Surfactant Attenuates Air Embolism-Induced Lung Injury by Suppressing NKCC1 Expression and NF-κB Activation

Excessive amounts of air can enter the lungs and cause air embolism (AE)-induced acute lung injury (ALI). Pulmonary AE can occur during diving, aviation, and iatrogenic invasive procedures. AE-induced lung injury presents with severe hypoxia, pulmonary hypertension, microvascular hyper-permeability, and severe inflammatory responses. Pulmonary AE-induced ALI is a serious complication resulting in significant morbidity and mortality. Surfactant is abundant in the lungs and its function is to lower surface tension. Earlier studies have explored the beneficial effects of surfactant in ALI; however, none have investigated the role of surfactant in pulmonary AE-induced ALI. Therefore, we conducted this study to determine the effects of surfactant in pulmonary AE-induced ALI. Isolated-perfused rat lungs were used as a model of pulmonary AE. The animals were divided into four groups (n = 6 per group): sham, air embolism (AE), AE + surfactant (0.5 mg/kg), and AE+ surfactant (1 mg/kg). Surfactant pretreatment was administered before the induction of pulmonary AE. Pulmonary AE was induced by the infusion of 0.7 cc air through a pulmonary artery catheter. After induction of air, pulmonary AE was presented with pulmonary edema, pulmonary microvascular hyper-permeability, and lung inflammation with neutrophilic sequestration. Activation of NF-κB was observed, along with increased expression of pro-inflammatory cytokines, and Na-K-Cl cotransporter isoform 1 (NKCC1). Surfactant suppressed the activation of NF-κB and decreased the expression of pro-inflammatory cytokines and NKCC1, thereby attenuating AE-induced lung injury. Therefore, AE-induced ALI presented with pulmonary edema, microvascular hyper-permeability, and lung inflammation. Surfactant suppressed the expressions of NF-κB, pro-inflammatory cytokines, and NKCC1, thereby attenuating AE-induced lung injury.

Air bubble contact with endothelial cells causes a calcium-independent loss in mitochondrial membrane potential

OBJECTIVE

Gas microembolism remains a serious risk associated with surgical procedures and decompression. Despite this, the signaling consequences of air bubbles in the vasculature are poorly understood and there is a lack of pharmacological therapies available. Here, we investigate the mitochondrial consequences of air bubble contact with endothelial cells.

METHODS AND RESULTS

Human umbilical vein endothelial cells were loaded with an intracellular calcium indicator (Fluo-4) and either a mitochondrial calcium indicator (X-Rhod-1) or mitochondrial membrane potential indicator (TMRM). Contact with 50-150 µm air bubbles induced concurrent rises in intracellular and mitochondrial calcium, followed by a loss of mitochondrial membrane potential. Pre-treating cells with 1 µmol/L ruthenium red, a TRPV family calcium channel blocker, did not protect cells from the mitochondrial depolarization, despite blocking the intracellular calcium response. Mitigating the interactions between the air-liquid interface and the endothelial surface layer with 5% BSA or 0.1% Pluronic F-127 prevented the loss of mitochondrial membrane potential. Finally, inhibiting protein kinase C-α (PKCα), with 5 µmol/L Gö6976, protected cells from mitochondrial depolarization, but did not affect the intracellular calcium response.

CONCLUSIONS

Our results indicate that air bubble contact with endothelial cells activates a novel, calcium-independent, PKCα-dependent signaling pathway, which results in mitochondrial depolarization. As a result, mitochondrial dysfunction is likely to be a key contributor to the pathophysiology of gas embolism injury. Further, this connection between the endothelial surface layer and endothelial mitochondria may also play an important role in vascular homeostasis and disease.

Carbonic anhydrase activities from the rainbow trout lens correspond to the development of acute gas bubble disease

Dissolved gas supersaturation is hazardous to fish and can result in gas bubble disease (GBD). Signs of GBD typically include bubbles in the eyes, fins, skin, lateral line, and gill filaments. Ocular abnormalities in diseased salmonids typically occur after aberrant gas production in the eyes. In this study, freshwater rainbow trout Oncorhynchus mykiss were exposed experimentally to percent total gas pressure (TGP%) levels of 104% (control) and 115%. No mortalities occurred during the 7-d experimental period. Effects of GBD were observed externally as a darkened skin, exophthalmia, localized hemorrhage in the eye, and gas bubbles on the operculum. Additional signs included increased swimming activity and, more frequently, panic episodes. Carbonic anhydrase (CA) enzyme activities from the lens and retina were determined at days 0, 1, 3, 5, and 7 of the study. Venous blood gases were also measured on day 7. Retinal pH did not differ between normal and affected fish, but blood characteristics such as the partial pressure of O2, partial pressure of CO2, carboxyhemoglobin level, and bicarbonate ion concentration were significantly elevated in affected fish relative to normal fish. Venous blood pH and oxyhemoglobin levels were not significantly different between affected and normal fish. Patterns of response to total dissolved gas levels differed between the lens and the retina. Mean CA activities in the lenses of fish exposed to a TGP% level of 115% were significantly below those of control fish. However, retinal CA activities did not significantly differ between the two groups over the course of the experiment. These findings show that dissolved gas supersaturation reduces CA activity in the rainbow trout lens.

