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 Table of Contents  
CLINICAL UPDATE
Year : 2020  |  Volume : 7  |  Issue : 6  |  Page : 352-363

Understanding facts about oxygen therapy


1 Department of Paediatrics, Pt. B D Sharma, PGIMS, Rohtak, Haryana, India
2 Department of Medicine, Pt. B D Sharma, PGIMS, Rohtak, Haryana, India

Date of Submission18-Sep-2020
Date of Decision25-Sep-2020
Date of Acceptance01-Oct-2020
Date of Web Publication11-Nov-2020

Correspondence Address:
Dr. Kundan Mittal
Department of Paediatrics, Pt. B D Sharma, PGIMS, Rohtak, Haryana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jpcc.jpcc_148_20

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  Abstract 


Oxygen supplement is administration of oxygen above normal atmospheric concentration or pressure in both acute and chronic conditions to prevent hypoxia or to treat certain conditions requiring hyperbaric oxygen. Delivery of oxygen depend on various factors. Oxygen should be used judiciously as its also associated with adverse effects. No concentration is safe in human.

Keywords: Oxygen, oxygen delivery device, respiration


How to cite this article:
Mittal K, Aggarwal H K. Understanding facts about oxygen therapy. J Pediatr Crit Care 2020;7:352-63

How to cite this URL:
Mittal K, Aggarwal H K. Understanding facts about oxygen therapy. J Pediatr Crit Care [serial online] 2020 [cited 2020 Nov 30];7:352-63. Available from: http://www.jpcc.org.in/text.asp?2020/7/6/352/300582




  Introduction Top


Oxygen, essential for aerobic respiration, is commonly used therapeutic agent in acutely ill and essential for life but misunderstood and used inappropriately in clinical situations. Oxygen should be used with well-recognized pharmacological principles. Oxygen is transported from air to each cell in the human body (by convection and diffusion methods) for cellular function.

Oxygen is a colorless, odorless, noninflammable, tasteless gas and slightly heavier than air but lighter than carbon dioxide, constituting approximately 20.94% of atmospheric air by volume and 23.2% by weight by earth atmosphere, and transferred from environment to mitochondria from higher pressure of 21.2, 19.9, and 13.4 kPa (concentration) to lower pressure of 1.5 kPa (concentration) of oxygen [Figure 1]. The difference between PAO2 of 104 mmHg and PVO2 of 64 mmHg causes oxygen to diffuse into the pulmonary blood. Diffusion of oxygen into the cell is limited by the distance between the cell itself and the source of oxygen. A highly complex capillary network (microcirculation) exists to distribute the oxygen to cells and tissues. Nitrogen in air stabilizes the alveoli. Oxygen should be warmed (31°–34°) and humidified (30%–40%) before delivery.
Figure 1: Oxygen cascade

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Objectives of oxygen therapy are to correct, hypoxemia, hypoxia, decrease work of breathing, and cardiopulmonary workload. It is essential to have adequate cardiac output (one of the determinants of oxygen delivery [DO2]) for proper delivery of oxygen at tissue level. DO2 is product of cardiac output and arterial hemoglobin content (DO2= CaO2[SaO2× Hb × 1.39 + PaO2× 0.003] CO). Thus, it is essential to have adequate cardiac output (one of the determinants of DO2) for proper delivery of oxygen at tissue level. During exercise, oxygen requirement increases 20 times from normal baseline, but still no oxygen deficiency occurs because oxygen diffusion capacity increases four-fold and also at the same time cardiac output increases during exercise. Transient time of blood may decrease in the pulmonary capillaries for RBC, which may lead to hypoxemia during exercise. Diffusion of oxygen with increased cardiac output is still possible as red cells still have sufficient time to achieve full saturation. Blood remains three times as long as blood to cause full oxygenation; thus, even during shortened time, blood can be fully oxygenated (oxygen diffuses from the alveolus to the capillary until the PCO2 is equal to that in the alveolus and this process is takes about 0.25 s. It is normally completed by the time the blood has passed about one-third of the way along the pulmonary capillary and total transit time through the capillary being 0.75 s). By 0.25 s, the red cell hemoglobin is completely saturated and the partial pressure of oxygen in the blood equilibrates with that in the alveolus and therefore diffusion stops. The difference between partial pressure of oxygen in the arterial and venous blood and venous blood and alveolus helps in diffusion of oxygen in the blood.

