Gas Infusion Research Articles

Systemic activation of coagulation and fibrynolysis in a porcine model of serogroup A streptococcal shock.

In a porcine model of Gram-positive sepsis, 28 juvenile pigs were studied to evaluate the effect of a continuous infusion of live serogroup A streptococci (GAS) on the activation of coagulation and fibrinolysis. Plasma levels of thrombin-antithrombin (TAT) complexes, tissue plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) activities were measured using commercially available kits. The continuous infusion of GAS [(3-5) x 10(8) colony-forming units/kg per h] caused early signs of severe septicaemia in the pigs, with pulmonary hypertension, systemic hypotension, reduced cardiac output and liver hypoperfusion, ultimately leading to shock with a high mortality. There was a sequential and ordered activation of the coagulation, fibrinolytic and antifibrinolytic systems. GAS infusion induced a gradual, maximally 2.5-fold increase in plasma TAT levels. Plasma t-PA activity levels peaked at 2 h (nine-fold increase), whereas the peak of PAI-1 activity was delayed (eight-fold increase at 4 h). These findings are similar to changes observed during endotoxin infusion. This procoagulant state favours disseminated intravascular coagulation and microthrombus formation, ultimately threatening tissue viability.

The Effect of Timing of Intravenous Muscle Relaxant on the Quality of Double-Contrast Barium Enema

AIM: To determine whether the timing of buscopan administration during double-contrast barium enema examination (DCBE) affects diagnostic quality.MATERIALS AND METHODS: In a prospective setting, 100 consecutive adult out-patients referred for DCBE received 20mg buscopan (hyoscine-N-butylbromide) intravenously, either before infusion of barium suspension (Group A) or after barium infusion and gas insufflation (Group B). A subjective assessment of ease of contrast medium infusion was made at the time of examination and the films subsequently analysed by two radiologists unaware of the mode of relaxant administration, who noted the quality of mucosal coating and made subjective and objective measurements of segmental distension.RESULTS: There was no significant difference in screening times, infusion difficulty or colonic contrast medium coating between the two groups. Subjective assessment of distension of the caecum, ascending colon, transverse colon and rectum were not significantly different. Patients receiving intravenous relaxant after barium and gas infusion had less subjective descending (P=0.05) and sigmoid (P=0.04) colon distension, but there was no significant difference with respect to maximal bowel diameter in any of the segments measured.CONCLUSION: The timing of intravenous administration during DCBE is likely to have no significant effect on the diagnostic quality of the study.Elson, E. M. (2000). Clinical Radiology55, 395–397.

Growth and characterization of oriented cadmium sulphide nanocrystals under Langmuir-Blodgett monolayer of arachidic acid

Cadmium sulphide nanocrystals were grown at room temperature (20 °C) under arachidic acid monolayers floating over an aqueous solution of CdCl2 inside an enclosed Langmuir-Blodgett set-up, through slow infusion of H2S gas. X-ray diffraction spectra suggest an oriented growth of the crystallites. The particle sizes were found to increase with duration of exposure to the H2S gas. Atomic force microscopy indicated that the particles were nearly circular pellets with uniform morphology throughout. In Raman spectra, the FWHM of the LO phonon was found to be large (≈20 cm-1) for all the films grown with different exposure times in H2S gas, and was found to reduce to 8 cm-1 after annealing a typical sample at 500 °C for 45 min.

Ultrasonographic appearance of exogenous isobutane gas in the mammary glands of dairy cows

Objective To determine whether exogenous isobutane gas infused into the udders of dairy cattle could be detected ultrasonographically, and if so, what effects volume of gas infused and infusion pressure had on how long after infusion exogenous isobutane gas could be detected. Design Randomized block design. Animals 8 Holstein cows 28 to 32 days after parturition. Procedure In each cow, 1 mammary gland was not treated and the other 3 received 1 of 3 treatments by means of intramammary infusion: low volume-high pressure, low volume-low pressure, and high volume-high pressure infusion of isobutane gas. Mammary glands were examined ultrasonographically 1 hour before and 1,3, 12, 24, 36, 48, 72, and 96 hours after treatment. Results After intramammary infusion of isobutane gas, bright echoes and associated acoustic shadows were seen ultrasonographically; echoes were no longer seen 72 hours after gas infusion. Percentages of mammary glands in which bright echoes were detected were not significantly different among the 3 treatment groups at any time during the study. Conclusions and Clinical Relevance Results suggest that exogenous isobutane gas infused into the mammary glands to enhance the appearance of the udder of show dairy cattle can be readily detected by ultrasonography. (J Am Vet Med Assoc 1999;215:366–368)

