**2. Mechanisms of stress hyperglycemia**

Stress Hyperglycemia, also called stress-induced diabetes, diabetes (6), is a multifact metabolic disorder that is characterized by the presence of hyperglycemia with hyperinsulinemia, peripheral resistance to insulin and an over-production of glucose by different mechanisms that result in incremented glycogenolysis and increased gluconeogenesis. Figure 1 shows how endogenous and exogenous predisposing factors may trigger the development of stress hyperglycemia, particularly during a cardiac surgery. The most important trigger is the surgical stress itself that conduces to a catabolic state that results in high levels of blood-glucose; the appearance of stress-hormones and a diminished peripheral response to insulin, cause hyperglycemia, and raises even more previous glycemia breeding an abnormal immune response (3, 6-9.). Stress hyperglycemia is caused mainly by the effects of counter-regulatory hormones (catecholamines, growth hormone, and cortisol) and by depletion of the functional reserve of the beta-cells in the Langerhans islets of the pancreas (7). During perioperative period of major surgeries, the counterregulatory hormones and the inflammatory response induced by surgical stress are the most important triggers of hyperglycemia (10). The degree of insulin resistance has been related to the magnitude and endurance of surgical stress. In addition, it has been reported that in the perioperative period, increased glucose reabsorption or decreased renal glucose clearance may enhance this phenomena that contribute to hyperglycemia (11).

#### **Predisposing factors**

Although stress hyperglycemia is mainly caused by six events: severe trauma, bleeding, hypothermia, septicemia, severe burns and great-magnitude surgeries, there are several factors that may contribute to this alteration, which could and should be explored in the preoperative examinations (Table 1). All of these predisposing factors can be divided in endogenous and exogenous factors.

a. Endogenous factors

The first and most important predisposing factor for developing stress hyperglycemia is to be previously diagnosed with Diabetes Mellitus; in this case glucose blood-levels after cardiac surgery may reach even 20 mmol/l (370mg/dl) or more, compared with nondiabetic patients which might reach 15 mmol/l (270 mg/ml) (12-14). With a previous diagnosis of diabetes, it is not just more likely to develop stress hyperglycemia, but this one is more severe.

Other endogenous factor is acute pain which inhibits the suppression of endogenous glucose by insulin; in addition, it releases diverse acute-stress hormones that contribute hyperglycemia, such as cortisol, glucagon, growth hormone, etc. (15). It has been impossible

the full standard care as a mean of reducing infections and further complications, and in

This chapter reviews the mechanisms of stress hyperglycemia, the evidence of the association between hyperglycemia and adverse outcomes in surgical patients particularly in cardiac surgery. Besides, it offers a general overview about discordant reports found in the literature on the strict glycemic control during the peri-operative period of a cardiac surgery. In addition to, it also recommends common approaches to control the glycemia in surgical intensive care unit (ICU) and post-surgical cardiovascular patients based on the

Stress Hyperglycemia, also called stress-induced diabetes, diabetes (6), is a multifact metabolic disorder that is characterized by the presence of hyperglycemia with hyperinsulinemia, peripheral resistance to insulin and an over-production of glucose by different mechanisms that result in incremented glycogenolysis and increased gluconeogenesis. Figure 1 shows how endogenous and exogenous predisposing factors may trigger the development of stress hyperglycemia, particularly during a cardiac surgery. The most important trigger is the surgical stress itself that conduces to a catabolic state that results in high levels of blood-glucose; the appearance of stress-hormones and a diminished peripheral response to insulin, cause hyperglycemia, and raises even more previous glycemia breeding an abnormal immune response (3, 6-9.). Stress hyperglycemia is caused mainly by the effects of counter-regulatory hormones (catecholamines, growth hormone, and cortisol) and by depletion of the functional reserve of the beta-cells in the Langerhans islets of the pancreas (7). During perioperative period of major surgeries, the counterregulatory hormones and the inflammatory response induced by surgical stress are the most important triggers of hyperglycemia (10). The degree of insulin resistance has been related to the magnitude and endurance of surgical stress. In addition, it has been reported that in the perioperative period, increased glucose reabsorption or decreased renal glucose

clearance may enhance this phenomena that contribute to hyperglycemia (11).

