2. Historical considerations
ī Carl Wilhelm Scheele â 1773
Discovered O2
ī John Pristley â 1774
Was the first to publish
a paper on O2
ī Antoine Lavoisier â 1777
Coined the term âO2â
3. Oxygen:
ī Colourless
ī Odourless
ī Tasteless
ī Transparent gas
ī Slightly heavier than air
ī Constitues 20-21% of atmospheric air
ī Essential for life
4. Importance of O2 in cell chemistry
ī Required in aerobic metabolism for:
1. Production of high energy phosphate compounds
(ATP)
2. Dehydrogenation of flavo proteins
3. Biotransformation of drugs
4. Oxidation of certain other substrates..
5. Definations:
ī Hypoxia: low level of oxygen at tissue level
ī Hypoxemia: low levels of oxygen in blood
ī Partial pressure: the pressure exerted on a surface by
the molecules of individual gases.
The partial pressure of oxygen can be calculated
for a given atmospheric pressure, by multiplying
concentration of a gas by the atmospheric or
barometric pressure.
Eg: 760 mm Hg 21% = 160 mm Hg
6. Oxygen cascade
ī Oxygen cascade refers to the progressive decrease in
the partial pressure of oxygen from the ambient air
to the cellular level.
PO2 in inspired air 150-160 mm Hg
PO2 in alveolar gas (PAO2) 100- 110 mm Hg
PO2 in arterial blood (PaO2) 98 mm Hg
PO2 in Capillary blood 50-80 mm Hg
PO2 in tissues 30- 50 mm Hg
PO2 in cell mitochondria 10- 20 mmHg
7. Factors affecting oxygenation at various levels in
O2 cascade:
Partial pressure Affected by:
Inspired oxygen
PiO2
Barometric pressure
PB
Oxygen concentration
FiO2
Alveolar gas
PAO2
Oxygen consumption
VO2
Alveolar ventilation
VA
Arterial blood
PaO2
Dead space ventilation
Increased V/Q
Shunt
Decreased V/Q
Cellular PO2 Cardiac output
CO
Hemoglobin
Hb
8. Oxygen therapy
Goals of oxygen therapy:
1. Correcting Hypoxemia
īˇBy raising Alveolar & Blood levels of Oxygen
īˇEasiest objective to attain & measure
2. Decreasing symptoms of Hypoxemia
īˇSupplemental O2 can help relieve symptoms of
hypoxia
īĸLessen dyspnoea/work of breathing
īĸImprove mental function
9. 3. Minimizing Cardiopulmonary workload
īˇ Cardiopulmonary system will compensate for
Hypoxemia by:
īĸ Increasing ventilation to get more O2 in the lungs & to the
Blood
īĸ Increased work of breathing
īĸ Increasing Cardiac Output to get more oxygenated blood to
tissues
īĸ Hard on the heart, especially if diseased
īˇ Hypoxia causes Pulmonary vasoconstritcion &
Pulmonary Hypertension
īĸ These cause an increased workload on the right side of heart
īĸ Over time the right heart will become more muscular & then
eventually fail (Cor Pulmonale)
10. ī Supplemental o2 can relieve hypoxemia & relieve
pulmonary vasoconstriction &
Hypertension, reducing right ventricular
workload!!
ī At our institution, minimal acceptable saturation
for post surgical patients who are cared for in non
critical setup is 92%
11. Assessing the need for oxygen therapy
3 basic ways:
Laboratory measures â invasive or noninvasive
PAO2, PaO2, SaO2, SpO2 monitoring
Clinical Problem or condition
postoperative patients, pneumonia, atelectasis,
pulmonary edema, etcâĻ
Symptoms of hypoxemia
Eg: tachycardia, tachypnoea, hypertension,
cyanosis, dyspnoea, disorientation, clubbing, etc
12. Methods of oxygen administration
ī Method selection depends upon required
concentration of oxygen.
ī However, during oxygen therapy the relative dangers
of hypoxia and O2 toxicity should be kept in mind.