Transient adhesion of platelets in pump-oxygenator systems: influence of SMA and nitric oxide treatments

We employed gamma scintigraphy to quantify the transient accumulations of platelets in pump-oxygenator systems employed in cardiopulmonary bypass (CPB). A flat sheet microporous polypropylene membrane oxygenator (Cobe Duo) was employed, with and without siloxane/caprolactone oligomer coating (SMA) (n = 8 each). The effect of nitric oxide gas infusion on platelet deposition was also evaluated for the uncoated Cobe Duo system (n = 10 each). Scintigraphic images of radiolabelled cells were obtained and converted to numbers of all platelets, labeled and unlabeled, adhering to the pump and oxygenator surfaces. These numbers were compared, by study group, for a 90-min period of normothermic CPB in the adult pig, employing standard prime and anticoagulation regimens. Platelets adhered in large numbers to control oxygenators, reaching maxima (> 20% of the circulating platelet mass) 30 min following institution of CPB, and decreasing for the duration of CPB. SMA treatment significantly decreased platelet adhesion following a 5-10-min transient accumulation period. Nitric oxide infusion significantly reduced platelet adhesion throughout the CPB period. Platelet accumulations on the high fluid shear centrifugal pump surfaces increased monotonically to maxima at about the same time as for the oxygenators, but did not decrease thereafter. Higher platelet surface densities were observed on the centrifugal pump surfaces than on the oxygenator surfaces. CPB with the untreated circuit tended to reduce circulating platelet counts vs theoretical values based on hemodilution alone. In contrast, SMA significantly increased the circulating platelet count versus the untreated control group. These results indicate that platelet adherence to the foreign surfaces of CPB equipment are influenced in characteristic ways by time and fluid shear. SMA treatment and nitric oxide infusion both reduce platelet adhesion to oxygenator surfaces. SMA treatment spares these cells for the circulation.

Nitric oxide gas infusion to the oxygenator enhances the biocompatibility of heparin coated extracorporeal bypass circuits.

Heparin coated bypass circuits have been reported to improve the biocompatibility of extracorporeal circulation, although it is still insufficient and improvable. Nitric oxide (NO) is known to inhibit platelet activation and inflammatory reactions. In this study, the authors evaluated exogenous NO infusion in enhancing the effect of a heparin coated bypass circuit on the biocompatibility of an extracorporeal circuit, especially in view of the attenuation of the inflammatory response. A miniature closed bypass circuit, including an oxygenator (BioActive surface; Carmeda, Stockholm, Sweden) was primed with fresh human heparinized blood and perfused with a centrifugal pump. Either pure N2 gas (control group: n = 7) or NO gas (NO group [100 ppm in N2]: n = 7) was infused to the oxygenator. NO metabolites (nitrite and nitrate), platelet count, thrombin-antithrombin III complex (TAT), alpha2-plasmin-plasminogen inhibitor complex (PIC), beta-thromboglobulin (beta-TG), platelet factor 4 (PF4), serotonin, complement 3 activation products (C3a), granulocyte elastase, and bradykinin were measured at 0, 30, 60, 120, and 180 min after starting perfusion. At every sampling point, platelet counts were significantly higher, and TAT, beta-TG, and bradykinin were lower in the NO group than in the control group. PF4, C3a, and granulocyte elastase were significantly lower in the NO group at 60, 120, and 180 min. These results suggest that NO gas infusion to the oxygenator enhances the biocompatibility of heparin coated extracorporeal circuits.

