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

Mechanical Circulatory Support Devices in Patients with High-Risk Pulmonary Embolism

Pulmonary embolism (PE) is a common acute cardiovascular condition. Within this review, we discuss the incidence, pathophysiology, and treatment options for patients with high-risk and massive pulmonary embolisms. In particular, we focus on the role of mechanical circulatory support devices and their possible therapeutic benefits in patients who are unresponsive to standard therapeutic options. Moreover, attention is given to device selection criteria, weaning protocols, and complication mitigation strategies. Finally, we underscore the necessity for more comprehensive studies to corroborate the benefits and safety of MCS devices in PE management.

Mechanical Circulatory Support and Critical Care Management of High-Risk Acute Pulmonary Embolism

Hemodynamically significant pulmonary embolism (PE) remains a widely prevalent, underdiagnosed condition associated with mortality rates as high as 30%. The main driver of poor outcomes is acute right ventricular failure that remains clinically challenging to diagnose and requires critical care management. Treatment of high-risk (or massive) acute PE has traditionally included systemic anticoagulation and thrombolysis. Mechanical circulatory support, including both percutaneous and surgical approaches, are emerging as treatment options for refractory shock due to acute right ventricular failure in the setting of high-risk acute pulmonary embolism.

Levosimendan, milrinone, and dobutamine in experimental acute pulmonary embolism

Acute pulmonary embolism is a frequent condition in emergency medicine and potentially fatal. Cause of death is right ventricular failure due to increased right ventricular afterload from both pulmonary vascular obstruction and vasoconstriction. Inodilators are interesting drugs of choice as they may improve right ventricular function and lower its afterload. We aimed to investigate the cardiovascular effects of three clinically relevant inodilators: levosimendan, milrinone, and dobutamine in acute pulmonary embolism. We conducted a randomized, blinded, animal study using 18 female pigs. Animals received large autologous pulmonary embolism until doubling of baseline mean pulmonary arterial pressure and were randomized to increasing doses of each inodilator. Effects were evaluated with bi-ventricular pressure–volume loop recordings, right heart catheterization, and blood gas analyses. Induction of pulmonary embolism increased right ventricular afterload and pulmonary pressure (p < 0.05) causing right ventricular dysfunction. Levosimendan and milrinone showed beneficial hemodynamic profiles by lowering right ventricular pressures and volume (p < 0.001) and improved right ventricular function and cardiac output (p < 0.05) without increasing right ventricular mechanical work. Dobutamine increased right ventricular pressure and function (p < 0.01) but at a cost of increased mechanical work at the highest doses, showing an adverse hemodynamic profile. In a porcine model of acute pulmonary embolism, levosimendan and milrinone reduced right ventricular afterload and improved right ventricular function, whereas dobutamine at higher doses increased right ventricular afterload and right ventricular mechanical work. The study motivates clinical testing of inodilators in patients with acute pulmonary embolism and right ventricular dysfunction.

Management of Right Ventricular Failure in Pulmonary Embolism

Acute right ventricular failure remains the leading cause of mortality associated with acute pulmonary embolism (PE). This article reviews the pathophysiology behind acute right ventricular failure and strategies for managing right ventricular failure in acute PE. Immediate clot reduction via systemic thrombolytics, catheter based procedures, or surgery is always advocated for unstable patients. While waiting to mobilize these resources, it often becomes necessary to support the RV with vasoactive medications. Clinicians should carefully assess volume status and use caution with volume resuscitation. Right ventricular assist devices may have an expanding role in the future.

Treating pulmonary hypertension post cardiopulmonary bypass in pigs: milrinone vs. sildenafil analog