Air bubble contact with endothelial cells in vitro induces calcium influx and IP3-dependent release of calcium stores

Gas embolism is a serious complication of decompression events and clinical procedures, but the mechanism of resulting injury remains unclear. Previous work has demonstrated that contact between air microbubbles and endothelial cells causes a rapid intracellular calcium transient and can lead to cell death. Here we examined the mechanism responsible for the calcium rise. Single air microbubbles (50-150 μm), trapped at the tip of a micropipette, were micromanipulated into contact with individual human umbilical vein endothelial cells (HUVECs) loaded with Fluo-4 (a fluorescent calcium indicator). Changes in intracellular calcium were then recorded via epifluorescence microscopy. First, we confirmed that HUVECs rapidly respond to air bubble contact with a calcium transient. Next, we examined the involvement of extracellular calcium influx by conducting experiments in low calcium buffer, which markedly attenuated the response, or by pretreating cells with stretch-activated channel blockers (gadolinium chloride or ruthenium red), which abolished the response. Finally, we tested the role of intracellular calcium release by pretreating cells with an inositol 1,4,5-trisphosphate (IP3) receptor blocker (xestospongin C) or phospholipase C inhibitor (neomycin sulfate), which eliminated the response in 64% and 67% of cases, respectively. Collectively, our results lead us to conclude that air bubble contact with endothelial cells causes an influx of calcium through a stretch-activated channel, such as a transient receptor potential vanilloid family member, triggering the release of calcium from intracellular stores via the IP3 pathway.

Nitric oxide modulates air embolism-induced lung injury in rats with normotension and hypertension

1. Air embolism the in lungs induces microvascular obstruction, mediator release and acute lung injury (ALI). Nitrite oxide (NO) plays protective and pathological roles in ALI produced by various causes, but its role in air embolism-induced ALI has not been fully investigated. 2. The purpose of the present investigation was to elucidate the involvement of NO and pro-inflammatory cytokines in the pathogenesis of ALI following air infusion into isolated perfused lungs from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. 3. The extent of ALI was evaluated by changes in lung weight, Evans blue dye leakage, the protein concentration in the bronchoalveolar lavage and pathological examination. We also measured nitrite/nitrate (NO(x)), tumour necrosis factor (TNF)-alpha and interleukin (IL)-1beta concentrations in lung perfusate and determined cGMP in lung tissue. 4. The NO synthase (NOS) inhibitors N(G)-nitro-l-arginine methyl ester (l-NAME) and l-N(6)-(1-iminoethyl)-lysine (l-Nil), as well as the NO donors sodium nitroprusside (SNP) and s-nitroso-N-acetylpenicillamine (SNAP), were administered 30 min before air embolism at a concentration of 10(-3) mol/L in the lung perfusate. 5. Air embolism-induced ALI was enhanced by pretreatment with l-NAME or l-Nil, but was alleviated by SNP or SNAP pretreatment, in both SHR and WKY rats. In both SHR and WKY rats, AE elevated levels of NO(x) (2.6 and 28.7%, respectively), TNF-alpha (52.7 and 158.6%, respectively) and IL-1beta (108.4 and 224.1%, respectively) in the lung perfusate and cGMP levels in lung tissues (35.8 and 111.2%, respectively). Pretreatment with l-LAME or l-Nil exacerbated, whereas SNP or SNAP abrogated, the increases in these factors, except in the case of NO(x) (levels were decreased by l-LAME or l-Nil pretreatment and increased by SNP or SNAP pretreatment). 6. Air embolism caused increases in the lung weight (LW)/bodyweight ratio, LW gain, protein concentration in bronchoalveolar lavage and Evans blue dye leakage. These AE-induced changes were less in lungs isolated from SHR compared with normotensive WKY rats. 7. The results suggest that ALI and associated changes following air embolism in lungs isolated from SHR are less than those in WKY rats. Nitric oxide production through inducible NOS isoforms reduces air embolism-induced lung injury and associated changes. Spontaneously hypertensive rats appear to be more resistant than WKY rats to air embolism challenge.

Effect of heliox, oxygen and air breathing on helium bubbles after heliox diving

In helium saturated rat abdominal adipose tissue, helium bubbles were studied at 101.3 kPa during breathing of either heliox(80:20), 100% oxygen or air after decompression from an exposure to heliox at 405 kPa for one hour. While breathing heliox bubbles initially grew for 15-115 minutes then shrank slowly; three out of 10 bubbles disappeared in the observation period. During oxygen breathing all bubbles initially grew for 10-80 minutes then shrank until they disappeared from view; in the growing phase, oxygen caused faster growth than heliox breathing, but bubbles disappeared sooner with oxygen breathing than with heliox or air breathing. In the shrinking phase, shrinkage is faster with heliox and oxygen breathing than with air breathing. Air breathing caused consistent growth of all bubbles. With heliox and oxygen breathing, most animals survived during the observation period but with air breathing, most animals died of decompression sickness regardless of whether the surrounding atmosphere was helium or air. If recompression beyond the maximum treatment pressure of oxygen is required, these results indicate that a breathing mixture of heliox may be better than air during the treatment of decompression sickness following heliox diving.

Increased expired NO and roles of CO2 and endogenous NO after venous gas embolism in rabbits

Venous gas embolism (VGE) is a feared complication in diving, aviation, surgery and trauma. We hypothesized that air emboli in the lung circulation might change expired nitric oxide (FeNO). A single intravenous infusion of air was given (100 mul kg(-1)) to three groups of anaesthetized mechanically ventilated rabbits: (A) one with intact NO production, (B) one with intact NO production and where end-tidal CO(2) was controlled, and (C) one with endogenous NO synthesis blockade (L: -NAME, 30 mg kg(-1)). Air infusions resulted in increased FeNO of the control group from 20 (4) [mean (SD)] ppb to a peak value of 39 (4) ppb within 5 min (P < 0.05), and FeNO was still significantly elevated [27 (2) ppb] after 20 min (P < 0.05). Parallel to the NO increase there were significant decreases in end-tidal CO(2 )(ETCO(2)) and mean arterial pressure and an increase in insufflation pressure. In group B, when CO(2) was supplemented after air infusion, NO was suppressed (P = 0.033), but was still significantly elevated compared with pre-infusion control (P < 0.05). In group C, all animals died within 40 min of air infusion whereas all animals in the other groups were still alive at this time point. We conclude that venous air embolization increases FeNO, and that a part of this effect is due to the concomitant decrease in ETCO(2). Furthermore, an intact NO production may be critical for the tolerance to VGE. Finally, FeNO might have a potential in the diagnosis and monitoring of pulmonary gas embolism.

