Endothelin abnormalities in patients with pulmonary embolism

Pulmonary vasodilation in acute pulmonary embolism - a systematic review

Acute pulmonary embolism is the third most common cause of cardiovascular death. Pulmonary embolism increases right ventricular afterload, which causes right ventricular failure, circulatory collapse and death. Most treatments focus on removal of the mechanical obstruction caused by the embolism, but pulmonary vasoconstriction is a significant contributor to the increased right ventricular afterload and is often left untreated. Pulmonary thromboembolism causes mechanical obstruction of the pulmonary vasculature coupled with a complex interaction between humoral factors from the activated platelets, endothelial effects, reflexes and hypoxia to cause pulmonary vasoconstriction that worsens right ventricular afterload. Vasoconstrictors include serotonin, thromboxane, prostaglandins and endothelins, counterbalanced by vasodilators such as nitric oxide and prostacyclins. Exogenous administration of pulmonary vasodilators in acute pulmonary embolism seems attractive but all come with a risk of systemic vasodilation or worsening of pulmonary ventilation-perfusion mismatch. In animal models of acute pulmonary embolism, modulators of the nitric oxide-cyclic guanosine monophosphate-protein kinase G pathway, endothelin pathway and prostaglandin pathway have been investigated. But only a small number of clinical case reports and prospective clinical trials exist. The aim of this review is to give an overview of the causes of pulmonary embolism-induced pulmonary vasoconstriction and of experimental and human investigations of pulmonary vasodilation in acute pulmonary embolism.

Subclinical Atherosclerosis in Patients with Prior Pulmonary Thromboembolism

OBJECTIVE

Recent studies have indicated that endothelial dysfunction is common in patients with a prior history of pulmonary thromboembolism (PTE). Based on the established relationship between endothelial dysfunction and atherosclerosis, we aimed to investigate carotid intima-media thickness (cIMT) as a marker of subclinical atherosclerosis in patients who have a prior history of PTE.

SUBJECTS AND METHODS

Medical records belonging to patients with a history of PTE and age- and gender-matched control subjects among those who underwent transthoracic echocardiography in Konya Training and Research Hospital were reviewed. Echocardiographic findings and cIMT measurements were recorded. cIMT measurements were compared between patient and control groups, and then independent correlates of cIMT were investigated using appropriate statistical methods.

RESULTS

A total of 110 patients (64.02 ± 12.67 years, males: n = 79 [71.81%]; females: n = 31 [28.19%]) were found to be eligible for assessment. Of these patients, 55 (50.00%) had a history of PTE. Patients with a history of PTE had significantly greater cIMT (p = 0.040). In this group of patients, cIMT positively and significantly correlated with basal right ventricular (RV) diameter (r = 0.271, p = 0.022), RV diastolic area (r = 0.376, p = 0.002), and systolic pulmonary artery pressure (r = 0.248, p = 0.037). In the multivariate linear regression analysis, only RV diastolic area was independently associated with cIMT in patients with PTE (p = 0.010).

CONCLUSION

Patients with a history of PTE have increased cIMT when compared to healthy subjects, and cIMT, which is a marker of subclinical atherosclerosis, is independently associated with RV diameter in these patients.

Endothelin ETA receptors predominate in chronic thromboembolic pulmonary hypertension

AIMS

Endothelin-1 levels are raised in chronic thromboembolic pulmonary hypertension. Our aim in this study was to identify the presence of endothelin receptors in patients with CTEPH by analysing tissue removed at pulmonary endarterectomy.

MAIN METHODS

Pulmonary endarterectomy tissue cross-sections were analysed using autoradiography with [(125)I]-ET-1 using ligands selective for ETA or ETB to determine sub-type distribution. The precise cellular localisation of ETA and ETB receptors was determined using selective antisera to both sub-types and compared with haematoxylin and eosin, Elastic Van Gieson and smooth muscle actin labelled sections.

KEY FINDINGS

Two patterns of ET-1 binding were found. In sections with frequent recanalised channels, ET-1 bound to the smooth muscle cells surrounding the channels. In sections where there was less organised thrombus with no obvious re-canalisation, minimal ET-1 binding was observed. Some contractile type smooth muscle cells not associated with recanalised channels and diffusely spread throughout the PEA material were associated with ET receptor antibody binding on immunohistochemistry. There was a greater expression of the ETA receptor type in the specimens.

