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Internal Respiration. Module F. Module F. Chapter 9 – Assessment of Hypoxemia and Shunting Chapter 10 – Treatment of Hypoxemia and Shunting Chapter 11 – Hypoxia: Assessment and Intervention. Objectives. At the conclusion of this session the participant will:

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module f
Module F
  • Chapter 9 – Assessment of Hypoxemia and Shunting
  • Chapter 10 – Treatment of Hypoxemia and Shunting
  • Chapter 11 – Hypoxia: Assessment and Intervention

At the conclusion of this session the participant will:

  • Still be awake! This covers 3 chapters!
    • Relax…most is a review and some will be covered in Winter 09.
  • Define oxygen extraction.
  • Describe the effects of anaerobic metabolism.
  • State the formula for calculating RQ.
  • List the 5 causes of hypoxemia.
  • State the effect of an increase or decrease in cardiac output on the shunt fraction.
  • List three methods, other than the shunt fraction, which can be used to assess the degree of physiologic shunting.
  • State three ways to treat acute hypoxemia.
  • Define anemia.
  • List three types of anemia and state the causes of the defect.
  • Describe the effect of anemia on the presence of hypoxia.
  • State the benefit, problems, and specific levels for each of the following as it relates to it being an indicator of cellular hypoxia:
    • Lactate
    • Mixed Venous Oxygenation
    • Oxygen Consumption & Utilization
    • Gastric Mucosal Acidosis
internal respiration5
Internal Respiration
  • Exchange of oxygen and carbon dioxide at the cellular level.
    • Some control by local vasculature.
      • Increased distance from capillary to tissue will result in decreased delivery.
    • Some organs use more than others.
      • Table 7-1 (p. 188).
      • Note: % of blood flow is not equal to volume of oxygen consumed.
internal respiration7
Internal Respiration
  • Normal metabolism exists when O2 is consumed and CO2 is produced.
    • Normal ratio of CO2 produced : O2 consumed is 0.8:1 (200/250)
    • Increased ratio with excess CHO utilization; decreased with fat & ETOH.
  • When insufficient oxygen is present, anaerobic metabolism results.
    • Less ATP produced.
    • Lactic Acid is produced.
adequacy and efficiency of oxygen delivery
Adequacy and Efficiency of Oxygen Delivery
  • Adequacy: Is there sufficient oxygen present? (Hint: Is hypoxemia present?)
    • Causes of Hypoxemia
      • Low PIO2
      • Hypoventilation
      • Absolute Shunts
      • Relative Shunts
      • Diffusion Defects
      • True or Absolute Deadspace (secondary mechanism)
  • Efficiency: Is the PaO2 appropriate for the FIO2?
    • If not…assume a shunt is present!
effects of cardiac output on p a o 2
Effects of Cardiac Output on PaO2
  • The normal decrease in PaO2 from alveolar oxygen levels is due to the small mixing of anatomically shunted blood (5%).
    • This blood is venous in nature and has a PO2 the same as the PO2.
  • Four situations exist that can affect the PaO2:
    • Decreased Cardiac Output with a Normal Shunt
    • Increased Shunting with a Normal Cardiac Output
    • Decreased Cardiac Output with an Increased Shunt
    • Increased Cardiac Output with an Increased Shunt
the normal ventilation perfusion relationship
The Normal Ventilation/Perfusion Relationship
  • Normal PAO2
    • Normal PćO2
