963CHAPTER 79 Cellular Respiration only two ATP molecules compared with the much more efficient process (10–15 times more efficient) of oxidative phosphoryla tion described later 13 Fatty Acid b Oxida[.]
CHAPTER 79 Cellular Respiration 963 only two ATP molecules compared with the much more efficient process (10–15 times more efficient) of oxidative phosphorylation described later.13 water, and oxaloacetate is regenerated to again react with a new acetyl-CoA molecule Fatty Acid b-Oxidation Fatty acids are primarily oxidized through b-oxidation in the mitochondrial matrix Entry of long-chain free fatty acids into the mitochondria is dependent on the carnitine transport system, while medium-chain fatty acids and ketone bodies enter the mitochondria without carnitine Catabolism of fatty acids by b-oxidation sequentially removes two-carbon units from the carboxyl terminal to generate one molecule of acetyl-CoA, one FADH2, and one NADH for each two-carbon fatty acid fragment cycle Like carbohydrate metabolism through glycolysis, lipid metabolism is a tightly regulated process Lipid stored in adipose tissue cycles continuously between triglycerides and free fatty acids When glucose and insulin concentrations are high (e.g., after a meal), fatty acid uptake into adipocytes is increased, resulting in the synthesis and storage of triglycerides In contrast, when glucose levels are low (e.g., during fasting), upregulation of catecholamines and glucagon stimulate lipases that release free fatty acids into the circulation to be used directly by peripheral tissues as fuel for the Krebs cycle or are transported to the liver to be converted into ketone bodies The ketone bodies acetoacetate and b-hydroxybutyrate are important substrates for oxidative phosphorylation in most tissues, including the heart, brain, kidney, and skeletal muscle Because generation of acetyl-CoA by fatty acid b-oxidation (either directly or through catabolism of ketone bodies) occurs independent of PDH, lipids provide an alternative fuel for oxidative phosphorylation when glucose availability is limited or PDH activity is impaired.14 Some to billion years ago, aerobic bacteria invaded and subsequently colonized a form of primordial eukaryotic cells that lacked the ability to use oxygen Over the ensuing millions of years, a critical symbiotic relationship developed between the cell and aerobic bacteria that has remained steadfast through evolution.16,17 The cell nucleus has evolved to regulate structure and overall cell function, while the oxygen-consuming bacteria evolved into mitochondria specialized in the energy production necessary for the development of increasingly complex species, including humans Despite extensive intercommunication between mitochondria and their nuclear-cytosol host, mitochondria have retained many features characteristic of their bacterial origins: (1) mitochondria arise only from other mitochondria; (2) mitochondria maintain their own genome that, in contrast to eukaryotic nuclear DNA, is circular in structure, lacking in associated histones, and is replicated up to dozens of times; (3) mitochondria synthesize their own proteins that, like bacterial proteins, include N-formyl methionine at the protein amino terminus; and (4) many antibiotics that inhibit protein synthesis in bacteria are also toxic to mitochondria.10 The independent mitochondrial genome contains genes that encode 37 proteins involved in the ETS, enzymes comprising the Krebs cycle, fatty acid b-oxidation, and pyruvate oxidation and the machinery needed for mitochondrial gene translation.17 Notably, nuclear DNA encodes approximately 1500 additional mitochondrial proteins, including 79 of the 92 subunits of the ETS complexes.18 Mitochondrial oxidative phosphorylation is fueled by pairs of elections originating from NADH and FADH2 (Fig 79.3) These electrons are shuttled along the mitochondrial ETS, ultimately reducing oxygen to water.8 To capture as much of the energy released in a usable form when strong reducing agents such as NADH and FADH2 react with a powerful oxidizing agent such as oxygen, mitochondria “step down” the reducing potential of these molecules through a controlled series of intermediate compounds in the ETS with progressively lower reducing potentials.5 Five complexes of proteins and cytochromes embedded within the inner mitochondrial membrane comprise the ETS, including NADH dehydrogenase–ubiquinone oxidoreductase (complex I), succinate dehydrogenase–ubiquinone oxidoreductase (complex