Role of the Endothelin System in Secondary Pulmonary Hypertension Related to Air Embolism: Lessons Learned from Testing Four Classes of Endothelin Blockers in a Rat Model

A rat model of acute pulmonary air embolism (APAE) was developed. These animals had a higher right ventricular systolic pressure (RVSP) (+ 69% at 15-minute peak, and 21-34% at 30-180 minutes), as well as a reduced mean arterial blood pressure (10-20% at 60-180 minutes), heart rate (20-26% at 60-180 minutes) and PaO2 (9-11% at 30-180 minutes) compared with control rats. The role of the endothelin (ET) system, known to be involved in pulmonary hypertension of various etiologies, was investigated by evaluating the effect of the four classes of ET blockers: ET-converting enzyme inhibitor (ECEi) (CGS 35066), selective endothelin-A receptor antagonist (ETA-Ra) (Atrasentan, ABT-627), endothelin-B receptor antagonist (ETB-Ra) (A-192621) or mixed endothelin-A/endothelin-B receptor antagonist (ETA/B-Ra) (A-182086) in this animal model. All four were effective, to various degrees, at reducing the APAE-induced rise in RVSP. The relative efficacy of those compounds in reducing the acute elevation (15 minutes) of RVSP was ECEi >or= ETA/B-Ra >> ETA-Ra = ETB-Ra. The sustained elevation (30-180 minutes) of RVSP was totally abolished by ECEi and attenuated by other ET blockers with a relative efficacy of ETA-Ra > ETA/B-Ra >or= ETB-Ra. ET receptor antagonists did not affect right ventricular basal tone (control rats) whereas ECEi reduced it by up to 12% after 2 hours. The APAE reduction in mean arterial blood pressure was unaffected by ETARa, was completely normalized by ETB-Ra, but was further reduced by either ETA/B-Ra or ECEi. The basal mean arterial blood pressure in control rats was unaffected by ETA-Ra, was elevated by ETB-Ra, but was depressed by ETA/B-Ra and ECEi. All ET blockers maintained normal oxygen saturation in APAE. These results support a role for ETs in rat APAE, since ET blockers can attenuate the cardiopulmonary deterioration and blood gas exchange. However, modulation of the central hemodynamic profile is more complex and may limit the usefulness of some ET blockers.

Surfactants Attenuate Gas Embolism-induced Thrombin Production

Background

There are no pharmacologic strategies to prevent embolism bubble-induced blood clot formation. The authors conducted experiments to measure thrombin production in sheared whole blood in the presence and absence of bubbles and three surface-active compounds.

Methods

Blood samples were obtained from six volunteers seven times. The thrombin-specific substrate Boc-VPR-MCA was added to citrated blood diluted with HEPES-buffered saline. Experimental groups were as follows: sparging (air microbubble embolization) with surfactant present; sparging alone; surfactant alone; and neither surfactant nor sparging. The surfactants were Dow Corning Antifoam 1510US, Perftoran, and Pluronic F-127. Blood was sheared by a cone-plate viscometer at 100 and 500 s-1 for 5, 10, and 20 min at 37 degrees C, pipetted into excess stop buffer, and evaluated fluorimetrically. Mean values of fluorescence intensity +/- SDs for each group were compared using ANOVA. Differences were considered significant at P &lt; 0.05 using the Bonferroni correction.

Results

For fixed shear rate, thrombin production increased 2.3- to 5.7-fold (P &lt; 0.05) as shear duration lengthened. For fixed shear duration, thrombin production increased 1.9- to 3.9-fold (P &lt; 0.05) with increasing shear rate. For fixed shear rate and duration, sparging increased thrombin production 2.1- to 3.7-fold (P &lt; 0.05). Surfactant addition without sparging did not change thrombin production (P &gt; 0.05). Surfactants attenuated thrombin production in sparged samples 31.8-70.9% (P &lt; 0.05).

Conclusions

Thrombin production is shear rate and duration-dependent. Sparging increases thrombin production. Surfactants added before sparging attenuate thrombin production. Surfactants may have a clinical application to attenuate gas embolism-induced clotting.

Model predictions of gas embolism growth and reabsorption during xenon anesthesia

BACKGROUND

It is not readily obvious whether an intravascular bubble will grow or shrink in a particular tissue bed. This depends on the constituent gases initially present in the bubble, the surrounding tissue, and the delivered gas admixture. The authors used a computational model based on the physics of gas exchange to predict cerebrovascular embolism behavior during xenon anesthesia.

METHODS

The authors estimated values of gas transport parameters missing from the literature. The computational model was used with those parameters to predict bubble size over time for a range of temperatures (18 degrees -39 degrees C) used during extracorporeal circulation.

RESULTS

Bubble size over time is highly nonlinearly dependent on multiple factors, including diffusivity, solubility, gas partial pressures, magnitude of concentration gradients, vessel diameter, and temperature. Xenon- and oxygen-containing bubbles continue to grow during xenon delivery. Bubble volume doubles from 50 to 100 nl in approximately 3-68 min, depending on initial gas composition and bubble shape. Bubble growth and reabsorption are relatively insensitive to temperature in the physiologic and surgical range.

