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OXYGEN THERAPYDR. RICHA JAIN

University College of Medical Sciences & GTB Hospital, Delhi

OVERVIEW Introduction Oxygen transport Indications Oxygen delivery systems Hyperbaric oxygen therapy Complications of oxygen therapy

OXYGEN THERAPY ….. WHAT? Administration of O2 in concentration

more than in ambient air

↑Partial Pr of O2 in insp. Gas (Pi o2)

↑Partial Pr of O2 in alveoli (PAo2)

↑Partial Pr of O2 in arterial blood (Pao2)

Why is O2 required for survival? O2 is required for the aerobic metabolism

Oxidative phosphorylation in mitochondria Glucose + 6O2 → 6H2O + 6CO2 + 36ATP

Lack of O2 causes Anaerobic metabolism in cytoplasm Glucose → lactic acid + 2ATP ↓ H+ + lactate-

“lack of O2 not only stops the machinery, but also totally ruins the

supposed machinery”

J.S.Haldane

What is the Oxygen Cascade?

The process of declining oxygen tension from atmosphere to mitochondria

Atmosphere air (dry) (159 mm Hg)

↓ humidification

Lower resp tract (moist) (150 mm Hg)

↓ O2 consumption and alveolar ventilation

Alveoli PAO2 (104 mm Hg)

↓ venous admixture

Arterial blood PaO2 (100 mm Hg)

↓ tissue extraction

Venous blood PV O2 (40 mm Hg)

↓

Mitochondria PO2 (7 – 37 mmHg)

O2 Cascade

Venous admixture

PA O2 = 104 mm HgAlveolar air

Arterial

bloodPa O2 = 100 mm Hg

A – a = 4 – 25 mmHg

PI O2

PV O2

Venous admixture(physiological shunt)

O2 Cascade

Low VA/Q Normal True shunt(normal anatomical

shunt)

Pulmonary(Bronchial veins)

Extra Pulm.(Thebesian veins)

Normal = upto 5 % of cardiac output

O2 Cascade

Utilization by tissue

Arterial

blood

Pa O2 = 100 mm Hg(Sat. > 95 %)

Mixed Venous blood

PV O2 = 40mm HgSat. 75%

Cell Mitochondria PO2 (7 – 37

mmHg)

O2 Cascade

Utilization by tissue

Arterial

blood

Pa O2 = 97mm Hg(Sat. > 95 %)

Mixed Venous blood

PV O2 = 40mm HgSat. 75%

Cell Mitochondria PO2 ( 7 – 37

mmHg)

PerfusionO2 content (Hb Conc.)

What is Pasteur point ?

The critical level of PO2 below which aerobic metabolism fails.

(1 – 2 mmHg PO2 in mitochondria)

Oxygen contentOxygen fluxOxygen uptakeO2 extraction ratio

O2 TRANSPORT

Oxygen Content (Co2)Amount of O2 carried by 100 ml of blood

Co2 =Dissolved O2 + O2 Bound to hemoglobinCo2 = 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 hemoglobinPo2 = arterial partial pressure of oxygen (mmHg)0.0031 = solubility coefficient of oxygen in plasma

O2Hb dissociation curve

0 20 40 60 80 100 120 140 1600

20

40

60

80

100

% H

b S

at w

ith O

2

PO2 mmHg

Oxygen FluxAmount of of O2 leaving left ventricle per minute.

= CO × Hb sat x Hb conc x 1.34 100 100 = 5000 x 97 x 15.4 x 1.34 100 100 = 1000 ml/min

CO = cardiac output in ml per minute. Do2 = oxygen flux

Oxygen Uptake (VO2) The Vo2 describes the volume of oxygen

(in mL) that leaves the capillary blood and moves into the tissues each minute.

VO2 = CO x C(a-v)o2 x 10 normal VO2 = 200–300 mL/min or 110–

160 mL/min/m2

Oxygen-Extraction Ratio (O2ER)

The fraction of the oxygen delivered to the capillaries and then to tissues.

An index of the efficiency of oxygen transport.

O2ER = VO2 / DO2 = CO x C(a-v)o2 x 10 CO x Cao2 x 10 = SaO2 - SvO2 / SaO2 Normal - 0.25 (range = 0.2–0.3)

Which patient is better placed – ?

