Venous air embolism in swine: transport of gas bubbles through the pulmonary circulation

The assumption that the lung is an effective filter for gas bubbles is of importance for certain occupations (e.g., divers, astronauts) as well as in the accomplishment of several medical procedures. The filtering capacity was tested in pigs by use of continuous air infusion into the right ventricle and a transesophageal echocardiographic transducer for detection of air in the left atrium. Twenty pigs, anesthetized with pentobarbital sodium and mechanically ventilated, were divided into groups that received air at infusion rates of 0.05 (group 1a, n = 7), 0.10 (group 2, n = 6), and 0.20 (group 3, n = 5) ml.kg-1.min-1. Two pigs served as controls. The breakthrough incidence was 0, 67, and 100%, respectively. Group 1a received a second infusion of 0.10 ml.kg-1.min-1 (group 1b, n = 7), and spillover of bubbles occurred in only 14% of these pigs. Infusion of gas caused a maximum increase in mean pulmonary arterial pressure (PAP) of 129 +/- 9% to 39.2 +/- 1.3 (SE) mmHg, with no significant difference between the groups. Breakthrough was observed only in animals with a dramatic reduction in mean arterial pressure and a PAP that returned to almost-normal values at spillover time. Our results suggest that the threshold value for breakthrough of air bubbles in pigs is reduced compared with that in dogs. The hemodynamic consequences at a given infusion rate are, however, greatly enhanced.     

Isolation and analysis of gas bubbles in the cavities of Azolla leaves

A procedure has been developed for the isolation of gas bubbles from the cavities of leaves of Azolla rubra. Mature leaves of exponentially growing Azolla plants were carefully cut under water with a razor blade. The gas bubbles in the cavities were driven out by a stream of water running from a glass capillary and collected in another capillary that was filled with distilled water. This allowed the collection of a sufficient number of gas bubbles for analysis. The concentration of oxygen in the gas obtained was slightly lower than that in the external air. The diffusion of external ¹⁵N₂ into the gas bubbles was low. The concentration of ¹⁵N₂ in gas bubbles was only 2% of that of the external air even after incubation of Azolla plants in ¹⁵N₂ in light or in darkness for 20 h. The method described here for isolation and analysis of gas bubbles should permit further studies of the properties of the gas bubbles in the cavities of Azolla leaves.     

Hyperbaric oxygen may reduce gas bubbles in decompressed prawns by eliminating gas nuclei.

It is accepted that gas bubbles grow from preexisting gas nuclei in tissue. The possibility of eliminating gas nuclei may be of benefit in preventing decompression sickness. In the present study, we examined the hypothesis that hyperbaric oxygen may replace the resident gas in the nuclei with oxygen and, because of its metabolic role, eliminate the nuclei themselves. After pretreatment with oxygen, prawns were 98% saturated with nitrogen before explosive decompression at 30 m/min. Ten transparent prawns were exposed to four experimental profiles in a crossover design: 1) 10-min compression to 203 kPa with air; 2) 10-min compression with oxygen; 3) 10-min compression with oxygen to 203 kPa followed by 12 min air at 203 kPa; and 4) 10 min in normobaric oxygen followed by compression to 203 kPa with air. Bubbles were measured after explosive decompression. We found that pretreatment with hyperbaric oxygen (profile C) significantly reduces the number of bubbles and bubble volume. We suggest that hyperbaric oxygen eliminates bubble nuclei in the prawn.     

Effect of combined recompression and air, oxygen, or heliox breathing on air bubbles in rat tissues.

The fate of bubbles formed in tissues during the ascent from a real or simulated air dive and subjected to therapeutic recompression has only been indirectly inferred from theoretical modeling and clinical observations. We visually followed the resolution of micro air bubbles injected into adipose tissue, spinal white matter, muscle, and tendon of anesthetized rats recompressed to and held at 284 kPa while rats breathed air, oxygen, heliox 80:20, or heliox 50:50. The rats underwent a prolonged hyperbaric air exposure before bubble injection and recompression. In all tissues, bubbles disappeared faster during breathing of oxygen or heliox mixtures than during air breathing. In some of the experiments, oxygen breathing caused a transient growth of the bubbles. In spinal white matter, heliox 50:50 or oxygen breathing resulted in significantly faster bubble resolution than did heliox 80:20 breathing. In conclusion, air bubbles in lipid and aqueous tissues shrink and disappear faster during recompression during breathing of heliox mixtures or oxygen compared with air breathing. The clinical implication of these findings might be that heliox 50:50 is the mixture of choice for the treatment of decompression sickness.     

The effect of influential outliers on parameter estimation in regression analysis.

A comment is made on an article recently published in this journal by Borgeat, Elie, and Castonguay (1991). It is noted that the raw data contained an outlier which is shown to have had a large influence on their estimation of the regression coefficient. Analysis of the data using a nonparametric statistic that is optimal for long-tailed distributions showed that the true regression coefficient is probably smaller than that reported. Other approaches to the analysis of data containing influential outliers are discussed.     

Hydrolysis of monodisperse phosphatidylcholines by phospholipase A2 occurs on vessel walls and air bubbles.

