Biomedical subjects

H M Chun

Publications and source records attributed to H M Chun.

6 recordsLinked to original sources

PubMed2001

High-precision fringe tracking invariant to systematic model errors.

We present a new approach to processing of interferometric data, which dramatically improves operation on the smallest fraction of the fringe (approximately 1/1,000 of a wavelength and beyond). In particular, this approach, dubbed the variation-invariant subspace tracking approach (VISTA), makes it possible to operate a Michelson interferometer in a highly stable mode in which the estimation of optical path delay becomes invariant (insensitive) to a large class of systematic model errors. This previously unknown invariance property of interferometry follows from the fundamental odd-even symmetry of the channeled spectrum and its derivatives. VISTA offers a powerful algorithmic alternative for alleviating technological challenges in the design of high-precision long-baseline spaceborne interferometers.

Journal Article↗

PubMed1999

Effect of adenosine and verapamil in catecholamine-induced accelerated atrioventricular junctional rhythm: insights into the underlying mechanism.

Accelerated AV junctional rhythm is postulated to be due to enhanced automaticity of a high AV junctional focus. The adenosine response of this rhythm was tested in 17 patients (7 males, 12-83 years). The indications of electrophysiology study were nonspecific palpitation (n = 5), unexplained syncope (n = 6), postablation of accessory pathways (n = 4), and postmodification of AV nodal reentry tachycardia (n = 2). The sinus node and AV nodal functions were normal. Pacing and programmed electrical stimulation failed to induce any arrhythmia at baseline. The accelerated junctional rhythm (cycle length = 553 +/- 134 ms) was initiated spontaneously in all patients after isoproterenol infusion (1-2 micrograms/min). It was not suppressible by overdrive pacing. Cessation of isoproterenol infusion terminated the rhythm in all patients. Adenosine (6 mg) reproducibly terminated the accelerated junctional rhythm in all patients. In six patients, adenosine suppressed the junctional rhythm without producing AV nodal block. In the other 11 patients, the junctional rhythm was terminated prior to the occurrence of AV nodal block. Verapamil was tested in ten patients and 5 mg of intravenous verapamil terminated the junctional rhythm in all patients. In conclusion, the mechanism of catecholamine-induced accelerated AV junctional rhythm is most likely enhanced automaticity, and catecholamine-induced accelerated AV junctional automaticity is sensitive to adenosine and verapamil. Adenosine appears to have differential effects on catecholamine-enhanced AV junctional automaticity and AV nodal conduction. This suggests that, under catecholamine stimulation, adenosine may have different mechanisms of action on AV nodal conduction and automaticity.

Adenosine↗

PubMed1998

Multiple atrioventricular nodal pathways in humans: electrophysiologic demonstration and characterization.

INTRODUCTION: Multiple AV nodal pathway physiology can be demonstrated in certain patients with clinical AV reentrant tachycardia. METHODS AND RESULTS: Evidence suggesting multiple AV nodal pathway conduction was present in seven (two males; age range 15 to 75 years) of 78 patients (9%) who underwent electrophysiologic studies for AV nodal tachycardia. The presence of two discrete discontinuities in the AV nodal conduction curves suggested triple AV nodal pathway conduction. Detailed mapping of their retrograde atrial activation sequence was performed along the tricuspid annulus from the coronary sinus ostium to the His-bundle electrogram recording site. Three zones (anterior, middle, and posterior) correspond to the upper, middle, and lower third of the triangle of Koch, respectively. The fast pathway exits were determined as anterior (4/7) or middle (3/7), the intermediate pathway exits as middle (4/7) or posterior (3/7), and the slow pathway exits as middle (1/7) or posterior (6/7). Other evidence suggesting multiple AV nodal pathway conduction includes: (1) triple ventricular depolarizations from a single atrial impulse; (2) sequential dual ventricular echoes; (3) spontaneous transformation between the slow-fast and fast-slow forms of AV nodal reentrant tachycardia; and (4) persistent cycle length alternans during AV nodal reentrant tachycardia. In four patients, all three pathways were shown to be involved in AV nodal echoes or reentrant tachycardia. CONCLUSION: Multiple AV nodal pathways are not uncommon and can be identified by careful electrophysiologic elucidation and mapping technique.