A B Hb 14gm (normal) 7gm

(Anaemic)

C.O. 5 L (normal) 4 L (Low)

SPO2 40 % 90 % PaO2 23 mm Hg 60

mmHg

O2 Flux 375ml 350ml

PO2 O2 content Per 100 ml 97mm Art. blood 14g x 1.39 x 100%=20ml40mm Ven. blood 14g x 1.39 x 75% = 15ml

Tissue extraction 25% = 5ml

97mm Art. blood 7g x 1.39 x 100% = 10 ml 27 mm Ven. Blood 7g x 1.39 x 50% = 5ml

Tissue extraction 50% = 5ml

Goal of oxygen therapy To maintain adequate tissue oxygenation

while minimizing cardiopulmonary work

O2 Therapy : CLINICAL OBJECTIVES

1. Correct documented or suspected hypoxemia

2. Decrease the symptoms associated with chronic hypoxemia

3. Decrease the workload hypoxemia imposes on the cardiopulmonary system

O2 Therapy : Indications Documented hypoxemia as evidenced by

PaO2 < 60 mmHg or SaO2 < 90% on room air

PaO2 or SaO2 below desirable range for a specific clinical situation

Acute care situations in which hypoxemia is suspected

Severe trauma Acute myocardial infarction Short term therapy (Post anaesthesia

recovery) Respir Care 2002;47:707-720

ASSESSMENT The need for oxygen therapy

should be assessed by 1. monitoring of ABG -

PaO2, SpO2 2. clinical assessment

findings.

PaO2 as an indicator for Oxygen therapy

PaO2 : 80 – 100 mm Hg : Normal 60 – 80 mm Hg : cold,

clammy extremities < 60 mm Hg : cyanosis < 40 mm Hg : mental

deficiency memory

loss < 30 mm Hg :

bradycardia cardiac

arrest

PaO2 < 60 mm Hg is a strong indicator for oxygen therapy

Clinical assessment of hypoxia

mild to moderate severeCNS : restlessness somnolence, confusion disorientation impaired judgement lassitude loss of coordination headache obtunded mental statusCardiac : tachycardia bradycardia, arrhythmia mild hypertension hypotension peripheral vasoconst.Respiratory: dyspnea increasing dyspnoea, tachypnea tachypnoea, possible shallow & bradypnoea laboured breathing Skin : paleness, cold, clammy cyanosis

MONITORING Physical examination for C/F of

hypoxemia Pulse oximetry ABG analysis

pH pO2 pCO2

Mixed venous blood oxygenation

O2 Delivery systems

CLASSIFICATIONDESIGNS Low- flow system Reservoir systems High flow system Enclosures

PERFORMANCES (Based on predictability and consistency of FiO2 provided) Fixed Variable

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

Reservoir System 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

How to judge the performance of an oxygen delivery system?

How much oxygen (FiO2) the system delivers?

Does the FiO2 remain fixed or varies under changing patient’s condition?

Low flow systems are Variable performance

High flow system are Fixed performance

Reservoir systems are Variable performance device

O2 Delivery deviceso Low flow (Variable performance devices )

Nasal cannula Nasal catheter Transtracheal catheter

o Reservoir system (Variable performance device) Reservoir cannula Simple face mask Partial rebreathing mask Non rebreathing mask Tracheostomy mask

o High flow (Fixed performance devices) Ventimask (HAFOE) Aerosol mask and T-piece with nebulisers

Low-Flow Devices

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 – ¼ - 8L/min (adult) < 2 L/min(child)

Nasal Cannula

Easy to fix Keeps hands free Not much

interference with further airway care

Low cost Compliant

Unstable Easily dislodged High flow

uncomfortable Nasal trauma Mucosal irritation FiO2 can be inaccurate

and inconsistent

Merits Demerits

Estimation of FiO2 provided by nasal cannula

O2 Flowrate (L/min

Fi O2

1 0.242 0.283 0.324 0.365 0.406 0.44

Patient of normal ventilatory pattern - each litre/min of nasal O2 increases the

FiO2 approximately 4%.E.g. A patient using nasal cannula at 4 L/min,

has an estimated FiO2 of 37% (21 + 16)

