Inhibition of vasoconstriction by phosphodiesterase III inhibitor milrinone in human conduit arteries used as coronary bypass grafts

Species comparison of compounds with known blood pressure effects in a vascular smooth muscle cell collagen contraction assay

INTRODUCTION

There is a great need for new approaches early in drug discovery that have the potential to improve clinical translation of compound-mediated cardiovascular effects. Current approaches frequently rely on in vivo animal models or in vitro tissue bath preparations, both of which are low throughput and costly. An in vitro surrogate screen for blood pressure using primary human cells may serve as a higher throughput method to quickly select compounds void of this secondary pharmacology and potentially improve late-stage drug development outcomes.

METHODS

In this study, we investigated 10 compounds with published in vivo blood pressure effects in a commercially available collagen contraction assay and evaluated rat, human, and canine (aortic) vascular smooth muscle cells (VSMCs). The aim of this study was to evaluate consistency between species and test their ability to predict the effects of known human vasodilators and constrictors. VSMCs were embedded at the same cell density in a collagen matrix which then floated freely in media containing test compounds. Collagen discs contracted faster than vehicle treated controls when incubated with a constrictor, and slower in the presence of a dilator.

RESULTS

Rat VSMCs responded as predicted of a VSMC-only culture to 9 out of 10 compounds. Human VSMCs responded as predicted to 8 out of 10 compounds, and canine VSMCs responded to 7 out of 10 compounds.

DISCUSSION

Our results suggest that rat VSMCs predict 90% of the effects of known vasoactive compounds in the collagen contraction assay while human and canine VSMCs were slightly less predictive (80% and 70%, respectively). Although blood pressure regulation is a multi-faceted and complex process, our data suggests the collagen smooth muscle contraction assay is useful as a qualitative early screen of compounds that act directly on smooth muscle cells of the arterial vasculature.

Sex differences in vascular reactivity of coronary artery bypass graft conduits

Females have increase in-hospital mortality and poorer outcomes following coronary artery bypass grafting (CABG). Biological differences in the reactivity of the graft conduits to circulating catecholamine may contribute to this sex difference. This study examined sex differences in the vasoconstrictor responses of internal mammary artery (IMA) and saphenous vein (SV) conduits to phenylephrine (PE) and endothelin-1 (ET-1). Functional IMA and SV were obtained from 78 male and 50 female patients undergoing CABG (67.7 ± 11 and 69 ± 10 years, respectively) and subjected to the following experimental conditions. (1) Concentration response curves for PE and ET-1 were generated in an intact IMA and SV and endothelium denuded IMA segments, (2) in the presence of the nitric oxide synthase inhibitor (L-NAME) or the cyclooxygenase inhibitor (indomethacin) in an endothelium-intact IMA and (3) the activity state (abundance and phosphorylation) of the α1-adrenergic receptor was investigated using Phos-tag™ western blot analysis. (1) Compared to male, female IMA and SV were hypersensitive to PE but not ET-1 (p < 0.05). The female IMA hypersensitivity response to PE was abolished following endothelial denudation, (2) persisted in the presence of L-NAME but was abolished in the presence of indomethacin and (3) there was no sex differences in the abundance and phosphorylation of the α1-adrenergic receptor in IMA. Female IMA and SV graft conduits are hypersensitive to α1-adrenergic stimuli. This endothelial cyclooxygenase pathway-mediated hypersensitivity may produce excessive IMA and SV graft constriction in females administered catecholamines and could contribute to their poorer CABG outcomes.

Milrinone Does Not Improve Free Flap Survival in Microvascular Surgery

Free flap microvascular surgery involves the transfer of a mobilised tissue flap with complete vascular reanastomosis at the new site. Ischaemia frequently threatens flap survival and may require a return to the operating theatre for anastomotic revision. Arterial spasm and hypoperfusion are recognised as factors in flap ischaemia. Phosphodiesterase inhibitors such as milrinone may improve flap blood flow and possibly flap survival by arterial dilation and increasing cardiac output. To investigate the role of milrinone in this type of surgery, a double-blinded randomised controlled trial was conducted with 88 patients receiving either a milrinone bolus and infusion throughout surgery or placebo (normal saline). We found that milrinone did not improve graft survival, return to theatre rate, or surgically graded arterial spasm, but did require more vasopressor support. We conclude that intraoperative milrinone did not improve flap outcomes in microvascular surgery.

Perioperative diltiazem or nitroglycerin in on-pump coronary artery bypass: A systematic review and network meta-analysis

Background

Arterial graft spasm is a severe complication after coronary artery bypass graft (CABG). Among numerous potential antispasmodic agents, systemic application of diltiazem and nitroglycerin had been investigated most frequently over the past three decades. However, it remains inconclusive if either or both agents could improve patient outcomes by preventing graft spasm when applied perioperatively, and, if so, which one would be a better choice. The current systematic review and network meta-analysis aims to summarize the data from all available randomized clinical trials of perioperative continuous intravenous infusion of diltiazem and/or nitroglycerin in patients undergoing on-pump CABG in order to define and compare their roles in graft spasm prevention and their impacts on perioperative outcomes.

