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Oxygen is essential for normal respiration in aerobic organisms. Upon entering the lungs, O2 molecules dissolve in the lipid and aqueous phase of tissues and diffuses across alveolar membranes and into alveolar capillaries. The vast majority of oxygen is bound to haemoglobin and only a small fraction, less than 2% is dissolved in the plasma. Following the transport to various tissues, oxygen diffuses into the cells for metabolism. The objective of Oxygen Therapy normally - a reduction in arterial oxygenation is compensated for by an increase in cardiac output to avert tissue hypoxia. When this ceases to be adequate, tissue hypoxia is accompanied by acidosis, a fall in standard bicarbonate and pH. Oxygen therapy becomes necessary when the arterial oxygen tension lies on the steep part of the oxyhaemoglobin dissociation curve (see below) i.e. below 50mm Hg.
Oxygen Administration Methods
Oxygen therapy devices for the spontaneously breathing patient fall into two categories, Variable performance and Fixed performance.
1. Variable Performance
These devices administer uncontrolled oxygen therapy, because the patient creates the inspired mixture by the act of breathing. Examples of these are nasal catheters, nasal cannulae and mask shells with or without a rebreathing bag (see pictures below). Various factors cause unpredictability in knowledge and control of inspired oxygen concentration when using these devices. Firstly, because the oxygen flow is less than the patient’s inspiratory flow, the patient obtains some oxygen and some air, but in unknown proportion because normal breathing has an irregular pattern:
The resulting oxygen concentration varies not only on a breath by breath basis, but also from patient to patient. Secondly, the size of the mask, size and placement of the vents and fit of the mask to the face will cause variation in inspired oxygen concentrations.
Variable Performance Devices
The disadvantage of using a variable performance device is that blood gas values cannot be easily interpreted since they only reflect performance variation of the mask/patient interface rather than a true change in the condition of the lungs under treatment.
2. Fixed Performance
These devices (see pictures below) allow controlled (known, fixed and selectable) oxygen dosage. They create a constant proportion of air/oxygen mixture in excess of patient inspiratory flow rate and are independent of patient factors or fit to the face. With gas flow constantly in excess of patient demand and with enhanced CO2 washout, rebreathing is virtually eliminated.
To work efficiently and to prevent air dilution a fixed performance device should incorporate
. A large volume face piece (not less than 280ml).
. Gas mixture flowing directly towards the nose and mouth.
. Vents positioned well away from the patient airway.
Fixed performance devices are sometimes referred to as Venturi or Air Entrainment masks.
