Off-pump coronary artery bypass grafting (OPCAB) is a prevalent cardiac surgical technique designed to avoid the complications of cardiopulmonary bypass (CPB). Despite its benefits, OPCAB poses significant hemodynamic challenges, such as reduced cardiac output, increased atrial pressures, and mitral regurgitation due to mechanical shifts during surgery. Milrinone, a selective phosphodiesterase III inhibitor, has been proposed as a valuable adjunct to address these challenges. The present study provides a broad overview of the efficacy of prophylactic Milrinone administration in patients undergoing OPCAB procedures.
Coronary artery bypass grafting is the most commonly performed cardiac surgical procedure globally, primarily aimed at improving outcomes for patients with coronary artery disease.
Milrinone is a potent inotropic agent that has been extensively studied in the context of cardiac surgery. It has been shown to improve hemodynamic parameters, reduce the incidence of low cardiac output syndrome, and potentially improve clinical outcomes in patients undergoing cardiac surgeries.
Milrinone is a selective phosphodiesterase III inhibitor that exerts its effects through a dual mechanism of action, combining positive inotropic effects with vasodilation.
The elevated cAMP activates protein kinase A within cardiac myocytes, which phosphorylates critical proteins involved in calcium regulation. This encompasses the L-type calcium channel, which enhances activity and increases calcium influx during the action potential.
Milrinone improves diastolic relaxation by influencing calcium reuptake, a process known as positive lusitropy. By facilitating the rapid sequestration of calcium into the sarcoplasmic reticulum, Milrinone reduces cytosolic calcium concentrations during diastole, allowing the myocardium to relax more effectively.
This effect is crucial for patients with diastolic dysfunction or conditions such as left ventricular hypertrophy, where impaired relaxation can compromise ventricular filling.
Milrinone’s effects extend beyond the myocardium to the vascular system. In vascular smooth muscle, elevated levels of cAMP enhance uptake by the sarcoplasmic reticulum, thereby reducing intracellular calcium concentrations.
The pharmacokinetic properties of Milrinone further support its clinical utility in perioperative settings. It has a short half-life of 48–56 minutes, facilitating rapid titration of its effects.
While Milrinone has shown beneficial impacts, it is not devoid of potential risks. The most typical adverse effects are hypotension, abnormal heart rhythms, and low platelet count.
Milrinone, an agent known to relax the smooth muscles of blood vessels, may induce a significant decrease in systemic blood pressure, particularly when administered intravenously in loading dose or in patients sensitive to changes in their vascular resistance. To reduce this risk, the rate of infusion should be regulated conservatively and closely observed.
The drug can also lead to various cardiac arrhythmias like ventricular tachycardia, atrial fibrillation, and supraventricular tachycardia because it overloads cardiac myocytes with intracellular calcium.
Prolonged exposure to Milrinone causes plasma concentrations that interfere with platelet aggregation and might, therefore, be associated with an increased risk of bleeding in surgical patients. The reason is that increased intracellular levels of cyclic adenosine monophosphate also inhibit adenosine diphosphate (ADP) and arachidonic acid (AA)-induced platelet aggregation.
The degree of heart dislocation during off-pump coronary artery bypass grafting can significantly impact global cardiac output. The anterior and lateral ventricular walls exhibit greater displacement during systole and diastole compared to the septal and posteroinferior walls, which play a significant role in stroke volume. Surgical access to the left anterior descending artery is relatively straightforward, but operations on the posterior or lateral walls require displacing the heart from its pericardial cradle.
The stabilizer device immobilizes the anastomosis site, restricts local ventricular motion, and decreases dimensions. The location of the stabilizer, combined with the predominant contribution of the septal and posteroinferior walls to stroke volume, means that compression on the anterior and lateral walls has more serious hemodynamic consequences than on the posterior wall.
Significant disturbances arise during lateral wall exposure for circumflex artery anastomosis, as the heart is lifted more extensively than in left anterior descending artery surgery. The vertical heart position also induces distortions in the mitral and tricuspid annuli, leading to increased mitral regurgitation.
The challenges associated with off-pump CABG, such as impaired cardiac output, increased atrial pressures, and mitral regurgitation, have led to the use of Milrinone in this setting.
Jebeli et al. studied Milrinone in a randomized clinical trial for patients with myocardial dysfunction scheduled for CABG. Milrinone treatment was associated with better PAT results, increased LVEF, reduced serum creatine kinase concentration, and a lower incidence of atrial/ventricular arrhythmias compared to the placebo group.
