Arterial Blood Gasesthe beginning

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Arterial Blood Gases---the beginning

• What you need to understand
• Where you need information
• How to use blood gases
• The basics of chemistry and physics
• Hemoglobin

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First some definitions---

• acidemia---decreased pH in the blood
• acidosis---decreased pH in the tissues
• alkalemia---increased pH in the blood
• alkalosis---increased pH in the tissues

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Definitions (contd)

• hypoxemia---decreased PaO2 in the blood
• hypoxia---decreased PO2 in the tissues
• hypercarbia, hypercapnia---increased PCO2 CO2
  retention
• acute---a condition in which there has been no
  compensation by the body to rectify the
  abnormality
• chronic---a condition in which there has been
  compensation by the body to rectify the
  abnormality

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Definitions (contd)

• acid---a proton (H) donor such as H2CO3
• base---a proton (H) acceptor such as HCO3-
• the 'P' in front of the respiratory gases
  indicates partial pressure from Dalton's Law of
  Partial Pressures, which states
• that with a MIXTURE of gases in an enclosed
  system, each gas will exert its own partial
  pressure and when added together,
• will yield the SUM TOTAL Pressure of the whole
  system...now...

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Why gas exchange takes place...

• After gases flow into the the respiratory zone,
  virtually no flows occur at this point...
• The movement of the gas molecules is vibratory in
  their motion...
• This is called Brownian Movement

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Why gas exchange takes place...

• The movement of molecules from an area of high
  concentration to low across a semi-permeable
  membrane is known as...
• Diffusion
• This remains to be the only reason for gas
  exchange either in external or internal
  respiration.

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Now...lets look at the concepts of arterial
blood gases...
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Concept of pH (-log H) or pH pKa log
Base/Acid Ratio

• 7.35-7.45
• 7.40
• Measured
• SANZ
• Acid-Base status

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PaCO2 (adequacy of alveolar ventilation)

• PaCO2
• 35-45 torr
• 40 torr
• Measured
• SEVERINGHAUS
• Ventilation

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PaO2 (adequacy of diffusion, a-c membrane)

• PaO2
• 80-100 torr
• 90 torr
• Measured
• CLARK
• Oxygenation

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HCO3 - (Bicarbonate)

• HCO3-
• 22-26 mEq/L
• 24 mEq/L
• Calculated
• Acid-base status

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Oxygen Saturation ()

• O2 Saturation
• gt88
• Measured (CO-oximetry)
• Infrared
• Hb efficiency O2

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COHb (Carboxyhemoglobin)

• COHb
• lt6
• Measured (CO-oximetry)
• Infrared
• Hb CO binding

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Henderson-Hasselbalch Equation, pH Concepts and
the Law of Mass Action

• Definitions
• acid--proton (H) donor
• base--proton (H) acceptor
• Law of Mass Action--when a number of reactants
  meet in chemical activity, a proportional amount
  of product will be produced. Therefore, if the
  amount of reactants is increased, the amount of
  product with be proportionately increased. This
  is the basic law of chemical equilibria...based
  upon Einsteins concept that neither energy or
  mass can be created nor destroyed. As in all
  systems, activities seek balance.

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Henderson-Hasselbalch Equation, pH Concepts and
the Law of Mass Action

• Consider
  CO2 H2O H2CO3 H HCO3- and
  the H-H equation
• pH pKa log HCO3 -
• 
  H2CO3

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• If carbon dioxide (CO2) increases, so does
  carbonic acid (H2CO3 ).
• pKa is the 1st dissociation constant of H2CO3 and
  has a value of 6.1 at 37 degrees C
• Under normal conditions, pH 6.1 log 20/1. The
  normal base-to-acid ratio is 201. The log of 20
  is 1.3. Therefore, pH 6.1 1.3 or a pH 7.4
  (which happens to be the normal mean pH of human
  arterial blood).
• Based upon the Law of Mass Action, carbon dioxide
  increases will result in carbonic acid increases
  therefore (and lets assume that carbonic acid
  has doubled)
• Now, pH 6.1 log 20/2...we have, the log of
  10, which happens to be 1.0...therefore pH 6.1
  1.0 7.1 (This is an example of acute
  respiratory failure.

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Hemoglobin
A large protein structure having a molecular
weight near 65,000, comprised of 4 heme units and
globin
The globin portion is a protein molecule, made up
of 2 ? and 2 ? polypeptide chains. Each of
these chains is bound to an iron (heme)
moiety. The iron attaches at a histidine residue
in both the alpha and beta chains.
Hemoglobin is a metalloporphyrin , that is,
porphyrin combined with a metal, in this case,
iron.
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Hemoglobin-a simplified structure
HEME
HEME
GLOBIN
HEME
HEME
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Hemoglobin

• In the life of a human being, 4 different types
  of hemoglobin
• will be produced, all with different globin
  chains
• Embryonic Hemoglobin (2 a and 2 e chains)
• Fetal Hemoglobin or HbF (2 a and 2 g chains)
• Normal Adult Hemoglobin or HbA (2 a and 2 b
  chains)
• Adult Hemoglobin Variant or HbA2 (2 a and 2 d
  chains)

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(No Transcript)
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100
Normal SaO2
90
80
Normal SvO2
70
60
Hemoglobin Saturation
50
40
Normal PaO2
30
20
Normal PvO2
10
0 10 20 30
40 50 60 70 80
90 100
PaO2 (torr)
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Hemoglobin--The Oxygen Dissociation Curve
Actually a misnomer, for the curve tells the
story of oxygen loading as well as unloading.
Increased Hemoglobin Affinity for Oxygen--a
left shift of the curve...easy to load
oxygen...difficult to unload Decreased
Hemoglobin Affinity for Oxygen-- a right shift of
the curve...difficult to load...easy to unload
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Hemoglobin

• Not only an oxygen carrier, it serves as one of
  bloods
• buffering systems.
• When oxygenated, hydrogen ions are released to
  play a
• vital role in lowering pH, helping to reverse the
  carbon
• dioxide reaction so that carbon dioxide can be
  exhaled as
• a free gas.
• When deoxygenated, hydrogen ions (primarily from
• the carbon dioxide reaction), are accepted
  thereby allowing
• the carbon dioxide reaction to proceed
  forward,placing
• carbon dioxide in solution in the form of
  bicarbonate.

