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Biomedical subjects

R D Braun

Publications and source records attributed to R D Braun.

8 recordsLinked to original sources
PubMed1992

Spontaneous fluctuations in oxygen tension in the cat retina.

Oxygen tension was recorded with double-barreled microelectrodes in the retina of anesthetized cats and monkeys. The oxygen tension (PO2) was often observed to undergo spontaneous irregular fluctuations with peak-to-peak amplitudes of about 3 to as much as 20 mm Hg. These fluctuations were observed only in the vascularized inner 40 to 50% of the retina, not in the avascular outer retina, and they were not correlated with the voltage recorded by the second barrel of the microelectrode. Oxygen records from the cat retina were subjected to Fourier analysis to determine their frequency content. The peak frequency was usually in the range of 3.5 to 10.5 c/min (0.06 to 0.18 Hz), considerably below the respiratory frequency of 20 to 30 c/min or the heart rate of 180 to 240 c/min. The power of the oxygen fluctuations fell gradually to less than 10% of the peak power by 60 c/min. It is suggested, based on the depth distribution and on comparison of these data with the literature on vasomotion, that the PO2 fluctuations are the result of changes in oxygen supply caused by normal fluctuations of arteriolar diameter (vasomotion) and/or capillary perfusion.

Animals↗
PubMed1992

Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia.

Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided approximately 91% of the photoreceptors' oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2's less than 60 mmHg the retinal circulation supplied approximately 10% of the oxygen. In the dark-adapted retina the decrease in choroidal PO2 caused a large decrease in photoreceptor oxygen consumption, from approximately 5.1 ml O2/100 g.min during normoxia to 2.6 ml O2/100 g.min at a PaO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was approximately 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects of hypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 +/- 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 +/- 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark-adapted retina, 30% of the values were less than 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2's less than 45 mmHg, and it was reduced less at these PaO2's in light adaptation.

Adaptation, Biological↗
PubMed1992

New perfluorocarbon emulsion improves tissue oxygenation in cat retina.

We have studied the conditions under which a perfluorocarbon emulsion of perfluorooctyl bromide (PFOB; Alliance Pharmaceutical, San Diego, CA) enhances tissue O2 delivery. Measurements of retinal tissue O2 tension (PO2) were made in anesthetized, artificially respirated, dark-adapted, normovolemic cats before, during, and after the infusion of three successive doses of 1 g PFOB/kg body wt each. There was little immediate effect of the infusion on the tissue PO2 when the cats were breathing room air, but the mean increase in tissue PO2 during 100% O2 breathing was 60 +/- 9% (SE; n = 8 cats) greater after infusion of 1 g PFOB/kg and approximately 136% greater after 3 g PFOB/kg. Similar infusions of the emulsifying medium alone had negligible effects on tissue PO2. These results suggest that PFOB emulsion may be clinically useful in treating tissue hypoxia in normovolemic patients breathing O2-enriched air.

Animals↗
PubMed1976

The oxytocin challenge test.

In a group of 152 high-risk pregnancies, 266 oxytocin challenge tests were reviewed. There were 137 patients with persistently negative oxytocin challenge tests with only one false-negative test being recorded. Fifteen patients who displayed a positive test at some time in their antepartum course are discussed along with other aspects of the oxytocin challenge test.

Adolescent↗