SIGNIFICANCE

The presence of ET-1 receptors in the chronic thrombus in proximal CTEPH suggests ET-1 could act not only on the distal vasculopathy in the unobstructed vessels but may also stimulate smooth muscle cell proliferation within chronic clot. The abundance of ET receptors within the tissue provides evidence that the ET pathway is involved in the pathology of chronic thrombus reorganisation leading to CTEPH providing a rationale for the repurposing of ET receptor antagonists in the treatment of this condition.

Pulmonary vascular diseases

Diseases of the pulmonary vasculature are a cause of increased pulmonary vascular resistance (PVR) in pulmonary embolism, chronic thromboembolic pulmonary hypertension (CTEPH), and pulmonary arterial hypertension or decreased PVR in pulmonary arteriovenous malformations on hereditary hemorrhagic telangiectasia, portal hypertension, or cavopulmonary anastomosis. All these conditions are associated with a decrease in both arterial PO2 and PCO2. Gas exchange in pulmonary vascular diseases with increased PVR is characterized by a shift of ventilation and perfusion to high ventilation-perfusion ratios, a mild to moderate increase in perfusion to low ventilation-perfusion ratios, and an increased physiologic dead space. Hypoxemia in these patients is essentially explained by altered ventilation-perfusion matching amplified by a decreased mixed venous PO2 caused by a low cardiac output. Hypocapnia is accounted for by hyperventilation, which is essentially related to an increased chemosensitivity. A cardiac shunt on a patent foramen ovale may be a cause of severe hypoxemia in a proportion of patients with pulmonary hypertension and an increase in right atrial pressure. Gas exchange in pulmonary arteriovenous malformations is characterized by variable degree of pulmonary shunting and/or diffusion-perfusion imbalance. Hypocapnia is caused by an increased ventilation in relation to an increased pulmonary blood flow with direct peripheral chemoreceptor stimulation by shunted mixed venous blood flow.

Blood flow redistribution and ventilation-perfusion mismatch during embolic pulmonary arterial occlusion

Acute pulmonary embolism causes redistribution of blood in the lung, which impairs ventilation/perfusion matching and gas exchange and can elevate pulmonary arterial pressure (PAP) by increasing pulmonary vascular resistance (PVR). An anatomically-based multi-scale model of the human pulmonary circulation was used to simulate pre- and post-occlusion flow, to study blood flow redistribution in the presence of an embolus, and to evaluate whether reduction in perfused vascular bed is sufficient to increase PAP to hypertensive levels, or whether other vasoconstrictive mechanisms are necessary. A model of oxygen transfer from air to blood was included to assess the impact of vascular occlusion on oxygen exchange. Emboli of 5, 7, and 10 mm radius were introduced to occlude increasing proportions of the vasculature. Blood flow redistribution was calculated after arterial occlusion, giving predictions of PAP, PVR, flow redistribution, and micro-circulatory flow dynamics. Because of the large flow reserve capacity (via both capillary recruitment and distension), approximately 55% of the vasculature was occluded before PAP reached clinically significant levels indicative of hypertension. In contrast, model predictions showed that even relatively low levels of occlusion could cause localized oxygen deficit. Flow preferentially redistributed to gravitationally non-dependent regions regardless of occlusion location, due to the greater potential for capillary recruitment in this region. Red blood cell transit times decreased below the minimum time for oxygen saturation (<0.25 s) and capillary pressures became high enough to initiate cell damage (which may result in edema) only after ~80% of the lung was occluded.

Computational models of the pulmonary circulation: Insights and the move towards clinically directed studies

Biophysically-based computational models provide a tool for integrating and explaining experimental data, observations, and hypotheses. Computational models of the pulmonary circulation have evolved from minimal and efficient constructs that have been used to study individual mechanisms that contribute to lung perfusion, to sophisticated multi-scale and -physics structure-based models that predict integrated structure-function relationships within a heterogeneous organ. This review considers the utility of computational models in providing new insights into the function of the pulmonary circulation, and their application in clinically motivated studies. We review mathematical and computational models of the pulmonary circulation based on their application; we begin with models that seek to answer questions in basic science and physiology and progress to models that aim to have clinical application. In looking forward, we discuss the relative merits and clinical relevance of computational models: what important features are still lacking; and how these models may ultimately be applied to further increasing our understanding of the mechanisms occurring in disease of the pulmonary circulation.

