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At least 595 records · Page 33Linked to original sources
PubMed2000

Optimization of transvenous coil position for active can defibrillation thresholds.

INTRODUCTION: Lead systems that include an active pectoral pulse generator are now standard for initial defibrillator implantations. However, the optimal transvenous lead system and coil location for such active can configurations are unknown. The purpose of this study was to evaluate the benefit and optimal position of a superior vena cava (SVC) coil on defibrillation thresholds with an active left pectoral pulse generator and right ventricular coil. METHODS AND RESULTS: This prospective, randomized study was performed on 27 patients. Each subject was evaluated with three lead configurations, with the order of testing randomized. Biphasic shocks were delivered between the right ventricular coil and an active can alone (unipolar), or an active can in common with the proximal coil positioned either at the right atrial/SVC junction (low SVC) or in the left subclavian vein (high SVC). Stored energies at defibrillation threshold were higher for the single-coil, unipolar configuration (11.2 +/- 6.6 J) than for the high (8.9 +/- 4.2 J) or low (8.5 +/- 4.2 J) SVC configurations (P < 0.01). Moreover, 96% of subjects had low (< or = 15 J) thresholds with the SVC coil in either position compared with 81% for the single-coil configuration. Shock impedance (P < 0.001) was increased with the unipolar configuration, whereas peak current was reduced (P < 0.001). CONCLUSION: The addition of a proximal transvenous coil to an active can unipolar lead configuration reduces defibrillation energy requirements. The position of this coil has no significant effect on defibrillation thresholds.

Aged↗
PubMed1979

The effects of stimulation of ear acupuncture points on the body's pain threshold.

Six ear acupuncture points, one non-acupuncture ear point, and the body locus Ho-Ku (LI-4) were electrically stimulated in order to compare the effects of stimulation on the body's pain threshold at selected loci on various points on the body by measurement with a radiation heat-type Pain Meter on 5 subjects. The ear points, with the exception of the non-acupuncture ear point, were found to be effective even in peripheral body regions in varying degrees. Ear stimulation did not increase the threshold as rapidly as Ho-Ku. In all cases where the pain threshold was raised, the effect persisted after electrical stimulation had stopped.

Acupuncture Therapy↗
PubMed2005

Measurement of the CO2 apneic threshold in newborn infants: possible relevance for periodic breathing and apnea.

We measured the PCO2 apneic threshold in preterm and term infants. We hypothesized that, compared with adult subjects, the PCO2 apneic threshold in neonates is very close to the eupneic PCO2, likely facilitating the appearance of periodic breathing and apnea. In contrast with adults, who need to be artificially hyperventilated to switch from regular to periodic breathing, neonates do this spontaneously. We therefore measured the apneic threshold as the average alveolar PCO2 (PaCO2) of the last three breaths of regular breathing preceding the first apnea of an epoch of periodic breathing. We also measured the PaCO2 of the first three breaths of regular breathing after the last apnea of the same periodic breathing epoch. In preterm infants, eupneic PaCO2 was 38.6 +/- 1.4 Torr, the preperiodic PaCO2 apneic threshold was 37.3 +/- 1.4 Torr, and the postperiodic PaCO2 was 37.2 +/- 1.4 Torr. In term infants, the eupneic PaCO2 was 39.7 +/- 1.1 Torr, the preperiodic PaCO2 apneic threshold was 38.7 +/- 1.0 Torr, and the postperiodic value was 37.9 +/- 1.2 Torr. This means that the PaCO2 apneic thresholds were 1.3 +/- 0.1 and 1.0 +/- 0.2 Torr below eupneic PaCO2 in preterm and term infants, respectively. The transition from eupneic PaCO2 to PaCO2 apneic threshold preceding periodic breathing was accompanied by a minor and nonsignificant increase in ventilation, primarily related to a slight increase in frequency. The findings suggest that neonates breathe very close to their PCO2 apneic threshold, the overall average eupneic PCO2 being only 1.15 +/- 0.2 Torr (0.95-1.79, 95% confidence interval) above the apneic threshold. This value is much lower than that reported for adult subjects (3.5 +/- 0.4 Torr). We speculate that this closeness of eupneic and apneic PCO2 thresholds confers great vulnerability to the respiratory control system in neonates, because minor oscillations in breathing may bring eupneic PCO2 below threshold, causing apnea.

Apnea↗
PubMed1984

The scotopic threshold of the human retina during static physical effort.

The scotopic threshold of 15 subjects was measured with a 1 degree circular white stimulus light placed at 15 degrees below the fixation level during 45 min at rest followed by 20 min constant pedalling on an ergometric bicycle with the head stationary. Loads were adjusted individually so that the effort extended by each subject required 60-75% of his maximum oxygen uptake. Similar results were obtained under both resting and effort conditions, indicating that the dark adaptation threshold remained unaffected by vigorous static effort.

Adolescent↗
PubMed1976

Proximity factor in color-difference evaluations.

The effect of dividing-strip width (sample proximity) upon sensitivity to suprathreshold and threshold color differences is explored by three methods that involve ratio comparisons, liminal determinations, and repeated color matchings. The results suggest, in general, that sample separation impairs lightness discrimination more than chromaticness discrimination and that it may be necessary to introduce a proximity factor into color-difference formulas when evaluating threshold or small-size color differences.

Adult↗
PubMed1975

Animal psychophysics: improvements in the tracking method.

