Evaluation of Self-Field Effects in Magnetometers Based on Meander-Shaped Arrays of Josephson Junctions or SQUIDs Connected in Series.

Arrays of superconducting quantum interference devices (SQUIDs) are highly sensitive magnetometers that can operate without a flux-locked loop, as opposed to single SQUID magnetometers. They have no source of ambiguity and benefit from a larger bandwidth. They can be used to measure absolute magnetic fields with a dynamic range scaling as the number of SQUIDs they contain. A very common arrangement for a series array of SQUIDs is with meanders as it uses the substrate area efficiently. As for most layouts with long arrays, this layout breaks the symmetry required for the elimination of adverse self-field effects. We investigate the scaling behavior of series arrays of SQUIDs, taking into account the self-field generated by the bias current flowing along the meander. We propose a design for the partial compensation of this self-field. In addition, we provide a comparison with the case of series arrays of long Josephson junctions, using the Fraunhofer pattern for applications in magnetometry. We find that compensation is required for arrays of the larger size and that, depending on the technology, arrays of long Josephson junctions may have better performance than arrays of SQUIDs.

Influence of flicker noise and nonlinearity on the frequency spectrum of spin torque nano-oscillators.

The correlation of phase fluctuations in any type of oscillator fundamentally defines its spectral shape. However, in nonlinear oscillators, such as spin torque nano-oscillators, the frequency spectrum can become particularly complex. This is specifically true when not only considering thermal but also colored 1/f flicker noise processes, which are crucial in the context of the oscillator's long term stability. In this study, we address the frequency spectrum of spin torque oscillators in the regime of large-amplitude steady oscillations experimentally and as well theoretically. We particularly take both thermal and flicker noise into account. We perform a series of measurements of the phase noise and the spectrum on spin torque vortex oscillators, notably varying the measurement time duration. Furthermore, we develop the modelling of thermal and flicker noise in Thiele equation based simulations. We also derive the complete phase variance in the framework of the nonlinear auto-oscillator theory and deduce the actual frequency spectrum. We investigate its dependence on the measurement time duration and compare with the experimental results. Long term stability is important in several of the recent applicative developments of spin torque oscillators. This study brings some insights on how to better address this issue.

Cardiac arrest diagnostic accuracy of 9-1-1 dispatchers: a prospective multi-center study.

INTRODUCTION: We sought to determine the ability of 9-1-1 dispatchers to accurately determine the presence of out-of-hospital cardiac arrest (OOHCA) over the telephone, and to determine the frequency with which CPR instructions are initiated and chest compressions delivered in patients not in cardiac arrest. METHODS: We conducted a multi-center, prospective cohort study of adult OOHCA patients not witnessed by EMS for which resuscitation was attempted. Dispatchers were not health care professionals and received 6 weeks of training followed by a 6-month preceptorship. We reviewed 9-1-1 call digital recordings for all unconscious patients for which the possibility of cardiac arrest was considered using a piloted standardized data collection sheet. RESULTS: We reviewed 2260 recordings occurring between January 2008 and October 2009. Among those, 1536 were confirmed OOHCA, and 724 were not. Among the 1536 confirmed OOHCA cases, 1012 were recognized by dispatchers and 524 were not. Among the 724 cases not in cardiac arrest, dispatchers suspected cardiac arrest was present in 490 and absent in 234. OOHCA diagnostic accuracy characteristics were: sensitivity 65.9% (95% CI 63.5-68.2%), specificity 32.3% (95% CI 29.0-35.9%), PPV 67.4%, and NPV 30.9%. Dispatchers believed that OOHCA was present in 490/2260 (21.7%) cases when it was not, resulting in 54/490 (11.0%) patients inappropriately receiving chest compressions, or 54/2260 (2.4%) of the whole cohort. CONCLUSIONS: Dispatchers had a fair sensitivity and modest specificity for the recognition of OOHCA. We found a very small number of patients receiving CPR when not in cardiac arrest, supporting the current use of dispatch-assisted CPR instructions.

Evaluating the effectiveness of dispatch-assisted cardiopulmonary resuscitation instructions.

OBJECTIVES: To determine the frequency of agonal breathing during cardiac arrest (CA), its impact on the ability of 9-1-1 dispatchers to identify CA, and the impact of dispatch-assisted cardiopulmonary resuscitation (CPR) instructions on bystander CPR rates. METHODS: A before-after observational study enrolling out-of-hospital adult CA patients where resuscitation was attempted in a single city with basic life support with defibrillation/advanced life support tiered emergency medical services. Victim, caller, and system characteristics were measured during two successive nine-month periods before (control group) and after (intervention group) the introduction of dispatch-assisted CPR instructions. RESULTS: There were 529 CAs between July 1, 2003, and December 31, 2004. Victim characteristics were similar in the control (n = 295) and intervention (n = 234) period; mean age was 68.3 years; 66.7% were male; 50.1% of CAs were witnessed; call-to-vehicle stop was 6 minutes, 37 seconds; ventricular fibrillation/ventricular tachycardia occurred in 29.9%; and the survival rate was 4.0%. Dispatchers identified 56.3% (95% confidence interval [CI] = 48.9% to 63.0%) of CA cases; agonal breathing was present in 37.0% (95% CI = 30.1% to 43.9%) of all CA cases and accounted for 50.0% (95% CI = 39.1% to 60.9%) of missed diagnoses. Callers provided ventilations in 17.2% and chest compressions in 8.3% of cases as a result of the intervention. Long time intervals were observed between call to diagnosis (2 minutes, 38 seconds) and during ventilation instructions (2 minutes, 5 seconds). Bystander CPR rates increased from 16.7% in the control phase to 26.4% in the intervention phase (absolute rate, 9.7%; 95% CI = 8.5% to 11.3%; p = 0.006). CONCLUSIONS: This trial demonstrates an increase in bystander CPR rate after the introduction of dispatch-assisted CPR. Agonal breathing occurred frequently and had a negative impact on the recognition of CA. There were long time intervals between call initiation and diagnosis of CA and during mouth-to-mouth ventilation instructions.