• Information
  1. Definitions
  2. Mechanisms of Coagulative Necrosis
  3. Reperfusion Injury
  4. Oxidative Damage and Disease
  5. Effects of Hypercarbia
  6. Ischemic Damage and Inflammation
  7. Leukocyte-Endothelial Adhesion Molecules

A. Definitions [1]

1. Hypoxemia
   1. Reduction of a normal oxygen supply to an organ from any of multiple causes
   2. Causes: ischemia, anemia, chemical modification of blood (eg. carboxyhemoglobin)
2. Anoxia: complete absence of oxygen supply to an organ
3. Ischemia
   1. Reduction or absence of blood supply to an organ or tissue
   2. Etiology: clot, atherosclerotic plaque, hypoxemic vasoconstriction
4. Infarction: death of an area of tissue as a result of ischemia
5. Necrosis
   1. Cells and tissues die by either necrosis or apoptosis
   2. Toxins, severe hypoxia, massive trauma, severe ATP depletion lead to necrosis
   3. Necrosis is abnormal tissue death
   4. Necrosis is nearly always focal and stimulates strong inflammatory response
   5. Cell debris is cleared by immigrant phagocytes (macrophages)
   6. Necrosis does not require energy
   7. DNA is broken down into randomly sized fragments
   8. Usually appears hypereosinophilic on hematoxylin-eosin (H and E) staining
   9. Cellular swelling, disruption of organelles, death of patches of tissue
   10. Plama membrane lysed; cell contents strewn out
   11. Contrast with apoptosis, which is programmed cell death without inflammation
6. Types of Necrosis
   1. Coagulative - usually ischemic, due to reactive oxygen species (ROS)
   2. Liquefactive: cell mediated, hydrolytic enzymes; includes brain and infection
   3. Fatty: fatty tissue degraded, broken down lipids; includes pancreas and liver
   4. Caseating: cheeselike, usually due to infection, surrounded by inflammatory cells
   5. Gangrenous: ischemic necrosis followed by infection leading to liquefactive necrosis
7. Apoptosis
   1. Programmed cell death (PCD) pathways
   2. Occur in both physiologic and pathologic settings
   3. Requires energy (ATP)
   4. DNA breakdown comprised of specific size fragments (multiples of 185 base pairs)
   5. Plasma membrane is intact, blebbed, with molecular changes
   6. Inflammation is minimal or does not occur (see below)
   7. Most types of ischemic damage include necrotic AND apoptotic components

B. Mechanisms of Coagulative Necrosis [7]

1. Hypoxemia leads to O2 depletion, leading to:
   1. Inhibition of fatty acid desaturation, and body cannot reacylate lipid
   2. Depletion of ATP leading to release of sequestered Ca2+
   3. Lipases are activated (some requiring calcium)
   4. Reperfusion with oxygen-rich blood leads to production of ROS
   5. ROS can oxidize lipids and proteins and lead to further cell damage and death
2. Net effect is degradation of phospholipid
   1. Decreased synthesis
   2. Increased degradation
3. Reduction in phospholipids leads to loss of Ca2+ permeability barrier
   1. Influx of Ca2+ occurs from intracellular and extracellular stores
   2. In addition, certain calcium channels may be activated in ischemia
4. Calpain
   1. This is a calcium dependent protease
   2. Increased Ca2+ concentrations leads to activation of calpain
   3. Calpain converts the enzyme Xanthine Dehydrogenase to Xanthine Oxidase (XOA)
5. Xanthine Oxidase (XOA) [10]
   1. Normal Xanthine Dehydrogenase Reaction: Xanthine + H2O + NAD + Uric Acid + NADH + H+
   2. Oxygen is available during reperfusion, and XOA is activated by calpain
   3. XOA catalyzes the conversion of xanthine and oxygen to superoxide, H2O and urate
   4. XOA is inhibited by carbon dioxide
   5. Thus, hypercarbia may reduce inflammation by inhibiting XOA [5]
   6. Xanthine oxidase may play a major role in inducing ROS in liver and intestine (but not heart)
   7. NADPH Oxidase is likely the major generator of ROS in heart and other tissues
6. Superoxide Dismutase (SOD)
   1. Superoxide O2· is highly reactive and toxic
   2. SOD catalyzes superoxide conversion to H2O2 (hydrogen peroxide)
   3. H2O2 is less toxic than than O2· but still highly reactive
   4. Glutathione peroxidase (GPO) is major enyzme for degrading H202
   5. Catalase also detoxifies H202, forming H20 and O2
7. Glutathione Peroxidase (GPO) [12]
   1. H2O2 is primarily detoxified by GPO in higher organisms
   2. GPO catalyzes reduction of H2O2 to H20 and oxidation of glutationine
   3. GPO also prevents oxidation of lipids to maintain biological membranes
   4. GPO implicated in protection against atherosclerosis
   5. GPO, but not superoxide dismutase (SOD), had strong predictive power for cardiovascular events in patients with suspected coronary artery disease
   6. Overall chronic coronary events reduced 70% in highest red cell GPO quartile versus lowest GPO quartile
   7. Inverse relationship between red cell GPO and risk of cardiovascular disease is strong
8. Fenton Reaction
   1. H2O2 is converted to OH· (hydroxide radical)
   2. This reaction ("Fenton") requires reduced (ferrous) iron, Fe2+, or Cu+1 (cuprous) copper
9. Reactive Oxygen Species (ROS) [10]
   1. Mainly H2O2 and OH· (and O2· if SOD is not present)
   2. Generated primarily by NADPH oxidase (neutrophils) and possibly xanthine oxidase
   3. Also generated by prostaglandin biosynthesis and mitochondrial electron transport
   4. H2O2 can combine with Cl- forming highly toxic hypochlorite (ClO-)
   5. These oxidize all biological macromolecules, destroying cell integrity
   6. Also damage DNA, can increase mutations
   7. May play role in variety of disease pathologies (see below)
10. Ischemic Cell Swelling
    1. Initially, lack of O2 leads to depletion of ATP, which is necessary for Na/K ATPase.
    2. If the Na/K ATPase does not function, then Na+ leaks into the cell leading to
    3. Increased cell osmolarity and phospholipid breakdown, then water influx
    4. The water influx causes cell swelling
11. Ischemic cell death stimulates inflammation, initiates inflammatory cascade

