First-principles study of silicon nanocrystals: structural and electronic properties, absorption, emission, and doping.

Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nanostructures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si--O--Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced, favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

Study of the efficiency of the head-up tilt test with nitroglycerin challenge in the diagnosis of vasovagal syncope.

UNLABELLED: Syncope is defined as a transient, self-limited loss of consciousness. It is an important cause of morbidity in general population and the vasovagal syncope (VVS) is a common clinical problem which often leads to hospital admission, multiple office visits, and performing of many diagnostic tests. Head-up tilt table testing (HUTT) is a widely used diagnostic tool, with proved efficiency in diagnosing the many types of VVS. The aim of our study was to assess the efficacy of the HUTT with sublingual nitroglycerin (NTG) challenge for the diagnosis of vasovagal syncope. PATIENTS AND METHODS: The study groups consisted of 72 patients (37 women, 35 men), mean age (+/-SD) 38.5 +/- 15.7 years, referred to our clinic for syncope of unknown origin and 16 healthy volunteers as control subjects (9 women and 7 men) with a mean age 26 +/- 6.5 years and no history of syncope. The HUTT protocol was performed in the morning after an overnight fast, with a 15 minutes supine equilibration phase followed by a 30 minutes drug free tilt phase at an angle of 70 degrees. If no positive response was recorded, 400 micrograms of NTG spray were given sublingually and the HUTT continued for 20 minutes. RESULTS: The HUTT was positive in 58 patients (80.5%) and negative in 14 (19.5%) while in the control subjects 4 (25%) had positive HUTT response. The sensitivity of the HUTT protocol with nitroglycerin challenge used in our study was 81% and the specificity was 75% (p<0.0001). CONCLUSION: The described HUTT protocol appears to be a simple and efficient tool for the diagnosis of syncope associated with normal ECG and no signs of organic heart disease. The sublingual nitroglycerin challenge was safe to use and showed no side effects while keeping the sensitivity and specificity of the test to an accepted level.

The use of light reflection rheography in diagnosing venous disease and arterial microcirculation.

The use of light reflection rheography (LRR) has reached an important level in the past few years. Using the originally construed and conceived RRL 001 device, we present the basic principles, the working technique and the limits of this method. We will use two magnetic amplifiers, one for the finger tip, which measures the capillary blood flow at this level and a venous magnetic amplifier which measures the blood flow in the venous plexuses at a 1-2 mm depth. The detection of the blood flow in the subcutaneous (hypodermic) venous areas leads to the diagnosis of profound thrombophlebitis and venous valves insufficiency. The detection of the capillary flow in the finger tip, its presentation as a curve, measuring the pressure of the inter digital arteries (using Riva Rocci method), establishing the index of systolic pressure make possible the understanding of the gravity of the existing hemodynamic disorders (especially in what concerns the Raynaud syndromes), the supervision of the original disease and the efficiency of the treatment. Weindorf was the first one to use the RRL technique in medicine in 1985 and, after 1997, it has been developed through Sproule's work. Within The Medical Clinic of the Municipal Hospital, a light reflection rheograph has been built and forwarded as innovation. The device has been used for 2 years in the diagnosis of venous disorders and peripheral microcirculation disorders. In this article we present the principles of the LRR examination of the lower limb venous system and of the microcirculation of the fingers of the upper limbs, an examination based upon our own experience which includes 72 examinations.

The efficacy of sublingual verapamil in controlling rapid ventricular rate in chronic atrial fibrillation.

The efficacy of sublingual verapamil (Verapabene, 40 mg) in acute control of high ventricular rate was assessed in 20 patients with chronic atrial fibrillation (AF) of ischemic origin (class NYHA II and III). The effect on irregularity of the rhythm was also studied using time domain parameters of heart rate variability. Four ECG recordings (before, and at 10, 30 and 60 minutes after the administration), of 10 minutes each, were performed in every patient in basal conditions. The parameters were compared using paired t-test and Wilcoxon's test. Mean heart rate became significantly lower (p = 0.0064) after 10 minutes, and reached progressively the lowest value after 60 minutes (from the initial 112.9 beats/min to 90.6 beats/min, after 60 minutes, p < 0.0001). The irregularity parameters (SDNN--from 114.7 ms to 148.6 ms, rMSSD--from 152 ms to 205.9 ms) and the NNmin interval (from 353 ms to 404.5 ms) increased significantly after 30 and 60 minutes (p < 0.0001), but the frequency corrected irregularity (coefficient of variability) remained unchanged (p > 0.4). Sublingually administered verapamil proved to be an efficient alternative in acute control of rapid ventricular rate in AF. The method has the advantage to be applicable by a properly instructed patient itself.

Computerized system for assessing heart rate variability.

The principal theoretical, methodological and clinical aspects of heart rate variability (HRV) analysis are reviewed. This method has been developed over the last 10 years as a useful noninvasive method of measuring the activity of the autonomic nervous system. The main components and the functioning of the computerized rhythm-analyzer system developed by our team are presented. The system is able to perform short-term (maximum 20 minutes) time domain HRV analysis and statistical analysis of the ventricular rate in any rhythm, particularly in atrial fibrillation. The performances of our system are demonstrated by using the graphics (RR histograms, delta RR histograms, RR scattergrams) and the statistical parameters resulted from the processing of three ECG recordings. These recordings are obtained from a normal subject, from a patient with advanced heart failure, and from a patient with atrial fibrillation.

