Biomedical subjects

M Valencia

Publications and source records attributed to M Valencia.

At least 19 recordsLinked to original sources

PubMed2006

Topography of cortical activation differs for fundamental and harmonic frequencies of the steady-state visual-evoked responses. An EEG and PET H215O study.

In humans, visual flicker stimuli of graded frequency (2-90 Hz) elicit an electroencephalographic (EEG) steady-state visual-evoked response (SSVER) with the same fundamental frequency as the stimulus and, in addition, a series of harmonic responses. The fundamental component of the SSVER is generated by increased synaptic activity in primary visual cortex (V1). We set out to determine the cortical origin of the harmonic responses in humans. For this purpose, we recorded the SSVERs at 5 different frequencies (5, 10, 15, 25, and 40 Hz) and measured regional cerebral blood flow (rCBF) with positron emission tomography-H(2)(15)O at rest and during visual stimulation at the same frequencies. The rCBF contrast weighted by the amplitude of the SSVERs first harmonics showed activation of a swath of cortex perpendicular to V1, including mostly the inferior half of the parieto-occipital sulcus. This area overlapped minimally with the primary visual cortex activated by the fundamental frequency. A different method, estimating EEG cortical source current density with low-resolution brain electromagnetic tomography, gave the same results. Our finding suggests that the inferior portion of the banks of the parieto-occipital sulci contains association visual cortex involved in the processing of stimuli that can be as simple as a flickering light source.

Adult↗

PubMed2006

Cortical gamma activity during auditory tone omission provides evidence for the involvement of oscillatory activity in top-down processing.

Perception is an active process in which our brains use top-down influences to modulate afferent information. To determine whether this modulation might be based on oscillatory activity, we asked seven subjects to detect a silence that appeared randomly in a rhythmic auditory sequence, counting the number of omissions ("count" task), or responding to each omission with a right index finger extension ("move" task). Despite the absence of physical stimuli, these tasks induced a 'non-phase-locked' gamma oscillation in temporal-parietal areas, providing evidence of intrinsically generated oscillatory activity during top-down processing. This oscillation is probably related to the local neural activation that takes place during the process of stimulus detection, involving the functional comparison between the tones and the absence of stimuli as well as the auditory echoic memory processes. The amplitude of the gamma oscillations was reduced with the repetition of the tasks. Moreover, it correlated positively with the number of correctly detected omissions and negatively with the reaction time. These findings indicate that these oscillations, like others described, may be modulated by attentional processes. In summary, our findings support the active and adaptive concept of brain function that has emerged over recent years, suggesting that the match of sensory information with memory contents generates gamma oscillations.

Acoustic Stimulation↗

PubMed2006

Slow oscillatory activity and levodopa-induced dyskinesias in Parkinson's disease.

The pathophysiology of levodopa-induced dyskinesias (LID) in Parkinson's disease is not well understood. We have recorded local field potentials (LFP) from macroelectrodes implanted in the subthalamic nucleus (STN) of 14 patients with Parkinson's disease following surgical treatment with deep brain stimulation. Patients were studied in the 'Off' medication state and in the 'On' motor state after administration of levodopa-carbidopa (po) or apomorphine (sc) that elicited dyskinesias in 11 patients. The logarithm of the power spectrum of the LFP in selected frequency bands (4-10, 11-30 and 60-80 Hz) was compared between the 'Off' and 'On' medication states. A peak in the 11-30 Hz band was recorded in the 'Off' medication state and reduced by 45.2% (P < 0.001) in the 'On' state. The 'On' was also associated with an increment of 77. 6% (P < 0.001) in the 4-10 Hz band in all patients who showed dyskinesias and of 17.8% (P < 0.001) in the 60-80 Hz band in the majority of patients. When dyskinesias were only present in one limb (n = 2), the 4-10 Hz peak was only recorded in the contralateral STN. These findings suggest that the 4-10 Hz oscillation is associated with the expression of LID in Parkinson's disease.

Action Potentials↗

PubMed2006

Imitating versus non-imitating movements: differences in frontal electroencephalographic oscillatory activity.

Non phase-locked oscillatory changes were studied in seven healthy volunteers during two different reaction time movement paradigms, in which the stimulus was a wrist movement (either extension or flexion) performed by another person seated in front of the subject (examiner). In the first paradigm (imitation), the subject was instructed to perform the same movement observed. In the second paradigm (non-imitation), the subject was instructed to perform the opposite movement (flexion when an extension was observed, and vice-versa). Changes in the 7-37 Hz range band were determined by means of Gabor transforms. A frontal energy increase (event-related synchronization, ERS) around 15 Hz could be observed in the frontal region after the examiner's movement; this frontal ERS was significantly larger in the non-imitation paradigm. A typical alpha and beta movement-related event-related desynchronization/synchronization (ERD/ERS) pattern was also observed in both paradigms in the central region. The beta-ERD was significantly larger in the imitation paradigm. Our results show that the motor preparation mechanisms involved in an imitated and a non-imitated movement are different.