II), ubiquinone–cytochrome c oxidoreductase (complex III), cytochrome c oxidase (complex IV), and ATP synthase (complex V) The cofactors ubiquinone (coenzyme Q10) and cytochrome c facilitate the transfer of electrons from complexes I and II to complex III and from complex III to complex IV, respectively Electrons essentially cascade along these protein/cytochrome complexes toward complex IV, where they are finally passed to molecular oxygen, generating water Protons generated during these reactions are pumped across the inner mitochondrial membrane by complexes I, III, and IV, generating a proton motive force The resulting electrochemical gradient across the inner mitochondrial membrane drives ATP synthesis by the chemiosmotic principle Energy for ATP synthesis arises from an influx of these protons back into the matrix through the rotary motor of ATP synthase.19 For each pair of electrons, three molecules of ATP are produced This process is termed oxidative phosphorylation because the reduction of molecular oxygen to water is linked to the addition of inorganic phosphate to ADP, producing the high-energy terminal pyrophosphate in ATP Aerobic respiration through Protein Catabolism Although typically protected as a fuel source, amino acids can be mobilized for energy production in times of energy need (such as starvation or critical illness).15 As part of the metabolic stress response mediated by a decrease in insulin and upregulation of cortisol, catecholamines, tumor necrosis factor-a, and interleukin (IL)-1 and IL-6, protein degradation can occur with release of amino acids from skeletal muscle (see also Chapter 80) Amino acid catabolites can either enter the Krebs cycle by way of acetylCoA or can be used to fuel gluconeogenesis.13 Krebs Cycle The Krebs cycle summarizes a circular series of nine reactions that occur in the mitochondrial matrix in which acetyl-CoA derived from glycolysis, b-oxidation, or protein catabolism is metabolized to two molecules of CO2, one molecule of guanosine triphosphate (GTP), three molecules of NADH, and one molecule of FADH2 GTP is equivalent to ATP in terms of energy charge and is ultimately converted to ATP by the enzyme nucleoside diphosphokinase Although oxygen itself is not part of the Krebs cycle, its presence at the end of the mitochondrial electron transport system ensures recycling of NAD1 and FAD required in the Krebs cycle The first step in the Krebs cycle catalyzes the formation of the six-carbon citrate molecule (hence, the alternative name of the Krebs cycle is the citric acid cycle) from the two-carbon acetyl-CoA and four-carbon oxaloacetate This reaction is catalyzed by the enzyme citrate synthase and is the rate-limiting step of the Krebs cycle With each revolution of the cycle, the fuel substrate that entered the cycle as acetyl-CoA is completely oxidized to CO2 and Mitochondrial Oxidative Phosphorylation 964 S E C T I O N V I I I Pediatric Critical Care: Metabolic and Endocrine Mitochondrial membrane potential H+ H+ H+ H+ H+ H+ I III IV V Q Intermembrane space Cyt C e- ee- II Matrix e- NAD + FADH NADH FAD ROS O2 H2O ADP + Pi ATP Krebs cycle • Fig 79.3 Oxidative phosphorylation and reactive oxygen species production through the mitochondrial electron transport chain (Modified from Protti A, Singer S Bench-to-bedside review: potential strategies to protect or reverse mitochondrial dysfunction in sepsis-induced organ failure Crit Care 2006;10:228.) oxidative phosphorylation is up to 15 times more efficient than anaerobic metabolism, producing 30 to 34 ATP molecules ATP molecules are transferred out of the mitochondria through a specific adenine nucleotide translocase (ANT) antiporter on the inner mitochondrial membrane and through a voltage-dependent anion channel (VDAC) on the outer mitochondrial membrane for use as energy currency for all cellular functions The overall metabolism of glucose through cellular respiration can be summarized as follows (the negative DG indicates that the overall reaction can occur spontaneously): Glucose + O2 → CO2 + H2O + heat G 2880 kJ per molecule of glucose Although multiple regulatory steps exist along the pathways of cellular respiration, the following three are preeminent: Oxygen availability to serve as the ultimate electron acceptor Availability of nutrient metabolism to generate reducing equivalents in the form of NADH and FADH2 The overall cellular energy state defined by the ratios of NADH/NAD1, ATP/ADP, and acetyl-CoA/CoA Oxygen Toxicity In health, approximately 1% to 