SEARCH · PubMed Health

Results for “Power”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 829 records · Page 46Linked to original sources

PubMed1991

Quantification of EEG irregularity by use of the entropy of the power spectrum.

A new method for quantifying irregularity of EEGs is proposed in this study. The entropy, an information measure, determines the uniformity of proportion distribution. The peakedness or flatness of the distribution of the EEG power spectrum, representing EEG rhythmicity, can be measured by the entropy, because the power spectrum consists of proportions of power at each frequency. The irregularity of the EEG was measured by the entropy of the power spectrum, called an irregularity index (II). The II was obtained from the power spectrum at F3, F4, C3, C4, P3, P4, O1 and O2 during rest and mental arithmetic in 10 normal subjects. Relative band powers of delta, theta, alpha and beta bands and alpha peak frequency were also obtained. EEGs during rest were significantly more irregular anteriorly than in the occipital areas. Alpha activity was also more irregular in the anterior region. A greater degree of EEG desynchronization during mental arithmetic was found over the left hemisphere and the right occipital area. The II was more sensitive to such desynchronization than alpha band power and alpha peak frequency. The differences in spectral structures between rest and mental arithmetic conditions, mainly over the left hemisphere, were also confirmed by the Kullback-Leibler information.

Adolescent↗

PubMed1996

Genetic architecture of EEG power spectra in early life.

We measured the electroencephalogram (EEG) in 209 5 year old monozygotic (MZ) and dizygotic (DZ) twin pairs to estimate the relative contribution of genetic and environmental factors to EEG power spectra in early life. Data from same-sex and from opposite-sex twin pairs were used to test for sex differences in genetic influences. Results showed high concordance for EEGs of MZ twins for absolute and relative power in delta, theta, alpha 1, alpha 2, beta 1 and beta 2 bands. A model with additive genetic and unique environmental influences explained individual differences in both absolute and relative power in almost all bands and all electrode positions. Heritability of EEG power spectra was high. For absolute power the highest heritabilities were observed in theta, alpha 1, alpha 2 and beta 1 power bands (mean heritability 81, 81, 78, and 73%, respectively). Somewhat lower heritabilities were found in delta and beta 2 bands (mean heritability 55 and 64%, respectively). For relative power heritabilities were 63, 76, 71, 72, 68, and 65 for delta, theta, alpha 1, alpha 2, beta 1, and beta 2, respectively. Virtually no sex differences in heritability were found. These findings indicate that the background EEG is one of the most heritable characteristics in early life.

Brain↗

PubMed1988

The power of analysis: statistical perspectives. Part 1.

Failure to consider statistical power when achieving apparently "negative" results prevents accurate interpretation of the results. A nonsignificant result can be obtained when one includes an insufficient number of subjects to permit observation of a true effect (low power to detect an effect), or when one has an adequate number of subjects, but a meaningful effect does not exist (high power, no effect); one can also have a situation of lower power and no real effect. Without considering power, one is unable to distinguish a "negative" experiment from an inadequate one. This article examines 154 published nonsignificant t-test results. When power is calculated with an effect size equal to a standardized difference of unity, over 50% of the tests have inadequate power.

Clinical Trials as Topic↗

PubMed1992

Power spectrum analysis of heart rate fluctuations and respiratory movements associated with cooling the human skin.

A length of respiratory movement and instantaneous heart rate was divided into 32 segments to adopt computerized power spectrum analysis. An array of frequency spectrum across all the segments was then constructed to demonstrate sequential changes in the respiratory rhythm (0.4 Hz), low (less than 0.1 Hz) and high (0.2-0.4 Hz)-frequency components of heart rate fluctuations associated with lowering the skin temperature. A significant increase in the power for the low and high frequency components of heart rate fluctuations accompanying bradycardia occurred by cooling the face to 0 and 10 degrees C, while cooling the foot to 0 degrees C yielded a significant increase and decrease in the power for the former and latter component, respectively, without change in heart rate. The spectral power for respiratory movements was not altered by these stimuli. The maximal change in the power for low- and high-frequency components as well as in the heart rate during cooling the face and foot to 0 degrees C was found to correlate negatively with the relative resting magnitude of the respective parameter in the pre-stimulus period. Similar relationship existed only with the power for low-frequency component of heart rate fluctuations when lowering the temperature of foot to 10 degrees C. The power spectrum array employed in this study thus enabled us to estimate the effectiveness of skin cooling on the three hemodynamic parameters.