Current understanding of the role of bosentan in inoperable chronic thromboembolic pulmonary hypertension

Chronic thromboembolic pulmonary hypertension may occur in the context of incomplete lysis of acute pulmonary emboli, resulting in the obstruction of pulmonary blood flow, as well as progressive right ventricular dysfunction and failure. The treatment of choice for this condition is surgical removal of the obstructing material. However, in many patients, surgery is not possible due to either an unfavourable distribution of the disease, the development of a concurrent small vessel pulmonary arteriopathy, or the presence of significant comorbid conditions. There is increasing evidence that the medical therapies that are used in other forms of pulmonary hypertension may also be effective in inoperable chronic thromboembolic pulmonary hypertension. This article examines the rationale for the use of the oral dual endothelin receptor antagonist bosentan in this life-threatening condition.

Plasma levels of endothelin-1 after a pulmonary embolism of bone marrow fat

BACKGROUND

During orthopedic surgery, embolization of bone marrow fat can lead to potentially fatal, intra-operative cardiovascular deterioration. Vasoactive mediators may also be released from the bone marrow and contribute to these changes. Increased plasma levels of endothelin-1 (ET-1) have been observed after pulmonary air and thrombo-embolism. The role of ET-1 in the development of acute cardiovascular deterioration as a result of bone marrow fat embolization during vertebroplasty was therefore investigated.

METHODS

Bone cement was injected into three lumbar vertebrae of six sheep in order to force bone marrow fat into the circulation. Invasive blood pressures and heart rate were recorded continuously until 60 min after the last injection. Cardiac output, arterial and mixed venous blood gas parameters and plasma ET-1 concentrations were measured at selected time points. Post-mortem, lung biopsies were taken for analysis of intravascular fat.

RESULTS

Cement injections resulted in a sudden (within 1 min) and severe increase in pulmonary arterial pressure (>100%). Plasma concentrations of ET-1 started to increase after the second injection, but no significant changes were observed. Intravascular fat and bone marrow cells were present in all lung lobes.

CONCLUSION

Cement injections into vertebral bodies elicited fat embolism resulting in subsequent cardiovascular changes that were characterized by an increase in pulmonary arterial pressure. Cardiovascular complications as a result of bone marrow fat embolism should thus be considered in patients undergoing vertebroplasty. No significant changes in ET-1 plasma values were observed. Thus, ET-1 did not contribute to the acute cardiovascular changes after fat embolism.

Endothelin receptor blockade does not improve hypoxemia following acute pulmonary thromboembolism

We studied the roles of endothelins in determining ventilation (V̇a) and perfusion (Q̇) mismatch in a porcine model of acute pulmonary thromboembolism (APTE), using a nonspecific endothelin antagonist, tezosentan. Nine anesthetized piglets (∼23 kg) received autologous clots (∼20 g) via a central venous catheter at time = 0 min. The distribution of V̇a and Q̇ at five different time points (−30, −5, 30, 60, 120 min) was mapped by fluorescent microspheres of 10 different colors. Five piglets ( group 1) received tezosentan (courtesy of Actelion) starting at time = 40 min for 2 h, and four piglets ( group 2) received only saline and served as control. Our results showed that, in all of the animals at 30 min following APTE but before tezosentan, the mean V̇a/Q̇ was increased, as was V̇a/Q̇ heterogeneity (log SD V̇a/Q̇), which represented a widening of its main peak. Afterwards, tezosentan attenuated the pulmonary hypertension in group 1 but also produced moderate systemic hypotension. However, it did not improve arterial Po2 or V̇a/Q̇ mismatch. We concluded that endothelin antagonism had minimal impact on gas exchange following APTE and confirmed our earlier observation that the main mechanism for hypoxemia in APTE was due to the mechanical redistribution of pulmonary regional blood flow away from the embolized vessels, resulting in the creation of many divergent low and high V̇a/Q̇ regions.

