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8/6/2019 30420289 Oxygen Therapy
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Oxygen Therapy
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Physiology of Oxygenation
� Alveolar Gas Equation
P AO2=(PB-PH2O)FiO2 ± PaCO2(FiO2+1-FiO2)
R� Normal transit time for blood through pulmonary
capillary is 0.3 to 0.7 sec
� Alveolar gas exchange is a major determinant of
P AO2
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Physiology of Oxygenation
� Oxygen delivery to the periphery and its
utilization depends on:
1. Oxygen content of arterial blood2. Amount of blood i.e. cardiac output
� DO2 = CO X CaO2 X 10
� CaO2 = (Hg X 1.34 X SaO2) + (PaO2X0.0031)
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Oxygen Cascade
� Dry air at sea level : 159 mmHg
� Trachea : 149 mmHg
� Alveolus : 101 mmHg� Mitochondrion : 3.8 ± 22.5 mmHg
� Mixed venous blood : 40 mmHg
� Oxygen consumption per minute at rest is225 to 250 ml.
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Oxyhemoglobin Dissociation Curve
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Causes of Hypoxia
A.Hypoxemic
� Decreased oxygen intake (high altitude)
� Ventilation-
perfusion imbalance (high V/Q)(obstructive airways disease)
� Shunt (low V/Q) (ASD, pulmonary AV fistula)
� Diffusion defect (interstitial pneumonitis)
� Alveolar hypoventilation (COPD)� Low mixed venous oxygen
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Causes of Hypoxia
B.Impaired Delivery
� Circulatory (forward flow) (hypovolemia,
heart-failure)� Distributive (sepsis, arterial insufficiency)
� Defective blood oxygen transport
(inherited abnormal hemoglobin, anemiaand acquired abnormal hemoglobin e.g.
carbon monoxide poisoning)
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Oxygen-tension based gas
exchange indices� (A-a)DO2 : normally <15 mmHg in young
healthy adults
� PaO2/P AO2
� PaO2/FiO2
� P(A-a)O2/PaO2 (Respiratory index)
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Indications of Short-Term Oxygen
Therapy� Arterial Hypoxemia
1. Hypoxemia with hypercapnia
2. Hypoxemia without hypercapnia
� Tissue hypoxia without hypoxemia
1. Anemic hypoxia
2. Circulatory hypoxia3. Cyanide poisoning
4. Carbon-monoxide poisoning
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Indications of Short-Term Oxygen
Therapy� Cardiac failure, trauma and hypovolemic
shock
� Acute myocardial infarction� Miscellaneous: sickle cell crisis,
acceleration of resorption of air in
pneumothorax, relief of dyspnea without
hypoxemia
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Indications of Long-Term Oxygen
Therapy1.Continous Oxygen
� Resting PaO2 < 55 mmHg or SaO2 < 88%
� Resting PaO2 56-59 mmHg or SaO2 89% in the
presence of any of the following indicative of cor -
-pulmonale
a) Dependant edema suggesting CHF
b) P-pulmonale on ECG (P wave >3 mm in I,II,aVF)
c) Polycythemia (hematocrit > 56%)d) Resting PaO2>59 mmHg or SaO2>89% justifying that
more conservative therapy has failed
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Indications of Long-Term Oxygen
Therapy2.Non-continuous Therapy
� During exercise PaO2 55 mmHg or SaO2
88% with a low level of exertion
� During sleep : PaO2 55 mmHg or 88% withassociated complications like P AH,daytime somnolence and cardiac
arrhythmias**Oxygen flow rate and number of hours per day
must be specified
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Goals of Oxygen
Therapy� To increase alveolar oxygen tension
� To decrease the work of breathing
required to maintain a given alveolar oxygen tension
� To decrease myocardial work necessary
to maintain a given arterial oxygen tension
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Guiding Principles of Oxygen
Therapy� Administer minimum therapeutic dose
needed to obtain the desired result and no
more� Prescribe either as percent (e.g. 24%) or a
fractional concentration (FiO2 : 0.24)
� Assess bedside cardiac, pulmonary and
neurological status before and beginning
oxygen therapy
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Evaluation of Oxygen Therapy
� Physical examination of cardiopulmonarysystem: pulse rate and rhythm, blood pressure(systolic and diastolic), perfusion state- skin