Adolescent↗

PubMed1995

Supraventricular tachyarrhythmias. Pharmacologic versus nonpharmacologic approaches.

Nonpharmacologic approaches to the long-term management of SVTs have evolved rapidly and now offer to patients a safe, effective alternative for symptomatic relief from many SVTs. By far, radiofrequency catheter ablation, a technology less than 10 years old, offers the least invasive and most cost-effective nonpharmacologic alternative for many SVTs. Knowledge gained through electrophysiologic and ablation studies has enlarged the understanding of SVTs and may enable electrophysiologists to approach the more common and morbid condition of atrial fibrillation. From a societal standpoint, catheter ablation can remain a cost-effective mode of treatment if patient selection is stringent. The next 10 years should see further refinement in technique and in understanding of SVTs, improved technology, and enlarging applications of radiofrequency energy to cure or modify cardiac arrhythmias.

Anti-Arrhythmia Agents↗

PubMed1993

Determinants of the prognosis after a first myocardial infarction in a migrant Japanese population. The Honolulu Heart Program.

BACKGROUND: Although numerous studies have been published on the prognostic assessment of myocardial infarction, little is known about determinants of the prognosis after a first myocardial infarction, especially regarding the role of standard risk factors for coronary heart disease (CHD) measured before the development of myocardial infarction. METHODS AND RESULTS: In a prospective study of CHD among men of Japanese ancestry living in Hawaii, 457 patients with a first myocardial infarction (age range, 46 to 84 years) were identified during 20 years of follow-up. The relations of clinical variables and CHD risk factors to mortality in early (< 30 days) and two stages of late (30 days to 5 years and 5 to 10 years) periods after myocardial infarction in these patients were investigated. In multivariate analyses using logistic regression models (for early mortality) and Cox regression models (for late mortality), age at myocardial infarction and severe complications (Killip classes 3 and 4) were independent predictors of both early and late mortality (up to 5 years after myocardial infarction). In addition, ventricular arrhythmias predicted only early mortality, whereas anterior myocardial infarction, radiological evidence of cardiomegaly and/or pulmonary congestion, and intraventricular block predicted only late mortality (up to 5 years after myocardial infarction). Only age was an independent predictor of all-cause mortality more than 5 years after myocardial infarction. After adjusting for age at myocardial infarction and these clinical variables, preinfarction-measured risk factors such as 1-hour postload serum glucose (positively) and 1-second forced expiratory volume (inversely) were significantly associated with late mortality up to 5 years, whereas systolic blood pressure was the only independent predictor of late mortality after 5 years. CONCLUSIONS: This study has confirmed the importance of age at myocardial infarction and clinical indicators of complications such as Killip class 3 or 4, radiological evidence of pulmonary congestion, and ventricular arrhythmias or intraventricular block as the prognostic determinants of myocardial infarction. In addition, some of the preinfarction-measured standard risk factors for CHD were found to predict long-term prognosis independent of age and clinical factors.

Age Factors↗

PubMedDate not supplied

Autoimmune hemolytic anemia associated exclusively with IgA of Rh specificity.

A nine-year-old boy had typical clinical, hematologic and blood group serologic findings of autoimmune hemolytic anemia except for one important exception; with most commercially available broad spectrum anti-human sera the direct antiglobulin tests were negative. With reagents prepared in the laboratory which were found to be relatively less potent in and anti-IgG and IgM activity but which utilize a short period of incubation after mixing with the washed cells, the tests were all clearly positive. If incubation were employed with the commercial reagents, the tests were positive. Further studies showed that the patient's cells were coated exclusively with IgA and that a commercial reagent, although potent in anti-IgG and anti-IgM, was relatively deficient in anti-IgA. It is proposed that incubation compensates for the latter and is cautioned that incubation, with reagents potent in anti-IgG, could lead to false negative reactions with weakly IgG sensitized cells due to antibody surplus prozones of inhibition. This case illustrates that the rare instances of "Coombs negative immune hemolytic anemias" may be based on mechanisms such as those reported here and, that to be ideal, a broad spectrum anti-human serum should have balanced anti-immunoglobulin activities in relation to its incubation time.

Anemia, Hemolytic, Autoimmune↗