Biochemistry BAHS 501

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Biochemistry BAHS 501

• Clinical Correlation
• Blood Gases

Dr. Bill Paulsen Associate Professor of
Anesthesiology Emory University School of Medicine
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Why Blood Gases are Obtained

• Assess the oxygenation capacity of the
  cardiopulmonary system
• Assessment of oxygen pressure to guide therapy
• Assessment of adequacy of ventilation
• Assessment of acid-base status
• Assessment of Hb concentration, electrolytes and
  glucose

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Blood Gases

• Arterial - peripheral arterial blood
• Venous - peripheral venous blood
• Mixed venous - pulmonary artery blood
• Capillary - scalp stick, finger stick

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Blood Gas Analysis
PO2 measured pH measured PCO2
measured HCO3- Calculated Hb measured
indirectly SO2 measured (functional,
fractional) Electrolytes Na, K, Cl-,
Ca,Mg Other Lactate, Glucose
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(No Transcript)
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Partial Pressures of Gases in Room Air
GAS Percent
Fraction Partial Pressure

Concentration
(mmHg) Oxygen 21 0.21 159.6 Nitrogen 78 0.78
592.8 Argon 0.9 0.009 6.8 Carbon
Dioxide 0.04 0.0004 0.3
Each gas contributes its fraction to the total
(barometric) pressure (e.g. Po2 0.21760 mmHg
159.6 mmHg)
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Oxygen Content in the Blood(mlO2/100ml whole
blood)
O2 dissolved in blood
O2 carried in blood by hemoglobin
Arterial Blood with Patient Breathing Room
Air Cao2 0.0031100 1.31150.97 0.31
19.06 19.37 mlO2/dl
Dissolved Hb
Po2 Partial pressure o2 (mmHg) Hb hemoglobin
(g/dl) So2 Hb saturation with o2
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Oxyhemoglobin Dissociation Curve
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Oxygen Delivery to Tissues
Delo2 Blood Flow Oxygen Content Q Cao2
5L/min 19.37mlO2/dL 968.5 mlO2/minute
Oxygen Uptake by Tissues
Vo2 Q (Cao2 - CvO2) Ficks law
5L/min (19.37 - 14.07)mlO2/dL 265 mlO2/min
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Hemoglobin Variants
Content equation generally assumes that all Hb is
capable of of carrying oxygen.

• Carboxyhemoglobin
• Methemoglobin
• Sulfhemoglobin
• Sickle Cell Hemoglobin
• Fetal Hemoglobin
• Genetically acquired hemoglobin abnormalities

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Measuring Hemoglobin Saturation

• Blood Gas Machine (computed, measured)
• Co-oximeter (measure fractional hemoglobins)
• Pulse Oximeter (measure functional hemoglobin)
• Intravascular fiber optic oximeter (measure
  functional hemoglobin)

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Co-Oximetry
tHb - total potential oxygen carrying
capacity O2Hb - oxygenated hemoglobin HHb -
non-oxygenated (deoxy) hemoglobin COHb - carboxy
hemoglobin (bound carbon monoxide MetHb -
methemoglobin Fe gt Fe HbF - fetal
hemoglobin (thalassemias) HbS - Sickle cell
hemoglobin SulfHb - Sulfhemoglobin (sulfur in
porphyrin ring, cannot bind O2)
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Pulse Oximetry
SpO2 only measures functional saturation -
limited by 2 wavelengths Behaves in unusual
manner in the presence of dysfunctional
hemoglobins - may be extremely misleading - not
useful
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SpO2 Carboxyhemoglobin
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SpO2 Methemoglobin
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Mixed Venous Oxygen Saturation
SvO2 - functional mixed venous oxygen saturation
using a fiber optic pulmonary artery catheter
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CarboxyhemoglobinMechanism of Action

• CO competes with O2 for binding site
• Binding affinity greater than 210 times that of
  oxygen
• Decreased oxygen delivery
• CO also binds to cytochrome A3 blocking
  mitochondrial electron transport and interfering
  with oxygen utilization in the cell.
• Decreased oxygen utilization

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CarboxyhemoglobinCommon Sources of CO

• Incomplete combustion of carbon containing
  materials
• furnaces and ovens
• exhaust gases from internal combustion engines
• kerosene heaters and stoves
• fires
• propane heaters and equipment (saws, forklift)
• charcoal grille

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CarboxyhemoglobinClinical Signs and Symptoms

