If speed is essential, deep breathing quickens preoxygenation (Fig 1). An O₂ concentration of 80% can be achieved with four vital capacity breaths within one min.

In order to meet the greater inspiratory flow requirements of vital capacity breathing, the fresh gas flow rate must be increased to at least 15 L/min to ensure the reservoir bag does not collapse. Alternatively, a larger reservoir bag may be inserted into the breathing system.

Within four breaths the alveolar concentration has almost reached the plateau concentration of ~85% O₂

Fig 2 shows the higher oxygen flow needed when vital capacity breathing is used.