Normally, 5 mL of oxygen is transported to the tissue by 100 mL of blood, and during exercise, 15 mL of oxygen is transported by 100 mL of blood. By increasing, FiO2 may not help in increasing oxygen content of hemoglobin which is already fully saturated, but some amount of oxygen dissolved in the plasma will increase. Oxygen is stored as functional residual capacity (FRC) in hemoglobin and plasma for shorter period of duration [Table 1]. Providing 100% oxygen will increase the oxygen store in the FRC, and 80% of oxygen can be consumed without reduction in hemoglobin saturation, thus making preoxygenation as an effective method to prevent hypoxemia during intubation. Carbon dioxide is transported as 7% dissolved in the plasma, 70% as bicarbonate in the RBC which is converted to carbon dioxide in the pulmonary capillaries, and 23% binds to hemoglobin.[1],[2],[3],[4],[5],[6],[7],[8]
Table 1: Principles of oxygen store in adults

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Use of opioids, sedatives, ketamine, and anesthetic agents is associated with hypoventilation, respiratory depression, decrease in FRC, hypoxic pulmonary vasoconstriction, and reduction in cardiac output and DO2, thus reduction in PAO2. FRC may drop below closing volume, thus increasing the shunt, resulting in increase in venous admixture from 1% to 10%. Reduced DO2 is partially compensated by decreased metabolic rate (15%) and use of artificial ventilation (6%). This explains use of supplementary oxygen (FiO2 of 25%–30%) during surgery. Preoxygenation for 3 min before intubation, apnea, and anesthesia increases oxygen in the FRC, thus preventing hypoxemia.


  Commonly Used Terms Top


  • FiO2: Fraction of inspired oxygen (%)
  • PaCO2: The partial pressure of CO2 in the arterial blood, a measure of ventilation. Hypercapnea is defined as increased carbon dioxide in the blood
  • Minute ventilation (MV): The total amount of gas moving into and out of the lungs per minute and calculated by multiplying the tidal volume (TV) by the respiration rate (RR), measured in liters per minute
  • PaO2: The partial pressure of oxygen in the arterial blood, a measure of oxygenation. Hypoxemia is low arterial oxygen tension in the plasma, and hypoxia is low oxygen level at the level of tissues
  • SaO2: Arterial oxygen saturation measured from blood while SpO2 is arterial oxygen saturation measured by pulse oximeter
  • High flow: High-flow systems are specific devices that deliver the patient's entire ventilatory demand, meeting or exceeding the patients peak inspiratory flow rate
  • Low flow: Low-flow systems are specific devices that do not provide the patient's entire ventilatory requirements; room air is entrained with the oxygen, thus diluting the FiO2
  • Peak inspiratory flow rate: The fastest flow rate of air during inspiration measured in liters per se cond
  • Humidification is the addition of heat and moisture to a gas
  • Ventilation-perfusion (VQ) mismatch: An imbalance between alveolar ventilation and pulmonary capillary blood flow.



  Clinical Indicators of Oxygen Deficiency (Respiratory, Cardiovascular, Neurological, or General) Top


  • Anxious look, restlessness
  • Older child complains headache, confusion, blurred vision, slow reaction time, poor coordination, fatigue, light headedness, dizziness, tingling, blurred night vision, hyperreflexia, coma
  • Increased work of breathing
  • Perspiration, hyperventilation
  • Decreased oxygen saturation (SpO2<92%)
  • Tachycardia, pallor, peripheral vasoconstriction, cyanosis
  • Arrhythmias, mild hypertension, and later hypotension
  • Peripheral vasoconstriction
  • Hypotonia (decrease muscle tone)
  • Lactic acidosis and sodium and water retention.



  Etiology of Hypoxia/hypoxemia Top


  • Decrease in oxygen content (decrease hemoglobin level, SaO2, PaO2)
  • Abnormal affinity of oxygen to hemoglobin (abnormal hemoglobin)
  • Decreased cardiac output
  • Inability of lung to oxygenate (gas exchange)
  • Low atmospheric pressure (high altitude)
  • Breathing in closed chamber with no fresh air: ambient hypoxia
  • Closed room fire
  • Rebreathing in face mask where oxygen flow is kept <5 L/min
  • V/Q mismatch (imbalance between alveolar ventilation and pulmonary capillary blood flow): Arterial hypoxemia due to diffusion defects responds to supplementary oxygen therapy
  • Intrapulmonary or cardiac shunts: True shunt does not respond to supplementary oxygen, while intrapulmonary shunt may respond partially to supplementation of oxygen therapy
  • Local tissue edema or ischemia.



  Classification of Hypoxemia Top


  • Acute: Rapid onset (<6 h)
  • Subacute: 6 h to 7 days
  • Sustained: 7–90 days
  • Chronic: >90 days
  • Mild
  • Moderate
  • Severe
  • Severity of Hypoxia is shown in [Table 2]
  • Generational: Cross-generational
  • Hyperoxia: PaO2≥120–150 mmHg.
Table 2: Severity of hypoxemia

