Oxygen therapy has been a mandatory requirement for more than 100 years unquestionably, to treat acute heart attack, i.e. Myocardial Infarction, most of the times on the basis of anecdotal evidence, expert opinion, and tradition. Recent compelling evidences have challenged this conventional thinking.
|myocardial infarction - Myokardinfarkt - scheme (Photo credit: Wikipedia)|
Patient with AMI has compromised myocardial perfusion and event arises due to myocardial hypoxia. It appears quite logical and biologically plausible to give oxygen in such situations to improve the oxygenation of the ischemic myocardial tissue and decrease ischemic pain.
On the other side, oxygen may be harmful for its’ paradoxical effect in decreasing coronary artery blood flow and increasing coronary vascular resistance, evidenced by intra-coronary Doppler ultrasonography. This effect leads to decrease in cardiac output and stroke volume. Excess Oxygen in blood (hyperoxia) causes increase in vascular resistance and reperfusion injury due to increased oxygen free radicals.
A survey among doctors managing AMI cases had showed that oxygen supplementation was given to 96% of their patients with acute coronary syndrome. About 50% of participants believed that oxygen reduces fatality, 25% thought it is helpful in decreasing pain, and 25% thought it has no effect.
Researchers like, Nicholson, Beasley et al. and Wijesinghe et al. have suggested that efficacy and safety of high flow oxygen in MI is not substantial. The existing evidence suggests that the routine use of high flow oxygen in uncomplicated MI can cause greater infarct size and possibly increase the risk of mortality.
Cochrane systemic review by Cabello et al., did not find any conclusive evidence from randomized controlled trials (four parallel-design, randomized controlled trials reported between 1976 and 2012) to support the routine use of inhaled oxygen in patients with acute AMI.
Recently published, Air Verses Oxygen in myocardial infarction study (AVOID Study) suggested routine oxygen supplementation to AMI patients from the ambulance through to the recovery room might actually be hurting their hearts. AVOID was a randomized, controlled, multicenter trial with the aim of comparing oxygen supplementation (6–8 L/min) with no oxygen in STEMI patients with oxygen saturation in the normal range pulse oximetry saturation >94%.
The study found a significant 25% increase in creatine kinase, suggestive of increased myocardial injury and cardiac magnetic resonance imaging (cardiovascular magnetic resonance) at 6 months suggestive of larger infarction size with oxygen therapy. Although, the mortality was similar in both groups, significant increases in recurrent MI and arrhythmias were observed in the oxygen group. Even though, AVOID Study used higher oxygen flow 6–8 L/min (more than usual clinical practice) and study was not powered for hard clinical end points, AVOID trial would really question the current practice of oxygen supplementation to all patients with acute myocardial ischemia and definitely to those with normoxia.
This subject is being further studied by researchers with the Swedish Coronary Angiography and Angioplasty Registry in an open-label randomized trial DETO2 X-AMI (with more than 5000 enrolled patients in multi centres) with mortality as the primary endpoint. Results are awaited which may have definitive conclusive evidence.
American Heart Association Guidelines for Cardiopulmonary Resuscitation (CPR) and Emergency Cardiovascular Care recommends oxygen in patients with dyspnea, hypoxemic, or with signs of heart failure and shock, based on monitoring of oxy-hemoglobin saturation, to ≥94%. However, evidence to support oxygen use in uncomplicated acute coronary syndromes is inadequate.
Oxygen overdose is not a new, but the way we use oxygen in coronary emergency needs reconsideration. Time has come to reassess oxygen treatment in acute coronary syndrome. Clinical practice should be based on proven benefits and safety, not on tradition. Oxygen is a life-saving drug and how much you give the patients, depends on how much they need.
The question of oxygen administration to all patients of AMI remains unanswered until new strong evidence comes.
The original article can be accessed here.