Although stress hyperglycemia is mainly caused by six events: severe trauma, bleeding, hypothermia, septicemia, severe burns and great-magnitude surgeries, there are several factors that may contribute to this alteration, which could and should be explored in the preoperative examinations (Table 1). All of these predisposing factors can be divided in

The first and most important predisposing factor for developing stress hyperglycemia is to be previously diagnosed with Diabetes Mellitus; in this case glucose blood-levels after cardiac surgery may reach even 20 mmol/l (370mg/dl) or more, compared with nondiabetic patients which might reach 15 mmol/l (270 mg/ml) (12-14). With a previous diagnosis of diabetes, it is not just more likely to develop stress hyperglycemia, but this one

Other endogenous factor is acute pain which inhibits the suppression of endogenous glucose by insulin; in addition, it releases diverse acute-stress hormones that contribute hyperglycemia, such as cortisol, glucagon, growth hormone, etc. (15). It has been impossible

consequence, the patient's improvement.

best performed randomized controlled trials.

**2. Mechanisms of stress hyperglycemia** 

**Predisposing factors** 

a. Endogenous factors

is more severe.

endogenous and exogenous factors.

Fig. 1. It is showed how endogenous and exogenous predisposing factors may trigger the developing of stress hyperglycemia, adding cardiac surgery's aggression. Physiologicalstress triggers a catabolic state that results in stress hyperglycemia; the appearance of stressreleased hormones and a diminished peripheral response to insulin augment glycemia, and condition the release of proteins and free fatty acids that in addition to hyperglycemia entail an abnormal immune response.

to quantify the real affection caused by pain in the glucose-metabolism, but the tendency of an aggressive-pain management might be helpful for decreasing the peripheral resistance to insulin (16).

Elderly patients represent an important group that undergoes cardiac surgeries; it is well known that insulin secretion is diminished in this group of age. This is reinforced by different studies; the comparison between patients over 60 years versus younger people shows incidence-reduction of hyperglycemia up to 38% in young adults (17). So, elderliness is a very important factor for elevated blood glucose levels (18).

b. Exogenous factors

Hypothermia, especially present in coronary bypass surgery due to cardioplegic solutions, provokes hyperglycemia by inhibiting the negative-feedback of the insulin response (19). Also, desaturation and arterial hypoxemia, increase a sympathetic autonomous response that favors glucagon release by an alpha-receptor action (20).

Many drugs commonly used with inpatient care might modify glucose metabolism. Some of them are well known as 'diabetogenic' medications such as glucocorticoids and opiates;


Modified from ref. 29: McCowen KC, Malhotra A, Bistrian BR. Stress-induced hyperglycemia. *Crit Care Clin* 2001; 17: 107-24.

Table 1. Predisposing factors for developing stress hyperglycemia

others are used for treating hypertension such as calcium channel blockers, clortalidone, and prazosin. Every synthetic catecholamine or catecholamine-agonist or blocker (such as epinephrine, norepinephrine, salbutamol, metoprolol, propanolol) and tricyclic antidepressants, might elevate glucose levels. Particularly, catecholamine-infusion during cardiac surgery contributes greatly to this metabolic state during surgery, as well as in the early postoperative period; it might also overlap and/or worsen stress response. Even alcohol and salicylates raise glycemia. All volatile anesthetic agents, including halothane, enflurane, and isoflurane, inhibit the insulin response to glucose in a dose dependent manner in vitro (21). The hyperglycemic response during inhaled anesthesia with isoflurane is a consequence of both impaired glucose clearance and increased glucose production (22,23).

Other factor that contributes to augment glycemia is bed resting. This condition reduces the sensitivity of the skeletal muscle to insulin, and in consequence increases fasting plasma insulin concentration as well as oral glucose challenge; insulin clamp studies showed no insulin secretion deficiency during repose. This response is mainly because of proteolysis that reduces total muscle mass and decreases the total number of transporters (24).

Other exogenous factor is the preoperative fasting. Fasting is a routinely action taken before undergoing a programmed cardiac surgery; fasting before surgery diminishes the glycogen reserve, induces protein breakdown (that releases alanine) and might disrupt insulin action and the stress responsiveness (25).

