Thermal History of CBb Chondrules and Cooling Rate Distributions of Ejecta Plumes.

It has been proposed that some meteorites, CB and CH chondrites, contain material formed as a result of a protoplanetary collision during accretion. Their melt droplets (chondrules) and FeNi metal are proposed to have formed by evaporation and condensation in the resulting impact plume. We observe that the skeletal olivine (SO) chondrules in CBb chondrites have a blebby texture and an enrichment in refractory elements not found in normal chondrules. Because the texture requires complete melting, their maximum liquidus temperature of 1928 K represents a minimum temperature for the putative plume. Dynamic crystallization experiments show that the SO texture can be created only by brief reheating episodes during crystallization, giving a partial dissolution of olivine. The ejecta plume formed in a smoothed particle hydrodynamics simulation served as the basis for 3D modeling with the adaptive mesh refinement code FLASH4.3. Tracer particles that move with the fluid cells are used to measure the in situ cooling rates. Their cooling rates are ~10,000 K hr-1 briefly at peak temperature and, in the densest regions of the plume, ~100 K hr-1 for 1400-1600 K. A small fraction of cells is seen to be heating at any one time, with heating spikes explained by the compression of parcels of gas in a heterogeneous patchy plume. These temperature fluctuations are comparable to those required in crystallization experiments. For the first time, we find an agreement between experiments and models that supports the plume model specifically for the formation of CBb chondrules.

Correlation of impedance minute ventilation with measured minute ventilation in a rate responsive pacemaker.

Although rate responsive pacing based on impedance minute ventilation (IMV) is now standard, there is almost no data confirming the relationship between IMV from an implanted pacemaker and measured minute ventilation (VE) during exercise. Nineteen completely paced adults implanted with Medtronic Kappa 400 pacemakers underwent symptom-limited maximal metabolic treadmill testing using a modified Minnesota Pacemaker Response Protocol. Minute ventilation (VE, L/min) was simultaneously measured using the flowmeter of a respiratory metabolic gas analysis system and the transthoracic impedance minute ventilation circuitry of the pacemaker. Correlation coefficients (r) were used to find the best fit line to describe the relationship between the two measurements. Mean (+/- SD) r values for the first, second, and third order polynomial equations and for log and exponential equations were: 0.92 +/- 0.08, 0.94 +/- 0.04, 0.95 +/- 0.04, 0.91 +/- 0.06, and 0.91 +/- 0.07, respectively. None of the r values were statistically different from the first order equation. Transthoracic IMV as measured by the Medtronic Kappa 400 is closely correlated to measured minute ventilation and is represented well by a first order (linear) equation.

R-wave detection by subcutaneous ECG. Possible use for analyzing R-R variability.

BACKGROUND: Atrial arrhythmia (AA) discrimination remains a technological challenge for implanted cardiac devices. We examined the feasibility of R-wave detection by a subcutaneous far field ECG (SFFECG) and analysis of these signals for R to R variability as an indicator of atrial arrhythmia (AA). METHODS: Surface ECG and SFFECG (from the pacemaker pocket) were recorded in sixteen patients (61.5 +/- 11.4 years) with AA. The SFFECG was recorded with a pacemaker sized four electrode array acutely placed in the pacemaker pocket during implantation. The signals were analyzed to obtain peak-to-peak R wave amplitude and R to R interval variability (indicative of AAs). RESULTS: In sixteen patients R waves were visually discernible in all recordings. The percentage over and under detection for automatic R wave recognition SFFECG was 3 and 9%, respectively. R to R variability analysis using the SFFECG produced results concordant to those using the surface ECG. CONCLUSION: SFFECG might be a helpful adjunct in implantable device systems for detection of R waves and may be used for measurement of R to R variability.

A comparative study of activity and dual sensor: activity and minute ventilation pacing responses to ascending and descending stairs.