The Effect of a Combination of Inhaled Nitric Oxide and an EndothelinA-Receptor Antagonist onHemodynamic Dysfunction in Experimental AcutePulmonary Thromboembolism

Although either inhaled nitric oxide (NO) or endothelinA receptor antagonist has been tried in the treatment of various forms of pulmonary hypertension, the effects of combination therapy have not been reported. We evaluated the effects of inhaled NO alone or a combination of inhaled NO and ZD2574 (an endothelinA receptor antagonist) in an experimental canine acute pulmonary thromboembolism model. Forty parts per million of inhaled NO alone, or a combination of inhaled NO and 10 mg/kg of ZD2574 was administered 1 hour after embolization with an autologous blood clot. We compared the hemodynamic and gas exchange parameters between the two treatment groups. Two treatment regimens decreased mean pulmonary arterial pressure and pulmonary vascular resistance and attenuated decrease in cardiac output. Moreover, systemic arterial hypotension or worsening of hypoxemia did not occur in either of the treatment groups. In the combined group, more favorable hemodynamic outcomes were maintained than in the inhaled NO alone group. And hemodynamic deterioration shown after NO withdrawal was attenuated in the combined group. These findings suggest that when inhaled NO is concomitantly administered with an ETA receptor antagonist, more favorable hemodynamic outcomes can be expected during and after NO inhalation in acute pulmonary thromboembolism.

Embolization itself stimulates thrombus propagation in pulmonary embolism

The role of active thrombosis in the pathophysiology of pulmonary embolism is unclear. We tested the hypothesis that venous thrombi significantly increase their thrombotic activity once they embolize into the high-flow circulation of the pulmonary arteries. Thrombotic activity was measured using an immunoassay that measures both fibrinopeptide B (FPB) as well as its most abundant metabolite des-arginine FPB. Thrombi were formed in the femoral veins of adult dogs. In one group, the thrombi were embolized without anticoagulation. In the second group, heparin (300 U/kg bolus, then 90 U x kg(-1) x h(-1) infusion) was administered before embolization to prevent subsequent thrombotic activity. Plasma FPB concentrations were significantly suppressed in the heparinized group relative to the nonheparinized group for 1 h postembolization (P = 0.038). We conclude that pulmonary embolization itself causes preexisting venous thrombi to greatly intensify their thrombotic activity and that embolization-associated thrombus propagation can be prevented by heparin.

Is there a place for inhaled nitric oxide in the therapy of acute pulmonary embolism?

Acute pulmonary embolism (PE) is a serious complication resulting from the migration of emboli to the lungs. Although deep venous thrombi are the most common source of emboli to the lungs, other important sources include air, amniotic fluid, fat and bone marrow. Regardless of the specific source of the emboli, very little progress has been made in the pharmacological management of this high mortality condition. Because the prognosis is linked to the degree of elevation of pulmonary vascular resistance, any therapeutic intervention to improve the hemodynamics would probably increase the low survival rate of this critical condition. Inhaled nitric oxide (iNO) has been widely tested and used in cases of pulmonary hypertension of different causes. In the last few years some authors have described beneficial effects of iNO in animal models of acute PE and in anecdotal cases of massive PE. The primary cause of death in massive PE that is caused by deep venous thrombi, gas or amniotic fluid, is acute right heart failure and circulatory shock. Increased pulmonary vascular resistance following acute PE is the cumulative result of mechanical obstruction of pulmonary vessels and pulmonary arteriolar constriction (attributable to a neurogenic reflex and to the release of vasoconstrictors). As such, the vasodilator effects of iNO could actively oppose the pulmonary hypertension following PE. This hypothesis is consistently supported by experimental studies in different animal models of PE, which demonstrated that iNO decreased (by 10 to 20%) the pulmonary artery pressure without improving pulmonary gas exchange. Although maximal vasodilatory effects are probably achieved by less than 5 parts per million iNO, which is a relatively low concentration, no dose-response study has been published so far. In addition to the animal studies, a few anecdotal reports in the literature suggest that iNO may improve the hemodynamics during acute PE. However, no prospective, controlled, randomized clinical trial addressing this issue has been conducted to date. Future investigations addressing the effects of iNO combined with other drugs such as vasoconstrictors and inhibitors of phosphodiesterase III or V, may increase the responsiveness to iNO in acute PE.

