Distinct patterns of calcium transients during early and delayed afterdepolarizations induced by isoproterenol in ventricular myocytes

BACKGROUND

The relation between early afterdepolarizations (EADs) and changes in intracellular Ca2+ concentration ([Ca2+]i) is still unclear. In the present study, we compared spatiotemporal changes in [Ca2+]i related to EADs and delayed afterdepolarizations (DADs) induced by isoproterenol.

METHODS AND RESULTS

Isolated patch-clamped guinea pig ventricular myocytes, loaded with fluo-3 acetoxymethyl ester (fluo-3 AM), were paced at 0.1 to 2 Hz. Isoproterenol (100 nmol/L) caused alterations in both phase 2 and phase 4 of the action potential (AP), consistent with EADs and DADs, respectively. During EADs (n = 16), similar to driven APs, increases in [Ca2+]i occurred simultaneously throughout the cell, whereas during DADs (n = 25), they originated in discrete cell sites and propagated as a wave. This difference was confirmed by analysis of eight EADs and DADs coupled to the same beat. Ca2+ transient linked to EADs reached a peak relative fluorescence level (expressed as percentage of the maximal level reached during the last stimulated beat) that was always higher than that reached during the DADs (77 +/- 3% versus 64 +/- 2%, P < .001). Spatial heterogeneity of Ca2+ transients was assessed by the maximal time interval between peaks monitored in different cell regions; this time lag was always greater during DADs than during EADs (290 versus 40 milliseconds, P = .006).

CONCLUSIONS

The present study had two main findings. First, even very modest notches occurring during the plateau of the AP may be accompanied by a marked secondary increase in [Ca2+]i. Second, these Ca2+ transients occurring during EADs are synchronous throughout the cell and differ significantly from those observed under identical conditions during DADs.

Improved late potential analysis in frequency domain

This work presents a technique to improve the identification of late potentials (LP) in patients affected by greater arrhythmogenic right ventricular disease (GARVD). Several authors have documented the correlation between GARVD and LP by means of time domain analysis. Moreover, the high incidence of bundle branch block in patients affected by GARVD suggests LP analysis in the frequency domain be performed. The method of spectral mapping of the ECG with Fourier transform was adopted. This consists in dividing the ST segment into 25 subsegments and estimating their frequency components by means of the fast Fourier transform. Recently, it was documented that this technique suffers from poor reproducibility of results. Low reproducibility is the consequence of an improper localization of the analysed QRS segments. An algorithm to increase the QRS end point identification reproducibility is proposed. An optimal QRS filter was adopted as well as a technique based on the Hilbert transform. This technique allowed the reliability of the normality factor estimates to be improved. The computed normality factors on the XYZ leads and on the vector magnitude were used to classify patients and healthy subjects; 28 patients affected by greater arrhythmogenic right ventricular disease and 35 healthy subjects were analysed in the study. High sensitivity was obtained with respect to GARVD and clinical sustained ventricular tachycardia by means of a cluster analysis technique. By applying the technique proposed in this paper the identification of LP in GARVD was increased from 47% to 88%, when clinical sustained ventricular tachycardia was documented, whereas in patients affected by GARVD but not prone to sustained ventricular tachycardia LP identification increases from 18% to 64%.

Ventricular beats induce variations in cycle length of rapid (type II) atrial flutter in humans. Evidence of leading circle reentry

BACKGROUND

Slight variation in cycle lengths of common and rapid atrial flutter in humans is an established phenomenon, but its mechanisms have not been completely clarified. In a previous study, we demonstrated that in common atrial flutter the variations in atrial cycle length were due to atrial stretch affecting the revolution time of a reentrant circuit. In the present study, we investigate the nature of atrial cycle length variations in the rapid type of human atrial flutter.

METHODS AND RESULTS

Atrial interval variations of 17 episodes of rapid atrial flutter in 14 patients were investigated by measuring the sequence of atrial intervals from intraesophageal or intra-atrial leads and the onset of QRS complexes from a surface lead (V1). To study whether interval variation in flutter cycle was related to ventricular activity, a phase plot was constructed in which the flutter cycle length was plotted against the time after the previous QRS complex. This showed that the interval fluctuations were strictly coupled to the moment of ventricular activation. After the onset of the QRS complex, the rapid atrial flutter interval gradually decreased by an average of 4.1% (P < .001) and reached a minimum value after 300 to 600 milliseconds. Thereafter, the intervals increased again until the next ventricular beat occurred. In 10 patients developing both common and rapid atrial flutter, two different phase relations were found. Whereas during common atrial flutter the atrial interval increased after the QRS complex, it decreased during rapid atrial flutter. In three patients, intra-atrial pressure was recorded together with the electrical activity during both common and rapid atrial flutter episodes. This showed that variations in atrial flutter cycle length were associated with the rise of atrial pressure during ventricular contraction.