color, texture and capillary refill� Urine output
� Level of consciousness
� Ventilatory pattern-respiratory rate, tidal volume
and work of breathing� Arterial blood gases : PaO2, PaCO2, acid-base
status
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Oxygen Delivery Equipment
Choice of equipment is based on:
a. Degree of hypoxemia
b. Requirement for precision of delivery
c. Patient comfort
d. Cost
Types of equipment
1. Rebreathing system
2. Non-rebreathing systemi. Variable performance low flow system
ii. Fixed performance high flow system
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Guidelines for Selecting mode of
OxygenLow flow system is adequate if:
� Tidal volume is between 300 to 700 ml
� Ventilatory rate is <25 per minute� Ventilatory pattern is regular and
consistent
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Variable Performance Low Flow
System� Gas flow is not sufficient to meet all inspiratory
demands
� Does not provide consistent and predictable
FiO2
� Factors affecting FiO2 are:a. ventilatory pattern of the patient
b. size of available oxygen reservoir
c. oxygen flow (liters per minute)� Advantages : cheap, comfortable, easy to set
up
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Variable Performance Low Flow
System� For every liter per minute (LPM) change in
flow rate, ther is approximately 0.04 (4%)
change in FiO2
� The larger the tidal volume or faster the
respiratory rate, the lower the FiO2
� The smaller the tidal volume or slower the
respiratory rate, higher the FiO2
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Estimation of FiO2 in Low-Flow
SystemExample: Healthy person with normal ventilatory
pattern; tidal volume=500 ml, respiratoryrate=20, inspiratory time=1 sec, expiratory time
=2 sec, anatomic reservoir=50 ml.� 50 ml of 100% O2 from anatomic reservoir
� 100ml of O2 supplied by cannula flow rate
� 350 ml of 20% O2 (room air); thus 0.20 x 350
ml=70 ml of 100% O2
Thus 500 ml of inspired gas contains 220 ml of 100% O2 or the FiO2 is 0.44 (44%)
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Variable performance Low Flow
System� When a constant FiO2 is required, as in
chronic carbon-dioxide retention, low-flow
systems should not be used
� A low flow system is not synonymous with
a low concentration of oxygen
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Guidelines for Estimating FiO2 with
Low-Flow Oxygen Devices
100% O2 Flow Rate (LPM) FiO2
Nasal cannula or catheter
1 0.242 0.28
3 0.32
4 0.36
5 0.40
6 0.44
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Nasal prongs
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Guidelines for Estimating FiO2 with
Low-Flow Oxygen Devices
100% O2 Flow Rate (LPM) FiO2
Oxygen mask
5-6 0.40
6-7 0.50
7-8 0.60
Mask with reservoir bag
6 0.60
7 0.70
8 0.80
9 0.80+
10 0.80+
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Oxygen Mask
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Fixed Performance High Flow
System� Gas flow is sufficient to meet all inspiratory
demands
� Provides consistent and predictable FiO2
� Patient¶s ventilatory pattern does not affect theFiO2
� Based on the principle of air -entrainment andgaseous jet-mixing
� Better control of humidity and temperature bothhigh and low concentrations can beadministered
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Air -Oxygen Ratio in High-flow
System
70
20
100
30
50
30 =0.6:1
50
³Magic-box´ method
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³Venturi´ Device with mask
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Oxygen Conserving Devices
� Trans-tracheal Oxygen Therapy (TTOT)
� Reservoir cannulaa. Moustache type
b. Pendant type
� Demand flow Oxygen Device System
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Respiratory Flow Cycle in relation to
Continuous Flow supplemental Oxygen
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Trans-tracheal Oxygen Therapy
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³Moustache´ type & ³Pendant´ type
³Oximizer´ devices
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³Moustache´ type ³Oximizer ́ Device during