CONCLUSIONS

Xenon anesthesia results in gas exchange conditions that favor bubble growth, which may worsen neurologic injury from gas embolism. The concentration gradients can be manipulated by discontinuation of xenon delivery to promote reabsorption of xenon-containing bubbles. Estimated growth and reabsorption rates at normothermia can be applied to temperature extremes of cardiopulmonary bypass.

Effects of cerebral air embolism on brain metabolism in pigs

OBJECTIVES

Cerebral air embolism was induced in pigs and changes in intracranial pressure (ICP), brain oxygen (PbrO2), brain carbon dioxide (PbrCO2), brain pH (brpH) and glucose, lactate and pyruvate levels were used to characterize this model.

METHODS

In seven anesthetized pigs, ICP, PbrO2, PbrCO2 and brpH were measured continuously with multiparameter sensors and brain glucose metabolism by microdialysis. After injection of air into the internal carotid artery, these parameters were recorded for 2 h.

RESULTS

ICP increased (433%) from 12 +/- 1 to 52 +/- 8 mmHg (P < 0.05). PbrO2 decreased from 25.7 +/- 6.2 to 11.9 +/- 5.2 mmHg. PbrCO2 increased (109%) from 57.7 +/- 2.7 to 120.4 +/- 21.5 mmHg (P < 0.05). Brain glucose decreased (38%) from 3.05 +/- 0.91 to 1.91 +/- 0.55 mmol, while brain lactate increased (384%) from 1.36 +/- 0.15 to 5.22 +/- 0.53 mmol/l (P < 0.05).

CONCLUSIONS

Cerebral air embolism has a deleterious effect on ICP and brain metabolism. Therefore, this model may be suitable for testing therapeutic regimens in cerebral air embolism.

Reduced perivascular PO2 increases nitric oxide release from endothelial cells

Many studies have suggested that endothelial cells can act as "oxygen sensors" to large reductions in oxygen availability by increasing nitric oxide (NO) production. This study determined whether small reductions in oxygen availability enhanced NO production from in vivo intestinal arterioles, venules, and parenchymal cells. In vivo measurements of perivascular NO concentration ([NO]) were made with NO-sensitive microelectrodes during normoxic and reduced oxygen availability. During normoxia, intestinal first-order arteriolar [NO] was 397 +/- 26 nM (n = 5), paired venular [NO] was 298 +/- 34 nM (n = 5), and parenchymal cell [NO] was 138 +/- 36 nM (n = 3). During reduced oxygen availability, arteriolar and venular [NO] significantly increased to 695 +/- 79 nM (n = 5) and 534 +/- 66 nM (n = 5), respectively, whereas parenchymal [NO] remained unchanged at 144 +/- 34 nM (n = 4). During reduced oxygenation, arteriolar and venular diameters increased by 15 +/- 3% and 14 +/- 5%, respectively: NG-nitro-L-arginine methyl ester strongly suppressed the dilation to lower periarteriolar Po2. Micropipette injection of a CO2 embolus into arterioles significantly attenuated arteriolar dilation and suppressed NO release in response to reduced oxygen availability. These results indicated that in rat intestine, reduced oxygen availability increased both arteriolar and venular NO and that the main site of NO release under these conditions was from endothelial cells.

Effects of a selective endothelin A receptor antagonist, ABT-627, in healthy normotensive anaesthetized rats developing acute pulmonary air embolism

Acute pulmonary air embolism (APAE) injures the vascular endothelium in the lung and results in pulmonary hypertension (PH). Endothelins (ETs), a family of potent vasoactive peptides, are known to be associated with PH of various aetiologies. We evaluated the effects of ABT-627, a selective ET(A) receptor (ET(A)-R) antagonist in a rat model of APAE over 3 h. APAE rats developed a higher right ventricular systolic pressure (RVSP), lower mean arterial blood pressure (MABP), and had lower PaO(2). At 3 h, arterial plasma levels of ET-1 were increased. ABT-627-treated controls showed no effects. However, ABT-627 significantly lowered RVSP during APAE, abolished the short recovery phase (within 10-25 min) of MABP without affecting the subsequent lowering of MABP, and improved oxygen saturation in APAE rats. These results show that ET(A)-R subtype is involved in the pathogenesis of APAE since a blockade of this receptor subtype attenuated the cardiopulmonary deterioration and improved blood gas exchanges in rats with this disease.

Role of endothelial nitric oxide and smooth muscle potassium channels in cerebral arteriolar dilation in response to acidosis

BACKGROUND AND PURPOSE

Potassium channels or nitric oxide or both are major mediators of acidosis-induced dilation in the cerebral circulation. However, these contributions depend on a variety of factors such as species and vessel location. The present study was designed to clarify whether potassium channels and endothelial nitric oxide are involved in acidosis-induced dilation of isolated rat cerebral arterioles.

METHODS

Cerebral arterioles were cannulated and monitored with an inverted microscope. Acidosis (pH 6.8 to 7.4) produced by adding hydrogen ions mediated dilation of the cerebral arterioles in a concentration-dependent manner. The role of nitric oxide and potassium channels in response to acidosis was examined with several specific inhibitors and endothelial damage.