Methods

We searched Ovid Medline, PubMed, CINAHL, Google Scholar and Cochrane Center for randomized controlled trials that reported outcome effects of perioperative continuous intravenous infusion of diltiazem and/or nitroglycerin in patients undergoing elective on-pump CABG. Conventional meta-analyses were conducted to evaluate the pairwise comparisons (diltiazem vs. placebo; nitroglycerin vs. placebo; diltiazem vs. nitroglycerin) on perioperative outcomes. Network meta-analyses were implemented to compare the three regimens through direct and indirect comparison.

Results

Twenty-seven studies involving 1,660 patients were included. Pairwise and network meta-analyses found no significant difference in mortality among the groups. There are four studies that reported blood flow measurements of internal mammary artery grafts intraoperatively after dissecting or immediately after distal anastomosis while patients were on continuous intravenous infusion of diltiazem and nitroglycerin. Although insufficient for data synthesis, the measured results from all four studies suggest that both diltiazem and nitroglycerin significantly increased blood flow of arterial grafts compared to placebo. For other perioperative outcomes, compared to diltiazem, patients that received nitroglycerin had higher odds of postoperative atrial fibrillation (OR = 2.67, 95% CI: 1.15 to 6.24) and higher peak serum cardiac enzymes. Patients that received placebo had higher odds of atrial fibrillation (OR = 3.00, 95% CI: 1.18 to 7.63) and lower odds of requiring inotrope support (OR = 0.19, 95% CI: 0.04 to 0.73) compared to diltiazem. Data from the network meta-analysis indicated that diltiazem had significantly lower odds of postoperative atrial fibrillation compared to nitroglycerin (OR = 0.39, 95% CI: 0.18 to 0.85). In fact, the rank from highest to lowest rates of postoperative atrial fibrillation was placebo>nitroglycerin>diltiazem. The rank from highest to lowest odds of requiring inotropic support is nitroglycerin> diltiazem>placebo. However, placebo had significantly higher odds of postoperative myocardial infarction than diltiazem (OR = 4.51, 95% CI: 1.34 to 15.25). The rank from highest to lowest odds of postoperative myocardial infarction, transient cardiac ischemic event and atrial fibrillation is placebo>nitroglycerin>diltiazem.

Conclusion

Compared to nitroglycerin and placebo, perioperative continuous intravenous infusion of diltiazem had stronger protective effects against postoperative ischemic cardiac injuries and atrial fibrillation although patients may need more inotropic support. The increased blood flow from diltiazem use in arterial grafts may potentially contribute to the drug’s outcome benefits.

AkrinorTM, a Cafedrine/ Theodrenaline Mixture (20:1), Increases Force of Contraction of Human Atrial Myocardium But Does Not Constrict Internal Mammary Artery In Vitro

Background: Intraoperative hypotension is a common problem and direct or indirect sympathomimetic drugs are frequently needed to stabilize blood pressure. Akrinor^TM consists of the direct and the indirect sympathomimetic noradrenaline and norephedrine. Both substances are covalently bound to the phosphodiesterase (PDE) inhibitor theophylline, yielding theodrenaline and cafedrine, respectively. We investigated pharmacodynamic effects of Akrinor^TM and its constituents on contractile force and tension in human atrial trabeculae and internal A. mammaria rings.

Methods: Isometric contractions were measured in human atrial trabeculae at 1 Hz and 37°C. CGP 20712A and ICI 118,551 were used to elaborate β₁- and β₂-adrenoceptor (AR) subtypes involved and phenoxybenzamine to estimate indirect sympathomimetic action. PDE-inhibition was measured as a potentiation of force increase upon direct activation of adenylyl cyclase by forskolin. Human A. mammaria preparations were used to estimate intrinsic vasoconstriction and impact on the noradrenaline-induced vasoconstriction.

Results: Clinically relevant concentrations of Akrinor^TM (4.2–420 mg/l) robustly increased force in human atrial trabeculae (EC₅₀ 41 ± 3 mg/l). This direct sympathomimetic action was mediated via β₁-AR and the effect size was as large as with high concentrations of calcium. Only the highest and clinically irrelevant concentration of Akrinor^TM increased the potency of forskolin to a minor extent. Norephedrine has lost its indirect sympathomimetic effect when bound to theophylline. Increasing concentrations of Akrinor^TM (4.2–168 mg/l) alone did not affect the tension of human A. mammaria interna rings, but shifted the noradrenaline curve rightward from -logEC₅₀ 6.18 ± 0.08 to 5.23 ± 0.05 M.

