Oxygen therapy and toxicity

Oxygen Therapy and Toxicity
PRESENTER
Dr Krishna Dhakal
Second year Resident
Department of Anesthesiology and Intensive Care
NAMS
MODERATOR
Dr. Santosh Parajuli
Department of Anesthesiology
Shahid Gangalal National Heart Centre

Learning Objectives
• To Define oxygen therapy
• To Explain the indication of oxygen therapy
• To know briefly about oxygen cascade
• To List the sources of oxygen
• To Classify and explain the sources of oxygen delivery devices
• To Explain the hazards of oxygen

Introduction
Oxygen therapy
It is the administration of oxygen at concentrations greater
than that in ambient air (20.9%) with the intent of treating or
preventing the symptoms and manifestations of hypoxemia or
tissue hypoxia.

Goals of Oxygen Therapy
• Correct documented or suspected hypoxemia
• Decrease the symptoms associated with chronic hypoxemia
• Decrease the workload hypoxemia imposes on the
cardiopulmonary system
• Removal of entrapped gas from the body cavities and vessels.

Indication
• Documented Hypoxemia
• Adults, children and infants > 1 month of age.
• Pao2< 60mm Hg
• Sao2 < or Spo2 < 92% at rest breathing room air
• Neonates
• Pao2< 50mm Hg
• Sao2 < or Spo2 < 88% at rest in room air
• COPD, Myocardial Infarction, pulmonary edema, ALI, ARDS,
pulmonary fibrosis, cyanide poisoning, carbon monoxide poisoning.
• Perioperative and postoperative period with general anaesthesia
• Prior to tracheal suctioning or bronchoscopy
• Severe Trauma
• Any clinical suspicion of hypoxemia or hypoxia

Oxygen Cascade
The process of declining oxygen tension from atmosphere to mitochondria
Atmosphere air (dry) (159 mm Hg) [ PIO2=PB x FIO2]
↓ humidification
Lower resp tract (moist) (150 mm Hg) [ PIO2=(PB-PH2O) x FIO2 ]
↓ O2 consumption and alveolar ventilation
Alveoli PAO2 (104 mm Hg) [ PAO2= PIO2- PaCO2/RQ]
↓ venous admixture
Arterial blood PaO2 (100 mm Hg)
↓ tissue extraction
Venous blood PV O2 (40 mm Hg)
↓
Mitochondria PO2 (7 – 37 mmHg)

The Alveolar arterial gradient
PAO2=104 mmHg PaO2=100mmHg
Venous admixture
A-a = 4 -25 mm Hg
PaO2 = 120 − Age/3
Alveolar
Air
Arterial
Blood

The A-a Gradient
● The alveolar-to-arterial O2 partial pressure gradient (A–a
gradient) is normally less than 15 mm Hg
● PaO2 = 120 − Age/3
The A–a gradient for O2 depends on
● the amount of right-to-left shunting,
● the amount of V/Q mismatch, and
● mixed venous O2 tension
● The A–a gradient for O2 is directly proportional to shunt

How Does Oxygen Therapy Work?
• Oxygen Transport
• Oxygen content
• Oxygen flux
• Oxygen uptake
• O2 extraction ratio

Oxygen cascade
Venous Admixture
( physiological shunt)
• V/Q mismatch Normal True shunt
• Bronchial vein Thebesian vein
• Normal : upto 5 % of the cardiac output

Oxygen Cascade
Pa O2 = 97mm Hg
(Sat. > 95 %)
PV O2 = 40mm Hg
Sat. 75%
Arterial
blood
Mixed
venous
blood
Cell
mitochondri
a PO2= 7-
37mmHg
Tissue utilization

Oxygen Content (Co2)
Amount of O2 carried by 100 ml of blood
Co2 =Dissolved O2 + O2 Bound to hemoglobin
Co2 = Po2 × 0.0031 + So2 × Hb × 1.34
(Normal Cao2 = 20 ml/100ml blood
Normal Cvo2 = 15 ml/100ml blood)
C(a-v)o2 = 5 ml/100ml blood
Co2 = arterial oxygen content (vol%)
Hb = hemoglobin (g%)
1.34 = oxygen-carrying capacity of hemoglobin
Po2 = arterial partial pressure of oxygen (mmHg)
0.0031 = solubility coefficient of oxygen in plasma