C. Reperfusion Injury [6,8,10]

1. Believed to be caused by formation of ROS at site of injury
   1. These ROS are generated primarily by neutrophils
   2. Generation of ROS is believed to require additional oxygen (not present in ischemic tissue)
   3. This oxygen is therefore provided during reperfusion
2. Irreversible injury therefore usually occurs during tissue reperfusion with oxygen
   1. Thus, cell death is often attributed to Reperfusion Injury
   2. However, clear demonstration of reperfusion injury in humans is lacking
   3. May be involved in myocardial infarction, stroke, and in acute peripheral arterial disease
3. Components of Reperfusion Reaction
   1. Complement activation appears to be central: C3a and C5a primarily involved
   2. Inflammatory cell infiltrate - neutrophils attracted by complement components
   3. Platelet activation
   4. Inhibition of complement activation generally reduces reperfusion injury in animal models
   5. At least in some systems, CD4+ T lymphocytes are requried for reperfusion injury [2]
   6. In cardiac cells, reperfusion is mediated through activated mitochondrial PTP membrane protein; protection is mediated through activating kinase family RJSK proteins [6]
4. Glutathione (GSH) Function
   1. Cellular reactive O2 metabolite removal system utilizes glutathione (GSH)
   2. GSH has a reduced sulfhydryl moiety which is highly reactive with oxygen:
   3. H2O2 + 2 GSH ±> 2H2O + GSSG
   4. Note, however, that GSSG will oxidize NADPH to NADP+
   5. This oxidizes normal cell thiols leading to formation of mixed thiols
   6. Mixed thiols are toxic to cells
   7. GSH plays a role in preventing lipid oxidation
   8. Lipid oxidation plays a major role in atherosclerosis (see below)
   9. GSH levels decrease with age and with declining health status [3]