Intestinal gas dynamics and tolerance in humans

Background & Aims: Abdominal symptoms are often attributed to intestinal gas. In humans, gas production and composition have been previously investigated, but intestinal gas dynamics and tolerance remain virtually unknown. The aim of this study was to establish the relationship between intestinal gas loads, evacuation, perception of symptoms, and objective abdominal distention in healthy humans. Methods: A dose-response study was performed in 46 healthy subjects; intestinal gas was infused for 3 hours (0, 1, 4, 12, and 30 mL/min), and anal gas evacuation, symptom perception, and abdominal distention were measured. A mixture of gases was infused in venous proportions to minimize diffusion. Anal gas recovery and calculations of gas retention were validated using sulfur hexafluoride as a nonabsorbable gaseous marker. Results: At all of the infusion rates, gas evacuation rapidly equilibrated and paralleled gas infusion without significant differences in perception. Only 6 subjects retained >400 mL gas, and 5 of them developed abdominal distention and symptoms. By contrast, all but 4 of the 41 subjects without retention tolerated the gas loads without discomfort. Conclusions: Intestinal gas tolerance is normally high, because expeditious gas transit and evacuation prevent gas pooling and symptoms. When this protective mechanism is insufficient, gas retention occurs, and it causes abdominal symptoms and distention. GASTROENTEROLOGY 1998;115:542-550

Mechanisms of gastro-oesophageal reflux in the ferret.

Transient lower oesophageal sphincter (LOS) relaxation is the major mechanism of gastro-oesophageal reflux in humans--an event unassociated with swallowing. Mechanisms involved in triggering transient LOS relaxation are poorly understood, and their further study requires a small animal model. In this study we aimed to establish methods for prolonged ambulant oesophageal manometry in ferrets, and to determine motor events associated with reflux episodes and their triggering by different gastric nutrient loads. Forty-two studies were performed on nine ferrets with chronic cervical oesophagostomies, through which a manometric assembly was introduced and secured to a collar, which incorporated a microphone for detection of swallows. The assembly included a gastric feeding channel, one gastric and four oesophageal manometric sideholes, a 2.5-cm-long LOS sleeve sensor, and an oesophageal pH electrode. Intragastric infusions were given over 2 min, the first after a 30-min control recording period, and in 29/42 studies, a second infusion was given 60 min later. Infusions were either 25 mL 10% dextrose solution, pH 3.5 (22 studies), 25 mL triglyceride emulsion (Intralipid) pH 3.5 (11 studies), or 25 mL air (nine studies). Episodes of oesophageal acidification were absent before gastric infusions. After infusion, 2.1 +/- 0.2 episodes occurred over the first 30 min. After glucose infusion, 15/18 acidification episodes (83%) occurred during transient LOS relaxation, and 3/18 (17%) occurred after gradual (< 1 mmHg sec-1) downward drifts in basal LOSP to < 2 mmHg. After lipid infusion two acidification episodes occurred, both during transient LOS relaxation. Mean duration of transient LOS relaxation was 8.0 +/- 0.4 sec. All infusions increased occurrence of transient LOS relaxation to a similar extent, each of which ended with primary peristalsis. We conclude that gastric infusion of glucose, lipid and gas are all effective in provoking gastro-oesophageal reflux in ferrets. Reflux occurs through similar mechanisms to those seen in humans, i.e. increased triggering of transient LOS relaxation. The conscious ferret is therefore an appropriate model for future studies of manipulation of mechanisms giving rise to gastro-oesophageal reflux.