RESULTS

The dilation was significantly inhibited by potassium chloride (30 mmol/L) and glibenclamide (3 micromol/L; ATP-sensitive potassium channel inhibitor). We found that 30 micromol/L BaCl2 (concentration-dependent potassium channel inhibitor) also affected the dilation; however, an additional treatment of 3 micromol/L glibenclamide did not produce further inhibition. Tetraethylammonium ion (1 mmol/L; calcium-activated potassium channel inhibitor) and 4-aminopyridine (100 micromol/L; voltage-dependent potassium channel inhibitor) as well as ouabain (10 micromol/L; Na-K ATPase inhibitor) and N-methylsulphonyl-6-(2-proparglyloxyphenyl) hexanamide (1 micromol/L; cytochrome P450 epoxygenase inhibitor) did not alter acidotic dilation. N(omega)-Monomethyl-L-arginine (10 micromol/L) and N(omega)-nitro-L-arginine (10 micromol/L) as nitric oxide synthase inhibitor blunted the dilation. Furthermore, the dilation was significantly attenuated after the endothelial impairment. Additional treatment with glibenclamide (3 micromol/L) further reduced the dilation in response to acidosis.

CONCLUSIONS

Endothelial nitric oxide and smooth muscle ATP-sensitive potassium channels contribute to acidosis-induced dilation of rat cerebral arterioles. Endothelial damage caused by pathological conditions such as subarachnoid hemorrhage or traumatic brain injury may contribute to reduced blood flow despite injury-induced cerebral acidosis.

Hemodilution during venous gas embolization improves gas exchange, without altering V(A)/Q or pulmonary blood flow distributions

BACKGROUND

Isovolemic anemia results in improved gas exchange in rabbits with normal lungs but in relatively poorer gas exchange in rabbits with whole-lung atelectasis. In the current study, the authors characterized the effects of hemodilution on gas exchange in a distinct model of diffuse lung injury: venous gas embolization.

METHODS

Twelve anesthetized rabbits were mechanically ventilated at a fixed rate and volume. Gas embolization was induced by continuous infusion of nitrogen via an internal jugular venous catheter. Serial hemodilution was performed in six rabbits by simultaneous withdrawal of blood and infusion of an equal volume of 6% hetastarch; six rabbits were followed as controls over time. Measurements included hemodynamic parameters and blood gases, ventilation-perfusion (V(A)/Q) distribution (multiple inert gas elimination technique), pulmonary blood flow distribution (fluorescent microspheres), and expired nitric oxide (NO; chemoluminescence).

RESULTS

Venous gas embolization resulted in a decrease in partial pressure of arterial oxygen (PaO2) and an increase in partial pressure of arterial carbon dioxide (PaCO2), with markedly abnormal overall V(A)/Q distribution and a predominance of high V(A)/Q areas. Pulmonary blood flow distribution was markedly left-skewed, with low-flow areas predominating. Hematocrit decreased from 30+/-1% to 11+/-1% (mean +/- SE) with hemodilution. The alveolar-arterial PO2 (A-aPO2) difference decreased from 375+/-61 mmHg at 30% hematocrit to 218+/-12.8 mmHg at 15% hematocrit, but increased again (301+/-33 mmHg) at 11% hematocrit. In contrast, the A-aPO2 difference increased over time in the control group (P < 0.05 between groups over time). Changes in PaO2 in both groups could be explained in large part by variations in intrapulmonary shunt and mixed venous oxygen saturation (SvO2); however, the improvement in gas exchange with hemodilution was not fully explained by significant changes in V(A)/Q or pulmonary blood flow distributions, as quantitated by the coefficient of variation (CV), fractal dimension, and spatial correlation of blood flow. Expired NO increased with with gas embolization but did not change significantly with time or hemodilution.

CONCLUSIONS

Isovolemic hemodilution results in improved oxygen exchange in rabbits with lung injury induced by gas embolization. The mechanism for this improvement is not clear.

Theoretical and experimental intravascular gas embolism absorption dynamics

Multifocal cerebrovascular gas embolism occurs frequently during cardiopulmonary bypass and is thought to cause postoperative neurological dysfunction in large numbers of patients. We developed a mathematical model to predict the absorption time of intravascular gas embolism, accounting for the bubble geometry observed in vivo. We modeled bubbles as cylinders with hemispherical end caps and solved the resulting governing gas transport equations numerically. We validated the model using data obtained from video-microscopy measurements of bubbles in the intact cremaster microcirculation of anesthetized male Wistar rats. The theoretical model with the use of in vivo geometry closely predicted actual absorption times for experimental intravascular gas embolisms and was more accurate than a model based on spherical shape. We computed absorption times for cerebrovascular gas embolism assuming a range of bubble geometries, initial volumes, and parameters relevant to brain blood flow. Results of the simulations demonstrated absorption time maxima and minima based on initial geometry, with several configurations taking as much as 50% longer to be absorbed than would a comparable spherical bubble.

Fatty acid composition of plasma lipids and erythrocyte membranes during simulated extravehicular activity

Ten subjects (from 27 to 41 years) have been participated in 32 experiments. They were decompressed from ground level to 40-35 kPa in altitude chamber when breathed 100% oxygen by mask and performed repeated cycles of exercises (3.0 Kcal/min). The intervals between decompressions were 3-5 days. Plasma lipid and erythrocyte membrane fatty acid composition was evaluated in the fasting venous blood before and immediately after hypobaric exposure. There were 7 cases decompression sickness (DCS). Venous gas bubbles (GB) were detected in 27 cases (84.4%). Any significant changes in the fatty acid composition of erythrocyte membranes and plasma didn't practically induce after the first decompression. However, by the beginning of the second decompression the total lipid level in erythrocyte membranes decreased from 54.6 mg% to 40.4 mg% in group with DCS symptoms and from 51.2 mg% to 35.2 mg% (p<0.05) without DCS symptoms. In group with DCS symptoms a tendency to increased level of saturated fatty acids in erythrocyte membranes (16:0, 18:0), the level of the polyunsaturated linoleic fatty acid (18:2) and arachidonic acid (20:4) tended to be decreased by the beginning of the second decompression. Insignificant changes in blood plasma fatty acid composition was observed in both groups. The obtained biochemical data that indicated the simulated extravehicular activity (EVA) condition is accompanied by the certain changes in the blood lipid metabolism, structural and functional state of erythrocyte membranes, which are reversible. The most pronounced changes are found in subjects with DCS symptoms.

