial activation. Furthermore, in patients with paroxysmal AF, we determined the atrial ERP, evaluated the ERP dispersion and assessed the presence of a correlation between the ERPs and the FF intervals.

RESULTS

In patients with chronic AF, we observed a shortening of the FF intervals and a greater prevalence of disorganized activity in all the atrial sites examined. In patients with paroxysmal AF, a significant dispersion of refractoriness was observed. The right lateral wall showed longer FF intervals and more organized atrial activity and, unexpectedly, the shortest mean ERPs. In contrast, the septal area showed shorter FF intervals, greater disorganization and the longest mean ERPs.

CONCLUSIONS

Electrical activity during AF showed a significant spatial inhomogeneity, which was more evident in patients with paroxysmal AF. The mean FF intervals did not correlate with the mean ERPs.

Reproducibility of spontaneous initiations of atrial fibrillation

INTRODUCTION

It has been suggested that some atrial regions may play a role in the maintenance of atrial fibrillation (AF), whereas little is known about the presence of critical areas for the initiation of AF. It is conceivable that the identification of such critical areas may lead to more localized and selective strategies of ablative therapy.

METHODS AND RESULTS

A patient suffering from paroxysmal AF was extensively mapped in both the atria with a multielectrode basket catheter in right atrium and two decapolar catheters placed in the coronary sinus and along the left septum. During the mapping, seven identical patterns of initiation of AF were recorded. AF was initiated by an atrial premature beat (APB) arising from the superior right septum, followed by a reentrant beat originating from the same area that slowly propagated through the atria and resulted in late activation of the right lateral wall. During sinus rhythm, the majority of the electrograms were single potentials, whereas during the APB, and particularly during the first atrial reentrant beat, a high percentage of fragmented complexes was present, mainly located in the right superior septum. These fragmented complexes were present in the same sites in each initiating episode and remarkably, they showed an almost identical morphology.

CONCLUSION

This case suggests that in some patients the initiation of AF could be caused by reentrant circuits localized in specific atrial regions and the reentrant circuits could be identical in the different episodes of AF. This highlights the importance of increasing our understanding of the mechanisms of the initiation of AF so that we can identify any critical area involved in the genesis of AF where selective RF lesions may be effective in curing this arrhythmia.

Atrial mapping and radiofrequency catheter ablation in patients with idiopathic atrial fibrillation. Electrophysiological findings and ablation results

BACKGROUND

Knowledge of the electrophysiological substrates and the cure of atrial fibrillation (AF) is still unsatisfactory. The goal of this study was to evaluate the electrophysiological features of idiopathic AF and their relationship to the results of radiofrequency (RF) catheter ablation of AF and the safety and effectiveness of this procedure.

METHODS AND RESULTS

Sixteen patients with idiopathic AF underwent atrial mapping during AF and then RF ablation in the right atrium. The atrial activation was simultaneously recorded in four regions in the right atrium: high lateral wall (HL), low lateral wall (LL), high septum (HS), and low septum (LS) and in the left atrium through the coronary sinus (CS). In these regions, we evaluated the atrial fibrillation intervals (FF) and the morphological features of AF recordings by Wells' classification. No complications occurred during RF ablation. Of the 16 patients, 9 (56%) without AF recurrences during the follow-up (11 +/- 4 months) were considered successfully ablated. These patients showed a significantly shorter mean FF interval in the HS and the LS (122 +/- 32 and 126 +/- 28 ms, respectively), than in the HL and LL (159 +/- 24 and 156 +/- 28 ms, respectively). Moreover, the septum had more irregular electrical activity with greater beat-to-beat changes in FF and a higher prevalence of type III AF than the lateral region. The CS had similar behavior to the septum. Conversely, patients with unsuccessful ablation had an irregular atrial activity in the lateral wall, septum, and CS with no significant differences between the different sites.

CONCLUSIONS

Right atrial endocardial catheter ablation of AF is a safe procedure and may be effective in some patients with idiopathic AF. The atrial mapping during AF showed a more disorganized right atrial activation in the septum than in the lateral wall in patients with successful ablation.