Adult↗

PubMed2006

High frequency oscillations in the somatosensory evoked potentials (SSEP's) are mainly due to phase-resetting phenomena.

A small series of high frequency oscillations (HFOs) overlapping the earliest part of the N20 wave can be observed in the somatosensory evoked potentials (SSEPs) of normal subjects. We tried to elucidate whether these high frequency components are mainly due to phase-resetting phenomena, to the emergence of new oscillations related to the stimuli, or to a combination of both. Averaged median-nerve SSEPs from seven healthy subjects were studied by means of time-frequency analysis. The presence of new oscillatory activities was evaluated by averaging the energy of the single-trial time-frequency transforms in the HFOs range (400-1000 Hz). To study phase-resetting phenomena, we measured inter-trial coherence (ITC) in the same frequency range. A marked inter-trial coherence related to the HFOs was found, whereas energy changes (related to the emergence of new oscillations) were minimal. The combination of these three different approaches suggests that the HFOs are mainly due to resettings of the ongoing EEG activity originated in response to the stimuli. The emergence of new activities does not seem to be a relevant mechanism in the formation of these components.

Adult↗

PubMed2005

Alpha and beta changes in cortical oscillatory activity in a go/no go randomly-delayed-response choice reaction time paradigm.

OBJECTIVE: Predictable movements induce oscillatory changes over the contralateral motor cortex that begin before the movement, but their significance has not been fully established. We studied non-phase-locked changes in cortical oscillatory activity in a S1-centred double-stimulus go/no go paradigm with random interstimulus interval. METHODS: About 58 reference-free EEG channels were analyzed by means of Gabor transforms in a group of 10 healthy subjects. A 2000 Hz tone (S1go, 84% probability) indicated the subject to make a brisk wrist extension after a second 2000 Hz tone (S2go). The S1-S2 interval was either 1.5, 3 or 4.5 s. A 1000 Hz tone (S1 no go, 16% probability) indicated the subject not to move (and wait for another S1 tone). RESULTS: A frontal 15 Hz synchronization was observed after S1 in all conditions. No further significant changes were observed in the no go condition. A small pre-S2 alpha and beta desynchronization could be observed only in the 3 and 4.5s-interval go conditions, being larger in the latter. CONCLUSIONS: These results suggest that the predictability of the timing of a movement influences the appearance of the pre-movement oscillatory changes; not only motor planning (the 'go' decision) is necessary, but also an estimation of when to move. SIGNIFICANCE: Our findings provide new insight on the relationship between the decision-making process, movement, and cortical oscillatory activity.

Adult↗

PubMed2005

Movement-related changes in oscillatory activity in the human subthalamic nucleus: ipsilateral vs. contralateral movements.

A voluntary movement is accompanied by a series of changes in neuronal oscillatory activity in the subthalamic nucleus (STN). These changes can be recorded through electrodes implanted for deep brain stimulation to treat Parkinson's disease in the time interval between the surgery and the internalization of the connections to the batteries. Both baseline activity and movement-related changes are different in the 'on' and 'off' medication motor states. In the 'off' state a low frequency activity in the alpha-beta range (8-25 Hz) that dominates the spectrum is interrupted during the movement, while in the 'on' state baseline frequencies are higher and a peri-movement gamma increase (70-80 Hz) is usually observed. Similar changes have been described with electrocorticographic recordings over the primary motor cortex but the gamma increase was only present during contralateral movements. We compared ipsi- and contralateral movement-related changes in STN activity, using a time-frequency analysis of the recordings obtained simultaneously in both STN and the scalp (electroencephalography) during right and left hand movements. The movement-related changes observed in the STN in the 'on' and the 'off' states were similar to those described previously in terms of predominant frequency bands, but we found bilateral changes in the STN during movements of either hand. A contralateral earlier start of the beta STN changes was mostly observed when the moving hand corresponded to the less-affected side, irrespective of hand dominance. These results suggest that movement-related activity in the STN has, by and large, a bilateral representation and probably reflects cortical input.

Aged↗

PubMed2004

Frontal and central oscillatory changes related to different aspects of the motor process: a study in go/no-go paradigms.

