Dehydration enthalpy of alkali-cations-exchanged montmorillonite from thermogravimetric analysis.

Dehydration of a series of homoionic alkali-exchanged montmorillonites is studied at different treatment temperatures by means of thermogravimetric analysis. More specifically, we investigate the last stages of dehydration when the number of adsorbed water molecules corresponds, at maximum, to a monolayer. Weight losses are measured at several constant temperatures as a function of time. Application of Van't Hoff's law yields the dehydration enthalpy. Trends and data similar to those reported from other experimental conditions are found. Comparison with X-ray data and with the dissociation enthalpy of alkali cation/water complexes shows that dehydration of weakly hydrated homoionic alkali montmorillonites results from the competition between opposite energy contributions due to (i) the cation solvation, (ii) the hydration of the silicate interlayer surface, and (iii) the structural swelling. So, depending on the balance between these various energy contributions, different behaviors are observed according to the nature of the alkali cations.

Hydration of a Na(+)-montmorillonite studied by thermally stimulated depolarization current.

Thermally stimulated depolarization current (TSDC) technique is a powerful tool for probing dipole re-orientational motions in condensed matter. In the case of cation-exchangeable aluminosilicates, it allows the assessment of the potential barrier related to the hopping mechanism of cations and, consequently, the measurement of its evolution when molecules, i.e. water, are adsorbed and interact with the cations embedded in the solid framework. Then, using suitable models based on thermodynamics, the analysis of TSDC signals obtained at various hydration states provides insights about the surface properties of the studied solid and the mechanism of adsorption at the cationic site. In this work, TSDC is used to study the first stage, i.e. when the number of adsorbed molecules is below the occurrence of the water monolayer, of water adsorption in a Na(+)-montmorillonite from Mostaganem (Algeria). It is shown that the hydration process follows two stages. Using the "chemical force" concept it can then be concluded that when the number of adsorbed water molecules per cation is lower than 2, cation-water interaction dominates the energetics of adsorption, whereas at higher water loading the water "chemical force" is also involved into water-water and/or water-clay framework interactions. The number of water molecules for the monohydrated state is found to be about 7.

Thermodynamic analysis of the immersion of a smectite substituted with Na or Ca: heat effect due to the cation.

The enthalpy produced during immersion in water of a smectite, identified as a montmorillonite, substituted by Ca or Na is analyzed using three different approaches. The heat of immersion is measured using the classical calorimetric method. It is compared to an estimate using adsorption techniques. And it is calculated from a theoretical estimate of the surface energy of a crystal model of montmorillonite. The comparison of the different values allows the enthalpy linked to the swelling phenomenon to be estimated. The Ca form appears in every case to interact more strongly with water molecules than the Na form.

Analysis of Thermally Stimulated Depolarization Currents (TSDC) Measured on Exchanged Clays.

This work shows that hydration of clays can be studied by means of a new interpretation of thermally stimulated depolarization currents technique. These measurements have been performed on four exchanged natural clays: Na-montmorillonite, Ca-montmorillonite, kaolinite, and illite. The results are analyzed using both the recently developed analysis of relaxation times distribution and the electronegativity equalization method. They provide evidence of the surface heterogeneity of clays. It is established that sites, identical from a crystallographic point of view, are different when the energy of the phenomenon is considered. The main interest of this work is to give for the first time a value of the hydration energy of cation onto clay surfaces. Copyright 2000 Academic Press.

[Alteration of sinus variability after cardiac surgery]. / Altération de la variabilité sinusale après chirurgie cardiaque.

