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

Does CO 2 flushing of the empty CPB circuit decrease the number of gaseous emboli in the prime?

Twenty (20) CPB-circuits were randomized to a CO2 group or a control group. In the CO 2 group, each circuit was flushed with CO2 (10L/min) at the top of the venous reservoir for 5 minutes, after which priming fluid was added without interruption of the CO2 inflow. Control group circuits were not flushed and contained air. A perfusionist, blinded to the study, started the pump (5L/min), ventilated the oxygenator (3L O2/min), and knocked on the oxygenator 20 times during the first and 14th minutes. Arterial line microemboli counts were registered with a Doppler for 15 minutes. In both groups, the median number of microemboli was highest during the first minute, 380.5 (288.75/422.25, 25th/75th percentile) counts in the control group versus 264.5 (171.75/422.25) counts in the CO 2 group (p=0.01). Throughout the experiment, the median microembolic count minute by minute in the CO2 group remained lower (p≤ 0 .004) than in the control group. Knocking on the reservoir (14th minute) increased the microemboli counts in both groups (p<0.01). The median values during the 15th minute were 15.5 and 0.5 in the control and the CO2 groups, respectively, which were 9% (15.5/173) and 0.5% (0.5/87), respectively, of the values registered after 14 minutes. In conclusion, CO 2 flushing of the empty circuit decreases the number of gaseous emboli in the prime compared with a conventional circuit that contains air before being primed with fluid. Knocking of the oxygenator releases gaseous emboli and the duration of re-circulating the circuit with prime influences the number of microemboli.

Pulmonary Air Embolization Inhibits Lung Lymph Flow by Increasing Lymphatic Outflow Pressure

BACKGROUND

Air embolization of the pulmonary vascular tree increases pulmonary microvascular filtration and induces pulmonary edema formation. Flow from cannulated pulmonary lymphatic vessels increases significantly following air embolization. However, in the intact animal, lymph flows into the venous system and the magnitude of lymph flow is directly affected by systemic venous pressure. We hypothesized that pulmonary air embolization would lead to systemic venous hypertension and that this increase in lymphatic outflow pressure would prevent an increase in pulmonary lymph flow.

METHODS AND RESULTS

Pulmonary air embolization was induced in dogs under general anesthesia. Flow from cannulated pulmonary lymphatic vessels was recorded for lymphatic outflow pressure set equal to atmospheric pressure (Q(LA)) and for outflow pressure set equal to systemic venous pressure (Q(LV)) both before and after embolization. Air embolization resulted in significant increases in systemic venous pressure from 6.4 +/- 0.3 to 12.4 +/- 1.2 mm Hg and in QLA from 48 +/- 9 to 175 +/- 29 microL . min(1). However, embolization did not increase Q(LV) (10 +/- 2 vs. 3 +/- 3 microl . min(1)).

CONCLUSIONS

Pulmonary air embolization impedes pulmonary lymph flow by increasing systemic venous pressure and, thereby, contributes to pulmonary edema formation.

Effect of BM-573 [N-Terbutyl-N′-[2-(4′-methylphenylamino)-5-nitro-benzenesulfonyl]urea], a Dual Thromboxane Synthase Inhibitor and Thromboxane Receptor Antagonist, in a Porcine Model of Acute Pulmonary Embolism

The aim of this study was to evaluate the effect of BM-573 [N-terbutyl-N'-[2-(4'-methylphenylamino)-5-nitro-benzenesulfonyl]urea], a dual thromboxane A2 synthase inhibitor and receptor antagonist, on the hemodynamic response to acute pulmonary embolism. Six anesthetized pigs were infused with placebo (placebo group) and compared with six other pigs receiving a continuous infusion of BM-573 (BM group). Pulmonary embolization with 0.3 g/kg autologous blood clots was carried out 30 min after the start of the infusion. Right ventricular pressure-volume loops were recorded using a conductance catheter, and end-systolic ventricular elastance was periodically assessed by varying right ventricular preload. Pulmonary vascular properties were studied by use of a four-element windkessel model. Hemodynamic data, including assessment of right ventricular-arterial coupling, were collected at baseline and every 30 min for 4 h. Blood samples were collected to assess gas exchange, thromboxane A2, and prostacyclin plasma levels and to evaluate platelet aggregation. Mean pulmonary arterial pressure in the placebo group increased significantly more than in the BM group, mainly because of an additional increase in pulmonary vascular resistance. Arterial and end-systolic ventricular elastances increased also more in the placebo group, whereas right ventricular efficiency decreased. BM-573 prevented both platelet aggregation induced by U-46619 (9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F2alpha) or by arachidonic acid, and thromboxane A2 overproduction, whereas prostacyclin liberation was preserved. Oxygenation, however, was not significantly improved. We conclude that in this animal model of acute pulmonary embolism, infusion of BM-573 reduced pulmonary vasoconstriction. As a result, right ventricular-vascular coupling values were maintained at a maximal efficiency level.