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  Types of Hypoxia Top


  • Tissue hypoxia: Inadequate tissue oxygenation
  • Cerebral hypoxia: Inadequate oxygen supply to brain
  • Ambient hypoxia: Reduced inspired oxygen
  • Altitude hypoxia: Due to reduced barometric pressure at altitude
  • Alveolar hypoxia: Decrease alveolar oxygen level
  • Anemic hypoxia: Inadequate/altered hemoglobin content
  • Ischemic hypoxia: Tissue oligemia
  • Circulatory hypoxia: Low cardiac output, hypotension
  • Stagnant hypoxia: Inadequate local, regional, systemic perfusion
  • Histotoxic hypoxia: Inability of tissues to utilize oxygen
  • Oxygen affinity hypoxia: Hemoglobin exists in two forms: Taut (T) type which is presently mainly in the tissues which has less affinity to oxygen and relaxed type (R) which is mainly present in the alveolar area having increased affinity to oxygen. Combination of Bohr and Haldane effects promotes oxygen binding and carbon dioxide release in the pulmonary capillaries. Decrease affinity of haemoglobin releases oxygen (increase PCO2, decrease pH, increase temperature, increase 2, 3-DPG, haemoglobin S, exercise and increase oxygen affinity results decrease in oxygen release (Foetal haemoglobin, Met-haemoglobin, carbon monoxide poisoning, decrease 2,3-DPG & temperature, increase pH and PCO2.



  Mechanisms of Hypoxemia Top


  • V/Q mismatch
  • Shunt
  • Hypoventilation
  • Diffuse limitation
  • Diminished inspire PAO2
  • Tissue hypoxia without hypoxemia
  • Cytotoxic effect.


Assessment of oxygenation using various variables

Hypoxemia is considered when PaO2 is <80 mmHg, and the indication for supplementary oxygen arises if PaO2<60 mmHg. DO2 to the tissues primarily depends on hemoglobin content and its saturation, rate of blood circulation, and efficiency of unloading of oxygen from hemoglobin. Patient who is anaemic, hypovolemic, have abnormal haemoglobin with increased affinity and low cardiac output with normal PaO2, the oxygen delivery may be inadequate. Hypoxia is not only determined by PaO2 or SPO2/SaO2 but also on hemoglobin, oxygen extraction, and metabolic demand of the body.

  • Look clinical features of oxygen and parameters of oxygen insufficiency
  • Various deranged physiological parameters are decreased PaO2, SaO2, SpO2, abnormal ABG (metabolic acidosis, respiratory alkalosis, and respiratory acidosis), and increased lactate level. The SPO2 will be 90%–95% when PaO2 is 60–80 mmHg in the patient with normal pH PCO2, temperature, and DPG
  • PaO2/FiO2: 389–500 in room air (anything below 250 is abnormal and points toward shunting and lung injury)
  • SpO2/FiO2
  • PAO2−PaO2: 5–15 mmHg in room air (abnormal value indicates alveolar gas diffusion)
  • PaO2/PAO2: 0.77–0.82 in room air
  • Oxygen index = Mean arterial pressure × FiO2/PaO2
  • SVO2
  • DO2/VO2


Choice of DO2 system is based upon:

  • Degree of hypoxemia
  • Requirement for precision of delivery
  • Patient comfort
  • Cost.



  Types of Oxygen Delivery Source Top
[Table 4]
Table 3: Composition of air

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Table 4: Types of oxygen cylinder

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  • Oxygen blenders and flowmeters: Flowmeter attached to 100% oxygen source or blender can deliver oxygen. Three types of flowmeters are available (standard 0–15 L/min and may be up to 60–90 L/min, low-flow flowmeters 1–3 L/min increment of 0.1 L/min, and micro-flow flowmeters 0.1 L/min with increment of 0.01 L/min).
  • Oxygen concentrators (stationary or portable): These are primarily used at home (if oxygen use is >1.4 h/day) and in primary healthcare settings. These devices use room air for oxygen supply using molecular sieve. They can deliver oxygen from 0.5 L to 10 L/min depending on the type of concentrator. Increasing flow rate will decrease oxygen concentration. Most of them need electricity for their operation and does not depend on oxygen source. There are two types of concentrator; molecular sieve and semi-permeable membrane type. Oxygen concentrator can deliver oxygen in continuous flow does or pulse mode
  • Compressed gas cylinders: Portable compressed gas cylinders in different sizes are commonly used in hospitals and home. Usually, it is available in two sizes, i.e., 3.2 kg and 2.1 kg, and lasts for approximately 3.5 and 2.5 h at 2 L/min. Duration can be increased if cylinder is made to deliver oxygen during inspiration only. There is increased risk of fire due to pasteurization. Devices are available which releases oxygen during inspiration only. Large cylinders if combined may be used as manifold system to deliver medical oxygen
  • Central gas supply: Compressed or liquid gas is used in larger hospitals (at a pressure of 4 bar, 400 kPa) attached with a flowmeter, which is capable of delivering oxygen at 15 L/min
  • Liquid oxygen (LOX: vacuum insulated evaporator): Oxygen can be stored in the liquid form at a temperature of −183°C and can be in the gas form at a temperature of −118.4°C and above at any given pressure. The refill unit last longer compared to compressed cylinder. If not used, the cylinder will evaporate in 2 days' time
  • Heliox is a mixture of helium and oxygen and less dense than oxygen; it is used to carry the oxygen past airway obstruction. Because heliox is less dense than pure oxygen, hence it has a faster flow. Multiply oxygen flow value by A factor of 1.8 (if ratio is 80:20), 1.6 (if it is 70:30), and 1.4 (if it is 60:40) to get actual flow. Nonrebreathing mask with reservoir is used to deliver heliox [Figure 2].
Figure 2: Heliox delivery system