Adult↗

PubMed1990

Power and sample size calculations. A review and computer program.

Methods of sample size and power calculations are reviewed for the most common study designs. The sample size and power equations for these designs are shown to be special cases of two generic formulae for sample size and power calculations. A computer program is available that can be used for studies with dichotomous, continuous, or survival response measures. The alternative hypotheses of interest may be specified either in terms of differing response rates, means, or survival times, or in terms of relative risks or odds ratios. Studies with dichotomous or continuous outcomes may involve either a matched or independent study design. The program can determine the sample size needed to detect a specified alternative hypothesis with the required power, the power with which a specific alternative hypothesis can be detected with a given sample size, or the specific alternative hypotheses that can be detected with a given power and sample size. The program can generate help messages on request that facilitate the use of this software. It writes a log file of all calculated estimates and can produce an output file for plotting power curves. It is written in FORTRAN-77 and is in the public domain.

Case-Control Studies↗

PubMed1988

The distribution of power and heat produced by interstitial microwave antenna arrays: II. The role of antenna spacing and insertion depth.

The distribution of power and temperature generated by 915 MHz interstitial microwave antenna arrays was studied in static muscle-equivalent phantoms and both perfused and non-perfused canine thigh muscle. These arrays, which would form the geometric basis of larger volume implants, consisted of four parallel antennas oriented such that transverse to their long axes they formed the corners of a square. Arrays with 2 and 3 cm sides were compared at various depths of insertion where the nodes for all four antennas were coincident at the same depth. The position relative to the antenna nodes of the maximum power and highest temperature within the array volume varied with the depth of insertion of the antennas. Though power dropped rapidly distal to the nodes at all depths, a shift in the location of the maximum power proximal to the nodes resulted in an increase in the effective heating volume at certain insertion depths. For 2 cm array spacing the highest power and temperature were measured along the central axis of the array at all insertion depths. However, arrays using 3 cm spacing generated their maximum power adjacent to the antennas with only 50% of this level occurring along the central axis. When the temperature produced by 3 cm arrays was measured in phantoms midway through simulated 30-minute hyperthermia treatments, the effect of thermal conduction on the temperature distribution was evident. Though power was only 50% centrally, the highest temperatures occurred there. This same pattern of central heating occurred in perfused canine muscle demonstrating the importance of conductive and convective heat redistribution in reducing thermal gradients within the array volume.

Animals↗

PubMed1987

Effect of anticonvulsants on pentylenetetrazol-induced power spectrum changes in electroencephalograms in rats.

The effect of anticonvulsants on pentylenetetrazol (PTZ)-induced EEG power spectrum changes was examined. When the minimum dose of PTZ (15 mg/kg) was administered intravenously twice, with an interval of 80 min, a clear EEG power spectrum change was observed after the second PTZ administration, irrespective of whether or not the first PTZ administration evoked marked EEG changes. We defined the values F/N, S/N and S/F. The value F/N, obtained by dividing the power spectrum area after the first PTZ administration by the normal power spectrum area, was 1.06 +/- 0.05 (mean +/- S.E.). The value S/N, obtained by dividing the power spectrum area after the second PTZ administration by the normal power spectrum area, was 2.00 +/- 0.21. The value S/F, obtained by dividing S/N by F/N, was 1.86 +/- 0.16. The S/F value was almost constant regardless of whether or not the first PTZ administration could evoke marked EEG changes. The effect of anticonvulsants was examined by S/F value changes, and 100 mg/kg of PHT completely inhibited the double PTZ effect. Phenobarbital, ethosuximide and sodium valproate also inhibited the double PTZ effect. Using the S/F value of the EEG power spectrum with minimum dose double PTZ administration, quantitative evaluation is possible for anticonvulsant drugs.

Animals↗

PubMed1994

Tumour destruction and proliferation kinetics following periodic, low power light, haematoporphyrin oligomers mediated photodynamic therapy in the mouse tongue.