CONCLUSIONS

These findings indicate a role of contraction-excitation feedback caused by atrial stretch after a ventricular activation. The shortening of the atrial interval after the onset of the QRS complex as found in patients during rapid atrial flutter can be explained by stretch-induced shortening of atrial refractoriness and consequent shortening of the revolution time of a functionally determined intra-atrial circuit.

Quantitative colorimetric analysis of liquid crystal films (LCF) for phantom dosimetry in microwave hyperthermia

A fully quantitative analysis of liquid crystal film (LCF) color patterns, in phantom thermal dosimetry for microwave hyperthermia, is presented. An accurate determination of absorption rate density (ARD) is achieved by color image computer processing. This work is proven to be an improvement upon the semi-quantitative or qualitative descriptions of LCF colors performed essentially by visual analysis of photographs. Temperature-induced chromatic distributions are acquired as R, G, B (red, green, blue) signals by a CCD camera connected to a PC frame grabber board. These data, stored into three 512 x 512 memory buffers, are then converted to H, S, I (hue, saturation, intensity) colorimetric system. Provided a suitable calibration of the LCF, the H quantity can be transformed to temperature using a monotonic relationship. In this way, a temperature accuracy lower than 0.2 degrees C and a spatial resolution less than 1 mm are obtained. A sequence of thermal maps can be acquired and stored on disk at a maximum rate of 1 image/2 s, and then the ARD is calculated at each pixel of the map using the least squares method.

Beat-to-beat measurement and analysis of the R-T interval in 24 h ECG Holter recordings

This study assesses the feasibility of beat-to-beat measurement of the R-T interval in Holter ECG recordings. The low sampling rate of the Holter system was increased by a specific interpolating filter, and the precision and accuracy of two T-wave fiducial point (T-wave maximum: Tm, T-wave end: Te) detection algorithms were compared. The results of the validation tests show better performance of the Tm measurement procedure in the presence of high noise levels. The overall process for the beat-to-beat R-T interval measurement was then tested on ECG Holter recordings collected during free and controlled respiration. Finally, the R-Tm and the corresponding R-R intervals were measured on 24 h ECG recordings of healthy subjects and the spectral analysis was applied to the constructed series. Both R-R and R-Tm spectra show two main frequency components (low-frequency approximately 0.1 Hz, high-frequency approximately 0.25 Hz) changing in their power ratios continuously throughout the 24 h period. The method described seems to provide a dynamic index of the sympatho-vagal balance at the ventricle that can be useful for a deeper understanding of ventricular repolarisation duration variability.

Spontaneous beat-to-beat variability of the ventricular repolarization duration

The spontaneous beat-to-beat variability of the ventricular repolarization duration was investigated in 21 healthy subjects (age 25-71 years; mean, 40 years) during the basal state in a recumbent position. For each subject, approximately 1,000 consecutive cycles were analyzed with an automated technique. The time series of the RR, QT, and RT intervals generate histograms that approximate normal distributions and have mean standard deviations of 57.0 ms, 5.4 ms, and 4.3 ms, respectively. Spectral analysis was used to detect rhythmical oscillations in these time series. The power spectra densities of both heart rate and ventricular repolarization during show peaks in the same frequency bands: low frequency (0.05-0.12 Hz) and high frequency (0.2-0.4 Hz). The power distribution between these two bands observed in the ventricular repolarization duration spectra was found to be the reverse of that in heart rate spectra (p less than 0.005).

Activation energy of the cardiac Na+/Ca2+ exchanger in sarcolemmal vesicles and reconstituted proteoliposomes

Sarcolemmal membrane vesicles isolated from bovine ventricular tissue accumulate Ca2+ through the Na+/Ca2+ exchanger when exposed to an outwardly directed Na+ gradient. This Ca2+ is then released by the same mechanism if the vesicles are transferred to a Ca(2+)-depleted Na+ buffer. Using the Ca+ indicator, arsenazo III, and a stopped-flow spectrophotometer, we can directly follow the kinetics of Ca2+ extrusion. We can thus measure the activity of the Na+/Ca2+ exchanger by the initial rate of Ca2+ release. We found that it depends upon the external Na+ concentration in a cooperative way, with a Hill coefficient of 2. By studying the temperature dependence of Na+/Ca2+ exchange, we found that it can be described by a single activation energy: Ea = 8.3 +/- 0.4 Kcal/mol. When the Na+/Ca2+ exchanger is reconstituted into lipid vesicles of defined composition, we observe a higher activity if cholesterol is among the lipids. The activation energy becomes 6.1 +/- 0.1 Kcal/mol in this system, but the Arrhenius plot shows a decreased slope for temperatures above 33 degrees C.