2% of oxygen consumption is directed toward the production of ROS and an even greater amount is “uncoupled” from energy production and lost as heat.2 Heat production varies substantially from tissue to tissue, being low in the heart and much higher in skeletal muscle and brown fat.20 Despite the terminology, uncoupling is itself a controlled process regulated by a series of uncoupling proteins (UCPs) that insert within the inner mitochondrial membrane and provide an alternative pathway for protons to move back into the matrix, bypassing ATP synthase For example, UCP-1 present in brown fat provides a vital source of heat generation for neonates and hibernating mammals.21 Successive one-electron additions to molecular oxygen result in the production of superoxide anion, hydrogen peroxide (H2O2), hydroxyl radical, and water, respectively.22 The partially reduced oxygen compounds are referred to as reactive oxygen species and are responsible for the so-called antagonistic pleiotropy characteristic of oxygen.23 Under normal conditions, small amounts of constitutively produced ROS, primarily in the form of H2O2, are necessary for regulation of a variety of intracellular signaling pathways However, increased production of ROS or decreased availability of endogenous antioxidant compounds (e.g., manganese superoxide dismutase, catalase, glutathione peroxidase, uric acid, vitamin C, and vitamin E) risks deleterious oxidative modifications in cellular membrane or intracellular molecules.24 Although the majority of cellular ROS is sequestered within the mitochondria—thereby protecting the redox state of other cellular molecules—an increase in ROS production can quickly outstrip antioxidant defense mechanisms and cause oxidant injury to cellular proteins, lipids, and DNA.22 Nitrogen is also involved in the generation of another group of toxic metabolic moieties known as reactive nitrogen species (RNS) Endogenous nitric oxide (NO) can react with the superoxide anion to form the powerful oxidant peroxynitrite, which can cause peroxidation of lipids within cell and organelle membranes, damage to various elements of the mitochondrial electron transport chain and ATP synthase, inhibition of glyceraldehyde 3-phosphate dehydrogenase, injury of the sodium-potassium adenosine triphosphatase pump, disruption of sodium channels, production of deoxyribonucleic acid (DNA) strand breaks, and activation of the polyadenosine ribosyl phosphate system Peroxynitrite can also irreversibly inhibit the Krebs cycle enzyme aconitase and increase mitochondrial proton leak.25 CHAPTER 79 Cellular Respiration Increased ROS- and RNS-mediated stress can promote the formation of the mitochondrial permeability transition pore (MPTP), leading to release of proapoptotic proteins, including cytochrome c, into the cytoplasm.22,26,27 This can trigger activation of the apoptotic pathway, orchestrating cell death Integrity of the mitochondrial ETS defines a fine line between maintaining mitochondrial homeostasis and efficient respiration versus excessive ROS/RNS production, mitochondrial dysfunction, cellular dysfunction, and cell death Impaired Cellular Respiration in Critical Illness Mitochondrial ATP production relies on a complex series of chemical reactions that, overall, are controlled by metabolic demand During periods of rest, metabolic needs are generally low and less energy is required Conversely, during periods of activity and stress, metabolic requirements increase with a concurrent rise in cellular oxygen utilization and ATP production An increase in ATP production ensures that sufficient energy is available to respond to the activity or stress and prevents the cell from depleting ATP levels below a critical threshold that can trigger apoptotic and necrotic cell death pathways With an acute decrease in oxygen delivery (DO2) to tissues, such as with cardiac arrest, shock, or hemorrhage, mitochondrial energy output will necessarily fall A concurrent increase in anaerobic respiration through glycolysis will attempt to partially offset the decline in ATP production, but if DO2 is not rapidly restored, bioenergetic homeostasis will be perturbed, leading to eventual cell injury and organ dysfunction.2 In critical illness, shock represents the imbalance between oxygen (and substrate) delivery and oxygen (and substrate) utilization such that cellular metabolic demands are not met Early in the course of shock and other inflammatory conditions, the total body metabolic rate rises with the surge in catecholamine