Previous studies with activity-based rate adaptive pacemakers have shown a somewhat paradoxical response when comparing ascending stairs to descending stairs. The objective of this investigation was to measure dual-sensor rate response provided by activity and minute ventilation (MV) compared with activity alone, and with a control group, during ascending and descending stairs. For dual sensor mode, measured mean peak pacing rate with 72 (92) steps per minute was 111 +/- 13 beats/min (124 +/- 14 beats/min) ascending stairs and 81 +/- 7 beats/min (97 +/- 13 beats/min) for descending. For activity mode alone, mean peak pacing rate was 90 +/- 12 beats/min (108 +/- 19 beats/min) ascending stairs and 97 +/- 12 beats/min (123 +/- 17 beats/min) descending. The mean peak control group heart rate ascending stairs for a step rate of 72 (92) steps/min were 116 +/- 11 beats/min (127 +/- 14 beats/min) ascending stairs and for descending 89 +/- 12 beats/min (95 +/- 11 beats/min). While for dual sensor controlled pacing there was a significant difference for ascending and descending stairs at both step rates, there was no difference between going upstairs and downstairs for activity mode alone. Rates with dual sensor did not significantly differ from respective rates of the control group. The mean correlation coefficient between MV and paced rate was 0.85. Pacing heart rates delivered by the dual sensor mode were appropriate for ascending and descending stairs. In contrast to activity mode alone, the peak heart rates for dual sensor mode are higher during ascending than during descending stairs.

Electrogram signals recorded from acute and chronic pacemaker implantation sites in pacemaker patients.

Electrogram signals recorded from typical pacemaker implantation sites may be useful for a variety of pacemaker system functions including pacemaker follow-up, atrial and ventricular sensing (event detection), and triggered electrogram storage. We quantified the electrical characteristics of pacemaker pocket electrograms using a subcutaneous electrode array (SEA) in a population of 48 patients undergoing initial or replacement pacemaker implantation. SEA recorded intrinsic R wave amplitudes measured peak to peak averaged 118 microV and 65 microV for the two recorded SEA electrograms and were significantly different (P < 0.001); paced R wave amplitudes averaged 180 microV and 110 microV. P wave amplitudes averaged 39 microV and 26 microV. No statistically significant difference in amplitudes were observed between acute versus chronic pacemaker pocket or indication for pacing (AV block, sick sinus syndrome). Signal to noise ratios, using R wave amplitude as signal, were lower in the SEA electrogram on average (11 dB) compared to the intracardiac electrogram (27 dB), but sufficient for diagnostic assessment. R wave/P wave ratios for SEA signals were lower than surface and intracardiac values 3.1 and 2.7 compared to a range of 6.2-9.8, indicating a relative enhancement of P waves to R waves in SEA signals. In summary, SEA electrograms are of sufficient amplitude and signal quality (signal to noise ratio) to hold promise for future implantable device features such as electrogram telemetry, enhanced sensing, and diagnostic data storage.

Horseback riding injuries among children and young adults.

BACKGROUND: Horseback riding injuries are responsible for an estimated 2300 hospital admissions annually among persons younger than 25 years of age in the United States, but injury rates, patterns of injury, and risk factors for this population have not been well studied. METHODS: Study participants were selected from a list provided by a national mail-order company that sells riding equipment. A total of 557 persons younger than 25 years of age who rode horses at least six times a year completed survey questionnaires. RESULTS: Among the study participants, 34 (6.1%) had been hospitalized at least once because of a riding injury and 153 (27.5%) had been treated by a physician within the previous 2 years for such an injury. The overall injury rate was 0.6 per 1000 riding hours. Among those injured, sprains or strains (41.8%), lacerations or bruises (40.0%), and fractures or dislocations (33.3%) were the most common types of injury. A total of 27.5% of those injured sustained concussions or other head injuries. Riding 15 to 24 hours per month (odds ratio [OR] = 2.04), being female (OR = 1.81), and riding English style (OR = 1.77) were the characteristics most strongly correlated with injury. CONCLUSIONS: Horseback riding injuries among participants in this study tended to be serious. Family physicians should inform their patients who ride horses about the risks associated with equestrian activities and should emphasize helmet use.

Helmets and horseback riders.

To determine patterns of helmet use and attitudes toward helmets among horseback riders, we analyzed a subset of data (n = 1,834) from a cross-sectional mail survey of horseback riders conducted from July to December 1991. Riders were randomly selected from the mailing list of a national mail-order company that sells horseback-riding equipment. Of 900 English-style riders, 517 (57.5%) wore helmets on their last ride, compared with only 81 of 684 (11.8%) Western-style riders. Among 546 riders who did not own helmets, their most commonly cited reasons included believing that helmets were unnecessary (43.8%) or uncomfortable (29.9%). Of the 1,263 helmet owners, nearly 62% had one or more complaints about helmets, but 41.1% of owners reported that their helmet had prevented at least one head injury. Increasing helmet use will require major educational efforts with horseback riders and design changes by manufacturers.