Photodynamic therapy (PDT) is an experimental modality in the treatment of cancer. It involves photochemical reactions that require the interaction of a photosensitising drug, light and oxygen. The development of an efficient protocol based on assuring oxygen availability through modulation of the incident light power density and its mode of delivery was addressed in this study. An estimated energy dose of 180 J/cm2 of 630 nm light from pulsed Nd:YAG dye laser was delivered 24 h after injection of 10 mg/kg haematoporphyrin oligomers in C3H/HeNCrj mice bearing the transplantable squamous cell carcinoma NR-S1, by either of these light regimens: (1) 5 mJ/cm2/pulse for 30 min, 1 h dark interval, followed by another 30 min exposure to the same power (low power, periodic light regimen) or (2) 15 mJ/cm2/pulse for 20 min (high power, continuous light regimen). Results showed a higher mean percentage area of tumour destruction with the low power, periodic light regimen at 54.34% in contrast to 12.44% of the high power, continuous light regimen 2 days after PDT. Furthermore, the mean bromodeoxyuridine labelling indices of the remaining viable-appearing cancer cells were 27.90 and 42.41, respectively, indicating a smaller tumour growth fraction with the former regimen. These results suggest that use of low power, periodically delivered light increases the antitumour efficacy of PDT.

Animals↗

PubMed2004

Investigation of a new heart contractility power parameter.

First derivative of arterial blood pressure, dp/dt is known to reflect the contractility power of the heart. We hypothesize that the calculated area under each cardiac cycle of the blood pressure curve is also another practical tool in revealing the heart contractility power. Of the 84 subjects, 61 patients were found to have adequate contractility power (high dp/dt) and their mean area calculation resulted in 64.2 mmHg s with a standard deviation of 2.9 mmHg s. The remaining 23 patients have indicated poor heart contractility power (low dp/dt) and stayed in serious condition for long time. This group had mean area of 41.4 +/- 3.1 mmHg s. Patients with poor contractility power had areas below 45 mmHg s, while all area calculations for patients with high contractility power stayed over 60 mmHg s. Therefore, small area of the arterial blood pressure curves seems to be a good indicator of a poor heart contractility power and the area calculation may be an adjunct parameter to the dp/dt that has been employed for the assessment of heart contractility.

Action Potentials↗

PubMed2006

The effects of innervation zone on electromyographic amplitude and mean power frequency during incremental cycle ergometry.

The purpose of this study was to examine the effects of electrode placements over the innervation zone (IZ), as well as proximal and distal to the IZ, on the patterns for the absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus power output relationships during incremental cycle ergometry. Fifteen men [mean +/- S.D. age = 24.3 +/- 2.4 years; VO2max = 47.3 +/- 4.9 ml kg(-1) min(-1)] performed incremental cycle ergometry tests to exhaustion. Surface EMG signals were recorded simultaneously from bipolar electrode arrangements placed on the vastus lateralis (VL) muscle over the IZ, as well as proximal and distal to the IZ. Polynomial regression analyses were used to describe the relationships for absolute and normalized EMG amplitude (microVrms and %max) and MPF (Hz and %max) versus power output (%max) for each subject at the three electrode placement sites. In addition, separate one-way repeated measures ANOVAs were used to examine mean differences between the three sites for absolute and normalized EMG amplitude and MPF at power outputs of 80, 110, 140, and 170 W. The results of the polynomial regression analyses revealed that the best fit model for each site for the absolute and normalized EMG amplitude versus power output relationship was linear for 11 subjects and quadratic for 2 subjects. The remaining two subjects exhibited both linear and quadratic patterns that were site-dependent. For EMG MPF, 10 subjects exhibited significant relationships (linear and/or quadratic) across power outputs for at least one site. In addition, there were significant (P < 0.05) mean differences between the electrode placement sites for absolute EMG amplitude, but not absolute EMG MPF at 80, 110, 140, and 170 W. There were no significant (P > 0.05) mean differences, however, between the three sites for normalized EMG amplitude or MPF at 80, 110, 140, and 170 W. These findings indicated that the placement of bipolar electrodes over the IZ, as well as proximal and distal to the IZ, had no effect on the pattern of the normalized EMG amplitude versus power output relationship or the mean normalized EMG amplitude and MPF values. Thus, during cycle ergometry, normalized EMG amplitude values (but not absolute values) can be compared between studies that have utilized various electrode placement sites on the VL.