Another portable quantitative capnometer

A new portable quantitative capnometer (Bruker CO2 Module) was tested in an animal lab (mini-pigs) during experiments on air embolism. The end-tidal CO2 values, as measured with this device, showed good agreement with the values of a stationary capnography unit. This device seems suited for quantitative capnometry in the prehospital setting.

Role of metabolic gases in bubble formation during hypobaric exposures

Our hypothesis is that metabolic gases play a role in the initial explosive growth phase of bubble formation during hypobaric exposures. Models that account for optimal internal tensions of dissolved gases to predict the probability of occurrence of venous gas emboli were statistically fitted to 426 hypobaric exposures from National Aeronautics and Space Administration tests. The presence of venous gas emboli in the pulmonary artery was detected with an ultrasound Doppler detector. The model fit and parameter estimation were done by using the statistical method of maximum likelihood. The analysis results were as follows. 1) For the model without an input of noninert dissolved gas tissue tension, the log likelihood (in absolute value) was 255.01. 2) When an additional parameter was added to the model to account for the dissolved noninert gas tissue tension, the log likelihood was 251.70. The significance of the additional parameter was established based on the likelihood ratio test (P < 0.012). 3) The parameter estimate for the dissolved noninert gas tissue tension participating in bubble formation was 19. 1 kPa (143 mmHg). 4) The additional gas tissue tension, supposedly due to noninert gases, did not show an exponential decay as a function of time during denitrogenation, but it remained constant. 5) The positive sign for this parameter term in the model is characteristic of an outward radial pressure of gases in the bubble. This analysis suggests that dissolved gases other than N2 in tissues may facilitate the initial explosive bubble-growth phase.

Computer simulation of microscopic cerebral air emboli absorption during cardiac surgery

Microscopic cerebral arterial air emboli (MCAAE) cause neurologic injury during cardiac surgery. We used a mathematical model of gas absorption to gain a preliminary assessment of what physical or physiologic parameters affect MCAAE absorption in the setting of cardiac surgery with its unique set of normal values. Simulated MCAAE of radii 50 and 200 microns have absorption times of 2 and 32 min, respectively. Predicted absorption times depend dramatically on PaN2. MCAAE are predicted to be absorbed twice as quickly at a PaN2 of 0 vs. 380 mmHg (FiO2 approximately equal to 0.50). Moderate hypothermia (27 degrees C) is predicted to cause only small decreases in absorption time. Changes in cerebral blood flow (for example, as affected by hemoglobin concentration, PaCO2, PaO2, collateral circulation, anesthetics, or cerebral metabolism) probably have only small effects on absorption time. Intravascular perfluorocarbons are predicted to cause small-to-moderate decreases in absorption time. In conclusion, there is probably only one important determinant of MCAAE absorption time during normothermic or moderately hypothermic CPB: arterial nitrogen partial pressure.

Recommendations for hyperbaric oxygen therapy of cerebral air embolism based on a mathematical model of bubble absorption

Transcranial doppler studies show that microscopic cerebral artery air emboli (CAAE) are present in virtually all patients undergoing cardiac surgery. Massive cerebral arterial air embolism is rare. If it occurs, hyperbaric oxygen therapy (HBO) is recommended as soon as surgery is completed. We used a mathematical model to predict the absorption time of CAAE, assuming that the volumes of clinically relevant CAAE vary from 10(-7) to at least 10(-1) mL. Absorption times are predicted to be at least 40 h during oxygenation using breathing gas mixtures of fraction of inspired oxygen approximately equal to 40%. When CAAE are large enough to be detected by computerized tomography, absorption times are calculated to be at least 15 h. Decreases in cerebral blood flow caused by the CAAE would make the absorption even slower. Our analysis suggests that if the diagnosis of massive CAAE is suspected, computerized tomography should be performed, and consideration should be given to HBO therapy if the CAAE are large enough to be visualized, even if patient transfer to a HBO facility will require several hours.

Effects of carbon dioxide embolism with nitrous oxide in the inspired gas in piglets

We have compared cardiorespiratory variables in anaesthetized piglets whose lungs were ventilated with oxygen in nitrous oxide (N2O group) or nitrogen (N group) after right ventricular carbon dioxide boluses (0.5 or 1 ml kg-1; n = 12) or slow graded injections (n = 6). Boluses affected all variables studied significantly (P < 0.05) except mean systolic arterial pressure. Significant changes in PE'CO2 (P = 0.012) and PaO2 (P = 0.048) values were observed in the N2O group. Changes in PaCO2 were related to volumes of injected carbon dioxide (P = 0.044). Boluses of 1.0 ml kg-1 induced severe circulatory collapse in two piglets in the N2O group. Slow embolization altered respiratory variables significantly (P < 0.001)). PaO2 decreased significantly in the N2O group (P < 0.0001). Mean pulmonary arterial pressure increased significantly over time (P = 0.001) and lasted longer in the N2O group (P < 0.05). Volumes and time required to induce a 50% increase in mean pulmonary arterial pressure differed significantly between groups (P < 0.05). We conclude that nitrous oxide worsened the effects of rapid and slow carbon dioxide emboli on cardiopulmonary variables. Rapid carbon dioxide embolism altered respiratory and haemodynamic variables, while slow carbon dioxide embolism changed only respiratory variables.