Conclusion: Akrinor^TM increased cardiac contractile force by direct sympathomimetic actions and PDE inhibition, did not constrict A. mammaria preparations, but shifted the concentration-response curve to the right, compatible with an α-AR antagonistic effect or PDE inhibition. The pharmacodynamic profile and potency of Akrinor^TM differs from noradrenaline and norephedrine in vitro. We anticipate metabolism of theodrenaline and cafedrine resulting in a different pharmacodynamic profile of Akrinor^TMin vivo.

Cafedrine/Theodrenaline (20:1) Is an Established Alternative for the Management of Arterial Hypotension in Germany-a Review Based on a Systematic Literature Search

A 20:1 combination of cafedrine:theodrenaline (Akrinor®) is widely used in Germany for the treatment of hypotensive states during anesthesia and in emergency medicine. Although this drug formulation has been available since 1963, there are few studies relating to its use and many of the data are only available in German. In this article, we summarize the available data and propose mechanisms for the effects of cafedrine/theodrenaline on cardiac muscle cells and vascular smooth muscle cells. Cafedrine/theodrenaline leads to a rapid increase in mean arterial pressure that is characterized by increased cardiac preload, stroke volume, and cardiac output. Systemic vascular resistance and heart rate remain mostly unchanged. Factors which impact the effects of cafedrine/theodrenaline are gender, high arterial pressure at baseline, use of β-blockers, and heart failure. Importantly, the drug is frequently used in obstetric anesthesia without detrimental effects on umbilical cord pH or APGAR score.

Pharmacologic Management of Acute Heart Failure: A Review

Because of improved social and medical standards in the western world, the average age of the population continues to increase. This altering demographic profile has its impact on the surgical population, resulting in a dramatic increase in the number of patients with exten sive cardiovascular disease, presenting for either car diac or noncardiac surgery. As a result, the incidence of perioperative acute congestive heart failure (CHF) is a major clinical problem at the present time. For the pharmacologic management of acute CHF, the β-adreno ceptor agonists continue to be the mainstay of therapy, due to their short duration of action and hemodynamic controlability by continuous intravenous infusion. Mono therapy with the phosphodiesterase type III (PDE) inhibi tors appears to be the treatment of choice in selected patients with high systemic vascular resistance or re sidual β-adrenoceptor blockade. Combination therapy especially deserves attention, because this approach allows the enhancement of contractile force, possibly without the side effects associated with the administra tion of high concentrations of one agent alone. New developments including ultrashort-acting PDE inhibi tors, calcium-dependent calcium sensitizers, and cal cium promotors are anxiously awaited.

Antispastic Management in Arterial Grafts in Coronary Artery Bypass Grafting Surgery

Arterial grafts have long-term patency superior to vein grafts but have a tendency to develop spasm that can lead to potentially life-threatening complications. A perfect antispastic protocol should include advanced surgical technique and adequate pharmacologic methods. All pharmacologic vasodilator drugs relax the vessel through specific mechanisms, and therefore, there is no perfect, single best vasodilator to prevent or treat spasm of the arterial graft against all mechanisms of contraction. One of the choices is to use a combination of pharmacologic vasodilators targeting different mechanisms of spasm to obtain the reliable and best effect.

Arterial grafts: clinical classification and pharmacological management

In comparison with standard saphenous vein grafts, use of the internal mammary artery (IMA) as a coronary artery bypass graft has achieved superior long-term results. This is related to the differences in the biological characteristics between the venous and arterial grafts. However, even arterial grafts are not uniform in their biological characteristics. The variation in the perioperative behavior of the grafts and in their long-term patency may be related to different characteristics. These factors should be taken into account in the use of arterial grafts, some of which are subjected to more active pharmacological intervention during and after the operation to obtain satisfactory results. To better understand the biological behavior of the grafts, their common features and their differences, a clinical classification may be useful for a practicing surgeon. Based on experimental studies of their vasoreactivity combined with anatomical, physiological and embryological considerations, we have proposed a functional classification for arterial grafts that may be useful clinically. Our classification suggests that there are three types of arterial grafts: Type I-somatic arteries; Type II-splanchnic arteries; and Type III-limb arteries. Type I arteries have enhanced endothelial function and release more nitric oxide and other relaxing factors. Type II arteries, such as the gastro-epiploic artery, and Type III arteries, such as the radial artery (RA), have higher pharmacological reactivity to vasoconstrictors. This classification explains why the IMA has the best long-term patency. Because Type II and III arteries are prone to spasms due to higher contractility, they require more active pharmacological interventions. Furthermore, the harvesting technique of the conduits, including the saphenous vein and IMA, are described and discussed in this article. Prevention of spasms using two cocktails of medications (verapamil + nitroglycerin and nicardipine + nitroglycerin) during harvesting of the conduits is described. These solutions have been demonstrated to be clinically effective.

