Oral Glutamine in Cardiopulmonary Bypass

INTRODUCTION Glutamine (GLN) is the most abundant free amino acid in the body, and commonly known as a non-essential amino acid due to the ability of most cells to produce it. It has many essential metabolic functions in the organism; it is an energy substrate for most cells (especially for enterocytes and lymphocytes), a precursor for nucleotide, glutamate, and (in particular) glutathione synthesis, and an important cellular antioxidant. It plays a central role in nitrogen transport within the body, and is the most important substrate for renal ammoniagenesis.

Studies have demonstrated that the presence of GLN in the medium regulates glutamine synthetase expression via a post-transcriptional mechanism, where the rate of glutamine synthetase protein degradation is diminished and its activity, augmented, in the presence of low GLN concentration. This amino acid modulates immune cell function and production of cytokines via attenuation of multiple pathways of inflammation, such as Nuclear Factor kB (NF-kB), protein kinases, inhibition of increases in Nitric Oxide Synthase (iNOS) expression.

It has been shown to be beneficial for the metabolically stressed patient, especially the critically ill patients. During acute illnesses, patients experience nutritional depletion, which correlates with low plasma and low mucosal GLN concentrations.

GLN levels are significantly decreased in critical illness, leading to an increase in infectious complications, organ failure and mortality. Moreover, in cases of ischemia/reperfusion injury, GLN increases the myocardial ATP/ADP ratio and prevents intracellular lactate accumulation.

GLN has been shown to be a key nutrient in the body's response to stress and injury described in multiple studies with in vitro and in vivo myocardial injury following I/R. These include enhanced myocardial glutathione, adenosine triphosphate (ATP), and glutamate (a major stress substrate for the stressed myocardium) post-I/R injury, and induction of heat shock proteins, especially Heat Shock Protein-70 (HSP-70) in critically ill patients. The stress tolerance provided by HSP-70 can protect against cellular injury, lung injury, ischemia/reperfusion injury, and septic shock, by the GLN influences on the inflammatory response, oxidative stress, apoptosis modulation, and the integrity of gut barrier.

Protection of the myocardium following ischemia and reperfusion injury is crucial in the perioperative management of patients after cardiopulmonary bypass (CPB). This ischemia is the most important factor of reversible perioperative risk for cardiovascular complications. Patients undergoing CPB with a heart-lung machine (termed cardiopulmonary bypass) are at an increased risk of having abnormal "inflammation" in their body after surgery. Such inflammation can contribute to slower recovery from surgery, an increased risk of infection, an increased risk of damage to organs other than the heart, and a more complicated course.

Numerous experimental and clinical trials have demonstrated the cardioprotective effects of GLN, including dose-dependent enhanced myocardial functional recovery following acute normothermic ischemia in the rat. GLN has also been shown to reduce infarct size to approximately 39% in a rabbit model following ischemia/reperfusion injury.[19] GLN treatment also increased load tolerance in patients with ischemic heart disease (IHD).

In 2012, a clinical study observed a significant decrease in TROP-I in patients with ischemic heart disease who underwent CPB with extracorporeal circulation who received (0.4 g/kg of GLN, Dipeptiven solution) one day before surgery. The case group (n=25) showed reduced levels of TROP-I compared to control group (1.2 ng/ml to 2.4 ng/ml), p=0.035 on the first postoperative day. Also, the median cardiac index and median stroke index were higher in case group following CPB, improving myocardial function.

In another study, the authors observed significantly decreased TROP-I levels in their pilot-clinical trial. They performed an oral intervention in patients undergoing CPB, giving 25 g twice of GLN supplement. TROP-I levels were significantly lower at 24, 48, and 72 postoperatively hours (all p < 0.05).

Despite the varied experimental data showing the cardioprotective effects of GLN, there is a lack of clinical trials with CPB patients. The finding that lower levels of cardiac injury markers are observed in patients treated with oral GLN prior to CPB can have major implications in these patients. The purpose of this study is to analyze the effect of a preoperative oral supplementation of GLN in reducing postoperative levels of Troponin-I in Mexican patients who underwent CPB under extracorporeal circulation. The oral GLN proposed in this study is based on what has been previously studied and what is considered safe.

MATERIALS AND METHODS. A controlled clinical trial performed in 28 patients who underwent cardiopulmonary bypass with confirmed diagnosis of ischemic heart disease. The protocol was made between January 2013 and December 2014 in the Specialty Hospital of the Western National Medical Center, Mexican Institute of Social Security.

The protocol received ethics approval by the Local Committee of Ethics in Research and Health. All patients gave informed consent prior to participation.

Participants were men or nonpregnant women, aged 40-70 years. Participants were excluded on the basis of preexisting kidney, liver dysfunction or other comorbid conditions such as drug or alcohol abuse, positivity to human immunodeficiency virus (HIV), hepatitis B / C or allergies against components of GLN. Participants were also excluded on the basis of signs of ongoing ischemia, defined by persistent elevation of TROP-I and CK-MB levels. They were also excluded if they were on a diet with any supplemental GLN.

Treatment Following study enrollment, patients were randomized (blinded envelopes that were opened sequentially by a blinded study pharmacist), to receive either oral GLN supplement group or control group (CONT) with maltodextrin (as isocaloric complex carbohydrate).

All investigators and clinical caregivers were blinded to study intervention. All patients in the GLN group received a GLN supplement. The total GLN/maltodextrin dose given to patients was standardized to 0.5 g/kg/day during 3 days prior to CPB, and taking one final dose of 0.25 g/kg/day of GLN/maltodextrin in the morning of surgery 4 hours prior to initiation of anesthesia.

Compliance with ingestion of the study drug was assessed via daily reminder calls from the study investigator and required empty package returns.

Patient Sample Collection and Analysis Blood was collected at baseline (one hour before surgery), and then one hour after surgery and 12 and 24 postoperative hours. After collection, blood was processed for analysis of cardiac injury markers: TROP-I, creatine phosphokinase (CPK) and creatine phosphokinase-Mb (CPK-Mb) and were analyzed using Meso Scale technology (Meso Scale Discovery, Gaithersburg, MD).

Clinical Data Collection All patients had essential demographic information collected as well as preoperative cardiac ejection fraction at study enrollment. In addition, postoperative complications were evaluated, including stroke, infections and postoperative vasopressor therapy. The time of time of aortic clamp during surgery and days in the Critial Care Unit (UCI) were measured. Mortality in the postoperative period was also collected.

Statistical Analysis Qualitative variables are expressed as raw numbers and proportions, whereas the quantitative variables are expressed as means ± standard deviations. To compare quantitative variables in the the results between the two groups, the U Mann Whitney test was used. Qualitative variables were measured using Chi 2 of Fisher's exact test in any expected values below 5. A p value of 0.05 or less was considered significant. The analysis was performed using Excel 2013 and Statistical Package for the Social Sciences (SPSS) version 20 for Windows (IBM Corp., NY, USA).

Ethical Considerations The study was conducted according to the principles of the Declaration of Helsinki and the Guidelines for Health Research in Mexico. The protocol was approved by the Local Committee on Health Research.