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Calculation of life of oxygen cylinders (various types) in minutes [Table 4] is given below:



When gauge fraction is known,

  • Capacity × 860 × gauge fraction/flow
  • PSI − 200 × cylinder factor/flow rate.


Liquid oxygen

Liquid oxygen is prepared at −183°C at 1 atm (pale blue liquid).

Duration in minutes = 344 × weight in pond/flow in L/min

One liter liquid oxygen weighs 2.5 lb.

One liter liquid oxygen = 860 L of oxygen in the gas form


  Color Coding of Different Gas Cylinders (Shoulder) Used in Acute Care Top


  • Oxygen: Green or white
  • Air: Yellow or black
  • Nitrous oxide: Blue
  • Nitrogen: Black
  • Carbon dioxide: Gray
  • Helium: Brown
  • Ethylene: Red
  • Carbon dioxide and oxygen: Gray and green.


Composition of Air

Composition of Air is a mixture of many gases as shown in [Table 3]. [Table 4] shows the various types of oxygen cylinder available along with capacity. Properties of various gases available for human use is shown in [Table 5].
Table 5: Properties of commonly used medical gases

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Factors affecting amount of FiO2 delivered to the patient

  • Flow/minute or percentage of oxygen
  • Device: High or low flow, fixed or variable flow
  • Respiratory rate, I:E ratio, TV, depth of respiration, pattern, and MV
  • Above may not hold true in hemodynamically unstable patients.


Example 1: Adult with 60 kg has TV 500 mL, RR 20/min, I:E 1:2 (inspiratory time 1 s and expiratory time 2 s), flow of oxygen 6 L/min (100 mL/s), dead space 150 mL (usually 1/3rd of TV: 2 mL/kg), nasopharyngeal space 1/3rd of dead space, i.e., 50 mL, and usually no expiratory flow during the last 1/4th time of expiratory time; the filling of reservoir occurs during the initial 1/4th of expiration time.

Example 2:

  • 10 kg child, breathing rate 40/min, flow of oxygen 2 L/min
  • TV: 10 × 6 = 60 mL
  • Anatomical reservoir: 2/3 kg × 10 = 6.6 mL
  • Respiratory cycle = 60/40 (1.5 s)
  • I: E ratio = 1:2
  • Inspiratory time: 0.5 s
  • Expiratory time: 1.0 s
  • Flow: 2000/60 = 33.4 mL/s
  • Filling time: 1/4th of expiratory time (no flow in the last part of expiration) = 0.25 s
  • Inspiratory time × flow/s: 0.5 × 33.4 = 16.7 mL
  • Anatomical reservoir: 0.25 × 33.4 = 8.4 mL
  • Actual is 6.6 mL
  • Room air volume: TV – anatomical reservoir = 60 − 16.7 + 6.6 = 36.7 mL
  • Oxygen concentration of room air volume: 36.7 × 0.21= 16.7 + 6.6 + 7.7 = 31
  • FiO2: 31/60 = 52% (larger tidal volume and higher rate will result lower FiO2).


Formula to increase FiO2 to get desired PaO2 is given below:



(Air flow × FiO2) + (Oxygen flow × FiO2) = Total flow × Expected FiO2

Flow depends on MV and I:E ratio.

Flow = MV × (I + E ratio)


  Humidification of Oxygen Top


Ideally, gas should be humidified to 37°C and 44 mg of water/L. Oxygen if not humidified can lead to dryness of secretion, atelectasis, increased airway resistance, risk of infection, substernal pain, and increased work of breathing. Oxygen can be humidified either by cold water or heated water. Heated water humidification is better, but the risk of injury is always there. Oxygen flow >4 L/min or FiO2>35% should be humidified. Humidified oxygen delivered through Venturi can decrease FiO2 since it will block the holes. Two types of humidification devices are available ([low flow or bubble or diffuse humidifier designated for flow of ≤10 L/min] and high flow designated for flow >10 L/min and heat the gas at desired temperature and is fully humidified). Water should be sterile and changed after 24 h. Bottle can be changed as per manufacturer instruction.