Vagal and mediator mechanisms underlying the tachypnea caused by pulmonary air embolism in dogs

We investigated the vagal and mediator mechanisms underlying the tachypnea caused by pulmonary air embolism (PAE) in anesthetized and spontaneously breathing dogs. PAE was induced by infusion of air into the right atrium (0.2 ml. kg(-1). min(-1) for 10 min). The first PAE induction caused an increase in respiratory frequency accompanied by a decrease in tidal volume in each of the 30 animals studied. Subsequently, animals were evenly divided into five groups, and a second PAE induction was repeated after various experimental interventions. The tachypneic response to PAE was not significantly altered by pretreatment with a saline vehicle but was largely attenuated by either perivagal capsaicin treatment (a technique that selectively blocks the conduction of unmyelinated C fibers), pretreatment with ibuprofen (a cyclooxygenase inhibitor), or pretreatment with dimethylthiourea (a hydroxyl radical scavenger). Ultimately, the tachypneic response was nearly abolished by a bilateral cervical vagotomy. These results suggest that 1) lung vagal unmyelinated C-fiber afferents play a predominant role in evoking the reflex tachypneic response to PAE and 2) both cyclooxygenase products and hydroxyl radical are important in eliciting this vagally mediated response.

Endothelin receptor blockade attenuates air embolization-induced pulmonary hypertension in sheep

We investigated the effects of two types of endothelin receptor antagonists on pulmonary hypertension induced by pulmonary air embolization in awake sheep. We prepared awake sheep with indwelling catheters inserted in blood vessels for continuous monitoring of pulmonary artery pressure, left atrial pressure and systemic arterial pressure. Cardiac output was measured every 30 min. The study consisted of two experiments, one with FR139317 (100 microg/kg/min; (R)2-[(R)-2-[(S)-2-[1-(hexahydro-1H-azepinyl)]-carbonyl]amino-4-++ +methy l-pentanoyl]amino-3-[3-(1-methyl-1H-indolyl)]propionyl)amino-3-(2-pyr idyl)propionic acid), a selective endothelin ET(A) receptor antagonist, and the other with TAK-044 (100 microg/kg/h; cyclo[D-alpha-aspartyl-3-[(4-phenylpiperazin-yl)carbonyl]-L-alanyl -L- alpha- aspartyl-D-2-(2-thienyl) glycyl-L-leucyl-D-tryptophyl] disodium salt), an endothelin ET(A) and ET(B) receptor antagonist. In the paired experiments, air was continuously (4.06 ml/min) infused into the main pulmonary artery for 3 h after the baseline pressures were stabilized. Sheep were treated or not treated with FR139317 or TAK-044. Pulmonary artery pressure was significantly higher than the baseline pressure after the start of air infusion. Both FR139317 and TAK-044 significantly attenuated the increase in pulmonary artery pressure during air embolization. Plasma endothelin -1 levels in both pulmonary and systemic arteries were equally and significantly increased after the start of air infusion. The results indicate that endothelin-1 release is attributable to the development of pulmonary hypertension during the course of air embolization in awake sheep.

Mechanisms underlying stimulation of rapidly adapting receptors during pulmonary air embolism in dogs

We investigated the mechanisms underlying stimulation of rapidly adapting receptors (RARs) during pulmonary air embolism (PAE). Impulses were recorded from RARs in 43 anesthetized, open-chest and artificially ventilated dogs. Forty one out of 56 RARs were stimulated by infusion of air into the right atrium (0.2 ml/kg per min for 10 min). As a group (n = 56), RAR activity increased from a baseline of 1.2 +/- 0.2 to a peak of 4.3 +/- 0.3 impulses/breath at 8-10 min after the onset of PAE induction. PAE also caused an increase in total lung resistance and a decrease in dynamic lung compliance. Studies were repeated in 33 RARs initially stimulated by PAE. In nine receptors, hyperinflation of the lungs performed at the termination of the second PAE induction largely reversed both the afferent and bronchomotor responses. Furthermore, the residual PAE-evoked activity subsequent to hyperinflation was almost eliminated by elevation in the heart position. In another 24 receptors, the responses of RARs to PAE were unaltered by pretreatment with saline (n = 8) or dimethylthiourea (a hydroxyl radical scavenger; n = 8), but was significantly attenuated by pretreatment with ibuprofen (a cyclooxygenase inhibitor: n = 8). These results suggest that: (1) the increased RAR activity following PAE is associated primarily with an increase in bronchomotor tone and secondarily with the mechanical action of the beating heart impacting on lung tissues; and (2) cyclooxygenase products may be involved in this sensory stimulation.