Oxygen Flux
Amount of of O2 leaving left ventricle per minute.
Do2 =CO x Cao2
= CO × SaO2 x Hb conc x 1.34
= 5000 x 0.97 x 15.4 x 1.34 /100
= 1000 ml/min
CO = cardiac output in ml per minute.
Do2 = oxygen flux

Oxygen Uptake (VO2)
• When blood reaches the systemic capillaries, oxygen
dissociates from hemoglobin and moves into the tissues.
The rate at which this occurs is called the oxygen uptake
(Vo2).
• VO2 = CO X ( CaO2 – CvO2 )
= CO X 1.34 X Hb X ( SaO2 – SvO2 )
• Normal VO2 = 200–300 mL/min or 110–160 mL/min/m2

• The fraction of the oxygen delivered to the capillaries
that is taken up into the tissue .
• An index of the efficiency of oxygen transport.
• O2ER = VO2 / DO2
= CO x C(a-v)o2
CO x Cao2
= SaO2 - SvO2 / SaO2
• Normal - 0.25 (range = 0.2–0.3)
Oxygen-Extraction Ratio (O2ER)

Sources of oxygen
• Atmosphere
• Compressed Oxygen cylinders : gaseous, liquid
(cryogenic container)
• Oxygen concentrators

Oxygen cylinders
• Medical gas distribution systems
• Central supply(G and H type cylinders)
• Patients transport, resuscitation, anesthesia machine (
E type)

Oxygen Source
Central supply system :
• Located outdoors,
restricted area
• Cylinder banks
connected to a common
manifold
• Primary and secondary
supply
• Reserve supply away
from main supply

Oxygen concentrators
• Based on Pressure swing
adsorber technology (adsorbs
nitrogen onto the molecular seive
made of zeolite)
• Deliver 90-96% oxygen
• Remote location , Domiciliary
use

Advantages:
• Cost effective, simple to use, portable
• Not affected by altitude changes
• Compatible with anaesthesia machine, vaporisers,
ventilators
Disadvantages:
• Maintanence: regular servicing of compressor,
filters
• Device malfuntion : excessive noise, overheating,
shut down
• Argon accumulation

O2 Delivery System
• Classification
DESIGNS
 Low- flow system
 Reservoir systems
 High flow system
 Enclosures
PERFORMANCES (Based on predictability and
consistency of FiO2 provided)
 Fixed
 Variable

Oxygen flow rate and concentration
Respiratory Distress Non respiratory Distress
Minute vol
(RR x TV)
20 l/min
(40bpm x 500ml)
5 l/min
(10bpm x 500ml)
O2 flow rate 2 l/min 2 l/min
Oxygen concentration 2 l/min of 100% oxygen
+
18 l/min air drawn into
mask (21%)
=
20 l/min minute volume
FiO2 =
(1.0x2) + (0.21x18) / 20
= 0.38 (38.8%)
2 l/min of 100% oxygen
+
3 l/min air drawn into mask
(21%)
=
5 l/min
FiO2 =
(1.0x2) + (0.21x3) / 5
= 0.53 (53%)

Low flow system
• The gas flow is insufficient to meet patient’s peak
inspiratory and minute ventilatory requirement
• O2 provided is always diluted with air
• FiO2 varies with the patient’s ventilatory pattern
• Deliver low and variable FiO2 → Variable performance
device

High flow system
• The gas flow is sufficient to meet patient’s peak
inspiratory and minute ventilatory requirement.
• FiO2 is independent of the the patient’s ventilatory
pattern
• Deliver low- moderate and fixed FiO2 → Fixed
performance device

Low Flow or Variable performance equipment
• Nasal cannula
• Nasal mask
• Simple Oxygen mask
• Oxygen tent
• Mask with gas reserviour
I. Partial rebreathing
II. Non-rebreathing
High flow or fixed performance equipment
• Anaesthesia bag (Bag mask valve system)
• Venturi mask (with or without nebulliser)

