Phase III Study of Intravenous Glutamate Infusion for Metabolic Protection of the Heart in Surgery for Unstable Coronary Artery Disease

Myocardial preservation in cardiac surgery has mainly focused on the period when the heart is arrested (cross-clamp time). Today the heart can be arrested for up to 2-3 hours without major consequences. However, in spite of comparatively short cross-clamp times approximately 10% of the patients undergoing coronary surgery sustain significant myocardial injury whereas perioperative myocardial infarction is rare in aortic valve surgery despite longer cross-clamp times. The reason for this is that preoperative ischemia, and to some extent postoperative ischemia, remain major risk factors for development of myocardial infarction in patients with ischemic heart disease. In light of this, we suggest that efforts to improve outcome and reduce permanent myocardial damage should focus on the preoperative and the postoperative phase of coronary surgery. Furthermore, efforts should be instituted to reduce reperfusion injury and minimize permanent myocardial damage in long-standing or severe myocardial ischemia.

Metabolic intervention with intravenous glutamate infusion, offers the prospect of addressing the issues above and extending myocardial protection into the pre- and postoperative phase. Glutamate is an important substrate for the intermediary metabolism of the heart, particularly in association with ischemia. The effects of glutamate are partly related to its role in the malate-aspartate shuttle, transporting reducing equivalents across the mitochondrial membrane, regulating the NAD/NADH balance in the cytosol of the cells, and thereby enhancing anaerobic glycolysis during ischemia. Furthermore, glutamate contributes to an alternative anaerobic pathway for regeneration of high-energy phosphates, by substrate level phosphorylation in the Krebs cycle. Glutamate also improves clearance of metabolic waste produced during ischemia such as lactate and NH3, by taking part in the reactions involving transamination of pyruvate to alanine and of glutamate to glutamine. During reperfusion glutamate contributes to the replenishment of Krebs cycle intermediates lost during ischemia, which is essential for recovery of oxidative metabolism.

Administration of glutamate to patients with stable angina pectoris has been found to increase tolerance to stress-induced ischemia. Ischemia before onset of cardiopulmonary bypass has been established as a major risk factor for postoperative myocardial infarction. Patients with unstable coronary artery disease may have critical ischemia at rest and are particularly vulnerable to the increased oxygen demands during the early stages of coronary surgery. In a pilot study on patients operated urgently for unstable angina we found metabolic signs compatible with improved tolerance to ischemia before surgery and improved recovery of oxidative metabolism during early reperfusion. These results warrant further studies to evaluate the potential clinical benefit of preoperative glutamate infusion extended into the early postoperative period.

Comparisons: Intravenous infusion of 0.125 M glutamic acid solution v saline at a rate of 1.65 ml/hour and kg body weight beginning with institution of anesthesia and stopping 2 hours after unclamping of aorta in patients operated for unstable coronary artery disease.

Preliminary power analysis (80% power; p<0.05) suggests that 2214 patients will be required with regard to primary end-point assuming 30% reduction of events occurring in 12% of untreated patients.