Milrinone has favorable impacts on patients with right ventricular dysfunction. In the study conducted by Jo et al. comparing the effects of Milrinone on right ventricular function, it was found to improve right ventricular ejection fraction significantly compared to the placebo.
Beyond its positive impact on cardiac contractility and relaxation, Milrinone directly elevated blood flow through the grafted mammary arteries following coronary artery bypass surgery. Lobato et al.
Milrinone is usually administered at a 50 µg/kg bolus over 10 minutes, followed by a continuous infusion of 0.375–0.75 µg/kg/min for an immediate hemodynamic response.
Compared to other inotropic agents like dobutamine or epinephrine, Milrinone offers several advantages in the perioperative setting. Milrinone is not dependent on beta-adrenergic receptor stimulation, which can be impaired in the setting of heart failure or patients receiving concomitant beta-blocker therapy.
The vasodilatory effects of Milrinone can reduce afterload and preload, thereby improving cardiac output without significantly increasing myocardial oxygen consumption. Conversely, traditional catecholamine inotropes like dobutamine and epinephrine increase heart rate and myocardial oxygen demand, potentially exacerbating ischemia in the postoperative setting.
Unlike levosimendan, it causes less reduction in systemic vascular resistance and profound hypotension.
The comparative analysis of inotropes summarizes in
| Parameter | Milrinone | Dobutamine | Levosimendan | Epinephrine |
|---|---|---|---|---|
| PDE III inhibitor | β-adrenergic agonist | Calcium sensitizer | α and β-adrenergic agonist | |
| Minimal | Increases | Minimal | Increases | |
| Moderate | Mild | Strong | Minimal | |
| No significant increase | Increases | No significant increase | Increases | |
| 0.5–1 hour | Short | Long (~80 hours) | Short | |
| Moderate | Mild | High | Low | |
| Moderate | Moderate | Low | High | |
| None | Reduced efficacy | None | Reduced efficacy |
The use of Milrinone in patients undergoing off-pump coronary artery bypass grafting has been extensively studied and has demonstrated several benefits. Milrinone can effectively alleviate the hemodynamic challenges associated with CABG, such as impaired cardiac output, elevated atrial pressures, and increased mitral regurgitation
Through its positive inotropic and vasodilatory effects, Milrinone can sustain cardiac output and improve right ventricular function during surgical manipulation and displacement of the heart.
However, some caution is necessary when administering Milrinone, as the loading dose should be carefully titrated to avoid potential adverse impacts such as hypotension and arrhythmias.
Despite promising results, there are certain limitations to validate and optimize the use of prophylactic Milrinone in off-pump CABG. There is a scarcity of well-designed, multicenter, randomized controlled trials that address the research gaps. Studies that focus on diverse patient populations, including high-risk groups with comorbidities, may provide further insights into the role of Milrinone in improving postoperative outcomes. Investigations into the impact of Milrinone on long-term survival rates, cardiac function, and quality of life post off-pump CABG are also limited.
Artificial intelligence (AI) can revolutionize the administration of Milrinone in off-pump CABG. AI models can analyze continuous hemodynamic parameters (e.g., cardiac output, blood pressure, and heart rate) and predict patient-specific responses to Milrinone. Machine learning algorithms can predict individual patients’ responses to Milrinone doses based on cardiac function, comorbidities, and intraoperative conditions.
AI-driven systems can continuously assess the hemodynamic status and adjust Milrinone infusion rates in real time. AI can consolidate data from electronic health records, intraoperative monitors, and laboratory results to comprehensively view patient status.
Artificial intelligence has the potential to revolutionize the administration of Milrinone in off-pump CABG. AI models could analyze continuous hemodynamic data and predict individualized patient responses to Milrinone.
In summary, the use of Milrinone in patients undergoing off-pump CABG has shown promising results in improving cardiac performance, reducing the need for additional inotropic support, and mitigating the hemodynamic disturbances associated with surgical manipulation of the heart. The current evidence supports the use of Milrinone as a valuable adjunct in managing patients undergoing off-pump coronary artery bypass grafting procedures.
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No industry funding
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Manoranjan Padhi, Dharmajivan Samantaray, Manmaya Padhy and Sweta Mohanty – Conceptualization, Writing – original draft, review and editing
Manoranjan Padhi
Commissioned and externally peer-reviewed
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