and A 965 catabolic hormones In healthy volunteers, injection of endotoxin leads to a systemic inflammatory response characterized by fever and a rapid rise (within hours) in total-body oxygen consumption (VO2).28 Over time, total-body VO2 and energy expenditure decrease with increasing illness severity in both animal models and critically ill patients.29 Following trauma, failure to attain a lownormal level of VO2 has been associated with increased risk of multiorgan dysfunction.30 Survivors of critical illness subsequently demonstrate a return of VO2 toward baseline levels.29 Changes in total-body VO2 and energy expenditure suggest that mitochondrial respiration is altered in critical illness The inability of cells to effectively use oxygen to generate ATP is termed cytopathic hypoxia.3,5 To date, most studies have focused on sepsis, though increasing evidence supports the development of cytopathic hypoxia after trauma, brain injury, and cardiac arrest Ultrastructural mitochondrial abnormalities in a variety of organ systems have been recognized in models of sepsis for over 30 years.31 For example, Crouser demonstrated marked swelling and disruption of mitochondrial architecture in the liver following cecal ligation and puncture (CLP) in a mouse model (Fig 79.4).4 Morphologic abnormalities have also been noted in mitochondria taken from the heart, kidney, liver, endothelial cells, intestinal epithelial cells, and skeletal muscle in animal models of sepsis.32 In human sepsis, heart and liver biopsies obtained immediately postmortem from adult nonsurvivors similarly show swollen and damaged mitochondria.33 Functional alterations in mitochondrial respiration have been more variably reported as increased, decreased, or unchanged in short-term sepsis models However, longer-term models that may better align with human critical illness more consistently demonstrate depressed mitochondrial function In human sepsis, most— but not all—studies demonstrate decreased mitochondrial VO2 in both immune and nonimmune cells.2,34,35 For example, in a study of pediatric patients with septic shock and multiorgan dysfunction, direct measurements of mitochondrial respiration in peripheral B • Fig 79.4 Electron micrograph of a mitochondrion at the site of cellular respiration (A) A normal invagi- nated cristae membrane provides the scaffolding for components of the electron transport chain. (B) Disrupted and edematous mitochondrial architecture observed in the liver 24 hours after cecal ligation and puncture in a mouse (Modified from Crouser ED Mitochondrial dysfunction in septic shock and multiple organ dysfunction syndrome Mitochondrion 2004;4:729–741.) 966 S E C T I O N V I I I Pediatric Critical Care: Metabolic and Endocrine blood mononuclear immune cells (PBMCs) showed that spare respiratory capacity—which reflects the ability of the ETS to keep pace with a rise in energy demand—was decreased and mitochondrial uncoupling was increased compared with noninfected controls.35 Similar findings have been reported in PBMCs in adults with sepsis,36 though one study demonstrated a progressive increase in mitochondrial respiration from day to day through day to day of sepsis compared with healthy controls.34 When considering bioenergetic impairment in sepsis, investigators have most commonly focused on NADH dehydrogenase– ubiquinone oxidoreductase (complex I) and cytochrome oxidase (complex IV) As the largest complex of the ETS, complex I is subject to impairment from changes in a variety of protein subunits Multiple studies have demonstrated decreased activity of ETS complex I in sepsis models and humans.37 Complex IV contains two heme subgroups (cytochrome a and a3) that assist in the final transfer of electrons to reduce oxygen to water A reduced cytochrome a,a3 redox state in the absence of tissue hypoxia indicates a defect in mitochondrial oxygen use and suggests impaired oxidative phosphorylation A number of investigators have demonstrated reduced cytochrome a,a3 redox status during endotoxemia and Gram-negative bacteremia in the heart, brain, skeletal muscle, and intestine in animals.38,39 In addition, decoupled cytochrome a,a3 was more likely in adults who did versus did not develop multiorgan system dysfunction following trauma despite normalization of DO2.40 Interestingly, Verma et al demonstrated that, when complex IV activity is stimulated by administration of caffeine, myocardial function and survival improved after cecal ligation and puncture in a rodent model.41 