Hydrolysis of monodisperse short chain phosphatidylcholines, far below their critical micelle concentration, by phospholipase A2 (PLA2) and other interfacial enzymes is characterized. Results show that virtually all the observed hydrolysis by pancreatic and human inflammatory PLA2 occurs on surfaces of the reaction vessel or air bubbles. Conditions to eliminate such extraneous contributions at low substrate concentrations are established. Premicellar aggregates are apparently formed near the critical micelle concentration. The observation window at low substrate concentrations is used to obtain an upper limit estimate of the rate of hydrolysis through the monodisperse Michaelis complex. A limit estimate of <0.1 s-1 is obtained for the hydrolysis of monodisperse substrates by pig pancreatic phospholipase A2. These results show that the observed rate of hydrolysis of dihexanoyl- and diheptanoylphosphatidylcholines with pig pancreatic phospholipase A(2) through the monomer path is insignificant compared to the rate of >1000 s-1 seen at the saturating levels of the micellar substrate. These protocols should be useful for evaluating reactions catalyzed at vessel walls. Implications of these results for assays and models of interfacial activation of pancreatic PLA2 are discussed.     

Plethysmographic estimation of thoracic gas volume in apneic mice.

Electrical stimulation of intercostal muscles was employed to measure thoracic gas volume (TGV) during airway occlusion in the absence of respiratory effort at different levels of lung inflation. In 15 tracheostomized and mechanically ventilated CBA/Ca mice, the value of TGV obtained from the spontaneous breathing effort available in the early phase of the experiments (TGVsp) was compared with those resulting from muscle stimulation (TGVst) at transrespiratory pressures of 0, 10, and 20 cmH2O. A very strong correlation (r2= 0.97) was found, although with a systematically (approximately 16%) higher estimation of TGVst relative to TGVsp, attributable to the different durations of the stimulated (approximately 50 ms) and spontaneous (approximately 200 ms) contractions. Measurements of TGVst before and after injections of 0.2, 0.4, and 0.6 ml of nitrogen into the lungs in six mice resulted in good agreement between the change in TGVst and the injected volume (r2= 0.98). In four mice, TGVsp and TGVst were compared at end expiration with air or a helium-oxygen mixture to confirm the validity of isothermal compression in the alveolar gas. The TGVst values measured at zero transrespiratory pressure in all CBA/Ca mice [0.29 +/- 0.05 (SD) ml] and in C57BL/6 (N = 6; 0.34 +/- 0.08 ml) and BALB/c (N = 6; 0.28 +/- 0.06 ml) mice were in agreement with functional residual capacity values from previous studies in which different techniques were used. This method is particularly useful when TGV is to be determined in the absence of breathing activity, when it must be known at any level of lung inflation or under non-steady-state conditions, such as during pharmaceutical interventions.     

New trend in sample preparation: on-line microextraction in packed syringe for liquid and gas chromatography applications. I. Determination of local anaesthetics in human plasma samples using gas chromatography-mass spectrometry

A new technique for sample preparation on-line with LC and GC-MS assays was developed. Microextraction in a packed syringe (MEPS) is a new miniaturised, solid-phase extraction technique that can be connected on-line to GC or LC without any modifications. In MEPS approximately 1mg of the solid packing material is inserted into a syringe (100-250 microl) as a plug. Sample preparation takes place on the packed bed. The bed can be coated to provide selective and suitable sampling conditions. The new method is very promising. It is very easy to use, fully automated, of low cost and rapid in comparison with previously used methods. This paper presents the development and validation of a method for microextraction in packed syringe MEPS on-line with GC-MS. Local anaesthetics in plasma samples were used as model substances. The method was validated and the standard curves were evaluated by the means of quadratic regression and weighted by inverse of the concentration: 1/x for the calibration range 5-2000 nM. The applied polymer could be used more than 100 times before the syringe was discarded. The extraction recovery was between 60 and 90%. The results showed close correlation coefficients (R>0.99) for all analytes in the calibration range studied. The accuracy of MEPS-GC-MS was between 99 and 115% and the inter-day precision (n=3 days), expressed as the relative standard deviation (R.S.D.%), was 3-10%.     

Effect of hypobaric air, oxygen, heliox (50:50), or heliox (80:20) breathing on air bubbles in adipose tissue.

The fate of bubbles formed in tissues during decompression to altitude after diving or due to accidental loss of cabin pressure during flight has only been indirectly inferred from theoretical modeling and clinical observations with noninvasive bubble-measuring techniques of intravascular bubbles. In this report we visually followed the in vivo resolution of micro-air bubbles injected into adipose tissue of anesthetized rats decompressed from 101.3 kPa to and held at 71 kPa corresponding to approximately 2.750 m above sea level, while the rats breathed air, oxygen, heliox (50:50), or heliox (80:20). During air breathing, bubbles initially grew for 30-80 min, after which they remained stable or began to shrink slowly. Oxygen breathing caused an initial growth of all bubbles for 15-85 min, after which they shrank until they disappeared from view. Bubble growth was significantly greater during breathing of oxygen compared with air and heliox breathing mixtures. During heliox (50:50) breathing, bubbles initially grew for 5-30 min, from which point they shrank until they disappeared from view. After a shift to heliox (80:20) breathing, some bubbles grew slightly for 20-30 min, then shrank until they disappeared from view. Bubble disappearance was significantly faster during breathing of oxygen and heliox mixtures compared with air. In conclusion, the present results show that oxygen breathing at 71 kPa promotes bubble growth in lipid tissue, and it is possible that breathing of heliox may be beneficial in treating decompression sickness during flight.