  • Normal PO2
    • Normal CO
    • Normal Oxygen Consumption
  • Normal PaO2
decreased cardiac output with a normal shunt
Decreased Cardiac Output with a Normal Shunt
  • PO2 decreases with a decrease in cardiac output because of an increased oxygen extraction (assumingO2 doesn’t change).
  • Any shunted blood will have a reduced PO2.
  • Because the amount of shunted blood is so small, the decrease in PaO2 isn’t significant.
increased shunting with a normal cardiac output
Increased Shunting with a Normal Cardiac Output
  • Example: ARDS
  • Normal Cardiac Output = Normal PO2
  • The problem here is a significant increase in intrapulmonary shunt, meaning more PO2 “contaminated” blood entering the pulmonary vein (arterial system).
decreased cardiac output with an increased shunt
Decreased Cardiac Output with an Increased Shunt
  • Similar to the first scenario, but here there is an increased intrapulmonary shunt.
    • Example: ARDS with an MI
  • Reduced Cardiac Output yields a reduced PO2 (higher extraction).
  • More of that low PO2 blood is shunted in the lungs, resulting in a large reduction in PaO2.
increased cardiac output with an increased shunt
Increased Cardiac Output with an Increased Shunt
  • Normal physiologic response to hypoxemia is to increase heart rate (peripheral chemoreceptors) and Cardiac Output.
    • PO2 is increased (better oxygen delivery).
  • With an increased intrapulmonary shunt, however, there still is an increased amount of PO2 “contaminated” blood entering the system.
so what
So What?
  • Don’t always assume that an improvement or deterioration in PaO2 is occurring solely because of a change in pulmonary gas exchange.
  • Suspect a change in cardiac output when an abrupt, unexplained hypoxemia is observed in critically ill patients.
  • Also, consider other non-cardiac causes of reduced PO2.
    • Anemia
    • Increased metabolism (fever)
    • Maldistribution of systemic perfusion
assessment of hypoxemia
Assessment of Hypoxemia
  • Definition of “Hypoxemia”.
    • Severity?
  • Causes of Hypoxemia
  • Differential Diagnosis of Hypoxemia
shunt substitutes
Shunt Substitutes
  • P(A-a)O2
  • PaO2/PAO2
  • PaO2/FIO2
p a o 2
  • PAO2 = [(PBARO - PH2O) x FIO2] – (PaCO2/0.8)
    • On FIO2 of less than 60%
  • PAO2 = [(PBARO - PH2O) x FIO2] – PaCO2
    • On FIO2 greater than 60%
  • Normal Values:
    • Room Air: 100 – 104 mm Hg
    • 100% Oxygen: 600
p a a o 2
  • Normal values is around 10 mm Hg on room air.
    • Values increase with increasing age and the supine position.
  • Normal values 25-65 mm Hg on 100%
  • Difficult to use when FIO2 varies from 21 or 100%
    • Normal values differ for each FIO2
      • Limited value when using supplemental oxygen.
p a a o 2 on room air
P(A-a)O2 on Room Air
  • Normal A-a gradient on 21% is seen with:
    • Pure hypoventilation
    • High altitude
    • Diffusion defect (patient at rest)
  • Abnormal A-a gradient on 21% is seen with
    • Relative shunt
    • Absolute shunt
p a a o 2 on 100
P(A-a)O2 on 100%
  • Relative Shunt will improve
    • A-a gradient less than 300 mm Hg
  • Absolute Shunt will not improve
    • A-a gradient is greater than 300 mm Hg
using p a a o 2 to estimate shunt
Using P(A-a)O2 to Estimate Shunt
  • On 100% FIO2, a 1% shunt is estimated for every 10 – 15 mm Hg P(A-a)O2
  • Example: A-a gradient is 140 mm Hg
    • 140 = 9.3% 140 = 14.0%