Nasal cannula
● A plastic disposable device consisting of two tips or
prongs 1 cm long, connected to oxygen tubing
● Inserted into the vestibule of the nose
● FiO2 – 24-40%
● Flow – 1-6 L/min

Nasal Cannula
Advantages
• Easy to use, well tolerated by most patients
• Patients can eat, drink and talk.
• May be used in patients with COPD requiring long term
oxygen therapy.
Disadvantages
• May cause pressure sores around ears and nose.
• May dry and irritate nasal mucosa.
• Not able to achieve high concentrations of inhaled O2 in
patients who have a high minute ventilation.
• Most of the oxygen wasted.
• May not be good for mouth breather

Reservoir systems
● Reservoir system stores a reserve volume of O2, that equals or
exceeds the patient’s tidal volume
● Delivers mod- high FiO2
● Variable performance device
● To provide a fixed FiO2, the reservoir volume must exceed the
patient’s tidal volume
Examples:
● Reservoir cannula
● Simple face mask
● Partial rebreathing mask
● Non rebreathing mask
● Tracheostomy mask

Reservoir masks
● Commonly used reservoir system
● Three types
● Simple face mask
● Partial rebreathing masks
● Non rebreathing masks

Simple face masks
● Reservoir - 100-200 ml
● Variable performance device
● FiO2 varies with
● O2 input flow,
● mask volume,
● extent of air leakage
● patient’s breathing pattern
● FiO2: 40 – 60%
● Input flow range is 5-8 L/min
● Minimum flow – 5L/min to
prevent CO2 rebreathing

Simple Face Mask
Merits
● Moderate but variable FiO2.
● Good for patients with blocked nasal passages and mouth breathers
● Easy to apply
Demerits
● Uncomfortable
● Interfere with further airway care
● Proper fitting is required
● Rebreathing (if input flow is less than 5 L/min)
Oxygen Flow
Rate(L/Min)
FiO2
5-6 0.4
6-7 0.5
7-8 0.6

Partial rebreathing face mask
• During inspiration: Draws air
from the mask, from the bag,
holes in the side of the mask.
• During expiration: First one
third of exhaled gases (from
anatomical dead space) will flow
back into the reservoir bag
• High enough O2 flow to keep the
bag from deflating more than one
third its volume during
inspiration, then exhaled CO2 will
not accumulate in the reservoir
bag.
• Maximum FiO2 of 0.7 to 0.8
• FGF > 8L/min

Nonrebreathing Face Mask
● Has 2 unidirectional valves
● Expiratory valves prevents
air entrainment
● Inspiratory valve prevents
exhaled gas flow into
reservoir bag
● FiO2 : 0.80 – 0.90
● FGF : 10 – 15L/min
● To deliver ~100% O2, bag
should remain inflated
● Factors affecting FiO2
● Air leakage and
● Pt’s breathing pattern

Tracheostomy Collar/ Mask
• Inserted directed into
trachea
• Is indicated for chronic o2
therapy
• O2 flow rate 8 to 10L
• Provides good humidity
• Comfortable ,more efficient
• Less expensive

High flow systems
The gas flow is sufficient to meet patient’s peak inspiratory and
minute ventilatory requirement.
FiO2 is independent of the the patient’s ventilatory pattern
Deliver low- moderate and fixed FiO2 → Fixed performance
device
Examples:
Air entrainment devices

Principle:
Based on Bernoulli principle –
A rapid velocity of gas exiting from a restricted orifice will
create subatmospheric lateral pressures, resulting in
atmospheric air being entrained into the mainstream.
Entrainment Ratio:
Air = 100 - %O2
O2 %O2 - 21

fi02 Flow Rate
(L/min)
Air
:02entrapme
nt ratio
Total Gas
flow(L/min)
24% 4 25:1 104
28% 6 10:1 66
35% 8 5:1 48
40% 10 3:1 32
60% 15 1:1 Twice O2 flow