Plasma levels of endothelin-1, big endothelin-1 and thromboxane following acute pulmonary air embolism

Acute pulmonary air embolism (APAE) was induced in nine piglets by repeated intravenous bolus injection of room air into a large bore central venous catheter at time=0 min so that the mean pulmonary artery pressure (MPAP) was maintained at two times the baseline value for 4 h. Another five animals served as controls. At time=0, 30, 60, 120, 240 min, circulating arterial plasma levels of endothelin-1 (ET-1), its precursor big ET-1, and thromboxane (Tx), were measured by RIA and EIA, respectively, along with hemodynamics and blood gases. The data showed that following APAE, there was a rapid increase in MPAP and a persistent decrease in Pa(O(2)), while the mean arterial blood pressure and cardiac output remained comparable. Plasma levels of ET-1, big ET-1 and Tx were also increased steadily in these first 4 h. These results showed that during APAE, the resulted changes in the pulmonary vascular and airway tones mediated by these potent mediators could explain the observed pulmonary hypertension and the deterioration of gas exchange.

Modulation and roles of the endothelins in the pathophysiology of pulmonary embolism

Recent research on the endothelins (ETs) and their pathways in acute pulmonary embolism (APE) has led to significant advances in the understanding of this disease. ETs are potent vasoconstrictors and bronchoconstrictors found abundantly in the lung and can be released by stimuli such as endothelial injury, hypoxia, or thrombin, a key product in the coagulation cascade. Many studies using different approaches and methods of inducing pulmonary embolization, both in vitro and in vivo in various species, have mostly shown that ETs play an important role in the pathophysiology of APE. These results were obtained by comparing the hemodynamic data in the presence or absence of various ETs inhibitors, but also by assessing the modulation of the ET-related elements of this system by molecular, cell biology, and pharmacological methods. Based on the current understanding, a mechanism involving the ET pathway in the pathophysiology of APE is proposed for the reader's considerations. We postulate that ETs are primary mediators in APE based on the following: (i) their source from pulmonary endothelial cells where the primary injury takes place; (ii) their direct vasconstrictive, bronchoconstrictive, and promitogenic effects via distinct ET receptors; and (iii) their indirect effects associated with the secondary release of thromboxane and other mediators, which are released from inflammatory cells and platelets, which together can potentiate the overall hemodynamic response, most specifically the pulmonary vascular bed. Such combined effects of ETs on bronchomotor and vasomotor tone in the lung can adversely affect ventilation perfusion matching and lead to severe hypoxemia without causing significant changes in the chest X-ray of these patients. Thus, we may consider ET inhibitors as future current therapeutic agents in patients with PE.

Pathogenic role of endothelin 1 in hemodynamic dysfunction in experimental acute pulmonary thromboembolism

The plasma endothelin-1 (ET-1) level is elevated in patients with acute pulmonary thromboembolism (APE). Whether ET-1 is a pathogenic mediator or a simple marker of APE is not known. We investigated the role of ET-1 in hemodynamic dysfunction in APE through evaluating the effects of ET(A) receptor antagonist in an experimental APE model. We also examined ET-1 expression in embolized lungs. In a canine autologous blood clot pulmonary embolism model, ET(A) receptor antagonist ZD2574 (10 mg/kg, intravenous; ZD2574 group; n = 6) or vehicle (control group; n = 5) was administered. Hemodynamic and gas exchange parameters and plasma levels of ET-1 were serially measured. Prepro-ET-1 mRNA expression and the distribution of ET-1 peptide in lung tissues were also examined. With ZD2574 pulmonary arterial pressure and pulmonary vascular resistance significantly decreased, and were lower compared with the control group. The decrease in cardiac output was also less in the ZD2574 group. Plasma ET-1 levels increased after embolization. Prepro-ET-1 mRNA expression increased in embolized lungs and ET-1 peptide expression also increased in embolized lungs, particularly in the muscular pulmonary arteries, compared with normal lungs. These findings suggest that ET-1 partially contributes to hemodynamic derangements of APE, and that ET(A) receptor antagonists might constitute a useful therapeutic tool for APE.