We studied alpha and beta EEG oscillatory changes in healthy volunteers during two different auditory go/no-go paradigms, in order to investigate their relationship with different components of the motor process. In the first paradigm (S2-centered), the initial tone (S1) was constant (warning), and the second tone (S2) indicated the subject whether to move or not. In the second paradigm (S1-centered), S1 indicated whether to move or not, while S2 just indicated the timing of the movement. A medial frontal beta energy increase was found in all conditions after the stimulus that forces the subject to decide whether to move or not (S1 or S2 depending on the paradigm). In both go conditions, a central alpha and beta energy decrease began after the go decision, reaching minimum values during the movement; it was followed by a beta post-movement increase, limited to the central contralateral area. In the no-go conditions, a marked fronto-central beta synchronization appeared after the decision not to move. In conclusion, our study was able to dissociate the beta oscillatory changes related to movement preparation and execution (central decrease/increase) from those associated with decision-making (medial frontal increase) and motor inhibition (fronto-central increase).

Adult↗

PubMed2004

Potentials evoked by chirp-modulated tones: a new technique to evaluate oscillatory activity in the auditory pathway.

OBJECTIVE: Steady-state potentials are oscillatory responses generated by a rhythmic stimulation of a sensory pathway. The frequency of the response, which follows the frequency of stimulation, is maximal at a stimulus rate of 40 Hz for auditory stimuli. The exact cause of these maximal responses is not known, although some authors have suggested that they might be related to the 'working frequency' of the auditory cortex. Testing of the responses to different frequencies of stimulation may be lengthy if a single frequency is studied at a time. Our aim was to develop a fast technique to explore the oscillatory response to auditory stimuli, using a tone modulated in amplitude by a sinusoid whose frequency increases linearly in frequency ('chirp') from 1 to 120 Hz. METHODS: Time-frequency transforms were used for the analysis of the evoked responses in 10 subjects. Also, we analyzed whether the peaks in these responses were due to increases of amplitude or to phase-locking phenomena, using single-sweep time-frequency transforms and inter-trial phase analysis. RESULTS: The pattern observed in the time-frequency transform of the chirp-evoked potential was very similar in all subjects: a diagonal band of energy was observed, corresponding to the frequency of modulation at each time instant. Two components were present in the band, one around 45 Hz (30-60 Hz) and a smaller one between 80 and 120 Hz. Inter-trial phase analysis showed that these components were mainly due to phase locking phenomena. CONCLUSIONS: A simultaneous testing of the amplitude-modulation-following oscillatory responses to auditory stimulation is feasible using a tone modulated in amplitude at increasing frequencies. The maximal energies found at stimulation frequencies around 40 Hz are probably due to increased phase-locking of the individual responses.

Acoustic Stimulation↗

PubMed2003

Hospital-acquired pneumonia: coverage and treatment adequacy of current guidelines.

The American Thoracic Society (ATS) guideline for hospital-acquired pneumonia (HAP) released in 1996 and the Trouillet classification published in 1998 supply different rational foundations for the classification of patients with HAP and for the selection of initial antibiotic therapy. The aims of this study were to assess the level of bacterial coverage and to assess and validate the adequacy of antibiotic strategy of each of these classifications. Intensive care unit-admitted patients (n=71) with suspicion of HAP were evaluated. The ATS and Trouillet classifications demonstrated an accuracy to predict the causative microorganism of 91% and 83% respectively. The ATS and Trouillet antibiotic treatment recommendations were adequate in 79% and 80% of the patients, respectively. The microorganisms implicated in the treatment inadequacy of the ATS guideline were Pseudomonas aeruginosa (n=3), Acinetobacter baumanii (n=1), Stenotrophomonas maltophilia (n=1) and methicillin-resistant Staphylococcus aureus (n=1). P. aeruginosa was implicated with Trouillet treatment inadequacy. The current recommendations for empirical antibiotic treatment of hospital-acquired pneumonia (American Thoracic Society and Trouillet) showed a good ability to predict the involved pathogen. However, considering the resistance pattern of the isolated pathogens, both classifications demonstrated a rather lower treatment adequacy; the main reason was the failure to treat highly resistant strains.

Aged↗

PubMed2001

NEJ1 controls non-homologous end joining in Saccharomyces cerevisiae.