Cellular mechanisms underlying the effects of milrinone and cilostazol to suppress arrhythmogenesis associated with Brugada syndrome

BACKGROUND

Brugada syndrome is an inherited disease associated with vulnerability to ventricular tachycardia and sudden cardiac death in young adults. Milrinone and cilostazol, oral phosphodiesterase (PDE) type III inhibitors, have been shown to increase L-type calcium channel current (ICa) and modestly increase heart rate by elevating the level of intracellular cyclic adenosine monophosphate.

OBJECTIVE

To examine the effectiveness of these PDE inhibitors to suppress arrhythmogenesis in an experimental model of Brugada syndrome.

METHODS

Action potential (AP) and electrocardiographic recordings were obtained from epicardial and endocardial sites of coronary-perfused canine right ventricular wedge preparations. The Ito agonist NS5806 (5 μM) and Ca(2+) channel blocker verapamil (2 μM) were used to pharmacologically mimic Brugada phenotype.

RESULTS

The combination induced all-or-none repolarization at some epicardial sites but not others, leading to ST-segment elevation as well as an increase in both epicardial and transmural dispersion of repolarization. Under these conditions, phase 2 reentry developed as the epicardial AP dome propagated from sites where it was maintained to sites at which it was lost, generating closely coupled extrasystoles and ventricular tachycardia. The addition of the PDE inhibitor milrinone (2.5 μM) or cilostazol (5-10 μM) to the coronary perfusate restored the epicardial AP dome, reduced dispersion, and abolished phase 2 reentry-induced extrasystoles and ventricular tachycardia.

CONCLUSIONS

Our study identifies milrinone as a more potent alternative to cilostazol for reversing the repolarization defects responsible for the electrocardiographic and arrhythmic manifestations of Brugada syndrome. Both drugs normalize ST-segment elevation and suppress arrhythmogenesis in experimental models of Brugada syndrome.

Fasudil is a superior vasodilator for the internal thoracic artery in coronary surgery

BACKGROUND

The internal thoracic artery (ITA) is a very useful conduit for coronary artery bypass artery (CABG), with excellent long-term patency. With the purpose to dilate the ITA graft and increase graft free flow (GFF) intraoperatively, we evaluated the usefulness of intraluminal injection of fasudil, a Rho-kinase inhibitor, in comparison to the conventional graft dilating agent, papaverine.

METHODS

Between June 2011 and January 2012, 30 patients with ischemic heart disease who underwent isolated CABG using ITA were enrolled. The patients were randomly assigned to 2 groups: the fasudil group (n = 15) in which fasudil solution 0.9 mg/dL was injected into the ITA, and the papaverine group (n = 15) in which papaverine solution (0.4 mg/mL) mixed with heparinized blood was used. Outcome measures were left ITA GFF, heart rate, and mean blood pressure during flow measurements, and histopathologic examination of the ITA.

RESULTS

In the fasudil group, GFF increased significantly (p < 0.01) from 19.7 ± 15.2 mL/minute at baseline to 66.9 ± 31.7 mL/minute after fasudil injection. In the papaverine group, GFF increased significantly (p < 0.01) from 22.9 ± 17.3 mL/minute at baseline to 44.8 ± 26.7 mL/minute after papaverine injection. Blood pressure and heart rate did not change significantly after drug injection in both groups. The GFF was significantly higher (p = 0.038) in fasudil-treated ITA than in papaverine-treated ITA. Histopathologically, the diameter of the ITA was markedly increased after fasudil injection. Elastica van Gieson staining showed that the multiple elastic lamellae structure was intact.

CONCLUSIONS

Fasudil exhibited very potent vasodilatory effect on the ITA compared with conventional papaverine resulting in increased GFF. This agent is a useful graft dilating agent.

Comparison of relaxation responses to multiple vasodilators in TxA(2)-analog and endothelin-1-precontracted pulmonary arteries

BACKGROUND

Peri-operative pulmonary hypertension can lead to right ventricular dysfunction and to an increase in morbidity and mortality. Altered function of the pulmonary vascular endothelium and vasoconstriction play a crucial role in the development of elevated pulmonary vascular resistance. Because pulmonary artery vasoreactivity is dependent on many factors including the constricting agent that precipitated the event therefore the aim of the current study was to investigate the effectiveness of different classes of vasodilator agents to reverse endothelin-1 (ET-1) or thromboxane A(2) (TxA(2))-induced vasoconstriction in porcine pulmonary artery (PA) in vitro.

METHODS

Relaxation responses to vasodilatory drugs were studied in PA precontracted with ET-1 (1 x 10(-8) M) or TxA(2) analog (U46619, 1 x 10(-8) M). All vasodilating drugs were added in a cumulative fashion and isometric tension measurements were obtained using an organ chamber technique.