  Oxygen Delivery Devices Top


DO2 devices can be classified into three categories:

Category based on the patient inspiratory flow rate

Low-flow devices

These devices have variable performance (deliver variable fraction of oxygen concentration (FiO2) and delivery of gas does not meet patient inspiratory flow demand, e.g., nasal cannula, moustache and pendant reservoir cannula, pulse-demand oxygen system, simple face mask, rebreathing mask (partial and non-rebreathing), and transtracheal catheter. Common problems encountered are inaccurate flow (when flow is ≤3 L/min), leak, obstruction, skin irritation, and device displacement.

High-flow devices

Air entrainment mask, oxygen hood, incubator, oxygen tents, oxygen blenders, ventilator.

  • Fixed performance devices
  • Variable performance devices.


Reservoir systems

These devices provide higher concentration of oxygen compared to low-flow system. These devices can provide comparable or even higher FiO2 with low flow. Various examples are reservoir cannula, simple face mask, partial nonrebreathing mask, and nonrebreathing reservoir circuit. System can deliver low (<35%), medium (35%–60%), and high (>60%) concentration of oxygen. Device can deliver variable (some inspired gas) or fix concentration (all inspired gas) of oxygen. Problems encountered are removal of device, erythema, and improper flow adjustments.

Categories based on FiO2 delivery

  • Low FiO2<35%
  • Moderate FiO235%–60%
  • High FiO2>60%
  • Full-range FiO221%–100%.


Low-flow devices

These devices deliver oxygen at flow rate less than the patient inspiratory flow rate/demands. The FiO2 depends upon patient's TV, inspiratory flow, minute volume, delivered oxygen flow, ventilatory pattern, and size of oxygen reservoir. Low-flow devices are useful in spontaneously breathing patients with fairly stable vitals.

Nasal cannula/prong [Figure 3]
Figure 3: Application of nasal cannula

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  • Two soft prongs are usually 1 cm long (straight or curved) attached with oxygen supply tube. Curved prongs enhance the laminar flow of gas while straight prongs deliver turbulent flow
  • Useful in patient who has good inspiratory efforts and require minimal oxygen therapy (FiO2<30%). Nasal cannula delivers variable fraction of oxygen concentration (FiO20.22–0.95 depending on flow rate and TV)
  • Available in variable designs and sizes for neonate, infant, child, and adult. Size should not be more than 50% of nares diameter
  • Use humidified oxygen when flow rate is greater than 4 L/min
  • Flow >6 L/min can cause nasal irritation and dryness
  • Flow should be kept <2 L/min below 2 years of age
  • Prong size should be approximately 50% of nares
  • Put the nasal cannula on the upper lip with prong pointed in the nostril and secure the cannula around head
  • Nasopharynx acts as reservoir (1/3rd of anatomical dead space)
  • If patient takes breath from mouth, flow produces Venturi effect
  • Flowmeter of different rate may be used depending on the age of patient (micro-low flowmeter, low flowmeter, and regular flowmeter)
  • Can deliver high flow in newborn if low is kept >4 L/min (inspiratory flow is 6 mL/kg × respiratory rate × 4) in normally breathing child
  • This device can be used as high-flow nasal cannula (HFNC) for short duration
  • Nasal, paranasal sinus and skin irritation are the risk associated with nasal prongs
  • Cannula to be changed 1–2 weeks.


Nasal reservoir cannula

Reservoir just placed below nose of 20 mL capacity approximately increases the FiO2 and conserves oxygen.

Nasopharyngeal catheters [Figure 4]
Figure 4: Nasopharyngeal catheters and measurement of length

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  • Available in various sizes for both children and adults (12–14 F)
  • Select size by comparing the external nostrils
  • Made of soft plastic having blind end with multiple holes on the side near tip
  • Measure length from nostril to tragus of ear for nasal catheter
  • Put the catheter from external nostril to just behind the uvula
  • Fix the catheter with tape
  • Nasal cavity acts as reservoir
  • Risk of blockade of catheter is high
  • Delivers variable FiO2
  • Give humidified oxygen if flow rate is ≥4 L/min in children ≤2 years, ≥6 L/min above 2 years of age, and >10 L/min in adolescents. Flow should not exceed >6 L/min through nasal prong in small children
  • Useful in less severe cases
  • No better than nasal prong
  • Not to be used in suspected nasal mucosal tear: risk of emphysema
  • Deep insertion causes gastric distension.


Simple oxygen face mask [Figure 5]
Figure 5: Face mask

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  • Simple, transparent, light weight mask covers both mouth and nose
  • Easily to apply and available for both pediatric and adult population
  • Minimum flow rate to be kept is 4–6 L/min. Rebreathing occurs at lower flow rate. Flow ≥40 L/min can be used to deliver higher FiO2. High flow becomes turbulent, has Venturi effects, thus diluting the FiO2 though generates some amount of Positive End Expiatory Pressure (PEEP)
  • Delivers variable/unpredictable FiO2
  • Mask reservoir capacity 100–250 mL
  • Useful only in spontaneously breathing patients with respiratory distress
  • Requires tight seal
  • Difficulty in feeding and communication
  • Difficult keep for long
  • Tusk mask if exhalation valves replaced by 6 inch tube to prevent air entrainment during inspiration [Figure 6].
Figure 6: Tusk mask

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Blow by oxygen

Children who cannot tolerate device may be given oxygen by tubing or simple face mask (FiO20.3–0.4 at flow of 10 L/min) for short-term use.