Action Potentials↗

PubMed2006

Conjugate eye movements and gamma power modulation of the EEG in persistent vegetative state.

BACKGROUND: Power in the gamma band EEG increases during saccades in normal subjects. OBJECTIVE: To develop a potential method to quantify signs of cortical responsiveness in persistent vegetative state (PVS) we quantified gamma range EEG in association with conjugate slow ballistic eye movements (SBEM). METHODS: The EEG and the simultaneous electro-oculogram were recorded in 14 (8F/6M) PVS patients. Clinical scoring was based on the Glasgow Coma Scale (GCS) and Coma Rating Scale (CRS). The Wavelet Transform, followed by Hilbert transform was applied to the EEG and gamma power distribution was quantified relative to the timing of an eye movement. We correlated the clinical and the neurophysiological measures. RESULTS: Gamma activity was present in all PVS patients. Its power was modulated in association with eye movements only in less severely affected patients, with minimum power prior to, and maximum power during the eye movement. In severely affected patients there was no evidence of a temporal relationship between gamma power and the phase of the eye movement. CONCLUSIONS: Detecting changes in the time course of gamma power in relation to conjugate ballistic eye movements provides a quantitative neurophysiological method for prospective longitudinal studies to explore if the preservation of this CNS function relates to the potential for recovery in PVS patients.

Adult↗

PubMed2006

Intraocular lens power calculation after myopic refractive surgery: theoretical comparison of different methods.

OBJECTIVE: To evaluate the reliability of different methods developed to calculate intraocular lens (IOL) power after corneal refractive surgery. DESIGN: Retrospective observational case series. PARTICIPANTS: Preoperative and postoperative data of all eyes that underwent myopic excimer laser surgery in a private practice (Centro Salus, Bologna, Italy) between 1999 and 2004 were reviewed. INTERVENTION: The following methods were analyzed: videokeratography, clinical history, Shammas' refraction-derived and clinically derived methods, Rosa's correcting factor, Ferrara's variable refractive index, separate consideration of anterior and posterior corneal curvature (with and without preoperative data), Feiz-Mannis' formula and nomogram, and Latkany's regression formulas (based on both average and flattest postrefractive surgery keratometry). The Holladay 1 formula was used for eyes with an axial length between 22 and 24.49 mm and the SRK-T for eyes longer than 24.49 mm. Double-K formulas were also evaluated, when applicable. Each IOL power determined with these methods was compared to a benchmark value, calculated using the preoperative axial length and corneal power and aiming for the preoperative spherical equivalent. MAIN OUTCOME MEASURE: Mean error in IOL power prediction. RESULTS: Ninety-eight eyes of 98 patients were analyzed. The double-K clinical history method, Feiz-Mannis' formula, double-K method based on separate consideration of anterior and posterior corneal curvature (with and without preoperative data), and both Latkany's regression formulas were the only methods resulting in a mean IOL power not statistically different (P>0.05) from the benchmark used for comparative purposes. CONCLUSIONS: When prerefractive surgery data are available, IOL power should be calculated using the double-K clinical history method. Alternative choices may be represented by the Feiz-Mannis' formula, Latkany's regression formulas based on average and flattest postrefractive surgery keratometry, and the double-K method based on separate consideration of anterior and posterior corneal curvatures. A variant of the latter can be used to calculate IOL power when prerefractive surgery data are not available. Further prospective studies based on patients undergoing phacoemulsification after refractive surgery are needed to validate the results of this theoretical comparison.

Adult↗

PubMed2006

Intraocular lens exchange due to incorrect lens power.