Similarities and Differences in the Microstructure of Attritor-milled Fe–Al–N Compositions

Although numerous studies of high-energy, ball-milled metal powders have been conducted, to date few studies have characterized the mechanical processing of identical elemental compositions of prealloyed powders and of powder blends. This study reports on the mechanical processing (attritor ball milling) in argon and nitrogen gas environments of (a) iron powder and prealloyed iron–2 wt.% aluminum powder, and (b) iron-aluminum, iron-aluminum nitride, and iron-iron nitride powder blends. When nitrogen was milled into iron particles either from nitride powder or by gas infusion, the nitrogen dissolved interstitially in bcc-Fe (principally at the grain boundaries) or was present as bct-Fe nanoparticles at the bcc-Fe nanograin boundaries. The resulting nitrogen distribution was independent of how the nitrogen was added. Milled blends of iron and aluminum powder and prealloyed iron-aluminum powder resulted in similar microstructures: micrometer size particles with similar nanograin size. The aluminum in the blended powder mixture developed an ultrafine distribution on the grain boundaries, but it did not become uniformly distributed within the bcc-Fe grains. In contrast, the aluminum in prealloyed Fe–Al powder remained in solid solution during the mechanical milling

Cardiopulmonary responses to intravenous infusion of soluble and relatively insoluble gases.

Carbon dioxide is the current gas of choice for pneumoperitoneum, but hemodynamic and acid-base effects secondary to its systemic absorption have been reported. Various studies have suggested inert gases as alternatives. We studied the cardiopulmonary responses to intravenous infusion of carbon dioxide, nitrous oxide, argon, helium, and nitrogen in anesthetized swine. The gas was infused into the femoral vein at a rate of 0.1 ml . kg-1 . min-1 for 30 min. The changes in end-tidal CO2, mean arterial pressure, hemodynamics, and arterial blood gases were compared to baseline values. No animals died during infusion of the soluble gases (CO2 and N2O). Three of the five pigs infused with nitrogen died suddenly at 20 and 30 min of infusion. The animals in the insoluble gas groups (Ar, He, N2) experienced clinical pulmonary gas embolism and severe acidemia, hypercapnea and tachycardia. Venous gas embolism is poorly tolerated when the gas is relatively insoluble. Insoluble gases should not be used for pneumoperitoneum when there is any risk of venous gas embolism.

Platelet and neutrophil distributions in pump oxygenator circuits. III. Influence of nitric oxide gas infusion.

The authors used quantitative gamma scintigraphy to evaluate the influence of nitric oxide gas on platelet and neutrophil deposition in Cobe Duo microporous oxygenators during cardiopulmonary bypass (CPB). The effects of nitric oxide gas on circulating platelet and neutrophil counts and platelet function also were assessed. Animals were prepared by standard methods. Cells were harvested, labeled (111 In platelet and 99mTc neutrophil), infused, and recirculated. Nitric oxide gas, a guanylate cyclase pathway promoter, was infused int he Duo gas port at 500 ppm (t = 0-60 min), increased to 1,000 (t = 60-80 min), and stopped (final, 10 min). Images were taken at 10-15 min intervals during CPB. Standard isotope image corrections were made. No differences between nitric oxide gas and control experiments were observed for flow, pressure, hematocrit, or replacement volume. Nitric oxide gas infusion significantly (p < 0.05) reduced both platelet adherence to the oxygenator and in vitro platelet aggregation. Neutrophil adhesion tended to be lower, and circulating platelet and neutrophil counts tended to be higher with nitric oxide gas infusion. Results of in vitro aggregometry studies using rabbit platelets indicate that the class V phosphodiesterase inhibitor zaprinast can strongly enhance the inhibitory effects of nitric oxide. The authors conclude nitric oxide gas is a promising platelet sparing agent in the setting of CPB.

Response to intraluminal gas in irritable bowel syndrome

Our aim was to evaluate the response to intraluminal gas in irritable bowel syndrome and to determine whether this response was consequent upon disordered motility or altered perception. We evaluated 10 patients who satisfied the clinical criteria for the diagnosis of irritable bowel syndrome and 10 healthy controls. An eight-lumen perfused catheter assembly was positioned to monitor motor activity in the duodenum and proximal jejunum; a separate side port in the distal duodenum permitted gas infusion. Subjects recorded symptoms of abdominal pain, bloating, and nausea throughout the study, using a visual analog scale. Following an overnight fast and a 60-min basal recording period in the fasted state, subjects ate a standard meal; 60 min later, "sham" gas was administered for 20 min, followed by the actual infusion of nitrogen gas at 40 ml/min. Subjects were randomized to receive atropine (7 micrograms/kg) or placebo intravenously during the period of actual gas infusion. Patients with irritable bowel syndrome described more pain (score, mean +/- SE, control versus irritable bowel: 0.22 +/- 0.16 vs 1.65 +/- 0.5, P < 0.01) and nausea (0.25 +/- 0.21 vs 1.45 +/- 0.64, P < 0.04) during sham gas; motility indices were similar in both groups. During active gas, irritable bowel syndrome patients reported more pain (0.40 +/- 0.39 vs 2.94 +/- 1.16, P < 0.03); motility indices at all sites were similar in both groups. Symptom severity in irritable bowel syndrome subjects randomized to receive atropine was similar to control subjects during active gas infusion; motility indices were similar.(ABSTRACT TRUNCATED AT 250 WORDS)