Performance assessment of standard algorithms for dynamic R-T interval measurement: comparison between R-Tapex and R-T(end) approach

Three automatic approaches to ventricular repolarisation duration measurement (R-Tapex, R-T(end threshold) and R-T(end fitting) methods) are compared on computer-generated and real ECG signals, in relation to their reliability in the presence of the most common electrocardiographic artefacts (i.e. additive broadband noise and additive and multiplicative periodical disturbances). Simulations permit the evaluation of the amount of R-T beat-to-beat variability induced by the artefacts. The R-T(end threshold) method performs better than the R-T(end fitting) one, and, hence, the latter should be used with caution when R-T(end) variability is addressed. Whereas the R-Tapex method is more robust with regard to broadband noise than the R-T(end threshold) one, the reverse situation is observed in the presence of periodical amplitude modulations. A high level of broadband noise dose not prevent the detection of the central frequency of underlying R-T periodical changes. Comparison between the power spectra of the beat-to-beat R-T variability series obtained from three orthogonal ECG leads (X,Y,Z) is used to assess the amount of real and artefactual variability in 13 normal subjects at rest. The R-Tapex series displays rhythms at high frequency (HF) with a percentage power on the Z lead (57.1 +/- 4.9) greater than that on the X and Y leads (41.9 +/- 4.6 and 46.1 +/- 4.9, respectively), probably because of respiratory-related artefacts affecting the Z lead more remarkably. More uniform HF power distributions over X,Y,Z leads are observed in the R-T(end threshold) series (31.8 +/- 3.8, 39.2 +/- 4.1 and 35.1 +/- 4.2, respectively), thus suggesting minor sensitivity of the R-T(end threshold) measure to respiratory-related artefacts.

An automated method for mapping human tissue permittivities by MRI in hyperthermia treatment planning

This paper presents an automatic method to obtain tissue complex permittivity values to be used as input data in the computer modelling for hyperthermia treatment planning. Magnetic resonance (MR) images were acquired and the tissue water content was calculated from the signal intensity of the image pixels. The tissue water content was converted into complex permittivity values by monotonic functions based on mixture theory. To obtain a water content map by MR imaging a gradient-echo pulse sequence was used and an experimental procedure was set up to correct for relaxation and radiofrequency field inhomogeneity effects on signal intensity. Two approaches were followed to assign the permittivity values to fat-rich tissues: (i) fat-rich tissue localization by a segmentation procedure followed by assignment of tabulated permittivity values; (ii) water content evaluation by chemical shift imaging followed by permittivity calculation. Tests were performed on phantoms of known water content to establish the reliability of the proposed method. MRI data were acquired and processed pixel-by-pixel according to the outlined procedure. The signal intensity in the phantom images correlated well with water content. Experiments were performed on volunteers' healthy tissue. In particular two anatomical structures were chosen to calculate permittivity maps: the head and the thigh. The water content and electric permittivity values were obtained from the MRI data and compared to others in the literature. A good agreement was found for muscle, cerebrospinal fluid (CSF) and white and grey matter. The advantages of the reported method are discussed in the light of possible application in hyperthermia treatment planning.

Adaptation of the QT interval to heart rate changes in isolated perfused guinea pig heart: influence of amiodarone and D-sotalol