UNLABELLED: Sinus tachycardia is frequent after cardiac surgery and this tachycardia is probably due to changes of the autonomic nervous system. The objective of this study was to evaluate the changes possibly induced by cardiac surgery, by studying sinus variability (SV) during a 24-hour Holter monitoring. The examination was performed in 28 patients who had undergone cardiac surgery 1 to 6 weeks previously. These patients had no alteration of left ventricular function, or any causes likely to modify SV and they had a normal postoperative course. Their results were compared to those of 4 subjects developing a postoperative complication (1 case of ventricular tachycardia and 3 cases of resuscitated cardiac arrest). The results were also compared to those of 24 age-matched adult controls without heart disease (control group). The study of SV included temporal and spectral analysis of SV with measurement of the standard deviation of normal RR intervals (SD), mean heart rate (HR), percentage of RR intervals differing by more than 50 m/sec from the adjacent interval (pNN50), coefficient of variability (CV) (SD/RR), square root of the differences between successive RR (rMSSD), spectral properties of low frequencies (LF) and high frequencies (HF) and the fractionated spectral property (LF/HF). RESULTS: (see tables, page 151 and 152). An alteration of SV was therefore observed in the surgical group, and lasted 4 to 6 months after surgery. No difference was observed between subjects without cardiac events and those presenting a cardiac event. IN CONCLUSION: cardiac surgery decreases all parameters of SV during the first few postoperative months. Certain unexpected cardiac accidents during this period could be explained by these changes.

Mechanism of QRS electrical alternans.

In patients with latent dual atrioventricular nodal pathways a 2:1 ventriculoatrial block often occurs during ventricular pacing and is generally associated with the concomitant appearance of QRS alternans. This type of QRS alternans is related to retrograde conduction, and a concealed retrograde conduction in the His Purkinje system could explain the QRS alternans. A case that confirms the hypothesis that electrical alternans is secondary to a 2:1 block in the activation of some part of the ventricles is reported.

Heterogeneous aortic response to acute beta-adrenergic blockade in Marfan syndrome.

Although prophylactic treatment with beta-blockers is used to retard aortic root dilatation in Marfan syndrome, it is not effective in all patients. To assess the effects of beta-adrenergic blockade on the aorta's elastic properties, aortic stiffness index and distensibility were calculated in 13 patients with Marfan syndrome and 10 control subjects before and after beta-adrenergic blockade. At baseline, patients with Marfan syndrome had a significantly increased stiffness index and decreased distensibility. After beta-adrenergic blockade, 8 patients with Marfan syndrome developed stiffness indexes and distensibility values that were closer to normal, whereas these variables deteriorated in 5 patients. Thus the benefit of beta-adrenergic blockade in Marfan syndrome may be the reduction in pulse pressure and myocardial contractility and also promotion of the elastic properties of the aorta. Moreover, the differential responses of aortic mechanics (normalizing or worsening) to beta-adrenergic blockade may possibly have implications for the prognosis in these patients.

[Development of indications in surgery of coronary revascularization. Current role of mammary-coronary anastomoses]. / Evolution des indications opératoires en chirurgie de revascularisation coronarienne. Place actuelle des anastomoses mammaro-coronariennes.

At a time when the cardio-surgical community has become aware that arterial revascularizations are superior to venous bypasses, GRUNTZIG (24) initiated the now well-known, and highly successful techniques of endoluminal angioplasty (P.T.C.A.) thus leading many teams to explore non-surgical revascularizations. These fast changing events somewhat overshadowed the interest placed in arterial anastomoses in general, and in particular, in mammary-coronary anastomoses. We believe that in 1993, many centres have not used this technique to its fullest and propose to the Académie de Chirurgie a programme of operative indications reinstating mammary-coronary anastomosis to its proper place in the treatment of coronary artery disease.

[Contribution of color echocardiography to the diagnosis of post-traumatic left ventricular-right atrial septal defect]. / Apport de l'échocardiographie couleur au diagnostic de communication ventricule gauche-oreillette droite post-traumatique.

The authors report the case of an acquired left ventricle--right atrial communication after closed chest trauma. This communication was associated with rupture of the aortic isthmus and complete atrioventricular block. The left-to-right shunt which was assumed for a long time to be a small, well-tolerated, ventricular septal defect, finally required surgical repair. The diagnosis was eventually made clear by Doppler color flow mapping which was of great value in orienting the surgical procedure.

