Oxygen is the most important single substance in animal biology.
It constitutes nearly 50% of the mass of our planet, 21% of the air
we breathe, 50% of rock and 89% of water. Yet it took man a
surprisingly long time to discover the significance of oxygen in
terms of combustion, respiration and animal metabolism. 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.
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
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
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) A large volume face
piece (not less than 280ml), b) Gas mixture flowing directly
towards the nose and mouth, c) Vents positioned well away from the
patient airway. Fixed performance devices are sometimes referred to
as venturi or air entrainment masks.