An underappreciated cause of hyperglycemia in critically ill patients is the provision of dextrose in excessive of amounts that can be easily oxidized or stored. In addition to hyperglycemia, other complications may result from the administration of dextrose over the rate of 4 mg/kg/min, inducing lipogenesis and increased carbon dioxide production (26).

Action Factor

Acute Pain

Overweight

Uremia Bed rest Fasting

Elderly Hypothermia Hypoxemia

Modified from ref. 29: McCowen KC, Malhotra A, Bistrian BR. Stress-induced hyperglycemia. *Crit Care* 

others are used for treating hypertension such as calcium channel blockers, clortalidone, and prazosin. Every synthetic catecholamine or catecholamine-agonist or blocker (such as epinephrine, norepinephrine, salbutamol, metoprolol, propanolol) and tricyclic antidepressants, might elevate glucose levels. Particularly, catecholamine-infusion during cardiac surgery contributes greatly to this metabolic state during surgery, as well as in the early postoperative period; it might also overlap and/or worsen stress response. Even alcohol and salicylates raise glycemia. All volatile anesthetic agents, including halothane, enflurane, and isoflurane, inhibit the insulin response to glucose in a dose dependent manner in vitro (21). The hyperglycemic response during inhaled anesthesia with isoflurane is a consequence of both impaired glucose clearance and increased glucose production

Other factor that contributes to augment glycemia is bed resting. This condition reduces the sensitivity of the skeletal muscle to insulin, and in consequence increases fasting plasma insulin concentration as well as oral glucose challenge; insulin clamp studies showed no insulin secretion deficiency during repose. This response is mainly because of proteolysis

Other exogenous factor is the preoperative fasting. Fasting is a routinely action taken before undergoing a programmed cardiac surgery; fasting before surgery diminishes the glycogen reserve, induces protein breakdown (that releases alanine) and might disrupt insulin action

An underappreciated cause of hyperglycemia in critically ill patients is the provision of dextrose in excessive of amounts that can be easily oxidized or stored. In addition to hyperglycemia, other complications may result from the administration of dextrose over the rate of 4 mg/kg/min, inducing lipogenesis and increased carbon dioxide production (26).

that reduces total muscle mass and decreases the total number of transporters (24).

Catecholamine-Infusion Glucocorticoid treatment

Septicemia/Sepsis/Septic Shock

Elevated Acute Physiology and Chronic Health Evaluation (APACHE) score

absolute)/Insulin resistance Diabetes mellitus

Insulin deficiency (relative or

Higher counterregulatory hormone levels

Insulin Peripheral resistance

Insulin impaired secretion

Table 1. Predisposing factors for developing stress hyperglycemia

*Clin* 2001; 17: 107-24.

(22,23).

and the stress responsiveness (25).

#### **The role of the glucose transporters**

Stress-induced diabetes seems to be supported mainly by peripheral resistance to insulin, i.e. the inability of skeletal muscles and adipocytes to uptake glucose. This condition appears because of the affection of the glucose transporter type 4, called GLUT4 which is dependent of insulin. This very important hexose-transporter is located in skeletal-muscle, cardiac muscle and adipocytes (27). This protein is member of a family of transmembranal proteins that are responsible for uptaking glucose in different cells which depend of insulin for their action. These transporters are responsible of plenty of physiological phenomena that maintain glucose homeostasis (28,29). The physiological stress inhibits -by different mechanisms- the insulin's action on the GLUT4, mainly by impairing the phosphorylation of several molecules of the intracellular signaling pathway of insulin (30); the result is a decreased function of the transporter and therefore, a diminution of insulin-mediated glucose uptake. Additionally, the transporters GLUT1 (present in endothelial and nonskeletal muscle cells) and GLUT3 (present in neurons) are affected –as well as GLUT4 -by diverse cytokines such as IL-1, IL-6 and C-Reactive Protein which are augmented in surgical stress (29, 31,32).

