Performance of breathing filters under wet conditions: a laboratory evaluation.

INTRODUCTION: Heat and moisture exchangers in combination with a bacterial and viral filter (HMEF) are widely used during general anaesthesia. Excess patient secretions occluding the HMEF have been responsible for previous case reports of airway obstruction. A previous study suggested that differences in HMEF design might contribute to filter obstruction under wet conditions. METHODS: We tested 14 types of HMEF under wet conditions to establish which design features contributed to HMEF obstruction. Incremental amounts of saline were added to each filter. The pressure across the filter was measured with an air flow of 60 litre min(-1). RESULTS: We observed that saline added to the filter was often not easily visible to the casual observer. This concealment volume varied between filters. Ceramic hydrophobic pleated-membrane filters did not absorb saline and their resistance did not change. The composite filter where the moisture exchange component was either polyurethane foam or cellulose absorbed saline and contributed to a rise in resistance of 70-480% with the higher value more typical of the cellulose-paper-based HMEF. CONCLUSION: The ideal HMEF for use during general anaesthesia should prevent the passage of viral, bacterial and prion material, should provide this filtration performance even under wet conditions, should supplement humidification of the inspired air and anaesthetic gases and should not increase respiratory work. We have identified large variations in HMEF performance under wet conditions. Users should be aware of performance variation in HMEFs and use a filter suited to the intended application.

Venus southern hemisphere: geologic character and age of terrains in the themis-alpha-lada region.

Arecibo high-resolution radar images of the southern hemisphere of Venus extending to 78 degrees S show that the surface of the Themis-Alpha-Lada region is characterized by linear deformation zones with volcanoes and corona-like features and by regional volcanic deposits (primarily plains, small shields, and large edifices). Large-scale areal deformation is limited to the tessera of Alpha Regio. Lada Terra, in the southern high latitudes, contains several large coronae, in contrast to Ishtar Terra in the northern high latitudes. The density of craters of possible impact origin is somewhat lower than that observed in the Venera 15 and 16 coverage; these data extend to 43 percent of the areas of the surface of Venus with ages of less than about 1 billion years.

Styles of volcanism on venus: new arecibo high resolution radar data.

Arecibo high-resolution (1.5 to 2 km) radar data of Venus for the area extending from Beta Regio to western Eisila Regio provide strong evidence that the mountains in Beta and Eisila Regiones and plains in and adjacent to Guinevere Planitia are of volcanic origin. Recognized styles of volcanism include large volcanic edifices on the Beta and Eisila rises related to regional structural trends, plains with multiple source vents and a mottled appearance due to the ponding of volcanic flows, and plains with bright features surrounded by extensive quasi-circular radar-dark halos. The high density of volcanic vents in the plains suggests that heat loss by abundant and widely distributed plains volcanism may be more significant than previously recognized. The low density of impact craters greater than 15 km in diameter in this region compared to the average density for the higher northern latitudes suggests that the plains have a younger age.

Reevaluation of patients on thyroxine therapy.

A 21-day withdrawal test was utilized at four primary care practice sites to reevaluate individuals on thyroxine therapy. Utilizing elevated thyroid stimulating hormone (TSH) concentrations as the standard for diagnosing hypothyroidism, 22 of 37 patients previously on thyroxine were found to be euthyroid. Of the 15 hypothyroid individuals identified, 13 had elevated serum TSH 21 days after withdrawal of thyroxine. Two individuals who had serum TSH concentrations in the normal range at 21 days became symptomatic at five and eight weeks, respectively, and were found to have elevated serum TSH. Several factors may account for the high percentage of euthyroid individuals being treated with thyroid medication. These include unreliability of symptoms as a basis for diagnosis of hypothyroidism, laboratory tests that are often difficult to interpret, the commonly held misconception that all forms of primary hypothyroidism are irreversible, the routine use of thyroxine by many surgeons after subtotal thyroidectomy, and the use of thyroxine to suppress diffuse or nodular goiter.