RESULTS

In both groups relaxation responses to the vasodilators were dose dependent. When ET-1 was used as a constrictor nitroglycerin and milrinone caused nearly complete (80-100%) relaxation, whereas other agents were of limited effectiveness (40-50%). On the other hand, in the vessels constricted with U46619, olprinone, indomethacin, prostaglandin E(1) (PGE(1)), nitroglycerin, milrinone and clevidipine induced complete (90-110%) vasodilatation but brain natriuretic peptide (BNP), L-arginine, and isoproterenol relaxed the vessels maximally by 45-60%.

CONCLUSIONS

Nitroglycerin and milrinone are very effective in reversing ET-1 and U46619-induced pulmonary vasoconstriction in vitro. The effectiveness of other drugs studied was dependent on the type of constrictor used. BNP, L-arginine and isoproterenol were shown to have minimal vasodilatory effects in porcine PA.

Update in computer-driven weaning from mechanical ventilation

Weaning from mechanical ventilation is a complex process requiring assessment and interpretation of both objective and subjective clinical parameters. For many years, automated computerised systems for various medical processes, including respiratory management, have been proposed to optimise decision-making and reduce variation amongst clinicians. SmartCare/PS, available since 2003 as a software application for the EvitaXL ventilator (Dräger Medical AG & Co. KG, Lübeck, Germany), is one of the first such ventilator systems to be made commercially available. SmartCare/PS can be described as a knowledge-based weaning system, which adjusts pressure support based on measurement of the patient's respiratory status, specifically the spontaneous respiratory rate, tidal volume and end-tidal carbon dioxide with the aim of optimising the weaning process. The primary proposed advantage of this system is an ability to provide management of ventilatory weaning through continuous physiological monitoring and real-time interventions. The relatively small number of available clinical studies indicate the system is able to deliver appropriate ventilation during pressure support weaning from both short-term and prolonged ventilation. Of potential clinical note, a recent study suggested that use of SmartCare/PS might be associated with useful reductions in the duration of weaning compared to existing clinical practice using weaning protocols. One recently published randomised trial supports this conclusion. However, given the known large variation in international critical care ventilatory practices further randomised trials are desirable.

Greater vasopressin-induced vasoconstriction and inferior effects of nitrovasodilators and milrinone in the radial artery than in the internal thoracic artery

BACKGROUND

Vasopressin may be a potential cause of spasm in both the radial artery and the internal thoracic artery. This study compared the vasopressin-induced contraction and the effects of milrinone, nitroglycerin, and nitroprusside in vasopressin-induced contraction between the human radial artery and the internal thoracic artery to find effective antispastic methods for arterial grafts.

METHODS

Concentration-contraction (normalized as force gram produced by each millimeter of the circumference tissue of the artery) curves for vasopressin with or without pretreatment of vasodilators and concentration-relaxation curves for the vasodilators listed were established in the human radial artery (n = 107) and internal thoracic artery (n = 122) segments.

RESULTS

Vasopressin induced a greater contraction in the radial artery than in the internal thoracic artery (1.9 +/- 0.2 g/mm vs 0.6 +/- 0.1 g/mm, P < .001) with a higher sensitivity (lower EC(50): -9.28 +/- 0.11 vs -8.91 +/- 0.05 log(10)M, P = .006). Milrinone was less potent than nitroglycerin and nitroprusside with higher EC(50) (P < .05) in both the internal thoracic artery and radial artery. Pretreatment with milrinone and nitroprusside significantly inhibited vasopressin contraction in the internal thoracic artery but had little effect in the radial artery. Pretreatment with nitroglycerin did not significantly inhibit the maximum vasopressin contraction in either the internal thoracic artery or radial artery.

CONCLUSION

The radial artery is more prone to develop spasm related to vasopressin than is the internal thoracic artery, and the effect of vasodilators in vasopressin-induced contraction is different in the radial artery from that in the internal thoracic artery. A more significant prophylactic antispastic effect of milrinone and nitroprusside is demonstrated in the internal thoracic artery than in the radial artery. Therefore, more intensive antispastic treatment is necessary in the radial artery than in the internal thoracic artery during coronary artery bypass grafting.