[Stable ventricular tachycardia in arrhythmogenic dysplasia of the right ventricle in sportsmen]

Right ventricular arrhythmogenic dysplasia (RVAD) is a typical asymptomatic arrhythmogenic cardiopathy in athletes, which is occasionally concurrent with normal ventricular function and life-threatening arrhythmias. A total of 32 athletes (28 males and 4 females, mean age, 23 years, mean follow-up, 6.7 years) were examined for severe cardiac arrhythmias with left bundle branch block. The conclusive diagnosis of RVAD was established from clinical, echocardio-, and angiographic evidence. The protocol of the examination involved Holter monitoring, loading tests, electrophysiological study, two-dimensional echocardiography, cardiac angiography of the right and left ventricles, coronary angiography. The most severe arrhythmias were observed in athletes whose mean age was 23.4 years, 20 patients had sustained ventricular tachycardia (it occurred only in 19 who were indulging in sports), 6 presented with transient ventricular tachycardia, and 1 had ventricular fibrillation. They all had been considered fit for sports. The disease proceeded severely in 16 of 32 athletes (in 13 of 16 while indulging in sports), the conditions close to syncope were seen in 9 patients (8 had sustained ventricular tachycardias and 1 had transient ventricular tachycardias), syncopes were observed in 5 patients (sustained ventricular tachycardias).

Absorption rate density (ARD) computation in microwave hyperthermia by the finite-difference time-domain method

A mathematical model has been developed, which is able to predict power distributions in biological tissues during microwave hyperthermia delivered by waveguide applicators. The numerical solutions of Maxwell's equations was obtained by the finite-difference time-domain (FDTD) technique. Two improvements with respect to the standard implementation of FDTD were introduced: a separation between the source and load calculations (based on the Schelkunoff equivalence principle) and a simple routine that automatically recognises the steady state. Two commercially available applicators, a dual-ridged and a side-loaded waveguide, were modelled using their theoretical aperture fields. The absorption rate density (ARD) distributions delivered by these applicators were measured through phantom thermal dosimetry and compared with the patterns estimated by the simulation.

Arrhythmia patterns in athletes with arrhythmogenic right ventricular dysplasia

Arrhythmogenic right ventricular dysplasia (ARVD) is a typical 'silent' arrhythmogenic cardiomyopathy in athletes, with the possibility of normal ventricular performance and life-threatening arrhythmias. We studied 32 athletes (28 M, 4 F), mean age 23 years, follow-up 6.7 years, all previously declared fit for sports activity. They were studied for significant ventricular arrhythmias with LBBB with a final diagnosis of ARVD based on accepted clinical, echocardiographic and angiographic criteria. The study protocol included Holter monitoring (HM), stress test (ST), electrophysiological endocavitary study (EES), 2D echocardiography, RV and LV cardioangiography and coronarography. The most serious arrhythmia appeared at a mean age of 23.4 years, 20 had clinical sustained ventricular tachycardia (VT) (19/20, 95% during sports activity), six non-sustained VT and one ventricular fibrillation (VF). Severe symptoms occurred in 16/32 athletes (50%) during sports activity in 13/16 (81%): presyncope in nine (non-sustained VT in one, sustained VT in eight); syncope in five (sustained VT); aborted sudden death (SD) in one, SD (follow-up) in one. The reproducibility (HM, ST, EES) of clinically severe arrhythmic manifestations which occurred during sports activity was not high. In fact, during sport many factors are at work which may activate several arrhythmogenic mechanisms not easily reproducible in the laboratory. We conclude that a cardioarrhythmological study is mandatory in suspected right ventricular arrhythmias, including morphological study of the RV, to avoid arrhythmic risk during athletic activity.