#### **Non-glucosydic substrates used during stress hyperglycemia**

The carbohydrate metabolism during periods of stress uses different non-glucosydic substrates for gluconeogenesis such as glycerol, alanine, pyruvate and lactate. The last two mentioned molecules are generated when aerobic glucolysis and Kreb's cycle is impaired (23). In particular, lactate is produced because of inhibition of the pyruvate-dehydrogenase enzyme, mainly by IL-1, IL-6 and Tumoral Necrosis Factor-alpha (TNF-α) (33,34). The aminoacids alanine and glutamine are substrate for gluconeogenesis of critical illness. They are derived from proteolysis of skeletal muscle (33). The alanine is converted to glucose via Cori´s cycle (29). Glycerol is a product of lypolisis, and might be elevated because of the action of several counter-regulatory hormones (29), glycerol is used as a substrate of 20% of hepatic-derived glucose (29).

#### **The role of the counterregulatory hormones**

Hyperglycemia severity is directly correlated with the intensity of the inflammatory response (6). It is well known that inflammation produces an endocrine response that releases the so called "stress hormones" that raise glucose plasma levels. A wide variety of both hormones and cytokines affect glucose homeostasis by different pathways, including the promotion of gluconeogenesis or provoking insulin resistance; when hypoglycemia becomes present (less than 70 mg/dl, 3.9 mmol/l) there is a correlation with the initial threshold for releasing counterregulatory hormones and cytokines (35,36).

In healthy people, when gluconeogenesis is augmented, glucagon production is inhibited and insulin is released to the circulation; in the post-operated patient, there is an inflammatory response in which endogenous –or exogenous- cytokines and cathecolamines may interfere with this negative-feedback system and allow a hyperglycemic state by augmenting hepatic gluconeogenesis and glycogenolysis (34).

Although adrenaline, noradrenaline and cortisol are the best-studied hormones in the metabolic response to trauma, the most important stress hormone in postsurgical stress is the Growth Hormone (37). In other hand, adrenaline and noradrenalin are cathecolamines that greatly impair carbohydrates' metabolism. Adrenaline has shown to increase hepatic gluconeogenesis (29, 38), favors glycogenolysis in skeletal muscle and impairs glucose uptake in peripheral tissues, and via the -3 adrenergic receptor, elevates free fatty acids (FFAs) in plasma (39). Likewise, noradrenaline may promote gluconeogenesis because its lipolytical effects and the marked glycerol supply to the liver (40). The glucogenolytic state mediated by these cathecolamines lasts no more than 36 hours (29). Additionally, cortisol is a major insulin counterregulatory hormone, which stimulates hepatic glucose production and enhances renal glucose production through glyconeogenesis, favors the presence of FFAs in plasma (34,41).

Growth Hormone is a very well known "diabetogenic" hormone and is the most important counterregulatory hormone present in surgical stress. It inhibits the insulin signaling cascade and it has been demonstrated in experimental animals to reduce the number of insulin receptors, as well as it reduces the phosphorylation on tyrosine residues triggered by insulin (42). Growth hormone also raises FFAs into plasma, and greatly affects the GLUT-1 and GLUT-4 activity (43). The Insulin-like Growth Factor (IGF-1 and -2) characterizes hepatic resistance to insulin's action, it has been proved that it keeps a direct association with mortality (44). There is not a clear relationship between every individual hormonal response that may result in a common phenomenon: hyperglycemia. More research is needed for augmenting our knowledge about hormonal regulation of this phenomenon, and also for understanding the role and importance of each one of these endocrine signals. Figure 2 shows how stress hormones might disrupt glucose metabolism by causing resistance to insulin's action and starting the usage of non-glucosydic substrates that contribute to raise glycemia.

Fig. 2. The release of stress hormones produces gluconeogenesis and peripheral resistance to insulin. Stress hormones might disrupt glucose metabolism by causing resistance to insulin's action in the GLUT-4 transporter and originates the usage of non-glucosydic substrates, that contribute to raise glycemia. GH: Growth Hormone. NGS: non glucosydic substrates.

uptake in peripheral tissues, and via the -3 adrenergic receptor, elevates free fatty acids (FFAs) in plasma (39). Likewise, noradrenaline may promote gluconeogenesis because its lipolytical effects and the marked glycerol supply to the liver (40). The glucogenolytic state mediated by these cathecolamines lasts no more than 36 hours (29). Additionally, cortisol is a major insulin counterregulatory hormone, which stimulates hepatic glucose production and enhances renal glucose production through glyconeogenesis, favors the presence of