Mechanisms of stimulation of vagal pulmonary C fibers by pulmonary air embolism in dogs

We investigated the involvement of the cyclooxygenase metabolites and hydroxyl radical (.OH) in the stimulation of vagal pulmonary C fibers (PCs) by pulmonary air embolism (PAE). Impulses were recorded from PCs in 51 anesthetized, open-chest, and artificially ventilated dogs. Fifty of 59 PCs were stimulated by infusion of air into the right atrium (0.2 ml.kg-1.min-1 for 10 min). As a group (n = 59), PC activity increased from a baseline of 0.4 +/- 0.1 to a peak of 1.7 +/- 0.2 impulses/s during the period from 1 min before to 2 min after the termination of PAE induction. In PCs initially stimulated by PAE induction, PAE was repeated after the intervening treatment (iv) with saline (n = 9), ibuprofen (a cyclooxygenase inhibitor; n = 11), or dimethylthiourea (a .OH scavenger; n = 12). The responses of PCs to PAE were not altered by saline vehicle but were abolished by ibuprofen and significantly attenuated by dimethylthiourea. Although hyperinflation of the lungs reversed the PAE-induced bronchomotor responses, it did not reverse the stimulation of PCs (n = 8). These results suggest that 1) cyclooxygenase products are necessary for the stimulation of PCs by PAE, whereas changes in lung mechanics are not, and 2) the functional importance of cyclooxygenase products may be mediated in part through the formation of .OH.

Specific neutrophil elastase inhibitor does not attenuate acute lung injury induced by air embolism in awake sheep

To investigate the role played by neutrophil-derived elastase in acute lung injury, this study examined the effects of treatment of ONO-5046, a competitive, reversible, and specific neutrophil elastase inhibitor, on lung dysfunction induced by air emboli in awake sheep. Sheep were prepared with chronic lung lymph fistula. In experiment 1, air (1.23 mL/min) was infused over a 2-h period into the pulmonary artery and hemodynamic monitoring was performed over a 5-h period. In experiment 2, air was infused in the same manner as in experiment 1, 1 h after the continuous infusion of ONO-5046 (10 mg/kg h-1) had begun. In experiment 1, pulmonary artery pressure (Ppa) increased from the baseline value of 17.0 +/- 0.7 cm H2O to 28.3 +/- 1.8 1 h after the beginning of the air infusion. Ppa returned to the baseline values within 1 h after the air infusion was stopped. Lung lymph flow (Qlym) increased from the baseline value of 3.7 +/- 0.7 mL/0.5 h to 8.5 +/- 1.9 after 1 h of air infusion. After the air infusion was stopped, Qlym continued to increase. When sheep were treated with ONO-5046, Ppa and Qlym increased in a fashion similar to that in the experiment 1. These findings suggest that ONO-5046 does not attenuate air embolism-induced lung injury and that neutrophil-derived elastase may not play an important role in air embolism-induced lung injury in awake sheep.

Endothelium-dependent relaxations in sheep pulmonary arteries and veins: resistance to block by NG-nitro-L-arginine in pulmonary hypertension