[Cardiac rhabdomyoma in children and Bourneville's tuberous sclerosis]. / Rhabdomyome cardiaque de l'enfant et sclérose tubéreuse de Bourneville.

Cardiac rhabdomyomas are rare congenital tumours resulting from an early dysembryoplastic disorder of organogenesis. They are generally benign hamartomas which may be the first manifestation of a phakomatosis, tuberous sclerosis (TS) (Bourneville disease), present in over half of cases. The cases of 11 children with cardiac rhabdomyomas are reported. All of them also had extracardiac lesions of TS. Their ages ranged from 1 day to 6 years of age. In one case, the diagnosis was made antenatally by foetal echocardiography. The clinical expression may be very serious when there are hemodynamic disturbances resulting from an obstructive syndrome (N = 3) or arrhythmias (N = 2). Nevertheless, the cardiac evolution is usually favorable (N = 9) in contrast to the neuropsychiatric outcome which is generally catastrophic (N = 8). The diagnosis is made by two-dimensional echocardiography and magnetic resonance imaging. Surgical resection of the tumours is reserved for forms complicated by cardiac failure due to intracardiac obstruction (N = 2) of life-threatening resistant arrhythmias. Genetic counseling should be directed towards prevention of TS because it is transmitted in an autosomal dominant mode. However, most of the cases are sporadic. Echocardiography should be systematic in all children with TS and is also recommended for members of their families, even those apparently unaffected, in order not to miss paucisymptomatic forms of TS.

[Cardiac pheochromocytoma. Failure of classic non-invasive diagnostic methods]. / Phéochromocytome à localisation cardiaque. Echec des méthodes diagnostiques non invasives classiques.

We report an exceptional case of cardiac pheochromocytoma which raised problems of localization. A 30-year old man who for several years had been hypertensive was admitted for attacks of paroxysmal hypertension. Very high levels of urinary catecholamines suggested a diagnosis of pheochromocytoma, but no tumour was found at computerized tomography (CT) and metaiodobenzylguanidine (MIBG) scintigraphy. However, regional venous samplings detected two para-carotid phaeochromocytomas which were surgically removed. Thereafter, the symptoms persisted and investigations were resumed. As new regional venous samplings persisted and investigations were resumed. As new regional venous samplings showed high levels of catecholamines in the right atrium, a mediastinal and, chiefly, cardiac phaeochromocytomas was suspected. No tumour was visible at CT or ultrasonography and another MIBG scintigraphy proved negative. Coronary angiography showed a very large tumour behind the left atrium, well supplied by the circumflex artery and by a branch of the right coronary artery. The patient was operated upon and is now totally asymptomatic after a 9-month follow-up. This case emphasizes the value of invasive methods (i.e. regional venous sampling and coronary angiography) in the localization of this ectopic tumour. In most cases, however, phaeochromocytomas can be localized by MIBG scintigraphy.

Sustained improvement in myocardial perfusion four to six years after PTCA in patients with a satisfactory angiographic result, six months after the procedure.

Of 46 consecutive patients who underwent percutaneous transluminal coronary angioplasty (PTCA) from April 1980 to August 1982 and who had a satisfactory angiographic result six months later, one died of cancer and 41 had a detailed evaluation with exercise single-photon emission computed tomography (SPECT, 40 patients) or repeat coronary angiography (one patient), 48-78 months after the procedure. During follow-up, two patients had recurrent angina due to progression of coronary artery disease requiring a second PTCA procedure of another coronary artery; one of them later had a limited myocardial infarction in an area supplied by the vessel initially dilated. At follow-up, only one patient had definite angina. All but one patient had a negative exercise test. Myocardial perfusion during exercise in the PTCA-related area, assessed by SPECT, was normal in 90% of the patients and showed a limited defect due to reversible ischaemia in the remaining four (10%). It is concluded that patients with a less than 50% stenosis six months after PTCA show sustained improvement in their functional status and myocardial perfusion, four to six years after the procedure suggesting continued patency of the coronary artery.