Oxygen is usually given by face mask, although nasal prongs or cannulas may be better tolerated. A fixed performance, high flow, air entrainment mask can provide a known fractional inspired oxygen concentration ([Fio.sub.2]) within the range 0.24-0.60. The fractional inspired oxygen concentration is not known with the more common variable performance masks. The maximum concentration is 0.6 unless a reservoir bag is added to the mask.

Non-invasive respiratory support

If the patient remains hypoxaemic on high flow oxygen (15 1/min) continuous positive airways pressure (CPAP) may be used. The technique improves oxygenation by recruiting underventilated alveoli and so is most successful in clinical situations where alveoli are readily recruited, such as acute pulmonary oedema and postoperative atelectasis. It is also helpful in immunocompromised patients with pneumonia. As intubation is avoided the risks of nosocomial pneumonia are reduced. The continuous positive airways pressure mask often becomes uncomfortable and gastric distension may occur. Patients must therefore be cooperative, able to protect their airway, and have the strength to breathe spontaneously and cough effectively.

Non-invasive ventilation refers to ventilatory support without tracheal intubation. This can be used as a first step in patients who require some ventilatory support and who are not profoundly hypoxaemic. Ventilation through a nasal or face mask may avoid the need for intubation, especially in exacerbations of chronic obstructive airways disease. Some patients with chronic ventilatory failure rely on long term non-invasive ventilation. It may also have a place during weaning from conventional ventilation. External negative pressure ventilation, historically provided by an "iron lung," is now provided by a cuirass system.

Ventilatory support

Endotracheal intubation and ventilation is the next step in the management of respiratory failure. Clinical symptoms and signs are generally more useful than arterial blood gas analysis or measurements of peak expiratory flow rate and vital capacity in deciding the need for intubation. However, some findings confirm the imminent need for ventilation. These include hypoxaemia in patients receiving maximum oxygen therapy ([Pao.sub.2] [is less than] 8 kPa, or [Sao.sub.2] [is less than] 90%), hypercapnia with impairment of conscious level, and a falling vital capacity in patients with neuromuscular disorders.  Medical Supplies.