COMPLICATIONS OF OXYGEN THERAPY

Complications of Oxygen therapy
1. Oxygen toxicity
2. Depression of ventilation
3. Retinopathy of Prematurity
4. Absorption atelectasis
5. Fire hazard

Oxygen Toxicity
• Oxygen toxicity is a condition resulting
from the harmful effects of breathing
molecular oxygen (O2) at
increased partial pressures.
• The metabolites of oxygen are more
damaging than the parent molecule
itself.
• These are formed in the process of
conversion of O2 to water in the ETC
(electron transport chain) in
mitochondria.
• The superoxide and hydroxyl radicals
are free radicals → very active
chemically

CNS effects
● Also known as Paul Bert effect
● Central nervous system toxicity - caused by short exposure to
high partial pressures of oxygen at greater than atmospheric
pressure(> 1 atm pressure)
● Symptoms may include disorientation, brief periods of
rigidity→ convulsions and unconsciousness, and seizures.
● Visual changes esp. tunnel vision, tinnitus, etc. are also
common
Oxygen toxicity,JIACM 2003; 4(3): 234

Pulmonary effects
• Also known as
Lorraine Smith’s effect
• First organ to be
exposed to high
concentration of O2
• Pulmonary toxicity
occurs only with
exposure to partial
pressures of oxygen
greater than 0.5 bar
(50 kPa)
ARDS like features

• Tracheobronchitis, or inflammation of the upper airways most
common findings,
• Others: cough, substernal chest pain, ARDS like features
• Preterm newborns are known to be at higher risk
for bronchopulmonary dysplasia with extended exposure to
high concentrations

How much O2 is safe?
100% - not more than 12hrs
Goal should be to use lowest possible FiO2
compatible with adequate tissue oxygenation

Hypoventilation
• COPD patients with chronic CO2 retention.
• Altered respiratory drive dependent on relative hypoxemia.
• Correction of hypoxemia leads to loss of respiratory drive.
• Increase oxygen concentration by 4-7% (i.e. Fio2 0.32-0.36)
with a goal of achieving Spo2 by 90-93%.
• Venturi mask when possible

Retinopathy of prematurity (ROP)
Premature or low-birth-weight infants
who receive supplemental O2
↑PaO2
↓
retinal vasoconstriction
↓
necrosis of blood vessels
↓
new vessels formation
↓
Hemorrhage → retinal
detachment and blindness
To minimize the risk of ROP - PaO2
below 80 mmHg

Absorption atelectasis
• Large volume of nitrogen
in the lungs is replaced
with oxygen,
• Oxygen is subsequently
absorbed into the blood,
• The effect is reducing the
volume of the alveoli,
resulting in a form of
alveolar collapse known
as absorption atelectasis

Risks of fire
• Improper use of oxygen
• Incorrect design of oxygen systems
• Incorrect operation and maintenance of oxygen
system

Summary
• Oxygen therapy is administration of oxygen in concentration
greater than that of environment (>20.9%).
• Oxygen can be obtained from different sources including
environment employing several industrial/non-industrial
techniques.
• It is very important to be aware the varying FiO2 with variable
performance devices and it is important to choose devices
appropriate for the patient.
• Oxygen in concentrations higher than required to maintain
normal PaO2 is associated with toxicity.
• The potential hazards of oxygen therapy are: absorpotion
atelectasis, CNS/Respiratory problems, depression of
ventilation, ROP, and fire hazards.
• Goal should be to use lowest possible FiO2 compatible with
adequate tissue oxygenation

Bibliography
• Marinos, The ICU Book, 4th Edition
• Barash, Paul G Cullen’s Clinical Anesthesia, 7th edition
• Mikhail & Morgan’s Clinical Anesthesiology, 5th edition
• Carvalho M, Soares M, Machado HS. Paradigms of Oxygen
Therapy in Critically Ill patients. J Intensive & Crit Care
2017,3:1
• Internet : Wikipedia.org , Uptodate
• BTS Guidelines for oxygen use in adults in healthcare and
emergency settings -2017

Oxygen therapy and toxicity

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