Abnormalities of renal endothelin during acute exacerbation in chronic obstructive pulmonary disease

Circulating and urinary levels of endothelin (ET), an endothelium-derived vasoconstrictive and mitogenic peptide have been reported to increase in patients with chronic obstructive pulmonary disease (COPD), but the mechanisms of these abnormalities are not fully understood. Our study objectives were to evaluate pulmonary and renal ET clearance in COPD patients during an acute exacerbation. Our participants included nine consecutive patients with moderate to severe COPD without signs of right heart failure admitted for acute exacerbation and ten healthy volunteers (HV) as controls. ET was detected by radioimmunoassay in venous and arterial blood as well as in a timed urine specimen. For each subject, arterial/venous immunoreactive ET ratio (ir-ETart/ir-ETven) was evaluated as an index of its pulmonary clearance. Creatinine clearance was employed in each case to obtain a corrected renal ir-ET clearance. Glomerular filtration rate (GFR) was also assessed by dynamic(99m)Tc-diethylenetriamine pentaacetic acid renal scintigraphy in six COPD patients during acute exacerbation and at recovery. The ratio ir-ETart/ir-ETven was comparable in COPD patients (0.75+/-0.12) and in HV (0.82+/-0.09). A significant difference was found with respect to 24 h ir-ET urinary excretion between COPD patients during exacerbation as well as at recovery (respectively 142.1+/-12.8 ng/24 h and 89.0+/-15.1 ng/24 h) and HV (65.1+/-10.1 ng/24 h). ET renal clearance was higher in COPD patients than in HV (29.2+/-5.2 ml min(-1)in COPD during exacerbation; 17.5+/-3.9 ml min(-1)at recovery and 13.6+/-2.4 ml min(-1)in HV, P<0.001). GFR was 69.4+/-10.0 ml min(-1)in COPD patients during exacerbation and it significantly increased at the recovery (95.5+/-20.9 ml min(-1)P<0.001). Corrected renal clearance of the peptide was significantly correlated to GFR values during the exacerbation (r=-0.81, P<0.05). Furthermore change in renal ET production resulted associated with changes in paCO(2)(r=0.83, P<0.001) and in paO(2)(r=-0.73, P<0.05). Acute exacerbation in COPD patients causes an increase in renal ET production which is partially reversible at the recovery, in the absence of significant changes in ET-1 circulating levels. ET might contribute to the renal response to hypoxaemia and hypercapnia in COPD.

Effects of oxygen and nitric oxide inhalation in a porcine model of recurrent microembolism

BACKGROUND

Inhalation of nitric oxide (iNO) has been proposed for the treatment of acute pulmonary embolism. The present study evaluates the effects of oxygen (O2) and nitric oxide inhalation in a porcine model of sustained pulmonary hypertension induced by recurrent pulmonary microembolism.

METHODS

Twelve pigs were embolized under general anesthesia with 300-microm microspheres intravenously three times over a period of seven weeks. Five pigs served as untreated controls. Hemodynamic and gas exchange responses to 100% oxygen and 40 ppm NO inhalation, and their combination (O2+iNO) were measured seven days after the last embolization.

RESULTS

Recurrent microembolism caused sustained pulmonary hypertension (pulmonary vascular resistance index; PVRI 408 +/- 57 dyn x s x cm(-5) x m(-2)) as compared to the control group (PVRI 143 +/- 20 dyn x s x cm(-5) m(-2); P<0.05). PVRI was significantly reduced by O2, iNO, and O2+iNO inhalation by 29 +/- 3, 28 +/- 4, and 32 +/- 3%, respectively.

CONCLUSION

We conclude that both O2 and iNO are selective pulmonary vasodilators in a porcine model of sustained pulmonary hypertension following recurrent pulmonary microembolism and, therefore, may be useful in the treatment not only in the acute phase of pulmonary embolism but also later in the time course of the disease.

Nonselective endothelin-receptor antagonism attenuates hemodynamic changes after massive pulmonary air embolism in dogs

STUDY OBJECTIVES

To evaluate the effects of nonselective endothelin (ET)-receptor antagonism on the hemodynamic changes and serum thromboxane (TX)-A(2) levels after a massive pulmonary air embolism (PAE) in dogs.

DESIGN

Prospective trial.

SETTING

University laboratory.