Criteria for selecting the method:
1. Patientâs GCS and patientâs comfort
2. Level & range of FiO2 required
3. Extent of humidification required
13. Classification of O2 therapy devices
Oxygen
delivery
systems
Low flow
systems
High flow
systems
14. Low flow O2 delivery system
ī Flow does not meet inspiratory demand
ī Oxygen is diluted with air on inspiration
ī These devices have limited reservoir to store
oxygen and are unable to deliver consistent
inspired oxygen concentrations in settings of
varying respiratory rates & tidal volumes.
17. High flow O2 delivery system:
ī Supplies given FiO2 at flow rates higher than
inspiratory demand.
ī They are suitable for delivering consistent and
predictable concentrations of oxygen.
ī Uses entrainment of air to maintain oxygen supply.
ī Eg: venturi mask, non rebreathing mask, puritan
face mask.
18. Air Entrainment system
īĄAmount of air entrained varies directly
with:
īˇport size
īˇVelocity
īĄThe more air
entrained:
īˇHigher flow
īˇLower FiO2
21. Indications for O2 therapy:
ī Arterial PO2 < 60 mmHg or SaO2 < 90%
ī Cardiac & respiratory arrest
ī Respiratory failure
ī Cardiac failure or myocardial infarction
ī Shock of any cause
ī Increased metabolic demands (eg. Burns, multiple
injuries, severe sepsis)
ī Post operative state
ī Carbon monoxide poisoning.
22. Hypoxia
ī HYPOXIA: A condition in which the oxygen
available is inadequate at the tissue level
ī Five types of hypoxia:
īĄ Anemic
īĄ Hypoxemic
īĄ Histotoxic
īĄ Circulatory
īĄ Hypermetabolic
23. Anemic Hypoxia
ī Having a decreased carrying capacity for oxygen, the pt
with decreased or abnormal Hb
ī Anemia
ī Carbon monoxide poisoning
ī Methemoglobinemia
ī Sickle Cell Anemia
ī Treatment involves blood transfusions, hyperbaric
chamber, bone marrow transplant
24. Hypoxemic Hypoxia
ī Low PAO2 due to the atmosphere
ī Hypoventilation â PCO2 is rising
ī Diffusion Defects
ī The PaO2 will be lower in all cases, but the PCO2 may or
may not be increased.
ī Treatment: Compensatory actions to reduce inequalities,
supplemental oxygen
25. Histotoxic Hypoxia
ī Inability for tissues to utilize oxygen available
ī Cyanide Poisoning will inhibit cellular metabolism
from occuring; the cells can not process the O2
ī Treatment: Reversal of poisoning, supplemental
oxygen and/or ventilation
26. Circulatory Hypoxia
ī A decrease in cardiac output results in a low BP and a
prolonged systemic transit time
ī The PaO2 can be high, but because of the time it takes
to get to the tissues, the pt is hypoxic
ī Cardiovascular instability or failure
ī Shock
ī Arrhythmias
ī Treatment include increasing cardiac output with use
of cardiovascular drugs and therapy, supplemental
oxygen
27. Hypermetabolic Hypoxia
ī In some disease states the body requires a slight
increase in metabolism (i.e. â wound healing
requires 5% increase)
ī Extensive burns and some cancers will cause large
increases metabolism to the point that supplemental
O2 is required
ī Treatment: Supplemental O2 or FiO2
28. Approach to selecting appropriate O2 delivery
system:
Purpose (Objective)
īˇ Increase FiO2 to correct hypoxemia
īˇ minimize symptoms of hypoxemia
īˇ Minimize Cardiopulmonary workload
Patient
īˇ Cause & severity of hypoxemia
īˇ Age
īˇ Neuro status/orientation
īˇ Airway in place/protected
īˇ Regular rate & rhythm (minute Ventilation)
Equipment Performance
The more critical, the greater need for high stable FiO2
īˇ Becomes more difficult the more critical due to pt varying pattern
29. īĄ Pt Categories
īˇEmergency
īĸ Highest FiO2 possible
īĸ Highest PaO2 possible
īˇCritical Adult
īĸ >60% O2
īĸ PaO2 >60mmHg
īĸ SpO2 >90%
īˇStable adult, acute illness, mild hypoxemia
īĸ Low to moderate FiO2
īĸ Response to therapy, not precise concentrations
30. īˇChronic dz adult, acute on chronic illness
īĸEnsure adequate oxygenation without depressing
Ventilation
âĸ SpO2 85-90%
âĸ PaO2 50-60mmHg
âĸ Use ventilating mask to control FiO2 precision
31. âĸ Assess response to therapy!!