Pocket mask or resuscitation mask

Pocket mask is available in pediatric and adult size and used to deliver rescue or manual breath during resuscitation.

Partial rebreathing and nonrebreathing masks

[Figure 7] and [Figure 8]
Figure 7: Partial nonrebreathing mask

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Figure 8: Nonrebreathing mask

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These are simple, transparent, disposable oxygen masks with reservoir. Nonrebreathing mask have two types of one-way valve (one present between reservoir bag and mask and second at exhalation port) so that higher FiO2 can be delivered. They are effective in spontaneously breathing patient for short period. They are available both in pediatric and adult size.

  • Indicated in all types of seriously ill patients who are spontaneously breathing and require high concentration of oxygen
  • Keep the reservoir bag full, i.e., flow of gas must be 6–8 L/min to avoid rebreathing of carbon dioxide. Flow should be adequate to maintain the reservoir bag at least one-third to one-half full on inspiration
  • Application is similar to simple face mask
  • Reservoir capacity 1 L
  • Partial rebreathing mask delivers FiO20.4–0.6 at a flow of 6–8 L/min depending on ventilatory pattern
  • First 33% of exhaled air fills the reservoir derived from anatomical dead space with little PCO2
  • During inhalation, first exhaled gas and fresh gas are inhaled making the device partial nonrebreather
  • Useful during transport
  • Conserve oxygen.
  • Guidelines for estimating FiO2 with low flow devices [Table 6]
Table 6: Guidelines for estimating FiO2 with low-flow devices

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High-flow oxygen delivery device

High-flow nasal cannula

HFNC (1–70 L/min) is classified as a fixed-performance DO2 system that is capable of delivering a specific warm and humidified (relative humidity 100%) oxygen concentration at flows that meet or exceed the inspiratory flow demand of the patient. Evidence suggests that HFNC can generate Continuous positive airway pressure therapy (CPAP) also. Care should be taken to use nasal cannula with not more than 50% diameter that of nares. HFNC can deliver FiO2 of 21%–100%. The flow should be set more than child inspiratory flow needs.

Oxygen hood or head box [Figure 9]
Figure 9: Oxygen hoods

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  • Primarily used in children below 1 year of age or <10 kg
  • Flow of oxygen should be kept more than 7–10 L/min. Flow of <6–7 L/min can lead to CO2 rebreathing
  • Delivers variable FiO2(0.22–0.80)
  • FiO2 should be measured near baby face due to layering effect of oxygen
  • Face may be compressed against wall and temperature variability of gas may cause certain problem
  • Keep sound level below 65 dB.


Self-inflating bag [Figure 10]
Figure 10: Parts of self-inflating bags

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This is a variable performance device (oxygen flow, TV, and RR) used in emergency. FiO2 can be increased by attaching oxygen and reservoir. Device may worsen the respiratory distress if used in patient having good spontaneous breathing efforts but are in distress. When the self-inflating bag is compressed during inspiration, flow from the oxygen supply is directed into the reservoir. If the reservoir becomes full, which may occur if MV is less than the oxygen flow, a pressure-release valve prevents a dangerous build-up of pressure within the device. During expiration, the self-inflating bag refills from both the oxygen supply and the reservoir bag. If the oxygen supply and the reservoir bag provide insufficient gas to refill the device, room air is entrained through the entrainment valve. The “duck-billed valve” at the patient end of the device ensures that minimal rebreathing of respiratory gas occurs and also allows the incorporation of a spring-loaded PEEP valve that is connected to the exhalation port of the breathing circuit if it is required.

These are primarily used during resuscitation or intubation. Reservoir bag is attached to the device to increase the oxygen concentration. Self-inflating or AMBU bag can also work without oxygen source. In seriously ill or trauma victim, open the airway and put the Guddel's airway (when gag reflex absent) or Laryngeal mask airways (LMA) or double-lumen tube and attach to the self-inflating bag. Squeeze 10–12/min in adolescent and 20/min in small infants and see the chest rise. Do not squeeze hard and fast. Only half of the size to be squeezed. There is risk of hypoventilation, hyperventilation, hypoxia, excessive pressure, gas wastage, gastric distension, aspiration, and rise in the intrathoracic pressure.

Flow inflating bags [Figure 11]
Figure 11: Flow inflating bag with JR circuit

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They are used in patients who are not breathing spontaneously and intubated with Endotracheal Tube (ET tube) or Laryngeal Mask Airway (LMA). This device gives 100% FiO2. Oxygen source is essential for its use. They are primarily used in operation theaters.