INTERVENTIONS

Anesthetized mongrel dogs (ET-receptor antagonist group; n = 6) received a bolus injection of 1 mg of the nonselective ET-A/ET-B-receptor antagonist PD 145065 (Sigma Chemical; St. Louis, MO), and dogs in the control group (n = 6) received saline solution. Hemodynamic data were recorded 5 min after the administration of antagonist or saline solution. Subsequently, each dog received 2.5-mL air/kg via the right femoral vein (the PAE), and the hemodynamic data were recorded for up to 60 min thereafter. Arterial blood samples were drawn at baseline and 15 min after PAE for the determination of plasma TX-A(2), measured by enzyme-linked immunosorbent assay as TX-B(2) (the stable metabolite of TX-A(2)).

RESULTS

PD 145065 alone produced no hemodynamic effects. However, dogs pretreated with PD 145065 had significantly lower increases in mean pulmonary arterial pressure and in pulmonary vascular resistance after the PAE (116% and 165%, respectively) compared to the control dogs (187% and 367%, respectively). The mean arterial pressure (MAP), cardiac index (CI), and plasma TX-B(2) levels were unaltered after PAE in the presence of ET-receptor antagonist, whereas CI and MAP decreased 5 to 10 min after PAE, and TX-B(2) concentrations increased 15 min after PAE in control dogs (p < 0.05 in all cases).

CONCLUSIONS

Nonselective antagonism of ET receptors attenuates the pulmonary hypertension and blunts the TX-A(2) release caused by massive PAE in dogs.

Ventilation heterogeneity does not change following pulmonary microembolism

By using the multiple-breath helium washout technique, ventilation heterogeneity (VH) after embolic injury in the lung can be quantitatively partitioned into the conductive and acinar components. Total VH, represented by the normalized slope of the phase III alveolar plateau, Sn(III (total)), was studied for 120 min in three groups of anesthetized and paralyzed mongrel dogs. Group 1 (n = 3) received only normal saline and served as controls. Group 2 (n = 4) received repeated infusions of polystyrene beads (250 microm) into the right atrium at 10, 40, 80, and 120 min. Group 3 (n = 3) was similarly treated, except that the embolic beads used were 1,000 microm in diameter. The data show that, despite repeated embolic injury by polystyrene beads of different diameters, there was no significant increase in total VH. The acinar component of Sn(III), which represents VH in the distal airways, accounts for over 90% of the total VH. The conductive component of Sn(III), which represents VH between larger conductive airways, remains relatively constant and a minor component. We conclude that pulmonary microembolism does not result in significant redistribution of ventilation.

The hemodynamic effects of endothelin receptor antagonism during a venous air infusion in dogs

Endothelin (ET) is involved in the humoral component of the vasoconstriction during pulmonary embolism. We examined the effects of selective ET receptor antagonists on the hemodynamic and respiratory changes and on serum thromboxane B2 (TXB2) levels, during a continuous venous air infusion (VAI) in anesthetized mongrel dogs. The VAI (0.2 mL x kg(-1) x min(-1)) was initiated 5 min after an injection of saline (controls, n = 7), 1 micromol of the selective ET(A) receptor antagonist JKC-301 (group A, n = 6), or 1 micromol of the selective ET(B) receptor antagonist BQ-788 (group B, n = 6). Hemodynamic evaluation was performed every 15 min of VAI, and blood samples were drawn for blood gas analysis and TXB2 determinations. The increase in pulmonary perfusion pressure after 30 min of VAI was attenuated in Group A compared with the controls and Group B (Group A = 7+/-1 mm Hg; Group B = 16+/-1 mm Hg; controls = 14+/-1 mm Hg; P < 0.05). Pulmonary vascular resistance showed a similar behavior. TXB2 concentrations increased after 60 min of VAI in the controls and in Group B, but not in Group A (controls = 48%; Group B = 104%; Group A = 18%; P < 0.05 for controls and Group B). Similar decreases in Pao2 and Sao2 were observed in the three groups. We conclude that antagonism of ET(A) receptors attenuates the hemodynamic changes and blunts the increase in thromboxane A2 production during a VAI in dogs.

IMPLICATIONS

We evaluated the effects of endothelin receptor antagonists during a venous air infusion in dogs. EndothelinA receptor antagonism attenuated the hemodynamic changes and blunted the increase in thromboxane A2 production in this setting.