PURPOSE: To evaluate patients who had intraocular lens (IOL) exchange for unexpected postoperative refractive errors, determine the sources associated with the errors, and derive an empiric approach to estimating the power for IOL exchange. DESIGN: Retrospective review of interventional case series. PARTICIPANTS: Twenty-two eyes that underwent IOL exchange for correcting unexpected refractive errors after cataract surgery with IOL implantation were reviewed. INTERVENTION: All the IOLs for IOL exchange were placed in-the-bag. The same type of IOLs was used for original IOL implantation and IOL exchange in 91% (20/22) of eyes. MAIN OUTCOME MEASURES: Uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), refraction, and reasons for IOL exchange. The equation derived from refractive change and change in IOL power was developed to calculate IOL power for exchange. RESULTS: Of the 22 cases, the identified reasons were keratometry errors in 5 (23%) and incorrect axial length (AL) determination in 3 (14%). In 3 other cases, a wrong IOL was implanted. After IOL exchange, 82% (18/22) of eyes were within +/-0.50 diopters (D) and 86% (19/22) within +/-1.00 D of emmetropia. Uncorrected visual acuity was 20/40 or better in 82% of eyes, and BSCVA was 20/40 or better in 95% (21/22) of eyes. The correlation between change of refraction and IOL power was significant (P<0.002). CONCLUSIONS: Among the identified causes, incorrect corneal power determination was the most frequent reason for incorrect IOL power implantation, followed by error in AL measurement and inserting a wrong IOL. The pre-exchange refraction can be used theoretically to calculate the IOL power for exchange.

Cataract Extraction↗

PubMed1995

Calculation of intraocular lens power after radial keratotomy with computerized videokeratography.

PURPOSE: Because standard methods to determine intraocular lens power are not adequate in eyes that have had radial keratotomy, we undertook this study to evaluate the corneal power derived from computerized videokeratography for use in intraocular lens power calculations. METHODS: We examined four eyes of three patients who had radial keratotomy and who underwent phacoemulsification cataract surgery with implantation of a posterior chamber intraocular lens. We used a computerized videokeratography-derived corneal curvature value in the Holladay formula for intraocular lens calculations. We determined the ideal intraocular lens power and the keratometric value that would have led to the ideal intraocular lens power from the postoperative refraction at 6.1 +/- 1.1 months after cataract extraction. The ideal keratometric value was compared with the keratometric values derived from computerized videokeratography, standard keratometry, contact lens overrefraction, and refractions before and after radial keratotomy. RESULTS: The postoperative refraction at approximately six months averaged -0.32 +/- 0.63 diopter (range, -0.88 to +0.75 diopter) different than the aim. The mean power in ring 3, which was the closest keratometric value to the ideal, disclosed only 0.09 +/- 0.73 diopter and -0.10 +/- 0.72 diopter of deviation from the ideal keratometric and intraocular lens powers, respectively. One to two weeks after phacoemulsification cataract surgery with implantation of a posterior chamber intraocular lens, the videokeratographic differential map disclosed steepening at the wound site with variable regression by six months in all patients. CONCLUSION: Results suggest that, after radial keratotomy, using the keratometric value derived from computerized videokeratography in intraocular lens calculations is more accurate than using keratometric values measured by routine methods.

Aged↗

PubMed2002

Beta power in the EEG of alcoholics.

BACKGROUND: In this study, the magnitude and spatial distribution of beta power in the resting electroencephalogram (EEG) were examined to address the possibility of an excitation-inhibition imbalance in the central nervous system of alcoholics. METHODS: Log transformed absolute power in the Beta 1 (12.5-16 Hz), Beta 2 (16.5-20 Hz), and Beta 3 (20.5-28 Hz) bands in the eyes-closed EEG of 307 alcohol-dependent subjects and 307 unaffected age- and gender-matched control subjects were compared using a multivariate repeated measures design. Effect of gender, age, and drinking variables was examined separately. RESULTS: Increased Beta 1 (12.5-16 Hz) and Beta 2 (16.5-20 Hz) absolute power was observed in alcohol-dependent subjects at all loci over the scalp. The increase was most prominent in the central region. Increased Beta 3 (20.5-28 Hz) power was frontal in the alcoholics. Age and clinical variables did not influence the increase. Male alcoholics had significantly higher beta power in all three bands. In female alcoholics the increase did not reach statistical significance. CONCLUSIONS: Beta power in all three bands of resting EEG is elevated in alcoholics. This feature is more prominent in male alcoholics. The increased beta power in the resting EEG may be an electrophysiological index of the imbalance in the excitation-inhibition homeostasis in the cortex.

Adolescent↗

PubMed1997

Season of birth and electroencephalogram power abnormalities in schizophrenia.