The inadequacy of the QT interval to shorten following heart rate increase is a feature of the inherited long QT syndrome and may have a role in the genesis of the typical arrhythmias associated with this syndrome (torsade des pointes). The aim of our study was to evaluate whether drugs that prolong the QT interval, such as amiodarone and D-sotalol, may also impair the ability of the QT interval to adapt to sudden heart rate changes. Experiments were carried out on isolated perfused guinea pig hearts (Langendorff preparation). Driving frequency was changed, in steps, every two minutes (Hz: 2.5-3-2.5-3.75-2.5-5-2.5), while epicardial ECG was continuously recorded on magnetic tape. QT interval was automatically measured by means of a beat-by-beat analysis program. D-sotalol was added to the perfusion medium at a concentration of 4 micrograms ml-1, while amiodarone was administered, before in vitro evaluation, for seven days (50 mg kg-1 per day, intraperitoneally). In control experiments two phases of QT adaptation were identified: an abrupt QT shortening at the first beat after frequency change (QT1), followed by a gradual, exponential QT shortening that reached a new steady state in about 1 min (half life: 13 sec). The electrical restitution curve (the relation between QT1 and the corresponding diastolic interval) had a rate constant of 57 +/- 8 ms. Neither drug changed the slow component of QT adaptation. However, both drugs increased the ability of QT to shorten upon premature stimulation: D-sotalol by increasing the rate constant of the restitution curve and amiodarone by decreasing the y-intercept. Our results indicate that D-sotalol and amiodarone do not impair QT shortening during tachycardia but, on the contrary, they may favour QT adaptation, thus reducing the likelihood of the potentially lethal 'R on T phenomenon'. This may be an additional mechanism by which these drugs can exert their antifibrillatory action.

Grading in superficial papillary bladder carcinoma, with an emphasis on nuclear orientation

OBJECTIVE

To develop a computerized analysis method that could reflect not only cellular anaplasia but also the orientation (PV) of neoplastic elements and could allow the separation of low (grade G 1 and G 2a) and high (G 2b) grades of superficial papillary bladder tumors (SPBTs).

STUDY DESIGN

Cellular anaplasia is based on classic morphometric parameters, such as mean and standard deviation of area, perimeter, roundness and aspect ratio (length/width ratio) of 50 papillary bladder tumors (16 G 1, 10 G 2a and 24 G 2b according to Pauwels). Orientation of the major axis of each nucleus with respect to the true basal membrane was calculated using the SD of valve distribution.

RESULTS

No statistically significant differences were found between cases showing contiguous grades (G 1 versus G 2a or G 2a versus G 2b). PV, SD of the area and SD of the perimeter were the parameters that could significantly distinguish between G 1 and G 2b and between G 1 plus G 2a and G 2b (P < .05). When comparing G 1 with G 2a plus G 2b, only PV and SD of the area were statistically significant.

CONCLUSION

Quantification of the irregularities in the orientation of nuclei with respect to the basal membrane may improve morphometric classification of low grade SPBTs.

[Effects of filtering techniques on time-domain analysis of signal-averaged ECG after acute myocardial infarction: a multicenter study, GISS-3 arrhythmia sub-project]

AIM OF THE STUDY

To evaluate the influence of different filtering techniques on the measurement of ventricular late potentials (VLP) the Sottoprogetto Aritmie of GISSI-3 collected signal-averaged ECG (SAECG) from 647 patients.

METHODS

Data were recorded after myocardial infarction (10 +/- 4 days) in 20 Italian Coronary Units. Three main filtering algorithms were used in the different commercial devices: Bidirectional Filter (ART, Aerotel, Fidelity Medical) (BF: 340 Patients), Spectral Filter (Marquette) (SF: 258 Patients) and Del Mar Filter (Del Mar Avionics) (DF: 49 Patients). QRS duration (QRSD), low amplitude signal duration (LAS40) and root mean-square-voltage (RMS40), were measured with various filters set at 40-250 Hz high and low pass frequencies.

RESULTS

After correction for clinical variables the measurements of VLP in the three different groups were different. QRSD value obtained by BF (100.6 +/- 13 ms) was shorter than that obtained by SF (109.1 +/- 12 ms). No differences were found in LAS40 and RMS40 values between SF and BF, while DF gave longer LAS40 and lower RMS40 than SF and BF. Residual noise was lower in BF (0.3 +/- 0.1 muV). than in SF and DF (0.5 +/- 0.1 muV). Applying standard criteria DF gave a higher prevalence of VLP (48.9%) than BF (23.8%) and SF (19%) groups.

CONCLUSIONS

This study demonstrates that the use of different filters produces discordant result on VLP measurements. For correct application of SAECG analysis in risk stratification after myocardial infarction, normal and abnormal values must be specifically established for the different filter techniques.