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Hemoglobin--reactions at the tissue level
Tissue
Plasma
Erythrocyte
Dissolved
CO2
Dissolved
CO2
CO2
H2O CO2
H2O
capillary membrane
H2CO3
CO2
HCO3- H
HCO3-
Cl-
Cl-
H HbO2
O2 HHb
HHb CO2
O2
O2
Carbamino compounds
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Hemoglobin--reactions at the alveolar level
Alveoli
Plasma
Erythrocyte
Dissolved
CO2
CO2
CO2
Dissolved
H2O CO2
H2O
capillary membrane
H2CO3
CO2
HCO3- H
HCO3-
Cl-
Cl-
H HbO2
O2 HHb
HHb CO2
O2
O2
Carbamino compounds
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Hemoglobin Breakdown Reuse
hemoglobin
Porphyrin ring split w/ open heme complexed
w/globin chains-- VERDOHEMOGLOBIN
Iron atom released
Open porphyrin ring-- BILIVERDIN
Reduction
Iron stored in Liver
BILIRUBIN excreted in bile
BILIRUBIN
LIVER
to liver
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Hemoglobin--Carbon Dioxide

• Attaches to the terminal amine groups of the
  globin chains, not to heme as oxygen does
• The reaction is reversible and is rapid, similar
  to oxygen

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Hemoglobin Myoglobin

• Hemoglobin
• 4 heme units, 1 globin chain
• Dissociation curve sigmoid
• attach 4 molecules O2
• oxygen release lt 60 torr

• Myoglobin
• 1 heme, 1 globin chain
• Dissociation curve--hyperbolic
• attach--1 molecule O2
• oxygen release lt 40 torr

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Oxygen Free Radicals

• We need oxygen to maintain metabolism...but,
  oxygen has its side effects as well
• Oxygen has two unpaired electrons spinning in the
  same direction in its outermost shell
• During its metabolic engagements, oxygen is
  reduced, but some of the oxygen may undergo
  univalent reduction--that is it gains one
  electron...
• When this happens, OXYGEN FREE RADICALS ,
  chemical intermediates of oxygen metabolism, are
  formed.

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Oxygen Free Radicals

• Those radicals considered cytotoxic (oxidants)
• O2- --superoxide radical
• 1O2 --singlet oxygen
• OH . -- hydroxyl radical
• .OH2 . --perhydroxyl radical

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Interpretation the 1st Steps

• PaCO2s gt 45 torr indicate hypoventilation
• PaCO2s lt 35 torr indicate hyperventilation
• For every 10 torr rise (acute) in PaCO2, above 40
  torr, the pH will fall 0.08 units
• For every 10 torr fall (acute) in PaCO2, below 40
  torr, the pH will rise 0.08 units

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Interpretation--An Example

• Given pH 7.32 and PaCO2 50 torr
• PaCO2 has risen 10 torr if this patient has an
  acute respiratory problem his predicted pH will
  fall 0.08 units and will equal 7.32.
• In this case, the pts pH 7.32, which means his
  problem is pulmonary related only...there is no
  compensation, nor is there a metabolic component

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Interpretation---

• If the patients actual pH is lt the predicted
  value, the patient has a metabolic acidosis
• If the patients actual pH is gt the predicted
  value, the patient has a metabolic alkalosis

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100
90
80
70
60
Legend Right Shift--Decreased Hb Affinity
for Oxygen Normal Left Shift-- Increased Hb
Affinity for Oxygen
Hemoglobin Saturation
50
40
30
20
10
0 10 20 30
40 50 60 70 80
90 100
PaO2 (torr)
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Causes of O2 Curve Shifts

• Left Shifts
• Hydrogen ion (Increased pH)
• PaCO2
• Temperature
• 2,3-DPG
• Abnormal Hbs such as
• Hb Kansas, Hb Bristol, Hb
• Seattle

• Right Shifts
• Hydrogen ion (Decreased pH)
• PaCO2
• Temperature
• 2,3-DPG
• Abnormal Hbs such as
• Hb Chesapeake , Hb Little Rock, Hb Rainier

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If...

• PaCO2 causes a right or left shift in the
  oxyhemoglobin dissociation curve...
• This is the... HALDANE EFFECT

• pH causes a right or left shift in the
  oxyhemoglobin dissociation curve,,,
• This is the...
• BOHR EFFECT

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The Carbon Dioxide Dissociation Curve
100
90
80
70
60
CO2 Content ml/100 ml (Vol)
50
40
30
20
10
0 10 20 30 40
50 60 70 80 90
100
PaCO2 (torr)
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The Carbon Dioxide Dissociation Curve
100
90
80
70
60
CO2 Content ml/100 ml (Vol)
50
40
30
20
10
0 10 20 30 40
50 60 70 80 90
100
PaCO2 (torr)
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A Change of 6 torr of carbon dioxide from 40 to
46 torr (the difference between arterial and
venous blood) causes an increase of about 5
Vol CO2... A similar change in oxygen will
cause only a 2 Vol change...