Broken DNA ends are rejoined by non-homologous end-joining (NHEJ) pathways requiring the Ku proteins (Ku70, Ku80), DNA ligase IV and its associated protein Lif1/Xrcc4 (ref. 1). In mammalian meiotic cells, Ku protein levels are much lower than in somatic cells, apparently reducing the capacity of meiotic cells to carry out NHEJ and thereby promoting homologous recombination. In Saccharomyces cerevisiae, NHEJ is also downregulated in meiosis-competent MATa/MAT alpha diploid cells in comparison with diploids or haploids expressing only MATa or MAT alpha. Diploids expressing both MATa and MAT alpha show enhanced mitotic homologous recombination. Here we report that mating-type-dependent regulation of NHEJ in budding yeast is caused in part by transcriptional repression of both LIF1 and the gene NEJ1 (YLR265C)--identified from microarray screening of messenger RNAs. Deleting NEJ1 reduces NHEJ 100-fold in MATa or MAT alpha haploids. Constitutive expression of NEJ1, but not expression of LIF1, restores NHEJ in MATa/MAT alpha cells. Nej1 regulates the subcellular distribution of Lif1. A green fluorescent protein (GFP)-Lif1 fusion protein accumulates in the nucleus in cells expressing NEJ1 but is largely cytoplasmic when NEJ1 is repressed.

DNA Damage↗

PubMed2001

[Indoor air pollution in a zone of extreme poverty of La Pintana, Santiago-Chile].

BACKGROUND: Indoor pollution can be an important risk factor for human health, considering that people spend more than 60% of their time in their houses. AIM: To investigate indoor pollution in a zone of extreme poverty in Metropolitan Santiago. MATERIAL AND METHODS: During 24 h, carbon monoxide (CO), sulfur dioxide (SO2), respirable particulate matter (PM10), polycyclic aromatic hydrocarbons absorbed in PM5, temperature and humidity, were measured in the interior of 24 houses in a La Pintana, Santiago. RESULTS: The higher pollutant concentrations were observed during hours when heating was used, in houses that used coal (mean PM10 250 micrograms/m3, CO 42 ppm, SO2 192 pph) or firewood (mean PM10 489 micrograms/m3, CO 57 ppm, SO2 295 pph). In all houses, polycyclic aromatic hydrocarbons were detected and they came from the interior of the house and not from external filtered air. Coal, firewood and cigarette smoke were important sources of carcinogenic and kerosene and gas were sources of non carcinogenic polycyclic aromatic hydrocarbons. CONCLUSIONS: In the houses studied, the population was exposed to an accumulation of highly toxic pollutants, caused by a lack of ventilation. A high relative humidity also contributed to the growth of biological pollutants.

Adolescent↗

PubMed2000

[Measurement of serum amiodarone by protein precipitation followed by high performance liquid chromatography with ultraviolet detection].

Our aim was to develop a quantitative method for serum amiodarone measurement using high performance liquid chromatography with ultraviolet photometric detection. We studied previous reports in the literature in order to obtain a simpler method to be used routinely in our TDM unit. Sample preparation was based on protein precipitation adding 2 parts of acetonitrile to 1 part of serum, followed by vortex-agitation for 45 s, incubation at 24 degrees C for 5 min, and centrifugation at 6000 x g for 2 min. Twenty microliters of the supernatant was directly injected into the chromatographic system. A microBondapak CN RP column (3.9 x 150 mm) at 45 degrees C, with a mobile phase consisting of KH2PO4 10 mM/methanol/acetonitrile (40:37:23 v/v/v), pH 3.5, were used. Eluting with a flow rate of 0.6 mL/mm the retention time of amiodarone was approximately 4.9 min. Detection was performed at 242 nm and the quantification was made by peak height comparison with external standards. The mass/response ratio is linear (r2 > 0.99) within a mass range of 2.96 to 18,930 ng of injected amiodarone, which exceeds the requirements for the monitoring of serum levels (0.3 to 6.0 micrograms/mL). Sample storage should be done with acetonitrile-extracted sera at -16 degrees C to avoid degradation. The method is very efficient, linear, sensitive and specific but it also simpler and cheaper than others reported in the literature.

Amiodarone↗

PubMed1998

Medical case management after laminectomy or craniotomy: do all patients benefit from admission to the intensive care unit?

To define severity of illness to identify most effectively patients for whom admission to the intensive care unit (ICU) is unnecessary, the authors performed a retrospective cost-effectiveness analysis. The authors studied the records of 113 patients who were admitted to the ICU after undergoing laminectomy (or other spinal cord surgery) or craniotomy for removal of neoplasm; the Acute Physiology and Chronic Health Evaluation III prognostic system had identified these patients as having a 10% or less risk of requiring intervention while in the ICU. No patient required active intervention during a mean stay of 3.26 days in the ICU. Combined use of a "step-down" postoperative care unit and ICU can optimize allocation of medical resources while providing high-quality care for some neurosurgical patients who are at low risk of requiring postoperative intervention.