• 5 COHb - Impaired visual and temporal
  discrimination
• 8 COHb - Alterations in mood and sleep
• loss of short term memory, mild hypotension,
  sinus tachacardia, pvcs
• 20 COHb - Headache and weakness
• atrial flutter/fibrillation, T-wave inversion,
• 40 to 60 COHb - unconsciousness and death
• ventricular tachycardia, fibrillation

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CarboxyhemoglobinTreatment

• Administer Oxygen to compete with CO for binding
  sites
• breathing room air
• half life of COHb is 4 - 5.5 hours
• administer 100 oxygen
• half life of COHb is 1 - 2 hours
• hyperbaric chamber
• at 2.5 - 3 atmA half life is 22 minutes

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MethemoglobinMechanism of Action

• Fe is oxidized to Fe
• Fe cannot bind O2
• One Fe molecule causes the affinity of the
  other 3 molecules of Fe to increase their
  binding affinity for oxygen - not releasing it to
  tissue.
• Oxygen content is reduced
• Oxygen delivery may be reduced without
  compensatory increase in cardiac output

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MethemoglobinCommon Sources

• Local Anesthetics
• benzocaines (topical anesthetics)
• Drugs
• primaquine (anti-malarial)
• dapsone (treating Pneumocystis Carinii)
• nitrites and nitrates (diet or drugs)
• nitroglycerine
• sodium nitroprusside
• Smoke inhalation

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MethemoglobinClinical Signs and Symptoms

• 0.6 - 1.5 MetHb - normal
• 10 - 15 MetHb - cyanosis is obvious
• blood is chocolate brown in color
• 20 - 45 MetHb - dizziness, shortness of breath,
  fatigue, lethargy, headache and syncope
• 45 - 55 MetHb - depressed consciousness
• 55 - 70 MetHb - seizures, coma, dysrhythmias
• gt 70 MetHb - results in Death

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Methemoglobin Normal
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MethemoglobinTreatment

• Goal is to reduce Fe to Fe
• Treatment begins at 20MetHb when the patient is
  symptomatic and 30 MetHb for asymptomatic
  patients. Concurrent diseases require treatment
  at lower levels of MetHb
• Methylene Blue
• 1 - 2 mg/kg over 3-5 minutes. Resolve by 30
  minutes
• use carefully in patients with G6PD deficiency -
  hemolysis
• total dose of 15 mg/kg may induce hemolysis

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SulfhemoglobinMechanism of Action

• Sulfur atom incorporated into the porphyrin ring
• incapable of binding oxygen at that site
• green pigmentation
• favors sickling in HbS
• Decreased oxygen content
• Decreased oxygen delivery without cardiovascular
  compensation

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SulfhemoglobinCommon Sources

• Drugs
• Sulfonamides
• Phenacetin
• Dapsone
• metoclopramide in repeated doses
• Industrial Compounds Environmental Pollutants
• sulfur dioxide
• hydrogen sulfide

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SulfhemoglobinClinical Signs and Symptoms

• Cyanosis without respiratory distress
• 7 -10 SulfHb produces obvious cyanosis
• Reduces affinity of unaffected Hb molecules for
  oxygen - unlike MetHb

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SulfhemoglobinTreatment

• No effective therapy
• Not reversible
• Disappearance related to red cell apoptosis
• RBC life span 120 days
• In extreme cases
• Red Cell Exchange Transfusion