Nasal catheter

Nasal catheter

Good stability Disposable Low cost

Difficult to insert High flow increases back

pressure Needs regular changing May provoke gagging, air

swallowing, aspiration Nasal polyps, deviated

septum may block insertion

Merits Demerits

Transtracheal catheter A thin

polytetrafluoroethylene (Teflon) catheter

Inserted surgically with a guidewire between 2nd and 3rd tracheal rings

FiO2 – 22-35% Flow – ¼ - 4L/min Increased anatomic

reservoir

Transtracheal catheter

Lower O2 use and cost

Eliminates nasal and skin irritation

Better compliance Increased

exercise tolerance Increased

mobility

High cost Surgical

complications Infection Mucus plugging Lost tract

Merits Demerits

Estimation of Fio2 from a low-flow system for patient with normal

ventilatory patternCannula 6 L/min VT, 500 mLMechanical reservoir None Rate, 20 breaths per

minAnatomic reservoir 50 mL I/E ratio, 1:2100% O2 provided/sec 100 mL Inspiratory time, 1 secVolume inspired O2    expiratory time, 2 sec Anatomic reservoir 50 mL   Flow/sec 100 mL   Inspired room air 0.2 × 350 mL = 70 mL  O2 inspired 220 mL   FiO2 220 O2 = 0.44

500 TV 

A patient with ideal ventilatory pattern who receives 6L/min O2 by nasal cannula is receiving  FiO2 of 0.44.

Estimation of Fio2 from a low-flow systemIf VT is decreased to 250 mL:  Volume inspired O2  Anatomic reservoir 50 mLFlow/sec 100 mLInspired room air (0.20 × 100 cm3) 0.2 × 100 mL = 20 mLO2 inspired 170 mL FiO2 170 = 0.68

250

The larger the Vt or faster the respiratory rate, the lower the Fio2.The smaller the Vt or lower the respiratory rate, the higher the Fio2.

↑minute ventilation → ↓ Fio2

↓minute ventilation → ↑Fio2

Reservoir systems

Reservoir cannula

NASAL RESERVOIR PENDANT RESERVOIR

Reservoir cannulaMerits

Lower O2 use and cost

Increased mobility Less discomfort

because of lower flow

Demerits Unattractive Cumbersome Poor compliance Must be regularly

replaced (3 weekly) Breathing pattern

affects performance (must exhale through nose to reopen reservoir membrane)

RESERVOIR MASKS Commonly used reservoir system Three types1. Simple face mask2. Partial rebreathing masks3. Non rebreathing masks

Simple face mask 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

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 Risk of aspiration in unconscious pt Rebreathing (if input flow is less

than 5 L/min)

O2 Flowrate (L/min)

Fi O2

5-6 0.4

6-7 0.5

7-8 0.6

Reservoir masks

Partial rebreathing mask Nonrebreathing mask

Partial rebreathing mask No valves Mechanics – Exp: O2 + first 1/3 of

exhaled gas (anatomic dead space) enters the bag and last 2/3 of exhalation escapes out through ports

Insp: the first exhaled gas and O2 are inhaled

FiO2 - 60-80% FGF > 8L/min The bag should remain

inflated to ensure the highest FiO2 and to prevent CO2 rebreathing

Exhalationports

O2

Reservoir

+

Non-rebreathing mask Has 3 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

O2

Reservoir

One-way valves

Tracheostomy Mask

Used primarily to deliver humidity to patients with artificial airways.

Variable performance device

Air entrainment devicesBlending systems

High-Flow systems

Air entrainment devices 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.

Principle of Air entrainment devices Principle of constant-pressure jet

mixing – a rapid velocity of gas through a restricted orifice creates “viscous shearing forces” that entrain air into the mainstream.

(Egan’s fundamentals of respiratory care;

Shapiro’s Clinical application of blood gases)

Mechanism of Air entrainment devices

oxygen

room air

exhaled gas

Characteristics of Air entrainment devices

Amount of air entrained varies directly with size of the port and the velocity of O2 at jet

They dilute O2 source with air - FiO2 < 100%

The more air they entrain, the higher is the total output flow but the lower is the delivered FiO2

Principles of gas mixing All High flow systems mix air and O2 to achieve a given FiO2 An air entrainment device or blending system is used

VFCF = V1C1 + V2C2 V1 and V2- volumes of 2 gases mixedC1 and C2- oxygen conc in these 2 volumesVF - the final volume CF - conc of resulting mixture