To determine the association between season of birth and electroencephalogram (EEG) power abnormalities in schizophrenia, this study examined the resting EEGs of 28 winter-born and 81 nonwinter-born schizophrenia patients. Eighteen winter-born and 58 nonwinter-born nonschizophrenic psychosis patients (e.g., bipolar disorder patients with psychotic features), and 97 normal subjects were also studied. Compared to normal subjects, nonwinter-born schizophrenia patients had augmented low-frequency power and diminished alpha band power, but winter-born schizophrenia patients failed to have any EEG power abnormalities. Nonwinter-born nonschizophrenic psychosis patients had the same low-frequency and alpha band power abnormalities as nonwinter-born schizophrenia patients. The winter-born non-schizophrenic psychosis group failed to show any EEG power abnormalities. The results of this study indicate that in psychosis the functional characteristics of the brain vary depending on the season in which a person is born. Low-frequency and alpha band EEG power abnormalities may help distinguish psychosis stemming from a seasonally varying pathogen from psychosis of other etiologies.

Bipolar Disorder↗

PubMed1997

Phosphorylation-dependent power output of transgenic flies: an integrated study.

We examine how the structure and function of indirect flight muscle (IFM) and the entire flight system of Drosophila melanogaster are affected by phosphorylation of the myosin regulatory light chain (MLC2). This integrated study uses site-directed mutagenesis to examine the relationship between removal of the myosin light chain kinase (MLCK) phosphorylation site, in vivo function of the flight system (flight tests, wing kinematics, metabolism, power output), isolated IFM fiber mechanics, MLC2 isoform pattern, and sarcomeric ultrastructure. The MLC2 mutants exhibit graded impairment of flight ability that correlates with a reduction in both IFM and flight system power output and a reduction in the constitutive level of MLC2 phosphorylation. The MLC2 mutants have wild-type IFM sarcomere and cross-bridge structures, ruling out obvious changes in the ultrastructure as the cause of the reduced performance. We describe a viscoelastic model of cross-bridge dynamics based on sinusoidal length perturbation analysis (Nyquist plots) of skinned IFM fibers. The sinusoidal analysis suggests the high power output of Drosophila IFM required for flight results from a phosphorylation-dependent recruitment of power-generating cross-bridges rather than a change in kinetics of the power generating step. The reduction in cross-bridge number appears to affect the way mutant flies generate flight forces of sufficient magnitude to keep them airborne. In two MLC2 mutant strains that exhibit a reduced IFM power output, flies appear to compensate by lowering wingbeat frequency and by elevating wingstroke amplitude (and presumably muscle strain). This behavioral alteration is not seen in another mutant strain in which the power output and estimated number of recruited cross-bridges is similar to that of wild type.

Animals↗

PubMed1994

Power spectral analysis of heart rate variability as a new method for assessing autonomic activity in the rat.

The authors studied power spectral analysis of heart rate variability in the rat, hypothesizing that the quantitative information provided by this analysis reflects the interaction between sympathetic and parasympathetic regulatory activities. For this purpose, an electrocardiogram was recorded from conscious and unrestrained Wistar rats (Nippon, Shizuoka) (12-16 weeks old) by a telemetry system and analyzed by a power spectrum. Because it was thought that the electrocardiogram recorded by the telemetry system could provide more reliable data to assess autonomic nervous activity than the tethering system, the telemetry recording system was used. There were two major spectral components in the power spectrum at low frequency (LF) (0.6 Hz) and high frequency (HF) (approximately 1.4 Hz). On the basis of these data, the authors defined two frequency bands of interest: LF (0.04-1.0 Hz) and HF (1.0-3.0 Hz). The power of LF was higher than that of HF in the normal rat. Atropine (2 mg/kg intraperitoneally) significantly reduced both HF and LF power. Propranolol (4 mg/kg intraperitoneally) also significantly reduced LF power; however, it had no significant effect on HF power. Thus, this study in the rat confirmed earlier observations in the conscious dog and human. Furthermore, the decrease in the parasympathetic mechanism produced by atropine was reflected by a slight increase in the LF/HF ratio. The LF/HF ratio appeared to follow the reductions of sympathetic activity produced by propranolol. From these results, the LF/HF ratio seemed to be a convenient index of parasympathetic and sympathetic interactions in the rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