1. The effect of the nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NOARG), on endothelium-dependent relaxation to a receptor-independent agent, ionomycin, was examined in isolated pulmonary arteries and veins from control, short-term and chronic pulmonary hypertensive sheep. All vessel segments were contracted to optimal levels of active force with endothelin-1 to record endothelium-dependent relaxation. 2. Pulmonary hypertension was induced by continuous pulmonary artery air embolization for 1 day (short-term) and 14 days (chronic) and was associated with a 2 and 3 fold increase in pulmonary vascular resistance respectively. 3. L-NOARG (0.1 mM) reduced the maximum relaxation (Rmax) to ionomycin in large and medium-sized pulmonary arteries from control sheep by approximately 70%. By contrast, L-NOARG (0.1 mM) did not inhibit the Rmax to ionomycin in matched vessels from short-term and chronic pulmonary hypertensive sheep. 4. Resistance of ionomycin-induced relaxations to inhibition by L-NOARG, was confined to the arterial vasculature in chronic pulmonary hypertensive animals, as relaxations to ionomycin in large and medium-sized chronic pulmonary hypertensive veins were, like those in control veins, abolished by L-NOARG. Both large and medium-sized pulmonary veins from short-term pulmonary hypertensive sheep, however, were resistant to block by L-NOARG. 5. Neither sensitivity (pEC50) nor Rmax to ionomycin in large, short-term pulmonary hypertensive arteries was affected when the extracellular concentration of K+ was increased isotonically to 30 mM. Nifedipine (0.3 microM) was present throughout to prevent high K(+)-induced smooth muscle contraction. In the presence of this high extracellular K+, however, L-NOARG (0.1 mM) caused complete inhibition of the relaxation to ionomycin, whereas in normal extracellular K+ (4.7 mM), L-NOARG only weakly inhibited ionomycin relaxations. 6. In conclusion, the onset of pulmonary hypertension in sheep following air embolization, is associated with the development of resistance of endothelium-dependent relaxations to block by L-NOARG. The mechanism of L-NOARG resistance appears to be due to the up-regulation of a K+ channel-mediated backup vasodilator mechanism which can compensate for the loss of nitric oxide (NO)-mediated relaxation. Although this mechanism remains functionally 'silent' in the presence of NO it is able to maintain adequate endothelium-dependent vasodilatation during pulmonary hypertension if NO synthesis is compromised.

Pulmonary hypertensive response to foreign body microemboli

Pulmonary hypertension and foreign body granulomas are recognized sequelae of chronic intravenous drug abuse. We have recently described the development of transient pulmonary hypertension and increased permeability pulmonary edema after the intravenous injection of crushed, suspended pentazocine tablets in both humans and dogs. To determine the role of vasoactive substances in the development of this transient pulmonary hypertension, we measured pulmonary hemodynamics and accumulation of arachidonic acid metabolites in dogs during the infusion of indomethacin, a cyclooxygenase inhibitor, diethylcarbamazine (DEC), a lipoxygenase inhibitor, and FPL 55712, a receptor antagonist for leukotriene C4/D4 (LTC4/D4). Following the intravenous administration of crushed, suspended pentazocine tablets (3-4 mg/kg of body weight), mean pulmonary artery pressure increased from 14 +/- 2 mmHg to 30 +/- 6 mmHg (p less than 0.05) at 60 secs with a concomitant increase in plasma concentrations of 6-keto-PGF1 alpha from 187 +/- 92 pg/ml to 732 +/- 104 pg/ml and thromboxane B2 from 206 +/- 83 pg/ml to 1362 +/- 117 pg/ml (both p less than 0.05). Indomethacin prevented the increase in both cyclooxygenase metabolites, but had no effect on the pulmonary hypertension. In contrast, DEC had no effect on the increase in cyclooxygenase products, but blocked the pulmonary hypertension. FPL 55712 did not effect either the increase in cyclooxygenase metabolites or the pulmonary hypertension. We conclude that the transient pulmonary hypertension, induced by the intravenous injection of crushed, suspended pentazocine tablets, is not mediated by cyclooxygenase products but may be mediated by lipoxygenase product(s) other than LTC4/D4.

Cyclooxygenase inhibition and effects of hypoxia on pulmonary circulation and gas exchange in anesthetized dogs

To investigate whether endogenously produced prostanoids are involved in hypoxic pulmonary vasoconstriction, pulmonary hemodynamic and gas exchange parameters and eicosanoid metabolites were measured in 5 anesthetized, artificially ventilated dogs (mean body weight 27 kg). Hypoxia elicited pulmonary vasoconstriction, but blood plasma levels of thromboxane B2 (TXB2) and 6-keto-prostaglandin F 1 alpha (6kPGF1 alpha) (stable metabolites of TXA2 and prostaglandin I2, respectively) remained unchanged. Administration of the cyclooxygenase inhibitor indomethacin blocked the synthesis of prostanoids, so that 6kPGF1 alpha and TXB2 levels decreased to values below the detection level (10 pg.ml-1) both during normoxia or hypoxia, but did not affect pulmonary vascular resistance or the alveolar-arterial PO2 difference (PAi-Pa)O2. The pulmonary vascular bed remained, however, responsive to TXA2 as evidenced by infusion of the TXA2 mimetic, U 46619, which significantly increased the pulmonary vascular resistance and (PAi-Pa)O2. Our data suggest that prostanoids are not involved in eliciting the effects of hypoxia on pulmonary hemodynamics and gas exchange efficiency.