Magnetic Reconnection in Extreme Astrophysical Environments

Magnetic reconnection is a basic plasma process of dramatic rearrangement of magnetic topology, often leading to a violent release of magnetic energy. It is important in magnetic fusion and in space and solar physics --- areas that have so far provided the context for most of reconnection research. Importantly, these environments consist just of electrons and ions and the dissipated energy always stays with the plasma. In contrast, in this paper I introduce a new direction of research, motivated by several important problems in high-energy astrophysics --- reconnection in high energy density (HED) radiative plasmas, where radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. I identify the key processes distinguishing HED reconnection: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and Compton resistivity); and, at the most extreme end, QED effects, including pair creation. I then discuss the main astrophysical applications --- situations with magnetar-strength fields (exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares and magnetically-powered central engines and jets of GRBs. Here, magnetic energy density is so high that its dissipation heats the plasma to MeV temperatures. Electron-positron pairs are then copiously produced, making the reconnection layer highly collisional and dressing it in a thick pair coat that traps radiation. The pressure is dominated by radiation and pairs. Yet, radiation diffusion across the layer may be faster than the global Alfv\'en transit time; then, radiative cooling governs the thermodynamics and reconnection becomes a radiative transfer problem, greatly affected by the ultra-strong magnetic field. This overall picture is very different from our traditional picture of reconnection and thus represents a new frontier in reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic reconnection). Article is based on an invited review talk at the Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA; February 8-12, 2010). 30 pages, no figure

Menus for Feeding Black Holes

Black holes are the ultimate prisons of the Universe, regions of spacetime where the enormous gravity prohibits matter or even light to escape to infinity. Yet, matter falling toward the black holes may shine spectacularly, generating the strongest source of radiation. These sources provide us with astrophysical laboratories of extreme physical conditions that cannot be realized on Earth. This chapter offers a review of the basic menus for feeding matter onto black holes and discusses their observational implications.Comment: 27 pages. Accepted for publication in Space Science Reviews. Also to appear in hard cover in the Space Sciences Series of ISSI "The Physics of Accretion onto Black Holes" (Springer Publisher

Effects of captopril and enalapril on renal function in elderly patients with chronic heart failure

OBJECTIVE: To compare the effects on renal function of captopril and enalapril in elderly patients with chronic heart failure. DESIGN: A multi-centre double-blind parallel-group comparison of the two angiotensin-converting enzyme (ACE) inhibitors, captopril (12.5 mg bid) and enalapril (2.5 mg bid). SUBJECTS: 80 elderly patients with chronic heart failure (41 in the captopril group, 39 in the enalapril group). MAIN OUTCOME MEASURES: The blood pressure and pulse rate response to the first dose of ACE inhibitor was assessed in all patients. Glomerular filtration rate (GFR) was measured radioisotopically by 99mTcDTPA or 51CrEDTA clearance after three and six months of each treatment. Subgroups were assessed for effective renal plasma flow (33 patients), exercise tolerance (25 patients) and by a symptom-oriented questionnaire (45 patients). RESULTS: No serious adverse effect on GFR was noticed. There was no significant difference between the two treatments in the mean baseline GFR or in changes from baseline at three and six months (captopril mean baseline GFR 49.6 ml min-1 1.76 m-2, enalapril 54.7 ml min-1 1.76 m-2; mean change (95% confidence interval) at three months captopril 12 ml min-1 (+3.0, +21.0), enalapril -2 ml min-1 (-13.0; +9.0); mean change at six months, captopril 3.7 ml min-1 (-6.7; +14.2), enalapril -6.0 ml min-1 (-21.0; +9.4). Significantly more patients given captopril had an improvement in GFR during the study period (26/31 compared with 20/31 enalapril-treated patients at three months, p = 0.0096, and 23/30 compared with 15/27 at six months, p = 0.021). There were no significant changes in effective renal plasma flow. Three patients treated with enalapril developed symptomatic hypotension within three days of starting treatment. Quality of life questionnaires revealed more gastrointestinal symptoms in the enalapril group (p = 0.039). CONCLUSIONS: Captopril seems marginally preferable to enalapril in the treatment of chronic heart failure in elderly patients

Immunocompromised patients with acute respiratory distress syndrome : Secondary analysis of the LUNG SAFE database

The aim of this study was to describe data on epidemiology, ventilatory management, and outcome of acute respiratory distress syndrome (ARDS) in immunocompromised patients. Methods: We performed a post hoc analysis on the cohort of immunocompromised patients enrolled in the Large Observational Study to Understand the Global Impact of Severe Acute Respiratory Failure (LUNG SAFE) study. The LUNG SAFE study was an international, prospective study including hypoxemic patients in 459 ICUs from 50 countries across 5 continents. Results: Of 2813 patients with ARDS, 584 (20.8%) were immunocompromised, 38.9% of whom had an unspecified cause. Pneumonia, nonpulmonary sepsis, and noncardiogenic shock were their most common risk factors for ARDS. Hospital mortality was higher in immunocompromised than in immunocompetent patients (52.4% vs 36.2%; p < 0.0001), despite similar severity of ARDS. Decisions regarding limiting life-sustaining measures were significantly more frequent in immunocompromised patients (27.1% vs 18.6%; p < 0.0001). Use of noninvasive ventilation (NIV) as first-line treatment was higher in immunocompromised patients (20.9% vs 15.9%; p = 0.0048), and immunodeficiency remained independently associated with the use of NIV after adjustment for confounders. Forty-eight percent of the patients treated with NIV were intubated, and their mortality was not different from that of the patients invasively ventilated ab initio. Conclusions: Immunosuppression is frequent in patients with ARDS, and infections are the main risk factors for ARDS in these immunocompromised patients. Their management differs from that of immunocompetent patients, particularly the greater use of NIV as first-line ventilation strategy. Compared with immunocompetent subjects, they have higher mortality regardless of ARDS severity as well as a higher frequency of limitation of life-sustaining measures. Nonetheless, nearly half of these patients survive to hospital discharge. Trial registration: ClinicalTrials.gov, NCT02010073. Registered on 12 December 2013