51-365 (Year 7) Blood oxygen level
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Shobha Shukla, Citizen News Service (CNS)
An optimum amount of oxygen is essential for the functioning and survival of all body tissues and even a few minutes deprivation can prove fatal. When saturation level of oxygen in the body falls due to some respiratory illness or injury then we need to replenish it artificially to maintain an optimum level by giving oxygen therapy to the patient. This method of dealing with 'respiratory failure' was explained succinctly by Dr Girija Nair, Head Department of Pulmonary Medicine, Dr DY Patil Medical College, Mumbai on the 1st day of 20th NESCON, (20th National Conference On Environmental Sciences And Pulmonary Diseases), organized by the Academy of Respiratory Medicine in Mumbai under the auspices of Environmental Medical Association.
Oxygen therapy is one of the key treatments in respiratory care. This therapy is used to treat documented hypoxemia (as in case of smokers), severe respiratory distress, severe trauma, interstitial lung disease (ILD), chronic obstructive pulmonary disease (COPD) including chronic asthma and chronic bronchitis, pulmonary hypertension, acute myocardial infarction, and chronic lung patients during exercise. Some may need it for long term oxygen therapy at home (LTOT).
Before prescribing oxygen therapy there should be a clear indication of its need and the type of therapy needed. The doctor should list the purpose of the therapy, demonstrate the procedure and also list possible complications. For safe and effective treatment with this therapy, it should be remembered that the oxygen used in oxygen therapy is a prescription drug which is to be used under a doctor's prescription and not like the oxygen we breathe naturally. The prescription should clearly state the air flow rate, delivery system, duration and instructions for monitoring. Regular evaluation of patient is necessary.
The basic purpose to ventilate patients through oxygen therapy is to improve gas exchange till lung functions improve. Oxygen saturation has to be within normal limits. Target should be pressure (PaO2) not less than 60mm Hg and SpO2 (saturation pressure) not less than 90mm Hg. Excess supply can do more harm than good and one should ensure that this does not happen. 100% Fi02 breathing is associated with decreased ventilation (obstruction) Dr. Nair shared her experience of having a tough time convincing the ICU staff to lower the oxygen flow while administering to patients of COPD requiring low flow oxygen, as COPD patients may worsen with high flow oxygen. Patients with chronic lung disease should never be delivered at more than 2-3 litre/minute oxygen .
Oxygen needed in the therapy can be dispensed/supplied from: Piped in oxygen cylinders which can supply oxygen for up to 57 hours at the desired regulated flow; Oxygen concentrator which has molecular sieves to supply 90% oxygen, and permeable membrane to supply 40% oxygen; and Liquid oxygen which can be used for 7 days at 2litres/minute; can be refilled, weighs less, but is costly and can cause thermal burns as liquid oxygen has a very low temperature
OXYGEN DELIVERY DEVICES FROM SOURCE TO PATIENT
Any system of oxygen delivery would require the following--oxygen supply, flowmeter, oxygen tubing, delivery device and a humidifier (if need be). There are (1) low flow systems which do not provide constant flow oxygen and contribute partially to inspired gas the patient breathes. Examples are nasal cannula, simple mask, non/partial re breather masks and (2) high flow systems which deliver specific and constant percentage of oxygen independent of patient's breathing. Examples are venturi mask, tracheostomy collar mask, and T piece.
Venturi mask mixes a specific volume of air and oxygen and is used for accurate delivery of low concentrations of oxygen between 24-60% at a flow of 4-15litre/minute. The mask is so constructed that there is constant flow of room air blended with a fixed concentration of oxygen. Valves are colour coded and flow rate required to deliver a fixed concentration is shown on each valve. Each colour code corresponds to a precise oxygen concentration and a specific flow rate.
Nasal cannula is a disposable plastic device (having 2 protruding prongs) and connected to an oxygen source for delivering low-medium concentrations of oxygen--24-44% at flow rate of 1litre/minute. They are easy and comfortable to use, but sometimes prolonged use may cause nasal irritation and pharyngeal mucosa.
The simple face mask is made of clear flexible plastic or rubber that can be moulded. It delivers 35-60% oxygen at a flow rate of 6 --10 litre/minutes. It is used when an increased delivery of oxygen is needed for short duration of time less than 12 hours. It is not very patient-friendly and requires frequent monitoring to check if it is placed correctly.
Partial re-breather mask has an oxygen reservoir bag which should remain inflated during inspiration and expiration. It can deliver oxygen concentrations up to 80%. The oxygen flow rate is maintained at a minimum of 6lit/min to ensure that the patient does not re breathe large amounts of exhaled air. The remaining exhaled air exits through vents.
The non re-breather mask also has an oxygen reservoir bag. It provides highest possible concentration of oxygen of 95-100% at a flow rate of 6-15 litre/minute. It has one way valves to prevent conservation of exhaled air. When the patient exhales air, the one way valve closes and all the expired air is deposited in the atmosphere and the patient does not re breathe any of the expired gas. The flow rate is set so that the mask remains two thirds full during inspiration. It is not very user friendly and is suitable for patients with severe hypoxemia but is impractical for long term therapy.
Tracheotomy collar mask is inserted directly into the trachea and is used for chronic oxygen therapy need. It provides oxygen concentration of 8-10%, provides good humidity, is comfortable and less expensive.
Dr Nair cautioned that one has to be careful about side effects of the therapy. Oxygen toxicity can occur with oxygen concentration more than 50% when used for long duration of more than 48 hours. Signs of oxygen toxicity are non-productive cough, nausea and vomiting, sub-sternal and chest pain. Suppression of ventilation can lead to increased carbon-di-oxide and its narcosis. Following simple instructions of maintaining proper hygiene of all instruments is also of paramount importance, said Dr Nair.
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