Cardiac and vascular effects of diltiazem, dobutamine and amrinone, drugs used after myocardial revascularization

Hemodynamic care during postoperative management of myocardial revascularization should include vasorelaxing drugs to insure adequate graft and coronary flow, and stimulation of stroke volume to maintain vascular perfusion pressure. We tested the cardiac (inotropic and lusitropic) and vascular (relaxant) effects of diltiazem (0.1 nM to 0.1 mM), dobutamine (10 microM to 10 mM) and amrinone (10 microM to 1 mM) on isolated rat atria and thoracic aorta, and also on isolated human saphenous vein (HSV) and human mammary artery (HMA). Dobutamine produced a maximal positive inotropic effect (+dF/dt max = 29 +/- 7%) at its ED50 for aortic relaxation (88 +/- 7 microM). Conversely, at their ED50 for aortic relaxation diltiazem depressed myocardial contractility and amrinone did not exhibit myocardial effects. In HSV and HMA contracted with 80 mM potassium, diltiazem and dobutamine (but not amrinone) had a vasorelaxant activity similar to that in rat aorta. Norepinephrine-contracted human vessels were significantly more sensitive than potassium-contracted vessels to the relaxant effect of amrinone (ED50 HMA = 15 +/- 5 microM, ED50 HSV = 72 +/- 31 microM, P < 0.05). We conclude that at concentrations still devoid of myocardial effects dobutamine and amrinone are effective dilators in graft segment vessels and rat aorta contracted by membrane depolarization. If the difference between aortic and myocardial tissue still holds in human tissues, at the appropriate concentrations these drugs should be expected to improve cardiac performance while still contributing to the maintenance of graft patency.

Smooth muscle and endothelial function of arterial grafts for coronary artery bypass surgery

1. Various small/mid-sized conductance arteries have been used for coronary artery bypass grafting as arterial grafts, but unanimous opinion to the best use of these grafts has not been formed. 2. Arterial grafts are not uniform in their biological characteristics. 3. The differences among the arteries may involve the contractility of the vessel. Based on studies of the contractility of arterial grafts, a clinical classification was formed to describe the tendency of vasospasm in the arterial grafts. 4. The differences among arterial grafts also involve endothelial function. This refers to both nitric oxide release from the endothelium, as well as endothelium-derived hyperpolarizing factor-mediated hyperpolarization and relaxation. 5. The difference in the peri-operative behaviour of the grafts and in their long-term patency may be related to different characteristics. These should be taken into account in the use of arterial grafts, some of which are subject to more active pharmacological intervention during and after operation to obtain satisfactory results. The clinical choice of grafts must be based on the general condition of the patient, the biological characteristics of the graft, the anatomy of the coronary artery, the match between the coronary artery and the graft and the technical considerations, including antispastic management.

Milrinone increases middle cerebral artery blood flow velocity after cardiopulmonary bypass

OBJECTIVE

To evaluate the effects of milrinone on middle cerebral artery blood flow velocity (Vmca) and pulsatility index (PI) during normocapnia and hyperventilation in adults after cardiopulmonary bypass (CPB).

DESIGN

A prospective study.

SETTING

University-affiliated hospital and Veterans Affairs Medical Center.

PARTICIPANTS

Twenty-five adults with left ventricular ejection fraction >40% undergoing coronary artery bypass graft surgery.

INTERVENTIONS

After separation from CPB, using transcranial Doppler ultrasonography, peak and mean Vmca and PI were recorded before and after the administration of 50 microg/kg of milrinone under normocapnia and with hyperventilation.

MEASUREMENTS AND MAIN RESULTS

Heart rate, arterial blood pressure, central venous pressure, and cardiac output were documented after each study period. Compared with baseline, milrinone increased peak Vmca by 20%, increased mean Vmca by 19%, and decreased PI by 16% (p < 0.001). Before the administration of milrinone, hyperventilation decreased peak Vmca by 20%, decreased mean Vmca by 26%, and increased PI by 24% (p < 0.01). After milrinone administration, hyperventilation also decreased peak Vmca by 22%, decreased mean Vmca by 21%, and increased PI by 19% (p < 0.01). Milrinone increased cardiac index and decreased mean arterial pressure and systemic vascular resistance (p < 0.05); however, heart rate and central venous pressure remained unchanged.

CONCLUSIONS

The administration of milrinone increases cerebral blood flow after CPB most likely as a result of cerebral vasodilation. The response to hyperventilation seems to be partially preserved.

Intravenous milrinone in cardiac surgery

Phosphodiesterase inhibitors including milrinone produce positive inotropic effects by slowing the hydrolysis of cyclic adenosine monophosphate in the myocardium. With a loading dose of 50 microg/kg followed by an infusion of 0.5 microg x kg(-1) x min(-1), milrinone increases stroke volume index and left ventricular velocity of circumferential fiber shortening after weaning from cardiopulmonary bypass. Milrinone has potential for the treatment and prevention of internal mammary artery spasm because of its vasodilative effect, which is similar to that of papaverine, and is a potent pulmonary vasodilator for patients with right ventricular dysfunction and pulmonary vasoconstriction. Low-dose milrinone may have antiinflammatory properties and potentially can improve splanchnic perfusion.