Kinetics of elimination and acute consequences of cerebral air embolism

The pathophysiology of arterial air embolism inducing brain injuries remains unclear. Previous experiments demonstrated the usefulness of computed tomography (CT) in the detection of air emboli in canine brain. This canine study investigates CT's ability to detect small air bubbles and to determine the kinetics of air elimination from cerebral arteries and its relationship with clinical, electroencephalographic (EEG), and histological manifestations. CT detects small air embolism, and intracerebral air volume strongly correlates with injected air dose (r2 = 0.86, p = 2 x 10(-3)). Air clearance time significantly depends on intracerebral air volume (r2 = 0.86, p = 0.04) and on the number of bubbles (r2 = 0.71, p = 0.03), whereas half-life of air elimination does not. No relationship was found between injected air dose, air clearance time, intracerebral volume of air, and clinical, EEG, and histological findings. The data indicate that CT accurately detects small air bubbles in the early course of cerebral air embolism, that air elimination from cerebral arteries follows a first-order compartment model, and that early CT findings do not correlate with clinical, EEG, and histological manifestations.

Simulation of gas bubbles in hypobaric decompressions: roles of O2, CO2, and H2O

To gain insight into the special features of bubbles that may form in aviators and astronauts, we simulated the growth and decay of bubbles in two hypobaric decompressions and a hyperbaric one, all with the same tissue ratio (TR), where TR is defined as tissue PN2 before decompression divided by barometric pressure after. We used an equation system which is solved by numerical methods and accounts for simultaneous diffusion of any number of gases as well as other major determinants of bubble growth and absorption. We also considered two extremes of the number of bubbles which form per unit of tissue.

RESULTS

A) Because physiological mechanisms keep the partial pressures of the "metabolic" gases (O2, CO2, and H2O) nearly constant over a range of hypobaric pressures, their fractions in bubbles are inversely proportional to pressure and their large volumes at low pressure add to bubble size. B) In addition, the large fractions facilitate the entry of N2 into bubbles, and when bubble density is low, enhance an autocatalytic feedback on bubble growth due to increasing surface area. C) The TR is not closely related to bubble size; that is when two different decompressions have the same TR, metabolic gases cause bubbles to grow larger at lower hypobaric pressures. We conclude that the constancy of partial pressures of metabolic gases, unimportant in hyperbaric decompressions, affects bubble size in hypobaric decompressions in inverse relation to the exposure pressure.

Severe pulmonary oedema after venous air embolism

We present a 59-yr-old Chinese male patient who developed acute pulmonary oedema and cardiovascular collapse following multiple episodes of venous air emboli while in the sitting position for removal of a cervical meningioma. The severity of the pulmonary oedema and cardiovascular disturbance were surprising. Postoperative ventilation and inotropic support were required and five litres of plasma were needed to replace the fluid lost as pulmonary oedema. We discuss the differential diagnosis of the pulmonary changes and review current ideas on the pathogenesis for pulmonary oedema following venous air embolism.

Insulin-like growth factor I and pulmonary hypertension induced by continuous air embolization in sheep

Chronic pulmonary hypertension is associated with arterial structural remodeling. Insulin-like growth factor I (IGF-I) has been proposed as one of the mediators of vascular change because of its ability to stimulate proliferation in, and elastin production by, cultured vascular smooth muscle cells. We have shown previously that 12 days of continuous air embolization into the pulmonary arterial circulation of sheep results in the functional and structural changes of chronic pulmonary hypertension. In the present study, measurements of IGF-I (by radioimmunoassay) and IGF-I binding protein activity in sheep lung lymph and plasma were made before and during the 12 days of air embolization in six sheep. Two untreated animals served as controls. Baseline lung lymph contained 23.5 +/- 3.6 ng/ml (mean +/- SEM) of IGF-I, and there was a slight increase to 36.7 +/- 9.8 on day 3, but by day 6 levels were back to baseline. The flux of IGF-I from the lung (concentration times lymph flow) increased significantly by day 2 embolization and remained elevated through day 12 (baseline = 37.2 +/- 11.1 ng/15 min; day 2 = 237.7 +/- 55.8; day 5 = 190.2 +/- 53.4; day 6 = 82.6 +/- 21.9; day 12 = 78.7 +/- 12.5). IGF-I binding protein activity was also present in lung lymph at baseline (29.6 +/- 3.0%) and was unchanged during air embolization. Plasma levels of IGF-I and plasma binding protein activity remained at baseline throughout the 12 days of embolization (71.51 +/- 34.48 ng/ml and 36.4 +/- 3.5%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Fate of air emboli in the pulmonary circulation

The lung serves an important nonrespiratory function by trapping and excreting venous air emboli. The site of trapping and the mechanism of excretion, however, are uncertain. To observe the behavior of bubbles in the pulmonary circulation, we injected venous air emboli into anesthetized dogs and videotaped their elimination from the pulmonary microcirculation by using in vivo microscopy. Small intravenous bubbles lodged exclusively in pulmonary arterioles and were eliminated from that site. To determine whether the gas was dissolving into nearby blood and then was carried to the capillaries for excretion, the rate of bubble radius change was measured during nonperfused conditions produced by balloon occlusion of lobar blood flow and compared with perfused conditions. Bubble volume decreased at the same rate during perfused and nonperfused conditions and thus was independent of regional blood flow. Molecular diffusion of gas directly across the arteriolar wall into alveolar spaces was the most likely mechanism of elimination because calculations based on the Fick equation for molecular diffusion predict an elimination rate nearly identical with those observed experimentally.