Procedures using cardiopulmonary bypass (CPB) and aortic cross-clamping are associated with a variable degree of ischemia/reperfusion of the lungs, leading to acute pulmonary hypertension (PHT). The purpose of this study was to compare the effects of the sildenafil analog (UK343-664), a phosphodiesterase type V(PDEV) inhibitor, with milrinone, a PDE type III inhibitor, in a porcine model of acute PHT following CPB. After the pigs were anesthetized, pressure-tipped catheters were placed in the right ventricle and carotid and pulmonary arteries. Cardiac output was measured with an ultrasound probe on the ascending aorta. After heparinization and placement of aortic and right atrial cannulae, non-pulsatile CPB was instituted and cardioplegia administered following aortic cross-clamping. After 30 minutes, the clamp was removed and the animals re-warmed and separated from CPB in sinus rhythm. The animals were randomized to 3 groups, and 16 animals were studied to completion: milrinone (n=5) 50 μg/kg; sildenafil-analog (n=5) 500 μg/kg; and normal saline (NS) (n=6). Hemodynamic data were collected at baseline pre-CPB and, following termination of CPB, at baseline, 5, 10 and 30 minutes after administration of the drug. Pulmonary hypertension was present in all groups following CPB. After administration of the drugs, mean pulmonary artery pressure decreased in all 3 groups; however, only in the sildenafil-analog group did pulmonary vascular resistance(PVR) decrease by 35%, from 820 to 433 dynes · cm · sec-5at 5 minutes (p<0.05), and continued to be decreased at 10 minutes by 26% (P<0.05). Pulmonary selectivity was demonstrated with sildenafil-analog, because there were no similar changes in systemic vascular resistance(SVR) and no significant changes in systemic hemodynamics. Sildenafil-analog, a PDEV inhibitor, shows a promising role for managing the PVR increases that occur following CPB.

The nitric oxide/cGMP signaling pathway in pulmonary hypertension

This article briefly reviews the background of endothelium-dependent vasorelaxation, describes the nitric oxide/cGMP/protein kinase pathway and its role in modulating pulmonary vascular tone and remodeling, and describes three approaches that target the nitric oxide/cGMP pathway in the treatment of patients with pulmonary arterial hypertension.

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.

Combination therapy with inhaled nitric oxide and intravenous dobutamine during pulmonary hypertension in the rabbit

Combination therapy with an intravenous inovasodilator and inhaled nitric oxide (NO) may be appropriate in patients with pulmonary hypertension and associated right ventricular failure. We examined whether dobutamine and inhaled NO would have additive pulmonary vasodilator effects in experimental pulmonary hypertension. Pulmonary hypertension was produced in anesthetized, mechanically ventilated rabbits by infusion of U46619, a thromboxane analogue. Dobutamine was administered in increasing doses (2.5-20 microg/kg/min) with and without inhaled NO (40 ppm). Dobutamine produced dose-dependent decreases in pulmonary vascular resistance (PVR) and mean arterial pressure (MAP) and increases in cardiac output (CO). Inhaled NO alone decreased pulmonary artery pressure (PAP) and PVR with no effect on MAP or CO. The effects of dobutamine and inhaled NO were additive, so that at each dose of dobutamine, inhaled NO decreased PAP and PVR with no effect on systemic hemodynamics. This study suggests that the combination of dobutamine and inhaled NO should produce additive pulmonary vasodilation in patients with pulmonary hypertension and associated right ventricular dysfunction.

Milrinone improves arterial oxygenation in dogs with acute lung injury induced by oleic acid

The aim of the study was to investigate effects of milrinone on pulmonary permeability in dogs with acute lung injury induced by oleic acid. To induce acute lung injury, we administered 0.08 mg/kg of oleic acid to 19 adult mongrel dogs and then measured hemodynamic parameters and performed blood gas analysis. An injection of oleic acid depressed the mean arterial pressure, cardiac index, and arterial oxygenation. Dogs were divided into three groups: six received a bolus of milrinone (50 microg/kg) followed by a continuous (0.5 microg/kg/min, low-dose), seven received a bolus (100 microg/kg) followed by a continuous (1.0 microg/kg/min; i.e., a low-dose twice; high-dose), and six no milrinone (control). Milrinone administration improved the cardiac index and arterial oxygenation and simultaneously depressed the intrapulmonary shunt fraction and the extravascular thermal lung water as extravascular water content of the lung. These changes produced by milrinone are different according to the doses. In conclusion, milrinone acts on the capillary endothelium and inhibits an accumulation in the extravascular water content of the lung, which may induce an improvement in arterial oxygenation. Milrinone may also improve arterial oxygenation through an inhibition of platelet aggregation and chemical mediators that are released from platelets. The latter mechanism also may improve arterial oxygenation, and the exact property responsible for causing the effect of milrinone has not yet been identified.

Pharmacological strategies for improving diastolic dysfunction in the setting of chronic pulmonary hypertension

BACKGROUND

Right ventricular (RV) hypertrophy is an adaptive process that occurs in the setting of chronic pulmonary hypertension (CPH) and can lead to alterations in normal RV diastolic properties. This study was designed to investigate the effects of NO and milrinone on RV diastolic dysfunction in the setting of CPH and RV hypertrophy by use of a canine model of monocrotaline pyrrole (MCTP)-induced CPH.