The prognostic value of late ventricular potentials for sudden death.

Sixty patients of whom 51 with old myocardial infarction--30 with late ventricular potentials and 30 without--were followed up for 5 years. The late ventricular potentials were recorded using an original method. During the follow-up period, in the group, with late ventricular potentials, sudden death was recorded in 6 patients and episodes of ventricular tachycardia in 2. No sudden death occurred in the patients without late ventricular potentials. The prognostic value of this simple, noninvasive technique is emphasized.

Intracardiac electrical discharge in terminating atrial fibrillation.

A new intracavitary technique for internal defibrillation in atrial fibrillation is presented. The discharge takes place between the distal electrodes of the catheter placed in the right atrium and a thoracic paddle. Four cases are discussed, and the indications for this type of defibrillation are considered. Internal defibrillation can be done when, for other purposes, an electrode catheter is already placed in the right atrium.

Preatrial activity recorded by intracavitary and transthoracic techniques.

Two methods to record preatrial activity, that is, sinus node potential are presented. One, during right atrial catheterization through an electrode catheter, and the other, using transthoracic recordings with a high amplification and derivation of the electric signals. The sinus node potential duration represents the sino-atrial conduction time (SACT), a useful parameter for the sinus node function evaluation.

Chest-wall and transesophageal stimulation in the functional evaluation of patients with permanent pacemakers.

For the functional estimation of artificial pacemakers, we have used two techniques: chest-wall stimulation and transesophageal stimulation. Transesophageal stimulation was always effective in inhibiting the generator, and it was used when chest-wall stimulation was ineffective. These two tests can be easily performed and are of remarkable usefulness in the follow-up of patients with permanent pacemaker.

A simple bedside technique in the study of the WPW syndrome.

A simple method for the study of the WPW syndrome, which can be performed at the patient's bedside, is presented. Using standard ECG, vagal maneouvers, ajmaline test, precordial mapping, vectorcardiography, electrode catheters positioned in the right atrium and esophageal catheters, the site of accessory pathways, and the complex arrhythmias occurring in these patients can be understood and medically treated.

Antiarrhythmic effects of verapamil given in sublingual way.

In order to eliminate the first pass through the liver where the drug is deactivated, 80 mgs verapamil was given in sublingual way in a) 10 patients with supraventricular tachycardias where in 7 instances sinus rhythm was obtained after 15-30 minutes, b) in 18 patients with atrial fibrillation and in 5 with atrial flutter with rapid ventricular rate, where a significant slowing down of ventricular rate was obtained after 20 minutes. The sublingual way of verapamil administration is a simple, effective and safe procedure in the treatment of tachyarrhythmias.

Three patients with branch to branch reentrant ventricular tachycardia. Diagnostic and therapeutic considerations.

Three patients with repetitive episodes of relatively slow rate (170 beats per minute) ventricular tachycardia, with right bundle branch block (RBBB) QRS morphology during arrhythmia are presented. The clinical and electrophysiological characteristics and the therapeutic problems raised are analysed. The branch to branch reentry mechanism was considered as the underlying mechanism. The peculiar characteristics found in our cases are: almost normal heart, except right bundle branch block in two and left anterior hemiblock in one patient; relatively slow ventricular rate during tachycardia; delayed retrograde ventriculo-atrial conduction during tachycardia; the onset of tachycardia related to atrial or ventricular premature beats; the possibility to stop the tachycardia through ventricular pacing; therapeutical point of view, DC shock, ventricular pacing and iv mexiletin HCl were highly effective, but not iv lidocaine.

Sinus node dysfunction occurring immediately after pericardiocentesis.

Performing in two cases three instances of pericardiocentesis, there always occurred sinus node dysfunction. Our explanation is of a mechanical involvement of the sinus node after the complete evacuation of the pericardial fluid.

Atrio-ventricular conduction disturbances occurring after strenuous physical effort.

Two young patients with a-v conduction disturbances occurring in relation with strenuous physical effort are presented. The possible mechanism may be an excessive vagal tone.

Experimental validation of sino-atrial conduction time using sinus node potential recording, right and left atrial continuous pacing method.

In an experimental study indirect sino-atrial conduction time (SACT) was measured, using continuous pacing method from right and left atrial sites, and compared with direct SACT obtained from sinus node potential recording, all the determination being done before and after autonomic blockade. The distance between the two sites of stimulation may be taken into account as a correction factor for SACT determination. The SACT obtained with sinus node potential technique is the same as the indirect SACT after autonomic blockade. These findings are valuable for SACT determination with transesophageal pacing method. Left atrial pacing, thus transesophageal pacing, is a valuable method in estimating the SACT, but a time correction and autonomic blockade is required in order to obtain a real value, eliminating the extrinsic influences.

Transesophageal pacing technique in the estimation of sinus node function.

A personal noninvasive transesophageal pacing technique for the estimation of sinus node function is presented. With a pacemaker device and an esophageal pentapolar catheter, using a correction factor for the left atrial site of stimulation, it is possible to calculate the sinus node recovery time, sinoatrial conduction time and sinus node effective refractory period, before and after autonomic blockade as accurately as by the intracavitary technique.