Congenital long QT syndrome inducing 2:1 atrioventricular block: early detection in fetal life

A case is reported of congenital long QT interval associated with fixed 2:1 atrioventricular block. The bradycardia was detected at 16 weeks of gestational age. The atrioventricular block was due to an extremely delayed ventricular repolarization. Early detection of bradycardia in fetal life and the demonstration of a normal positive correlation between QT duration and ventricular rate suggest that, in this case, the syndrome may be due to an anomaly of the myocardial cells rather than to imbalance of the sympathetic nervous system.

On-line interval measurement during invasive cardiac electrophysiologic testing

A microprocessor-based timer has been developed for routine and research use in cardiac electrophysiologic studies. The cycle length (A-A) and the conduction times through the right atrium (A(HRA)-A(HIS], the AV node (A-H) and the His-Purkinje system (H-V) can be automatically measured beat-to-beat in real-time both during sinus rhythm and during atrial pacing. The design has been refined during three years of use in over 80 clinical studies. A comparison between manual and automatic measurements has been executed for randomly chosen portions of 10 studies. With the manual measurements as reference procedures the sensitivities of the automatic measurements of the A, H and V waves were 0.98, 0.88 and 0.92, respectively, while the specificities were 0.93, 0.92 and 0.93, respectively. For the true positive measurements the correlation coefficients of the intervals were r(A-A) = 0.9998, r(A-H) = 0.987 and r(H-V) = 0.988. The statistical analysis of the differences between manual and automatic readings for the intervals A-A, A-H and H-V yielded mean values of -1 ms, -0.8 ms and 0.6 ms and standard deviations of 3 ms, 4 ms and 4 ms which are smaller than the accuracy of the common manual measurements.

Absorbed power distributions from single or multiple waveguide applicators during microwave hyperthermia

A theoretical model has been developed to calculate the power distributions in biological-like tissues in direct contact with waveguide applicators. A numerical solution of the coupled integral equations is performed by means of their transformation into matrix equations by using the method of moments, where the biological tissue and the applicator are described by two independent matrices. The model permits an accurate evaluation of the electric field at the aperture. The application of this method for the evaluation of arrays is also discussed. Power contour plots produced by applicators routinely used in clinics have been computed and compared with experimental results obtained with tissue-equivalent phantoms for three typical bodies: homogeneous, stratified and inhomogeneous.

Real-time beat-to-beat measurement of conduction intervals during cardiac electrophysiological studies

The conduction times through different myocardial structures have been measured in real-time during clinical intracardiac investigations by means of a low-cost microprocessor-based system. Two endocavitary electrograms, a surface ECG and a pacemaker stimulus are analysed to detect the stimulus as well as atrial, His and ventricular waves and to measure, beat to beat, the time intervals between them. Measurements have been made in the course of 40 clinical studies, randomly chosen from about 500, and including different conditions such as sinus rhythm, acute drug tests and programmed atrial pacing. Our results show that the real-time measurement of the conduction intervals is feasible and may offer several advantages: it allows immediate data analysis which enables the investigator to better make decisions during the study; it reduces the time and effort required to perform and analyse the study; and it increases the quantity and quality of information gained on the cardiac status of the patient.

Electrical properties of ionic channels formed by Helix pomatia hemocyanin in planar lipid bilayers

Helix pomatia hemocyanin forms ion-conducting channels in planar lipid bilayer membranes when added at mg/ml concentration. These channels have several original features. They fluctuate between one conducting and some poorly conducting states and fluctuations can be grouped in bursts. Different channels can have widely different conductance amplitudes. Both channel conductance and burst lifetime are dependent on the applied voltage. Fluctuations within a burst show a complex kinetic behaviour which has been explained developing a multistate model. The model calls for one single open state and six different closed states. Transitions are allowed only between one of the closed states and the open one and obey first order kinetics. This model is able to fit all our experimental curves obtained in single channel experiments.

The dependence of the conductance of the hemocyanin channel on applied potential and ionic concentration with mono- and divalent cations

Incorporation of Megatura crenulata hemocyanin into phosphatidylcholine black lipid membranes results in the formation of ion channels. Channel properties depend on many factors, three of which are examined in this work: type and concentration of electrolyte and applied voltage. Eight cations at different concentrations have been used. Instantaneous conductance of the channel is a saturating function of both applied voltage and ionic strength of the bathing solution with monovalent cations, but only of ionic strength with divalent cations. Steady-state voltage-conductance relations are nonlinear for both signs but show slight saturation with ionic strength. Relaxation towards the steady state can be fitted by two exponentials with different time constants. All experimental data are fitted postulating the existence of a mechanism of voltage gating of the channel, and of discrete negative charge near its mouth. Specific and nonspecific binding of cations is required.