Oxygen - CriticalCareMedicine

Report
Oxygen: What’s it good
for anyways?
 Outline
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Basic Concepts
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Diffusion
Hemoglobin binding
Oxygen equations
Mitochondrial function
Type IV Respiratory Failure
• Critical DO2
• Cytopathic hypoxia
• Microcirculation shunting
Oxygen Diffusion
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Partial pressure of O2 at standard pressure and
temperature is 21.3 kPa but falls to 14.7 kPa at the
alveoli.
Diffusion of O2 into the blood and then into the tissue is
determined by Fick’s law.
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K=permeability of O2 within the diffusion medium
S=surface area
P=pressure gradient
=diffusion distance
S
Diffusion  K  P
 
Oxygen Diffusion
 In
the lung, the diffusion barrier is the
alveolar-capillary membrane.
 The PO2 is 100 mmHg on the alveolar side
and 90 mmHg on the capillary side.
 At the tissue level, the capillary wall is the
primary barrier.
 The diffusion distance can vary but the
pressure gradient is much higher as the
PO2 at the mitochondria is about 1 mmHg.
Hemoglobin Binding

Once oxygen has crossed the capillary
membrane, it enters the red blood cells and
binds to hemoglobin.
 Why is the oxyhemoglobin dissociation curve
sigmoid?
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
Cooperativity
When the curve shifts to the left or right, it alters
the P50 (oxygen tension at which hemoglobin is
50% saturated).
 Shift to the left – P50 decreases (i.e. lower PO2
needed to saturate 50% of the hemoglobin)
 Shift to the right – P50 increases (i.e. higher PO2
needed to saturate 50% of the hemoglobin).
Hemoglobin Binding

Name four conditions that shift the oxyhemoglobin curve to the left.
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Name four conditions that shift the oxyhemoglobin curve to the right.
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Hyperthermia
Acidosis
Hypercarbia
Increased 2,3-DPG
What happens to red cells from the blood bank?
What is the purpose of 2,3 DPG?


Hypothermia
Alkalosis
CO
Decreased 2,3-diglycerophosphate
Binds deoxyhemoglobin to stabilize the T-state and forces release of
oxygen. A lack of 2,3 DPG mimics fetal hemoglobin.
Trivia – What is the normal shifting of the oxyhemoglobin curve in
the lungs and the tissue called?
Oxygen Equations

1 gm of hemoglobin binds 1.34 mL of O2.
 The solubility of oxygen in serum is 0.03 mL of
O2/ (L)(mmHg).
 Since there is no other way to transport oxygen,
the total oxygen content of blood is the sum of:


That bound to hemoglobin: (1.34 mL/g)(Hgb
g/L)(Saturation)
That dissolved in serum: (0.03 mL/(L)(mmHg))(PO2
mmHg)
CaO2  1.34 Hgb SaO2   0.03 PaO2 
Oxygen Equations

In order to calculate the total amount of oxygen
delivery (global), multiply the cardiac output by
the oxygen content.
DO2  CO( L / min) CaO2 (mL/ L)

Normal oxygen delivery is 1000 ml O2/min
(assuming a cardiac output of 5 L/min and
hemoglobin of 150 g/L)
Oxygen Equations

The amount of oxygen consumed in any tissue can be
calculated by measuring the oxygen content in both the
arterial and venous limb of the tissue.

VO2  CO  CaO2  CvO2 
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The normal global oxygen consumption is 250 mL/min.
What would be the required cardiac output in the
absence of hemoglobin to support a VO2 of 250 mL/min?
Oxygen Equations
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The ratio of VO2/DO2 is the oxygen extraction ratio (ER).
How can you calculate the ER without knowing the Hgb?
The ER increases in conditions such as exercise, CHF, and anemia
as a result of a lower CvO2.
The converse occurs in sepsis.
Each organ has its own metabolic needs so individual organ ER
vary.
The brain and the heart extract much more oxygen and thus are
more susceptible to decreased delivery.
Mitochondrial Function

All reversible reactions proceed in the direction
that results in a net decrease in the Gibbs
energy for the system. (G=H-TS)
 In order for living systems to carry out reactions
that require a positive Gibbs energy, they must
be coupled to a reaction that is energically
favorable.
 If the total Gibbs energy for the two reactions is
negative then the reactions can proceed.
Mitochondrial Function
 Aerobic
generation of ATP occurs as a
result of series of stepwise reactions that
couple the oxidation of substrates to
oxygen with the phosphorylation of ATP.
 To review:
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Reducing agents donate electrons.
Oxidizing agents accept electrons.
Oxygen is a very strong oxidizer while NADH
and FADH are very strong reducers.
Mitochondrial Function
 The
reaction of oxygen to NADH or FADH
has a very negative Gibbs energy whereas
the phosphorylation of ADP to ATP has a
low positive Gibbs energy.
 To capture the released energy efficiently,
mitochondria step down the reaction.
 First it has to generate NADH and FADH
via the citric acid cycle.
Mitochondrial Function

