Oxygen Therapy

Outline:-

Oxygen transport in the blood

Indications for oxygen therapy

Types of Hypoxia

Methods of oxygen administration

Complications of oxygen therapy

Oxygen transport in the blood:

• O₂ is transported in 2 forms:-
• Combined with Hb- 1g carries 1.34mls of O₂
• in solution in plasma-100mls carry 0.3mls O₂

Oxygen content:- 100mls of blood with 15g Hb will carry 20.4mls O₂ of which 20.1ml is combined with Hb and 0.3mls in plasma solution

The oxygen available to tissues depends on: oxygen content, cardiac output, n local circulaton to tissues. i.e. oxygen content is 20/100ml=200ml/L, and C.O. 5L/min = 200x5=1000ml/min of oxygen

Only a max of 80% can be extracted by the tissues.

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Indications for oxygen therapy:

• Hypoxia
• Displacement of air from body cavities e.g pneumoencephalogram, pneumothorax, air embolism, caisson’s disease etc.
• Non-aerobic infections e.g. clostridium welchi infection, gas gangrene
• Severe anaemia
• Hypermetabolic states e.g. malignant hyper-thermia, thyroid storm, etc

Types of hypoxia :

• Hypoxic hypoxia- inadequate oxygen is reaching the pulmonary capillaries i.e breathing gases low in oxygen content.
• Anaemic hypoxia- low Hb e.g. anaemia, severe haemorrhage, carbon monoxide poisoning
• Stagnant hypoxia- circulation to tissues is compromised e.g hypovolaemic shock
• Histotoxic hypoxia-tissues unable to utilise oxygen supplied e.g. cyanide poisoning
• Demand hypoxia- increased demand for oxygen makes supply inadequate e.g malignant hyper-thermia, thyroid storm and shivering.

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Methods of oxygen administration:

• Nasal prongs (cannulae)- in nostrils @ 2-3l/min. it’ comfortable but easily displaced, and unsuitable for mouth breathers
• Nasal catheter- distal end in the nasopharynx. Flow rate 2-3l/min. It’s less easily displaced, but less comfortable, and can lead to gastric distension from swallowing oxygen.
• Simple face mask- covers both mouth & nose, should be transparent. Flow rate 3-6l/min, suitable for both nasal & mouth breathers, but interferes with feeding & talking.
• Ventimask- for controlled oxygen therapy esp in patients with COPD- 24%, 28%, 32% etc. It’s expensive.
• Oxygen hood-placed over patient’s head, usually children. It’s well tolerated, but access to head interrupts therapy.
• Oxygen tent-for neonates and infants, usually put in oxygen rich environment
• Ventilator-spare work of breathing, but very expensive
• Hyperbaric oxygen chamber- oxygen therapy under pressure, so more oxygen dissolved in plasma. It’s useful in severe non-aerobic infection & decompression illness. It’s very expensive & elaborate/cumbersome

The choice of method depends on- whether high or low FIO₂ (fraction of inspired oxygen) is required, whether control of FIO₂ is important, whether patient is intubated or not.

Precautions during Oxygen therapy

• Do not use open fire nearby
• Avoid electric sparks near oxygen source
• Do not store oxygen cylinders in the hot sun or near heat
• Avoid accident of cylinder falls – use an oxygen stand/trolley
• Do not use grease on oxygen regulator/head
• Always use humidifier, do not deliver dry gases to patients

Complications of oxygen therapy- prolonged use:

• Retro-lental fibroplasia especially in preterm babies given high FIO₂ for prolonged period-, leading to blindness
• Pulmonary oxygen toxicity- FIO₂ greater than 40% for prolonged period will lead to stiff lungs, due to impairment of lung surfactant
• Cerebral oxygen toxicity- especially in hyperbaric oxygen chamber which manifests as convulsions
• Apnoea in COPD in patients.