D. Oxidative Damage and Disease [7,8]

1. Increasing evidence that reactive oxygen species (ROS) play key roles in disease
   1. Arthritis
   2. Vasculitis
   3. Glomerulonephritis
   4. Systemic Lupus Erythematosus
   5. Adult respiratory distress syndrome (ARDS)
   6. Cardiovascular Disease: heart disease and stroke
   7. Alcoholism
   8. Smoking related diseases
   9. Alzheimer's Dementia [11]
   10. Amyotrophic Lateral Sclerosis (ALS)
   11. Many others
2. Problems occur when normal detoxification systems are overwhelmed
   1. Superoxide Dismutase
   2. Glutathione S-Transferase
   3. Glutathione Peroxidase
   4. Available reduced glutathione (GSH) [3]
   5. Nitric oxide production stimulated (mixed effects on free radicals) [9]
3. Free Radicals (ROS)
   1. Have unpaired electrons and are highly reactive chemically
   2. Lead to lipid and protein peroxidation with consequent dysfunction
   3. OH· and O2· are particularly damaging
   4. Nitric oxide can combine with these radicals forming toxic peroxynitrite
4. Oxygen Toxicity
   1. Increased O2 tension (including treatment for hypoxia, especially in lung disease)
   2. Increased mitochondrial autoxidation
   3. Increased formation of superoxide and peroxide
   4. Mixed thiols are toxic and dysfunctional in cells
5. Examples of Oxygen Toxicity [10]
   1. DNA damage leading to oncogenesis
   2. Lipid peroxidation leading to atherosclerosis
   3. Protein oxidation leading to protein dysfunction, various pathologies
   4. Mixed thiol formation - abnormal protein function
   5. Protein dysfunction may exacerbate damage to DNA and lipids
   6. Acute pancreatitis - major component with ROS damage
   7. Free Radical Damage occurs also in muscular dystrophies
   8. Many disease of mitochondrial origen
6. Anti-oxidants including Vitamins E and C reduce free radicals

E. Effects of Hypercarbia [5]

1. Anti-Inflammatory
   1. Reduced neutrophil function
   2. Reduced phagocyte (mainly macrophage, some neutrophil) function
   3. Reduced platelet activating factor (PAF)
   4. Reduced phospholipase A2 function (signal transduction)
   5. Reduction in cell adhesion molecule level and function (see below)
   6. Reduced lipid peroxidation
   7. Reduced free radical formation and superoxide function
   8. Reduction in xanthine oxidase (see above)
   9. Increased nitric oxide (vasodilator) and cAMP formation
2. Oxygen Supply-Demand
   [Figure] "Oxygen-Hemogloblin Dissociation"
   1. Shifts oxygen dissociation curve to the right
   2. This means that more oxygen is delivered to tissue for a given pO2 level
   3. Increases packed-cell volume
   4. Causes vasodilation leading to increased tissue blood flow
   5. Increased tissue oxygen and nutrient delivery
   6. REduces sarcoplasmic calcium release
   7. Reduces mitochondrial respiration
   8. Inhibits production of organic acids and metabolic intermediates]
3. Cardiac and Vascular Effects
   1. Increases cardiac output
   2. Increases hypoxic pulmonary vasoconstriction
   3. Improves ventilation - perfusion (V/Q) matching
4. Permissive Hypercarbia
   1. Appears to improve lung dynamics in mechanically ventilated patients
   2. Allows lower tidal volumes and pressures, thereby reducing overdistension
   3. Reduced overdistension also leads to reduced production of stress responses
   4. Has been shown to improve outcomes / mortality in critically ill patients
   5. Likely that these systemic effects are key to improved outcomes
   6. Permissive hypercarbia should strongly be considered in sick patients
   7. Maintenance of adequate (but low normal) oxygenation is also critical

F. Ischemic Damage and Inflammation [4]

1. Ischemic damage leads to both necrosis and apoptosis
2. Necrotic components stimulate inflammatory cascades
3. In addition, ischemia itself leads to upregulation of inflammatory molecules
4. Induction of cell adhesion molecules (CAM) on endothelium
5. Up-regulation of CAMs leads to leukocyte trafficking into tissue (see below)
6. Apoptotic cell death does not typically lead to inflammation
7. Reperfusion leads to increased neutrophil infiltration [10]
   1. Causes direct tissue damage with neutrophil enzyme release
   2. Production of reactive oxygen species

References

1. Hetts SW. 1998. JAMA. 279(4):300
2. Zwacka RM, Zhang Y, Halldorson J, et al. 1997. J Clin Invest. 100(2):279
3. Nuttall SL, Martin U, Sinclair AJ, Kendall MJ. 1998. Lancet. 351(9103):645
4. Molitoris BA and Marrs J. 1999. Am J Med. 106(5):583
5. Laffey JG and Kavanagh BP. 1999. Lancet. 354(9186):1283
6. Yellon DM and Hausenloy DJ. 2007. NEJM. 357(11):1121
7. McCord JM. 2000. Am J Med. 108(8):652
8. Babior BM. 2000. Am J Med. 109(1):33
9. Kubes P and McCafferty DM. 2000. Am J Med. 109(2):150
10. Lefer DI and Granger DN. 2000. Am J Med. 109(4):315
11. Pratico D and Delanty N. 2000. Am J Med. 109(7):577
12. Blankenberg S, Rupprecht HJ, Bickel C, et al. 2003. NEJM. 349(17):1605