Blood Gases: Pathophysiology and Interpretation

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Blood Gases Pathophysiology and Interpretation

• Tintinalli
• Chapter 26
• Sept. 1, 2005

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Definitions

• Ventilation Is a function of the rate and depth
  of breathing and determines the clearance of
  carbon dioxide from the body
• Oxygenation Diffusion of oxygen from the lungs
  to the bloodstream for subsequent delivery to the
  tissues

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Minute Ventilation

• Total amount of new air moved in and out of the
  airways and lungs each minute
• Equals tidal volume multiplied by the respiratory
  rate
• Normal tidal volume is 7mL/kg, or 500mL in adult
• Normal rate is 12 breaths/minute
• Normal minute ventilation is 6L/min

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Dead Space

• Anatomic dead space occurs in the trachea,
  bronchi, and bronchioles
• Alveolar dead space (high V/Q mismatch) occurs
  when ventilation of the alveolar-capillary is
  normal but perfusion is absent
• The combined dead space is physiologic dead space
  and is about 30 of the tidal volume
• ARDS and COPD can increase dead space to 60
• Dead space over 60 typically requires intubation

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Partial Pressures

• Normal atmospheric pressure is 760mm Hg
• Partial pressure of H20 is 47mm Hg and is
  subtracted from atmospheric pressure (760-47713)
• Remaining gases are
• Nitrogen 79 (563mm Hg)
• Oxygen 21 (149mm Hg)
• CO2 0.04 (0.3mm Hg)

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Alveolar Gas Equation

• For each mL of O2 leaving the alveolus, 0.8 to
  1.0 mL of CO2 enters it
• This is the respiratory quotient (RQ)

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A-a Gradient

• Determines the degree of lung function impairment
• The A-a gradient is the partial pressure of
  alveolar oxygen minus the partial pressure of
  arterial oxygen (PAO2-PaO2)
• Normal is 2-10mm Hg or 10 plus one tenth the
  persons age

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A-a Gradient

• PAO2(PB-PH2O)(FIO2)-PaCO2/RQ
• PAO2(760-47)(0.21)-40/0.08
• PAO2100mm Hg at sea level in room air
• PaO2 in a normal, healthy adult in room air at
  sea level is 90-100mm Hg
• So, the PAO2-PaO2 is 100 minus 90, or about 10mm
  Hg

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A-a Gradient

• PAO2-PaO2 of 20-30mm Hg on room air indicates
  mild pulmonary dysfunction, and greater than 50mm
  Hg on room air indicates severe pulmonary
  dysfunction
• The causes of increased gradient include
  intrapulmonary shunt, intracardiac shunt, and
  diffusion abnormalities

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PaO2

• Factors affecting the PaO2 include alveolar
  ventilation, FIO2, altitude, age, and the
  oxyhemoglobin dissociation curve
• Relation between PaO2 and SaO2
• PaO2 corresponds to SaO2
• 60mm Hg 90
• 50mm Hg 80
• 40mm Hg 70
• 30mm Hg 60

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Alveolar Ventilation

• During hyperventilation, the PaCO2 falls and the
  PaO2 rises
• If the PaCO2 falls by 1mm Hg, the PaO2 rises by
  about 1.0-1.2mm Hg

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Factors Affecting Oxyhemoglobin Dissociation

• pH
• The more acidic the blood, the more readily
  hemoglobin gives up oxygen and the higher the
  PaO2
• With alkalosis, hemoglobin binds more tightly to
  oxygen
• A rise or fall in pH of 0.10 causes a fall or
  rise in the PaO2 of about 10, respectively

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Factors Affecting Oxyhemoglobin Dissociation

• Partial pressure of CO2
• CO2 entering blood from tissues shifts the curve
  to the right
• Oxygen is displaced from the hemoglobin and
  delivers oxygen at a higher PO2 than normal

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Factor Affecting Oxyhemoglobin Dissociation

• Temperature
• With a rise in blood temperature, hemoglobin
  releases oxygen more readily, which increases the
  PO2 in the plasma

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Factors Affecting Oxyhemoglobin Dissociation

• Exercise
• With exercise, muscles release large amounts of
  CO2 and acids
• Muscle temperature can rise 3-4oC
• Combined, these shift the oxyhemoglobin
  dissociation curve to the right, which releases
  O2 more readily

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Factors Affecting Oxyhemoglobin Dissociation

• 2,3-Diphosphoglycerate (2,3-DPG)
• With prolonged hypoxia over several hours,
  2,3-DPG quantities increase, which shifts the
  dissociation curve to the right
• If the concentration of 2,3-DPG falls, such as in
  banked blood or sepsis, the curve shifts left and
  the PaO2 falls

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PaO2/FIO2 Ratio

• To estimate the impairment of oxygenation,
  calculate the PaO2/FIO2 ratio
• Normally, this ratio is 500-600
• Below 300 is acute lung injury
• Below 200 is ARDS
• Along with diffuse infiltrates, normal PCWP, and
  appropriate mechanism

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Pulse Oximetry

• Factors that affect the pulse ox effectiveness
• Impaired local perfusion (hypothermia,
  vasopressors)
• Ambient light (fluorescent)
• Nail polish (particularly blue)
• Abnormal hemoglobin
• Very high PO2
• Carboxyhemoglobin falsely raises readings
• Methemoglobin falsely lowers the readings

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Questions

• 1. A SaO2 of 90 corresponds to a PaO2 of
• A. 40
• B. 50
• C. 60
• D. 70
• E. 80
• 2. Which of the following affects the pulse
  oximetry readings?
• A. Blue nail polish
• B. Ambient fluorescent lighting
• C. Hypothermia
• D. Carboxyhemoglobin
• E. All will adversely affect the pulse ox
  readings
• 3. Which is false regarding oxyhemoglobin
  dissociation?
• A. As temperature rises, hemoglobin binds oxygen
  with more affinity
• B. Exercise shifts the dissociation curve to the
  right, releasing oxygen more readily
• C. Hemoglobin binds oxygen with more affinity in
  an alkylotic state
• D. During sepsis, the curve shifts left, causing
  a decrease in the PaO2

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Questions

• 4. ARDS is defined as bilateral diffuse
  infiltrates, normal PCWP, appropriate mechanism,
  and a PaO2/FIO2 ratio of what?
• A. Below 500
• B. Below 400
• C. Below 300
• D. Below 200
• E. Below 100
• 5. True or False
• Minute ventilation is the respiratory rate
  multiplied times the tidal volume

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Answers

• 1. C
• 2. E
• 3. A
• 4. D
• 5. T