An indirect calorimetry system for ventilator dependent very low birthweight infants.

With neurodevelopmental outcome of very low birthweight (VLBW) infants being adversely affected by inadequate nutrition during the first few weeks of life, there is an urgent need for more specific nutritional data on the sick VLBW ventilator dependent infant. The development of a new mass spectrometry gas analysis indirect calorimetry system which is non-invasive and can operate over several hours or days is described. Technical evaluation of each of the components of the system indicates a total random error of less than 5%. Systematic error was determined using gas infusions which simulated carbon dioxide production and oxygen consumption. The relative error in the measurement of carbon dioxide production was less than or equal to 1.5% (coefficient of variation (CV) 6.0%)) with carbon dioxide infusion rates ranging from 3.86 to 13.98 ml/min. The relative error in oxygen 'consumption' was less than or equal to 4.3% (CV 2.8%) for infusions of oxygen at rates of 7.5 to 14.80 ml/min. With nitrogen infusions simulating oxygen consumptions of 2.0 and 5.5 ml/min the relative error in the calculated nitrogen infusion was 1.5% (CV 4.1%) and 1.4% (CV 5.7%) respectively. Clinical studies on 10 infants demonstrated a mean energy expenditure of 161.7 kJ/kg/day and a respiratory quotient in excess of 1.0. The energy expenditure of ventilated VLBW infants may be less than previously indicated and the energy mix and nitrogen content of parenteral nutrition regimens recommended for these infants may be inappropriate.

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.

The use of carbon dioxide gas to displace flowing blood during angioscopy

Carbon dioxide (CO2) gas is a safe and effective arterial contrast agent that images arteries by displacing blood. The use of CO2 gas to displace blood during angioscopy may improve intraoperative angioscopy and allow percutaneous angioscopy. To study this, 46 angioscopic evaluations in the femoral arteries of four dogs were done without inflow occlusion. The ability of CO2 gas to clear the flowing blood was compared to a high-pressure infusion of saline solution. Carbon dioxide gas successfully displaced flowing blood in 20 of 25 evaluations (80%) as compared to three of 21 evaluations (14%) with infusion of saline solution (p < 0.0001). After the CO2 gas infusion was stopped, the image remained clear for 9 ± 1 seconds. When saline was used, the image was clear only as long as the infusion was continued. The quality of clear images for the two media were similar through the eyepiece of the angioscope, but images observed through the motion picture camera had better contrast and depth of field in CO2 gas. The use of a CO2 gas infusion can clear the field of flowing blood without the risk of volume overloading and potentially allow performance of a successful percutaneous angioscopy.

Simulating Norwegian troll gas prospects in a competitive spatial model

The 1986 Troll gas agreement between Statoil and a European consortium of gas buyers is the largest ever signed in Europe. Uncertainty surrounds this most expensive North Sea gas to date with its long lead time and dependence on an unstable oil market. Uncertain contract profitability is a key concern extending to both sides of the market. Importing and exporting governments as well as those more directly involved have a keen interest in deriving the bounds such profits might have. We provide quantitative evidence on profitability for these decision makers by simulating this infusion of gas into the European gas market using a spatial model that includes both gas and oil explicitly. The model simultaneously solves for equilibrium gas prices at demand and supply regions, given transportation costs as well as supply and demand assumptions in the oil and gas markets. The model generates gas prices under alternative assumptions about oil prices; own and cross price elasticities for gas and oil; income growth and elasticities; as well as the behavior of Norway's major competitors. Combining these prices with cost information, we compute a range of rates of return for the project that vary from 7.4% to 24.8%. We find these rates of return to be highly dependent on oil prices, income growth, income elasticities, and alternative supplies of gas, but much less dependent on own and cross price elasticities for gas. Using a probability distribution derived from oil price forecasts, we find expected real rates of return to be 15.9% to 21.9%. We conclude that under our model assumptions, if contract prices are adjusted to market forces, the Troll contract appears to be quite promising.