Effects of potassium channel opener KRN4884 on human conduit arteries used as coronary bypass grafts

AIMS

The effects of a new potassium channel opener KRN4884 on human arteries have not been studied. This study was designed to investigate the effects of KRN4884 on the human internal mammary artery (IMA) in order to provide information on possible clinical applications of KRN4884 for preventing and relieving vasospasm of arterial grafts in coronary artery bypass grafting.

METHODS

IMA segments (n = 140) taken from patients undergoing coronary surgery were studied in the organ chamber. Concentration-relaxation curves for KRN4884 were established in the IMA precontracted with noradrenaline (NA), 5-hydroxytryptamine (5-HT), angiotensin II (ANG II), and endothelin-1 (ET-1). The effect of glibenclamide (GBC) on the KRN4884-induced relaxation was also examined in NA or 5-HT-precontracted IMA. Concentration-contraction curves for the four vasoconstrictors were constructed without/with pretreatment of KNR4884 (1 or 30 microM) for 15 min.

RESULTS

KRN4884 induced less relaxation (P < 0.05) in the precontraction induced by ET-1 (72.9 +/- 5.5%) than by ANG II (94.2 +/- 3.2%) or NA (93.7 +/- 4.1%) with lower EC50 (P < 0.05) for ANG II (-8.54 +/- 0.54 log M) than that for NA (-6.14 +/- 0.15 log M) or ET-1 (-6.69 +/- 0.34 log M). The relaxation in the IMA pretreated with GBC was less than that in control (P < 0.05). KRN4884-pretreatment significantly reduced the contraction (P < 0.05) induced by NA (151.3 +/- 18.4% vs 82.7 +/- 8. 7%), 5-HT (82.7 +/- 12.2% vs 30.1 +/- 7.3%), and ANG II (24.3 +/- 6. 3% vs 5.4 +/- 1.6%), but did not significantly reduce the contraction induced by ET-1 (P > 0.05).

CONCLUSION

KRN4884 has marked vasorelaxant effects on the human IMA contracted by a variety of vasoconstrictors and the effect is vasoconstrictor-selective.

Vasorelaxant effect of phosphodiesterase-inhibitor milrinone in the human radial artery used as coronary bypass graft

OBJECTIVE

The radial artery is a spastic coronary bypass graft. We investigated the effect of the phosphodiesterase III inhibitor milrinone on the human radial artery.

METHODS

Radial artery segments (n = 76) taken from 15 patients were studied in an organ chamber. Concentration-relaxation curves for milrinone were established in the radial artery precontracted with 3 vasoconstrictors (phenylephrine, K(+), and U46619). In radial artery rings incubated with therapeutic plasma concentrations of milrinone (7 and 70 micromol/L) for 10 minutes, concentration-contraction curves for the 3 vasoconstrictors were constructed.

RESULTS

Milrinone caused a submaximal relaxation in phenylephrine- (98.6% +/- 1.4%), K(+)- (89.1 +/- 4.5%), or U46619- (74.2 +/- 8.0%) precontracted radial arteries at -4.5 log(10) M. The EC(50) was higher against K(+) (-5.85 +/- 0.24 log(10) M, P =.02) or U46619 (-5. 21 +/- 0.61 log(10) M, P =.03) than phenylephrine (-6.68 +/- 0.11 log(10) M). Pretreatment with milrinone depressed the contraction by phenylephrine from 70.0% +/- 7.9% to 23.5% +/- 9.3% (P =.003) and by K(+) from 138.6% +/- 5.8% to 73.0% +/- 13.9% (P =.006) and shifted the EC(50) 3.8-fold higher (P =.03) for phenylephrine and 2.2-fold higher for K(+) (P =.01). Milrinone reduced the U46619 contraction at low concentration (-8.5 log(10) M) but had little effect on the maximal contraction.

CONCLUSION

Milrinone is a potent vasodilator for the radial artery, with possibly higher potency in alpha-adrenoceptor- and depolarizing agent K(+)-mediated, but less potency in thromboxane A(2)-mediated, contraction. Because it also has a positive inotropic effect, this vasodilator may be particularly indicated for use in patients receiving radial artery grafts in coronary artery bypass grafting.

Vasorelaxant actions of enoximone, dobutamine, and the combination on human arterial coronary bypass grafts