expiration & inspiration
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³Pendant´ type ³Oximizer ́ device
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Intermittent Flow Oxygen
conservation devices
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Enclosures
� Oxygen Tents
� Oxyhoods
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Oxygen Supply Methods
Home Oxygen is supplied from:
� Compressed oxygen cylinders
� Liquid oxygen cylinders
� Oxygen concentrators and enrichers
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Compressed Oxygen Cylinders
Advantages
Good for small-volume users no waste or loss, stores oxygen indefinitely,
widespread availability
Disadvantages
Large cylinders are heavy and bulky, high
pressures are a safety hazard (2200 psi),provides limited volume of oxygen,frequent deliveries may be necessary
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Cylinder Oxygen Systems
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Liquid Oxygen Systems
Advantages
Provides large quantities of oxygen, low pressure
system, portable units can be refilled from
reservoir (up to 8 hours supply at 2 LPM),valuable for rehabilitation
Disadvantages
Loss of oxygen due to venting when system is not
in use, LOX must be delivered as needed, low
temperature may be a safety hazard
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Liquid Oxygen Reservoirs (Stationary
System)Portable Liquid Oxygen Unit
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Oxygen Concentrator AdvantagesNo waste or loss, low pressure system (15 psi),
cost-effective when continuous supply of oxygenis needed, eliminates need for oxygen delivery
DisadvantagesDisruption in electrical service renders system
inoperable, back-up oxygen is necessary,cannot operate ventilators or other high pressure
devices, concentration of oxygen decreases withflow rate, electrical costs for operating systemmay be substantial
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Oxygen Concentrator
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Limitations of Oxygen Therapy
� Refractory Hypoxemia:
*PaO2 < 55 mmHg at FiO2 > 0.35
*PaO2 > 55 mmHg at FiO2 < 0.35 ANDresponse to oxygen challenge of 0.2 FiO2
is less than 10 mmHg
� Hypoxic Vasoconstrction (HPV)
� Denitrogenation Absorption Atelectasis
(DAA)
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Adverse Effects of Oxygen Therapy
� Altered physiology
*Pulmonary (hypoventilation, absorptionatelectasis, pulmonary vasodilatation, decreasedmucociliary clearance)
*Extra-pulmonary (suppressed (erythropoesis,decreased caediac output, systemicvasoconstriction)
� Pulmonary Tissue Injury Syndrome (tracheo-
bronchitis, ARDS,B
roncho-pulmonarydysplasia)
� Retinopathy of prematurity
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� Oxygen is addicting: in its grip are all mitochondria-richeukaryocytes who learned to depend on it during the past 1.4 billion years. This, the first atmospheric pollutant, is the waste product of Stromatolytes(formation of algal plankton), which excreted it at least 2.3 billion years ago. Since then all sediments have beenrusted or oxidised. Oxygen is toxic. It rusts a person in acentury or less. With oxygen came the danger and blessing of fire. If introduced today, this gas might havedifficulty in getting approved by the Food and Drug administration
Severinghaus JW, astrup PB. History of blood gas analysis;In: Leland Clark¶s electrode. J Clin Monit 1986;2;125-139
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Cellular Mechanisms of Oxygen
Toxicity� Step 1 produces superoxide molecule
� Step 2 produces hydrogen peroxide
� Step 3 produces hydroxyl ion
� Step 4 produces water Followed by free radical reactions: lipid
peroxidation, enzyme inactivation and nucleicacid damage
Cellular defences: superoxide dismutase,glutathione peroxidase, ascorbic acid, alpha-tocopherol and beta-carotene
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Factors that Increase Susceptibility
to Oxygen Toxicity� Adrenergic stimulation
� Corticosteroids
� Hypothermia
� Hyperthyroidism
� Vitamin E deficiency
� Protein deficiency
� Premature birth
� Bleomycin