Ultrastructural Studies on the Change of Eosinophil Granules in the Lung of Guinea Pigs with Ethylene Oxide Gas Infusion into the Subcutaneous Tissue

Ultrastructural Studies on the Change of Eosinophil Granules in the Lung of Guinea Pigs with Ethylene Oxide Gas Infusion into the Subcutaneous Tissue

Modification of maternal and fetal oxygenation with the use of tracheal gas infusion

A new technique is described for producing changes in fetal blood gases in the chronically instrumented pregnant sheep. Gas mixtures were infused directly into the maternal trachea. Maternal and fetal carotid arterial blood gases and pH were measured. Air infusion at 16 L · min−1 produced no change. Oxygen infusion caused significant increases in maternal PaO2 at 2 L·min−1 and in fetal PaO2 at 4 L·min−1. Nitrogen infusion significantly decreased maternal and fetal PaO2 at 4 L·min−1. During 4 L·min−1 oxygen infusion, maternal PaO2 rose rapidly to plateau at 314 ± 47 mm Hg at 4 minutes and fetal PaO2 rose to plateau at 28.7 ± 2.8 mm Hg after 7 to 8 minutes. Maternal PaO2 fell to 56.4 ± 4.3 mm Hg during nitrogen infusion (4 L·min−1) while fetal PaO2 fell to 15.9 ± 1.8 mm Hg. Continuous infusion for 5- to 6-hour periods produced a consistent rise in maternal PaO2 during oxygen infusion and a consistent decrease during nitrogen infusion. Tracheal infusion of gases can be used to change maternal and fetal PaO2 rapidly and predictably.

Determinants of hypoxemia during the acute phase of pulmonary embolism in humans.

The determinants of hypoxemia were studied in 10 patients with acute pulmonary embolism demonstrated by pulmonary angiography. Two patients were mechanically ventilated, and in the 8 who breathed room air spontaneously, the mean arterial PO2 was 61.5 mmHg. Measurements of the distributions of ventilation (VA) and perfusion (Q) against VA/Q ratios by the multiple inert gas infusion technique demonstrated an increase in VA/Q inequality. The major part of pulmonary blood flow was distributed in a mode near to, or slightly above, a VA/Q ratio of 1. The cumulative fraction of blood in true shunt and low VA/Q mode (VA/Q less than 0.01) was 9.1%. For a small part of the AaDO2 (13%), an oxygen diffusional component was found. The remaining hypoxemia was due to the fall in the mixed venous PO2 (PVO2), irrespective of its cause: low cardiac output, low hemoglobin concentration, high oxygen consumption, low P50. The fall in PVO2 led to a fall in end-capillary blood PO2 in both shunt or ventilated and perfused units. We conclude that the major determinant of hypoxemia in these patients suffering from acute pulmonary embolism is the fall in PVO2. This is enhanced by a moderate increase in the fraction of blood flowing through low VA/Q units. Diffusion impairment plays only a minor role.

An experimental study of gas exchange in laminar oscillatory flow

Experiments were conducted to determine the effective diffusivity for axial transport through a tube of circular cross-section of a contaminant gas in oscillatory flow. Results were compared with the theoretical predictions of Watson (1983) and found to be in excellent agreement. The experiments differ from the theoretical situation in that the oscillations are superimposed upon a steady flow due to a constant infusion of tracer gas, and a buoyancy-induced flow associated with spatial variations in gas density. The influence of both artifacts is found to be negligible.