Enoximone (a type III-selective phosphodiesterase inhibitor) and dobutamine (a beta-receptor agonist) are positive inotropic drugs frequently used in the postoperative management of coronary bypass surgery. The purpose of this study was to characterize their relaxant effects on the human internal mammary artery (IMA) and the gastroepiploic artery (GEA) and to test the hypothesis that their combination may have greater than additive relaxant effects. In organ baths, the relaxant effects of enoximone and dobutamine were tested on rings of IMA (n = 86) precontracted with U46619 (a thromboxane A2 mimetic), norepinephrine (NE), or KCl. The relaxant effects of dobutamine and enoximone also were tested on rings of GEA (n = 42) precontracted with U46619 and NE. The effect of the combination of enoximone and dobutamine were tested on rings of IMA (n = 24) precontracted with U46619 or NE. With respect to maximal relaxations induced by papaverine (10(-4) M), enoximone (< or =10(-3) M) caused full relaxations of IMA precontracted with NE, U46619, or KCI. Dobutamine (< or =10(-3) M) caused full relaxations of IMA precontracted with NE or KCI but only 46% (95% CI, 27-65) relaxation in the rings precontracted with U46619. Similar patterns of relaxation were observed in GEA rings, with dobutamine inducing partial relaxation in GEA precontracted with U46619. The pD2 values of enoximone and dobutamine were both significantly lower in segments precontracted with U46619. The in vitro threshold relaxant concentrations were in the upper limits or over the range of therapeutic plasma concentrations. The relaxant effect of the combination was significantly more important than the theoretic additive effect in IMA contracted with U46619 or NE. Enoximone and dobutamine are potent in vitro vasodilators but exert weak relaxant effects in IMA and GEA at concentrations in the therapeutic range. There is, however, a greater than additive vasorelaxant effect of the combination, suggesting that the vasorelaxant effect of the combination, in addition to the additive inotropic effect, may be beneficial to patients undergoing coronary bypass grafting.

Inhibition of vasoconstriction by the thromboxane A2 antagonist GR32191B in the human radial artery

AIMS

The newly revived coronary bypass graft, the radial artery (RA), is more spastic than the internal mammary artery. Thromboxane A2 is a potent vasoconstrictor for arterial grafts. This study was therefore designed to determine whether the specific thromboxane A2 (TP) receptor antagonist, GR32191B, is effective in inhibition of prostanoid or nonprostanoid receptors in the RA.

METHODS

The effect of GR32191B was studied in human RA segments, taken from coronary bypass patients, in organ chambers. Two effects of GR32191B were tested: (1) the relaxation induced by GR32191B in the RA precontracted with the TP receptor agonists U46619 and PGF2alpha or nonprostanoid vasoconstrictors (noradrenaline [NA], angiotensin II [AII], and K+ ) and (2) the inhibitory effect of GR32191B on TP receptor agonists and nonprostanoid vasoconstrictors.

RESULTS

In U46619 (10 nm, n=7) and PGF2alpha (1 microm, n=7) precontracted RA, GR32191B induced 100% relaxation (10-100 microm ) but not after precontraction with nonprostanoid stimuli (5.8% for K+, 25 mm, n=6, 24.4% for NA, 3 microm, n=8, and 53.2% for AII, 3 nm, n=5) (P<0.001). Treatment with GR32191B (30 nm ) significantly depressed the contraction with U46619 (from 160.1+/-11.0% to 116.8+/-13.1%, P<0. 05) or PGF2alpha (from 91.3+/-12.3% to 42.2+/-9.2%, P<0.01). The contraction was further abolished by 3 microm GR32191B. However, GR32191B at 3 microm did not significantly inhibit the contraction induced by either NA, AII, or K+.

CONCLUSIONS

GR32191B is a highly potent and specific TP receptor antagonist for the human RA. It may be particularly useful in inhibiting TXA2-mediated vasoconstriction and therefore in reducing the complications related to vasospasm in this graft.

Pharmacology of coronary artery bypass grafts

Spasm of arterial and venous graft conduits can occur both during harvesting and after the graft is connected. Attempts to overcome spasm during harvesting by probing or hydraulic distension can cause structural damage to the graft, which may impair short- and long-term patency. After a coronary artery bypass graft is connected, spasm can cause major problems with myocardial perfusion. To select the best pharmacologic agent to prevent or reverse vasoconstriction in a graft requires an understanding of the reactivity of that particular type of graft to vasoconstrictor and vasodilator agents. The pharmacologic reactivity of venous and arterial graft conduits has been documented through extensive studies of isolated vessels in the organ bath and of in situ grafts in the body. In this review we summarize the current state of knowledge of the reactivity of arterial and venous grafts to vasoconstrictor and vasodilator agents and describe the practical application of this knowledge in the operating room and in the postoperative period.

Arterial grafts for coronary artery bypass grafting: biological characteristics, functional classification, and clinical choice

Various arterial grafts have been used for coronary artery bypass grafting, but a unanimous opinion on how to best use these grafts has not been formed. Arterial grafts are not uniform in their biological characteristics. Differences between the perioperative behavior of the grafts and their long-term patency may be related to different characteristics. These characteristics should be taken into account in the use of arterial grafts, some of which are subject to more active pharmacologic intervention during and after operation to obtain satisfactory results. Clinical choice of grafts must be based on the general condition of the patient, the biological characteristics of the graft, the anatomy of the coronary artery, the match between the coronary artery and the graft, and technical considerations, including antispastic management.