15 10

using p a a o 2 to estimate shunt24
Using P(A-a)O2 to Estimate Shunt
  • Normal Shunt is 5%
  • Add 5 % to the normal 5% shunt for every 100 mm Hg gradient; Example:
    • 100 mm Hg – 10%
    • 200 mm Hg – 15%
    • 300 mm Hg – 20%
shunt equation
Shunt Equation
  • Classic Shunt Equation
    • “Gold Standard”
  • Clinical Shunt Equation
  • A shunt greater than or = 15% is significant
  • Increased shunts will correlate with
    • “White out on x-ray unless its cardiac in origin.
    • Atelectasis, pneumonia, pulmonary edema, ARDS
classic shunt equation
Classic Shunt Equation
  • Where:
    • CćO2= (1.34 x Hb x 1.0) + (PAO2 x .003)
      • Assumes 100% saturation in the ideal alveolus
  • Requires a Pulmonary Arterial Catheter (BTFDC)
clinical shunt equation
Clinical Shunt Equation
  • Requires a Pulmonary Arterial Catheter (BTFDC)
  • Only accurate at lower FIO2
p a o 2 p a o 2 a a ratio
PaO2 /PAO2 (a-A ratio)
  • Normal value is greater than 75% on any FIO2
  • Example: 100/104 = 96%
    • 96% of oxygen is diffusing across the A-C membrane
p a o 2 f i o 2 ratio
PaO2/FIO2 ratio
  • Normal value is 400 – 500
  • Example: 100 mm Hg/.21 = 476
  • Value between 200 – 300 = ALI
  • Value less than 200 = ARDS
    • Values less than 200 correlate with a shunt of greater than 20%
treatment of hypoxemia
Treatment of Hypoxemia
  • Increase FIO2
  • Increase MAP
    • PEEP,­Inspiratory Time, ­ Vt
  • Body Positioning
    • Prone Positioning
    • Lateral decubitus (good lung down)
  • Good bronchial hygiene
    • Suction, bronchodilators, CPT/Flutter/PEP
oxygen administration
Oxygen Administration
  • Treat hypoxemia/Hypoxia
  • Decrease the work of breathing
  • Decrease the work of the heart
hazards of oxygen therapy
Hazards of Oxygen Therapy
  • Absorption atelectasis
  • Oxygen Toxicity
  • Retinopathy of prematurity
  • Oxygen induced hypoventilation in COPD
    • Look for oxygen levels above 60 mm Hg and a rising PaCO2
    • Evaluate FIO2 patient is receiving
    • Patient symptomatic: sleepy, lethargic
  • PaO2 greater than 100 mm Hg
    • Usually undesirable
    • Very little oxygen content is gained
  • A PaO2 above 130 mm Hg indicates the patient is breathing supplemental oxygen.
  • Hyperoxemia is indicated in COHb%.
  • SpO2% of 100% means the PaO2 could be between 100 mm Hg & 600 mm Hg
    • Very dangerous in infants
oxygen administration in chronic hypercapnia
Oxygen Administration in Chronic Hypercapnia
  • PaO2 will increase 3 mm Hg for each 1% increase in FIO2
  • Keep PaO2 around 60 mm Hg
  • FIO2 = 60 - PaO2 on room air


  • You are asked to draw an ABG on a CO2 retainer. The PaO2 is 39 mmHg on 21%

Where should the FIO2 be set?

FiO2 = 60 - 39 = 7% Add to 21%


  • Set FIO2 at 28%
calculating the maximal p a o 2 for any given f i o 2
Calculating the maximal PaO2 for any given FIO2
  • The PaO2 on room air during hyperventilation may go up to 130 mm Hg
  • A PaO2 more than 5 times the % of oxygen is suspicious.
    • 30 x 5 = 150
    • 40 x 5 = 200
    • 50 x 5 = 250
    • 60 x 5 = 300
  • pH 7.32, PaCO2 48, PaO2 200, FIO2 .30
  • PAO2 = 760 – 47 x 0.30 – 48/.8