Sites of leakage in three models of acute lung injury

Fluid leaking from arterial and venous extra-alveolar vessels (EAV's) may account for up to 60% of the total transvascular fluid flux when edema occurs in the setting of normal vascular permeability. We determined if the permeability and relative contribution of EAV's was altered after inducing acute lung injury in rabbits by administering oleic acid (0.1 ml/kg) into the pulmonary artery, HCl (5 ml/kg of 0.1 N) into the trachea, or air emboli (0.03 ml.kg-1.min-1) into the right atrium for 90 min. Subsequently, the lungs were excised and continuously weighed while they were maintained in a warmed, humidified chamber with alveolar and pulmonary vascular pressures controlled and the lungs either ventilated or distended with 5% CO2 in air. The vascular system was filled with autologous blood and saline (1:1) to which papaverine (0.1 mg/ml) was added to inhibit vasospasm. Vascular pressures were referenced to the lung base. After a transient hydrostatic stress to maximize recruitment, vascular pressures were set at 5 cmH2O, and lungs were allowed to become isogravimetric (30-60 min). A fluid filtration coefficient (Kf) was determined by the use of a modification of the method of Drake and colleagues [Am. J. Physiol. 234 (Heart Circ. Physiol. 3): H266-H274, 1978]. EAV's were isolated by zoning techniques. In control preparations arterial and venous EAV's accounted for 26% (n = 9) and 38% (n = 11) of the total leakage, respectively. In all three models Kf increased two- to fourfold when the lungs were in zone 3 (alveolar vessels and arterial and venous EAV's contributing to the leakage).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thromboxane synthase inhibition on air emboli lung injury in sheep

We tested the effects of OKY-046, a thromboxane synthase inhibitor, on lung injury induced by 2 h of pulmonary air infusion (1.23 ml/min) in the pulmonary artery of unanesthetized sheep with chronic lung lymph fistula so as to assess the role of thromboxane A2 (TxA2) in the lung injury. We measured pulmonary hemodynamic parameters and the lung fluid balance. The concentrations of thromboxane B2 (TxB2) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) in plasma and lung lymph were determined by radioimmunoassay. Air infusion caused sustained pulmonary hypertension and an increase in pulmonary vascular permeability. The levels of TxB2 and 6-keto-PGF1 alpha in both plasma and lung lymph were significantly elevated during the air infusion. TxB2 concentration in plasma obtained from the left atrium was higher than that from the pulmonary artery at 15 min of air infusion. When sheep were pretreated with OKY-046 (10 mg/kg iv) prior to the air infusion, increases in TxB2 were prevented. The pulmonary arterial pressure, however, increased similarly to that of untreated sheep (1.8 X base line). The increase in lung lymph flow was significantly suppressed during the air infusion. Our data suggest that the pulmonary hypertension observed during air embolism is not caused by TxA2.

Arterial air embolism: structural effects on the gerbil brain

Air injection into the carotid artery of adult mongolian gerbils caused, within 10 minutes, multifocal brain lesions. The extracellular spaces were widened and neurons, oligodendrocytes and myelin sheaths remained unchanged. The "delayed" effects of air embolism (first seen after 3 h) were similar to those observed in gerbils after unilateral carotid ligation. The histologic alterations after 3 h consisted of astrocytic swelling and shrinkage/necrosis of neuronal soma. The observations reported here illustrate the temporal and spatial separations that exist between a) brain water retention, and b) intraparenchymal entry of horseradish peroxidase. Both alterations can be a consequence of either decreased blood flow or arterial air embolism. Edema and protein leakage in each situation may be initiated by different mechanisms.

Regional distribution of circulating microspheres in the femur of the rabbit

In an attempt to explain the distribution of lesions of caisson disease of bone in the human femur, the regional distribution of circulating microspheres which had been labelled with scandium-46 was studies in the femur of the rabbit. Microspheres with a diameter of 15 microns were equally distributed between the two ends of the bone and between the upper and lower halves of the shaft. However, microspheres with a diameter of 50 microns congregated in the upper end of the femur and in the lower half of the shaft, the two sites most commonly affected by caisson disease. A large percentage of the microspheres in the shaft, especially the larger spheres, were retained in the marrow. It is suggested that the microcirculation of the marrow may act as a filter and that the nature and distribution of its vessels determine the site of impaction of circulating emboli. This would explain why lesions of the shaft mainly affect the medulla of the bone and not the cortex.

Blood-brain barrier dysfunction to peroxidase after air embolism, aggravated by acute ethanol intoxication

Ethanol-intoxicated and non-intoxicated rats were injected with horseradish peroxidase and Evan's blue-labelled albumin and given a small air embolus in the right common carotid artery after ligation of the external carotid branch. In both ethanol-intoxicated and non-intoxicated rats, some endothelial cells, mainly in arteries and arterioles, showed a diffuse distribution of peroxidase in the cytoplasm. In some arterioles with a diffuse endothelial distribution of peroxidase there was a detachment of endothelial cells from the vessel wall, with an exposure of the adluminal basement membrane to blood elements. This endothelial detachment was mainly observed in ethanol-intoxicated rats. The vascular basement membranes underlying detached endothelial cells contained peroxidase, both in ethanol-intoxicated and in non-intoxicated rats. There was a considerable leakage of peroxidase via endothelial pinocytotic vesicles into the vascular basement membranes, mainly in arterioles, but also in capillaries and venules of the embolised hemisphere. This transendothelial pinocytotic transport of peroxidase was more prominent in ethanol-intoxicated than in non-intoxicated rats.