METHODS AND RESULTS

Sixteen mongrel dogs (22 to 24 kg) were used. Animals underwent percutaneous pulmonary artery (PA) catheterization to measure pulmonary hemodynamics before and 8 weeks after injection of 3 mg/kg MCTP (n=8) or placebo (control, n=8). Eight weeks after injection, all hearts were instrumented with a PA flow probe, sonomicrometric dimension transducers, and micromanometers. Data were collected at baseline and after both NO and milrinone administration. Diastolic properties were quantified by use of the end-diastolic pressure-volume relationship and the time constant of ventricular isovolumic relaxation. Eight weeks after injection, significant increases in the PA pressure and pulmonary vascular resistance were observed in MCTP dogs. Significant worsening of RV diastolic function occurred in association with significant increases in the ratio of RV dry weight to LV+septal dry weight. NO and milrinone administration both led to significant improvements in RV diastolic properties.

CONCLUSIONS

In the setting of MCTP-induced CPH, significant worsening of RV diastolic function was observed in association with significant increases in the ratio of RV dry weight to LV+septal dry weight, suggesting that these changes are partially due to RV hypertrophy. The significant improvement in RV diastolic properties after both NO and milrinone administration suggests that these agents may be effective forms of pharmacological therapy for improving RV diastolic dysfunction in the setting of CPH.

Right ventricular failure--insights provided by a new model of chronic pulmonary hypertension

This study was designed to examine the effects of both nitric oxide and milrinone on pulmonary hemodynamics and right ventricular function using a newly established model of monocrotaline pyrrole-induced chronic pulmonary hypertension. Sixteen mongrel dogs (23-25 kg) were used. All animals underwent percutanous pulmonary artery catheterization to measure right heart hemodynamics prior to and 8 weeks after a right atrial injection of either monocrotaline pyrrole (MCTP, n=8) or placebo (CTL, n=8). Eight weeks postinjection, all hearts were instrumented with a pulmonary artery flow probe and intracavitary micromanometers. Data were collected at baseline as well as following both nitric oxide and milrinone administration. There was no significant difference in the baseline hemodynamic measurements between the two groups. Eight weeks postinjection, significant increases in the pulmonary artery pressure and pulmonary vascular resistance were observed in MCTP compared with CTL. Both nitric oxide and milrinone resulted in significant improvements in pulmonary vascular resistance, pulmonary blood flow, and right ventricular contractility. In addition, nitric oxide caused a significant improvement in pulmonary artery pressure and transpulmonary efficiency, while milrinone led to a significant increase in right ventricular hydraulic power. This study demonstrates the well-known clinical effects of nitric oxide and milrinone in improving pulmonary hypertension, which were also associated with an increase in pulmonary blood flow, transpulmonary efficiency, and right ventricular hydraulic power in the setting of monocrotaline pyrrole-induced chronic pulmonary hypertension.

Effects of milrinone on lung water content in dogs with acute pulmonary hypertension

To evaluate the effects of milrinone (MIL) on hemodynamics and lung water content, we used 10 mongrel dogs with pulmonary hypertension (PH). To induce pulmonary hypertension, we administered two injections of glass beads stirred in saline to dogs. Mean pulmonary arterial pressure (PAP) and pulmonary vascular resistance significantly increased following induction. Milrinone, which inhibits cyclic AMP phosphodiesterase-(PDE) demonstrated pulmonary vasodilation, indicated a reduction in these two parameters. To clarify the drug mechanism, we measured lung water content as extravascular lung thermal volume (ETVL) using a thermo/sodium double-indicator dilution method. The induction of pulmonary hypertension produced a transient reduction in extravascular lung thermal volume. The parameter remained constant following milrinone administration, whereas the control showed a gradual increase. Of the 10 dogs, five were killed to measure gravimetrically the volume of lung water content as a comparison with extravascular lung thermal volume. We concluded that milrinone produced pulmonary vasodilation which induced a reduction in the transmural capillary pressure gradient according to Starling's hypothesis. This study suggests that the reduction in the transmural pressure gradient induced by milrinone may also prevent the re-elevation in extravascular lung thermal volume. Milrinone increases the cyclic AMP level in the endothelium and in the platelet which may affect either directly or indirectly the permeability of capillary endothelium.