= 154 mm Hg

  • Can’t have a PaO2 greater than PAO2, so…
    • Either the FIO2 was not recorded accurately
    • Lab error (air bubble)
evaluating f i o 2
Evaluating FIO2
  • High flow devices may not be delivering the FIO2 that is set
    • If the patient’s total flowrate is exceeding the flow from the oxygen delivery device, the FIO2 will decrease
    • Water in the aerosol tubing will increase FIO2
  • High flow oxygen delivery systems should be analyzed
analyze high flow systems
Analyze High Flow Systems
  • Polarographic (battery and electrolyte solution)
  • Galvanic (fuel cell)
  • Troubleshooting: If analyzer is not reading the FIO2 within + 2% then:
    • Calibrate analyzer first
    • Change fuel cell (galvanic) or
    • Change battery/electrolyte level (polarographic)
correlating abg to the patients condition
Correlating ABG to the Patients Condition
  • A patient who looks good but has bad ABG
    • Suspect a lab error
    • Venous blood gas sample
    • COPD (high PaCO2 and HCO3-)
correlating abg to the patients condition43
Correlating ABG to the Patients Condition
  • A patient who looks and feels bad but ABG are good.
    • CO poisoning, MetHB%
    • Tissue hypoxia
      • Anemic hypoxia
      • Histotoxic hypoxia
      • Circulatory hypoxia
    • Pulmonary embolism – high Vd/Vt ratio and highE
analyzing an abg
Analyzing an ABG
  • On 21%, add PaCO2 and PaO2 to see if greater than 150.
  • If one of the three acid base parameters is abnormal, there is an error.
    • pH 7.58, PaCO2 40, HCO3- 24
  • PaO2 cannot be greater than PAO2 on any FIO2.
analyzing an abg45
Analyzing an ABG
  • Know normal venous values and suspect when a venous sample may have been drawn
  • Inaccurate FIO2
    • Improperly recorded
    • Patients total flow exceeds flow from delivery device
    • FIO2 recorded from low flow system
    • Water in the aerosol tubing
  • Define anemia.
  • List three types of anemia and state the causes of the defect.
  • Describe the effect of anemia on the presence of hypoxia.
  • State the benefit, problems, and specific levels for each of the following as it relates to it being an indicator of cellular hypoxia:
    • Lactate
    • Mixed Venous Oxygenation
    • Oxygen Consumption & Utilization
    • Gastric Mucosal Acidosis
  • Definition: Reduced oxygen levels at the tissue.
  • No “best” index for assessing tissue oxygenation.
  • Begin assessment by assessing the components of oxygen delivery:
    • Dissolved Oxygen
    • Bound Oxygen
    • Hemoglobin
    • Cardiac Output (This will be covered in RSPT 2420)
  • Then look at markers of the effects of possible tissue hypoxia.
types of hypoxia
Types of Hypoxia
  • Hypoxemic Hypoxia
  • Circulatory (Stagnant) Hypoxia
  • Anemic Hypoxia
  • Histotoxic Hypoxia
oxygenation indices
Oxygenation Indices
  • Dissolved Oxygen as an index of hypoxia.
    • Not very useful
      • Pretty good bet hypoxia is present with severe hypoxemia
        • Be careful at extremes!
    • Keep PaO2 above 60 mm Hg.
  • Combined Oxygen (SaO2) as an index of hypoxia.
    • Make sure how you know HOW it is reported
      • SaO2 with nomogram, 2-wavelength oximetry, CO-Oximetry, Pulse Oximetry
    • Better than PaO2, but has its faults.
      • Abnormal species of hemoglobin
      • Insensitive in telling deterioration or at high PaO2 levels.
  • RBC:
    • 5 million/mm3 in men; 4.5 million/mm3 in women.
  • Hemoglobin
    • 15 g% in men, 13 to14 g% in women.
  • Anemia defined as a reduction in the amount of circulating RBC or hemoglobin.
  • Hematocrit (formed elements in blood)
    • 47% in men, 42% in women.
    • Too low is bad; too high is bad.
types of anemia
Types of Anemia
  • Presence of anemia means one of two things:
    • Decrease in production of RBC or Hb
      • Bone Marrow Failure (Aplastic Anemia)