Injuries in Equestrian Sports.

In brief The authors surveyed equestrians to determine an injury profile. Based on responses from 2,195 frequent riders, the study confirms previous findings regarding injury rates, injury sites, and helmet use. New findings include a large number of neck and back injuries and a likelihood of injured riders to be between 15 and 44 years of age, ride English style, and have less than 10 years of riding experience. Data also show that many horseback-riding injuries are treated in physicians' offices. The high percentage of injured patients who suffered prolonged disability underscores the need for physicians to counsel horseback-riding patients about hazards and safety measures-especially helmet use.

Strategies of a successful campaign to promote the use of equestrian helmets.

Although the risk of serious head injury for horse riders is higher than for most other sports, few equestrians regularly wear protective headgear. This study indicates that riders are well informed about the need for helmets and that the main reason for nonuse is inadequate helmet design. In particular, riders perceive that existing helmets are uncomfortable, expensive, and inappropriate for some riding styles. Based on these findings, the authors developed strategies to increase usage and incorporated them into a successful program. These strategies included working with manufacturers to develop a low-cost, versatile helmet; efficiently distributing educational literature among the horse riding community; and encouraging individual clubs and equestrian organizations to mandate a helmet policy.

Conformational studies of aqueous melittin. Characteristics of a fluorescent probe binding site.

The structural basis of the interaction of melittin with amphipathic molecular assemblies, i.e. membranes, was investigated by studying the binding of 2-p-toluidinylnaphthalene-6-sulfonate (TNS) to melittin by ultraviolet and fluorescence spectroscopy. Monomeric melittin did not significantly bind TNS as judged by UV and fluorescent spectroscopy. Tetrameric melittin bound two TNS molecules per protomer with dissociation constants (Kd) of 4.2 X 10(-6) M. TNS binding to tetrameric melittin led to an increase in fluorescence quantum yield of 180-fold over the value for TNS alone in aqueous buffer (phi H2O = 0.004). Five independent experimental findings suggest that the arginine residues of melittin provide one portion of the TNS binding site (presumably by formation of an especially stable "ringed-structure" salt bridge between the tetrahedral sulfonyl anion of TNS and the argininyl residues of melittin): 1) the Kd for binding is independent of pH from 6.0 to 10.8, the range in which the alpha-aminoglycine and epsilon-aminolysines titrate; 2) TNS binding fails to perturb the kinetics of the reaction of 2,4,6-trinitrobenzenesulfonate with Lys-21 or Lys-23; 3) 1,7,21,23-acetyl-melittin, in which the NH2 terminus and all lysines are acetylated, binds TNS with a Kd similar to that for normal melittin; 4) guanidination of the NH2-terminal glycines and lysines of melittin (forming N-guanidoglycine and homoargininyl residues, respectively) increases the number of TNS molecules bound per protomer to approximately 5; 5) conversion of Arg-22 and Arg-24 to the anionic N7, N8-(2,3-dihydro(7,7-dimethyl))bicyclo[2.2.1] heptane - 1 - methanesulfonylborate complex abolishes TNS binding, as judged by fluorescence.

Conformational studies of aqueous melittin: thermodynamic parameters of the monomer-tetramer self-association reaction.

The self-association reaction in which four melittin molecules associate to form an aqueously soluble tetramer was studied by fluorescent spectroscopy. At 23 degrees C, pH 7.15, gamma/2 0.50, the dissociation constant, Kd, is 3.20 x 10(-16) M3. At 23 degrees C, gamma/2 0.60, melittin has an amino acyl group with a proton ionization constant at ca. 10(-6) M, which must be un-ionized for tetramer formation to occur. The change in Kd with temperature indicates the forward reaction (tetramer formation) proceeds primarily by entropic changes, with delta H degrees = -20.3 kJ/mol of monomer and delta S degrees = 211 J/(K . mol of monomer). The observed enthalpic and entropic values for the tetramerization reaction are consistent with the expected contributions of both nascent hydrogen bonds and hydrophobic stabilization to the reaction. The ionic strength dependence of the tetramerization reaction was found to be consistent with an Edsall-Wyman treatment of activity coefficients. Specifically, the calculated charge of melittin varied from 2.5 (pH 10.53, gamma/2 less than 0.08) to ca. 6 (pH 7.15, gamma/2 greater than 0.3) and showed a strong dependence on gamma/2.