% O2 = ( air flow x 21) + (O2 flow x 100) total flow

Air-to O2 entrainment ratio:Air = 100 - %O2

O2 % O2 - 21

Calculation of Air to O2 Entrainment Ratio using a magic box

20

100

60

20

40 60 = 3 : 120

Approximate Air Entrainment Ratio and Gas Flows for different Fio2

Fio 2 (%) Ratio

Recommended O2 Flow (L/min)

Total Gas Flow (to Port)

(L/min)24 25.3:1 3 7926 14.8:1 3 4728 10.3:1 6 6830 7.8:1 6 5335 4.6:1 9 5040 3.2:1 12 5050 1.7:1 15 41

2 most common air-entrainment systems are

1. Air-Entrainment mask (venti-mask)

2. Air-Entrainment nebulizer

Venturi / Venti / HAFOE Mask

Mask consists of a jet orifice around which is an air entrainment port.

FiO2 regulated by size of jet orifice and air entrainment port

FiO2 – Low to moderate (0.24 – 0.60)

HIGH FLOW FIXED PERFORMANCE DEVICE

Varieties of Venti Masks

 A fixed Fio2 model  A variable Fio2 model

Air entrainment nebulizer Have a fixed orifice, thus, air-to-O2 ratio

can be altered by varying entrainment port size.

Fixed performance device Deliver FiO2 from 28-100% Max. gas flows – 14-16L/min Device of choice for delivering O2 to

patients with artificial tracheal airways. Provides humidity and temperature

control

Air entrainment nebulizer

Aerosol mask

Face tent Tracheostomy collar

T tube

How to increase the FiO2 capabilities of air-entrainment nebulizers? 1. Adding open reservoir (50-150ml aerosol tube)2. Provide inspiratory reservoir (a 3-5 L

anaesthesia bag) with a one way expiratory valve

3. Connect two or more nebulizers in parallel 4. Set nebulizer to low conc (to generate high

flow) and providing supplemental O2 into delivery tube

Blending systems With a blending system,

separate pressurized air and oxygen sources are input.

The gases are mixed either manually or with a blender

FiO2 – 24 – 100% Provide flow > 60L/min Allows precise control over

both FiO2 and total flow output - True fixed performance devices

OXYGEN BLENDER

Oxygen tent Hood Incubator

ENCLOSURES

OXYGEN TENT Consists of a canopy placed

over the head and shoulders or over the entire body of a patient 

FiO2 – 40-50% @12-15L/minO2 Variable performance device Provides concurrent aerosol

therapy Disadvantage

Expensive Cumbersome Difficult to clean Constant leakage Limits patient mobility

OXYGEN HOOD An oxygen hood covers

only the head of the infant

O2 is delivered to hood through either a heated entrainment nebulizer or a blending system

Fixed performance device

Fio2 – 21-100% Minimum Flow > 7/min to

prevent CO2 accumulation

INCUBATOR Incubators are

polymethyl methacrylate enclosures that combine servo-controlled convection heating with supplemental O2

Provides temperature control

FiO2 – 40-50% @ flow of 8-15 L/min

Variable performance device

Hyperbaric O2 Therapy (HBOT)

DEFINITION

A mode of medical treatment wherein

the patient breathes 100% oxygen at a pressure greater than one Atmosphere Absolute (1 ATA)

1 ATA is equal to 760 mm Hg at sea level

Basis of Hyperbaric O2 TherapyDissolved O2 in plasma :0.003ml / 100ml of blood / mm PO2

(Henry’s Law -The concentration of any gas in solution is

proportional to its partial pressure.) Breathing Air (PaO2 100mm Hg)0.3ml / 100ml of bloodBreathing 100% O2 (PaO2 600mm Hg)1.8ml / 100ml of bloodBreathing 100% O2 at 3 AT.A (PaO2 2000 mm Hg)6.0ml / 100ml of blood

The basis is to increase the concentration of dissolved oxygen

Physiological effects of HBO Bubble reduction ( boyle’s law) Hyperoxia of blood Enhanced host immune function Neovascularization Vasoconstriction

INDICATIONS OF HBOT

Decompression sickness Air embolism Carbon monoxide

poisoning Severe crush injuries Thermal burns Acute arterial insufficiency Clostridial gangrene Necrotizing soft-tissue

infection Ischemic skin graft or flap

Radiation necrosis Diabetic wounds of

lower limbs Refratory

osteomyelitis Actinomycosis

(chronic systemic abscesses)