        • Usually due to chemical or physical agent (normocytic)
      • Inadequate Hemoglobin synthesis
        • Iron deficiency 2° chronic blood loss or pregnancy (microcytic)
          • Pagophagia: Ice chip craving
        • Thalassemias – genetic disorder (microcytic)
      • Inadequate RBC formation
        • Folic Acid deficiency: Green vegetables & alcoholics (macrocytic)
        • B12 deficiency: Pernicious anemia 2° lack of intrinsic factor (macrocytic)
    • RBC & Hb are being lost or destroyed at an accelerated rate.
      • Blood loss
        • Acute bleeding (normocytic)
        • Excessive hemolysis
      • Sickle cell disease
analyzing f i o 2
Analyzing FIO2
  • Always correlate ABG to patients condition.
  • When drawing from an A-line, always remove all heparin from the lines – this means withdrawing 3-5 cc and discarding.
  • Understand the relationship of increased metabolism with leukocytosis (leukemia).
anemia and hypoxia
Anemia and Hypoxia
  • Mild anemia (10 g%) usually won’t cause hypoxia
    • 25% extraction
    • Cardiac output reserves (acute)
    • Changes in levels of 2,3 DPG (cardiac)
  • Probably significant with Hb < 6 g%
  • Transfuse when Hb levels fall below 7 g%
key indicators of hypoxia
Key Indicators of Hypoxia
  • Lactate
  • Mixed Venous Oxygen
  • Oxygen Consumption/Oxygen Extraction
  • Gastric Tonometry
  • Vital Organ Function
  • Immediate response to a reduced oxygen delivery is the onset of anaerobic metabolism.
    • Glycolysis: Pyruvate reduction to lactate.
    • Normal lactate is 0.9 to 1.9 mM/L or 8 to 17 mg/dL
    • Metabolic Acidosis + hypoxemia + ¯ CO = Hypoxia
    • Increase in mortality at levels above 2.5 mM/L; 90% at levels above 8 mM/L
    • Problem is lactate elevation is not linear (not a good early predictor)
      • Reduction is by liver. Poor perfusion/Liver failure worsens prognosis.
    • Cyanide poisoning (Histotoxic hypoxia) should be suspected with high lactates and no increase in HbCO with smoke inhalation.
mixed venous oxygenation
Mixed Venous Oxygenation
  • Requires a pulmonary artery catheter.
  • Assessment of oxygen supply vs. demand
  • SO2: Continuous vs. Spot Check
    • Normal 75%
        • Decreased with increasedO2, decreased SaO2, decreased Hb or decreased CO.
  • PO2:Average end-capillary driving pressure.
    • Usefulness depends on distribution of cardiac output.
    • Decreases are associated with decreased supply or increased demands.
    • Increases are associated with reduced utilization (NOT ALWAYS A GOOD THING!)
oxygen uptake and utilization
Oxygen Uptake and Utilization
  • Normal oxygen uptake (consumption) by the tissue remains constant despite changes in cardiac output because of huge reserve (25% normal extraction).
    • Hypoxia is present when O2del falls below 8 to 10 ml/kg/min.
  • Covert Hypoxia: Normally, increasing oxygen delivery is not needed; in some situations (MOF secondary to ARDS, septic shock, ARF). The cause is suspected to be an altered oxygen utilization.
gastric tonometry
Gastric Tonometry
  • Blood shunting to key organs occurs with reduced oxygen supply at the expense of non-vital organ systems (GI tract).
  • If hypoxic crisis is present, GI involvement will be a primary source.
  • The mixing of gases to a point of equilibration is called tonometry.
  • Use of a specialized catheter with a balloon can measure the gastric carbon dioxide and infer gastric blood flow.
sublingual tissue p c o 2
Sublingual Tissue PCO2
  • Improvement on gastric tonometry.
  • Uses CO2 sensor in “temperature” like probe.
  • Results within 60 seconds.
vital organ function
Vital Organ Function
  • If compensatory mechanisms are intact, the presence of these compensatory mechanisms may be an indication that hypoxia is present.
    • Urine output
    • Mental status
    • Skin coolness
    • Great toe temperature