Venturi mask/venti mask/jet mixing systems/high flow with oxygen enrichment system [Figure 12] and [Figure 13]
Figure 12: Principles of Venturi mask function

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Figure 13: Venturi mask

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Venturi mask is a high-flow device to deliver fixed oxygen concentration (24%–50%) device based on Bernoulli's principle and useful in patient's having variable respiratory rate, I:E ratio, TV, and MV.

  • Mixes a specific volume of air and oxygen
  • Useful for accurately delivering the low concentrations of oxygen
  • Valves are color coded and flow rate required to deliver a fixed concentration is shown on each valve [Table 7]
  • Deliver oxygen concentrations between 24% and 60%
  • Due to high-flow rate no risk of rebreathing and no increase dead space
  • The total flow must be equal or greater than peak inspiratory flow (four to six times of MV). More may be required if patient is in respiratory distress
  • Peak inspiratory flow rate is the maximum flow at which a set tidal volume breath is delivered (inspiratory flow = TV in liter/inspiratory time in seconds). To calculate flow in L/min multiply above by 60
  • Total flow can be raised by increasing oxygen flow in a particular color code without affecting the FiO2. One may analyze the FiO2 inside the mask
  • Total flow can be calculated by adding the ration and multiplying with oxygen flow rate.
Table 7: Guide to colors of Venturi valves

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FiO2= (Air flow × 0.21) + (Oxygen flow × 1.0)/Total flow

As the oxygen flow increases, the total flow decreases; hence, device may behave low flow device.

Example: Air:O2 ratio for an air entrainment mask at FiO240%?

Air:oxygen = 100 − FiO2/FiO2– 21 = 100 − 40/40 − 21

  • Ratio for 40% is 3.2:1
  • If the O2 flow meter is set at 10 L/min, entrained air will be 10 × 3.2 = 32 L/min


Total flow = (Air + O2) = (10 + 32) = 42 L/min

Nebulizers

Air entrainment nebulizers can provide FiO2 between 28% and 100% with humidification.

Child with endotracheal tube or tracheostomy tube should receive heated aerosol to eliminated humidity deficiency.

Closed incubators

Oxygen is attached to the incubators at higher flow rate.

Ventilators

Ventilator can give fixed FiO2 up to 100%.

Face tents

  • Useful in children who cannot wear face mask or nasal cannula
  • Mask fit around neck, jaw, cheeks, ear, and above the level of nose
  • Heated humidified oxygen at flow of 5–10 L/min is given
  • Monitor FiO2 around the face
  • Oxygen being heavier than air may settle or pour down from depending on the posture of child so difficult to know FiO2.


Tracheostomy collar

These can be used to deliver oxygen in tracheostomized child. The air should be humidified as upper airway is bypassed.

T-piece adaptor

This can be used in child who has endotracheal tube in situ. It has three adaptors one of 15 mm diameter to fit with ET tube and two with oxygen and nebulizer.

Hyperbaric oxygen therapy

Use of oxygen under more than one atmospheric pressure (usually 2–3 atm).

Monitoring a child with oxygen therapy

  • Pulse rate
  • Respiration: Rate/min, shallow or deep breathing
  • Respiratory effort: Strong or poor
  • Oxygen flow rate: L/min
  • Oxygen saturations: Usually, after 5 min of start of therapy. Limitations are poor perfusion, anemia, CO poisoning, skin pigmentation, nail varnish
  • Transcutaneous PO2
  • Type of device used
  • Connections to check leaks
  • Capillary refill time: >3 s is always abnormal
  • Blood pressure
  • ABG si o´pus sit (SOS)
  • Lactate level (SOS).


Risk associated with oxygen supplementation

It depends on FiO2, PaO2, and duration of exposure. Higher the duration of exposure and PaO2, more the side effects. Limit the high oxygen concentration (100%) beyond 24 h and decrease gradually (concentration to be decreased by 70% within 2 days and 50% within 5 days). It is the PaO2 and not the FiO2 responsible for harmful effects. Physiological response in healthy individual exposure to 100% oxygen includes:

  • 0–12 h: Normal Pulmonary Function Test (PFT), tracheobronchitis, chest pain
  • 12–24 h: Decrease vital capacity
  • 25–30 h: Decrease lung compliance, increase PA-aO2
  • 30–72 h: Decreasing diffusing capacity.