ACUTE CONDITIONS CHRONIC CONDITIONS

METHODS OF ADMINISTRATION of HBOT

Problems with HBOT Barotrauma

Ear/ sinus trauma Tympanic membrane rupture Pneumothorax

Oxygen toxicity Fire hazards Clautrophobia Sudden decompression

Complications of Oxygen therapy

Complications of Oxygen therapy1. Oxygen toxicity2. Depression of ventilation3. Retinopathy of Prematurity4. Absorption atelectasis5. Fire hazard

1. O2 Toxicity Primarily affects lung and CNS. 2 factors: PaO2 & exposure time CNS O2 toxicity (Paul Bert effect)

occurs on breathing O2 at pressure > 1 atm

tremors, twitching, convulsions

Pulmonary Oxygen toxicityC/F acute tracheobronchitis Cough and substernal pain ARDS like state

Pulmonary O2 Toxicity (Lorrain-Smith effect)

Mechanism: High pO2 for a prolonged period of time

↓ intracellular generation of free radicals e.g.:

superoxide,H2O2 , singlet oxygen ↓ react with cellular DNA, sulphydryl proteins

&lipids ↓

cytotoxicity

↓ damages capillary endothelium, ↓

Interstitial edema Thickened alveolar capillary membrane.

↓ Pulmonary fibrosis and

hypertension

A Vicious Cycle

How much O2 is safe? 100% - not more than 12hrs

80% - not more than 24hrs 60% - not more than 36hrs

Goal should be to use lowest possible FiO2 compatible with adequate tissue oxygenation

Indications for 70% - 100% oxygen therapy

1. Resuscitation2. Periods of acute cardiopulmonary

instability3. Patient transport

2. Depression of Ventilation Seen in COPD patients with chronic hypercapnia Mechanism ↑PaO2 suppresses peripheral V/Q mismatch chemoreceptors depresses ventilatory drive ↑ dead space/tidal

volume ratio ↑PaCO2

3. Retinopathy of prematurity (ROP) Premature or low-birth-weight infants who

receive supplemental O2 Mechanism

↑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

4. Absorption atelectasis100% O2

oxygennitrogen

PO2 =673PCO2 = 40PH2O = 47

A B

A – UNDERVENTILATEDB – NORMAL VENTILATED

Denitrogenation Absorption atelectasis

The “denitrogenation” absorption atelectasis is because of collapse of underventilated alveoli (which depends on nitrogen volume to remain above critical volume )

↓ Increased physiological shunt

5. Fire hazard High FiO2 increases the risk of fire Preventive measures

Lowest effective FiO2 should be used Use of scavenging systems Avoid use of outdated equipment such as

aluminium gas regulators Fire prevention protocols should be

followed for hyperbaric O2 therapy

Oxygen challenge concept ↑ FiO2 by 0.2

↑ PaO2 > 10 mmHg ↑ PaO2 < 10 mmHg ( true shunt – 15 %) ( true shunt – 30

%)

↑ PaO2 < 10 mmHg in response to an oxygen challenge of 0.2 – refractory hypoxemia

Implications of Oxygen challenge concept

To identify refractory hpoxemia (as it does not respond to increased FiO2)

Refractory hpoxemia depends on increased cardiac output to maintain acceptable FiO2

Potentially deleterious effect of increased FiO2 can be avoided

SUMMARY Therapeutic effectiveness of oxygen

therapy is limited to 25% - 50%• Low V/Q hypoxemia is reversed with less than

50%• DAA occurs with FiO2 more than 50%• Pulmonary oxygen toxicity is a potential risk

factor with FiO2 more than 50%

Bronchodilators, bronchial hygiene therapy and diuretic therapy decreases the need for high FiO2

Oxygen is a drug. When appropriately used, it is extremely

beneficialWhen misused or abused, it is potentially

harmful

References Medical gas therapy. Egan’s Fundamentals of

respiratory care. 9th ed. Oxygen delivery systems, inhalation therapy

and respiratory therapy. Benumof’s Airway management. 2nd ed.

Shapiro BA. Hypoxemia and oxygen therapy. Clinical application of blood gases. 5TH ed.

Oxygen and associated gases. Wiley 5th ed. Miller’s Anaesthesia 7th ed. Paul L. Marino. The ICU Book. 3rd ed.

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