âĸ If not maintainable on Cannula, use masks
īPt may remove mask frequently due to
âĸ Discomfort
âĸ Convenience
âĸ Change in mental status
īEncourage Cannula use between mask use if
mask must come off for periods
32. Precautions & Hazards
īĄ O2 Toxicity
īˇPrimarily affects Lungs & CNS
īˇ2 determining factors of O2 toxicity
īĸ PO2
īĸ Time of exposure
īĸ i.e., higher the PO2 & exposure time the greater the
toxicity.
īˇCNS effects occur with Hyperbaric Pressures
īˇPulmonary effects can occur @ clinical PO2 levels
īĸ Patchy infiltrates on x-ray, prominent in lower lung
fields
īĸ Major alveolar injury
33. īˇPathophysiology
īĸHigh PO2 damages capillary endothelium
īĸFollowed by interstitial edema & AC
membrane thickening
īĸType I cells are destroyed (cells that create
new lung tissue, gas exchange cells)
īĸType II cells proliferate (trigger inflamatory
response)
34. īĸ Exudative phase
âĸ Alveolar fluid buildup (from inflamatory response)
leads to
ī low ventilation/perfusion ratio (shunting)
ī hypoxemia
ī Hyaline membranes form @ alveolar level
âĸ Proteinaceous eosinophilic (basic) material
âĸ Composed of cellular debris & condensed plasma
proteins.
ī Pulmonary fibrosis develop
ī Pulmonary Hypertension develops
35. Treatment:
īĸ Try to keep pt alive while reducing FiO2
īˇCause:
īĸ Overproduction of O2 free radicals
âĸ Byproducts of cellular metabolism
âĸ Toxic in excessive amounts
âĸ Normally antioxidants & other special enzymes dispose of excess
free radicals
âĸ Neutrophils (WBCâs) & macrophages flood the infiltrate the tissue
& mediate inflammation response, leading to more free radicals
36. How much is too much?
īĸ>50% for very extended times
īĸ>PO2 the less time it takes
īˇGoal of ideal oxygen therapy:
īĸUse the lowest FiO2 possible to maintain
adequate tissue oxygenation
37. Other side effects
Growing lungs are more sensitive to O2
Retinopathy of Prematurity (ROP), more later
Bronchopulmonary Dysplasia (BPD), chronic lung
dz, Absorption Atelectasis, Fire hazards, etc
Depression of Ventilation
īļ Hypercarbic drive is blunted
High PCO2 no longer stimulates pt to
increase Ventilation
īļSuppression of hypoxic drive
The only stimulus left to increase Ventilation
is due to hypoxia
38. īļ When you add to much O2, (remove the hypoxia) you
effectively remove the neurological stimulus to breathe.
(peripheral chemoreceptorâs)
âĸ Hypoventilation occurs
īļ CO2 continues to elevate to sedative levels
âĸ Pt stops breathing until hypoxic again
âĸ If CO2 is too high, they will remain sedated & causes
Cardiopulmonary arrest
âĸ Never withhold O2 therapy from a
Hypoxic pt (PaO2)
39. Take home message!!
ī Oxygen is a drug, prescribe it as other
drugs, ie, amount, device and time should be
specified.
ī If patientâs SpO2 is not good with nasal
cannula, consider changing the device instead
of increasing flow rate.
ī Overzealous use of oxygen is often without
justification & consideration of toxic effects of
oxygen therapy. So think before such
unaccounted for use of oxygen.