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10 20 30 40 50 60 70 80 90 100 Inspired oxygen concentration (%)

10 20 30 40 50 60 70 80 90 100 Inspired oxygen concentration (%)

of the cardiac output bypasses functioning lung units creating a large alveolo-arterial oxygen gradient. Oxygen administered does not come into contact with pulmonary capillaries and may not completely reverse this hypoxaemia (Fig. 2.3).

The performance of different oxygen face masks depends on several factors. These are the flow of oxygen, the shape of the mask, total gas flow, and the respiratory pattern of the patient, in particular the peak inspired flow rate (PIFR). Face masks are divided into those of fixed or of variable performance.

Variable performance face masks

These masks (Hudson, M. C., nasal cannulae, etc.) provide an inspired oxygen concentration which varies breath by breath depending on the patient's PIFR and breathing pattern (expiratory pause). They are perfectly satisfactory for the majority of healthy patients undergoing non-major surgery. If there is any doubt about a patient's ability to maintain adequate oxygenation, they probably should not be used.

Fixed performance face masks

They provide a constant known inspired oxygen concentration, independent of the patient's breathing pattern. They

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