Magnus Holm

Calculations of the electronic structure of strained InAs quantum dots in InP

We have calculated the electronic structure of InAs quantum dots embedded in InP as a function of size, using strain dependent eight-band k⋅p theory in the envelope function approximation. A realistic three-dimensional shape was used for the simulations and the piezoelectric polarization of the system was included. In order to avoid spurious solutions, an extra term was added to the Hamiltonian. Polarization dependent dipole matrix elements were calculated as well as the exciton binding energies. A comparison between measurements and calculated transition energies shows good agreement.

Global Repolarization Sequence of the Ventricular Endocardium: Monophasic Action Potential Mapping in Swine and Humans

YUAN, S., et al.: Global Repolarization Sequence of the Ventricular Endocardium: Monophasic Action Potential Mapping in Swine and Humans. The aim of this study was to evaluate the global sequence of repolarization over the ventricular endocardium. Disturbances in myocardial repolarization are associated with the genesis of arrhythmias. However, little is known about the global sequence of repolarization. Monophasic action potentials (MAPs) were recorded from 61 ± 18 LV and/or RV sites in ten healthy pigs and from 43 ± 15 LV or RV sites in eight patients using the CARTO system. Local activation time (AT), end‐of‐repolarization (EOR) time, and MAP duration were calculated and three‐dimensional global maps of AT, EOR, and MAP duration constructed. LV maps were obtained from all ten pigs and RV maps from three pigs. Five RV maps and five LV maps were obtained from the eight patients. (1) EOR sequence was recognizable in 12 of 13 pig maps and in all the patient maps. (2) EOR followed the sequence of activation in 12 of 13 pig maps and 8 of 10 patient maps. (3) The longest MAPs were recorded in or near the earliest activation area, and the shortest ones in or near the latest activation area in all the pig maps and in nine of ten and eight of ten patient maps, respectively. (4) In all maps, MAP duration and AT were negatively correlated, and EOR and AT positively correlated. In conclusion, repolarization gradients exist over the pig and the human ventricular endocardium. The activation sequence is a determinant for the repolarization sequence. The magnitude of the progressive MAP shortening with progressively later activation, relative to local AT, is a critical factor governing the direction and pattern of the EOR.

Photon mapping of quantum dots using a scanning tunneling microscope

Scanning tunneling microscopy (STM) and scanning tunneling luminescence (STL) have been used to investigate the geometric and optical properties of individual self-assembled InP quantum dots overgrown with a thin layer of GaInP. STL spectra and monochromatic photon maps were used to correlate the surface topography with the optical properties of single quantum dots. We find a spatial resolution of about 10 nm in the photon maps. Theoretical emission spectra were calculated by six-band k.p theory using a realistic shape of the dot as well as of the cap layer. The calculated emission spectrum of a single dot is in good agreement with the experimental findings

Global Repolarization Sequence of the Right Atrium: Monophasic Action Potential Mapping in Healthy Pigs

The aim of the study was to explore the global sequence of atrial repolarization and its correlation to that of activation. Endocardial monophasic action potentials (MAPs) were sequentially recorded from 51 ± 14 sites in the right atrium of ten healthy pigs using the CARTO electroanatomic mapping system. Local activation time (AT), MAP duration, and 90% repolarization time (RT) were obtained, and from these data, color coded three‐dimensional maps of AT and RT sequences and spatial distribution of MAP duration were reconstructed. The results of the study were: (1) An activation sequence was recognizable in all maps, starting from the posterosuperior wall and ending in the posteroinferior wall near the tricuspid annulus. (2) The repolarization sequence was also recognizable in all maps, and mainly followed the sequence of activation. (3) A significant positive correlation between the RT and AT was observed in all maps with an average r value being 0.571 ± 0.159 (P < 0.01 – 0.0001) , suggesting that progressively later AT associates with progressively longer RT. (4) No consistent correlation between the MAP duration and AT was found. In conclusion, repolarization gradients exist over the atrial endocardium in healthy pigs. The repolarization sequence follows the same sequence as the activation, suggesting that the spatiotemporal pattern of activation is an important determinant of the characteristics of the repolarization sequence. (PACE 2003; 26:1803–1808)

Electroanatomic mapping of right atrial activation in patients with and without paroxysmal atrial fibrillation

Inter-atrial conduction delay in patients with atrial fibrillation (AF) has been reported. However, the area of this conduction delay has not been well identified. The activation time and conduction velocity over the right atrial endocardium were evaluated during sinus rhythm using the CARTO mapping technique in 6 patients with paroxysmal AF (AF group) and 11 patients without history of AF (control group). No significant differences were observed between the 2 groups in the mean activation times and conduction velocities from the earliest activation site to the superior septum, His bundle area and coronary sinus ostium, or in the total activation times of the right atrium. There was no significant difference between the two groups in the local conduction velocity between 2 adjacent sites in the free wall, septum and bottom of the right atrium. This study suggests the previously reported conduction delay in the posteroseptal region in patients with paroxysmal AF might locate within the posterior inter-atrial septum.

