PubMed Health⌕ Search

Biomedical subjects

H J Gerrits

Publications and source records attributed to H J Gerrits.

14 recordsLinked to original sources
PubMed1995

Continuous infusions of alfentanil and propofol for coronary artery surgery.

OBJECTIVE: To study the anesthetic efficacy of two different background infusion rates for alfentanil in a total intravenous anesthesia (TIVA) technique using propofol. Therefore, the effects of these infusions on hemodynamic stability and on the suppression of hemodynamic and somatic responses to noxious stimuli were compared. DESIGN: Prospective and randomized. SETTING: The study was performed in a university hospital setting. Two patient groups were compared. INTERVENTIONS: Anesthesia was induced in group 1 (n = 16) with alfentanil 50 micrograms/kg and in group 2 (n = 14) with alfentanil 75 micrograms/kg, infused in 4 min, as well as with an infusion of propofol at a rate of 10 mg/kg/h in both groups. After 4 min, the alfentanil infusion was reduced to 1 microgram/kg/min in group 1 and to 2 micrograms/kg/min in group 2. The propofol infusion was reduced following sternal spread to 3 mg/kg/h. Responses indicating inadequate anesthesia were treated with additional alfentanil bolus doses. MEASUREMENTS AND MAIN RESULTS: Induction of anesthesia in group 1 was associated with significant decreases in systolic and diastolic (-13%) blood pressures, cardiac index (-16%) and left ventricular stroke work index (-31%). Hemodynamic changes were similar in group 2, except for the greater fall in systemic vascular resistance during maintenance of anesthesia. There was no difference in the incidence of breakthrough hypertension between the two groups (in 44% and 43% of the patients, respectively) and in the number of alfentanil bolus supplements. There were also no differences in the incidence of ischemia, myocardial infarction or duration of postoperative ventilation. CONCLUSIONS: Because both infusions provided equally stable anesthesia, the lower infusion regimen for alfentanil is the more appropriate technique. Using this technique, the administration of additional alfentanil boluses just before stressful surgical episodes will further improve hemodynamic stability.

Alfentanil↗
PubMed1988

Afterimages: a collective term for percepts of different origin.

Exposure of the eye to a strong photoflash results in a so-called "afterimage", which may last for 20 min or longer. In contrast, the true afterimage, which fluctuates in brightness and is best seen in complete darkness, lasts only a few minutes. This true afterimage can be attributed to the strong oscillatory neuronal responses immediately initiated by the flash. Thereafter dark and light regions, insensitivity percepts, are observable against bright and dark backgrounds, respectively. These percepts can be adequately explained by a reversal of the response behaviour of rod and cone driven ganglion cells situated along the contour of the flash-exposed area. The slow recovery of the rods explains why insensitivity percepts can be seen for many minutes.

Afterimage↗
PubMed1988

Apparent size and receptive field properties.

A light square against a dark background besides an equisized dark square against a light background shows a difference in apparent size such that the light square seems larger than the dark one. This illusion was studied for squares of 109 x 109 min arc. Their centres were located 90 min arc out of the centre of the fovea. The apparent size appeared to be dependent on the contrast between the square and the background and not on the mean luminance in the vicinity of the contour. The addition of a small white or dark line to the luminance step that constituted the border of the square changed the illusion dramatically. It is argued that this explains the observations where an inversion of the illusion for very small contrast values was found. The results are explained on the basis of a model with receptive fields of different sizes overlapping the same retinal location and having different sensitivities.

Fixation, Ocular↗
PubMed1979

Apparent movements induced by stroboscopic illumination of stabilized images.

Stroboscopic illumination of stabilized images causes habituation effects and illusory phenomena which, at low stimulation frequencies, show up as brighter patches. When a line is used as a stimulus, these patches are observed as brighter spots in the line surrounded by dimmer patches in the background. These locally brighter parts function as sources from which brightness spreads in the direction of the line at light-on and as sinks to which the brightness retracts again after light-off. This spread and retraction of the brightness induces the perception of movements. Higher stimulation frequencies or a diminished stimulus-background contrast enables the brightness to spread also in the direction perpendicular to the line contour, i.e., into the background. Again a perception of movement is induced, local displacements of a part of the field are observed as a result of the brightness spread. Tiny pinpoints of light, the smallest foveal perceptive elements, visible in lines narrower than 10 min arc (Gerrits, 1978), are also observed when these lines are illuminated with stroboscopic light. These tiny elements do not spread their brightnesses and so also no movement is induced. The results are discussed in relation to the properties of the perceptive elements and the cells activating them (habituation, barriers, filling-in).

Afterimage↗
PubMed1978

Differences in peripheral and foveal effects observed in stabilized vision.

Foveal images fade much faster than peripheral images, at light-on as well as at light-off. An opposite result can be obtained, however, in case of unsatisfactory stabilization. The amount and extension of the brightness spreading before the fading depend on the stimulus brightness and its location on the retina. The intensity threshold and the maximal obtainable sharpness of the image decrease towards the periphery, the image becomes wider and sharper on increasing the stimulus intensity. In the foveal area brightness or darkness can be observed (even from stimuli below the foveal threshold) by a filling-in from the periphery, which has a lower threshold. On increasing the stimulus intensity very small pinpoints of light, tiny dots are observable in the foveal area. Each dot measures less than 1 min arc in diameter and is visible for a few hundred milliseconds only. The final sharply contoured foveal percept of a narrow line, observed at higher intensities, is made up of a crowding of these tiny dots. When lines wider than about 10 min arc are used as stimuli these small foveal dots are only seen along the inside contours but not on the (diffuse) interior of the line. In the periphery no tiny dots but only spots of larger size can be observed. At each location of the visual field the final percept seems to be built up from the co-operation of a number of perceptive elements of different sizes. Small elements fade faster, have higher thresholds, habituate more easily on repeated stimulation and become easier activated by small-amplitude stimulus displacements than larger elements. The results described, contradicting some literature date, depend heavily on the quality of the stabilization.

Color Perception↗