Despite structural, biochemical, and functional evidence for mitochondrial dysfunction in sepsis, trauma, and other critical illness, the literature is less clear regarding these effects on tissue ATP availability Several studies have reported that ATP levels are preserved in septic myocardium, but others have shown decreased high-energy phosphates with endotoxemia.2,5 In a study of 28 human adults with severe sepsis, 12 (43%) of whom died of sepsis-related MODS, nonsurvivors had lower levels of ATP in skeletal muscle compared with survivors.37 However, even preservation of ATP does not imply an absence of mitochondrial dysfunction in sepsis.42 When DO2 and cellular hypoxia are limited (as in shock), cells may adapt to maintain viability by downregulating VO2, energy requirements, and ATP demand.2,5,9 Thus, a fall in ATP utilization may help to preserve ATP even if energy production is diminished In the heart, this response is referred to as myocardial hibernation and classically occurs following an acute coronary syndrome If cellular metabolic activity continued unchanged despite mitochondrial dysfunction, then ATP levels would inevitably diminish and cell death pathways would be activated Because cell death does not appear to be a primary feature of sepsis-induced organ dysfunction, it follows that cells may instead adapt to cope with the falling energy supply.31,43 Thus, finding preserved ATP during sepsis reveals little about the integrity of oxidative phosphorylation The precise etiology by which mitochondrial function is altered in critical illness remains unclear A number of mutually compatible mechanisms may contribute to a clinical state of cytopathic hypoxia under pathologic conditions, including: Decreased tissue oxygen (and substrate) delivery impairing oxidative phosphorylation Inhibition of PDH with decreased flux of acetyl-CoA through the Krebs cycle NO-mediated inhibition of ETS complexes Peroxynitrite-mediated inhibition of the Krebs cycle and ETS complexes I, II, and V Depletion of cellular NADH due to activation of the enzyme poly(ADP-ribose) polymerase (PARP)-1 in response to ROSmediated nuclear DNA damage Insufficient mitochondrial turnover due to impaired mitochondrial biogenesis, fission/fusion, or mitophagy Hormonal alterations, including relative hypothyroidism Pharmacologic inhibition of mitochondrial function as a side effect of many drugs commonly used in the intensive care unit (ICU) setting, including some antibiotics, catecholamines, and sedatives (propofol in particular) Injection of lipopolysaccharide (LPS) endotoxin into healthy human volunteers also results in widespread suppression of genes regulating mitochondrial energy production and protein synthesis.44 Differential expression of mitochondrial genes in blood cells has been reported for several diseases in which bioenergetic failure is a postulated mechanism.45,46 In a study of adult sepsis, decreased ETS gene expression correlated with impaired activity of ETS complexes I and IV in skeletal muscle.47 In pediatric sepsis, suppression of nuclear-encoded mitochondrial genes was greatest in the subgroup of patients with the highest rates of MODS and death.48 Accumulating evidence also supports a propagative role for mitochondrial damage to feed-forward the systemic inflammatory response and contribute to distant organ injury in trauma and sepsis.9 Oxidative injury due to an increase in mitochondrial ROS and RNS can fragment mitochondrial DNA (mtDNA) Oxidatively damaged mtDNA fragments can be exported from the mitochondrial matrix through the MPTP to the cytosol or the extracellular space In the cytosol, mtDNA promotes the formation of the Nod-like receptor-P3 (NLRP3) inflammasome, a supramolecular platform that increases the release of the proinflammatory cytokines IL-1b and IL-18.49 In the circulation, mitochondria are recognized by the innate immune systems as a danger-associated molecular pattern (DAMP) via the pattern recognition receptor, toll-like receptor 9, due to their evolutionary similarities with bacterial pathogens.50 Several studies in human sepsis and trauma have demonstrated that an increase in circulating levels of mtDNA is associated with adverse outcomes.50,51 Consequently, mtDNA has been proposed as a potential novel biomarker linked to mitochondrial dysfunction in critical illness Despite demonstration of altered mitochondrial structure and function, there is actually little evidence that impaired mitochondrial respiration causes organ dysfunction in