Growth Hormone is a very well known "diabetogenic" hormone and is the most important counterregulatory hormone present in surgical stress. It inhibits the insulin signaling cascade and it has been demonstrated in experimental animals to reduce the number of insulin receptors, as well as it reduces the phosphorylation on tyrosine residues triggered by insulin (42). Growth hormone also raises FFAs into plasma, and greatly affects the GLUT-1 and GLUT-4 activity (43). The Insulin-like Growth Factor (IGF-1 and -2) characterizes hepatic resistance to insulin's action, it has been proved that it keeps a direct association with mortality (44). There is not a clear relationship between every individual hormonal response that may result in a common phenomenon: hyperglycemia. More research is needed for augmenting our knowledge about hormonal regulation of this phenomenon, and also for understanding the role and importance of each one of these endocrine signals. Figure 2 shows how stress hormones might disrupt glucose metabolism by causing resistance to insulin's action and starting the usage of non-glucosydic substrates that

**Hyperglycemia**

Stress Hormones

GH Cathecolamines Glucocorticoids Glucagon

Fig. 2. The release of stress hormones produces gluconeogenesis and peripheral resistance to

Resistance to insuline

GLUT 4 transporter

insulin. Stress hormones might disrupt glucose metabolism by causing resistance to insulin's action in the GLUT-4 transporter and originates the usage of non-glucosydic substrates, that contribute to raise glycemia. GH: Growth Hormone. NGS: non glucosydic

FFAs in plasma (34,41).

contribute to raise glycemia.

Piruvate Lactate Alanine Glutamine Glycerol

Usage of NGS

substrates.

#### **The role of the inflammatory response**

Surgical stress establishes an acute inflammatory response that promotes the release of different cytokines –specifically TNF-α, IL-1, IL6- from mononuclear cells, this contributes to insulin resistance (29) and raising glucose levels in blood; at the same time, induces the production of diverse pro-inflammatory and mitogen cytokines like the nuclear factor kappa-B (NF-kB), the Early Growth Response-1 gene, the Plasminogen Activator Inhibitor-1 (PAI-1), Intracellular Adhesion Molecule-1 (iCAM-1), Monocyte Chemotactic Protein-1 (MCP-1) and matrix metalloproteinases-1, -2 and -9.

The mechanism whereby TNF- mediates stress hyperglycemia has been well studied. This cytokine causes insulin resistance in both liver and skeletal muscle (6) most likely through the modification of signaling properties of insulin receptor substrates. In particular, endotoxin derived from cell wall of Gram-negative bacterial is also a potent stimulant of secondary production of TNF- and various proximate interleukins, mainly IL-1 and -6 which disrupt both the insulin post-receptor signaling and the phosphorylation of molecules associated to the tyrosine-kinase receptor that lastly affects GLUT's activity, and glucose uptake (32, 45).

The Nuclear Factor kappa-B (NF-kB) is a pleiotropic transcription factor that is present in almost all cell types and is involved in many biological processes such as inflammation, immunity, cell differentiation, growth and apoptosis, and carcinogenesis. The early growth respone-1 gene is a nuclear protein that functions as a transcriptional regulator that favors cell differentiation and mitogenesis. The intracellular Adhesion Molecule-1 (iCAM-1) favors leucocyte adhesion, and metalloproteinases help in extracellular matrix remodeling (46-48). Hyperglycemia affects immune cellular responsiveness by reducing neutrophyle activation, chemotactism, fagocytosis, and diminishing bactericidal activity of the reactive oxygen species (ROS). Humoral-cell response is affected by immunoglobulin glycosylation and altered synthesis of IL-6 and TNF- (6,9). As mentioned, hyperglycemia has been associated to an altered immune response, an enhanced proinflammatory response, endothelial dysfunction, a hypercogulability state as well as neural damage with an augmented oxidative stress and secondary release of ROS from leucocytes (8,49).

All conditions above mentioned make up a not favorable setting for the surgical patient and represent a challenge to reach out a condition of stable normoglycemia. Figure 3 shows the relationship in between the abnormal glucose usage and how does physiological stress modifies the carbohydrate metabolism, leading to a deleterious non-homeostatic condition.