Journal Article↗

PubMed1997

A single, double lumen high-flow catheter for patients undergoing peripheral blood stem cell transplantation. Experience at the National Cancer Institute in Mexico.

Peripheral blood stem cell transplantation (PBSCT) requires a high-flow catheter for adequate cell collection by apheresis and long i.v. support, this is usually achieved by multiple catheters. We analyzed our experience with Mahurkar or Permacath for apheresis and long-term i.v. support in PBSCT, cared for exclusively by an i.v. therapy team. Fifty-six catheters were used in 53 patients that completed PBSCT (28 Permacath and 28 Mahurkar). In 10 patients (19%) the same catheter was used for multiple PBSCT. The average stay was 58.4 days (7-219), Permacath 76.8 days (14-219) and Mahurkar 42 days (7-106). The incidence of infectious complications was 2.2 x 1000 catheter-days (1.7 Permacath and 3.0 Mahurkar); during neutropenia it was 3.7 x 1000 cathether-days. The incidence of thrombosis was 0.9 x 1000 catheter-days. There was a total of seven infectious episodes (12.7%). Five (9%) were local and two were (3.6%) bacteremias. The microorganism most commonly isolated was Staphylococcus sp. (57%). Four catheters (7.1%) were removed because of complications: one thrombosis and three infections. Both catheters have proven useful and safe for long-lasting vascular access in patients undergoing PBSCT. No statistical difference was found in infectious and non-infectious complications between either catheters.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed1997

Superoxide production and antioxidant enzymes in ammonia intoxication in rats.

Injection of large doses of ammonium salts lead to the rapid death of animals. However, the molecular mechanisms involved in ammonia toxicity remain to be clarified. We have tested the effect of injecting 7 mmol/kg of ammonium acetate on the production of superoxide and on the activities of some antioxidant enzymes in rat liver, brain, erythrocytes and plasma. Glutathione peroxidase, superoxide dismutase and catalase activities were decreased in liver and brain (both in cytosolic and mitochondrial fractions) and also in blood red cells, while glutathione reductase activity remained unchanged. Superoxide production in submitochondrial particles from liver and brain was increased by more than 100% in both tissues. Both diminished activity of antioxidant enzymes and increased superoxide radical production could lead to oxidative stress and cell damage, which could be involved in the mechanism of acute ammonia toxicity.

Ammonia↗

PubMed1997

[Usefulness of preoperative location tests in primary hyperparathyroidism].

The utility of preoperative parathyroid location in primary hyperparathyroidism (pHPT) remains controversial. In this retrospective study are analysed noninvasive techniques prior to surgical neck exploration of 11 patients with pHPT, ten adenomas and one hyperplastic parathyroid glands. Localization test were performed 24 times. Ultrasonography allowed correct localization of 5 of 7 cases, computer tomography 2 of 6, scintigraphy 2 of 4 and nuclear magnetic resonance 1 of 7. This exploration was doubtful in a patient, and two false-positive nuclear magnetic resonance were found. The low yield of these tests makes them unnecessary in the evaluation of patients undergoing surgery for pHPT, because the cost is not justified.

Adult↗

PubMed1995

Echographic evaluation of primary cysts of the iris pigment epithelium.

PURPOSE: Primary cysts of the iris pigment epithelium frequently appear to be focal elevations of the peripheral iris. To differentiate them from other anterior segment tumors requires the ability to assess the distribution, tissue characteristics, and progression of such lesions, which we accomplished by using anterior segment echography. METHODS: We reviewed the medical records of 49 consecutive patients referred to the Doheny Eye Institute for echographic evaluation of focal bulging of the peripheral iris. Echography of the anterior segment was performed on each patient by using an immersion technique. Clinical examination was also performed on 20 (41%) patients at the time of initial examination. RESULTS: We identified 52 iris pigment epithelial cysts in 52 eyes of the 49 patients studied. All lesions were single, localized to the posterior iris surface, and demonstrated a well-circumscribed, echolucent structure on B-scan echography. Average maximal sagittal thickness was 1.6 +/- 0.4 mm. Cysts were most frequently located in the inferotemporal quadrant (31%). Minimal increase in size or regression of lesions was noted in the nine patients (ten eyes) for whom follow-up echographic examinations were available. CONCLUSIONS: Primary iris pigment epithelial cysts possess characteristic features with regard to location, size, and internal structure, which can be accurately documented with echography. Clinically evident lesions tend to be between 1 and 4 mm in maximum sagittal thickness, conform to the contour of posterior chamber structures without distortion, produce anterior bulging of the peripheral iris, are preferentially located in the inferotemporal quadrant, and demonstrate limited potential for growth.

Adolescent↗