critical illness In fact, the decrease in oxygen utilization and overall metabolic rate seen in sepsis parallels similar physiologic changes observed in hibernating mammals that reduce their metabolism to promote survival amid decreased substrate availability.43 Indeed, despite physiologic and biochemical mitochondrial dysfunction in many tissues recovered from animal models and human patients with sepsis, there tends to be minimal evidence of cell death in dysfunctional organ systems Moreover, survivors of critical illness rapidly recover organ function Therefore, although a profound and prolonged metabolic downregulation can trigger cell death, the observation that mitochondrial hibernation is a conserved physiologic response across models and species suggests that some degree of transient energetic swoon early in sepsis (and other critical illnesses) could be an adaptive response to ischemia, hypoxemia, and shock.2,43,52 The use of induced hypothermia to decrease metabolic demand and protect vital organs during cardiopulmonary bypass supports the notion CHAPTER 79 Cellular Respiration that a decrease in metabolic demand may help to restore bioenergetic homeostasis when oxygen (and substrate) delivery—and thus ATP production—is limited Although this may manifest clinically as organ dysfunction, protected bioenergetic homeostasis may help to prevent cell death and offer the prospect of organ recovery once oxygen (and substrate) supply is restored.2 Clinical Assessment of Oxygen Utilization Biochemical pathways and cellular energetics may seem far removed from the daily care of patients in the pediatric ICU, where discussions about the respiratory and circulatory systems seem to dominate However, at its core, the goal of critical care medicine is to maintain and support the basic physiologic functions of the patient until the patient’s own homeostasis returns This means that the underlying goal of cardiopulmonary and other organ support is to optimize cellular function and that function requires energy derived from the metabolism of oxygen Inherent in this goal is the need to assess the adequacy of DO2 and utilization at the bedside Lactate Lactate is a commonly used surrogate for tissue hypoxia, as it is produced as a by-product of anaerobic metabolism In health, blood lactate concentration is maintained in the approximate range of 0.5 to 1.5 mmol/L In and of itself, lactate is not harmful but rather promotes ongoing glycolysis and provides an alternative metabolic fuel through gluconeogenesis However, lactate can accumulate in the absence of oxygen or with mitochondrial dysfunction, such that hydrogen ions are not able to be used to convert ADP to ATP through oxidative phosphorylation This combination of hyperlactatemia and acidosis is called lactic acidosis Blood lactate monitoring is frequently performed in critically ill patients, usually with the aim of detecting tissue hypoxia leading to anaerobic respiration, traditionally termed type A lactic acidosis However, other processes not related to tissue hypoxia can also result in increased blood lactate levels, such as with excess adrenergic stimulation that increases aerobic glycolysis and pyruvate production beyond the rate of mitochondrial metabolism, termed type B lactic acidosis and common in severe asthma, therapy with epinephrine, impaired PDH activity, or delayed lactate clearance seen in patients with hepatic dysfunction.53–55 The causal relationship between anaerobic hyperlactatemia and tissue hypoxemia has been confirmed in studies that limit VO2 by decreasing DO2 below a threshold with a resulting increase in lactate levels.56,57 Accordingly, in the early phase of septic shock, hyperlactatemia is accompanied by oxygen supply dependency,58 and initial lactate correlates with increased mortality in children with septic shock.59 Moreover, lactate normalization with hours of sepsis recognition predicts survival in pediatric sepsis.8,60 In adults, an elevated blood lactate concentration may be sufficient evidence that tissue perfusion is inadequate even if hypotension is not present (so-called cryptic shock).61 Moreover, the revised Sepsis-3 criteria now include both hypotension requiring vasopressors and hyperlactatemia greater than mmol/L in the operational criteria for septic shock in adults Despite wide availability and an association with both global tissue hypoxia and poor outcome, the exact utility of measuring initial and serially trending lactate remains unclear For example, in a study