Oxygen therapy

Inspired oxygen concentration (Fio2) is titrated to arterial oxygen tension (PaO2) and arterial oxygen saturation (SaO2). The risks of oxygen toxicity are reduced by administering the lowest concentration compatible with normoxemia or modest hypoxemia. In refractory acute respiratory failure, targets of PaO2 >8kPa (60mmHg) or Sa02>90per cent represent reasonable goals. Benefits depend on the degree of right-to-left shunt present in the pulmonary circulation. Large shunts (above 30per cent) effectively nullify the effects of increasing FiO 2. In the spontaneously breathing patient, face-mask oxygen is administered from variable- or fixed-performance devices. The efficacy of variable-performance systems (e.g. Hudson or MC masks) depends on the oxygen flow and the respiratory pattern of the patient. As patient peak inspiratory flow rate can exceed 60 l/min, and thus the fresh gas delivery to such devices, entrainment of ambient air occurs. This results in variations in FiO 2 and rebreathing of dead-space gases. Fixed-performance systems (e.g. Ventimask) are preferable, as they rely on the Bernoulli principle to deliver fresh gas flow at a rate greater than patient peak inspiratory flow rate. The ratio of entrained air to oxygen determines FiO 2. By definition, acute lung injury and ARDS do not respond to oxygen therapy alone and further intervention is necessary.

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