Oxygen has the potential for toxicity if given in excess. Recognition that supra-physiological doses cause arterial constriction and may increase mortality has altered recommendations for acute coronary syndromes; oxygen should only be administered when the arterial saturation is subnormal. Reflecting on-going uncertainty, guidelines variably define subnormal as saturation below 90 – 95%. For now, raising the saturation above 96% is sufficient; anymore and you may cause harm. Note this only applies to acute coronary syndromes; patients sick for other reasons my benefit from supplemental O2.
At a partial pressure of oxygen of 60-70mmHg (about 10-11kPa) the oxyhaemoglobin dissociation curve plateaus – the inflection on the sigmoid-shaped curve is normally at a saturation of around 90%.
Oxygen saturations above this level do not significantly increase the dissolution into plasma. In order to do this, you need a higher Hb, or very high supplemental O2 or preferably hyperbaric O2.
Quotations from recent Guidelines:
?Oxygen Insufflation (4?8 L/min) if oxygen saturation is
?A recent Cochrane meta-analysis identified three trials with a total of 387 patients evaluating the value of oxygen therapy in whom 14 deaths occurred. The relative risk of death for those receiving oxygen therapy was 2.88 (95%CI 0.88?9.39) by intention-to-treat analysis and 3.03 (95%CI 0.93?9.83) amongst patients with confirmed acute MI. Although these analyses lacked adequate power the findings suggest increased hazard and the routine use of supplemental oxygen is not recommended. IIa A Oxygen therapy is indicated for patients with hypoxia (oxygen saturationIIa C?
?Oxygen (by mask or nasal prongs) should be administered to those who are breathless, hypoxic, or who have heart failure. Whether oxygen should be systematically administered to patients without heart failure or dyspnoea is at best uncertain.30 Non-invasive monitoring of blood oxygen saturation greatly helps when deciding on the need to administer oxygen or ventilatory support.?
?Oxygen is indicated in patients with hypoxia (SaO2 I C?
?Few data exist to support or refute the value of the routine use of oxygen in the acute phase of STEMI, and more research is needed. A pooled Cochrane analysis of 3 trials showed a 3-fold higher risk of death for patients with confirmed acute MI treated with oxygen than for patients with acute MI managed on room air. Oxygen therapy is appropriate for patients who are hypoxemic (oxygen saturation
Note that the above analysis may not apply to patients who are unwell for reasons other than an acute coronary syndrome. Alterations in pH or pCO2 shift the oxyhaemoglobin dissociation curve and carbon monoxide displaces O2 from haemoglobin. Basal atelectasis occurs frequently in those bed ridden and the associated shunting is partially relieved by supplemental O2.