of 123 adult patients with vasopressor-dependent septic shock, mortality remained high (20%) in the 45% of patients found to be “nonlactate expressers” (defined as a lactate 967 level ,2.4 mmol/L).62 Additionally, no data are currently available to support monitoring lactate levels beyond the immediate resuscitative period, especially once DO2 has been restored The numerous etiologies of increased lactate and the lack of sensitivity and specificity for regional tissue perfusion and respiration remain important limitations in using lactate to monitor cellular bioenergetics A common clinical scenario occurs during volume resuscitation of patients (iatrogenic hyperchloremia) or epinephrine infusion, in which a chloride load and type B lactic acidosis can be interpreted as ongoing shock, thus prompting additional fluid and more vasoactive-inotropic support This scenario may initiate a vicious cycle with the potential for overresuscitation.63 Therefore, the treatment of the patient with lactic acidosis should be aimed at the underlying disease, with resuscitation guided by the overall clinical improvement of the patient and not just the lactate concentration itself Accordingly, the absence of an elevated lactate should not slow resuscitative efforts in children with other indices of altered perfusion Venous Oxygen Saturation Bedside evaluation of DO2 and VO2 would be of great value to the clinician Unfortunately, most of the methods of calculating VO2 rely on cardiac output (via the Fick principle), which results in a situation in which a single measured variable is included in two parts of a regression analysis, so-called mathematical coupling This dependency leads to amplification of any error in that measurement and may result in an apparent relationship between variables that does not truly exist Other methods to measure VO2 in intubated critically ill patients require use of a metabolic cart,64 but routine use is currently lacking As in many areas of critical care medicine, when something cannot be measured directly, a surrogate measure must be used In practice, the central venous oxygen saturation (Scvo2) or mixed venous oxygen saturation (Smvo2) can be used as a continuous or intermittent measure to assess the global balance of Do2 and consumption in a patient with septic shock.65 As central venous catheters have largely replaced the use of pulmonary arterial catheters, particularly in pediatrics, Scvo2 is now more commonly measured than Smvo2.66 Scvo2 is ideally measured from a catheter with its tip at the superior vena cava–right atrial junction Measurements taken from a femoral catheter with its tip in the inferior vena cava are thought to be less reliable due to greater variability in splanchnic oxygen utilization.67 An Scvo2 less than 70% indicates that tissue oxygen extraction is increased from normal, suggesting inadequate Do2 due to either low cardiac output, decreased arterial oxygen content, or both.65 However, Scvo2 may also be low if oxygen utilization is increased, as in states of high metabolic demand (e.g., fever, seizure, or hyperthyroidism) Alternatively, elevated Scvo2 above 80% suggests that oxygen utilization may be impaired due to mitochondrial dysfunction, reflecting a global state of cytopathic hypoxia.5 In a trial of early goal-directed therapy in adult septic shock by Rivers et al.,68 an Scvo2 greater than 70% was targeted as a primary hemodynamic end point Although three more recent trials failed to demonstrate benefit from routine Scvo2 monitoring,69–71 an Scvo2 greater than 70% remains the recommended target in current pediatric shock guidelines A trial in children with fluid refractory shock found that targeting an Scvo2 greater than 70% for 72 hours compared with standard therapy reduced mortality from 39.2% to 11.8% (P 002).72 Furthermore, in a pediatric prospective cohort trial evaluating the effect of intermittent Scvo2 ... oxygen to generate ATP is termed cytopathic hypoxia.3,5 To date, most studies have focused on sepsis, though increasing evidence supports the development of cytopathic hypoxia after trauma, brain injury,... availability of endogenous antioxidant compounds (e.g., manganese superoxide dismutase, catalase, glutathione peroxidase, uric acid, vitamin C, and vitamin E) risks deleterious oxidative modifications... cellular membrane or intracellular molecules.24 Although the majority of cellular ROS is sequestered within the mitochondria—thereby protecting the redox state of other cellular molecules—an increase