Monophasic action potential mapping in swine and humans using modified-tip ablation catheter and electroanatomic mapping system

Objective : To evaluate the feasibility of monophasic action potential (MAP) mapping using a modified-tip NaviStar catheter in swine and humans. Methods : MAP mapping was performed using the modified-tip catheter at 71 - 21 atrial and 60 - 16 ventricular sites in 10 healthy pigs and at 56 ventricular sites in one patient, and using an ordinary Navi-Star catheter at 30 atrial sites in one patient and 50 - 14 ventricular sites in four patients. In an additional 20 patients, MAPs were also recorded at 9 - 2 atrial sites using the modified-tip catheter or at 12 - 9 atrial sites using the ordinary catheter. Results : In pigs, the plateau amplitudes of the MAPs recorded using the modified-tip catheter were 4.1 - 3.2 mV for the atrial and 9.5 - 4.3 mV for the ventricular MAPs. In patients, both the ventricular and atrial MAPs recorded using the modified-tip catheter were significantly higher than using the ordinary catheters, 15.7 - 8 and 3.0 - 0.9 mV vs 9.5 - 3.9 and 2.0 - 0.6 mV for the ventricular and atrial MAPs, respectively ( p < 0.0001). The baseline disturbances were <10% of the MAP amplitude in 95% of the pig and 96% of the patient MAPs. Conclusion : A modified-tip Navi-Star catheter could be used in swine and in humans for prompt recording of MAPs with acceptable amplitudes and baselines. MAP mapping using the modified-tip catheter is safe and feasible for clinical use.

The Optimal Oesophageal Pacing Technique: The Importance of Body Position, Interelectrode Spacing, Electrode Surface Area, Pacing Waveform and Intra-oesophageal Local Anaesthesia

In order to improve the technique of transoesophageal atrial stimulation (TAS), the effects of body position, interelectrode spacing and electrode surface area on pacing threshold were assessed in two substudies. The effects of intra-oesophageal local anaesthesia and of two different pacing wave configurations on pacing threshold and discomfort were also assessed. Substudy I comprised 16 subjects (3 patients with a history of paroxysmal supraventricular tachycardia and 13 healthy volunteers) and substudy II comprised 16 healthy volunteers. TAS was performed using a hexapolar luminal prototype oesophageal electrode catheter. In substudy I bipolar pacing was performed in the semi-supine and left decubitus body positions for different pulse durations (20, 10, 6 and 2 ms), interelectrode pole distances (10 to 24 mm) and electrode pole surface areas (0.22 to 0.66 cm2). In substudy II TAS was performed with square wave and triangular waveform pulses after intra-oesophageal saline and lidocaine 20 mg/ml. These solutions were given in random order. Neither the interelectrode distance nor electrode surface areas had any significant influence on pacing thresholds. Stimulation thresholds were not affected by body position. Intraoesophageal lidocaine did not affect the discomfort experienced. Peak pacing thresholds using a triangular waveform were significantly higher than thresholds using a square waveformn (p < 0.001). The optimal pacing technique for TAS remains to be defined. The TAS-induced pain is probably not generated from the oesophageal mucous membrane. There is a significant difference in pacing thresholds between triangular and square waveforms.

Spectral analysis and time-dependent properties of atrial fibrillation in the surface ECG

A new non-invasive technique is presented for the characterization of time-dependent spectral properties of atrial fibrillation in the surface ECG. The method uses the Wigner-Ville distribution for time-frequency analysis and the cross Wigner-Ville is used to compute trends which describe the instantaneous frequency of the atrial activity. Preliminary results indicate that short-term variations exist in the fibrillation cycle lengths and that the variations can exhibit similar behavior in the leads V/sub 1/-V/sub 3/.

System identification of atrial activation during chronic atrial fibrillation in man.

Abstract To further clarify the mechanisms maintaining chronic atrial fibrillation (CAF). a method for system identification of activation patterns of the atria was developed. Data were obtained as repeated recordings of 8 s duration, each of 56 atrial bipolar electrograms, simultaneously acquired during open-heart surgery at multiple sites in the right atrial free wall in 12 patients with stable sinus rhythm (SR) and during artificial pacing and 16 patients with chronic atrial fibrillation (AF). Realization theory was applied to fit and reproduce the multivariate electrograms corresponding to individual beats during SR and type I activations during AF, respectively—in all cases and for all subjects tested with good accuracy. The local electrogram was a predictable signal in all cases tested and the proposed method may be used for further investigations concerning the deterministic and predictive nature of atrial activation during atrial fibrillation.

System identification of atrial activation during chronic atrial fibrillation in man

To further clarify the mechanisms maintaining chronic atrial fibrillation (CAF), a method for system identification of activation patterns of the atria was developed. Data were obtained as repeated recordings of 8 s duration, each of 56 atrial bipolar electrograms, simultaneously acquired during open-heart surgery at multiple sites in the right atrial free wall in 12 patients with stable sinus rhythm (SR) and during artificial pacing and 16 patients with chronic atrial fibrillation (AF). Realization theory was applied to fit and reproduce the multivariate electrograms corresponding to individual beats during SR and type I activations during AF, respectively-in all cases and for all subjects tested with good accuracy. The local electrogram was a predictable signal in all cases tested and the proposed method may be used for further investigations concerning the deterministic and predictive nature of atrial activation during atrial fibrillation.