The electrons are transferred through a series of
intermediate compounds that have progressively lower
reducing potentials.
 This respiratory chain is located on the inner membrane
of the mitochondria.
 The energy thus released is used to pump protons from
the mitochondrial matrix to the intermembrane space.
 The protons then follow their gradient through the
F0F1ATPase that catalyzes the formation of ATP.
 Oxygen’s only job is to act as the final electron acceptor
in the respiratory transport chain.
Type IV Respiratory
Failure
Critical DO2
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With moderate reductions in DO2, the ER
increases to satisfy VO2.
What is the ER when DO2 is 1000 mL/min?
(assume VO2 = 250 mL/min)
What is the ER when DO2 is 500 mL/min?
What is the ER when DO2 is 150 mL/min?
The level at which VO2 begins to decline with
declining DO2 is the critical DO2.
At this point, VO2 becomes supply dependant
and the tissues turn to anaerobic metabolism.
The average critical DO2 is 4.2 mL/min/kg.
Cytopathic Hypoxia
 There
are four different but mutually
compatible mechanisms to explain
decreased oxygen consumption in sepsis:
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Inhibition of pyruvate dehydrogenase
NO mediated inhibition of cytochrome a,a3
Peroxynitrite inhibition of mitochondrial
enzymes
Poly(ADP-ribose) polymerase
Inhibition of Pyruvate
Dehydrogenase (PDH)

End product of glycolysis
is pyruvic acid.
 It can be reduced to
either lactate or enter
TCA.
 PDH converts pyruvate to
acetyl-coenzyme A in the
presence of NAD+ and
coenzyme A.
 PDH kinase
phosphorylates PDH to
inactive form.
Inhibition of Pyruvate
Dehydrogenase (PDH)
 In
sepsis, the activity of PDH kinase is
increased.
 The inactivation of PDH limits the flux of
pyruvate through TCA cycle.
 Excess pyruvate accumulates and leads to
increased production of lactate.
 Therefore, hyperlactatemia is not just
evidence of low DO2.
NO-mediated inhibition of
Cytochrome a,a3
 Sepsis
induces iNOS to produce NO.
 When NO binds to cytochrome a,a3 (last
step in the ETC) it out competes O2 for the
same binding site.
 This causes a rapid but reversible
inhibition of the enzyme.
 Since the reaction is reversible, this should
not pose a major problem BUT…
Peroxynitrite Inhibition of
Mitochondrial Enzymes
 NO
also can react with O2- to form
peroxynitrite (ONOO-) with is a powerful
oxidizing and nitrosating agent.
 ONOO- inhibits F0F1 ATPase and Complex
I and II.
 ONOO- also inhibits aconitase (TCA
enzyme).
 Unlike NO, these inhibitions are
irreversible.
Poly(ADP-ribose) Polymerase
(PARP-1)
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PARP-1 is a nuclear enzyme involved
in the repair of single strand breaks of
DNA.
It catalyzes the cleavage of NAD+ into
ADP-ribose and nicotinamide and then
polymerizes the ADP-ribose into
homopolymers.
ROS and ONOO- can induce single
strand breaks in DNA which activates
PARP-1.
The PARP-1 causes the NAD+/NADH
content to fall which impairs the cells
ability to use O2 in ATP production.
Microcirculation shunting
 The
endothelium is an important regulator
of oxygen delivery.
 In response to local blood flow and other
stimuli, it signals upstream to dilate
feeding arterioles.
 RBC can sense hypoxia and release
vasodilators such as NO and ATP.
 The goal is to control local flow patterns to
ensure global oxygenation.
Microcirculation shunting
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In sepsis, endothelial cells:
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Are less responsive to vasoactive agents.
Lose their anionic charge and glycocalyx.
Become leaky
Massively over express NO.
RBC and WBC cell deformability reduces,
causing plugging.
 The WBC and endothelium interact in ways to
induce inflammation and coagulation pathways.
Microcirculation shunting
 Inflammatory
activation of NO is one of the
key mechanism responsible for shunting.
 Inhomogeneous expression of iNOS
interferes with regional blood flow and
promotes shunting from vulnerable
microcirculatory units.
 Inhomogeneous expression of endothelial
adhesion molecules also contribute
through their effects on WBC kinetics.
Lactate
Sympathy for the Devil
Background
 For
years lactate was considered a waste
product of metabolism.
 Recent bench work points to its important
role in intracellular signaling and energy
transport in the muscle, brain and sperm.
 In sepsis, the classic explanation for
hyperlactatemia has been anaerobic
glycolysis due to insufficient oxygen
delivery.
The Case

Recent it has been suggested that the
hyperlactatemia in sepsis may be from an
adrenaline surge that stimulates Na/K/ATPase
activity and coupled aerobic glycolysis.
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To review, adrenaline binds B2-adrenoreceptors,
activating adenylate cyclase, catalyzing ATP to
cAMP.
cAMP activates PKA which activates Na/K/ATPase.
The ATPase pump derives its energy from glycolysis.
Therefore, hyperlactatemia in the face of
hemodynamic stability may be adrenaline
stimulated aerobic glycolysis rather than tissue
hypoxia.
The Evidence
 Aerobically
incubated muscle from septic
rats had an increased rate of lactate
production that was partially inhibited by
ouabain (Na/K/ATPase inhibitor).
 Endotoxemia in heathy humans causes a
rise in adrenaline and lactate levels.
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The lactate rise in the leg was matched by a
fall in regional potassium levels in the blood
and increase in uptake.
No evidence of hypoxia or hypoperfusion.
More Evidence
 A similar
mechanism is in play with
hemorrhage.
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Blocking adrenergic receptors in bleeding rats
caused a significant fall in lactate levels with a
increased Na/K ratio (implying decreased
Na/K/ATPase activity).
Administering ouabain to muscles that were
either bled or infused with adrenaline reduced
the lactate level equally compared to controls.
Conclusion
Tissue hypoperfusion, hypoxia and
anaerobic glycolysis are probably not
the only cause of increased lactate
production in shock.

Outline

Basic Concepts
•
•
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•
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Diffusion
Hemoglobin binding
Oxygen equations
Mitochondrial function
Type IV Respiratory Failure
• Critical DO2
• Cytopathic hypoxia
• Microcirculation shunting
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Lactate – Maybe not the boogie man after all.
Questions???

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