Selective pulmonary vasodilatory effect of ZSY-27 in dogs with pulmonary hypertension due to 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.

Increase in exhaled nitric oxide and protective role of the nitric oxide system in experimental pulmonary embolism

BACKGROUND AND PURPOSE

Pulmonary embolism (PE) represents a real diagnostic challenge. PE is associated with pulmonary hypertension due to pulmonary vascular obstruction and vasoconstriction. We recently reported that pulmonary gas embolism transiently increases exhaled nitric oxide (FENO), but it is not known whether solid emboli may alter FENO, and whether an intact endogenous NO synthesis has a beneficial effect in experimental solid pulmonary embolism.

EXPERIMENTAL APPROACH

We used anaesthetised and ventilated rabbits in these experiments. To mimic PE, a single intravenous infusion of homogenized autologous skeletal muscle tissue (MPE) was given to rabbits with intact NO production (MPE of 60, 15, or 7.5 mg kg(-1); group 1) and to another group (group 2) with inhibited NO synthesis (L-NAME 30 mg kg(-1); MPE of 7.5, 15 or 30 mg kg(-1)).

KEY RESULTS

In group 1, after MPE, FENO increased rapidly and dose-dependently and FENO was still significantly elevated after 60 min with the two highest emboli doses. All these animals survived more than 60 min after embolization. In group 2, MPE of 7.5, 15 and 30 mg kg(-1), in combination with NO synthesis inhibition, resulted in 67%, 50% and 25% survival at 60 min respectively, representing a statistically significant decrease in survival. Cardiovascular and blood-gas changes after MPE were intensified by pre-treatment with NO synthesis inhibitor.

CONCLUSIONS AND IMPLICATIONS

We conclude that solid PE causes a sustained, dose-dependent increase in FENO, giving FENO a diagnostic potential in PE. Furthermore, intact NO production appears critical for tolerance to acute PE.

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.

Effects of milrinone on platelet aggregation in swine with pulmonary hypertension

PURPOSE

The purpose of this study was to investigate whether the effect of milrinone on platelet aggregation was related to the selectivity of vasodilation vasculature in a swine model with PH.

MATERIALS AND METHODS

To induce pulmonary hypertension, we injected two sets of acid-washed glass beads in 15 swine, which were divided into two groups (those receiving milrinone or not) and compared with each other.

RESULTS

The induction of pulmonary hypertension decreased the platelet count and increased the plasma levels of thromboxane B2 and 6-keto-prostaglandin F1alpha.

CONCLUSION

A locally high concentration of prostaglandin I2, at least in part, may produce selectivity of vasodilation in the pulmonary vasculature.

Effects of carbon dioxide pneumoperitoneum in pigs with impaired pulmonary function

The objective of our study was to investigate the possible adverse hemodynamic effects of a CO2 pneumoperitoneum in an experimental model in pigs with impaired pulmonary function. Thirteen animals were anesthetized with azaperon/ketamine and ventilated with 67% nitrous oxide in oxygen. By intravenous injection of dextran microspheres, a capillary pulmonary embolism was induced. After embolization, three animals served as controls (Group 1), five underwent open nephrectomy (Group 2), and five underwent laparoscopic nephrectomy (Group 3). Intra-abdominal pressure was kept constant at 15 mm Hg. At intervals, hemodynamic parameters were measured, and blood gas measurements were performed. Data were analyzed using a general linear model analysis of variance for differences between groups, and a paired t-test was applied for differences within groups from one condition to the next. The groupwise comparison revealed a significant rise of cardiac output in the laparoscopy group compared with the open nephrectomy group. No differences were noted for heart rate, systemic arterial pressure, central venous pressure, mean pulmonary arterial pressure, or pulmonary arterial wedge pressure. Impairment of pulmonary function caused no negative hemodynamic effect during laparoscopic nephrectomy.

Inhaled nitric oxide selectively decreases pulmonary artery pressure and pulmonary vascular resistance following acute massive pulmonary microembolism in piglets

Acute massive pulmonary embolism increases pulmonary artery pressure (PAP) and pulmonary vascular resistance (PVR), which may lead to early right ventricular failure and subsequent cardiocirculatory deterioration. Inhaled nitric oxide (NO) selectively dilates pulmonary vessels in vivo. Thus, inhaled NO may be useful in preventing cardiocirculatory deterioration following pulmonary embolism. We investigated the effects of inhaled NO in the acute phase of massive pulmonary microembolism in 10 anesthetized and mechanically ventilated piglets (body weight, 18 +/- 2 kg). Microspheres of 300-microns diameter were injected i.v. in an amount sufficient to initially increase mean PAP to 45 mm Hg. Forty-five minutes after pulmonary embolization, the pretreatment control values were recorded. Thereafter, the piglets inhaled 40 ppm NO, and subsequently 80 ppm NO. When 40 ppm NO was inhaled, there was a significant decrease in systolic PAP (-10.3%; 44.5 +/- 2.2 to 39.9 +/- 2.4 mm Hg; p < 0.05) and mean PAP (-9.4%; 32.9 +/- 1.3 to 29.8 +/- 1.3 mm Hg; p < 0.05). PVR was changed by -13.6% (p = 0.07). Administration of 80 ppm NO resulted in a significant decrease in systolic PAP (-12.6%; to 38.9 +/- 1.9 mm Hg; p < 0.05), mean PAP (-11.9%; to 29.0 +/- 1.4 mm Hg; p < 0.05), and PVR (-19.4%; p < 0.05) compared with pretreatment values. Discontinuation of NO inhalation was associated with an immediate return to pretreatment values. Systemic hemodynamics and the arterial and mixed venous oxygen concentrations remained unchanged. We conclude that inhaled NO following acute massive pulmonary microembolism selectively decreases PAP and PVR without influencing systemic hemodynamics in piglets.

Vasodilatory effects of milrinone on pulmonary vasculature in dogs with pulmonary hypertension due to pulmonary embolism: a comparison with those of dopamine and dobutamine

1. The limited therapeutic role of pulmonary vasodilation reflects lack of their selectivity for the pulmonary vasculature, and many drugs have been evaluated for effectiveness; however, none has gained widespread clinical use. 2. Milrinone (MIL) is a newly synthetized phosphodiesterase inhibitor, which has potent positive inotropic and vasodilatory effects. 3. The present study shows the effects of MIL on the pulmonary circulation in dogs with pulmonary hypertension due to autologous muscle-induced pulmonary embolism, and also demonstrates a comparison with those of dopamine and dobutamine. 4. As MIL showed potent vasodilatory effects on the pulmonary vasculature, it had a potential clinical role in the treatment of pulmonary hypertension.