  • Worsening of hypercapnia in patient with Chronic Obstructive Pulmonary Disease (COPD) (depression of ventilation by 20%)
  • Retinopathy of prematurity in preterm neonates
  • Hyperoxemia leads local vasoconstriction, paradoxical myocardial hypoxia
  • Free radical damage, oxidative stress, alveolar fibrosis, hypertension
  • FiO2>0.50 is associated with absorption atelectasis
  • FiO2 of 1.0 for 3 h may give symptoms similar to bronchopneumonia
  • Chest pain, cough, tinnitus, paresthesia, anorexia, nausea, vomiting, seizure
  • Bronchopulmonary dysplasia
  • V/Q mismatch, reduced compliance, and forced vital capacity
  • Depression of lung barrier function: ciliary function, decreased tracheal mucus velocity, lymphocytic infiltration
  • Coronary vasoconstriction, reduced cardiac index, reperfusion injury, reoxygenation injury in children with cyanotic heart disease
  • Necrotizing enterocolitis
  • Intraventricular bleeding
  • Central nervous system toxicity
  • Risk of fire or explosion during defibrillation and bronchoscopic laser therapy
  • Drugs enhancing oxygen consumption are epinephrine and nonepinephrine, and free radical production is nitrofurantoin and bleomycin or impairing endogenous antioxidant system (cyclophosphamide), thereby increasing oxygen toxicity.


Alternative methods to improve oxygen delivery

  • Keeping airway patent by various well-defined methods
  • Position of comfort, e.g., Fowler position in congestive cardiac failure
  • Optimizing circulation to maintain tissue perfusion
  • Improving cardiac output
  • Correcting anemia
  • Avoid respiratory depressant drugs
  • Recruitment of alveoli by various means by noninvasive and invasive me thods
  • It is essential to oxygenate before and after suctioning
  • Treatment of underlying etiology.



  Conclusion Top


  • FiO2 remains constant at all altitude
  • FiO2 multiplied by 5 is usual PaO2
  • Flow rate does not increase FiO2, it is oxygen saturation which is important
  • Inspiratory flow rate: 30 × 21 = 630%


  • 630 ÷ 30 = 21%

    • If flow 10 L/min of 100% oxygen and 20 L/min 21%


      • (10 × 100) + (20 × 21) =1420%
      • 1420 ÷ 30 = 47%


    • •If flow rate is 50 L/min


      • (10 × 100) + (40 × 21) =1840%
      • 1840 ÷ 50 = 37%


    • •If flow rate is 20 L/min


      • (10 × 100) + (10 × 21) =1210%
      • 1210 ÷ 20 = 60%


  • Body does store small amount of oxygen
  • No need to humidify oxygen if flow is <4 L/min
  • FiO2 depends on breathing rate, TV, respiratory efforts, depth of respiration, MV
  • DO2 depends on arterial oxygen content ([Hb × 1.34 × SaO2] + [PaO2×0.00031]) and cardiac output
  • Oxygen is a drug; its proper prescription should be written and target should be well defined before prescription. Delivery system and flow rate should be defined and documented
  • Monitoring device should be available for child on oxygen supplementation
  • Critical DO2 is 4 mL/kg/min
  • Prolonged exposure of FiO2>0.5 is dangerous
  • Every 50 mmHg difference in P(A-a) O2 shunt increases by 2%
  • P(A-a) O2 increases by 4 every 10 years of age
  • Small decrease in SpO2 below 90% may lead to dangerous fall in PaO2
  • At high PaO2 change in pressure leads to small change in SpO2
  • Conservative oxygen therapy includes target SaO2>90% (88%–92%).
  • Hyperoxemia leads to decrease in heart rate, cardiac output, coronary blood flow, and brachial artery blood flow and increase in vascular resistance.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Chang DW, White GC, Waugh JB, Restrepo RD. Respiratory Critical Care. Burlington: Jones & Bartlett Learning; 2020.  Back to cited text no. 1
    
2.
Mastropietro CW, Valentine KM. Pediatric Critical Care: Current Controversies. USA: Springer; 2019.  Back to cited text no. 2
    
3.
Landsberg JW. Manual for Pulmonary & Critical Care Medicine. USA: Elsevier; 2017.  Back to cited text no. 3
    
4.
Stockwell JA, Kutko MC. Comprehensive Critical Care: Pediatric. USA: Society of Critical Care Medicine; 2016  Back to cited text no. 4
    
5.
Volsko TA, Barnhart S. Foundation In Neonatal and Pediatric Respiratory Care. Burlington: Jones & Bartlett Learning; 2020  Back to cited text no. 5
    
6.
Wilmott RW, Sly P, Deterding R, Zar HJ, Li A, Bush A, et al. Kendig's Disorders of Respiratory Tract in Children. Philadelphia: Elsevier; 2019.  Back to cited text no. 6
    
7.
Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. Missouri: Elsevier; 2017.  Back to cited text no. 7
    
8.
American Academy of Pediatrics Section on Pediatric Pulmonology and Sleep Medicine Pediatric Pulmonology, Asthma & Sleep Medicine. USA; American Academy of Pediatrics: 2018.  Back to cited text no. 8
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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Abstract
Introduction
Commonly Used Terms
Clinical Indicat...
Etiology of Hypo...
Classification o...
Types of Hypoxia
Mechanisms of Hy...
Color Coding of ...
Humidification o...
Oxygen Delivery ...
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Types of Oxygen ...
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