It is difficult to maintain stimulus control in animal psychophysical studies using the tracking method. Loss of stimulus control is characterized by wandering thresholds and responses in the absence of the stimulus. Rats were trained to make a variable number of licking responses to turn on an auditory stimulus. A response on a lever in the presence of the stimulus was reinforced with food. Two precedures were added to improve stimulus control. First, lever responses in the absence of the stimulus raised the intensity of the stimulus at the next presentation; second, rate of reinforcement in the threshold region was maintained at about the same level as that in the suprathreshold region by reducing the number of licking responses required to turn on the stimulus. Using these two procedures, stimulus control was improved and maintained and reliable auditory intensity thresholds were obtained.

Acoustic Stimulation↗
PubMed1987

Sympathetic modulation of the pressure-dependent renin release in conscious dogs.

The relationship between mean renal artery pressure and renal venous-arterial plasma renin activity-difference (renin stimulus-response curve; RSRC) was studied in 15 conscious dogs by a stepwise reduction of renal artery pressure down to the lower limit of renal blood flow (RBF)-autoregulation. The RSRC has a flat section above threshold and a steep slope - indicating a 100% increase of renin release per 2.5 mmHg - below a well defined threshold pressure (Pth). Pth may remain unchanged for 4 weeks. A reflex activation of the renal sympathetic nerves by common carotid occlusion increased Pth by 16.5 +/- 3.2 mmHg (P less than 0.01); this effect was abolished by intrarenal alpha-blockade (prazosin). A low dose intrarenal infusion of methoxamine, which did not change RBF, increased Pth by 8.5 +/- 0.7 mmHg (P less than 0.001). We conclude that in the resting conscious dog renal perfusion pressure is a powerful factor in the control of renin release. The renal sympathetic nerves modulate the pressure-dependent mechanism within the autoregulatory range of renal blood flow by an alpha-adrenergic adjustment of threshold pressure.

Animals↗
PubMed1997

CD19 and CD22 expression reciprocally regulates tyrosine phosphorylation of Vav protein during B lymphocyte signaling.

B cell development and humoral immune responses are controlled by signaling thresholds established through the B lymphocyte antigen receptor (BCR) complex. BCR signaling thresholds are differentially regulated by the CD22 and CD19 cell surface receptors in vivo. B cells from CD22-deficient mice exhibit characteristics of chronic stimulation and are hyper-responsive to BCR crosslinking with augmented intracellular Ca2+ responses. By contrast, B cells from CD19-deficient mice are hypo-responsive to transmembrane signals. To identify signaling molecules involved in the positive and negative regulation of signaling thresholds, the signal transduction pathways activated after BCR crosslinking were examined in CD22- and CD19-deficient B cells. These comparisons revealed that tyrosine phosphorylation of Vav protein was uniquely augmented after BCR or CD19 crosslinking in CD22-deficient B cells, yet was modest and transient after BCR crosslinking in CD19-deficient B cells. Ligation of CD19 and CD22 in vivo is likely to positively and negatively regulate BCR signaling, respectively, because CD19 crosslinking was more efficient than BCR crosslinking at inducing Vav phosphorylation. However, simultaneous crosslinking of CD19 with the BCR resulted in a substantial decrease in Vav phosphorylation when CD22 was expressed. Thus, the differential regulation of Vav tyrosine phosphorylation by CD19 and CD22 may provide a molecular mechanism for adjusting BCR signaling thresholds.

Animals↗
PubMed2004

Effects of stimulation duration on electrogustometric thresholds.

The ability to detect low electrical currents presented to the lingual surface is widely used to assess taste function in humans. Despite this fact, the influence of stimulus duration on electrical taste thresholds is not well established. In this study, we evaluated the effects of current duration (0.5, 1.0, and 1.5 s) on electrogustometric detection thresholds for two regions of the anterior tongue in 24 college students. Anodal stimulation was produced using a stainless steel 12.5-mm(2) electrode, and thresholds were determined using a single-staircase procedure. A non-monotonic function for the threshold values was observed for both tongue regions across the stimulus durations, with the 1.0-s duration stimulus resulting in a lower threshold value (i.e., higher sensitivity) than either the 0.5- or 1.5-s durations, which did not differ in magnitude from one another. These data suggest that stimulus-duration-related enhancement of lingual detection threshold sensitivity to electric currents disappears at some point after 1 s.

Adolescent↗
PubMed1987

Different degrees of ischemic threshold stratified by the dipyridamole-echocardiography test.

Dipyridamole-echocardiography (echo) testing, exercise stress testing and coronary arteriography were performed in 141 patients with effort chest pain. Patients were separated into 5 groups according to the dose of dipyridamole needed to induce ischemia (0.56 mg/kg over 4 minutes vs 0.84 mg/kg over 10 minutes) and to the time of onset of the asynergy with the small dose (within vs beyond 3 minutes after the end of dipyridamole administration): group 1--early positive response to a small dose (33 patients); group 2--late positive response to a small dose (29 patients); group 3--negative response to a small dose, positive response to a large dose (17 patients); group 4a--negative response to both large and small doses, with significant coronary artery disease (CAD) (32 patients); and group 4b--negative response to small and large doses, without CAD (30 patients). All patients in groups 1, 2 and 3 had significant CAD. The rate-pressure product on exercise stress testing was measured at 0.10 mV of ST-segment shift in patients with a positive response and at peak exercise in patients with a negative response. Rate-pressure product significantly separated group 1 and group 2 from each other (157 +/- 46 and 229 +/- 33 mm Hg X beats/min X 1/100, respectively, mean +/- standard deviation) and from group 3, group 4a and group 4b (284 +/- 40, 290 +/- 51, and 298 +/- 45 mm Hg X beats/min X 1/100); values in the 3 latter groups overlapped.2+ Thus, the dipyridamole-echo test can stratify groups of patients with different levels of ischemia threshold on effort.

Adult↗