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Case Report 1
A thirty year old woman attending a funeral
accidentally drinks a bitter liquid that was
stored in a cola can. Upon presentation to the
emergency department 2 hours later, her only
complaint is shortness of breath. The can from
which she drank was not available in the
emergency department. Initial Examination Blood
Pressure 130/70 Pulse 120 Respirations
24 Temperature afebrile SpO2 85 on room
air Lungs are clear Heart exam is normal Patient
is noted to have deep cyanosis of the trunk, face
and extremities
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Case Report 1
Patient is given 100 oxygen by mask Patient
remains deeply cyanotic in the trunk, face and
extremities Arterial blood gas is drawn (blood
appears to be brown in color) Po2 575 torr pH
7.45 Pco2 35 SpO2 87
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Case Report 1
Co-oximetry O2Hb 46 COHb 1.5 MetHb
50 HHb 2 Diagnosis - methemoglobinemia from
ingestion of a phenol based preservative which
was left in a cola can. Treatment - Methylene
blue 1-2 mg/kg intravenously over 5
minutes MetHb should resolve over next 30 minutes
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Case Report 2
A 43 yo Caucasian female arrived in the emergency
room complaining of fatigue. She appeared
cyanotic and reported dyspnea on exertion. She
also reported that 10 days earlier she had
applied 4 ounces of DMSO (dimethyl sulfoxide) to
her lower abdomen for treatment of interstitial
cystitis, and that 24 hours afterward she
developed fatigue, cyanosis and heavy breathing
in response to mild exertion. Initial
Examination Blood Pressure 135/82 Pulse
76 Respirations 12 Temperature afebrile SpO2
85 on room air Lungs are clear Heart exam is
normal Patient is noted to be cyanotic in the
trunk, face and extremities
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Case Report 2
Patient is given 100 oxygen by mask Arterial
blood gas is drawn Po2 535 torr pH
7.41 Pco2 34 SpO2 85
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Case Report 2
An co-oximeter in the ER stat lab reported a
MetHb of 47. The patient was treated with two
doses of methylene blue (1 mg/kg) given
intravenously. There was no significant
improvement in her cyanosis or methemoglobin
level . The patient was admitted and a blood
sample was sent to an outside laboratory for a
toxicology screen.
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Case Report 2
The next day the outside lab report indicated a
SulfHb concentration of 8.2 and a MetHb of lt
0.1. Diagnosis - Sulfhemoglobinemia Treatment
- wait for red cell apoptosis since this is not
severe enough for an exchange transfusion.
Explain to the patient that she will feel better
gradually as the damaged red cells are replaced
by the new ones.
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Case Report 3
A 46 yo male with metastatic carcinoma was to
have an MRI of the brain. The patient is
claustrophobic and unable to hold still for the
scan. An anesthesia consult was requested by the
radiologist, who thought that this patient should
be anesthetized for the scan. During the
examination by the anesthesiologist the patient
reported that he almost died during his last
surgery, and he had been told to tell anyone who
was going to put him to sleep that he had a very
difficult airway. The anesthesia team decided to
perform an awake fiber-optic oral intubation.
Upon inspection of his oropharynx a number of
weeping/bleeding mucosal lesions were noted. The
patients oropharynx was topically anesthetized
using Hurricane spray, and an oral endotracheal
tube was placed without difficulty. The patient
was breathing spontaneously as he entered the
bore of the magnet. During the scan, the
patients respiratory rate and heart rate
increased, he became hypotensive, started moving,
and his SpO2 intermittently fell to 85. The
level of anesthesia, blood pressure and heart
rate were treated believing this to be an effect
of anesthesia in the presence of hypovolemia. The
SpO2 reading was ignored as movement artifact and
interference from the MRI.
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Case Report 3
The scan was completed in 30 minutes, and when
the patient was removed from the magnet he was
cyanotic and dyspneic, despite the fact that he
was breathing 100 oxygen through his
endotracheal tube. An arterial blood gas was
drawn Po2 598 mmHg pH 7.38 Pco2 31
mmHg THb 15 g/dl BE -6 mEq/L
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Case Report 3
Co-oximetry O2Hb 30 COHb 1.5 MetHb
68 HHb 1 Diagnosis - methemoglobinemia from
topical anesthetic sprayed in the mouth prior to
intubation Treatment - Methylene blue 1-2 mg/kg
intravenously over 5 minutes MetHb should resolve
over next 30 minutes
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Case Report 4
A 27 yo male is brought to the emergency room
comatose. Friends reportedly found him
unconscious at home. There is a history of
cigarette smoking and a questionable history of
drug abuse, but no one stayed around the ER to
answer any questions. Initial
Examination Blood Pressure 90/45 Pulse
110 Respirations 8 Temperature afebrile SpO2
90 on room air Lungs are clear ECG sinus
tachycardia no evidence of coronary ischemia.
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Case Report 4
Send blood samples for toxicology screen as well
as other panels Draw arterial blood gas Room
air blood gas results PaO2 92 mmHg pH
7.34 PaCO2 42 mmHg tHb 16 g/dL SpO2 91
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Case Report 4
Co-oximetry O2Hb 53 COHb 45 MetHb
0.9 HHb 1
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Case Report 4
Diagnosis - carboxyhemoglobinemia from space
heater found in his apartment. Treatment -
administer 100 oxygen as soon as possible
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Know What is Being Measured when you Request a
Lab Test

• Blood Gas Machine
• is SO2 calculated from oxyhemoglobin desaturation
  curve or is it measured optically? Wavelengths?
• Pulse Oximeter
• limited to 2 wavelengths cannot discern HbO2 in
  presence of dysfunctional forms of Hb
• Co-oximeter
• what is it measuring and what can interfere -
  primarily how many wavelengths does it use?